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Technical Manual THERMOFORMING

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Technical Manual THERMOFORMING Powered By Docstoc
					 Technical Manual
THERMOFORMING
    INDEX


    Thermoforming principles                                4
            -History of thermoforming industry
            -Products manufactured by thermoforming

    Suitable polymers for thermoforming                     7
              -Thermal properties
              -Temperature
              -Heat measurement
              -Specific heat
              -Thermal conductivity

    Heating plastics                                       11
             -Heat transfer: conductivity, convection
              and radiation
             -Thermal properties of plastics
             -Heat transmission media
             -Temperatures and forming cycles
             -Establishing the right temperature
              for the material

    Thermoforming equipments                               17
            -Gas furnaces with pressured air circulation
            -Infrared heating furnace
            -Lineal heating electric resistors

    Complementary equipment: vacuum, pressured air
    and mechanical forces                                  25
            -Vacuum forming
            -Pressured air forming
            -Mechanical forming
            -Combined techniques
            -Mechanical support design

    Thermoforming molds                                    31
            -Choosing thermoforming technique
            -Criteria to design thermoformed products
            -Criteria to design thermoforming molds
            -Considerations in designing thermoforming
             molds
            -Materials used to manufacture tthermoform -
             ing molds

2   Thermoforming
    Thermoforming techniques                              46
            -Bi-dimensional thermoforming
            -Tri-dimensional thermoforming (with molds)
            -Molding techniques in infrared heating
             furnace

    Cooling thermoformed products                         51
             -Conventional cooling methods
             -Non-conventional cooling methods

    Cutting thermoformed products                         53
              -Cutting equipment
              -Cutting techniques


    Thermoforming variables                               58
            -Material variables
            -Mold variables
            -Pre-stretching variables
            -Mechanical support variables

    Problem and solution guide                            62

    Appendix                                              68
                    -Glossary
                    -Glass fiber reinforced plastic
                    -Unit conversion table




3   Thermoforming
                  Thermoforming principles

     History of   Since the beginning of the XX century some techniques to form sheets, with materials
thermoforming     such as metal, glass and natural fibers, have been known. The true thermoforming
      industry    principles emerged as thermoplastic materials were developed, which happened dur-
                  ing the second world war. The post-war period brought about mass commercialization
                  and rapid development of equipment and machinery capable to adapt to the manu-
                  facturing modern methods, to make more useful and income yielding products.

                  In the 50s, the volume of thermoplastic material production and the products made
                  with it reached impressive figures. In the 60s, by developing the thermoforming indus-
                  try, the foundations for the future were established. Then huge consumers and prod-
                  uct competitiveness, in the 70s, required high speed productive machinery. Equipment
                  manufacturers met those needs by making machinery capable to produce about one
                  hundred thousand thermoformed individual containers per hour. Sophisticated controls
                  were also required.

                  Since the 80s up to the present, thermoformers have so much relied on their process
                  that they have gone beyond their expectations and have established production lines
                  that can produce finished thermoformed products, not only from sheets but also from
                  resin pellets; besides, they recycle the scrap with minimum control. Equipments have
                  been computerized and at present, they can perform auto-monitoring and diagnostic
                  functions. Nowadays, very complex equipment does not require more than one work-
                  er to handle and control it thanks to electronic advances. Thus, it is believed that the
                  thermoforming industrial labor market will undergo a shortage of technically trained
                  and experienced personnel, since traditional knowledge will no longer be enough.
                  Therefore, lectures, seminars, courses, etc., would be useful to increase thermoform-
                  ers´ general knowledge, and would further advance this well established industry.

Manufacturing     Many of the thermoformed products in use at present have been manufactured to
thermoformed      replace their original use forms. This has taken place so fast that those original ones
     products     have been almost forgotten. For example: it is not easy to remember in what ham-
                  burgers were packed before the arrival of the one piece polystyrene package or what
                  kind of material lined the interior of refrigerators.

                  The following list begins with the area with the most number of thermoformed pieces
                  and continues in a decreasing order up to the one with the fewer pieces.

                  Packaging industry
                  Since the beginning of the thermoforming process, the packaging industry has been
                  the most benefited due to the high productivity and benefits (cost-profit) that it offers.



              4   Thermoforming
    At present, most of the packaging equipments (blister) are high speed automatically
    sustained. These equipments are called "form-fill-seal" and are used to pack cosmet-
    ics, cold cuts, sodas, candies, stationery, etc.

    Take away food industry
    In the growing "take away food" industry, a great deal of thermoformed products are
    used, ranging from a complete meal container (divided containers), to hamburgers and
    sandwich packages, sodas, etc.

    Usually, that industry requires printed thermoformed packages. This printing can be
    made before or after thermoforming. Some examples of this are trays, cups, sandwich,
    hamburger, hot-dog packages, etc.

    Food packaging industry
    Supermarkets are the great consumers of thermoformed containers. The materials
    used are low-cost thermoplastics. These are designed to be piled or placed in differ-
    ent forms. Examples: meat, fruit, eggs and vegetables containers.

    Transport
    Public and private transport such as bus, train subway, plane, car, etc., has within its
    equipment many thermoformed plastic parts. Most of these are used for inside finish-
    ing or non-structural exterior parts. In others, they are used for seats, backs and arms
    of seats, fronts of doors, service tables, wind-shields, instrument protectors, guards,
    spoilers, etc.

    Signaling and advertisements
    These are usually made of acrylic and can consist of only one piece and can be very
    large. Transparent (clear) acrylic is generally used and it is painted on the inside using
    acrylic based paint.

    The use of acrylics for exteriors makes advertisements weather resistant and they vir-
    tually need no maintenance; furthermore, they can stand extreme cold or hot weather
    conditions. Exterior lighted bill-boards, interior advertisements, signaling in public
    places, offices, etc., are some examples.

    Household products
    There is a great deal of products that have thermoformed parts; actually, they are pro-
    duced in great quantities. They can be found in cabinet, washing machines, dish
    washers, dryers, refrigerators, air conditioning outlets, humidifiers, T.V. and radio cab-
    inets, etc.

    Food industry
    One of the oldest and greatest thermoformed product consumers is the food industry.
    The use of trays and other accessories has a greater potential use, besides the great

5   Thermoforming
    users like hospitals, nurseries, schools fairs and others, there are the military sector
    and international aid organizations. Some examples of products are: trays, cups and
    plates.

    Medical industry
    The medical industry requires a great variety of products and sterilized packaging for
    hospitals, clinics and doctors´ offices. The specifications for these products are usual-
    ly very strict and recycling materials is unacceptable.

    The use of acrylic , since it is physiologically harmless, is growing every day. Some
    examples are: chirurgical equipment, syringes and needles, chirurgical tables, cabi-
    nets, incubators, dentists´ seats and exercise platforms.

    Agriculture and horticulture
    Commercialization of decoration plants in supermarkets and specialized shops has
    generated, for some time, the need to make flower pots and small containers, includ-
    ing with multiple divisions for exhibiting and selling. This kind of containers are made
    of recycled plastic at low cost. Flower pots, different size and divided containers, small
    green houses, trays for growing seeds, planting containers, etc., are some examples.

    Constructión and housing
    For some years, construction industry has used thermoformed products, which have
    become quickly popular. Thermoformed parts have easily replaced a lot of products.
    Actually, there are products that cannot be manufactured any other way, such as sky-
    lights or cannon arches. In this sector, acrylic is used a lot because of its weather
    resistant properties and its thermoforming quality.

    Examples of these are: skylights, cannon arches, hydro-massage tubs, bath modules,
    wash basins, bathroom screens and cabinets, tables, chairs, lamp stands, kitchen
    items, stairs, frontages, partings, windows, aquariums, etc.

    Luggage
    Some luggage manufacturers are deciding in favor of using the thermoforming
    process, since it has advantages over the injection products. Because it is molded
    effortlessly, the possibility of thermoformed products fracturing is reduced. Examples:
    all kinds of suitcases, briefcases, etc.

    Photography equipment
    One of the oldest thermoformed products is the tray used for developing photos, also
    flash bulbs (metallic reflector) and the magazine for standing cameras, even though its
    manufacturing requires a precision thermoforming technique.




6   Thermoforming
             Suitable polymers for thermoforming

             Basically, every thermoplastic polymer is suitable for the thermoforming process.
             Those materials, when exposed to heating, show an elasticity, hardness, and resist-
             ance capacity, under load variation in their module. With an increased temperature over
             the H.D.T., the material will tend to become rubber-like, having as critical value the
             temperature of annealing of the thermoplastic polymer. This can be seen in the rapid
             bending of the hot sheet, when the force of gravity is strong enough to cause this
             deformity.

             Table 1 shows the suitable and most common polymers for thermoforming, as well as
             their temperature.



                                                   HEATING DEFLECTION           THERMOFORMING
                                                      TEMPERATURE                 TEMPERATURE
                             POLYMERS            AT 264   AT 66     WITHOUT   SHEET   MOLD       AID
                                                  PSI      PSI      CHARGE    TEMP.   TEMP  .   TEMP
                                                  (ºF)     (ºF)        (ºF)    (ºF)    (ºF)      (ºF)

               Extruded acrylic                  201.2     208.4            275-347   149-167
               Cell-cast acrylic                 204.8      230             320-356   149-167
               Cellulose acetobutyrate          149-167   167-176   248-302 284-320
               High density polyethylene                  140-176     212   293-374    203      338
               Polypropylene                    131-149   230-239     284   293-392
               Polystyrene                      158-203   158-212     212   284-338   113-149   194
               High impact polystyrene          185-203   194-203     248   338-356   113-149   194
               SAN                                212       221             428-446
               ABS                              167-239   176-248     203   248-356   158-185   194
               Polyvinyl chloride (RV.C.)         158       167       230   275-347     113     176
               Polycarbonate                      266       248       320   356-446   203-248   284



  Thermal    One of the least considered aspects in thermoforming practice, is that of the ther-
properties   mal properties of polymers which is one of the most relevant and critical aspects of
             the process. Wholly understanding these factors will reduce the risk of long pre-pro-
             duction run or bad adjusting of the product to the outline.

             When we talk about thermal properties, it is indispensable to establish the concepts
             related to this topic. First, it must be remembered that energy often dissipates
             through friction and then it appears as heat or the inner thermal energy of a body.
             Of course, some times, heat in a substance is increased deliberately to change its
             temperature or its form.

         7   Thermoforming
                Specific heat and thermal conductivity are two of the physical properties of polymers
                that are extensively used in thermoforming.

Temperature     In the thermal phenomenon debate some terms and concepts must be included. The
                first thermal property is temperature. Temperature is the measurement of the degree of
                "heat" or "cold" in an object. A temperature scale must be established, water properties
                have been taken as a parameter, specially the degree of ice fusion and water boiling.
                There are three scales to measure the temperature of a substance: the scale in centigrade
                degrees (°C), Fahrenheit (°F), and Kelvin (°K), the first two are the most commonly used.

       Heat     Heat is simply a form of energy, therefore, the suitable physics unit to measure heat is
measurement     the same as the one for mechanical energy and it is the joule (J). As in the case of tem-
                perature, water is used as parameter of substance to define the heat unit. The amount
                of heat needed to raise the temperature of 2.2 pounds of water by one degree [at pres-
                ent it is taken as 58.1ºF to 59.9 ºF (14.5 °C to 15.5 °C) is defined as 1 calorie (cal)].

     Specific   When a calorie is added to 2.2 pounds of water, the water temperature increases 33.8
        heat    degree, for example: if the same amount of heat is added to the same amount of methyl-
                alcohol, the temperature rises about 35.06 degrees, or if 1 cal. is added to 2.2 pounds
                of aluminum, the temperature of the metal rises about 41 degrees. In fact, each sub-
                stance will respond differently when exposed to heat. The amount of heat needed to
                raise 33.8 degree in 2.2 pounds substance is called specific heat of that substance.
                Water works as a parameter and it has been determined as 1 cal./pounds, and it is taken
                as a basis to compare every material. Excepting water, most materials have a specific
                heat, lower than plastics.

    Thermal     Thermal conductivity is one of the three ways by which heat energy can be transferred
 conductivity   from one place to another; it results from the molecular movement and therefore, it
                needs the presence of matter. Heat energy is transferred by collisions where the rapid
                movement of atoms and molecules of the hotter object transfers part of the energy to
                the colder object or the one with a slower movement of atoms and molecules. When a
                substance is heated, it expands, heat increases the volume of a substance and dimin-
                ishes its density. The thermal conductivity of acrylic is 0.0005 cal./seg. cm2

    Thermal     Thermal expansion derives from increasing the temperature of a substance, and as a
  expansion     consequence it expands, actually, almost every substance: solid, liquid or gas has the
                property to increase its size, as its temperature rises. As for thermoforming, when a
                polymer is heated the mobility of molecular chains increases, therefore, they tend to
                separate from each other, increasing the volume and area of the polymer. This proper-
                ty is extremely important especially in thermoformed pieces, which are exposed to
                sudden changes of temperature or weather conditions. In thermoforming, the plastic
                sheet is expanded more rapidly than the metal frame, creating some wrinkles near the
                frame, which disappear when the sheet contracts. The numeric values of the coeffi-
                cients for heating and cooling are identical; this means that it takes the same time for

            8   Thermoforming
    a piece to get hot as to get cool. It must be taken into consideration that there might
    be problems when the thermoformed parts have to be within a very close dimensional
    tolerance. There might be other kinds of problems when there is shrinkage in a male
    mold, making it difficult to remove the part from the mold. The thermal expansion coef-
    ficient of acrylic is 0.00009 cm./cm./°C.




9   Thermoforming
                 Heating plastics

Heat transfer:   In the thermoforming process, the heating operation is one of the longest stages in
 conduction,     which there might be present the most difficulties and material and human resources
  convection     waste. That is why this chapter is devoted to heat transfer, aiming at trying to clarify
and radiation    phenomena that might occur in plastics heating operation.

                 Although scientists have divided heat transfer into three different phenomena: con-
                 duction, convection and radiation, in practice, the three phenomena are concurrent.

                 Conduction
                 This is heat transfer from one part of a body to another part of the same body, or from
                 one body to another which is in physical contact with it, without a substantial dis-
                 placement of the particles of the body.

                 Convection
                 This is heat transfer from one point to another, in a fluid, gas or liquid (by mixing one
                 part of the fluid with another). In natural convection, the movement of the fluid totally
                 derives from the difference in density as a result of different temperatures. In the forced
                 convection, which is the one we are interested in, the movement is produced by
                 mechanical means. When velocity is relatively low, it must be noted that free convec-
                 tion factors, such as different temperature and density, may have an important influence.

                 Radiation
                 This is heat transfer from one body to another that is not in contact with it, by means
                 of a wavy movement through space.

