This study investigates the molding characteristics and mechanical properties of injection-molded foaming polypropylene (PP) parts and coinjection-molding PP parts for foaming core material embedded in nonfoaming skin material. Effects of processing parameters including injection-velocity, melting temperature, mold temperature and back pressure on part weight and part mechanical properties including tensile strength, flexural strength and stiffness are investigated. Influence of part thickness and foaming agent content on the degree of foaming is also studied. Based on the measured results, it was found that for thin-wall tensile foam specimens with 0.5 mm thickness, weight reduction is about 4–9% whereas in the thick-wall bending foam specimens with 15 mm thickness, the reduction is approximately 43–50%. Part thickness is a dominant factor to determine the degree of foaming. For injection-molded foaming parts, part weight, tensile strength, flexural strength and stiffness decrease with increasing melt temperature, mold temperature and injection velocity, whereas they increase with increasing back pressure. Foaming results in weight reduction but it also reduces part mechanical properties. The foaming agent content increases from 0.8 to 1.6%, the mechanical properties reduces significantly. Moreover, the coinjection-molding thin-wall parts process for foaming core material embedded in nonfoaming skin material results in a foam part with very good esthetics and higher tensile strength than injection-molded foaming parts. Meanwhile, the foam core is well bonding to the nonfoam skin in the coinjection-molding parts. The present investigation provides a molding guideline for injection-molded foaming parts and coinjection-molding parts to achieve a specific objective of part quality.
