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PLASTICS MATERIALS PROCESSING Why Plastics Polymer Plastics

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PLASTICS MATERIALS PROCESSING Why Plastics Polymer Plastics Powered By Docstoc
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 PLASTICS MATERIALS & PROCESSING
                                   Plastics ?
                          Why Plastics ?
• Most versatile and design flexibility
• Require little or no finishing, painting, polishing etc
• Can be coloured during manufacture
• Can be easily printed, decorated or painted
• Corrosion resistant, and generally waterproof
• Lighter than metals


                              Polymer ?!
                           Solid Materials
Plastics
      Thermal Transition of Plastics
           Importance of Tg
• To evaluate the flexibility of the polymer molecule
• Tg value of polymer decides whether material at “use
  temperature” will behave like rubber or Plastic
• Tg & Tm values gives an idea of processing temperature


                              Definitions
• Polymer: Macromolecules prepared from the
  polymerization reaction of monomer

• Thermoplastics: Materials that soften on heating and
  harden on cooling

• Thermosets: Undergo some chemical change on heating
  and convert themselves into an infusible mass
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• Commodity: These are low performance materials
  characterized by HDT< 100oC & tensile strength< 50MPa

• Engineering: These materials characterized by HDT >
  100oC & tensile strength> 40MPa

• Speciality: These are materials characterized by high
  modulus, tensile & impact strength




          Examples for Commodity Plastics
• Low Density Polyethylene (LDPE)
• Linear Low Density Polyethylene (LLDPE)
• High Density Polyethylene (HDPE)
• Ultra High Molecular Weight High Density Polyethylene
  (UHMHDPE)
• Polypropylene (PP)
• Polyvinylchloride (PVC)
• Polystyrene (PS)
  etc
         Properties of Commodity Plastics
        Applications of Commodity Plastics
•   Agriculture
•   Automotive
•   Building
•   Electrical
•   Packaging
•   Medical
•   Electronics
•   Household
•   Marine: PVC
•   Furniture: SAN
•   Optical: PMMA
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          Examples for Engineering Plastics
•   Polyethylene Terephthalate (PET)
•   Polybutylene Terephthalate (PBT)
•   Polyamides (PA)
•   Polyoxymethylene (POM)
•   Polycarbonate (PC)
         Properties of Engineering Plastics
        Applications of Engineering Plastics
•   Automotive
•   Electrical
•   Packaging
•   Medical
•   Electronics
•   Household

• Films, photographic films: PET
• Bottles: PET
• Optical: PC: CDs, contact lenses


            Examples for Speciality Plastics
•   Polytetrafluoroethylene (PTFE)
•   Polysulphone (PSO)
•   Polyetheretherketone (PEEK)
•   Polyacrylates
•   Liquid Crystal Polymers (LCP)
            Properties of Speciality Plastics
                             Polystyrene
                            Polyethylene
                            Polypropylene
                                PVC
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                       Nylon 6/6
                        ACETAL
                      PC (Lexan)
                          PPS
                         PTFE
                         PEEK
                  Various Processes
              Injection moulding process
              Sequences in IM operation.

          Advantages of Injection Molding
• High production rates

• High tolerances are repeatable

• Wide range of materials can be used

• Low labor costs

• Minimal scrap losses

• Little need to finish parts after molding
     Disadvantages of Injection Molding

• Expensive equipment investment

• Running costs may be high

• Parts must be designed with molding consideration
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                                 Classification Of Machine
• Based on Injection Unit
  1.plunger machines
  2.screw plunger machines
  3.reciprocating screw machine
• Based on clamping unit
  1.toggle clamping machines
  2.hydraulic clamping machines
  3.hydromechanical clamping machines
• Based on control unit
  1.conventional control machines
  2.microprocessor control machines
          Hydraulic Clamping System
Toggle Clamping System
   GAS ASSISTED INJECTION MOULDING
                                            Concepts
• Introducing a pressurized inert gas (nitrogen) into the melt
  stream of a part

• The original intention of the GAIM process was to be able
  to mold thick walls without surface imperfections

                                            Advantages
•   [1] Reduction in operation cost due to the possibility of using injection molding machines with
    smaller clamping force.

•   [2] Shorter molding cycle time especially of thick wall moldings.

•   [3] Reduction in the weight of moldings.

•   [4] Elimination of sink marks.

•   [5] Better dimensional accuracy due to reduction in internal strain.

•   [6] Designing flexibility by neglecting the principle of uniform wall thickness distribution.

•   [7] Reduction in assembly cost by making hollow moldings in one step.

                                            Limitations
•   [1] A hole for gas injection remains.
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•   [2] Difference in surface gloss between solid and hollow portions in some cases. The defect in
    appearance so-called "switch over mark" remains at the tip of molten resin at the time of gas injection.

•   [3] Difficulty in controlling the position and the shape of the hollow.

•   [4] Muti-cavity molding is difficult. If higher dimensional accuracy is required, 2-cavity molding is at most.

•   [5] Poor appearance so-called blush appears on the surface of the channel portion, which is a thick wall
    portion installed to lead gas.

•   [6] Degree of deformation is more sensitive to mold temperature in comparison to ordinary injection
    molding.