                 For the purposes of thermoforming process, three media for heat transfer are considered:

                 A) Contact with a solid, liquid or hot gas.
                 B) Infrared radiation.
                 C) Internal excitation or by microwaves.

                 The first two ones are very much used in plastic thermoforming and for several of them
                 the scope of temperature is between 120°C and 205°C (250°F and 400°F).




            10   Thermoforming
  Thermal     Plastics are poor heat conductors; therefore, thick sheets need a considerably long
properties    time to heat. In table 8, there are some thermal properties of some materials to be com-
of plastics   pared. In plastic thermoforming the method and size of the heating equipment must be
              taken into consideration.

              Heating a sheet on both sides (sandwich-like heating) helps to reduce the time taken
              in this operation. In some cases, heating time can be reduced if the sheet is pre-heat-
              ed and kept at a medium temperature; however, this is rarely done with less than 6mm.
              thick materials.

              In addition, the amount of heat required to raise the temperature of plastics is high,
              compared with any other material; except water. To estimate the needed heat for a
              sheet, the following formula can be used.

              Required heat = Length X width X thickness X density of material X (specific heat X dif-
              ferent temperature + fusion heat)

              Table 8: Thermal properties of some materials.

                                                                                              THERMAL
                                 SPECIFIC     SPECIFIC      FUSION        THERMAL
                                                                                             COEFFICIENT
                  MATERIALS      GRAVITY        HEAT         HEAT       CONDUCTIVITY
                                                                                              of LINEAL
                                  g/cm3       Btu/ Ib 0F     Btu/lb     Btu ft/sq ft hr 0F
                                                                                              in/in 0F10-5

               Air                0.0012         0.24                        0.014
               Water                 1             1           144           0.343
               Ice                 0.92           0.5          144            1.26                2.8
               Soft wood            0.5           0.4                        0.052                1.5
               Hard wood            0.7           0.4                        0.094                1.5
               Phenol R.            1.5           0.3                          0.2                3-5
               Epoxy R.           1.6-2.1         0.3                       0.1-0.8             1.5-2.8
               Polyethylene        0.96          0.37          55             0.28                 7
               Acrylic             1.19          0.35                        0.108                3.5
               Polycarbonate        1.2          0.30                        0.112                3.7
               Graphite             1.5          0.20                          87                0.44
               Glass                2.5          0.20                        0.59                 0.5
               Quartz               2.8          0.20                        4y8               0.4 y 0.7
               Aluminum             2.7          0.23          171             90                1.35
               Steel                7.8          0.10          171             27                0.84
               Copper               8.8         0.092           88            227                0.92




        11    Thermoforming
   Heat    For practical purposes we will divide the media for heat transfer into 4 types:
transfer
  media    Heating by contact
           The fastest heating method is placing a plastic sheet directly in contact with a hot
           metal sheet. It is specially used in mass production of small and thin items.

           Heating by immersion
           With this method, a plastic sheet is immersed in some liquid that transmits heat as
           evenly and quickly as possible, but its use is restricted to molding parts out of huge or
           very thick sheets, since handling and cleaning of the piece are very difficult

           Heating by convection
           Furnaces with air convection are widely used, because they provide even heating and
           can, to a certain degree, dry some materials that contain some degree of moisture.
           These furnaces provide a huge safety margin as for time variations in thermoforming
           cycles.

           Important note:
           All the above mentioned heating media require a considerable amount of time to pre-
           heat the equipment.

           Infrared heating:
           This method can supply instant heating and therefore, its exposition cycles are very
           short, and sometimes it takes only a few seconds. The main sources of this kind of
           energy are:

           -Quartz lamps that emit in the visible and near infrared.
           -Ceramic or metal resistors that emit more energy in the far infrared.

           The surface of these radiation heaters can be between 599 ºF and 1301 (315°C and
           705°C). It must be noticed that at the highest temperatures, the mass of radiation
           occurs at shorter wave lengths. On the other hand, at lower temperatures, radiation
           expands on longer wave lengths; and this is extremely important, since each plastic
           absorbs infrared radiation in different areas. Only the radiation absorbed is used to heat
           plastic directly.

           Internal heating
           This method has not had enough application in thermoforming because the equipment
           used is very expensive. Besides, it is not suitable for every plastic, and cooling time is
           very long. It is useful in forming processes where localized heating is required on a spe-
           cific area of the material. For example, when forming edges of material which has a
           high loss factor, such as P.V.C.




      12   Thermoforming
                In certain applications, thermoformed products show uneven parts, even when a sheet
                has been uniformly heated. Heterogeneous shrinkage of a sheet is due to the very
                design of that part. In those special cases, controlling heat by section will give more
                uniform wall areas. This procedure is called shading or screening and it consists in
                placing a non-flammable filter to regulate heat (a wire net, asbestos, etc.) between the
                sheet and the source of heat, this will reduce the flow of heat to certain areas of the
                material, and will prevent excessive stretching on that area.

                In more sophisticated equipments, at present, there are electronic controls and ceram-
                ic parabolic elements that allow variability when heating different areas of the sheet.

Temperatures    Before we start with temperatures and forming cycles, we will establish some termi-
 and forming    nology:
       cycles
                a) Temperature to remove items off a mold
                b) Operation: bottom limit
                c) Normal temperature to form
                d) Operation: top limit

                Temperature to remove item off a mold
                It is the temperature at which an item can be removed off the mold without distortion.
                Some times an item can be removed at higher temperature if cooling devices are used.

                Operation bottom limit
                This represents the lowest temperature at which the material can be formed without
                internal effort. This means that the plastic sheet must touch each corner of the mold
                before it reaches its bottom limit. The material processed under this limit will show
                internal effort that later will cause distortions, glow loss, cracking and other physical
                changes in the finished product.

                Normal temperature to form
                This is the temperature at which a sheet must be formed in a normal operation. It must
                cover the whole sheet. Shallow thermoformed items with the aid of air or vacuum will
                allow a bit lower temperatures, and this translates into shorter cycles. On the other
                hand, deep forming requires high temperatures, as well as for pre-stretching opera-
                tions, details or intricate radiuses.

                Operation top limit
                Under this temperature a thermoplastic sheet begins to degrade, and it also turns too
                fluid and cannot be handled. These temperatures can be exceeded, but only with mod-
                ified formulations that improve the physical conditions of the sheet. Injection and extru-
                sion molding, actually use much higher temperatures, but only for very short periods of
                time.


           13   Thermoforming
     General recommendations
     a) The characteristics of a finished product are determined by the kind of thermoform-
        ing technique used.
     b) The material must be heated evenly at the annealing and forming point, before it
        cools below its molding temperature.
     c) Acrylic must cool slowly and evenly while it is in the mold.
     d) The formed piece must be cool before any finishing is done, like spraying paint or
        serigraphy.
     e) In the design of a piece, a 2% shrinkage in both directions and a 4% increase in
         thickness must be taken into consideration, as well as a 0.6% contraction at 1%
        when cooling

     Temperatures and forming cycles
     As it was previously mentioned, one of the most important steps of the thermoforming
     process is determining the right temperature of the material. For acrylic, the right selec-
     tion of annealing or normal temperature will prevent:

     At a low temperature:
     Internal effort concentrates in the thermoformed piece which later, under sudden envi-
     ronmental temperature changes, will emerge as fissures or cracking.

     At high temperature:
     Bubbles and mold marks, due to extreme heating.

     Table 9 shows the ranging temperatures for Plastiglas acrylic sheet, for general use,
     and Sensacryl FP¨, deep molding sheet.

     Table 9

                                                               TEMPERATURE RANGE
                     KIND OF MATERIAL
                                                       BOTTOM LIMIT             TOP LIMIT
                                                           (OF)                    (OF)

       Plastiglas (general use)                             320                    356

       Sensacryl (deep molding)                             356                    392




14   Thermoforming
                   In Mexico, due to the high cost of electricity, it is more common to use a convection
                   furnace with pressured air re-circulation by means of gas, for which a very practical for-
                   mula is very useful to determine the permanence time for an acrylic sheet, taking into
                   consideration the annealing temperature range previously adjusted.

                   Formula: 53.3 X E (inches) = T (min.)
                   Where : 53.3 = Factor, E = Thickness of material, T = time.

                   This formula can be used for thin (0.04 to 0.24 inches) Chemcast sheets. For thicker
                   sheets, the factor has to be changed as follows:

                   Formula: 3 X E (inches) = T (min). Ex: 53.3 X 0.118 = 6.30 min.

                   As it has already been mentioned, there are variables that may modify these formulas,
                   such as: environmental temperature of the place where the furnace is located, cure
                   (especially in extreme weather conditions), material thickness fluctuation and the con-
                   ditions of the equipment among other things.

                   Forming temperature
                   Every thermoplastic material has a process specific temperature. These ranges apply
                   without taking into consideration the way the material will be processed. The most
                   used materials compared with acrylic are mentioned in table 10:

                   Table 10, Ranges of forming temperature

                                        SHEET     BOTTOM NORMAL        TOP       REMOVAL   MOLD       MECHANICAL
                          MATERIAL      TEMP.      LIMIT   (0F )       LIMIT               TEMP   .    SUPPORT
                                                                                  TEMP.                  TEMP
                                         (0F )      (F)
                                                    0
                                                                         (0F )     (0F )    (0F )         . (°F)


                    Acrylic CHEMCAST   320- 356    320       338        356        248     149-167
                    Sensacryl FP       356-392     356       374        392        266     158-176
                    ABS                257-356     257       329        356        185     158-185       210
                    Polycarbonate      392-482     392       455        482        284     194-248       248
                    AD Polyethylene    320-428     320       374        428        185     194-212       338



  EstablishIng     Another important factor in the thermoforming process, is establishing the right tem-
       the right   perature for plastic material. You must bear in mind that apart from the heat trans-
  temperature      mission medium, a sheet must be heated at the recommended range of temperature
of the material    (annealing range), besides, a sheet has to be heated in an evenly way.

                   In practice, it is not easy to accurately establish the temperature of the sheet, even
                   when using contact thermometers; therefore, this determination is based on the per-
                   formance of a sheet. The gradual change in which a sheet yields during the heating


              15   Thermoforming
     process (annealing point), is one of the cues to establish the right temperature. Some
     controls for infrared radiation thermoforming equipment have been developed, where
     a sheet is fastened horizontally, and the "yielding" or "bending" phenomenon is used,
     and photo-electric cells control heating time and/or temperature.



                             Clamp




                Frame
                                                                          Vacuum box




              Photo-electric cells
                                                                 Solenoid valve controlled by
                                                                 photoelectric cells.



     However, this criterion cannot be applied indiscriminately to every plastic, since some
     materials may over-heat before they begin to yield or bend. Although a range of tem-
     perature is established, the expected temperature of a sheet may not be achieved; this
     may be caused by:
     a) Fluctuations in the thickness of the material
     b) Temperature changes in the equipment and/or environment
     c) Minimum fluctuations in the line voltage (in infrared equipment).
     d) The regulator of the pressured air circulation gas equipment may not be the right
        one, there is not enough gas pressure, the burner is not the right one or it may be
        blocked with soot, etc.

     There are cone formed pyrometers, infrared radiation or gas (hot air) heating tablets,
     that can render a more accurate measurement. Although probably, the best way to
     measure the temperature of a sheet is by means of an infrared pistol, which measures
     by zones; though the equipment is expensive, it is the only one that measures the tem-
     perature of a sheet accurately and reliably.




16   Thermoforming
     Thermoforming equipments

     Originally, convection furnaces were the first equipments to heat plastic sheets that
     were going to be thermoformed, and up to now, that kind of heating is still preferred
     for sheets of different thickness, and for temperature even distribution.

     Heat can be applied with gas or electric resistor units. To produce air circulation from
     4,500 to 6,100 cm3/min. (150 to 200 feet3/min), pressured air re-circulation and deflec-
     tors are crucial to get homogeneous temperatures. The furnace temperature must be
     adjusted to the plastic forming temperature.

     Infrared radiation heating, compared with oil immersion or contact heating (the two lat-
     ter very limited in practice), is extremely rapid. For example, a 3.0 mm sheet heating
     time by infrared radiation can be achieved in one min. at about 10 watts/inch2.

     Because infrared radiation heating takes very little time, heat energy absorbed by a
     sheet may cause over-heating, that may even affect the degrading of the material
     (bubbles or burning) if it is not controlled. It is important to consider that in long runs,
     the furnace temperature has to be gradually reduced.

     In some cases, when the product has intricate or very deep sections, there is the risk
     of the thickness of the material considerably thinning; in this case screens must be
     used (they may be made of perforated plate or metallic display) to prevent over-heat-
     ing.

     The elements of infrared radiation can be obtained in a very wide range of designs,
     according to their importance they are:

     1.- Tungsten filaments in quartz tubes or lamps, temperature 3992 ºF (2,200 °C).
     2.- Spring- like nichrome resistor on refractory ceramic bases.
     3.- Nichrome resistors protected by plate or stainless steel tubes.

     There are manufacturers who make infrared radiation thermoforming machines in a
     wide variety of sizes, capacity, degree of automation and versatility.

     The specifications to acquire a thermoforming machine vary depending on the finished
     product that you want to get and therefore, it is necessary to consider:

     Voltage, wattage, amperage, useful area of forming, number of heaters (lower and
     upper), controls to regulate temperatures by zones, degree of automation, capacity to




17   Thermoforming
                   accept mechanical support, type of sheet fastening device, (clamps, mechanical,
                   pneumatic, etc.), ventilators to cool the product, general dimensions, production
                   capacity, cost- profit.

 Gas furnaces      This kind of furnace supplies uniform heat and constant temperature, with a minimum
with pressured     risk of over-heating an acrylic sheet. Electric ventilators must be used to force hot air
 air circulation   circulation on the acrylic sheet at a speed about 4,500 to 6,100 cm3/min., and
                   devices to distribute the air in every zone of the furnace.

                   Gas furnaces need heat inter-changers to prevent accumulation of soot due to the
                   gas flow, as well as controls to interrupt the gas flow, when necessary.

                   Electric furnaces can be heated, using sets of 1000 watts resistors. When using a fur-
                   nace with a 10 m3 capacity, about 25,000 power watts will be consumed and half of
                   this will be used to compensate heat loss due to leakage, insulating transmission and
                   the use of doors. A minimum 2" thick insulation is advised and the doors of the fur-
                   nace should be as narrow as possible, to reduce most of the temperature loss.

                   Automatic devices must be used to strictly control temperature between 32 ºF and
                   482 ºF (0 °C and 250 °C). To get a more uniform sheet heating, it is important to hang
                   it vertically, and this can be done with a system that fastens the material all along with
                   clamps or canals with springs which move on wheels that slide on rails, like the ones
                   used for closets.

