The process for molding Expanded Polystyrene (EPS) beads is a well known process where the EPS beads are placed into a closed mold and heated with steam for consolidation. Expanded Polypropylene beads molding is not as simple because of two factors. Polystyrene is amorphous material, Polypropylene is semicrystalline material and Polystyrene Tg is around 100^oC, Polypropylene Tm is around 165^oC. Therefore much higher level of steam pressure is necessary to mold Polypropylene expanded beads. However, the service temperature for articles molded from Polypropylene beads is much higher. The expanded beads molding setup at PPI consisted of a manually operated molding press, a mold, a small steam generator capable of generating steam at a pressure of 300 psi and associated electronics. The molding process was controlled from a remote station. The mold, shown in the picture, produces a cube.

The protocol of purging, autoclaving and cooling is extremely important for the production of parts from PP foamed beads. All these operations were controlled from a remote station. Cycle time and the design of the mold are also important factors. Normally a test mold can be made very inexpensively from aluminum.

Part molded from PP foamed bead is being taken out of the mold.