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					                     MECH152-L19 (1.0) - 1




Polymer Processing
                                          MECH152-L19 (1.0) - 2


                  Contents
• Properties of Polymer Melt
• Processes
  –   Extrusion
  –   Spinning
  –   Coating
  –   Injection molding
  –   Transfer & Compression molding
  –   Blow molding & Rotational molding
  –   Thermoforming
  –   Casting
  –   Polymer foaming & shaping
• Design Considerations
                                   MECH152-L19 (1.0) - 3


      Advantages of Polymer
• Wide range of processes.
• Able to cater for complex geometry.
• Net shape manufacturing.
• None or minimum decorative finishing.
• Low temperature, low energy, low handling
  processes - low operating cost
• Low material cost
                                                   MECH152-L19 (1.0) - 4


    Properties of Polymer Melt
• Viscosity
  – Non-Newtonian fluid in liquid phase

                                τ = Shear stress

                τ = k (γ& ) n   γ& = shear rate
                                η = shear visocosity
                                k = constant coefficient

              τ = ηγ&           n = flow index ( >1 for
                                polymer)
                                        MECH152-L19 (1.0) - 5


    Properties of Polymer Melt
• Viscoelasticity - the strain is time and
  temperature dependent.
                                      MECH152-L19 (1.0) - 6


                Extrusion
A compression process in which the material is
 forced to flow through a die orifice to provide
 long, continuous product with regular shape
 controlled by the orifice shape.
                                          MECH152-L19 (1.0) - 7


                 Extrusion
• Diameter of barrel: 25 ~ 150 mm
• length/diameter of barrel: 10 ~ 30 (high of
  thermplastics, low for elastomers)
• Rotational speed ~ 60 r.p.m.
• Sections:
  – Feed section - material feeding
  – Compression section - full melting and
    compression
  – Metering section - feeding through the die
    under pressure
                                       MECH152-L19 (1.0) - 8


                      Extrusion
                                  Drag flow between
                                  plates
                                  Qd =0.5vdw
                                  Flight angle
                                  tan A = Pitch/πD




Back pressure flow


Resulting flow rate
            MECH152-L19 (1.0) - 9


Extrusion
                    MECH152-L19 (1.0) - 10


Extrusion
        Die swell
            MECH152-L19 (1.0) - 11


Extrusion
                  MECH152-L19 (1.0) - 12


Extrusion




            Wire coating
                      MECH152-L19 (1.0) - 13


Extrusion Defects

             Melt fracture




Sharkskin    Bambooing
                          MECH152-L19 (1.0) - 14


Sheet & Film Extrusion
                      Slit die




    Water Quenching       Chill rolls
                        MECH152-L19 (1.0) - 15


 Extrusion




                    Calendering



Blown-film processing
                    MECH152-L19 (1.0) - 16




Injection Molding
                                 MECH152-L19 (1.0) - 17


    Injection Molding Process
• Typical cycle time: 10 - 30 seconds
• Molding cost: HK$100,000 - HK$1,000,000
  upwards
• Most widely used process
• Two units:
  – Injection unit
  – Clamping unit
                                                  MECH152-L19 (1.0) - 18


     Injection Molding Machine




The injection molding process depends on the injection
  pressure, injection velocity, melt temperature, mold
  temperature, and holding time.
                                                    MECH152-L19 (1.0) - 19


                       Molding Cycle




(1) Mold closing            (2) Melt injection into cavity
(3) Screw retraction        (4) Mold opening and part ejection
                                                 MECH152-L19 (1.0) - 20


                Injection Mold
                  Closed mold               Opened mold




• The mold consist of two halves each attached to the
  stationary platen and the moving platen of the molding
  machine.
• The mold halves each contains the core and cavity to form
  the part geometry.
                                                   MECH152-L19 (1.0) - 21


