 materials (fiber/resin) preweighed to a specific size charge, placed in a (closed) mold, and heat and pressure applied in a press

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 resins
– polyester (most common)
 low shrink systems
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– up to 10% thermoplastic polymers by weight mold shrinkage as low as 0.1% easily pigmented
 low profile systems
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10% - 15% thermoplastic polymers by weight mold shrinkage as low as @ 0% - 0.05% not readily pigmented most common fiber is glass

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 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)

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 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

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 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)

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 mixing process usually consists of two separate mixing stations
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– one station prepares the resin paste
 resin system, initiator, mold release, fillers
 high speed, high shear mixer resin paste transferred to second station
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(glass) fibers (0.5 in) added
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 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

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 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
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– molding temp charge pattern and placement molding pressures press closing speed cure time

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 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
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– improve productivity reduce internal stresses in molded part ensure reproducible part properties

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 should be as simple as possible consistent placement of charge in mold
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– 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

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 factors to consider in establishing molding pressures
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– 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

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 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

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 factors controlling cure time
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– 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