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The size of the dependence of the shrinkage anisotropy in different locations of injection molded parts is not frequently reported (shrinkage calculated by comparing the actual plastic parts to the metal mold, while anisotropy refers to differences in the melt flow (MD) and the transverse (TD) directions). In this study, the effects of packing pressure (from 20 to 100MPa) in the injection molding process on the shrinkage anisotropy at two positions in a molded part were investigated in two ABS resins. The shrinkage anisotropy decreased as a function of packing pressure at both locations, although at different rates. It was found that the shrinkage anisotropy at these two positions was quite similar at around 60MPa. It was also found that the screw head moved forward (backward) at pressures higher (lower) than 60MPa. At a packing pressure of 60MPa the screw head position was almost constant during the packing process in a plunger type injection molding machine where the polymer melt can hardly flow backwards.In order to get further insight into the causes of the shrinkage anisotropy, birefringence measurements were performed with a transparent ABS resin. The measured birefringence at the two locations of the injection molded part were in good agreement with the respective shrinkage anisotropy values. Moreover, the packing pressure dependence seen in the shrinkage anisotropy was found to be similar for that of birefringence. It was demonstrated that a difference in molecular orientation in the two directions (MD and TD), caused the shrinkage anisotropy, and that the ratio of molecular orientation difference becomes almost identical at the two locations at around 60MPa of packing pressure.

Effect of Packing Pressure on the Shrinkage Anisotropy of Injection Molded Parts of Acrylonitrile-butadiene-styrene (ABS) Resins