Thermoforming principles 4
-History of thermoforming industry
-Products manufactured by thermoforming
Suitable polymers for thermoforming 7
Heating plastics 11
-Heat transfer: conductivity, convection
-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
-Pressured air forming
-Mechanical support design
Thermoforming molds 31
-Choosing thermoforming technique
-Criteria to design thermoformed products
-Criteria to design thermoforming molds
-Considerations in designing thermoforming
-Materials used to manufacture tthermoform -
Thermoforming techniques 46
-Tri-dimensional thermoforming (with molds)
-Molding techniques in infrared heating
Cooling thermoformed products 51
-Conventional cooling methods
-Non-conventional cooling methods
Cutting thermoformed products 53
Thermoforming variables 58
-Mechanical support variables
Problem and solution guide 62
-Glass fiber reinforced plastic
-Unit conversion table
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.
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.
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.
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,
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.
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-
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
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
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.
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.
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.
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
Table 1 shows the suitable and most common polymers for thermoforming, as well as
HEATING DEFLECTION THERMOFORMING
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.
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
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.
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.
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.
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.
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).
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.
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.
SPECIFIC SPECIFIC FUSION THERMAL
MATERIALS GRAVITY HEAT HEAT CONDUCTIVITY
g/cm3 Btu/ Ib 0F Btu/lb Btu ft/sq ft hr 0F
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
Heat For practical purposes we will divide the media for heat transfer into 4 types:
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
All the above mentioned heating media require a considerable amount of time to pre-
heat the equipment.
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
-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
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.
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:
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
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
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%
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.
KIND OF MATERIAL
BOTTOM LIMIT TOP LIMIT
Plastiglas (general use) 320 356
Sensacryl (deep molding) 356 392
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.
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
(0F ) (F)
(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
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.
Solenoid valve controlled by
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.
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-
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
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:
B) Fastening acrylic sheet
C) Electric system
D) Gas installation
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
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
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
Plate "U" bearings of 1/4”
FASTENING SYSTEM FOR ACRYLIC SHEETS
1/4” iron plate
5/16" Cold rolled bar
Type C profile cal.# 18
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
Hook formed 1/2"
1/2” cold rolled bar.
Joint of the fastening
system for acrylic
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”
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.
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
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.).
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.
Acrylic Sheet Heating zone Acrylic Sheet Heating zone
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.
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
• 10 to 20 lt. container
• Garden water pump
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
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
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
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.
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
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.
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
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
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-
tages of this method are: improvement
Mold on dimensional tolerance, forming speed
can be considerably increased and fine
details are better defined.
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
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
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.
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
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-
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.
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
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.
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
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.
Table 12 Basic differences between Injection and thermo-shaping processes.
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
Time to make a piece (design, From 3 to 6 months. Maximum 1 month.
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.
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
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.
4. - When the dimensions of a formed
piece are critical, molds must be built big-
ger to compensate for the contraction of
Expected contraction from molding tem-
perature to environment temperature is
5.-A slight curving of the flat big areas of
the mold will allow flat areas when the
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.
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
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.
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
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
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
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
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
A X B X E (2C + 2D)
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-
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
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-
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.
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
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
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
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
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
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
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.
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-
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.
Undulated alternative flow
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
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.
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:
3) Plastic resins.
Table 13. Use of materials for thermoforming molds
GROUP ADVANTAGES AND DISADVANTAGES
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.
GROUP ADVANTAGES AND DISADVANTAGES
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.
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
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:
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
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
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:
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.
Mechanical forming with matrix and
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-
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-
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.
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
Vacuum with return and male mold
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
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.
Vacuum forming, matrix and mechanical support Pressured air pre-stretching,
mechanical support and vacuum
Vacuum forming, matrix and mechanical support Vacuum forming, matrix
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.
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.
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:
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
Table 14. Cutting specifications for circular, radial, or travel saw.
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
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.
WIDTH MIN TEETH X
Thickness (inches) HP RPM
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
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
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.
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
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,
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.
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.
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-
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
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
A smooth surface with well polished radiuses, dust-free and rubbish-free, will produce
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.
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.
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
• 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
• Slow vacuum displacement • Check vacuum system for
• Use vacuum canals in possi-
• Insufficient heating of a sheet. • Increase temperature or heat-
• Color change of a sheet • Excessive heating • Reduce heating time.
