UNIT Molding

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					                                                                                               Welding and Plastics
      7.1    Introduction

      7.2    Properties of Plastics
      7.3    Industrial Uses of Plastics
      7.4    Injection Molding
      7.5    Classifications and Important Features of Injection Molding Machine
      7.6    Some Special Injection Molding Processes
      7.7    Defects and Precautions in Injection Molding Process
      7.8    Compression Molding Process
      7.9    Principle of Compression Molding
      7.10 Advantages of Compression Molding Process
      7.11 Plastics and Environmental Issues
      7.12 Summary
      7.13 Answers to SAQs

Plastics can be used to make different types of parts of molding them to different shapes,
extruding them to different sections, insulation coatings on electrically conducting wires.
Plastics are also used as ingredient material for other materials like paints, varnishes, to
improve their adhering capability, various polymer matrix composites. The present unit
describes the technologies at their introductory level, which are used for plastics
processing with their relative advantages, disadvantages and industrial applications. In
industrial applications plastics are being considered as a suitable replacement of metals
due to the following properties :
      (a)    Their easy availability.
      (b)    Low processing cost in case of mass production.
      (c)    Low material cost as compared to cost of metal.
      (d)    Long product life.
      (e)    Resistance to corrosion and water proofing capability.
      (f)    Better surface finish, etc.
Normally plastics are divided into two broader categories thermoplastics and thermo
sets. The major difference between these two type is thermo sets under a curing process
during heating and shaping, which cause a permanent chemical change in their molecular
structured once, these plastics are processed, these cannot be melted again by reheating.
Thermoplastics are just opposite to this fact. Thermoplastics do not cure, their chemical
structure remains unchanged upon reheating even though they transform solid to fluid.
Due to the above difference thermoplastics contributes to more than 80% of the total
plastic tonnage.
Plastic is processed by plastic shaping processes which can be classified according to the
resulting product geometry as describe below :
      (a)    Continuous extruded products with constant cross-section other than sheets,
             films and filaments.
      (b)    Continuous sheets and films.                                                                   91
Manufacturing Processes-III         (c)    Solid molded parts.
                                    (d)    Hollow molded parts with thin sectioned walls.
                                    (e)    Discrete parts made of formed sheets and films.
                                    (f)    Casted parts made with the help of special costing processes.
                                    (g)    Plastic parts made by forming operations.
                                    (h)    Foamed products of plastics.
                              Different processing and shaping processes used for plastics are described in later
                              sections of this unit.
                              After studying this unit, you should be able to understand
                                          introduction of plastics and their classification,
                                          favourable properties of plastics,
                                          reasons for suitability of plastics for industrial uses,
                                          industrial applications of plastics,
                                          process of different types of injection molding,
                                          compression molding process, and
                                          plastics and its related environmental issues.

                              7.2 PROPERTIES OF PLASTICS
                              Before discussing more about the processing methods of plastics, it is important to
                              describe the important properties of plastics.
                                    (a)    Density of plastics is very low (1 to 2 gm/cm3) as compared to metals.
                                           Strength of plastics is sufficient to make low weight, high strength machine
                                    (b)    Antifriction and self lubricating properties of plastics enables it to be a good
                                           replacement of metal parts.
                                    (c)    Plastics components offer noiseless operation of moving parts, corrosion
                                           resistance, water proofing, leak proof joints, etc.
                                    (d)    Their costing is very easy due to low melting point and excellent flow
                                           ability in liquid state into the mold cavity.
                                    (e)    Low cost of manufacturing in case of plastics.
                                    (f)    Plastics exhibits insulation for heat and electricity.
                                    (g)    Plastics are chemically stable when subjected to solvents, oxidizing agents,
                                           gases, etc.