                 Injection Mold
• The mold has water channels to control the temperature of
  the mold.
• The cavity may have multi-cavities to improve the
  efficiency of the production.
• The core may consist of sliding cores to cater for intricate
  geometry.
• The sprue, runner and gate form the feeding system.
• Hot runners are sometimes used to eliminate melt
  solidification at the sprue and runner.
• The ejector pins are used for part ejection.
• The gate and ejector location should be chosen not to
  affect the aesthetic design of the part.
• Air venting should be provided for via the ejector pin
  clearance or air vents.
                                                MECH152-L19 (1.0) - 22


                Injection Mold




Three-plate mold provides better automation and more room
  for gate adjustment.
                                                    MECH152-L19 (1.0) - 23


      Injection Molding Machine




Injection units: The two types being shown are the screw
   preplasticizer and plunger type. The most common
   injection system is to use the plasticizing screw to work
   also as an plunger.
                                                 MECH152-L19 (1.0) - 24


     Injection Molding Machine

 Open




 Closed


Clamping units: The clamping unit could be mechanical or
  hydraulic operated. The mechanical clamping system uses
  toggle clamps while the hydraulic system uses a hydraulic
  cylinder to provide the actuation.
                              MECH152-L19 (1.0) - 25


            Molding Defects
•   Shrinkage
•   warpage
•   short shots
•   Flashing
•   Sink marks and voids
•   Weld lines
      Other Injection Molding              MECH152-L19 (1.0) - 26




             Processes
• Thermoplastics foam injection molding
  – introducing gas into melt or mixing a gas-
    producing ingredient with starting pellet.
  – Producing part with high stiffness-to-weight ratio.
• Multi-injection molding process
  – sandwich molding
  – bi-injection molding
• Injection molding of thermosets
  – mostly with high filler content
      Other Injection Molding              MECH152-L19 (1.0) - 27




             Processes
• Reaction Injectin Molding
  – Mixing of two highly reactive liquid and
    injected it immediately into the cavity to create
    thermosets typically of large size.
  – Low energy requirement.
  – Equipment and mold cost are less than injection
    molding.
  – Good process reliability.
  – Wide choice of chemical systems.
      Other Injection Molding  MECH152-L19 (1.0) - 28




             Processes
• Reaction Injection Molding
                                            MECH152-L19 (1.0) - 29


          Design Considerations
• Production Cost:
   – material cost
   – mold cost
   – operating cost
      •   cycle time
      •   labor cost
      •   QC cost
      •   finishing cost
The unit cost per part is the most important item.
                                          MECH152-L19 (1.0) - 30


       Design Considerations
• Part complexity
  – The part complexity will increase the cost of
    the mold, not only in terms of mold finishing,
    but may incur sliding cores and other
    mechanisms.
  – The trade off is between mold cost and the
    additional cost of having more parts.
                                              MECH152-L19 (1.0) - 31


           Design Considerations
• Material selection
  –   Viscosity (at processing temperature and pressure)
  –   Coefficient of thermal expansion (shrinkage factor)
  –   Strength and stiffness
  –   Impact resistance
  –   Service temperature
  –   Degradation
                                       MECH152-L19 (1.0) - 32


       Design Considerations
• Wall and rib thickness - even thickness is
  molded part is important.
• Corner radii and fillets - avoid sharp corners
  which leads to poor melt flow and stress
  concentration.
• Holes - Careful treatment in design to avoid
  obstruction of mold opening.
• Drafts - thermosets 1/2 ° to 1°
           thermoplastics 1/8° to 1/2°
                           MECH152-L19 (1.0) - 33


       Design Considerations
• Tolerances
                   MECH152-L19 (1.0) - 34




Compression and
Transfer Molding
                                               MECH152-L19 (1.0) - 35


          Compression Molding




•   Loading of molding compound into heated mold
•   mold closing
•   holding at selected process temperature
•   part ejection
                                        MECH152-L19 (1.0) - 36


         Compression Molding
• Material - thermsets
• Pre-heat of charge is possible
• Pros
  – simpler molds with lower cost and maintenance
  – low scrap
  – low residual stresses
• Cons
  – longer cycle time
  – limited geometry
                                               MECH152-L19 (1.0) - 37


              Transfer Molding




Pot transfer molding - The charge is heated.
                                                   MECH152-L19 (1.0) - 38