                   Basic criteria to construct a gas furnace with pressured air circulation.
                   The best advice in this case, is asking any industrial furnace manufacturer to build
                   one with the mentioned characteristics, since the construction of one, specially the
                   heating and operation systems, is very risky for anybody who has only little knowl-
                   edge on the subject.

                   This kind of equipment must be approved by specialists in gas installations, it also
                   has to be registered before the corresponding authorities.

                   It is also relevant to point out that the information provided here, is only related to the
                   metallic structure and fastening system for acrylic sheets. A furnace construction can
                   be divided into the following sub-systems:

                   A) Structure
                   B) Fastening acrylic sheet
                   C) Electric system
                   D) Gas installation
                   E) Controls




              18   Thermoforming
     Recommendations to build a furnace
     Building the structure with commercial iron tubular of 11/2" X 11/2" or 2 X 2".

     a) Cut it according to the measurements and requirements of design.
     b) Weld the lateral walls.
     c) Weld the upper wall, the lower one and the back one; to join them with the lateral ones,
        and build the whole structure.
     d) Line the inner part of the structure with a black plate cal. 18 and weld it or rivet it with
        "pop".
     e) Cover the holes (thickness of the tubular) with a rigid sheet of glass fiber to get ther-
        mal insulation, code RF-4100, or a similar one.
     f) Line the exterior with a black plate cal. 18 and rivet it with "pop" or weld it.
     g) Make the doors with a structure of tubular PTR 1" X 1", and follow the same instruc-
        tions as for the walls, they should be shorter to leave room for the rails.
     h) Attach the doors to the furnace with hinges.
     i) Put the closet-type rails, they should be twice as long as the furnace. They are fixed
         with screws on the upper part of the furnace. Once they are fixed to the furnace and
        the furnace on its place where it will operate, using bearings fasten the rails to the ceil-
         ing or structure of the place.



      GAS STRUCTURE WITH AIR
      RE-CIRCULATION
                                                                            The electric ventilator is
                                                                            placed in this section to
          Rectangular tubular profile                                       force the air
          of 11/2” X 11/2” ó 2” X 2”


                                                                          Every joint must be welded
                                                                          with electric welding

        Closet-type rails

                                                             Plate "U" bearings of 1/4”




19   Thermoforming
          FASTENING SYSTEM FOR ACRYLIC SHEETS
                                                           1/4” iron plate

                      5/16" Cold rolled bar
                                                                 Iron hinge


                                                             Spring
                      1/4” crossbar
                     handle




                                                          Washer
                                                                              Nut




                                                          Type C profile cal.# 18



                                                       Acrylic sheet




       FURNACE FRONT VIEW AND DOOR DETAIL AND RAILING SYSTEM

                                                      1   1/2”   x 11/2” iron angle
            Steel cable to fix it to
            the ceiling of the place.
                                                       1 3/4” x 2” (1500 rail)
                                                       closet- type profile
                          No. 50 wheels

                                                          Furnace door


           Hook formed 1/2"
           cold-rolled bar.

                                                                        1/2” cold rolled bar.




           Joint of the fastening
           system for acrylic
           sheets.




20   Thermoforming
                            LATERAL VIEW AND DETAIL OF THE FURNACE DOOR AND RAILING SYSTEM
                                                 Steel cable to fix it to the
                                                 ceiling of the place

               1 1/2” x 1   1/2”
              iron angle                                                          1 3/4” x 2” (riel 1500)
                                                                                  closet -type profile




              No. 50 wheel




                           2 1/2” x 2 1/2”
                           iron angle

                                                                                Furnace door




Infrared   It is normally used in automatic thermoforming machines, heating a sheet by means of
 heating   radiation at a speed 3 to 10 times faster than in a pressured air circulation furnace,
furnace    thus, with very short heating cycles. It should be noted that the ratio temperature/time
           becomes critical and it is harder to heat the material uniformly.




      21   Thermoforming
                  Infrared energy is absorbed by the acrylic surface exposed, rapidly reaching tempera-
                  tures over 356 ºF (180 °C), that later on, is transmitted to the center of the material due
                  to temperature conduction.

                  Infrared radiation heating can be obtained using tubular metal elements, spring elec-
                  tric resistors, or by grouping infrared light lamps. To get a more uniform heating distri-
                  bution, a net or metallic mesh can be placed among the heating elements and the
                  material which can work to expand the temperature. It is also convenient to place an
                  infrared heating plate, about 12”from the material and 20” from the bottom plate.


                  To regulate energy input into the equip-
                  ment, we recommend using devices such
                  as different transformers or percentage
                  meters that will help to control tempera-
                  ture. Planning electric energy charges and
                  great capacity equipment is also advis-
                  able, an electric sub-station will also be
                  needed.


Lineal heating    An electric resistor can only be used to make bends in a straight line; to achieve this,
       electric   you also need a spring type electric resistor (20) or armored type (about 1KW X 1.2 m.).
     resistors
                  Lineal resistors are made of wire, inside Pyrex ceramic tubes. The material must not be
                  in contact with the tube to avoid marks on the surface. A distance of 6 mm. from the
                  tube to the material is recommended to get uniform heating on thin material.

                  When more than 3.0 mm thick material is going to be heated with this procedure, the
                  resistors should be placed on both sides of it. In the next picture, it is shown how an
                  asbestos plate bender at the beginning of production will provide a suitable bend, but
                  as production advances, the heating area expands making a bigger radius bend, that
                  is why a resistor with water re-circulation is much better for acrylic bending.




             22   Thermoforming
       Acrylic Sheet        Heating zone             Acrylic Sheet        Heating zone




                                        Asbestos                                      Asbesto
                                        plate                                         s plate



                        Electric resistor                             Electric resistor



     Basic criteria to build a lineal heating electric resistor.
     Bi-dimensional thermoforming or lineal bending, can be made with a spring type resis-
     tor or a tubular one. Building these equipments is conditioned to thickness, kind of
     bending and volume to be produced. Generally, a 1.32 yd. long resistor is the most
     common, though a 24” one is also acceptable, the specifications for this resistor are
     1Kw for each 1.32 yd., thus, with a rule of three consume can be deduced both for a
     longer or a shorter resistor.

      Acrylic benders are more common than the ones built with asbestos plates on the lat-
     eral walls, these are suitable as long as you do not have to produce a huge volume,
     since when asbestos plates are exposed to the same infrared radiation they tend to get
     hot and therefore, the heating area will expand changing a piece production standard.
     In other words, at the beginning of production, there will be small radiuses and as
     production advances, the heating area will be wider creating a bigger radius.

     An electric resistor bender with water re-circulation will be more effective and produce
     better quality bent pieces. This equipment needs tubular profiles that allow water re-
     circulation, which will keep the surface cool and will only allow a heating zone. The
     required materials to build this kind of bender are listed below.

     It is important to include a rheostat to control temperature intensity on an acrylic sheet,
     since it will provide the suitable pace of production and, obviously, it will reduce costs
     of electric energy.




23   Thermoforming
                     ASBESTOS PLATE FOLDER                     WATER RE-CIRCULATION FOLDER

     • Spring-like, tubular or nichrome tape resistor   • Spring-like, tubular or nichrome tape resistor
     • No. 16 or 18 cable with glass fiber insulator    • No. 16 or 18 cable with glass fiber insulator
     • Terminals.                                       • Terminals
     • 2 X 14 Heavy duty cable                          • 2 X 14 Heavy duty cable
     • Plug                                             • Plug
     • 500, 1000, 2000 or 3000 watts dimmer             • 500, 1000, 2000 or 3000 watts dimmer
     • 1/8", 3/16" o 1/4" asbestos plate                • 3/4" x, 3/4" aluminum tubular profile
                                                        • 6.6 yd. hose
                                                        • Clamps
                                                        • 10 to 20 lt. container
                                                        • Garden water pump




24   Thermoforming
          Complementary equipment: vacuum, pressured
          air and mechanical forces

          The thermoforming process consists in heating and softening a sheet of any kind of
          thermoplastic material and making it adopt the form of the corresponding mold to get
          an almost finished product with a particular form.

          Some times, an external force has to be used to turn a flat sheet into a different form
          and to make it copy the outline and details of the mold. The level of energy or use of
          this force must be adjusted, so that the plastic sheet can be easily forced to take
          another form.

          The most common used forming forces in the thermoforming process are: vacuum or
          pressured air, mechanical forces and the combination of these three. Choosing a form-
          ing force in the forming process generally depends on the size of the product, the vol-
          ume to be produced and the speed of the forming cycles.

          In addition, the following factors must be considered, since any of these can make a
          difference in selecting the forming force:

          a) Intrinsic limitations of each thermoplastic material
          b) Construction and material of the mold
          c) Thermoforming equipment available

Vacuum    The oldest method to form a plastic sheet into a utilitarian piece is vacuum forming. The
forming   original description of the thermoforming process was precisely "vacuum-forming".

          The basic principle of the vacuum-forming process is having a softened thermoplastic
          sheet in a mold perfectly sealed and where the air inside is evacuated by the vacuum
          force or suction. As the air is evacuated from the mold, it creates a negative pressure
          on the surface of the sheet and therefore,
          natural atmospheric pressure yields, forc-
          ing the hot sheet to take the place of the
          empty spaces, as it can be seen in the
          picture.
                                                          Acrylic
                                                          sheet




     25   Thermoforming
     Vacuum equipment
     There is a great variety of vacuum pumps: reciprocal piston, diaphragm, blades, eccen-
     tric rotor, etc. All these provide a good vacuum but cannot evacuate great volumes of
     air at high speed; that is why a stock tank has to be connected to be used as "vacu-
     um accumulator". On the other hand, there are compressors that can evacuate a great
     volume of air but are limited for vacuum force.

      A suitable vacuum system needs a pump that can displace from 28 to 29" Hg or from
     0.5 to absolute 1 Psi (710 to 735 mm of Hg.) in the stock tank before the forming cycle.

     The line, duct or pipe between the stock tank and the mold should be as short as pos-
     sible with a minimum of angles. It is important to eliminate air leaking due to damaged
     piping, perforated hoses, loose couples or nipples, as well as unnecessary valves.

     Rapid action or globe valves should be used. Vacuum pumps are available in one or
     two steps. A two step vacuum pump can evacuate pressures below 10 Psi; displace-
     ment capacity or evacuation for a one step pump is reduced by half. Table 11 shows
     vacuum pumps typical capacities


     Table 11: Vacuum pump typical specifications

                SPECIFICATIONS                        VACUUM THEORETICAL CAPACITY
       No. OF        DIAMETER      RUN      ONE STEP TWO STEPS     SPEED   POWER     DIAMETER
       CYLIN-         (inches)   (inches)    (yd3/min) (yd3/min)   (RPM)   NEEDED    OF PIPING
       DERS                                                                 (Kw)      OUTLET
         1             3.04        2.8        0.280      ----       800     0.56        19
         2             3.04        2.8        0.561     0.280       800     0.74        25
         2             4.08        2.8        0.996     0.498       800     1.48        32
         2             5.08        3.2         1.87     0.935       750    2.2/3.7      38
         2              5.6       4.08         3.08      1.54       900      3.7        52
         3              5.6       4.08         4.64      3.08       900      5.6        52




     Vacuum tanks
     Excepting some vacuum equipments, most have a stock tank. Bearing in mind that
     work pressure is about 10 Psi (about 21 inches Hg/530 mm. Hg) vacuum, then the vol-
     ume of the tank should be 2.5 times bigger than the volume between the molds, the
     vacuum box and the piping. Doubling the volume of the stock tank (along with other
     similar conditions) pressure can be increased 15% (11.5 Psi), according to what is
     established, the theoretical limit for the vacuum forming process is only 14.5 Psi.




26   Thermoforming
     In many cases, a rapid displacement of vacuum is very important. This can only be
     made by placing the vacuum tank as near the mold as possible and reducing the pip-
     ing friction as much as possible, which can be done by:

     a) A bigger piping diameter.
     b) Piping with wide curves, avoiding 90° angles.
     c) Changes in the transversal section of the piping (diameter changes).

     Many equipments in the market do not meet these requirements. In general, the piping
     must be 1" diameter to displace 1 ft3 of air, for big pieces a 2" or 3" diameter is suit-
     able. There should also be a flexible plastic hose internally reinforced with wire or a
     similar material that prevents it form collapsing; it should be connected between the
     mold and the piping, as shown in the picture.


                                   Stock tank (400 lt.)

                                                2” flexible hose




                                                                                Bearings
            globe valve


              Solenoid valve                          Air deflector




     Vacuum forces, applications.
     In general, pumps work constantly to keep vacuum in the stock tank, there is a varia-
     tion on the vacuum-meter readings in each cycle. The vacuum generated on the
     formed part must be kept enough time to cool and stand the internal force of the mate-
     rial which will tend to keep the original form, causing waves and bending.

     As a general rule, the faster the vacuum is made the better the piece will be formed.
     Occasionally, slow forming speed for deep forming pieces or intricate sections is rec-
     ommended. When the matrix is very deep and when the configuration is problematic,
     slow vacuum can allow plastic more time to contract in the transversal section, this
     way a deficient configuration can be avoided.

27   Thermoforming
Pressured air   In operations where vacuum force is replaced by pressured air, it should be considered
    forming.    that it is harder to seal the mold satisfactorily. The forming force can easily multiply up
                to 10 times if the pressured air is at 100 Psi. However, the molds can stand such pres-
                sure very few times.

                To form by using pressured air, it is necessary to take as many precautions as possi-
                ble. A regular size mold requires a closing pressure of some tons, which obviously a
                common vise (type "C") cannot stand. Then, various clamps or rapid action fasteners,
                which are very useful in this case, should be used. With the pressure exerted, a badly
                built mold may explode like a bomb. An aluminum or machine finished metal mold is a
                good choice; resin or wooden molds must not be used unless they are reinforced with
                metal.

                Pressure forming equipment must be stronger than the vacuum forming one. It must
                have a similar tank for the compressor as well. Piping does not need strict specifica-
                tions since pressure drop is not considerable. If in a piping pressure drops 5 Psi, pres-
                sure loss in the system will be 10 Psi, 50% of the pressure. But if the pressure system
                is 100 Psi, it will be 5%. A valve to reduce pressure and a manometer should be also
                installed, as well as a baffle or filter at the entrance of the mold, so that cold air is never
                in direct contact with a hot sheet. Some times, heaters should be incorporated to the
                air system, since they will help in great blows, which must be kept hot until a piece is
                formed on the mold.

                If possible, there should also be filters to eliminate water that tends to condense in the
                system and in the long run can make the equipment rusty, in addition, combined with
                air particles, it can block air ventilation orifices in the molds. Periodical maintenance is
                a must.

                  Vacuum                            Acrylic      When needed, the mold should have ori-
                                                                 fices to eliminate the air caught inside
                                                    Mold         and avoid wrinkles or deficient forming.