                              7.3 INDUSTRIAL USES OF PLASTICS
                              Above mentioned exclusive properties of plastics and other advancements are making
                              plastics very much popular for industrial uses. Plastics are replacing metal in many
                              engineering industries. These are replacing sheet metal parts like, zinc and aluminium
                              alloy castings and cast iron parts. Plastics are being used in automobile industries, where
                              weight reduction is one of the most important considerations due to fuel economy.
                              Earlier plastics were used in automobiles to make fiscal panels, interiors and other
                              non-load bearing components. Now-a-days, it is gaining popularity to make load bearing
parts and other components like bumpers, brake, fluid tanks and body parts. In addition       Welding and Plastics
to the above other industrial and commercial applications of plastics are listed below.
      (a)   Polysters are also a type of plastic fibers and it is used in textile industry.
      (b)   Polyethylene is used to make wide variety of different size begs used for
            packaging purpose.
      (c)   A type of plastic (PET) is used for making containers and bottles used in
            pharmaceutical, food processing, cold drinks, washing power industries as
            packaging of material.
      (d)   Computer hardware industry uses plastics for making various hardware
      (e)   Plastics are used for surface coating of metal parts to protect them against
      (f)   There are some special purpose plastics and in medical implant
            applications, aerospace industries and bearing industries.
Normally plastics are used in situations determined on the basis of their advantageous
properties as compared to metals. Their relative low cost, ease of manufacturing,
imperviousness to water, low weight make them a good substitute of metals. However,
plastics may create some environment related problems which are discussed in later
sections of this unit.

Most of the plastic articles are produced by injection molding method. Thermoplastic is
used as raw material in this process.
Working Principle of Injection Molding
      In this process the thermoplastic polymer is heated to a plastic state. The hot
      plastic in plastic state is forced to flow under high pressure into a prepared mold
      cavity, where it is allowed to solidify. The solidified plastic is the final product
      which is removed form the mold cavity.
      The process of injection molding is very fast, the product cycle time is very short,
      it is of the order of 10 to 30 seconds only. A single mold may have more than one
      mold cavities so molding of more than one component is possible in one cycle
      By this process complex and intricate shapes are easily produced. Size of the
      products produced by this way may vary largely from very small to very large.
      Major limitations of injection are described below.
            (a)    Capability to fabricate the mold cavity. Accuracy and quality of the
                   product largely depend on the dimensional accuracy and quality of
                   the mold cavity produced for injection molding. The mold cavity
                   should be the replica of the product to be produced.
            (b)    Proper provisions should be provided with the mold cavity to take out
                   the solidified final product after its molding. The process of product
                   removal should not damage the mold cavity as same mold cavity has
                   to be used many number of times. Molds are normally made with
                   ultra care so a mold of large size for complex parts can cost more than
                   five lakh rupees. So injection molding is economical for mass
Equipment for Injection Molding
      Equipment for injection molding resembles with the equipment for metal die
      casting. This equipment is named injection molding machine. It consists of two
      principal parts (i) molten plastic injection unit and (ii) mold clamping unit.                       93
Manufacturing Processes-III        Molten Plastic Injection Unit
                                         Molten plastic injection unit consists of a barrel having a feed hopper at its
                                         one end. The hopper supplies the plastic pellets. Inside the barrel, there is a
                                         screw like extruder screw. This screw can be rotated to mix the prepared
                                         material in the barrel and it also acts as a ram used to inject molten plastic
                                         into the mold. A non-return valve mounted near the screw prevents the
                                         molten plastic from flowing backward along the screw threads. The screw
                                         retracts to its former position without making the flow of molten plastics
                                         back. So due to the double action of Rota table screw it is called
                                         reciprocating screw. The injection unit of the molding machine, does three
                                         functions :
                                                (a)    melts the plastics (polymer),
                                                (b)    homogenize the molten plastics, and
                                                (c)    then properly injects the plastics to the mold cavity.
                                   Mold Clamping Unit
                                         Other important unit of injection molding machine is the clamping unit. It is
                                         concerned with the operation of mold. It does three functions are discuss
                                         below :
                                                (a)    It holds the two halves of the mold in proper alignment with
                                                       respect to each other.
                                                (b)    It keeps the mold closed during the process of injection by
                                                       applying clamping force. The clamping force resists the
                                                       injection force.
                                                (c)    It opens and closes the mold at the appropriate timings in the
                                                       molding cycle.
                                         The clamping unit consists of two platens, a fixed platen and other movable
                                         platen along with a mechanism for translating the latter. The mechanism is
                                         operated with the help of hydraulically operated power press, capable to
                                         apply a clamping force of several thousand tons. The clamping force value
                                         depends on the size of molding machine. A typical molding machine with
                                         its cycle by cycle operation is shown in Figure 7.1.
                                         Feed hopper

