Injection molding shrinkage begins as free volume decreases during cooling, making material choice and process control critical to dimensional stability.
Shrinkage is a natural part of injection molding. It affects part dimensional stability and, if not understood correctly, can cause delivery problems and high added costs. This article reviews why shrinkage occurs and how both material selection and process conditions affect it.
You can also read: Read Your Material Datasheet to Cut Molding Costs and Defects.
Shrinkage refers to the dimensional contraction of an injection-molded part. It is a completely natural phenomenon in injection molding. Every molder must deal with dimensional change in molded components, and only accurate prediction can ensure that a part performs as intended. When a plastic is above its glass transition temperature and enters the mold, its molecules have high energy and therefore greater intermolecular spacing. The free space between molecules, often called free volume, increases as temperature rises. As the molded part transfers heat to the mold, it cools and solidifies. Consequently, the free volume decreases. That reduction appears as a reduction in part size. A part measured immediately after molding can be up to 2% larger in every dimension than the same part after it has fully cooled and reached its final shape, which may take roughly 16 hours.
The amount of shrinkage depends strongly on the material. Semi-crystalline polymers generally shrink more because the formation of crystalline regions, called spherulites, further reduces free volume. A simple analogy helps illustrate this behavior. Imagine a drawer full of T-shirts. If the shirts are mixed loosely, they occupy more space. If you fold and organize them, they take up less room. Polymers behave in a similar way: the more organized the molecular structure, the less space it occupies.
In amorphous polymers, by contrast, the occupied volume depends mainly on the available free volume. The change in volume is steeper above the glass transition temperature, but once the part solidifies, the volume continues to decrease more gradually.
PvT diagram of a semi-crystalline material: Akulon F223-D, PA6 from Envalior. Courtesy of Datasheet Campus.
PvT diagram of an amorphous material: Makrolon 2205, PC from Covestro. Courtesy of Datasheet Campus.
Pressure also affects free volume. Higher pressure compresses the molecules more strongly and therefore reduces shrinkage. It is also important to understand that a polymer’s solidification temperature rises with increasing pressure. In other words, even if more thermal energy is present, excessive molding pressure can still restrict molecular flow.
PvT curves help explain how part volume changes with temperature and pressure. In this notation, P stands for pressure, v for specific volume, and T for temperature. These curves can be measured for a given polymer and are commonly used as input for injection molding simulations that predict shrinkage.
If you compare the PvT curves of an amorphous material and a semi-crystalline polymer, as shown in Figure 1, the main difference appears in the shape of the curve. A semi-crystalline polymer shows a distinct curved region, or “belly,” just below the glass transition range. This section reflects the volume change associated with crystalline organization.
In general, amorphous materials can exhibit shrinkage values that are nearly ten times lower than those of semi-crystalline polymers. That is one reason why a processor may prefer ABS or PC over PP for an assembly application. Performance and cost may differ, but amorphous materials can reduce the dimensional-variation problems that create assembly headaches.
Processors should also account for the effects of fiber reinforcement. Fibers limit the ability of the polymer matrix to shrink along the fiber direction. As a result, fiber-reinforced polymers often show anisotropic shrinkage: greater shrinkage perpendicular to the fiber orientation and much lower shrinkage parallel to it.
Any parameter that affects the temperature gradient or the rate of heat transfer will influence shrinkage. If residual heat remains inside the molded part after ejection, the part size will continue to change. Molecules keep moving as long as thermal energy is still present, and that continued movement contributes to shrinkage.
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RJG teaches master molders to identify when we can no longer increase the weight of the part by continuing to pack the part after the gate has already frozen. By charting your process and weighing your parts you can identify when your pack and hold is no longer effective. Knowing when pack & hold is finished helps contribute to minimizing excessive shrinkage and it's a very important stage of the molding process.