                   Pressured air            Vacuum orifices      Pressured air forming has become popu-
                                                                 lar, specially for small pieces. The advan-
                                                    Acrylic
                                                                 tages of this method are: improvement
                                                    Mold         on dimensional tolerance, forming speed
                                              Air exhaust
                                                                 can be considerably increased and fine
                                                                 details are better defined.




           28   Thermoforming
Mechanical   The thermoforming process is not limited
   forming   to pneumatic techniques. There are sev-
             eral mechanical forces that can be
             applied. The simplest form of mechanical
             forming is used for bi-dimensional form-
             ing. In this case, a heated sheet is placed
             on the surface of a curved mold which is
             usually a smooth surface and gravity is
             enough to curve the sheet; the edge of
             the sheet should be fastened to keep it in
             position until it cools. That is the case for
             the manufacturing of the cannon arch whose sides are tightly fastened and there is not
             thickness variation.

             Mechanical forming, matrix and male mold.
             Matrix-male molding is used, among other things, to shape complicated pieces. In this
             molding technique, a heated sheet is shaped between 2 opposing but similarly outlined
             molds (matrix-male). When the molds are joined, the outlines force the sheet to take
             the same shape, in the space left between the two molds. Any protuberance on the
             male mold, mechanically, will force the plastic into the counterpart (matrix). For big or
             medium production, mechanical equipment is used to close the molds; in other cases,
             the movement is created by servomotors. If both molds have a controlled temperature,
             cooling time can be reduced.

             There are three basic criteria to achieve good thermo-shaping performance when using
             this technique.

             The first, is applied force, regardless of its source (pneumatic, hydraulic or mechani-
             cal), it must be strong enough to make plastic deform, of course, a huge surface or an
             intricate mold will need a bigger pressure force.

             The second refers to suitable elimination of the air caught inside. The pressure exert-
             ed between the two molds causes that air gets caught between them and the sheet,
             and air must be removed to shape the piece well. Boring some holes in one or the two
             molds in the areas where this anomaly is spotted, can eliminate the air.

             The third is related to the depth limit of stretching, that derives from the forces used in
             the process. It can be easily understood that maximum stretching is only successful
             when the mold has exit angles bigger than 5° and very big and smooth curve radius-
             es, the angles close to 90° may diminish stretching and even tear the plastic material.

             This sophisticated thermo-shaping method should not be used on the whole mold, its
             use is limited to only some parts of the mold.


        29   Thermoforming
 Combined    Mechanically forming with matrix-male molds does not only depend on the forces
techniques   used, usually, this kind of forming can be combined with vacuum, pressured air or both
             at the same time. Therefore, the matrix-male mold does not have to coincide accu-
             rately, the male mold may be relatively inferior in dimensions and have a substantially
             different form from the matrix.

             When male molds are made like this, they can act as "pushers" of a plastic sheet. This
             kind of support is called mechanical support, because it presses the softened materi-
             al into the matrix. The purpose of this support is to stretch the material so that the final
             form is accomplished in combination of vacuum and/or pressured air.

             Using mechanical support in the process has the advantage of a better distribution of
             the thickness of a product, than using any other process. Many variations in the
             process can be obtained combining these techniques. Those variations can be vacu-
             um pressure changes, vacuum or pressure application time, mold closing speed time
             or forming cycles.

Mechanical   Usually, mechanical supports are made of wood. Hard or tropical wood is the most
   support   used to make supports. In some cases, pieces of other plastic material such as: nylon,
    design   rigid polyurethane, acrylic, aluminum or steel, which are easily machine finished, can
             be incorporated.

             If production volume requires it, a cooling and/or heating system can be incorporated.
             The decision to heat or cool the support, must be made from the beginning of the
             design, since later on it will be harder if not impossible to try to adapt a heating ele-
             ment, that is why required machine finishing should be made to incorporate the sys-
             tem.

             When a support is very cold, a sheet will surely get cold on it. Cooling usually takes
             place between the points of a support and a sheet and the sheet and the mold. In
             extreme cases, the sheet may shrink on the support during the forming.

             The form of a support has a determining
             influence on the wall or thickness of a fin-
             ished piece. In the next picture, there are
             three different kinds of support.




        30   Thermoforming
     Flat surfaced and blunt edged support
     This allows a sheet to stretch between the support and the edge of the mold, and
     meanwhile, the part of the sheet in contact with the edge of the support gets cool. A
     piece formed this way will have a thick bottom and thin walls

     Tin-like support
     In this second alternative, a sheet is in contact with the support and cools fast only on
     the perimeter of the support. Stretching is similar to that of the flat support, but the
     central area of the support allows extra stretching.

     Sphere-like support
     On the other hand, in this case, only a small area is in contact with the support. There
     might be a significant stretching as the support moves forward, therefore, the area of
     the perimeter between the edge and the support decreases.




              Flat surfaced and blunt      Tin-like support         Sphere-like support
                   edged support




31   Thermoforming
                  Thermoforming molds

     Choosing     One of the most important aspects to be taken into consideration in thermoforming
    the type of   pieces is the thermoforming technique to be used. Depending on the characteristics of
thermoforming     the product if the wrong technique is used, there may be problems before you can get
     technique    a piece with the specifications initially determined, finished. And many times the oper-
                  ation will fail, with the consequence of a waste of time, money and resources. Thus,
                  before manufacturing a mold, the following should be considered:

                  1.- Form and dimensions of the piece.
                  2.- Desired aspect.
                  3.- Thermoforming technique.

                  Based on these factors, you can plan and anticipate possible defects in the pieces. In
                  this chapter all the variables that emerge when a thermoforming mold has to be man-
                  ufactured, are analyzed.

    Criteria to   It must be mentioned that: products made using thermoforming technique, though this
        design    technique is versatile and flexible, regarding aspect and characteristics, differ from
thermoformed      products manufactured using injection molding. In the following comparative table the
     products     basic differences can be analyzed. To conclude, to design thermoformed pieces the
                  following criteria must be established:

                  1. - Thinning of material should be considered, this mostly depends on form, size and
                       technique used (chapter 8). Generally, thinning of material is directly proportional
                       to the height of a piece.

                  2.- A 3° and 5° exit angle of the mold should be considered.

                  3. - It must be taken into consideration that a piece will contract 0.6 to 1% when it cools.

                  4. - In general, the surface of a thermoformed piece will be smooth, though some tex-
                       tures can be obtained.

                  5.- In designing a piece, big radiuses should be included; there may be edges but they
                      can tear the material.




             32   Thermoforming
     Table 12 Basic differences between Injection and thermo-shaping processes.

                                                                           PROCESO
                     VARIABLES
                                                     INJECTIÓN                        TERMOFORMING
      Thickness                                       Constant                              Variable
      Mold exit angles                                0.5° to 1°                             3° - 5°
      Molding temperature                  392ºF-464ºF (200°C – 240°C)          320ºF-356ºF (160°C – 180°C)
      Dimensional tolerance                           Excellent                Relatively good, not for accuracy.
      Inserts                             Possible insertion of elements in     Mold surface can be prepared
                                                   other materials.                        for inserts
      Surface finishing                    Smooth surfaces or any other         Only smooth surfaces, some
                                              texture can be obtained.                  shallow textures
      Production                           High production, hundreds or         Medium, some dozens a day.
                                            thousands of pieces a day.
      Mold                                 Steel with alloys or expensive       Variety of materials, rather low
                                            treatment, complex design,          cost, simple design, may use
                                                 matrix-male mold.                    matrix-male mold.
      May create ribbings, all types of                  Yes.                                No.
      holes, coils, etc.
      Scrap, material waste                    Very little, recoverable.        Depends on the shape, about
                                                                                 25% waste and recoverable.
      Radius                                Must blunt edges, about 1.5        Larger radiuses, 0.4” to 2” need-
                                              thickness of material.             ed. Depending on shape and
                                                                                            depth.
      Time to make a piece (design,             From 3 to 6 months.                   Maximum 1 month.
      mold, tests).
      Subsequent treatment and            Any treatment or finishing, paint-   Any treatment or finishing, paint-
      finishing                            ing, hot-stamping, serigraphy,       ing, hot-stamping, serigraphy,
                                                 metallization, etc.                  metallization, etc.




33   Thermoforming
    Criteria to   The following criteria are key factors to successfully produce thermoformed pieces.
        design    They are the core of any development, but it is also vital to thoroughly analyze these
thermoforming     concepts and later we will see in detail each consideration in the design of molds.
        molds     Then, these basic criteria and considerations will be the fundamental parameters to
                  manufacture thermoforming molds, regardless of their complexity. It should be noted
                  that when these molds are manufactured, the following concepts must be assessed.

                  1. - Form and dimensions of the piece.

                  2.- Aspect of the piece.

                  3.- Estimated production volume.

                  Probably the most important of these concepts is the estimated production volume,
                  since it will depend on the definition of the kind of mold, material, finishing, thermo-
                  forming technique, etc. Next, the model designs are shown:

                                                                1. - A male mold is easier to use, less
                                                                expensive and more suitable to form
                                                                deep pieces. In general, a matrix should
                                                                not be used to form pieces deeper than
                                                                half the width of the piece. The matrix is
                                                                used when the concave face of the fin-
                                                                ished piece must not be in contact with
                                                                the mold.

                                                                2.- The molds must have enough vacuum
                                                                orifices so that an annealed sheet can
                                                                conform to the critical parts of the mold,
                                                                the vacuum orifices have to be made in
                                                                the deepest parts and areas where air is
                                                                caught, and must be small enough not to
                                                                leave marks (1/32" to 1/8" diameter).
                                                                Vacuum can be more effective if the hole
                                                                is enlarged from the inside.

                                                                3.- There must be ducts that allow water
                                                                or oil circulation through the mold when
                                                                temperature control in it is needed.




             34   Thermoforming
     4. - When the dimensions of a formed
     piece are critical, molds must be built big-
     ger to compensate for the contraction of
     the material.

     Expected contraction from molding tem-
     perature to environment temperature is
     1% maximum.


     5.-A slight curving of the flat big areas of
     the mold will allow flat areas when the
     material cools.




                                                    30
     6. - Pieces with 90° walls cannot be
     obtained; the mold must have an exit
     angle of at least 3°.




     7. - Edges should be blunt, since vertex
     form accumulates internal efforts. A piece
     will be more resistant designing blunt
     edges and corners.




     8.- The thin or weak parts can be rein-
     forced with reinforcement ribs, which will
     also reinforce big flat areas.




35   Thermoforming
                   9.- If it is necessary to mold using a per-
                   manent incrustation, you should consid-
                   er: the difference between the expansion
                   coefficient and the various materials, oth-
                   erwise, there can be a failure due to a
                   forced insert, because of different expan-
                   sions and contractions of the materials in
                   contact.

                   10.- The surface of the molds can be lined
                   with cotton flannel, felt, velvet, suede, etc,
                   to diminish mold marks. The most com-
                   mon is cotton flannel.


 Considerations    One of the advantages of the thermoforming process is the diversity and kinds of
in the design of   molds that can be made at a very low cost and relatively fast, being highly accepted
 thermoforming     for other applications, over other processes.
          molds
                   Usually and unlike injection molds, only half the mold is needed and it depends on the
                   form of the product, desired aspect and chosen technique (may be male mold or
                   matrix).

                   Choosing the right one is much more important when the part to be thermoformed is
                   very deep. When the pieces are shallow, profiles are small or when thinning is irrele-
                   vant, choosing will depend on the aspect of the piece. If details of the mold are impor-
                   tant, then the side of a plastic sheet in contact with the mold surface should be the
                   front of the piece.

                   Some times, a bigger radius or smooth aspect is desirable if a sheet of material shows
                   a nice surface, then the surface which does not touch the mold will be the front of the
                   piece, besides, a dimensional control closer to the surface of the mold can be
                   obtained.

                   Thinning of the material
                   Under every condition of thermoforming when pieces are formed of a plastic sheet, the
                   area of the surface will get bigger, there will be some stretching and the material will
                   get thinner.

                   One of the decisive factors of this thinning is the ratio, generally defined as maximum
                   depth or height ratio with a minimum space through the opening. To estimate this thin-
                   ning, the area of the available sheet to be thermoformed must be determined and divid-
                   ed into the area of the finished piece, including waste. It is always desirable that the


              36   Thermoforming
     molds and thermoformed pieces have generous curving radiuses. Theoretically, there
     is a formula to determine the thinning percentage of the material, considering that the
     material is uniformly annealed and stretched.

                                    Final thickness of the material        available area of a sheet   =
                     Thinning % =                                     =
                                    Original thickness of material        total area of shaped piece

                                               AXB
                                         A X B X E (2C + 2D)




                 A=3                                            A=3
                 B=4                                            B=4
                 C=2                                            C=2
                 D=1                                            D=1
                 E=1                                            E=1



     In practice, with a micrometer or calibrator you can determine thickness directly on the
     thermoformed piece, cutting small pieces on different sections. Other methods use
     translucent sheets and correlate color intensity vs. thinning of the sheet. Thickness can
     also be determined making squares with an oil marker on the sheet before thermoform-
     ing it and observing stretching of the material.

                                                               One should consider the possibility of
                                                               wrinkling on some critical areas or on the
                                                               bottom of a male mold or matrix. If an
                                                               annealed sheet cannot contract from the
                                                               dimension A to E, excess material will cre-
                                                               ate wrinkles.


     In a matrix the opposite happens, the
     sheet will expand to the 4 vertexes of the
     mold surface, becoming very thin. This
     can be seen in most of the thermoformed
     tubs.


37   Thermoforming
     Next, some techniques to prevent wrinkling are shown:




     When low molding temperature is used, a sheet will keep a greater tenacity and elas-
     ticity. For big pieces, molding time and temperature should be increased on difficult
     zones to be thermoformed, minimizing this kind of defect. For deep molding sheets,
     because of their partially cross-linked structure, they tend to minimize wrinkling. When
     there are many molds, there should be enough room to prevent wrinkling, a distance
     1.75 times the height of a piece, is suitable.

     Dimensional shrinking and tolerance.
     Dimensional shrinkage and tolerance in thermoforming vary for pieces formed on
     matrix or male mold. On a male mold, shrinkage can be reduced if the piece cools most
     of the time on the mold. If cooling reaches environmental temperature on the mold,
     shrinkage will be minimum. Thus, the internal dimension of the piece will be very close
     to the one of the mold, but then a production cycle will not be productive.

     However, the fact is that a piece must be removed from the male mold when it is still
     hot, otherwise removal will be difficult. This is exactly thermal shrinkage, which is the
     proportional difference between the environmental temperature and the one at the time
     of removal. Thus, to keep the specified dimension of a piece, the model must be slight-
     ly bigger.

