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COMPRESSION MOLDING (MATCHED-DIE MOLDING)

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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  1. COMPRESSION MOLDING (MATCHED-DIE MOLDING)

  2. 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

  3. 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

  4. 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)

  5. 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

  6. 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)

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

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