Injection overmolding of unidirectional fibers and preforms is an attractive process for many good reasons. It’s fast, consistent and repeatable, and it can be performed with a machine that is relatively easy to acquire, program and control. Injection molding’s disadvantage, however, is it requires tooling that is typically very expensive — certainly more expensive than tools for compression molding. Further, overmolding requires that the composite be encapsulated within the tool, and if it’s a larger part, that means larger tooling. Add it all up and the cost of composites overmolding can exceed its benefits.
Robert Davies, CEO of Fibrtec (Atlanta, Texas, U.S.), has developed an interesting solution to this particular overmolding dilemma. His system uses a hybrid injection/compression overmolding process of his own design to combine smaller injection molded parts or inserts within a larger compression molded part. In essence, the approach flips the script on the overmolding strategy. Rather than injection mold around a composite preform, Davies’ solution involves compression molding around an injection molded part.
“What [Fibrtec] decided to do was injection mold the bosses, features, ribs and those kinds of things, and position them in place into the same compression molding tool that the laminate was going to be consolidated in,” says Mike Favaloro, president and CEO of composites consultancy CompositeTechs LLC (Amesbury, Mass., U.S.), who spoke about the process at the Composites Overmolding conference in Novi, Mich., U.S., in 2018.
“Since you have to consolidate the laminate anyway, why not consolidate the laminate with the injection molded parts in place?” he adds.
With Fibrtec’s process, ribs, bosses and similar features are injection molded off-line using thermoplastic resins, lower cost tooling and conventional injection molding machines. The injection molded insert is then robotically placed in a compression mold with a preform.
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