When the dependence of molding shrinkage on molding conditions is evaluated by orthogonal experiments, it is found that the dependence on holding pressure is significant. This is thought to be due to the decrease in volume shrinkage per unit volume when the molten resin is cooled to room temperature due to the decrease in free volume resulting from the increase in density of the molten resin as holding pressure increases.
Figure 1 shows the change in shrinkage in the flow direction (MD) and in the orthogonal direction (TD) as a function of holding pressure, indicating that mold shrinkage decreases as holding pressure increases for ABS resin. This is thought to be due to the fact that the specific volume of the molten resin decreases as the holding pressure increases, resulting in a smaller difference from the specific volume that shrinks less under room temperature and atmospheric pressure. In addition, the shrinkage rate in the TD direction tends to be smaller than that in the MD direction as the pressure-retaining force increases. This is thought to be due to anisotropy caused by the difference in linear expansion coefficient due to molecular orientation caused by flow.
Figure 1: Distribution of various shrinkage rates versus filling pressure for ABS parts;
○: Flow direction in region 1 (near the gate) □: flow direction in region 2 (far from the gate)
●: lateral direction in region 1 ■: lateral direction in region 2
Figure 2 shows the change in molding shrinkage anisotropy values versus holding pressure for the region close to the gate (region1) and the region far from the gate (region2). It can be seen that the molding shrinkage anisotropy is smaller in region 1, which is closer to the gate, than in region 2, which is farther from the gate. This indicates that the molding shrinkage anisotropy has a distribution within the molded product. Shrinkage anisotropy in Region 1 becomes almost constant when the holding pressure exceeds 60 [MPa], as the effect of holding pressure disappears. However, the shrinkage anisotropy of Region 2 shows no clear pressure holding force at which the effect disappears, and tends to decrease as the pressure holding force increases. This suggests that the distribution of shrinkage anisotropy within the molded product varies with the pressure holding force.
Figure 2: Shrinkage anisotropy of ABS parts due to filling pressure
(■: Region 1 (near gate) ●: Region 2 (far from gate))
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