This paper presents foam injection molding of polypropylene (PP) with mold-opening technique for achieving foams with high cell densities. The process of mold-opening injection molding fills and packs a polymer/gas mixture at high-pressure into a closed mold cavity. Subsequently, foam expansion is initiated through the opening of the mold cavity to a desired distance. In this work, control over the polymer/gas mixture temperature within the mold cavity was achieved by varying the dwelling time as the material was cooled. Foaming experiments were conducted with two different grades of homopolymer polypropylenes: a branched PP and a linear PP. A crystal nucleating agent (CA), 1,3:2,4-bis-O-(4-methylbenzylidene)-D-sorbitol (i.e. MDBS), was also used in the study. The foaming experiments of the two grades of PP were performed in a 50-ton injection molding machine, Arburg ALLROUNDER 270C, equipped with the Trexel MuCell® dosing unit. Poor quality foams with low cell densities and large cell sizes were produced as a result of the low melt viscosity at high foaming temperatures with short dwelling times. As the matrix was allowed to further cool before foaming was initiated, increases in the cell densities were observed, and microcellular foams with cell densities of up to 1010cells/cm3 were attained. Non-uniform cellular morphologies with vitrified regions were observed from samples excessively cooled in the mold cavity prior to foam expansion. The addition of CA was found to have a more prominent effect on promoting the cell densities for the linear PP. The crystals developed during cooling before mold opening caused stress variations, and consequently, the cell density was increased.
