Research on cellular morphology and mechanical properties of microcellular injection-molded BCPP and its blends

Microcellular injection molding (MIM) is an environmentally friendly processing technology to produce lightweight microcellular plastic parts with nitrogen or carbon dioxide as a blowing agent. For the MIM process, the poor mechanical performance of the prepared parts is one of the toughest challenges that limit its wide applications. Here, an experimental study was conducted to investigate the foaming behavior of block copolymer polypropylene (BCPP), BCPP/random copolymer polypropylene (RCPP) blend, and BCPP/high-density polyethylene (HDPE) blend. Factorial experiments were conducted to clarify the influences of the major processing parameters including injection speed, melt temperature, and mold temperature on cellular morphology of the foamed BCPP, BCPP/RCPP, and BCPP/HDPE samples. Furthermore, tensile tests and Gardner impact tests were conducted to evaluate the mechanical properties of the foamed samples. The interrelationships of processing conditions, cellular morphology, and mechanical properties were discussed. The results show that the foamed BCPP/RCPP samples have the best tensile properties, while the foamed BCPP/HDPE samples show the best impact toughness. In particular, the foamed BCPP/RCPP samples even show much better tensile toughness than the solid samples. Tensile strength is independent of cellular morphology and processing parameters. Cell density has little effect on elongation at break, while the thickness of solid skin has a significant effect on elongation at break. Thicker solid skin benefits tensile toughness. The effects of cell density and solid skin thickness on impact strength do not show an obvious trend. Slower injection speed leads to improved impact strength, while melt temperature and mold temperature do not show pronounced effect on impact strength.