     On the other hand, a piece formed in a matrix will begin shrinking as soon as the tem-
     perature of the material is below the one of forming. To keep a close continuous toler-
     ance, the mold dimension must be considerably increased and vacuum pressure kept
     during the whole operation.




38   Thermoforming
     As a guideline it can be assumed that shrinkage on male molds it is .127 mm/mm
     (0.005 in/in) and in a matrix it is bigger. For acrylic, polycarbonate, thermoplastic poly-
     ester and oriented polystyrene .203 mm/mm (0.008 in/in) can be considered. Anyway,
     one should be cautious about these values, since the following conditions can signifi-
     cantly alter them.

     1.-Mold temperature: a difference of 15°F (10°C) can change shrinkage over 0.001
        in/in. (0254 mm/mm).

     2.- Size and thickness: this refers to the exit angle limited by the mold and the effect
         of greater thickness regarding temperature profile.

     3. - Final use temperature: Due to expansion and contraction proportional to lineal
          expansion coefficient, a thermoformed piece will keep on varying with environmen-
          tal temperature changes.

     4.- Use extreme conditions: Shrinking can reach top values after the first exposition to
         the highest temperature of use.

     5.- Molecular orientation: There might be bigger shrinkage related to the molecular ori-
         entation of the material.

     Some times, to prevent distortion and shrinkage, cooling templates are needed until a
     piece reaches the environmental temperature. Further more, the pieces thermoformed
     at a temperature below the one specified, tend to go back to their original state due to
     the plastic memory of the material. It is advised to monitor shrinkage and deformation
     during production.

     Aspect of the mold.
     It must be clarified that the surfaces obtained by injection and extrusion processes
     cannot be reproduced by conventional thermoforming techniques. Even highly brilliant
     materials may lose their glow during the process. In addition, they tend to emphasize
     mark and waving when they touch a cold mold and undergo thickness changes. A
     change of thickness will cause small distortions. Thus, cleaning the working area is a
     must. All the outlines should be rounded, actually, a mold with big radiuses will bene-
     fit the thermoforming operation, since the material will tend to stretch better




                     NO           YES



39   Thermoforming
     If you want a sheet to copy details of a mold, like non-skid textures or similar ones,
     those detail should be at least three times bigger than the thickness of the material.
     Actually, it is better to have a not so smooth molding surface, this way, the piece will
     not copy the mistakes of the mold. It may even be sand-blasted with glass fiber micro
     spheres or an abrasive material. This way you can eliminate the air caught between the
     mold and the piece. Some times it is a good idea to sand the surface using rough
     sandpaper, this helps at the time of removal, to break the vacuum between the mold
     and the piece.




                     Superficie lisa, bien pulida                  Superficie áspera


     Vacuum bores
     When using thermoforming techniques with vacuum or pressured air, it is very impor-
     tant to eliminate most of the air between a mold and a sheet in a minimum of time.
     Depending on the kind of mold, 1/2" or 1" orifices can be used, as in the case of ther-
     moformed skylights, up to homogenous distribution in all the vertexes of the mold.

           Metallic frame                 Acrylic




                                        Base
       1/2” or 1” piping


                                                    These pictures show the distribution of
                                                    the vacuum pressured air bores, typical
                                                    for pressure-free forming molds, male
                                                    mold and matrix

     In general, the diameter of vacuum bores should be slightly smaller than the thickness
     of the material. As a starting point, the vacuum bores will have a diameter equivalent
     to the final thickness of a thermoformed piece. This rule does not apply when the
     material is very thin or very thick, or when the marks of these orifices are irrelevant. It
     can be considered that a suitable range is from 1/32" to 1/8" diameter. To eliminate a

40   Thermoforming
     great volume of air, 1/8" or _" diameter holes can be drilled. Depending on the manu-
     facture of the mold, the bores can be widened on the inside of the mold, as shown in
     the picture. To reduce the time to eliminate the volume of air round a softened sheet
     and a vacuum box, the space can be refilled with polystyrene foam balls or
     polyurethane pieces.




        Widened bores on   Increased diameter
        the inside         bore




     Another function of a mold is to contribute along with a frame to stabilize the position
     of a sheet and provide good sealing all around the mold. In some cases, a canal
     around the piece is helpful, exactly on the external zone of the cutting line.

     Mold cooling
     Some times when production runs are very long, the mold should have a cooling sys-
     tem, generally copper piping is used. It should be placed adequately and have enough
     capacity to carry a considerable volume of water or refrigerant. A relationship between
     the temperature of the sheet and the mold should be established so that the material
     does not get too cold and it does not thermoform below the bottom limit of the mold-
     ing temperature.

     There are different methods to cool a mold, for example, when there are critical mold-
     ing zones, plastic or poly-tetra-fluorine-ethylene inserts can be incorporated. In some
     cases, a plastic covering can be applied to reduce thermal conductivity, or even after
     thermoforming, pressured air can be injected through the bores or holes. Three cool-
     ing systems are shown in the next picture: First an undulated cooling system, the sec-
     ond is a branch system and the third is an external multiple alternative flow branch
     system with 2 inputs and 2 outputs.

                                     Branch system


                                                                           External multiple
         Undulated                                                         alternative flow
         system                                                            branch




41   Thermoforming
                  Mold supports
                  As it has been mentioned before, when thermoforming a piece the material always gets
                  thinner. Molding supports are used to get a better distribution of material in a thermo-
                  formed piece. Their purpose is to stretch a softened sheet, as a pre-forming. This tech-
                  nique is very important, specially with very deep pieces. In general terms, the molding
                  supports can be made of the same material as molds. There are three categories of
                  mold supports:

                  Metallic supports
                  Usually they are made of iron or aluminum, must be very smooth, with radius on the
                  edges. The range of temperature is 10 to 15°C (10°F) below the temperature of the
                  material, if their temperature is too high the sheet will stick to them.

                  Thermal material supports
                  These are made of wood, plastic or metal and they are built under the principle of a
                  good thermal insulator. The surface may be of soft wood, plastics like nylon, or anoth-
                  er thermofixed, synthetic foam or any other material including soft flannel.

                  Skeleton type support
                  Skeleton or frame type supports are only rounded bars welded forming intersections,
                  which should be totally rounded to avoid tearing the material.

                  Support dimensions are related to the size of a piece, since they have a great influ-
                  ence on the thickness distribution of the material. It must be noted that in some
                  cases, by only changing the depth penetration of a support (75% depth of the piece),
                  the thickness of the material between the faces and the surface can be controlled.
                  Therefore, the equipment must have the required depth adjustment capacity, pene-
                  tration power and speed.

 Materials used   Materials used.
to manufacture    Unlike other plastic molding processes, such as injection or compression, thermo-
 thermoforming    forming has the advantage of using relatively low pressure and temperature. That is
         molds    why a great variety of materials can be used. Usually, wooden molds can be used, they
                  are ideal for low production and as wood has a low thermal conductivity, it helps the
                  annealed sheet not to cool quickly at first contact, but these molds are not good for
                  medium or high production. Manufacturing molds with phenol laminates are better
                  because they are not seriously affected by heat or humidity.

                  There are also molds made of mineral or metallic charges and polyester or epoxy or
                  rigid polyurethane resins. These are easy to remove off a mold and may even have a
                  mold with multiple cavities. The thermal properties of epoxy and polyester resins make
                  them suitable for medium production. Copper piping can be used as cooling system to
                  better control the mold temperature, but even then, it is not enough for high production.


             42   Thermoforming
     Aluminum molds are the best for high production, but because of the thermal conduc-
     tivity of aluminum, the mold has to be pre-heated by means of circulating hot water
     through the cooling/heating system or radiating heat with electric resistors, or even
     heating the mold with the same material to be thermoformed. For long runs, a thermo-
     stat has to be incorporated, to ensure there is the least temperature fluctuation on the
     surface of the mold, thus, preventing over cooling. Applying poly-tetra-fluorine-ethyl-
     ene to aluminum can improve its properties.

     Summarizing, there are 4 groups to manufacture thermoforming molds:

     1)   Wood.
     2)   Minerals.
     3)   Plastic resins.
     4)   Metals.

     Table 13. Use of materials for thermoforming molds


                            MATERIALS      PRODUCTION
             GROUP                                         ADVANTAGES AND DISADVANTAGES
                              USED           VOLUME
             Woods              Pine           Low         These are low cost molds, their time of
                             Mahogany                      manufacturing is short and they have
                               Cedar                       good surface finishing, though in some
                               Maple                       cases the grain of the wood leaves
                                Triply                     marks. Wood should be seasoned, for
                            Agglomerated                   better finishing and preventing dimen-
                                                           sional changes due to humidity, molds
                                                           must be sealed with casein, phenol-
                                                           varnish or epoxy resin diluted in methyl-
                                                           ethyl ketone. For better finishing the
                                                           grain of the wood must be parallel to
                                                           the length of the mold. Triply or agglom-
                                                           erated molds last longer, which can be
                                                           prolonged by reinforcing the intersec-
                                                           tions with metal.




43   Thermoforming
                        MATERIALS         PRODUCTION
          GROUP                                            ADVANTAGES AND DISADVANTAGES
                          USED              VOLUME
      Minerals              Cast             Low       Cast Molds are more durable than wooden
                          (Calcium          Medium     ones and can be cast of a composite of low
                         Carbonate)                    shrinking cast, highly resistant and interiorly
                           Sodium                      reinforced with metallic mesh, glass fiber or
                       Fluoric-silicate                materials that do not absorb humidity. Cast on
                                                       the molding is left to cure 5 to 7 days at envi-
                                                       ronmental temperature. If surface is good it
                                                       does not need finishing. Polyester, epoxy or
                                                       phenol resin coverings provide more resistant
                                                       surface. Care must be taken not to chip cast
                                                       when making vacuum holes, which may be
                                                       eliminated if pieces of wire are inserted previ-
                                                       ously and removed after hardening.

      Plastic resins     Polyester          Medium     Plastic resin molds are more expensive and
                           Epoxy                       elaborated than cast or wooden ones but more
                           Phenol                      durable, smoother surfaces and dimensional
                           Plastic                     stability. These resins can be charged with alu-
                         laminated                     minum powder which provides a more homo-
                           Nylon                       geneous temperature of the mold or with
                                                       kaolin, glass fiber etc. A vacuum system can
                                                       be incorporated to these molds, fitting a card-
                                                       board pipe at the back of the mold.

      Metallic           Aluminum            High      They are ideal for big production runs, high
                         Beryllium-                    pressure or metallic forming. Aluminum,
                          copper                       bronze, or any other low point fusion alloy
                            Iron                       founding molds can be used, and also
                                                       machine finished steel, brass or bronze. They
                                                       are the must expensive, making them takes a
                                                       long time, have better surface finishing, main-
                                                       tenance low cost and better dimensional sta-
                                                       bility. Cooling system must be used, and avoid
                                                       rapid cooling of the piece.




44   Thermoforming
     Recommendations for thermoforming molds
     1. For wooden molds, the best remover is baby powder or flour.
     2. For metallic or plastic resin molds, removing waxes are recommended.
     3. Soft wood must not be used with very sensitive materials such as polystyrene,
        foamed or acrylic P.V.C., since they get marked because of the grain of the wood.
     4. For long production runs, wood must not be used since slow cooling makes the
        mold expand, creating separations on the joints.
     5. For plastic resin or metallic molds, aerosol removers can also be used.
     6. For wash basins, tubs, or bath room modules, a porcelain-like glow can be achieved
        sand-blasting the surface of the mold, roughness will achieve a finish with these
        characteristics.
     .




45   Thermoforming
                 Thermoforming techniques

                 Thermoforming is the simplest and most used process to form an acrylic sheet. Being
                 a thermoplastic material it softens and it is easy to handle and can take any form when
                 heated at suitable temperature and time.

                 As it cools it recovers its rigidity and keeps the form it was exposed to. The cost of
                 equipment and molds is relatively low and bi or tri-dimensional forms can be obtained
                 by means of a great variety of processes.

Bi-dimensional   This is a bending process that can be achieved through two methods:
thermoforming

                 Lineal heating bending.
                 A Chemcast acrylic sheet is heated on a lineal resistor, bending at the desired angle.
                 To bend, remove the protector paper of the bending line (the rest of the paper may be
                 left to protect the areas that are not to be worked on), then place the sheet on the sup-
                 ports with the bending line directly on the heating line, bending on the heated side.
                 Heating time varies according to the thickness of a sheet. To bend an acrylic sheet over
                 0.16” thick it should be heated on both sides to obtain a suitable bend. Heat the sheet
                 until it gets soft on the bending zone. Do not try to bend the sheet before it is well heat-
                 ed, this may cause irregular or creased corners.

                 Heat carefully, irregular heating may cause arching on the bending line. Some times
                 this is hard to avoid, specially on pieces over 24” long. Arching may be diminished fas-
                 tening the recently formed material with some clamps or a template until it cools.
                 Templates can be made of wood, fixed or adjustable.


                                            Acrylic          Top           Support with      Acrylic
                                                                          adjustable hinge
                                                                            at any angle
                         YES     No          Electric resistor                                         Butts


                 With suitable heating,   Place the sheet on the     Use fixed or
                 clean shining corners    support with the fold-     adjustable templates
                 are obtained             ing line directly on the   to keep the piece at
                                          heating line               the desired angle




            46   Thermoforming
                  Cold forming

                  Chemcast acrylics sheet can be cold formed on curved frames, as long as the radius
                  of the curve is 180 times bigger than the thickness of the material used.

                  Formula: R (radius) = 180 X T (Thickness of material in inches.)




                                                              R=180 X E




        Three-    The procedures for tri-dimensional forming in general, require using vacuum, pressured
   dimensional    air, mechanical equipment, or a combination of these to mold Chemcast acrylic sheets
thermoforming     to a desired form. These techniques are described next:
  (with molds).

                                    Acrylic
                                                               Free or gravity shaping
                                                               This method is the simplest of all,
                                                               because once the material is softened,
                                                               the sheet is placed on the mold and the
                                                               material adopts the form by its own
                                                               weight. The edges of the material can be
                                                               fastened to the mold to avoid waves that
                                                               tend to occur when cooling.
                                     Mold
                                                               Mechanical forming with matrix and
                                                               male mold.
                                  Male mold
                                                               A Chemcast acrylic sheet can be formed
                      Frame                                    pressing the annealed material between
                                                               the male mold and the matrix, to produce
                                                               pieces of very accurate dimensions. This
                                                               procedure requires excellent finishing of
                                                               the molds to reduce their marks to a min-
                                                               imum.
                                    Matrix


             47   Thermoforming
     Free, pressure or vacuum forming
     The pieces that require optical clarity like
     skylights, helicopter cabins, etc., can be
     formed without mold, Chemcast acrylic
     can be vacuum or pressured air formed.
     The form of the finished piece is given by
     the form and size of the ring that fixes it to
     the frame and by the given height.
     However, these forms are limited to
     spherical outlines or bubbles freely
     formed. Vacuum is better for this kind of
     forming, or pressure if it is over 1 atmos-
     phere.

