Why do they occur?
Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature (see Specific volume (pvT diagram)). During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses. These so-called residual stresses (see Residual stress) act on a part with effects similar to externally applied stresses. If the residual stresses induced during molding are high enough to overcome the structural integrity of the part, the part will warp upon ejection from the mold or crack with external service load.
FIGURE 1. The pvT curves for amorphous and crystalline polymers and the specific volume variation () between the processing state (point A) and the state at room temperature and atmospheric pressure (point B). Note that the specific volume decreases as the pressure increases.
Causes of excessive part shrinkage
Excessive shrinkage, beyond the acceptable level, can be caused by the following factors. The relationship of shrinkage to several processing parameters and part thickness is schematically plotted in Figure 2.
Problems caused by part shrinkage
Uncompensated volumetric contraction leads to either sink marks or voids in the molding interior. Controlling part shrinkage is important in part, mold, and process designs, particularly in applications requiring tight tolerances. Shrinkage that leads to sink marks or voids can be reduced or eliminated by packing the cavity after filling. Also, the mold design should take shrinkage into account in order to conform to the part dimension. Part shrinkage predicted by C-MOLD offers a useful guideline for proper mold design.
Warpage due to differential shrinkage
Differences in filled and unfilled materials
Differential shrinkage for filled and unfilled materials is shown in Figure 3 below. When shrinkage is differential and anisotropic across the part and part thickness, the internal stresses created can lead to part warpage.
Non-uniform mold cooling across the part thickness
Non-uniform cooling in the part and asymmetric cooling across the part thickness from the mold cavity and core can also induce differential shrinkage. The material cools and shrinks inconsistently from the mold wall to the center, causing warpage after ejection.
Part thickness variation
Shrinkage increases as the wall thickness increases. Differential shrinkage due to non-uniform wall thickness is a major cause of part warpage in unreinforced thermoplastics. More specifically, different cooling rates and crystallization levels generally arise within parts with wall sections of varying thickness. This causes differential shrinkage, resulting in part warpage, as shown in Figure 5 below.
Part geometry asymmetry or curvature
Geometric asymmetry (e.g., a flat plate with a large number of ribs that are aligned in one direction or on one side of the part) will introduce non-uniform cooling and differential shrinkage that can lead to part warpage, as shown in Figure 6 below.