Numerous efforts to increase the speed of compression molding for structural parts are underway. The Alfond Lab at the University of Maine is working with industry to demonstrate the efficacy of combining automatic tape laying (ATL) with compression molding. Erb says that the lab uses an ATL machine to rapidly lay down pre-consolidated structural tapes in various orientations before they are ultrasonically welded together and compression molded. The lab has used the process to develop a prototype of an automobile differential cover.
Meanwhile, an industry consortium based in France, the IRT-M2P, has developed an innovative processing method called Fast RTM. The resin transfer molding process enables two-minute cycle part production of structural thermoplastic and thermoset composite parts for the automobile and aerospace industries. Last year, consortium member Arkema introduced Elium, the first liquid thermoplastic that can be cured at room temperature. Like thermosets, it also cures in a two-step process that uses an initiator. The Fast RTM platform infuses CFRT with Elium and molds it in a Pinette P.E.I. short-stroke ECS-Press. Because Elium is a liquid reactive system and does not have to be heated to the same high temperatures as other thermoplastics, the process can be used to mold net shape CFRP thermoplastic parts.
Overmolding refers to hybrid processes in which two different materials are molded together, with the base layer molded first and additional layers molded into, on or around the original part. In the most common type of overmolding, CFRT is placed into an injection molding tool and then short or long fiber thermoplastics are injected into the mold.
“We consider continuous fiber with a short fiber injection-molded material one of, if not the, most important advancement in thermoplastic composites,” says Baker. “It provides a lot of design flexibility.” For example, you can build in integral ribs and attachment points and reduce the weight and cost compared to a completely injection-molded part, he says.
Overmolding can also be used with CFRT compression molding. For example, PolyOne’s Polystrand continuous fiber thermoplastic reinforcements are strategically placed into a tool and then compression overmolded with long fiber polypropylene in certain automotive applications. This provides reinforcement that keeps parts from unlatching upon impact. Pilpel believes that this kind of selective reinforcement, using overmolding, is an extremely important innovation. “This is clearly where the market is going,” he notes.
In a different type of overmolding, thermoplastic is injected onto the surface of basic compression molded parts. This allows for increasingly complex details, such as channels, bolts, inserts and outserts, according to Erb. He says this technique could be used to create an automobile dashboard, for example.