     Vacuum and pressure forming, matrix.
     This procedure allows forming pieces, on
                                                      Presión de aire
     1 piece molds whose form requires more
     accuracy than the ones vacuum formed.
     However, high pressure leaves marks of
     the mold on the piece. As high pressure is
     required, the molds should be of metal,
     epoxy resins or other materials that can
     stand high pressure without deforming.
     Good finishing of the molds is a must to
     obtain quality pieces.
                                                          Vacío
     Pressure forming with the help of a pis-
     ton and matrix
     The technique of piston help is used to
     reduce thinning at the bottom of the
     formed pieces. The piston stretches the
     material before pressure is applied. Piston
     speed of 6.6 yd./min., is required, it may
     damage the material at initial contact.
     Forming pressure 6.16 pounds/in2




48   Thermoforming
                         Vacío
                                 Vacuum with return and male mold
                                 forming.
                                 This technique is useful to form pieces
                                 that require uniform thickness on the
                                 walls and fewer forming marks. An
                                 annealed sheet is stretched in a vacuum
                                 box until it reaches the necessary depth
                                 for the mold; once it is inside it, vacuum
                                 is freed gradually so that the acrylic
                                 returns to its original form meeting it.
                                 More defined forms can be obtained if at
                     Vacío       the point of returning, vacuum is applied
                                 to the male mold


                                 Pressure forming with the help of a pis-
                                 ton, matrix and vacuum.
                                 This is the most sophisticated of all, since
                                 it is a combination of almost all the oth-
                                 ers, it is generally used for very deep
                                 thermoforming which requires more con-
                                 trolled thickness and when breaking is
                                 possible because of excessive molding
                                 depth.
                     Vacío




49   Thermoforming
        Infrared   In this section we will try to expand the techniques mentioned before. Although these
heating furnace    examples are designed for infrared heating equipments, it is possible to apply them to
        molding    the conventional molding systems.
     techniques

                     Vacuum forming, matrix and mechanical support      Pressured air pre-stretching,
                                                                       mechanical support and vacuum




                     Vacuum forming, matrix and mechanical support         Vacuum forming, matrix




              50   Thermoforming
                                                     Pressured air pre-stretching, matrix,
                     Free pressured air forming       mechanical support and vacuum




                                                         Free pressured air forming




              Pressured air stretching, mechanical
                     support and vacuum



                                                     Vacuum forming, matrix, mechanical
                                                         support and pressured air.




51   Thermoforming
               Cooling thermoformed pieces

               Cooling a thermoformed piece is as important as heating it, but in some cases, it takes
               longer than heating. That is why it is important to choose the right method. Some
               times, when very thick pieces that can stand less internal effort are formed, normal
               cooling should be delayed, covering the piece with soft cloth or flannel. If the piece is
               fastened with clamps, fastening force diminishes as cooling takes place and shrinkage
               will show the great efforts of this process.

               Most of the heat absorbed during the heating cycle should dissipate off the plastic
               before it is removed off the mold, otherwise, the piece might get distorted and warped.
               If the piece is formed on a male mold, it should be removed before shrinkage, which
               will make it hard to remove.

Conventional   Conduction and convection are practically the only methods to dissipate heat, since
    cooling    thermal conductivity is low, pieces over 0.08” thick require long cooling. The most
   methods     common is using electric ventilators to cool the piece; this method has the advantage
               of allowing cooling the piece on the mold. The disadvantage is that the air draft is not
               enough to cool the mold in each cycle, and the mold will be too hot, interfering with
               the normal heating cycle.

               Cooling a piece in contact with a mold is very efficient if it is a metallic mold and has
               cooling ducts with water re-circulation. In these cases, enough volume of refrigerant
               liquid should be used to keep a constant temperature on the mold. If the cooling water
               is kept at a certain temperature, marks on the piece (usually known as undulations on
               its surface) due to a cold mold, can be minimized. Aluminum or epoxy resin and/or
               polyester molds are very suitable if you want to include a refrigeration system. Wooden
               molds are not convenient for long runs because they do not dissipate heat quickly.


        Non    There are faster cooling methods that use a spray or a very thin de-ionized water cur-
conventional   tain or liquid carbon dioxide, which rapidly cools a thermoformed piece. This method
    cooling    is not common because of its cost, but both methods can be justified, specially if they
   methods     are applied locally to prevent thermal tearing of very deep pieces. Irregular fast cooling
               of a formed piece causes great efforts that affect durability.




          52   Thermoforming
             Cutting thermoformed pieces

             Once the forming cycle is finished, pieces have to be cut to eliminate excess material.
             It is very rarely that a finished piece does not need cutting, as in the case of lighted
             signs. Most thermoformed products need some kind of cutting.

             The right equipment and technique must be chosen. Anyway, there are some factors
             that determine the choice, as sheet measures, size and depth of a piece, acceptable
             level of roughness of the cutting surface, required dimensional tolerance and cutting
             speed among others.

   Cutting   There are several equipments to cut thermoformed pieces:
equipment.
             Electric tools.

             Circular saw.
             A circular saw must have straight teeth to help cooling and not to soften the material.
             Tungsten carbide teeth provide excellent cutting and keep sharp longer. Cutting must
             be slow to prevent heating or stretching the material. The saw has to be operated at
             relatively high speed and before starting, make sure that the saw has reached its high-
             est speed. The thicker the material, the bigger the diameter of the saw must be, and
             have the least number of teeth (minimum 2 teeth per 0.8”.). When a hand circular saw
             is used, the sheet has to be held and pressed firmly as it cuts at a steady speed to
             avoid chipping.

             Table 14. Cutting specifications for circular, radial, or travel saw.

                      SHEET                                                  DISK
                 Thickness inches         DIAMETERS (inches)          Thickness (inches)           No. TEETH (*)
                      0.06-0.12                     8                      1/16-1/32                    96
                      0.12-0.16                    10                       3/32-1/8                   82-96
                        0.2-0.4                    10                          1/8                     82-96
                       0.48-0.6                    12                          1/8                     82-96
                      0.72-0.84                    12                          1/8                     48-52
                        1-2.08                    12-14                     1/8-5/32                   48-52

             *Teeth with tungsten carbide bit, teeth with straight surface at the center, combined or alternated




        53   Thermoforming
     Band saw
     A band saw is the right one to make curves in flat sheets and rethread formed pieces.
     A band saw with variable speed up to 5000 feet/min. and minimum 10" deep groove is
     recommended . It is convenient to use the special bands to cut metal or plastic; the
     guide must be adjusted as close as possible to the material to avoid chipping on the
     cutting line and to reduce the vibration of the saw to a minimum. Next, cutting speci-
     fications with a band saw are listed:

     Table 15, cutting specifications with a band saw.


                                          BAND                                   ENGINE
           SHEET
                             WIDTH MIN            TEETH X
      Thickness (inches)                                                 HP               RPM
                               (inches)            (inch)

           0.06-0.12            3/16                18                     1
           0.16-0.24            3/16                14                   1.5               DE
           0.32-0.48            1/4                 10                   1.5              2500
             0.6-1              3/8                  8                  1 .5-2             A
             1-2.08             3/8                  8                     2              3500



     Table 16 Radial cutting specifications, with band saw

             SHEET                                              BAND
        MINIMUM RADIUS             WIDTH OF                 TICKNESS OF               TEETH X
        TO CUT IN (inches)        BAND (inches)             BAND (inches)             INCHES

                 0.48                  3/16                      .028                     7
              0.52-0.76                 1/4                     .028                      7
               0.8-1.52                 3/8                     .028                      6
              1.56-2.28                 1/2                     .032                      5
              2.32-3.04                 5/8                     .032                      5
              3.08-4.56                 3/4                     .032                      4
               4.6-8.12                  1                      .035                      4
              8.16-12.2                1 1/4                    .035                      3
              12.24-20                 1 1/2                    .035                      3


     Router
     Chemcasts acrylic sheets can be cut with a portable or fixed router (electric or pneu-
     matic). A 1.5 HP and 20,000 to 30,000 RPM electric router is recommended, and bits
     or cutters with tungsten carbide bits with 1/4 or 3/8" diameter and ideally 1/2" to avoid
     that vibrations break the bit.



54   Thermoforming
     This method provides very uniform cut and is good to form as well as to make big
     diameter holes. The router can be fixed to a table and a copying guide can be used for
     intricate designs.

     The cutting tool of a circular saw or router can be changed for an abrasive normal disk
     or even a diamond one; this kind of disk should not be used when an acrylic formed
     piece is reinforced with glass fiber, as in the case of tubs, wash basins, phone booths,
     etc.

     Automatic equipment.
     This kind of cutting equipment is used when a high automatic level is required; gener-
     ally, this equipment has a computing system and specialized software, like CAD-CAM-
     CAE, which is used to design the cutting pattern, and later send the information to a
     peripheral one, that in this case may be 1 or 5 head routers, pressured water system
     or laser. Cutting capacity is not limited to a direction or plane, it can perform any kind
     of cut or perforation.

     Pressured water cutting
     The abrasive system with pressured water eliminates many of the problems related to
     the machinery and cutting operations of conventional cutting. A very fine jet of pres-
     sured water 50.000 Psi, is concentrated, at a speed of about 3.3 yd./min and a pres-
     sure of +/- 0.04”.

     Using a combination of highly pressured water and abrasive materials, such as silica
     powder, the water jet can cut every material without heating and provide an exceptional
     finishing on the cutting surface.

     The advantages of this cutting system on acrylic are: eliminating heating distortions,
     any cutting angle can be performed because of its multi-directional type integrated to
     computing systems, it eliminates secondary operations like sanding, and reduces
     material waste since the cutting area is very reduced.

     Cutting with laser
     Cutting with laser is a technique that has already been used in other industrial sectors
     for several years and its main characteristics are:

     • High pressure cutting
     • Manufacturing flexibility
     • Reduced cost

     An advantage of the laser cutting is its application versatility, since apart from its direct
     use to cut acrylic sheets, it offers the possibility of processing many other materials.




55   Thermoforming
             With a laser device you can cut, weld and hew surfaces up to 1.2” thick, because laser
             energy is concentrated on one spot and heat generation can be limited to a minimum
             zone, which avoids any heat deformation or structural changes in the material. Very fine
             cuts with accurate edges can be obtained which is good for acrylic pieces with intri-
             cate forms. You can make 0.004” diameter bores at a speed up to 150,000 holes per
             hour. A laser equipment can cut 1/2" of acrylic at a speed of 12”./min.

             Swaging
             This technique is not much used because of its limitations; it may be used on thermo-
             formed pieces when they are still hot and are not over 0.08” thick, the blades should
             be at a temperature between 104ºF and 140ºF (40°C and 60° C). Even then cutting
             quality is not very good. This kind of cutting is better for plastics-like acetate poly-
             styrene and foamed P.V.C.

   Cutting   Although there are non conventional cutting techniques and highly automatic ones,
techniques   their practical application is far from popular, because of their high investment and
             maintenance cost compared with traditional techniques like router or circular saw cut-
             ting.

             Some cutting alternatives of thermoformed pieces are shown next. As long as it is pos-
             sible, you should build a cutting template as support for the thermoformed piece, this
             way you will avoid variations on a piece and production will be standardized.


                        Cutting with router and bullet bit   Cutting with router, straight bit and copying guide




        56   Thermoforming
              Cutting with bench saw and iron or           Cutting with bench saw and wooden butt
                     aluminum angle butt




              Cutting with router and cookie cutter          Cutting with router and cookie cutter
                or abrasive disk on the outside                 or abrasive disk on the inside




      Cutting with radial saw and template on the inside     Cutting with radial saw and template
                                                                        on the outside




57   Thermoforming
            Thermoforming variables

            In the thermoforming process there are variables that can affect aspect, quality, dimen-
            sions and distribution of the material of a formed piece. Knowing these variables can
            help to solve difficult production problems in the thermoforming process. Following, the
            most frequent variables as deviations in the thermoforming process are shown.

 Material   Thickness of a sheet
variables   When electric resistors or infrared radiation is used to heat, changes on caliber of the
            thickness of the material can cause an uneven heating, creating variations in the
            formed part. In pre-stretching or deep forming, close dimensional tolerance is needed
            to avoid breaking the material in very thin areas, because of the force exerted by vac-
            uum or pressured air. In very deep pieces there is a variation in the thickness of the
            material which depends on the thickness used, the area and maximum depth of a
            piece. When there is a thickness variation between each sheet, the heating tempera-
            ture must be reduced to prevent material from over softening. If the temperature of a
            sheet is homogeneous, even a piece with thin areas can be well made.

            Sheet pigmentation
            In the case of radiation heating (electric resistors) the different colors of the same mate-
            rial can cause temperature changes and heating cycle changes. In a convection fur-
            nace (hot air re-circulation) this variable does not apply.
            .
            Size of a sheet.
            To get a better distribution of the material of a very deep piece, it is more economic to
            increase the size of a sheet instead of its thickness.

            Temperature uniformity of a sheet
            When the temperature of any material is increased, tension force is reduced and there-
            fore the sheet becomes malleable. Simple or deep forming made at a lower range than
            annealing temperature provides the best results.

            For high quality pieces, it is important that a sheet heats evenly at annealing point
            length-wise and width-wise. The sheets that are not evenly heated will be deficiently
            formed: there will be more stretching in the normal temperature zones than in the ones
            that were not softened.




       58   Thermoforming
Mold variables   Vacuum bores or orifices
                 Vacuum speed is directly proportional to the quality of a piece. A slow vacuum makes
                 the part of the sheet where the first contact with the mold takes place to cool faster
                 than the rest. Therefore, there are sections with very thin walls or incomplete pieces.
                 To eliminate air quickly, 1/8" and _" vacuum bores should be used. When possible,
                 there should be vacuum canals or ducts since they display a greater volume of air.

                 Mold surface
                 When a thermoplastic sheet is formed it will take the form of the mold, one with opaque
                 finishing, will give an opaque finishing, a very polished finishing (mirror finishing) of
                 course, will provide a shining piece.

                 Mold temperature.
                 A mold surface temperature influences directly the duration of the forming cycles, the
                 size and a better aspect of a formed piece. A thermoformed piece final shrinkage
                 depends on having a mold temperature similar to the thermal expansion coefficient of
                 the material.

                 Mechanical support temperature
                 To prevent a sheet from getting cold during a pre-stretching operation causing "cool-
                 ing marks" and deformations, a mechanical support should be heated at a tempera-
                 ture over the distortion point.