•   [7] There is no rule which is applicable before hand to the quantitative estimation of molding shrinkage.


      Classification by the methods of gas control
• Volume-control method

     – The method to meter the gas in a vessel of variable volume,
       typically a piston-cylinder for example, and then to send this gas
       into the inside of the molding under pressure. Through this
       process the amount of gas is decided.
     – Systems : CINPRES,Engel,Mannesmann,Krauss-Maffei
     – Suitable for commodity plastics
                         Pressure-control method
• The gas is compressed and stored in a vessel under high pressure.
  In the process of injection, the pressure of the gas is reduced to an
  arbitrary level and then the gas is sent to the inside of the molding.
  Through this process the amount of gas is decided, depending on
  the gas pressure.
• Systems : AGI,Battenfeld,
  Idemistu GIM,Engel,
  Kloeckner,Mannesmann,
  Krauss-Maffei
• Suitable for Engineering plastics.



• In phase three, the resin shot is completely injected into the mold
  as gas injection continues. This gas injection keeps the flow front
  moving as the bubble forms inside the article thus stretching the
  skin to the end of the mold.
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• In phase four, the material has completely filled the article, the
  skins are fully established, and the gas bubble continues to be
  pressurized thus creating an internal cushion to compensate for
  resin shrinkage


• In the final phase, the article has cooled adequately to
  establish skin strength so the gas pressure can be vented.
  The gas must be vented prior to mold opening to avoid
  explosion. The gas pin retracts to accomplish this venting
  action
                      Design Guidelines
• Guideline 1: Process feasibility for the final application
  should be considered prior to adopting the gas-assisted
  injection molding process

  – Different from Blow moulding (Large vol of air used)
  – Air for hollowing out thick sections and compensate for
    Volumetric shrinkage.


• Guideline 2: For part design, first determine the thick
  sections to be cored out by the gas within the part, before
  connecting those gas passages to lay out the gas-channel
  network

  – for tube- or rod-like parts, it is typically desirable to let gas
    penetrate the entire part length
  – For large, sheet-like, structural parts, gas channels can be built-
    in by incorporating ribs with enlarged bases or a thick passage
  – In general, gas channels should be continuous, but a closed-
    loop design should be avoided if complete coring is required



• Guideline 3: Gas channel dimensions should be defined
  clearly to enclose the gas penetration with a limit to
  minimize the racetrack effect.
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  – The dimensions of the gas-channel network should be distinctly
    larger than those of the adjacent areas to give the gas a well-
    defined path in which to travel
  – As a rule of thumb, the ratio of channel dimension to nominal
    wall thickness for structural parts is 2 to 3
  – Use extreme care to avoid air traps and gas permeation caused
    by improper layout and sizing of the gas-channel network
  – the recommended approach is to start with relatively small gas-
    channel dimensions to minimize the racetrack effect



• Guideline 4: Extend gas channels as close to the last-
  filled areas as possible to avoid gas permeation into those
  areas at the end of cavity filling.

  – The gas-channel network should guide the gas penetration to
    the extremities of the cavity
  – avoid gas permeation into thin sections
  – Gas channels should also be oriented somewhat in the direction
    of melt flow



• Guideline 5: Overflow wells can be incorporated at the
  ends of the gas channels to promote a better gas
  penetration pattern

  – The advantages of using overflow wells are to reduce the
    complexity in the filling dynamics of resin and gas, and at the
    same time, to offer an additional control to guide the gas
    penetration. The inherent disadvantages are the secondary
    trimming operation that might be required, and the additional
    material used to fill the overflow well



• Guideline 6: The design of the tool and selection of
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 polymer and gas entrances should deliver a balanced
 filling pattern to promote even gas penetration

  – Flow balancing is one of the key factors in designing the part and
    tool and in selecting the polymer and gas entrances
  – A short flow length from the gas tip to the polymer melt front.
  – Thick gas channels in which polymer melt is still hot and less
    viscous.
  – The polymer to be displaced by the gas has some place to flow.




• Guideline 7: The volume of unfilled areas prior to gas
  injection should not exceed 50 percent of the total volume
  of the gas channels
  – Gas should be confined within the gas channels without gas
    blow-through or gas permeation into thin sections
  –
    For example, to hollow out a simple circular tube with an
    average polymer skin thickness of half the radius, the part has to
    be pre-filled at least 75 percent by volume with polymer. In other
    words, 25 percent of the part volume will be cored out by gas to
    form a hollowed, circular tube with thickness equal to half the
    part radius



• Guideline 8: Mold-wall temperature and shot-size control,
  as well as part dimensions, are more critical in gas-
  assisted injection molding than in conventional injection
  molding

  – To ensure product repeatability, shot-size control within 0.5
    percent is desirable
  – With only a small variation in mold-wall temperature, shot
    volume, or part dimension, gas penetration can change
    dramatically.
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• Guideline 9: The effect of material properties and process
  variables on the gas penetration should be taken into
  account in determining the processing window.
  – the primary gas penetration is determined by the polymer
    volume fraction and is strongly coupled with flow dynamics,
    whereas the secondary gas penetration depends on amount of
    polymer shrinkage, occurs only along the thick sections, and
    extends in all directions.

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