Basell Pro-fax 6331 plastic part shrinking then expanding after molding

[nady_sophy]

TL;DR

Need to know upon shrinkage and expansion of plastic parts.

Hi,

I'm currently doing a shrinkage study upon this plastic part. I hv been observing how much can the part able to shrink over the time after molding, and now hv been on the 5th month observation. However, on the 5th month, the part starting to expand, compared with the data from the 4th month. Is this possible? Are there any plastic material that can behave this way?

By the way, this plastic part material is Basell Pro-fax 6331 Natural.

 

[jrmichler]

Plastics absorb water. Are you controlling for water absorption due to room humidity? Plastics have high thermal coefficients of expansion. Are you measuring at the same temperature? How accurate are you measurements? Note that accuracy is not the same as the number of significant digits on a digital measuring device. Is the part warping from residual molding stresses?