Injection molding is a fast, high-volume, closed molding process that uses, most commonly, reinforced thermoplastics, such as nylon with chopped glass fiber. In the past 20 years, however, automated injection molding of BMC has taken over some markets previously held by thermoplastic and metal casting manufacturers. They include electrical and automotive components, appliance housings and motor housings, to name a few. The use of BMC must be justified by production volume because the cost of both molds and presses is relatively high.
When injection molding BMC, a ram- or screw-type plunger forces a metered shot through a heated barrel and injects it (at 5,000 psi to 12,000 psi) into a heated mold, where the liquefied BMC flows easily along runner channels and into forming cavities. Heat build-up is controlled to minimize curing time. After cure and ejection, parts need only minimal finishing. Injection speeds are typically one to five seconds, and nearly 2,000 small parts can be produced per hour in a multiple-cavity mold.
Long-fiber reinforced thermoplastics (LFRT) are materials with long (6.35 mm/0.25 inch or greater) fiber reinforcement and make up one of the fastest growing fabrication categories. Leading this expansion is one of the oldest forms, glass mat thermoplastic (GMT) and two of the segment’s newest: precompounded (pelletized) LFRTs (long-fiber reinforced thermoplastics), also known as LFTs, and inline compounded (ILC) or direct LFTs (D-LFTs).