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Design for Manufacturability_ - Introduction - MechSE - Illinois_9_

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					ME 350 – Lecture 10 – Chapter 13

 SHAPING PROCESSES FOR PLASTICS
  Properties of Polymer Melts
  Injection Molding
  Extrusion
  Extrudate Production
  Other Molding Processes
  Thermoforming
  Casting
  Polymer Foam Processing
  Product Design Considerations
Polymer Melt - Viscosity




Shear rate and viscosity:   Temperature & viscosity:
thinner at higher shear     thinner at higher
rates                       temperature
 Die Swell, aka: Viscoelasticity
Extruded polymer "remembers" its previous shape when
in the larger cross section of the extruder, tries to return
to it after leaving the die orifice




                                                            Swell ratio:

                                                              rs = Dx / Dd




    Die swell, a manifestation of viscoelasticity in polymer melts.
 Injection Molding
Polymer is heated to a highly plastic state and forced to flow
  under high pressure into a mold cavity where it solidifies
  and the molding is then removed.
• Produces discrete components almost always to net
  shape or near net shape
• Typical cycle time 1 to 30 sec
• Mold may contain multiple cavities, so multiple moldings
  are produced each cycle
• Some thermosets and elastomers are injection molded,
  but equipment and operating parameters must be
  modified to avoid: premature cross-linking
Injection Molding Machine




Two principal components:
1. Injection unit (operates similar to an extruder)
   – Melts and delivers polymer melt (plunger for injection)
2. Clamping unit
   – Opens and closes mold each injection cycle
Injection Molding
1. Mold Closes                    2. Inject Plastic




3. Cooling Time   4. Mold Opens
Injection Molding Time/Cost Model

        Injection time: machine and material manufacturers data
        tinjection = {(Ncav x Vcav) + Vrunner} / Rmax_injection x Matfactor

        Cooling time: Ballman / Shusman model
                     - Pmax_thickness2              (Tdeflect – Tmold)
        tcooling   =                   x ln
                       2ThDiffusivity              4(Tmelt – Tmold)

        Ejection time: from machine manufacturers data
        tejection = f(tdry_cycle)


       Cycle Time:
       tcycle = tinjection + tcooling + tejection

       Direct Mc/Overhead Cost:
       CM/c_overhead = Mcrate x tcycle / Ncav

                      Labor Cost:
                      Clabor = Labratio x Labrate x tcycle / Ncav
The
Mold
(Tooling)
Moving Side Cores or ‘Slides’
Moving Internal Cores or ‘Lifters’
Avoiding Moving Side Cores and Lifters (1)
Avoiding Moving Side
Cores and Lifters (2)
Two-Plate Mold




• Cavity – slightly oversized to allow for shrinkage
• Distribution channel
   – Sprue - leads from nozzle into mold
   – Runners - lead from sprue to cavity (or cavities)
   – Gates - constricts flow to: decrease viscosity
• Ejection system – pins built into moving half of mold
• Cooling system – typically water
• Air vents – at end of flow path
Three-Plate Mold
Uses three plates to separate parts from sprue
  and runner when mold opens (Fig 13.24)
• Advantages over two-plate mold:
  – As mold opens, runner and parts disconnect
    and drop into two separate containers under
    mold
  – Allows automatic operation of molding machine
  – Allows material to be injected at the mold base
    or middle, rather than side injection, which a
    two-plate mold must do.
Hot-Runner Mold

• Heaters are located around the runner

  channels which eliminates solidification of the:

  sprue and runner

• This type of mold improves mold flow as

  material is heated right up to when in enters

  the cavity.
Shrinkage
• Polymers have high thermal expansion coefficients, so
  significant shrinkage occurs during solidification
• Typical shrinkage values:
   Plastic           Shrinkage, mm/mm (in/in)
   Nylon-6,6                 0.020
   Polyethylene              0.025
   Polystyrene               0.004
   PVC                       0.005
• Dimensions of mold cavity must be larger than
  specified part dimensions: Dc = Dp + DpS + DpS2
     where Dc = dimension of cavity; Dp = molded part dimension,
     and S = shrinkage value and the third term on right hand side
     corrects for shrinkage in the shrinkage
Shrinkage Factors

