Structure of PP structural foam moldings made by the gas-counterpressure process

An experimental study was carried out to investigate the gas-counterpressure process by egression of a part of the polymer melt from the core of the molded body towards the accumulator. A systematic study of bubble morphology development and structural parameters of structural foam moldings is reported. The structural foam samples were produced on a two-stage molding machine (SIEMAG Structomat 2000/70) with passively transporting accumulator and on an in-line injection molding machine (KuASY 800/250) with FIFO-type accumulator, the melt temperature being varied in the range 473 to 533 K. The polymer used was isotactic polypropylene into which chemical blowing agent (azodicarbonamide) was added. The structural properties studied were overall density, local density, and density distribution. It was found that the distance from the sprue to the extreme of flow and the melt temperature have a profound influence on the bubble sizes and their distribution. It was established that the use of the two types of accumulator causes remarkable differences in the structural organization of the structural foam moldings.