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Shrinkage
Amorphous resins such as ABS, Polycarbonate, and Polystyrene have much lower shrinkage values than the polyolefins. The higher shrinkage of polyolefins is due to the fact that, in their molten state, they take up more volume than in the solid state because polyolefin resins are semi-crystalline. When the resin solidifies, the chains in the crystalline regions pack tightly together resulting in a reduction in volume. In general, the polyolefins can be ranked for shrinkage: HDPE ≥LLDPE ≥LDPE≥PP
Once a resin has been selected, shrinkage can be controlled, to some extent, through mold design and processing conditions (Table 5). Studies on a test mold in which the thickness and gate area of a flat plaque can be varied, indicate the following:
• Shrinkage is reduced as part thickness decreases. The response to a thickness change is more pronounced with HDPE than PP.
• Shrinkage is reduced when the gate area is reduced.
Since the degree of shrinkage is partly a result of cooling, it can be reduced by molding at lower injection temperatures and running a colder mold. Packing the part more will also minimize shrinkage. This is done either by molding at moderate temperatures and high pressures or by molding at fairly high temperatures and moderate pressures. However, excessive temperature or pressure can result in flash.
Another means of reducing shrinkage is the use of higher pressure and longer packing time. This allows additional resin to flow into the mold as the material in the mold cools and shrinks, packing out the mold as much as possible but may also increase cycle time and higher molded-in stress.
Longer cooling time in the mold before ejection is especially useful whenever an inside dimension is critical. As the molded article cools and contracts around the core, the core will maintain the critical inside dimension of the part. Generous draft, or tapering, will allow easier part ejection. A longer cooling time will mean an increase in cycle time, therefore many molders will increase the mold cooling to reduce shrinkage.
Shrinkage is a time-dependent function. In general, a polyolefin part has achieved about 90% of its total shrinkage after 48 hours. Shrinkage can continue for several more days if the parts are packed hot and/or are stored in a warm warehouse. Parts that have shrunk after packaging typically exhibit ‘nesting’ problems if the parts are stacked inside each other.
Warpage
Warpage results from non-uniform shrinkage of the molded part caused by non-uniform cooling. When a part warps after being ejected, it is assuming its ‘natural’ shape by relieving the stresses forced upon it while being cooled in the mold. The problem, often a difficult one to solve, is to minimize the ‘locked-in’ stresses, which the part might later ‘remember’ and relieve during cooling to room temperature. In cases where parts are fixtured after ejection, subsequent exposure to higher temperatures may cause relaxation and warpage. Part designs incorporating significant differences in cross-sectional thickness are more prone to warpage than those with a more uniform thickness, due to higher residual temperatures in the thicker sections.
In addition to non-uniform cooling, locked-in stresses are generated in the mold by such operating conditions as excessive molding pressures, slow fill times, low backpressure, or too low a melt temperature.
There is no single, clear-cut remedy for warpage. Adjusting mold conditions, redesigning the part or the mold, switching to a material with a narrower MWD, or a combination of these may reduce the internal stresses. Generally, the least warpage occurs when the melt temperature is set at the maximum, the mold temperature is high, injection pressure is a minimum and the injection time is short (Table 5).
Molding at high temperatures allows the stresses induced during injection to be reduced before the part sets. Running a warm mold also allows the stresses to relax before the melt sets. Differential cooling between the mold halves is often required to produce warp-free parts especially those having large, flat sections.