Shrinkage, it’s no laughing matter (especially if you need to find an alternate resin)

Shrinkage, (with all due respect to the Seinfeld skit), happens to your plastic/composite at the end of the injection molding process and needs to be dealt with. Molded resin shrinkage is both going to happen, and a factor to account for. The change in density between, A – the resin temperature during the molding process, and B – the resin losing heat and gradually equalizing to ambient room temperature, is what results in the plastic shrinking.

Every composite injection mold designed is based upon the resin that the part is destined to be made in that mold. The shrinkage of molded plastic parts can be as much as 20 percent by volume, when measured between the ranges of processing and ambient temperature.

If your molder is experiencing delivery delays or even availability, you may be asking about alternative resins. (Unless of course you were forward thinking enough to determine an alternate resin at the time of mold design).

Resin suppliers are more than happy to offer up alternative resins. You can ask them about performance properties on your existing resin and similar capabilities of alternates. Shrinkage rates rarely come up in the conversation. If you don’t think to ask, we certainly will.

It is usually possible to sample a mold in an alternative resin, but the shrink rate may make that easy or difficult.

So, it is easiest to identify a primary and secondary resin when developing a new component, so the mold can be designed for both. An alternate can be specified in after the fact, but it may limit your available options, due to the mold and a specific shrinkage being planned upon. It is possible yet not typical to be able to modify a mold to a new resin’s shrinkage factor. Lastly, a whole new mold may be required if an alternate resin must be specified. And that IS no laughing matter!!

Resin issues? Talk to us, we are the material experts.