15 May Understanding Shrinkage in Injection Molding Materials
Shrinkage is a critical factor in injection molding. It refers to the reduction in volume that occurs as molten polymer cools and solidifies in the mold. Understanding how and why materials shrink helps engineers design better tooling and ensure dimensional accuracy in finished parts.
Measuring Shrinkage: % vs. in/in
Shrinkage is commonly expressed as a percentage (%) or in inches per inch (in/in). For instance, a shrinkage value of 0.005 in/in equals 0.5%. Both units are used across datasheets and simulations, so it’s essential to be comfortable interpreting either format.
Material Type Matters: Amorphous vs. Semicrystalline
Shrinkage behavior depends heavily on the polymer’s molecular structure:
- Amorphous materials (e.g., ABS, Polystyrene, Polycarbonate) have a random molecular arrangement and do not crystallize upon cooling. They typically exhibit lower, more uniform shrinkage (0.3%–0.7%), making them easier to predict and control.
- Semicrystalline materials (e.g., Nylon, Polypropylene, PEEK) form tightly packed crystalline regions during cooling. This leads to higher shrinkage (1.0%–3.0%) and greater differences between flow and crossflow termed and is also significantly impacted by the wall thickness of the part.
Directional Shrinkage: Flow vs. Crossflow
Shrinkage isn’t uniform—it varies based on flow direction:
- In-flow direction shrinkage occurs along the path of molten plastic and is typically lower because polymer chains align with the flow.
- Crossflow shrinkage occurs perpendicular to flow and is usually higher due to less molecular alignment.
When materials have different shrinkage values in different directions they are termed “anisotropic” and must be considered during the design process to prevent warping.
Using Fibers and Fillers to Control Shrinkage
Engineers pay close attention to anisotropy and can impact dimensional stability with two main additives:
- Fiber reinforcements add strength and align with flow thereby reducing shrinkage in that direction. This can cause a larger difference between in-flow and crossflow shrinkage thereby causing additional warpage if not done carefully.
- Mineral fillers (like talc or calcium carbonate) do not shrink during cooling and reduce the amount of dimensional change lessening warp concerns.
Careful filler selection can fine-tune a material’s behavior, helping match shrinkage characteristics across different components or improve dimensional stability.
Conclusion
Shrinkage affects the accuracy, appearance, and function of molded parts. By understanding how shrinkage varies with material type, flow direction, and additives, manufacturers can design more precise molds and choose materials that minimize warpage and distortion. With thoughtful material selection and processing, consistent, high-quality injection molded parts are well within reach.
Shrinkage is part of the molding process, but with the right material and expertise, it doesn’t have to be a problem.
Let’s Connect! PolySource is here to help you select the right material and tackle shrinkage challenges head-on. Reach out to our team for expert guidance and support in achieving the best results for your molded parts.