Foam injection molding of glass fiber reinforced polypropylene composites with laminate skins

Sandwich panels which consist of discontinuous glass-fiber reinforced polypropylene composite foam core and continuous glass-fiber reinforced polypropylene laminate skins were manufactured using industry-scale equipment in a streamlined manner. The streamlined process included two stages: (1) continuous glass-fiber reinforced polypropylene laminates as the face skins were produced using an automated, rapid tape layup machine and a hydraulic press, and (2) discontinuous glass-fiber reinforced polypropylene composite as the core was foam injection-molded onto the laminate skins (i.e., overmolding), which eliminated bonding process of the foam core and laminate skins, with a physical blowing agent, nitrogen. Optical microscopy results suggested that the increase of void fraction by the core-back (or mold opening) technique increased the average cell size and cell size distribution, but had little effect on the cell density of the foam core. Mechanical test results suggested that the addition of laminate skins increased the flexural modulus and flexural strength of the solid as well as foam composites significantly. In addition, the material indices were calculated for the sandwich panels manufactured in this study, and the results suggested that the addition of the laminate skins and foaming by the core-back (or mold opening) technique can greatly reduce the weight of material needed to have the same bending stiffness and maximum bending force.