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How does shrinkage affect TPEs?

Shrinkage

As TPEs cool from their molten state, the molecules align with each other and shrink the overall size of the molded part. While usually only in the thousandths of an inch per inch range, shrinkage can dramatically affect the molding and removal of the part as well as the appearance of the final part.

If shrinkage is uneven, a part meant to lay flat can bend or warp. Additionally, in applications with tight tolerances, unexpected shrinkage may affect the fit of a part in an assembly.

For these reasons, shrinkage generally must be taken into account in production.

Part Removal

When parts contain cores or cut-outs, as the elastomer shrinks it can tighten around the tooling, making removal difficult. Mold design, mold surface finish and processing conditions can lessen this effect, and make even automatic removal possible.

Molding Conditions

Molding conditions can dramatically affect the amount and nature of shrinkage. Going quickly from a state of high stress to one of low stress can increase the amount of shrinkage. Quick cooling of a part as well as very high injection speeds or pressures can also affect shrinkage. Consult GLS for more information on how molding conditions affect shrinkage.

Design Considerations

Due to shrinkage, a mold must be cut larger than the desired size of the part. Generally, exact shrinkage values cannot be known until a specific part is molded, so it is always best to be conservative and use a prototype mold if at all possible.

As with other elastomer properties, shrinkage often varies with the direction of polymer flow. Gate location will determine the direction of flow into the part and thus the shrinkage direction. Also, some TPEs are more isotropic than others, meaning that they shrink more in one direction than another. This must be taken into consideration when designing the mold. Consult your TPE supplier for more information on how design considerations affect shrinkage.

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