Shrinkage Factor for Polypropylene

TL:DR – 1.9%.


When injection molding parts, the mold sizes need to be slightly bigger than the final parts to allow for thermal contraction of the plastic. How much bigger? I wasn’t sure.

I wanted to create a lot of data fast, so I designed molds to create four parts with the same thickness, and I (arbitrarily) chose 0.100” as the thickness.

After molding around 60 parts, I measured them all in this scheme:



All of my measurement data is in this Google Sheets Link.

I used a scatterplot and linear regression because I wanted to see the effect of part size if it was present.  The data show a nice linear relationship between the mold size and the finished part size, with a shrinkage factor of 0.981.  A typical workflow for drawing molds includes scaling up the original part and subtracting it from the mold blanks. The amount to scale the part is 1/0.0981, or 1.019.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s