               Transfer Molding




Plunger transfer molding - thermosets in a heat well.
                                                  MECH152-L19 (1.0) - 39


                 Blow Molding
For making hollow, seamless part out of thermoplastic
  polymers. Blow molding uses air to inflate the softened
  plastic to solidify on the wall




                    Extrusion Blow Molding
                    MECH152-L19 (1.0) - 40


Injection Blow Molding
                                                  MECH152-L19 (1.0) - 41


          Stretch Blow Molding




The stretching makes the part more rigid, have higher
  transparency and better impact resistance.
                                    MECH152-L19 (1.0) - 42


         Rotational Molding
• Using a rotating mold to achieve a hollow
  form.
• Favours more complex external form, larger
  parts and lower production quantities than
  blow molding.
• Molds are simpler and less expensive than
  blow molding and injection molding.
• Perform on multi-cavity machine.
                   MECH152-L19 (1.0) - 43


Rotation Molding
                                     MECH152-L19 (1.0) - 44


              Thermoforming
• Flat thermoplastic sheet is heated and
  deformed into the desired shape.
• Used in making large consumer products
  such as bath tub, motor cycle body, etc.
• Process -
  – heating
  – forming
                 MECH152-L19 (1.0) - 45


Vacuum Forming
                                                   MECH152-L19 (1.0) - 46


              Vacuum Forming




The alternative is to use positive mold. It will replicate the
  pattern onto the interior surface.
Positive mold thermoforming has a less uniform thinning than
  the negative mold. This may be overcome by pre-
  stretching of the plastic sheet.
                                                    MECH152-L19 (1.0) - 47


        Pressure Thermoforming




The process uses air pressure to facilitate the forming process.
                                                     MECH152-L19 (1.0) - 48


     Mechanical Thermoforming




Air pressure is used as an external energy source.
                     MECH152-L19 (1.0) - 49


Mechanical Thermoforming
                                          MECH152-L19 (1.0) - 50


                   Casting
Casting is conducted by allowing molten
  plastics to flow into a mold under gravity.
Pros:
  – simpler and less costly mold
  – free of residual stress and viscoelastic memory
  – suited or low production quantity
                                          MECH152-L19 (1.0) - 51


             Polymer foaming
Polymer-gas mixture giving the material a
  porous cellular structure.
  –   Low density
  –   High strength-to-weight ratio
  –   Good thermal insulation
  –   Good energy absorbing qualtities


   Closed                                Opened
   cell                                  cell
                      MECH152-L19 (1.0) - 52




Injection Molding Process
                                                                                  MECH152-L19 (1.0) - 53


                                 Process Control
                   I      II                       III
    Cavity
                                      I : Filling                 Phold = Holding pressure
   Pressure
                                      II : Holding                thold = Holding time
      Phold                           III : Cooling               tpostfill = Postfill time
                                                                  Tmold = Mold temperature
                                                                  Tdemold= Demolding temperature
                                                                  tfill = Filling time
                                 Hydraulic Pressure               tcool = Cooling time
                                                                  Tmelt = Melt temperature
                                                           Time
                                                                  Tsolidify= Solidifying temperature
                 tfill   thold                 tcool
  Cavity
Temperature                            tpostfill

      Tmelt
                                        Gate Temperature
     Tsolidify


      Tdemold
      Tmold
                                                           Time
                                                                MECH152-L19 (1.0) - 54


                   Molding Defects
                                      Burnt Streak: Visible streak on the
                                      surface    as  brownish  or   silvery
                                      discoloration.

                                      Delamination: A separation or layering of
                                      the surface of a molded part. It can also
                                      appear as fish-scaling.
1. Burnt Streak    2. Delamination
                                      Diesel Effect: A small, dark brown or
                                      black discoloration on the surface of a
                                      molded part, usually found at the end of
                                      the material flow path or in blind pockets.

                                      Discoloration: A change in the original
                                      color of a polymer material.

