COMPRESSION MOLDING PROCESS materials fiberresin 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 ID: 920041
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Slide1
COMPRESSION MOLDING (MATCHED-DIE MOLDING)
Slide2COMPRESSION MOLDING PROCESS
materials (fiber/resin) preweighed to a specific size charge, placed in a (closed) mold, and heat and pressure applied in a press
Slide3RESINS 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
Slide4SHEET 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)
Slide5DIFFERENT 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
Slide6MATURATION 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)
Slide7BULK 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 pressalternately, may be extruded in log form for easier weighing at the press
Slide8COMPRESSION 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
Slide9MOLDING 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
Slide10CHARGE 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
Slide11MOLDING 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
Slide12PRESS 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
Slide13CURE 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