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The automotive and aerospace industries ask for lightweight and cost effective materials, material combinations and structures for their products. To achieve these goals certain composite material groups have to be optimized with respect to their lightweight potential and production methods.Thermoplastic sandwich composites - which consist of a core structure (transferring the load) and two face-layers (absorbing tensile and compression loads occurring at bending) - suite the need of minimizing weight per area under bending loads. Reduction of process steps can be achieved by connecting the face layers and core in-situ via in mold assembly process using foam injection molding (FIM). FIM uses a single material system and – as within this work – physical blowing agents (PBA) for foaming. To increase the strength and stiffness of FIM parts, (long glass) fibers are in cooperated to create long fiber reinforced thermoplastic (LFT) materials.A commercially available version of the LFT-FIM process is the MuCell® process (Trexel, Inc.). LFT granulate (~ 11 mm length) is fed into the injection molding machine, melted and combined with nitrogen as PBA. To skip the needed compounding process step of the rod granules Fraunhofer ICT developed a Direct LFT-FIM process where polymer and continuous fibers are fed into a twin-screw extruder, melted and mixed with nitrogen. This single phase solution then is transferred to an injection unit.Within this work, these two foam injection molding processes will be compared concerning their achievable fiber length. For that purpose, (foamed) long fiber reinforced polypropylene (PP) blanks were manufactured using identical raw materials such as polymer, additives and glass fibers. The semi-finished product, starting material for the MuCell® process, were manufactured by EASICOMP GmbH using the same raw materials as with the D-LFT process. The different blanks – foamed and fiber reinforced PP (30 wt% and 40 wt %) – were manufactured using one injection unit for the MuCell® process and one for the D-LFT-FIM process. To compare the fiber length the same mold optimized to reduce fiber breakage was used in both processes.

The MuCell process is a special injection molding process which utilizes supercritical gas (nitrogen) to create integral foam sandwiches. The advantages are lower weight, higher specific properties and shorter cycle times. In this study, a series of glass fiber-reinforced polyphenylene sulfide foam blanks are manufactured using the MuCell injection molding process. The different variations of the process (low-pressure also known as structural foam injection molding) and high-pressure foam injection molding (also known as “core back expansion,” “breathing mold,” “precision opening,” decompression molding) are used. The sandwich structure and mechanical properties (tensile strength, bending strength, and impact behavior) of the microcellular and glass fiber-reinforced polyphenylene sulfide foams are systematically investigated and compared to compact material. The results showed that the injection parameters (injection speed, foaming mechanism) played an important role in the relative density of microcellular polyphenylene sulfide foams and the mechanical properties. It could be shown that the specific tensile strength decreased while increasing the degree of foaming which can be explained by the increased number of cells and the resulting cell size. This leads to stress peaks which lower the mechanical properties. The Charpy impact strength shows a significant dependence on the fiber orientation. The specific bending modulus of the high-pressure foaming process, however, surpasses the values of the other two processes showing the potential of this manufacturing variation especially with regard to bending loads. Furthermore, a high dependence of the mechanical properties on the fiber orientation of the tested specimens can be found.