• Fillers in the plastic tend to: reduce shrinkage
• Injection pressure – higher pressures in the
  mold cavity tend to: reduce shrinkage
• Compaction time – longer time tends to:
  reduce shrinkage
• Molding temperature - higher temperatures
  lower polymer melt viscosity, which tends to:
  reduce shrinkage
Extrusion

Heated plastic is forced to flow through a die orifice to
provide a long continuous product (tube, sheet, etc.)
whose cross-sectional shape is determined by the die
orifice. The extrudate is then cut into desired lengths.




    Three zones in an extruder: feed, compression, & metering.
    Die End of Extruder

•   Progress of polymer melt through barrel leads
    ultimately to the die zone
•   Before the die, the melt passes through a series of
    wire meshes supported by a stiff plate containing
    small axial holes called a: screen pack
•   Functions:
    1. Filter (remove contaminants and any hard lumps)
    2. Build pressure in the metering section
    3. Straighten flow - remove "memory" of circular
       motion from screw
Melt Flow in Extruder

• Archimedian screw forces polymer melt
  toward die
• Principal transport mechanism is Qd, resulting
  from friction between the viscous liquid and
  the rotating screw: drag flow
• Compressing the polymer melt through the die
  creates a back pressure that reduces drag
  flow transport, Qb called: back pressure flow
• Resulting flow in extruder is: Qx = Qd – Qb
 Extruder Screw Melt Flow (pg 264-265)
                                  Qd = 0.5 π2 D2 N dc sinA cosA
                                  Where, D – flight screw diameter
                                          N – screw rotational speed
                                          dc – screw channel depth
                                          A – flight angle


                                       pDd c3 sin 2 A
                                  Qb ≈
                                          12L
Q x = Qd - Q b                    Where, p – head pressure (die)
        Drag Flow Qd →
                                          η – melt viscosity
        Back Pressure Flow Qb ←
Flight angle ‘A’?                         L – length of the barrel

        tan A = p / πD            Assumes leakage flow is negligible
   Extruder Screw Melt Flow (pg 266)
                                                     Boundary Conditions:
                                                     1) With no back pressure
                                                       Qx = Qmax = Qd
                                                     2) With no flow
                                                       Qx = 0 = Qd – Qb,
                  Extruder characteristic
                                                                6DNL  cot A
                     Qx = Qmax – (Qmax/pmax)p          pmax =
Qmax                                                                d c2
                                Die characteristic
    Melt flow




                                   Qx = Ksp                Dd4
                                                     Ks =        (for round opening)
                                                          128Ld

                Head pressure                          Dd – effective die opening
                                  pmax
                                                       Ld – effective die opening length
Intersection (Qx,p) known as the operating point
Thermoplastic Foam Injection Molding
Molding of thermoplastic parts that possess dense
  outer skin surrounding lightweight foam center
• Part has high stiffness-to-weight ratio suited to
  structural applications
• Produced either by introducing a gas into molten
  plastic in injection unit or by mixing a
  gas-producing ingredient with starting pellets
• A small amount of melt is injected into mold
  cavity, where it expands to fill cavity
• Foam in contact with cold mold surface
  collapses to form dense skin, while core retains
  cellular structure
 Injection Molding of Thermosets

• Temperatures in the injector are generally: lower
• The barrel length of the injection unit is generally:
  shorter
• Melt is injected into a heated mold, where
  cross-linking occurs to cure the plastic
   – The most time-consuming step in the cycle:
      curing in the mold
 Compression Molding
• A widely used molding process for thermosets
• Also used for rubber tires and polymer composites
• Molding compound available in several forms:
  powders or pellets, liquid, or preform
• Amount of charge must be precisely controlled




  (1) charge is loaded, (2) and (3) charge is compressed and
  cured, and (4) part is ejected and removed.
Molds for Compression Molding
• Simpler than injection molds
• As opposed to injection molding, there is no:
  sprue and runner system
• Limited to simpler part geometries due to lower
  flow capabilities of TS materials
• Mold must be heated, usually by electric
  resistance, steam, etc
• Typical molding materials: phenolics,
  melamine, epoxies, urethanes, and elastomers
• Typical compression-molded products:
  – Electric plugs, sockets, and housings; pot
    handles, and dinnerware plates
Transfer Molding
TS charge is loaded into a heated chamber; pressure is
applied to force the soft polymer into the heated mold.


