Gas-Counter Pressure Injection Molding
Gas-Counter Pressure Injection Molding.
Normally, as plastic enters a mold it displaces the air within the cavity. The air is vented to the atmosphere via specially designed vents specific to the type of plastic being molded. In gas-counter pressure molding an O-ring is installed around the perimeter of the cavity and a gas pressurizes the cavity. During injection this counter pressure gas, usually between 345 and 2760 kPa (50–400 psi), prevents gases, water, nitrogen, etc from emerging to the surface. Gas-counter pressure molding will provide a smooth skin
and nice appearance when processing plastic with a foaming agent. The counter pressure prevents the gas bubbles from developing at the surface.
Structural Foam Molding.
Plastic pellets are blended with a chemical blowing (foaming) agent, usually in pellet form, and injected into the mold under normal molding conditions. The foaming agent can also be a gas or liquid. The cavity is filled only to 70–95% full and then the foaming agent releases a gas, usually nitrogen, to finish fill. The parts surface is usually splayed or rough. The resulting part has a significant density reduction, saving material costs. Density reduction ranges from 5 to 35% with an average in the 8–20% range, yet the parts are stiffer than solid wall parts. In combination with gas-counter pressure the rough surface caused by the gas bubbles coming to the surface can be eliminated. The counter pressure gas prevents the foam bubbles from developing at the surface; cosmetics are as good as regular injection molding. Injection and cavity pressures are lower than that with conventional injection molding and sometimes the term low pressure molding is applied. Machine and mold costs are lower due to the lower pressure requirements. Trexel Inc. has recently brought out a new variation to foam molding, the Mucel® Process. It injects high pressure gas (N2 or CO2) into the barrel achieving supercritical fluid conditions to dissolve the gas into the polymer. A special screw aids in dispersing the gas into the molten plastic before injection. Once in the cavity, small bubbles evolve as pressure is lowered as the part cools.
Encyclopedia of Polymer Science and Technology.
Copyright John Wiley & Sons, Inc. All rights reserved.
