When thermoplastic elastomers (TPE) begin to cool from a melted state, their molecules align with each other, causing an overall shrinkage in the size of the molded part. Although this shrinkage is usually only in the range of a few thousandths of an inch, it can significantly affect the molding and demolding of the part, as well as the appearance of the finished product.
If the shrinkage is uneven, a part that should be flat may warp or curl. In addition, unexpected shrinkage in applications with strict tolerances can affect the fit of a component with the overall assembly.
For these reasons, shrinkage must typically be taken into account during production.
When a part contains a core or hollow section, as the elastomer shrinks, it tightly grips these areas of the mold, making demolding difficult. Mold design, surface finish, and processing conditions can all minimize this effect or even allow for automated demolding.
Molding conditions can significantly affect the degree and nature of shrinkage. If a high-stress state is quickly transitioned to a low-stress state, the degree of shrinkage will increase. Rapid cooling of the part and high injection speed or pressure can also affect shrinkage.
Due to shrinkage, the mold must be machined slightly larger than the size required for the part. Typically, the actual shrinkage value can only be determined once a specific part is molded, so it is always best to err on the side of caution. Prototype molds can be used if possible.
Like many other properties of thermoplastic elastomers, shrinkage often varies with the flow direction of the polymer. The location of the sprue will determine the direction in which the molten material flows into the mold, and thus the direction of the shrinkage. Additionally, some TPEs are more anisotropic than others, meaning they may shrink more in one direction than in another. This factor must be taken into account when designing the mold.
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