Injection Molding without Pre-drying
A group of researchers in Korea has developed a processing technique that can eliminate the need to pre-dry plastics prior to injection molding. The underlying principle is to prevent the moisture present in the material from vaporizing in the injection mold and causing voids or surface flaws in the molded part. To achieve this, the researchers employ the gas counterpressure process (GCP), the same process used to injection mold structural foam. This involves establishing a defined gas counter-pressure in the injection mold prior to injection and then injecting against this gas cushion. During injection, the plastic melt forces the gas out of the mold.
In order to pressurize the cavity, it must be sealed from the surrounding air. With the aid of a gas reservoir and controller, pressure is built up in the cavity and maintained at a constant level during injection of the melt. As soon as the melt solidifies, the pressure can be released. The pressure in the cavity must be higher than the saturation vapor pressure of the water at the temperature of the melt. When this requirement is satisfied, no voids or surface flaws are created.
In tests on an all-electric injection molding machine, PMMA test bars and production parts made from recycled PC were molded. The mold temperature was 60°C; the injection rate was 50 mm/s. Although the material was not pre-dried, no bubbles formed in the PMMA test bars when the gas counterpressure process was employed. According to the information from the researchers, it was even possible to mold production parts from PC with a flawless, high-gloss surface.