How to Compare Plastic Shrink Rates

Find a mold that closely resembles the size and flow paths that best match the characteristics to be taken up for comparison. During molding, the polymer chains of the plastic tend to line up in directions based on how the plastic flows through the mold. Finding a mold with a similar wall thickness and width-to-length ratio compared to the planned part will help to better characterize how the planned part will shrink.

Mold 10 samples from each of the selected resins at the processing conditions (temperatures, pressures, injection speeds and times, pack and hold times) that meet the planned production conditions. Molding conditions are selected with several goals in mind. It is desirable to mold as quickly as possible to minimize costs, but still there are physical limitations from the melt characteristics of the plastic and the dimensions of the part that impact the actual molding conditions. Material suppliers and molding process engineers can help in determining the optimal conditions for production.

Allow the samples to cool for 24 hours in a temperature and humidity controlled room. This will help the samples to attain full dimensional stability. Plastic shrinks as it cools, and it is important to get it to a point where it has reached its final size and is stable (no longer shrinking), especially when comparisons on the thousandths of an inch are being considered.

Measure parts with the appropriate devices. A CMM is ideal, although calipers or other tools may suffice. Measure the parts along the main flow path as well as in a perpendicular direction, as shrink rates often vary by flow direction.

Tabulate measured data for desired comparison.