                                                                                          Mold cutting
                                                       Heaters        screw


                                                 Figure 7.1 : Line Diagram of Injection Molding Machine

                              Working of Injection Molding Machine
                                   Working of injection molding machine is explained sequentially cycle by cycle as
                                   given below.
                                   The first action is mold open and machine is ready to start fresh molding. The first
                                   portion of molding cycle starts when mold is closed and clamped. Raw material,
                                   plastic or polymer is fed through hopper to the barrel, which is heated and brought
                                   to right temperature and viscosity. It is injected under pressure into the mold
                                   cavity by the ram. The process of injection is the second part of the molding cycle
      when the screw (ram) moves towards the mold cavity. The injected plastic coals             Welding and Plastics
      and begins to solidify when it comes in contact with the cold surface of mold
      cavity. Ram pressure is maintained to pack the molten plastic into the cavity that
      compensate contraction of plastic during solidification. Third part of molding
      cycle is the screw is rotated and retrated back with the non-return valve open to
      permit fresh polymer to flow into the forward portion of the barrel. Till the
      completion of third part the polymer in the mold cavity has completely solidified.
      The fourth part of the molding cycle is mold is opened and product is taken out of
      the mold.

Classification of injection molding machines into different categories depends on its two
major components, the injection unit and clamping unit. On the basis of injection
molding unit these machines are classified as discussed below.
Reciprocating Screw Machines
      This is designed in such a manner so that same barrel can be used for melting,
      mixing and injection of the molten plastics. This type of molding machines are
      preferred where low intensity of mixing of different additives are required to make
      injectable molten plastic mixture to the mold cavity.
Screw Pre-plasticizer Machines
      This is also called two stage molding machine. In this machine two different
      barrels are used, one for heating and mixing then prepared plastic mix is
      transferred to another barrel which is used for injection of the mix to mold cavity.
      The first barrel is screw driven which prepares the plastic mix. The second barrel
      is plunger driven it maintains the temperature of prepared mix and injects it to the
      mold cavity. This machine is also named plunger type injection molding machine.
      On the basis of clamping unit there can be three types of different design of
      molding machine as described below.
      Toggle Type
             In this case an actuator moves the crosshead forward, extending the toggle
             links to push the moving platens towards a closed position. The open and
             closed positions are shown in Figure 7.2. In this mechanism, at the
             beginning of stroke speed is high but mechanical advantage is low, when
             the stroke is likely to complete, the mechanical advantage is high but speed
             is very low so speed and force are maintained as these are desirable in the
             molding cycle. The way the machine can give lower values of cycle time.
              Actuating      Crosshead               Fixed
               cylinder         links                platen        Moved platen
                                                                                  Direction of

      Open position head
                                    Moving platen      position

                           Figure 7.2 : Working of Toggle Clamping Unit
Manufacturing Processes-III         Hydraulic Type Clamping Unit
                                          Hydraulic type clamping works similar to toggle type clamping unit, here
                                          the moving platen is moved by hydraulic pressure. These clamping units are
                                          used for high tonnage injection molding machines. These clamping units are
                                          relatively flexible as compared to toggle type clamping units with reference
                                          to adjustment of tonnage applied and length of stroke.
                                    Hydro-mechanical Clamping Unit
                                          This clamping unit explores advantages of both of the above categories.
                                          These clamping units are designed for large tonnage values. Movements of
                                          pushing plastic to mold are faster. Locking of the position of hydraulic
                                          cylinder is done by mechanical means. This maintains the tonnage value by
                                          building high pressure in the locked position.
                              There are some special injection molding processes which are described below.