• Reduce furnace temperature.
• Low mold temperature. • Heat mold.
DEFECT POSIBLE CAUSE SUGGESTED SOLUTION
• Color change of a sheet. • Low temperature of mechanical • Heat mechanical 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
• 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
• 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
• 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
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
• Bending variation of sheet. • Sheet irregular temperature. • Check there are no drafts in
furnace, deflectors must be
• 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
• Excessive bending of sheet. • Reduce molding range tem-
• Insufficient vacuum.. • Check vacuum system.
• Increase vacuum canals or ori-
• Very shiny lines or zones. • Over heating sheet on shine • Use screens to reduce heat on
• As far as possible more dis-
tance between 2 heaters and
sheet (only infrared heating fur-
• 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-
• If marks are isolated, increase
number of vacuum orifices in
DEFECT POSIBLE CAUSE SUGGESTED SOLUTION
• Piece surface bad aspect • Excessive mold temperature. • Reduce mold temperature.
• IInsufficient mold tempera- • Increase mold temperature.
• 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
• Material thickness variation. • Check material meets quality
norms and /or complain.
• Uneven sheet heating. • Check furnace operation.
• Sheet at excessive tempera- • Reduce furnace temperature.
• Reduce heating time.
• Cold mold. • Heat mold.
• Sheet not firmly fastened to • IIncrease closing pressured.
• Check possible sheet thick-
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-
• 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-
• Longer heating time at lower
• Incorporate an air distribution
system with deflectors.
• In deep forming, thin cor- • Wrong forming technique. • Change forming technique.
• Thin sheet • Increase sheet thickness.
• Sheet unevenly heated • Check furnace operation.
• Use screens to change heat
• Mold wrongly heated. • Change furnace temperature.
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-
• Make sure piece is wholly
formed before it cools below
Fraction of radiant energy taken by a sheet.
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.
Energy transferred by directly touching a solid.
Energy transferred by the movement of a fluid current.
Marks caused by using wrong temperature on a plastic sheet, derived from inadequate
Polymer composed of tow different kinds of monomers.
Complete repetitive sequence in the thermoforming process, which consist in: heating,
forming, cooling and removal.
Capacity of a piece to keep the accurate shape and dimension of the mold used.
Inner energy of a system.
HEAT TRANSFER COEFFICIENT
Effectiveness measure of energy transported between a fluid current and a solid surface.
Polymer made of only one monomer
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”
Range of temperature at which a crystalline polymer turns from a solid rubber-like state
into a viscous-elastic liquid.
A piece temperature at which it can be removed without deforming.
PRESSURED AIR SHAPING
Difference of pressure exceeding two atmospheres (30 Psi.).
It is the transfer or exchange of electromagnetic energy.
Fraction of radiant energy reflected on a sheet surface.
Another name to call a polymer or plastic material.
Material waste that is not part of the final piece.
External charge exerted on a defined area.
Transmission index of calorific energy in a material.
Fraction of energy that is transmitted through a sheet.
Polymer composed of three different kinds of monomers.
Tank between the vacuum pump and the mold, that allows you to apply pressure even-
ly during forming.
Plastic reinforced with glass fiber
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-
These are the ones modified with oil, used for decorative and/or protective coverings,
for example: paints, varnishes, printing inks, etc.
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.
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-
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-
The most important reinforcing material are:
1.- Cellulose fibers.
2.- Synthetic fibers.
Polyvinyl alcohol fibers.
3.- Asbestos fibers.
4.- Special fibers.
Carbonate and graphite fibers.
Boron and tungsten fibers.
5.- Reinforcing charges.
6.- Glass fibers.
In reinforced plastic industry, the material most used is glass fiber because of its fea-
1.- Tension high resistant.
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:
Following, processes to obtain these and their features are mentioned.
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.
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.
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.
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.
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:
Salwort, henequen, jute.
Polyvinyl alcohol fibers.
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
Boron tungsten filaments.
Carbonate and graphite fibers.
Adhesion promoting agents.
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:
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.
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.
It is made of a resin that provides a film whose characteristics are:
2.-Avoiding that reinforcing material crops up.
3.-Improving weather resistant properties.
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
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.
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
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 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.
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
Table: unit conversions.
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
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
Temperature scale conversion formulas.
ºF = °C x l.8 + 32
ºC = ºF - 32/1.8
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.