3. Diesel Effect   4. Discoloration
                                                    MECH152-L19 (1.0) - 55


             Molding Defects
                              •Flash: An excess plastic material
                              forced out of the mold, usually at
                              parting lines but also at any point
                              where two surfaces meet.

                              •Flow Mark: Very fine grooves
                              show up on the surface of the molded
                              part, which is very similar to those of
5. Flash       6. Flow Mark   records. It is also known as Record
                              Grooves Effect.

                              •Jetting: A “snake-like” pattern on
                              the surface of a molded part.

                              •Low Gloss: A dulling of part surface
                              finish.

7. Jetting    8. Low Gloss
                                                        MECH152-L19 (1.0) - 56


                Molding Defects

                                 •Short Shot: An incomplete molded
                                 part.

                                 •Shrinkage: An extreme decrease in the
                                 dimensions of a molded part after it has
9. Short Shot    10. Shrinkage   cooled.

                                 •Sink Mark: A depression, resembling
                                 a dimple or groove.

                                 •Void: A voided area trapped within a
                                 molded part.


11. Sink Mark    12. Void
                                                       MECH152-L19 (1.0) - 57


              Molding Defects

                              •Warpage: An excessive dimensional
                              change in a molded part after it is
                              ejected from the mold.

                              •Weld Line: An inability of two flow
                              fronts to weld together during the filling
                              stage. This normally occurs around
                              holes or obstructions and causes very
                              weak areas in the molded part.
13. Warpage   14. Weld Line
Table 6.8   Machine setting for film-gated plate and PP
                                                                                           MECH152-L19 (1.0) - 58


               Typical Molding M/C Setting
              Temperature                            Coolant Temperature = 41                       °C
                                                      Nozzle Temperature = 190                      °C
                                                          Barrel Temperature = 185, 180, 175, 170   °C
              Injection                                    Injection Pressure = 100                 bar
                                                              Injection Speed = 35                  mm/s
                                                             Injection Stroke = 16.5                mm
              Holding                                       Holding Pressure = 12                   bar
                                                              Holding Time = 3.5                    sec.
              Cooling & Clamping                               Cooling Time = 18.7                  sec.
                                                             Clamping Force = 70                    ton
              Metering                                        Back Pressure = 6                     bar
                                                             Rotating Speed = 150                   rpm
                                                                                                 MECH152-L19 (1.0) - 59


                            Molding Simulation
         Filling phase:                               Arrhenius temperature dependence: of
                                                      viscosity:
         ∂ρ
             + ∇ • ρu = 0
         ∂t                                                                     ηo (T )
                                                        η (T , γ& ) =
         ∂
            ( ρu ) + ∇ • ( ρuu - σ ) = ρg                                   (
                                                                        1 + ηoγ τ *
                                                                               &          )
                                                                                          1− n


         ∂t                                              with
                        (
         σ = − pI+ η ∇u + ∇uT        )                                    Tb 
                                                                          
                                                        ηo (T ) = Be     T 
               ∂T          
         ρC p     + u • ∇T  = ∇(k∇T ) + ηγ 2
                                           &
               ∂t                                     where n is the power law index,
         where u is the velocity vector,                ηo the zero shear viscosity, τ* is
         T the temperature, t the time,                 the parameter that describes the
         p the pressure, σ the total stress tensor,     transition region between zero
         ρ the density, η the viscosity,                shear rate and the power law
         k the thermal conductivity,                    region of the viscosity curve.
         Cp the specific
         heat and γ the shear rate.
                   &

INTEGRATED NUMERICAL SIMULATION OF INJECTION MOLDING USING TRUE 3D APPROACH, Moldex3D
                                                                                                 MECH152-L19 (1.0) - 60


                            Molding Simulation
         Filling phase:                               Arrhenius temperature dependence of
                                                      polymer melt viscosity:
         ∂ρ
             + ∇ • ρu = 0
         ∂t                                                                     ηo (T )
                                                        η (T , γ& ) =
         ∂
            ( ρu ) + ∇ • ( ρuu - σ ) = ρg                                   (
                                                                        1 + ηoγ τ *
                                                                               &          )
                                                                                          1− n


