Calculate Plastic Mold Shrinkage

Plastic injection molding shrinkage is the contraction of a plastic molded part as it cools after injection. Most of the part shrinkage occurs in the mold while cooling, but a small amount of shrinkage occurs after ejection, as the part continues to cool (especially for Delrin or POM). After that, the part may continue to shrink very slightly for several hours or even days until the temperature and moisture content stabilize. Thus dimensional inspection should wait at least a day.

Plastic injection molded part shrinkage units are expressed as thousandths of an inch per linear inch (0.00X /in/in). Typical shrink rates vary between 0.001/in/in and .020/in/in, with the most common being around 0.006/in/in.

When calculating shrinkage, the tooling engineer simply scales the part by 1.00X. In pre-CAD days, the engineer would expand your part by simply multiplying every number on the drawing by 1.00X.

Plastic injection molding shrinkage varies with wall thickness also. The material supplier will usually provide a range such as 0.005-0.007/in/in with a 0.100 inch wall. If your wall thickness is 0.100 you would go right in the middle with 0.006.

The molder can fine tune the shrinkage of the parts by adjusting the density of the material, i.e. how hard he packs it out, and how long he lets it cool in the tool. If the part is large, tolerances are critical, or you are using a new or unusual material, then it’s a good idea to do some test shots.

Many injection molders have a huge rack of obsolete tools. Find one of these that makes a part somewhat similar in size, shape and wall thickness to your part. Then pay your molder to shoot your resin into it and use the parts to calculate a precise shrinkage for your material. The cost of doing this is small compared to that of reworking or scrapping a tool.

Asymmetrical Shrinkage

A few plastics shrink differently in one direction than in the other. For example, polymers filled with long glass fibers will shrink more in the cross (transverse) direction than the longitudinal (flow) direction. This poses an interesting dilemma for the mold designer. The material supplier will tell you that you have to use a different shrink in the X axis than in the Y. This is fine if you are making popsicle sticks or rulers.

But if your part is complex, like with holes and flow fronts meeting at different angles and running different directions at different places in the part, it is impossible to do, and would be outrageously expensive even if you could. Even round holes would now become elliptical in the tool so that standard components such as core pins cannot be used.

What winds up happening is that the average between longitudinal and cross shrinkage is assumed, everybody signs off on it, and then critical features are then altered or added after first shots.

The bottom line is, do not use asymmetrical shrinkage resins if close tolerances matter.