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Steve Ham Plastics

Product Design and Process Technologist


Structural Foam

Structural Foam molding has been around since the sixties but remains unknown to many product designers.  It is a way to injection mold very large parts.  Traditional thermoplastic resins are used along with a blowing agent which produces a molded article with a cellular core and solid skins.  These Polyethylene water tanks convey the concept of size capability:HDPE structural foam- thank you Wilmington MachineHDPE structural foam- thank you Wilmington Machine





The following sequence illustrates how the process works:

Prior to resin injection the mold is clamped with 1/4 ton per square inch minimum clamping pressure.  The pre-blended material is held in the machine in an accumulator prior to injection.Clamped mold prior to resin injectionClamped mold prior to resin injection






The machine injects a precisely measured short shot of material.  Structural Foam's thicker wall section along with the blowing agent boost allows rapid and low pressure filling.Short shot of resin injection beginsShort shot of resin injection begins





After resin injection is completed, the cavity pressure is isolated and the blowing agent pressure continues to fill the mold cavity.



 The blowing agent pressure continues to pack the mold even after the mold cavity is completely filled.  This blowing agent pressure compensates for volumetric shrinkage as the plastic cools.  The blowing agent becomes an internal "cushion".Blowing agent pressure does final packingBlowing agent pressure does final packing

Structural Foam molded as a low pressure short shot will exhibit "Swirl" or flow marks on the surface of the molded part. This natural occurrence results from the pressure drop on the leading edge of resin flow front during the mold filling phase of the process.