         ∂t                                              with
                        (
         σ = − pI+ η ∇u + ∇uT        )                                    Tb 
                                                                          
                                                        ηo (T ) = Be     T 
               ∂T          
         ρC p     + u • ∇T  = ∇(k∇T ) + ηγ 2
                                           &
               ∂t                                     where n is the power law index,
         where u is the velocity vector,                ηo the zero shear viscosity, τ* is
         T the temperature, t the time,                 the parameter that describes the
         p the pressure, σ the total stress tensor,     transition region between zero
         ρ the density, η the viscosity,                shear rate and the power law
         k the thermal conductivity,                    region of the viscosity curve.
         Cp the specific
         heat and γ the shear rate.
                   &

INTEGRATED NUMERICAL SIMULATION OF INJECTION MOLDING USING TRUE 3D APPROACH, Moldex3D
                                                                           MECH152-L19 (1.0) - 61


                        Molding Simulation
        Filling phase:
       A volume fraction function f tracks the evolution of the melt
       front within a cell:
       f=0 the air phase,
       f=1 the polymer melt phase,
       The melt front is located within the cell, then 0<f<1.
       The melt front f over time is governed by the transport
          equation:
                               ∂f
                                  + ∇ • (uf ) = 0
                               ∂t
       The flow rate or injection pressure is prescribed at mold inlet.
       No slip is assumed at mold wall.
INTEGRATED NUMERICAL SIMULATION OF INJECTION MOLDING USING TRUE 3D APPROACH, Moldex3D
                                                                           MECH152-L19 (1.0) - 62


                            Molding Simulation
     Packing phase: Modified Tait equation
                                             P 
          V (P, T ) = V (0, T )1 − C • ln1 +
                                           B (T )  + Vt ( P, T )
                                                   
                                                 
          where
                    b1m + b2 m , T > Tt , melt state
          Vo (T ) = 
                    b1s + b2 s , T < Tt , solid state
                   b exp(− b4 mT ), T > Tt , melt state
          B (T ) =  3m
                   b3 s exp(− b4 sT ), T < Tt , solid state
                                0, T > Tt , melt state
          Vt (P, T ) = 
                       b7 exp(b8T − b9 P ), T < Tt , solid state
          T ≡ T − b5
          transition temperature : Tt = b5 + b6 P
          for amorphous polymers, b1m = b1s
          for Crystalline polymers, b1m > b1s
INTEGRATED NUMERICAL SIMULATION OF INJECTION MOLDING USING TRUE 3D APPROACH, Moldex3D
                                                                                 MECH152-L19 (1.0) - 63


                          Molding Simulation
     Cooling phase:
     Transient heat transfer problem with convective boundary
     condition represented by a 3D Poisson equation:
           ∂T      ∂ 2T ∂ 2T ∂ 2T 
      ρC p    = k  2 + 2 + 2  for r ∈ Ω
                   ∂x
           ∂t           ∂y   ∂z  
      where T is the temperature, t is the time, x, y, and z are the Cartesian coordinates, ρ is
      the density, PC is the specific heat, k is the thermal conductivity.

      Warpage analysis
      σ = C (ε - ε 0 − α ∆ T )
         1
         2
              (
      ε = ∇u + ∇uT          )
       Where σ is the stress tensor, C is a 4th tensor related to the material mechanical
       properties, ε is the strain tensor, α is CLET tensor and u is the displacement tensor.
INTEGRATED NUMERICAL SIMULATION OF INJECTION MOLDING USING TRUE 3D APPROACH, Moldex3D
                                                                           MECH152-L19 (1.0) - 64


                        Molding Simulation




INTEGRATED NUMERICAL SIMULATION OF INJECTION MOLDING USING TRUE 3D APPROACH, Moldex3D
           MECH152-L19 (1.0) - 65


Examples
          MECH152-L19 (1.0) - 66


Example
                                                                             MECH152-L19 (1.0) - 67


                      Example – Mold insert




THREE-DIMENSIONAL INSERT MOLDING SIMULATION IN INJECTION MOLDING, Moldex3D

				
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