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COMPRESSION MOLDING (MATCHED-DIE MOLDING). COMPRESSION MOLDING PROCESS. materials (fiber/resin) preweighed to a specific size charge, placed in a (closed) mold, and heat and pressure applied in a press. RESINS AND REINFORCEMENTS USED IN COMPRESSION MOLDING PROCESS. resins

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COMPRESSION MOLDING (MATCHED-DIE MOLDING)


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compression molding process
COMPRESSION MOLDING PROCESS
  • materials (fiber/resin) preweighed to a specific size charge, placed in a (closed) mold, and heat and pressure applied in a press
resins and reinforcements used in compression molding process
RESINS AND REINFORCEMENTS USED IN COMPRESSION MOLDING PROCESS
  • resins
    • polyester (most common)
      • low shrink systems
        • up to 10% thermoplastic polymers by weight
        • mold shrinkage as low as 0.1%
        • easily pigmented
      • low profile systems
        • 10% - 15% thermoplastic polymers by weight
        • mold shrinkage as low as @ 0% - 0.05%
        • not readily pigmented
  • most common fiber is glass
sheet molding compound smc
SHEET MOLDING COMPOUND (SMC)
  • automated, continuous flow process
  • resin paste (resin, filler, initiator, mold release and thickening agents) properly proportioned and mixed
  • uniformly metered onto lower plastic (polyethylene, nylon) film
  • reinforcement is placed onto lower plastic film
  • top plastic film is coated in a similar manner with resin paste
  • top plastic film feeds into moving belt to form continuous sandwich of fiber/resin
  • compacted under controlled pressure
  • taken up on rolls (2 ft - 5 ft width)
different forms of smc
DIFFERENT FORMS OF SMC
  • random chopped (0.5 in - 2 in) fibers (SMC-R)
  • continuous unidirectional fibers (SMC-C)
  • combination of above (SMC-C/R)
  • thickness varies between 0.25 in - 0.5 in
  • thick molding compound (TMC) available in thicknesses up to 2 in
maturation of the molding compound
MATURATION OF THE MOLDING COMPOUND
  • thickens between processing and molding in order to provide tack free molding compound that releases cleanly from plastic film
  • viscosity of resin paste depends on resin formulation
  • viscosities between 10 - 100 million cps usually chosen for molding
  • storage time varies from 1 day to 7 days depending on resin formulation (most common formulation 3 days)
bulk molding compounds bmc
BULK MOLDING COMPOUNDS (BMC)
  • mixing process usually consists of two separate mixing stations
    • one station prepares the resin paste
      • resin system, initiator, mold release, fillers
      • high speed, high shear mixer
    • resin paste transferred to second station
      • (glass) fibers (0.5 in) added
      • sigma blade, spiral blade or combination of blades and single or twin screws
      • final mixing @ 3 mins
  • packaged in vapor barrier for maturation of resin paste or transported directly to molding press
  • alternately, may be extruded in log form for easier weighing at the press
compression molding process8
COMPRESSION MOLDING PROCESS
  • measured charge of compound placed between halves of split mold
  • heated mold is closed
  • pressure is applied so that compound flows to fill mold cavity
  • compound is allowed to cure before removal from the mold
  • process variables to consider in compression molding
    • molding temp
    • charge pattern and placement
    • molding pressures
    • press closing speed
    • cure time
molding temperature
MOLDING TEMPERATURE
  • temperature control is extremely important - variations of a few degrees can result in premature gelling in areas of mold if it is too hot and longer cure times than expected if some areas of mold are too cold
  • objective is to achieve fastest possible cure settings that are easy to maintain and result in near uniform heating throughout the charge and no degradation
  • accurate temp control
    • improve productivity
    • reduce internal stresses in molded part
    • ensure reproducible part properties
charge pattern and placement
CHARGE PATTERN AND PLACEMENT
  • should be as simple as possible
  • consistent placement of charge in mold
    • assures reproducible flow time to all parts of the mold
    • important for proper temp distribution
  • charge placement affects fiber orientation due to flow in mold - can result in variation in part properties
  • obstructions cause interruption and subsequent rejoining of flowing material resulting in knit lines (local regions of weakness)
  • various automated loading and unloading devices have been developed for molding presses
molding pressures
MOLDING PRESSURES
  • factors to consider in establishing molding pressures
    • viscosity of resin paste
    • part design
    • quality of part surfaces
  • pressures up to 2,000 psi can be required for relatively large molds with deep draws
  • for parts with flat surfaces satisfactory flow and fillout will be obtained at slightly lower pressures
press closing speed
PRESS CLOSING SPEED
  • depends primarily on mold temperature and resin paste gel time
  • higher molding temp and faster curing formulations require faster press closure for rapid pressure build in mold
  • if closing too slow, material will pre-gel in mold
  • if closing too fast, fiber wash will result
  • typical closing times 1 - 10 secs
cure time
CURE TIME
  • factors controlling cure time
    • resin initiator
    • mold temp
    • part thickness
  • cure times typically range from 1 - 5 min
  • cure time is presently limiting factor with respect to increasing production rates
  • actually molding of part often occupies only about 10% of total press cycle time, remaining 90% is cure time