STUDIES ON GAS COUNTER PRESSURE AND LOW-PRESSURE STRUCTURAL FOAM MOLDING .2. EFFECT OF PROCESSING CONDITIONS ON STRUCTURE OF MOLDED PARTS

Studies involving the structure of the parts molded from the low pressure structural foam injection molding (LPSF) and the gas counter pressure structural foam injection molding (CPSF) were conducted via varying processing conditions. The bubbles near the skin layer were observed to be less uniform in size and more elongated for the LPSF parts than for the CPSF parts. The CPSF parts, however, exhibited a more uniform cellar core. The skin layer thickness from both processes increased under one of the following conditions: higher melt temperature, higher reduced density, lower injection speed, or lower blowing agent concentration. As regards the effect of mold temperature, a higher mold temperature resulted in an increase of the skin layer thickness for the LPSF parts but a decrease for the CPSF parts. The skin layer of the CPSF part would, become thin if the gas counter pressure was not high enough to prevent foaming during melt injection. With a sufficient amount of gas counter pressure, the CPSF parts were observed under the same processing conditions to become superior to the corresponding LPSF parts owing to a thicker skin layer with a greater integrity and smoother surface.