Pre-stretching   Vacuum box
     variables   In vacuum with return and free forming it is very effective to use a vacuum box of 3.2”
                 to 4.8” longer than the total depth of the formed bubble to prevent cooling on the
                 perimeter of the sheet in contact with the mold. Before forming the bubble, the sheet
                 must be strongly sealed on the mold. In a vacuum with return operation, maximum
                 thinning will occur at the bottom of the formed bubble. To get thicker walls, there must
                 be a two step edge in the vacuum box which will cool the top area making it thicker.

                 Air temperature
                 Sometimes the air of the system should be pre-heated. When air at room temperature
                 gets into the system, it may cool the sheet, affecting its size and form. With thin mate-
                 rials, the cooling problem is more serious. With pre-heated air, the temperature should
                 be about 10% below the temperature of the sheet. An air deflector or an air diffuser
                 should be used at the intake of the mold since they can prevent a sudden cooling in
                 some areas of the material.




            59   Thermoforming
Mechanical   Mechanical support form
   support   This must be closely adapted to the form of the cavity of the mold, but must be 10 to
 variables   20% smaller length-wise and width-wise (or diameter). When these dimensions are
             4.8” or larger, the small supports must allow at least 1/4" margin between the final part
             and the support, to prevent thickness irregularities of the material as far as possible.

             When the mold has canals (corrugated tin) with sudden changes from flat to narrow
             zones it is important that the support is made with detachable parts that fit into the
             canals of the mold. These parts will help add more material to increase thickness in a
             particular area. For boxes in the mold, the same projection of the support must be
             applied. In the case of deep depressions on the walls of the mold, a support mecha-
             nism should be incorporated to take material to that zone, all the corners should be
             softened and have generous radiuses.

             Support materials.
             To get good results, the mechanical support must have excellent qualities to transfer
             heat, and must have constant and prolonged resistance at high temperatures.
             Aluminum is one of the best materials. For short or prototype runs hard wood is better
             and to prevent it from getting too dry or cracking because of the heat, the surface has
             to be greased frequently.

             Support temperature.
             The temperature of a support must be kept below that of the forming of a sheet. The
             support may have low working temperatures, anyway, if the temperature drops, the
             cooling marks will be more visible.

             It is not so critical to strictly control the temperature of a mold. In the case of a sup-
             port, a maximum uniform heating of 50°F must be kept without variation, with suitable
             regulated temperature the molding marks are generally eliminated.

             Support surface.
             A smooth surface with well polished radiuses, dust-free and rubbish-free, will produce
             good pieces

             Support height
             An effective mechanical support is the one that is longer than the depth of the mold,
             since it can regulate adjusting.

             Support vacuum speed.
             Increasing the speed of a support raises air compression capacity in the cavity of the
             mold. The vacuum system capacity and its duration related to the run of the support
             affects pressure in the cavity of the mold. Normally, the vacuum cycle must start at the
             same time as the support touches the material.


        60   Thermoforming
     Support, depth of action
     The best results are achieved when a support penetrates 78 or 80% in the cavity of the
     mold. This creates the best combination between the thickness of the bottom and the
     walls of a piece.

     Material variables when forming with support
     The kind of material used will affect the amount of pressure needed to keep the right con-
     tact of the material around the support. High resistance of materials such as acrylic and
     ABS, need air pressure between 15 and 50 Psi.




61   Thermoforming
     Problems and solutions guide
                     DEFECT                      POSIBLE CAUSE                    SUGGESTED SOLUTION
       • Bubble or blister on the sheet   • Excessive moisture                 • Pre-dry sheet.
                                                                               • Dry both sides of sheet at
                                                                                  140°F (60°C)
                                          • Heating too fast                   • Reduce furnace temperature.
                                                                               • Increase distance between
                                                                                 sheet and heater.
                                          • Irregular heating.                 • Check and fix the furnace.
                                                                               • Check heating elements.

       • Incomplete forms and             • Insufficient vacuum                • Eliminate obstructions in vac-
         details                                                                 uum system
                                                                               • Increase number of holes
                                                                               • Increase their diameter
                                                                               • More tank and vacuum pump
                                                                                 capacity.
                                                                               • Leakage.
                                          • Slow vacuum displacement           • Check vacuum system for
                                                                                 possible leaks.
                                                                               • Use vacuum canals in possi-
                                                                                 ble areas.
                                          • Insufficient heating of a sheet.   • Increase temperature or heat-
                                                                                 ing time.

       • Color change of a sheet          • Excessive heating                  • Reduce heating time.
                                                                               • Reduce furnace temperature.
                                          • Low mold temperature.              • Heat mold.




62   Thermoforming
                     DEFECT                 POSIBLE CAUSE                   SUGGESTED SOLUTION
      • Color change of a sheet.     • Low temperature of mechanical     • Heat mechanical support.
                                       support
                                     • Too much thinning of a sheet.     • Increase sheet thickness.
                                     • Sheet cooling before its formi-   • Place sheet more quickly on
                                       ing is completed.                   the mold.
                                                                         • Increase vacuum speed.
                                                                         • Heat mold and mechanical
                                                                           support.
                                     • Mold wrongly designed.            • Reduce mold depth.
                                                                         • Improve vacuum air flow.
                                                                         • Use more curved radiuses.
                                     • Inadequate material               • Change material.

      • Excessive warping or bend-   • Sheet too hot                     • Reduce heating time.
        ing of a sheet                                                   • Reduce furnace

                                     •Sheet too big.                     • If possible, reduce sheet size
                                                                         • Use screens, mainly on cen-
                                                                           ter of sheet (only infrared
                                                                           heating furnaces).

      • Cooling marks on a formed    • Sheet too hot.                    • Reduce mold temperature. .
        piece.                                                           • Reduce heating time.

                                     • Insufficient temperature of       • Raise support temperature. .
                                       support.                          • Use soft flannel filter on sup
                                                                           port surface
                                     • Mold low temperature              • Raise mold and/or support -
                                      (Shrinking stops at contact          temperature, without exceed
                                      with mold or cold support).          ing temperature range.
                                                                         • Soften and/or round mold
                                                                           critical areas.




63   Thermoforming
                     DEFECT                    POSIBLE CAUSE                 SUGGESTED SOLUTION
       • Small wrinkles or circular     • Sheet too hot                  • Reduce mold temperature.
         marks..                                                         • Reduce heating time..

                                        • Too big vacuum bores.          • Refill and bore again smaller
                                                                           diameter.

       • Bending variation of sheet.    • Sheet irregular temperature.   • Check there are no drafts in
                                                                           furnace, deflectors must be
                                                                           incorporated.

       • Wrinkles while forming.        • Excessive heating of sheet.    • Reduce furnace temperature.
                                                                         • Reduce heating time.
                                                                         • As far as possible, more dis-
                                                                           tance between 2 heaters and
                                                                           sheet (only infrared heating
                                                                           furnaces)
                                        • Excessive bending of sheet.    • Reduce molding range tem-
                                                                           perature.
                                        • Insufficient vacuum..          • Check vacuum system.
                                                                         • Increase vacuum canals or ori-
                                                                           fices.

       • Very shiny lines or zones.     • Over heating sheet on shine    • Use screens to reduce heat on
                                          area.                            zone.
                                                                         • As far as possible more dis-
                                                                           tance between 2 heaters and
                                                                           sheet (only infrared heating fur-
                                                                           naces).
                                                                         • Reduce heating time.

       • Bad surface aspect of piece.   • Defect caused by air caught    • Sandblast mold surface.
                                          on flat surface of mold.
                                        • Insufficient vacuum.           • Increase number of vacuum ori-
                                                                          fices
                                                                         • If marks are isolated, increase
                                                                           number of vacuum orifices in
                                                                           affected area.




64   Thermoforming
                     DEFECT                     POSIBLE CAUSE                 SUGGESTED SOLUTION
      • Piece surface bad aspect         • Excessive mold temperature.     • Reduce mold temperature.
                                         • IInsufficient mold tempera-     • Increase mold temperature.
                                           ture..
                                         • Superficie del molde demasia-   • Soften mold surface.
                                           do áspera o rugosa.             • Make mold of other material.
                                         • Dirty sheet.                    • Clean sheet.

      • Excessive distortion or          • Piece removed too fast.         • Prolong cooling cycle.
       shrinking after removing a                                          • Move the piece to a cooling
       piece off a mold.                                                     template.
                                                                           • Use refrigerant.
                                                                           • Use water spray steam to
                                                                            reduce piece temperature.
                                                                           • use electric ventilators to cool
                                                                            piece inside the mold.

      • Excessive thinning of walls of   • Inadequate forming tech-        • Use different forming techni-
        a piece                            nique.                           que: vacuum with return, pres-
                                                                            sured air and mechanical sup-
                                                                            port, pressured air and return
                                                                            with vacuum.
                                         • Material thickness variation.   • Check material meets quality
                                                                            norms and /or complain.
                                         • Uneven sheet heating.           • Check furnace operation.
                                         • Sheet at excessive tempera-     • Reduce furnace temperature.
                                           ture.
                                                                           • Reduce heating time.
                                         • Cold mold.                      • Heat mold.
                                         • Sheet not firmly fastened to    • IIncrease closing pressured.
                                           frame.
                                                                           • Check possible sheet thick-
                                                                            ness variation.




65   Thermoforming
                     DEFECT                  POSIBLE CAUSE                   SUGGESTED SOLUTION
      • Pieces twist                  • Piece cooled wrongly.             • Adjust cooling cycle.
                                      • Uneven wall thickness distri-     • Use pre-stretching mechanical
                                        bution.                             or technical support.
                                                                          • Sheet might be unevenly heat-
                                                                            ed.
                                      • Wrongly designed mold.            • Increase vacuum orifices.
                                                                          • Modify mold
                                      • Wrongly designed piece.           • As far as possible, curve a little
                                      • Insufficient mold temperature.      flat areas.
                                                                          • Increase mold temperature.

      • Shrinking marks on corners.   • Mold surface too smooth.          • Sandblast mold surface.
                                      • Insufficient vacuum.              • Check vacuum system.
                                                                          • Add more orifices.

      • Bubble stretches unevenly.    • Insufficient sheet temperature.   • Check furnace operation con-
                                      • Sheet uneven thickness.             dition
                                      • Insufficient pressured air.       • Use cooling screens (Only
                                                                            infrared radiation heating fur-
                                                                            nace.).
                                                                          • Longer heating time at lower
                                                                            temperature.
                                                                          • Incorporate an air distribution
                                                                            system with deflectors.

      • In deep forming, thin cor-    • Wrong forming technique.          • Change forming technique.
        ners.
                                      • Thin sheet                        • Increase sheet thickness.
                                      • Sheet unevenly heated             • Check furnace operation.
                                                                          • Use screens to change heat
                                                                            distribution.
                                      • Mold wrongly heated.              • Change furnace temperature.




66   Thermoforming
                     DEFECT                 POSIBLE CAUSE                   SUGGESTED SOLUTION
      • Piece sticks to mechanical   • Mechanical support (wood).      •   Apply removing agent.
        support.                                                       •   Cover with soft felt or flannel.
                                     • Mechanical support (metal).     •   Apply removing agent.
                                                                       •   Lower support temperature.
                                                                       •   Cover with felt or flannel.

      • Piece sticks to mold.        • Piece high temperature.         •   Longer cooling time.
                                                                       •   Reduce mold temperature.
                                     • Mold insufficient exit angle.   •   Give 1° and 3° angle
                                                                       •   Change matrix.
                                     • Wooden mold.                    •   Apply removing agent.

      • Corners of formed piece      • Piece wrongly designed.         • Redesign piece.
        shatter once in use..        • Effort concentration on a       • Increase mold curve radius.
                                       piece.                          • Increase thermoforming tem-
                                                                         perature.
                                                                       • Make sure piece is wholly
                                                                         formed before it cools below
                                                                         forming temperature.




67   Thermoforming
           APPENDIX

Glossary   ABSORBENCY
           Fraction of radiant energy taken by a sheet.

           CAVITY
           Depression of a vacuum made mold, machine finished or a combination of both,.
           depending on the number of depressions, it may have one or several cavities.

           CONDUCTION
           Energy transferred by directly touching a solid.

           CONVECTION
           Energy transferred by the movement of a fluid current.

           COOLING MARKS
           Marks caused by using wrong temperature on a plastic sheet, derived from inadequate
           heating.

           CO-POLYMER
           Polymer composed of tow different kinds of monomers.

           CYCLE
           Complete repetitive sequence in the thermoforming process, which consist in: heating,
           forming, cooling and removal.

           DIMENSIONAL STABILITY
           Capacity of a piece to keep the accurate shape and dimension of the mold used.

           ENTHALPY
           Inner energy of a system.

           HEAT TRANSFER COEFFICIENT
           Effectiveness measure of energy transported between a fluid current and a solid surface.

           HOMO-POLYMER
           Polymer made of only one monomer

           INFRARED
           Part of electro-magnetic spectrum, between the range of visible light and the range of
           radio waves. Radiant heating is the range at which infrared heaters are used to heat a
           sheet. Wave length is 0.08” to 0.4”


      68   Thermoforming
     MELTING TEMPERATURE
     Range of temperature at which a crystalline polymer turns from a solid rubber-like state
     into a viscous-elastic liquid.

     MOLDING TEMPERATURE
     A piece temperature at which it can be removed without deforming.

     PRESSURED AIR SHAPING
     Difference of pressure exceeding two atmospheres (30 Psi.).

     RADIATION
     It is the transfer or exchange of electromagnetic energy.

     REFLECTIVITY
     Fraction of radiant energy reflected on a sheet surface.

     RESIN
     Another name to call a polymer or plastic material.

     SCRAP
     Material waste that is not part of the final piece.

     TENSION
     External charge exerted on a defined area.

     THERMAL DIFFUSIVENESS
     Transmission index of calorific energy in a material.

     TRANSMITTANCE
     Fraction of energy that is transmitted through a sheet.

     TRI-POLYMER
     Polymer composed of three different kinds of monomers.

     VACUUM TANK
     Tank between the vacuum pump and the mold, that allows you to apply pressure even-
     ly during forming.




69   Thermoforming
                  Plastic reinforced with glass fiber
                  Introduction
                  Reinforced plastics are those thermo-plastic or thermo-fixed materials, in whose shap-
                  ing process, some reinforcing material is used to improve their mechanic characteris-
                  tics. This reinforcing material can be continuous or discontinuous. As examples of the
                  former there are fiber materials like: salwort, jute, henequen, rayon, etc., but the most
                  used is glass fiber.

        Resin,    A polyester is made by the reaction of a poly-basic acid and a polyhydric-alcohol, at
     polyester    temperatures over 212°F (100ºC), getting one polyester and water. Depending on the
and reinforced    type of acids and alcohol used and modifications performed, the following kinds of
        plastic   products will be obtained.

