How Material Shrinkage Affects Dimensional Accuracy of Plastic Parts
Material shrinkage has a major influence on the dimensional accuracy of injection molded plastic parts. Even when the molding machine and mold are manufactured with high precision, it is still difficult to achieve tight dimensional tolerances if the selected resin has a high shrinkage rate.
In general, the greater the shrinkage of the molding material, the more difficult it becomes to control the final size of the plastic part. For this reason, resin shrinkage must be carefully evaluated during material selection, especially for precision plastic components.
1. Why Shrinkage Matters in Plastic Part Accuracy
After injection molding, plastic materials shrink as they cool and solidify. This dimensional change directly affects the final size and tolerance stability of the molded part.
If the shrinkage rate of the selected resin is too large, even a well-designed precision mold may not be able to consistently produce parts within the required dimensional tolerance range. Therefore, the allowable shrinkage range of the resin should always be considered together with the dimensional accuracy requirements of the finished plastic part.
2. Shrinkage Differences Between Resin Types
The shrinkage behavior of different plastic resins varies significantly. One of the main reasons is the degree of crystallinity of the material.
In general:
- Crystalline and semi-crystalline resins usually have higher shrinkage rates
- Non-crystalline resins usually have lower shrinkage rates
- Crystalline materials also tend to show a wider shrinkage variation range
Because of this, crystalline and semi-crystalline resins are generally more difficult to control in precision molding applications than amorphous, non-crystalline materials.
3. Relationship Between Crystallinity and Shrinkage
For crystalline resins, higher crystallinity usually leads to greater volume reduction during cooling, which results in larger shrinkage in the molded plastic part.
As the molecular structure becomes more ordered, the material contracts more during solidification. This is why highly crystalline plastics often require more careful mold design, shrinkage compensation, and process control.
4. Influence of Spherulite Size on Plastic Part Shrinkage
In crystalline plastics, the size of the resin spherulites also affects the shrinkage rate and mechanical performance of the molded part.
When the spherulites are smaller, the gaps between molecules are also smaller. This usually leads to lower shrinkage of the plastic part and can also improve impact strength.
When spherulites are larger, shrinkage may become more pronounced, and the dimensional consistency of the molded part may become more difficult to control.
5. Material Selection for Precision Plastic Parts
When selecting a molding resin for precision plastic components, manufacturers should evaluate not only strength, appearance, and processing performance, but also the shrinkage behavior of the material after molding.
For parts that require high dimensional accuracy, it is important to choose materials with stable and predictable shrinkage characteristics. Proper resin selection helps reduce dimensional variation, improve mold compensation accuracy, and ensure better consistency in mass production.