| This study quantified the effects of process variables on the molding performance and selected part properties in intermittent extrusion blow molding. The three major processing parameters, (a) melt temperature, (b) injection velocity, and (c) platen delay time, affected the formation of the parison, and hence, the final properties of high density polyethylene (HDPE) and polycarbonate (PC) bottles. Parison sagging increased with melt temperature, thereby decreasing bottle mass, increasing scrap mass, producing thinner-walled bottles with thicker bases, and reducing tensile modulus. Very high temperatures facilitated parison blowout or break-off while melt fracture occurred at very low temperatures. Since higher injection velocities increased parison swell, bottle mass and thickness increased while scrap mass decreased. Low velocities also enhanced parison sag and increased the incidence of blowouts. Increasing platen delay time increased bottle mass, but had no definable effect on thickness or modulus. The effects of injection speed were more pronounced for HDPE than PC, and platen delay time was less influential than melt temperature and injection velocity. Blow pressure only impacted the inflation of very thick or thin parisons whereas blow time and mold temperature did not affect the measured properties. In general, HDPE had a larger process window than polycarbonate; it was less sensitive to changes in melt temperature, injection velocity, and platen delay. |
