Fabrication of Microcellular Injection‐Molded Polypropylene with Super High Expansion Ratio by Precision Mold Opening Technology

In this study, microcellular injection molding with the combination of the precision mold opening and rapid heat cycle mold technology is conducted to fabricate the 30% glass fiber reinforced polypropylene (PP/GF30) foam. By optimizing supercritical fluid, mold temperature, mold opening distance, and other process conditions, the injection-molded PP/GF30 foam with a super high expansion ratio, density of 0.32 g cm(-3), and porosity of 75%, is fabricated. The crystallization and rheological properties of the PP/GF30 samples are tested to determine the suitable injection molding parameters. The results show that glass fiber promotes the crystallization of polypropylene (PP) while improving its storage modulus and complex viscosity. The fiber distribution and orientation as well as the cellular structure are investigated by scanning electronic microscopy (SEM) and X-ray computed tomography. Due to the high mold temperature, the shear effect and melt cavity filling are greatly changed. The oriented fibers in the core layer are randomly moved by the cell growth and presented as a scaffold to support the cell wall structure. The glass fiber increases the PP melt stiffness and avoids cell collapse so that the cellular structure with small cell size and even cell distribution is fabricated. The compression properties, surface roughness, and thermal conductivity are examined.