                              7.6 SOME SPEICAL INJECTION MOLDING
                              Thermoplastic Foam Injection Molding
                                    This is one of the special purpose injection molding process. It involves the
                                    molding of thermoplastic parts that posses a dense or thick outer skin surrounding
                                    a lightweight foam core. These parts are recommended for structural applications.
                                    These produced either by introducing gas with molten plastic into the injection
                                    unit or by mixing a gas producing ingredient with the starting pellets. During
                                    injection, insufficient amount of molten plastic is filled to the cavity, where it
                                    expends as foams to fill the mold completely. This is used to make causing of
                                    machines, furniture, tanks, etc.

                              Multi Injection Molding Processes
                                    In this process different polymers are injected sequentially. These polymers may
                                    be injected into same mold cavity to have different layers of polymers on the same
                                    produce or in to different mold cavities mutual connected. So the part produced by
                                    this may consists of two integrally connected plastics. The first method is called
                                    sand witch molding and later one is called Bi injection molding method.

                              Injection Molding of Thermosets
                                    The difference between injection molding of thermoplastics and thermosets is,
                                    thermosets barrel length is relatively shorter to avoid premature curing and so
                                    solidification of molten plastic. Sufficient amount of molten plastic is accumulated
                                    ahead of the screw before its injection to the mold cavity. Thermosets are used to
                                    make composite with the reinforcement of glass fibres and wood fibres.

                              Reaction Injection Molding
                                    This process involves mixing of two highly reactive liquid ingredients (plastics)
                                    and the mixture is quickly injected to the mold cavity, where a chemical reaction
                                    takes place and solidification occurs. Normally the ingredients used are thermoset
                                    plastics. Urethanes, epoxies and urea formaldehyde are the examples of
                                    ingredients. This process is used to produce larger automotive components such as
                                    bumpers, etc. There is a need to control the process very precisely as quantity of
                                    ingredients to be mixed together is determined very precisely relative to the mold
                                    cavity. Once the process of reaction injection molding is designed and established
                                    it is found cheaper in long run (mass production).
                                                                                             Welding and Plastics
Some of the common defects that can occur in an injection molding process their remedy
and related precautions are described below.
Pour Short
      This defect resembles with costing defect, it is the result of incomplete filling of
      the mold cavity. This can be corrected by increasing temperature and filling
      pressure of the molten plastic. The pressure and temperature suitably can be
      determined at the time of process design. The barrel of molding machine should
      large enough to accommodate slightly more volume of molten mix then required.
      Flashing is the result of squeezing of molten polymer between the mold plates. If
      injection pressure is very high or clearance between the mold plates is large then
      this defect appears.
Sink Marks and Voids
      Sink marks and voids are two different molding defects but these are being
      discussed here simultaneously as their cause is same. A sink mark occurs when the
      outer surface of the mold solidifies but contraction of internal material causes the
      skin to the depressed below its intended profile. Sometimes void is formed due to
      contraction of internal material during cooling of mold. The chances of these
      defects are more in case of thicker sections. It is recommended to design sections
      of uniform thickness to avoid the occurrence of sink marks and voids.
Weld Lines
      When flowing molten plastic flow around a convex shaped portion of a mold
      cavity, the two different streams of flowing material meet in opposite direction
      and a boundary form that is called weld line. Mechanical properties of weld line
      may be different (inferior) from rest of the molded product. The defect can be
      checked by maintaining high melting temperature, high injection pressure and
      proper ventilation.
      Plastics and polymers have high value of thermal expansion coefficients so
      appreciable shrinkage takes place on cooling of molded plastic parts. To
      compensate the shrinkage, the dimension of mold cavity is made larger than the
      desired dimensions of the processed plastic part. The amount of enlargement of
      mold cavity can be worked out on the basis of type of plastic, its injection
      temperature molding size, etc. The above mentioned data should be considered
      while designing the process and so designed and established procedure can be
      used for mass production.