                                   Pot transfer molding:
                                   charge is injected from a
                                   "pot" through a vertical
                                   sprue channel into cavity


                                   Plunger transfer molding:
                                   plunger injects charge
                                   from a heated well
                                   through channels into
                                   cavity
Compression vs. Transfer Molding

• In both processes, scrap is produced each
  cycle as leftover material, called the: cull
• The TS scrap cannot be recovered
• Transfer molding is capable of molding more
  intricate part shapes than compression
  molding but not as intricate as injection
  molding
• Transfer molding lends itself to molding with
  inserts, in which a metal or ceramic insert is
  placed into cavity prior to injection, and the
  plastic bonds to insert during molding
Blow Molding
•   Molding process in which air pressure is used
    to inflate soft plastic into a mold cavity
•   Material limited to: thermoplastics
•   Accomplished in two steps:
    1. Fabrication of a starting tube, called a: parison
    2. Inflation of the tube to desired final shape
•   Two methods:
    1. Extrusion blow molding
    2. Injection blow molding
  Extrusion Blow Molding
(1) extrusion of parison; (2) parison is pinched at the top and
sealed at the bottom around a metal blow pin as the two halves
of the mold come together; (3) the parison is inflated; and (4)
mold is opened to remove the solidified part.
  Injection Blow Molding
(1) parison is injection molded around a blowing rod; (2) injection
mold is opened and parison is transferred to a blow mold; (3)
parison is inflated; and (4) blow mold is opened and product
removed.
  Vacuum Thermoforming
• Starting
  material:
  thermoplastic
  sheet or film
• To soften, heat
  is supplied by
  radiant electric
  heaters located
  on one or both
  sides
   Products: bathtubs, contoured skylights, door liners for refrigerators,
   boat hulls, shower stalls, advertising displays and signs, etc.
Polymer Casting
Pouring liquid resin into a mold, using gravity to
 fill cavity, where polymer hardens
• Both thermoplastics and thermosets are cast
  – Thermoplastics: acrylics, polystyrene,
    polyamides (nylons) and PVC
  – Thermosetting polymers: polyurethane,
    unsaturated polyesters, phenolics, and epoxies
• Simpler mold
• Suited to low quantities
    Polymer Foams
•    A polymer-and-gas mixture, with gas added by:
    1. physical mixing or injection (air, nitrogen, CO2)
    2. chemical blowing agent that decomposes at elevated
       temperatures
•    Two types:
    1. closed cell (a)
    2. open cell (b)
•    Product methods:
    1. make the beads first, then feed the beads into a cavity
       to be fused together (e.g. styrofoam cups)
    2. injection mold using a chemical blowing agent
    3. extrude sheets (least expensive method)
Product Design Guidelines: General

• Strength and stiffness
  – Plastics are not as strong or stiff as metals
  – Avoid applications where high stresses will be
    encountered
  – Creep resistance is also a limitation
  – Strength-to-weight ratios for some plastics are
    competitive with metals in certain applications
Product Design Guidelines: General
• Impact Resistance
  – Capacity of plastics to absorb impact is generally
    good; plastics compare favorably with most
    metals
• Service temperatures
  – Limited relative to metals and ceramics
• Thermal expansion
  – Dimensional changes due to temperature
    changes much more significant than for metals
Product Design Guidelines: General

• Many plastics are subject to degradation from
  sunlight and other forms of radiation
• Some plastics degrade in oxygen and ozone
  atmospheres
• Plastics are soluble in many common solvents
• Plastics are resistant to conventional corrosion
  mechanisms that afflict many metals

				
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posted:10/20/2012
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