As energy costs continue to be unstable, the need for lightweighting in the automotive segment and the broader transportation industry is ever present, as is the need to develop materials and manufacturing processes that can improve performance, aesthetics and cost. Polymer composites can be an important materials/process option for achieving such goals. The mechanical benefits gained (with resulting ability to reduce nominal wall thickness and therefore part weight) by preserving fiber length in the final part are well documented in the literature and evidenced by the growing market for a broad range of composite materials / processes in transportation. No family of composite materials have grown faster over the past decade than long-fiber thermoplastics (LFTs) - both in pellet form (for injection molding) and with inline-compounded (ILC) direct molding processes (D-LFT) for injection and compression molding. However, post-mold fiber lengths attained with traditional LFT injection methods are typically restricted to 5 mm / 0.2 in., while compression molding is limited in its three-dimensional (3-D) design capabilities and associated post-mold trimming requirements. A new variant on injection-LFT technology has emerged to offer significant benefits over traditional thermoplastic composites molding processes through rapid cycles, excellent surface finish and 3-D design opportunities. This is achieved in a closed molding process similar to LFT injection, yet produces parts whose mechanical properties are closer to those produced by D-LFT compression molding, since post-mold fibers are far longer - typically 10 mm / 0.4 in. even in very-complex designs, and up to 50 mm / 2.0 in. in simpler structures. The generic name for this new technology, 3-D-LFT, is derived from the abovementioned benefits: 3-D injection molding design capabilities coupled with the mechanical properties typically only found in D-LFT compression molding. This paper summarizes the research and results of a comprehensive study on the effects of and benefits demonstrated by this new molding process through an analysis of its design flexibility, material formulation and fiber-length retention, as well as cycle times and shear reduction. These benefits are illustrated through a case study using test piece results as well as operating parameters obtained from molding a large, complex, long-glass fiber polypropylene (LGF-PP) part.
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