Shrinkage and warpage

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.

The shrinkage of molded plastic parts can be as much as 20 percent by volume, when measured at the processing temperature and the ambient temperature. Crystalline and semi-crystalline materials are particularly prone to thermal shrinkage; amorphous materials tend to shrink less. When crystalline materials are cooled below their transition temperature, the molecules arrange themselves in a more orderly way, forming crystallites. On the other hand, the microstructure of amorphous materials does not change with the phase change. This difference leads to crystalline and semi-crystalline materials having a greater difference in specific volume () between their melt phase and solid (crystalline) phase. This is illustrated in Figure 1 below. We'd like to point out that the cooling rate also affects the fast-cooling pvT behavior of crystalline and semi-crystalline materials.

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.

FIGURE 2. Processing and design parameters that affect part shrinkage

Warpage is a distortion where the surfaces of the molded part do not follow the intended shape of the design. Part warpage results from molded-in residual stresses, which, in turn, is caused by differential shrinkage of material in the molded part. If the shrinkage throughout the part is uniform, the molding will not deform or warp, it simply becomes smaller. However, achieving low and uniform shrinkage is a complicated task due to the presence and interaction of many factors such as molecular and fiber orientations, mold cooling, part and mold designs, and process conditions.

Warpage due to differential shrinkage
Warpage in molded parts results from differential shrinkage. Variation in shrinkage can be caused by molecular and fiber orientation, temperature variations within the molded part, and by variable packing, such as over-packing at gates and under-packing at remote locations, or different pressure levels as material solidifies across the part thickness. These causes are described more fully below.

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.

Filled materials  
For fiber-filled thermoplastics, reinforcing fibers inhibit shrinkage due to their smaller thermal contraction and higher modulus. Therefore, fiber-filled materials shrink less along the direction in which fibers align (typically the flow direction) compared to the shrinkage in the transverse direction. Similarly, particle-filled thermoplastics shrink much less than unfilled grades.

Unfilled materials  
On the other hand, if an unfilled molded part contains high levels of molecular orientation, shrinkage is anisotropic because aligned chains shrink to a greater extent in the direction of orientation.

Liquid crystal polymers  
For liquid crystal polymers (LCPs), the tightly ordered self-reinforcing structure tends to exhibit anisotropic shrinkage.

FIGURE 3. Differential shrinkage for both unfilled and filled materials

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.

FIGURE 4. Part warpage due to (a) non-uniform cooling in the part, and (b) asymmetric cooling across the part thickness.

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.

FIGURE 5. Larger volumetric shrinkage due to the high crystallization level in the slow cooling areas (e.g., the thick sections) leads to differential shrinkage and thus part warpage

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.

FIGURE 6. The poor cooling of the mold wall on the ribbed side causes a slower cooling of the material on that one side, which can lead to part warpage