Compression molding is used for molding process of thermosetting plastics. The raw
material used for compression molding can be in any form like powder, pellets, liquid or
perform. The amount of polymer must be precisely controlled to obtain repeatable
consistency in the molded product. Normally the charge is preheated prior to its
placement into the mold. Preheating of raw material softens the plastics and shortens the
production cycle time. Preheating is done with the help of infrared heaters, ovens.
Reheating is also done by the use of hot rotating screw in the barrel of molding machine.
The process of compression along with its working principle is described below.                           97
Manufacturing Processes-III
                                  7.9 PRINCIPLE OF COMPRESSION MOLDING
                                  The process of compression molding is explained with the help of line diagram shown in
                                  Figure 7.3. The process in normally carried out in three sequential steps.
                                         (a)   Loading of the charge (raw material or plastic) into the lower half of mold.
                                               As we know that mold of compression molding process is designed into two
                                               parts. This is important as the mold has to be opened after solidification of
                                               molded parts. The amount of the charge is calculated very precisely so that
                                               it should be fully consumed during the operation of molding. If the charge
                                               kept there in, fall short the molded part will be of rejected quality. So
                                               precise determination of quantity of charge is done at the time of designing
                                               and established the process. The lower half of the mold is first heated to
                                               maintain the temperature of charge to a particular point during the process
                                               of molding.
                                                                              Upper mold      Movement of upper
                                Hot chores                                                     molding half to
                              loaded to mold                                                 compress the charge

                                                                             lower mold                            Compressed charge to
                                                     Knockout pin                                                   the required shape
                                                                                                  Knockout pin


                                                           Figure 7.3 : Process of Compression Molding

                                         (b)   The placed charge to lower half of mold is compressed by bringing both
                                               halves of the mold close together. This pressing of charge is done under
                                               pressure which enable the charge to flow and acquire the shape of the mold
                                               cavity with ultra precision and dimensional accuracy. The precision and
                                               dimensional accuracy depends upon the quality of mold design and exact
                                               calculation of the amount of charge to be molded.
                                         (c)   The step is executed along with the previous step. The charge is heated by
                                               means of the hot mold to polymerize and cure it into a solidified desired
                                               shaped molded plastic component.
                                         (d)   Last step of compression molding is opening of molding halves and removal
                                               of the molded plastic part by pressing the knockout pin towards inside. This
                                               is done after molding, solidification and cooling of the molded part.
                                  Compression molding machine is a type of press which is oriented vertically and
                                  contains two platens molding halves are fasten to these platens. The operation of
                                  applying pressure is possible by two types of actuations :
                                         (a)   upstroke of bottom platen, or
                                         (b)   by downstroke of top platen.
                                  Pressing is normally hydraulically powered. A pressure upto few hundred tons can be
                                  applied by hydraulic power. In addition to pressing pressure a clamping pressure of the
                                  order of 100 ton can be used for proper clamping to avoid flushing.
                                  Molds for compression molding are simpler than the molds used for injection molding.
                                  As the material is not powered to the cavity so required getting system is not needed.
                                  There is almost no scope of appearing defects like powered short, etc. Mold cavities
should be designed to consider the flowing capabilities of charge on pressing. Any            Welding and Plastics
design interfering the flow of charge should be avoided.
Common raw materials (plastics) used for compression molding are phenolics,
melamine, urea, formaldehyde, epoxies, elastomers, etc. Major applications of
compression molding using thermosetting plastics are done in making plugs, sockets,
switches, handles, dinner plates.