                  Non saturated polyesters
                  These are lineal polyester resins obtained when dibasic acids and polyvalent alcohols
                  react, and can polymerize in a cross-linking way with vinyl monomers to make ther-
                  mofixed plastic..

                  Alkyd polyesters.
                  These are the ones modified with oil, used for decorative and/or protective coverings,
                  for example: paints, varnishes, printing inks, etc.

                  Plasticizing polyesters
                  Polyesters totally saturated that are used to soften other plastics, they are also known
                  as polymeric plasticizers. They are used to make vinyl with or without reinforcement,
                  for example: the one used for car upholstery, wall paper, etc.

                  Fibers and films
                  They are polyesters of heavy molecular weight, molecularly oriented and for which spe-
                  cific acids and alcohol are used. Example: polyethylene, polypropylene, etc.

                  Polyester foam.
                  Polyesters with a great number of hydroxyl groups and that react with interlinked
                  chains with isomeric acid groups, to make foams, elastomers, coverings, etc.

                  According to the previous classification, polyesters are a great variety of chemical
                  composites and products. However, they are generally used to name composites
                  defined as non-saturated polyesters, so unless something else is suggested, this
                  denomination will be adopted.

                  Polyester resins are used in a wide variety of applications, in different industries, for
                  example: forming with reinforcing materials (reinforced plastic), encapsulating, protec-


             70   Thermoforming
     tive covering, decorative objects, buttons, etc. Reinforced plastic industry is the one
     that has the most polyester consumption.

     Increasing demand and application of plastic reinforced items are basically due to their
     properties and features, among which, the following can be mentioned:

     1) Composites are easy to handle (polyester resin is applied in liquid form).
     2) Easy curing and using.
     3) Excellent dimensional stability in the final product.
     4) Good dielectric properties.
     5) Excellent physical and mechanical properties. A reinforced plastic sheet, equivalent
     to three times steel thickness, has mechanical resistance to tension, weighs about half
     and is more resilient.
     6) Rust resistant and also to a great amount of chemical agents.
     7) Easy finishing (coloring, painting, machine finishing, etc.).

     To obtain optimal reinforced plastic features, the reinforcing material must have the
     best mechanical and chemical properties. Next, reinforcements most used are men-
     tioned.

     Reinforcing materials
     The most important reinforcing material are:

     1.- Cellulose fibers.
         Cellulose alpha.
         Cotton.
         Jute.
         Salwort.
         Rayon.

     2.- Synthetic fibers.
         Polyamides (nylon).
         Polyester (Dacron).
         Polyacrylonitrile.
         Polyvinyl alcohol fibers.

     3.- Asbestos fibers.

     4.- Special fibers.
         Carbonate and graphite fibers.
         Boron and tungsten fibers.
         Ceramic fibers.




71   Thermoforming
     5.- Reinforcing charges.

     6.- Glass fibers.




72   Thermoforming
     Glass fiber

     In reinforced plastic industry, the material most used is glass fiber because of its fea-
     tures:

     1.- Tension high resistant.
     2.- Incombustible.
     3.- Biologically inactive.
     4.- Excellent weather resistant as well as to a great deal of chemical agents.
     5.- Excellent dimensional stability.
     6.- Low thermal conductivity.

     The main uses of glass fiber reinforcements are:

     Roving.
     Mat.
     Woven roving.
     Surfacing mat.
     Chopped strand.

     Following, processes to obtain these and their features are mentioned.


     Roving
     Roving is one of the most used glass fiber products, and it is indispensable when rein-
     forced plastic items are made by sprinkling, directed filament and hot forming (pre-
     form manufacturing). Roving comes wound on bobbins, and it usually has 60 threads.




73   Thermoforming
     Mat
     This is the most popular and known glass fiber product in reinforced plastic industry
     and it is made of fiber mono-filaments about 2”, long.

     Woven roving
     This is roving strings woven at 90° angles as to their longitudinal axes. Combined with
     mat, it is used as secondary reinforcement, to manufacture boats and big structures.

     Surfacing mat
     This material is made of glass fiber sections like the mat, though with less weight/unit
     area. It is mainly used to improve the finishing of reinforced plastic products and to
     increase their weather resistance features; since when it is put on the reinforcement
     material, usually a mat, it does not allow the fiber to crop up and as it absorbs resin,
     finishing gets smoother.

     Chopped strand
     This glass fiber presentation is not much used, it is made by the machine that makes
     mat. Its size varies from 2/2" to 2" long (1.25 to 5.0 cm). It is mainly used to make items
     by methods of pre-mixing.

     According to reinforcing materials classification, there is another type of products used
     in the manufacture of reinforced plastics, the most important are:

     Asbestos.
     Salwort, henequen, jute.
     Synthetic fibers.
     Ceramic fibers.
     Polyvinyl alcohol fibers.
     Special reinforcements.

     To improve reinforced plastic efficiency and application, several reinforcing elements
     have been developed. Their main characteristic is a high elasticity module, which con-
     siderably increases mechanical resistance of laminated products. This is specially
     important in specialized fields like aero-spatial vehicles, submarines, etc. Among these
     reinforcements are:

     Boron tungsten filaments.
     Carbonate and graphite fibers.
     Metallic filaments.
     Whiskers
     Adhesion promoting agents.
     Hybrid reinforcements.
     Metalized reinforcements.


74   Thermoforming
     Mechanical resistance of a plastic/reinforcing composite derives from joining, gener-
     ally mechanically a system composites. This joint, satisfactory in most cases, may
     reduce composite or product aging as well as moisture, when glass fiber systems are
     used to reinforce, since the fiber is hydrophilic and tends to absorb water which weak-
     ens or destroys plastic joints.

     To prevent this, chemical composites hydrogen siliceous type are added to the inor-
     ganic charge, resin or reinforcing material. They provide a chemical consolidation in the
     interface of the joint, improving and keeping the mechanical properties of composites,
     apart from improving dielectric characteristics of the system.

     Manufacturing reinforced plastic molds.
     To make a mold, a model or original of the piece to be made is required. When there
     are only specifications and blue prints, the model can be made of cast, wood, or epoxy
     paste, depending on how difficult the piece is and how skilled the operators are. Some
     times, the model can be made by combining polyurethane foam or polystyrene plates
     covered with a thin coat of cast or epoxy.

     When the model is finished, roughness can be smoothed with emery cloth and then
     applying a sealer to eliminate porosity. In most of the cases it can be a nitro-cellulose
     lacquer, which is spayed, or shellac dissolved in alcohol. To polish the model, a remov-
     ing agent is applied, its specific function is avoiding adherence of the resin to the mold.
     Removing agents can be classified in three groups:

     Solutions.
     Generally aqueous polyvinyl-alcohol, methyl-cellulose, etc. This kind of removers must
     be applied in each molding operation.

     Waxes and wax emulsions
     This agent is applied with a flannel or felt, and polished manually.

     Internal removers
     These agents are mixed with gel-coat. When they mix with the tooling gel-coat, remov-
     ing characteristics improve, making molding easy.

     Once the removing agent has been chosen, the mold is coated with a resin prepara-
     tion known as gel-coat or finishing coat.

     Gel-coat
     It is made of a resin that provides a film whose characteristics are:

     1.-Uniform surface.
     2.-Avoiding that reinforcing material crops up.
     3.-Improving weather resistant properties.

75   Thermoforming
     Some times, a glass fiber mat should be put to reinforce gel-coat, getting a resin rich
     coat and avoiding that the reinforcing material crops up. Gel-coat is usually sprayed,
     but it can also be applied with hair brushes. In that case, accelerator/catalyst quanti-
     ties should be less than for immersion. Finishing film thickness depends on the use and
     characteristics of the piece to be made and it can be measured with a calibrator of
     humid film.

     Manual finishing process
     It is often used since it does not require any special equipment. Its process is:

     A mold prepared with removing agents (wax, removing film or both) is coated with a
     finishing product using a soft brush or spraying equipment, thickness varies depend-
     ing on the use of the piece and the supplier’s specifications. Once the gel-coat thick-
     ness is determined and it has been cured, the glass fiber mat is placed. Next, using a
     brush and with vertical movements, a resin of styrene monomers or methyl methacry-
     late, or both, is applied to the mold, as well as the accelerator, whiskers and/or toxi-
     tropic agents, heat concentrator, catalyst, etc.

     Later, and before the resin jells rolling is done, with a plastic or metal 0.36” to 1” (9.0
     to 25mm.) diameter and 2” to 8” (5 to 20 cm.) long roller generally grooved, depending
     on the case.

     Rolling the roller in several directions pressing evenly helps to eliminate air caught in
     the resin and reinforcing material, as well as to obtain good adhesion with the gel-coat.
     Finishing and rolling should be done by sections no bigger than 1m2 when a piece is big.




76   Thermoforming
     Often, commercial measurements of mat and woven roving (which are always applied
     with this procedure) are not enough to cover the whole mold; therefore, they have to
     be joined by sections. Overlapping them 5cm is suggested. The resin to join them
     should have the least accelerator and catalyst to avoid problems created by material
     contractions which derived from a bigger amount of resin, that reduces curing time and
     increases exo-thermal temperature.

     Some times, one or more woven roving layers have to be used as reinforcement. They
     must be put between the 2 mat sections, or even better, as a final layer and never
     directly with the gel-coat, because if the finishing coat is not properly applied, the
     woven roving will be visible, which will give the product a bad aspect.

     The brushes and rollers have to be washed intermittently with a solvent like acetone,
     ethyl acetone, methyl ethyl acetone, etc., since as the resin cures it hardens and they
     may get damaged. Most of the time it is enough to put them in a container with a sol-
     vent or a monomer mixture.

     Reinforced plastic machine finishing.
     Reinforced plastic product manufacturing, often includes machine finishing or adjust-
     ing, operations that are not highly specialized, but must be done carefully to get good
     results. Among machine finishing operations are: cutting, perforating, joining, etc. The
     most important are detailed next

     Cutting on the mold.
     It is also known as trimming and it is cutting the material (glass fiber and resin) that sur-
     passes the mold or piece made. This is done with steel blades along the edge of the mold
     when the resin is jelled and has not been totally cured. In the case of products manufac-
     tured with pressure or temperature, cutting must be done immediately after removing the
     piece; otherwise, it becomes harder to do so.

     Cutting with equipment.
     It is performed on totally finished products. Abrasive products are recommended, since
     metallic disks are not as fast, accurate, and ergonomic as the ones suggested. Water
     should be used as cutting takes place; since water acts as refrigerant and lubricant
     helping to eliminate reinforced plastic dust and the cut is cleaner.

     Reinforced plastic joints
     Often, 2 or more sections have to be joined to get a final piece; The systems commonly
     used are




77   Thermoforming
     Joining with adhesives
     Although this kind of joining can be done in 2 ways, on the edges or overlapping, the
     latter is the most used. Because contact surface is bigger. The adhesives most used
     are: polyester resin (modified with flexible resin) or epoxy resin that provides excellent
     adhesion. Adhesive material can be put directly on the plastic surface, though apply-
     ing it on a layer of reinforcing material is suggested, placing the layer between the sur-
     face to be joined, and pressing next, to obtain uniformity in the joint.

     Rivets.
     Rivets are not often used in this industry, but if needed, aluminum or bronze ones are
     recommended. They must not be bigger than 4.5mm (3/16") diameter. The minimum
     distance from the edge is three times their diameter. Besides, flat washers should be
     used to reduce the rivet tendency to penetrate laminated.

     Using screws
     Screws are the most commonly used to join reinforced plastic pieces, excepting adhe-
     sives. The use of setscrews is not advisable. Using a screw and nut has the advantage
     that they are easy to place, are adjustable and available. To get the most efficiency, the
     following rules must be followed:

     • Distance between the center of the screw and the edge of a laminated must be min-
       imum three times the screw diameter.

     • Separation between the center of each screw must be 2.5 times perforation diameter.

     • Flat washers should be used on both sides of laminated, thus, charge and mechan-
       ical efforts are uniformly distributed

     • Perforations must be perpendicular to reinforcing layer, and the screws must adjust
       perfectly in. (Both screw and perforation diameter must be the same)..

     • Screws allow using adhesives, which will provide a better quality and more resistant
       joint.




78   Thermoforming
     Table: unit conversions.



                             g          lb
       Specific gravit: 1       = 62.4
                            cm3        cu ft



       Specific heat: 1 Btu = 1 cal
                        lb° F   g° C


                         Btu ft        Btu in                cal cm              W cm
       Heat fusion: 1            =12             = 0.00413            = 0.0173
                        sq hr° F     sq ft hr° F           cm2 sec °C            cm2 °C



       Thermal conductivity: 1     in = 1.80 cm
                                 ln° F       cm° C




79   Thermoforming
     Table: temperature scale conversion.
                  ºF                ºC         ºF     ºC
                  50                10        275    135
                  55               12.8       280   137.8
                  60               15.6       285   140.6
                  65               18.3       290   143.3
                  70               21.1       295   146.1
                  75               23.9       300   148.9
                  80               26.7       305   151.7
                  85               29.4       310   154.4
                  90               32.2       315   157.2
                  95               35.0       320   160.0
                 100               37.8       325   162.8
                 105               40.6       330   165.6
                 110               43.3       335   168.3
                 115               46.1       340   171.1
                 120               48.9       345   173.9
                 125               51.7       350   176.7
                 130               54.4       355   179.4
                 135               57.2       360   182.2
                 140               60.0       365   185.0
                 145               62.8       370   187.8
                 150               65.6       375   190.6
                 155               68.3       380   193.3
                 160               71.1       385   196.1
                 165               73.9       390   198.9
                 170               76.7       395   201.7
                 175               79.4       400   204.4
                 180               82.2       405   207.2
                 185               85.0       410   210.0
                 190               87.8       415   212.8
                 195               90.6       420   215.6
                 200               93.3       425   218.3
                 205               96.1       430   221.1
                 210               98.9       435   223.9
                 215              101.7       440   226.7
                 220              104.4       445   229.4
                 225              107.2       450   232.2
                 230              110.0       455   235.0
                 235              112.8       460   237.8
                 240              115.6       465   240.6
                 245              118.3       470   243.3
                 250              121.1       475   246.1
                 255              123.9       480   248.9
                 260              126.7       485   251.7
                 265              129.4       490   254.4
                 270              132.2       495   257.2
                                              500   260.0

     Temperature scale conversion formulas.
     ºF = °C x l.8 + 32
     ºC = ºF - 32/1.8

80   Thermoforming
     IMPORTANT: CHEMCAST is not legally liable for the recommendations or information given in this manual,
     which are based on information we consider to be true, we offer it bona fide, but we do not guarantee it, since
     transformation conditions and use of products are beyond our control.


81   Thermoforming

				
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