How to achieve low shrinkage?
Last update on Jan 14, 2026
Low shrinkage in plastics refers to a material's ability to retain its intended size and shape with minimal dimensional change during and after processing — especially after cooling from the molten state during injection molding, extrusion, or thermoforming.
Why is low shrinkage important in polymers?
Low shrinkage is important in polymers because it helps ensure that molded or formed parts maintain their intended dimensions and shape after cooling. This is especially critical for applications requiring precision, tight assembly fits, or high aesthetic standards.
Key reasons:
- Ensure dimensional accuracy
- Reduce warpage and distortion
- Improve fit in assemblies
- Minimize surface defects
- Support consistency in high-volume production
How does shrinkage occur in plastics, and in what forms?
Linear shrinkage: The reduction in length or dimension of the plastic as it cools and solidifies in the mold.
- Post-molding shrinkage: Dimensional change that occurs after the part is ejected from the mold, often due to continued cooling or crystallization. It might take a day for the component to settle to its final dimensions.
- Directional shrinkage: 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
- Post-treatment shrinkage: Size reduction that occurs during secondary processes like annealing or sterilization.
- Moisture absorption expansion: Dimensional increase in hygroscopic plastics (like nylons) when they absorb moisture from the environment.
What factors influence shrinkage in polymers?
Several factors influence shrinkage in polymers, primarily related to the material’s nature, processing conditions, and part/mold design. Understanding these helps in minimizing dimensional variations and defects.
Polymer type: Semi-crystalline polymers shrink more than amorphous ones due to their crystalline structure. This shrinking takes place when the polymer chains pack into crystals. When packed, they need less room, and this leads to higher shrinkage.
Filler content: Adding glass fibers or mineral fillers reduces shrinkage by limiting polymer movement during cooling
- Moisture absorption: Hygroscopic polymers can swell post-molding, affecting shrinkage over time
- Mold design: Uneven wall thickness, poor cooling, or improper gate placement can lead to non-uniform shrinkage. Here is a detailed importance of the parameters mentioned above:
- When plastic is injected into the cavity, it begins to cool and shrink. The intention is to compensate for this shrinkage by maintaining holding pressure and feeding in additional material. However, at some point, the plastic near the gate solidifies, making it impossible to push more material into the cavity. Thicker sections continue to cool and shrink, but there’s nothing the operator can do to correct this once the gate is frozen. This is why plastic products should be designed with uniform wall thickness, and the gate should be positioned to effectively fill all areas of the cavity.
- Processing conditions: Higher melt or mold temperatures, fast injection speeds, or inconsistent cooling can increase shrinkage. If there’s an area in the mold that is hotter than the rest due to improper cooling system, that section of the part will continue to shrink for a longer time, leading to sink-marks and warpage. But this is overall due to poor tool (or part) design, and not specifically processing parameters.
- Orientation and flow: Polymer chains align along flow direction during molding, which can cause directional (anisotropic) shrinkage