TPE products' shrinkage rate is determined by the difference between the mold size M and the product size D (D-M), divided by the mold size D, then multiplied by 100%. This measures the extent of shrinkage that occurs during production.
The shrinkage rate for TPE materials usually falls within the range of 0.5% to 2%, depending on the formulation of the material. For example, SBS typically has a lower shrinkage rate than SEBS. Additionally, TPE materials with higher filler content have lower shrinkage rates.
Shrinkage can be influenced by several factors, including molding process, mold structure, material properties, and the geometry of the product. Understanding how these variables affect shrinkage performance is crucial for optimizing the production process and ensuring product quality for each specific application.
There are several methods to adjust shrinkage rate. For instance, increasing injection pressure or holding pressure may reduce shrinkage, while higher melt temperature may slightly decrease it as well. On the other hand, too high mold temperatures will cause an increase in shrinkage, and longer mold cooling times can also help to reduce shrinkage. The specific values of these parameters should be fine-tuned based on the specific application.
Moreover, the geometry of the product also affects shrinkage. In general, thick-walled parts have higher shrinkage than thin-walled ones, but exceptions exist. The presence of inserts may also lower shrinkage, and more complex geometries may result in lower shrinkage. Long, slender parts often exhibit smaller shrinkage along their length, while internal holes typically have higher shrinkage than external contours.
Lastly, mold design and fabrication are key factors as well. Larger gate sizes and vertical gate locations can reduce shrinkage. Areas further from the gate generally have lower shrinkage, while areas with greater freedom may experience larger shrinkage. By carefully designing and selecting appropriate molds, shrinkage can be controlled and predicted to some degree.