Main advantages of compression molding process are listed below :
      (a)   Less expending process, low maintenance cost.
      (b)   Negligible scrap.
      (c)   No residual stress into the molded part.
Its major limitation is the time of compression molding is slightly higher than that of
other similar processes so it gives slightly more cycle time. The process is recommended
for flat thin parts.
In addition to the above processes some other processes are also used for making plastic
components like transfer molding, blow molding, rotational molding, thermoforming
casting and molding. The detailed description of these process is not included in this

Some of the environmental issues related to use of plastics are discussed below :
      Due to their insolubility in water and inertness pure plastic generally have low
      toxicity in their finished state. Plastics pass through the digestive system with no
      ill effects. However, large pieces of plastics can do mechanical damages and
      obstruction to digestive system. Plastic components are manufactured by adding a
      variety of toxic additives to pure plastics. Traces of these chemicals can be pass on
      when it comes with contact of food items.
      Plastics are very durable and degrade very slowly, the molecular bonds that make
      plastic so durable make it very much resistant to natural process of degradation.
      Billions of ton of plastic discarded in last 75 years may persist for another
      thousand years.
      Sometimes plastic is burnt to dispose off which is also dangerous and not
      recommended because burning plastic can release toxic and environmentally
      unfriendly gases. These gases may cause ozone layer depletion and other
      environmental damages. Manufacturing process involved in making plastic
      articles often creates large quantities of chemical pollutants. Use of plastic begs
      created a problem of their disposal and begs disposed off improper way have
      started problem of chocking drainage system of cities.
      It is advised to use biodegradable plastics at the place of commonly used plastics.
      Starch powder is mixed with plastic as a filler material to make it easily
      There is a great necessarily to work more in this area to make easily degradable
      plastic or to recycle it. Manufacturing processes involved in making plastic
      articles should also be modified to make them environment safe.                                      99
Manufacturing Processes-III
                              7.12 SUMMARY
                              Now-a-days plastic is considered as one of the commonly used engineering material as a
                              suitable replacement of metal. This is due to its easy availability, low cost and low
                              manufacturing cost and compatible mechanical properties. Normally plastics are divided
                              into two categories thermoplastic and thermosetting depending upon their capability of
                              being reused. Plastics are widely being used for commercial and industrial applications
                              due to the exclusive advantages over other materials. Two major processes used for
                              making plastic components are injection molding and compression molding processes.
                              These processes resembles with casting and metal forming respectively which are used
                              for metal processing.
                              In case of injection molding the molten plastic with additives is injected into a specially
                              designed mold cavity by the injection molding machine where it is allowed to cool down
                              and solidify. This gives the plastic molded part according to the pre-designed mold
                              cavity. Injection of molten plastic is done under heavy pressure to fill the mold cavity
                              completely. Equipment of injection molding is divided into two parts injection unit and
                              clamping unit. The clamping unit basically constitutes the accurately and precisely
                              designed mold cavity. On the basis of these two units injection molding machine is
                              classified into different categories. The details of injection molding machine and its
                              categorization are explained in the unit in details. Like costing defects, there are chances
                              of appearance some defects into injection molded products like pour short, flashing,
                              weld lines and voids. These defects can be predicted in advance and necessary care can
                              be taken to avoid them from the molded products.
                              In case of compression molding process the hot charge premixed with all additives is
                              kept in the lower half of molding box and it compressed by the upper half of the molding
                              box under high pressure. The compressed part is cooled and allowed to solidify to give
                              molded plastic part. High pressure application provides proper flow ability to the charge
                              to fill all parts of the mold cavity and to get a good quality product. The whole process is
                              designed and established by predicting all possible defects which may appear in a
                              molded component and precautionary measures are incorporated in the design of the
                              process itself.
                               Use of plastics is not very much encouraged from environment point of view, because
                              there is a problem of disposal of plastic waste as plastic is not a degradable item.
                              Disposal difficulties may give different type of problems. In addition to this plastic
                              manufacturing processes may produce harmful chemical pollutants. It is suggested to
                              work more to develop biodegradable type of plastic and to modify the related
                              manufacturing processes to make them friendly for mankind in future.

                              7.13 ANSWERS TO SAQs
                              Refer the preceding text for all the Answers to SAQs.