If we compare this to the tolerance of ☐.005 in., the molder might be successful at molding good parts with a single lot of material, and that might even be a stretch. Based on our previous calculations, the molder could see a range of shrinkage of 0.2619 in. Now, we need to calculate the range of shrinkage the molder could expect to see if we used a single gate at one end of the part (0.3395 in. x 0.035 in/in = 0.3395 in.), then multiply the nominal dimension by the lowest shrink rate (9.700 in. So, let’s walk through a small example using the previously mentioned tensile test bar.įirst, we need to take the nominal dimension and multiply the highest shrink rate (9.700 in. We often hear the question, “Why does it matter what the range is? I’ll just select the middle, and it’ll be fine.” It matters because the molder will struggle to make parts with different lots of material that roll in through the door. For example, the shrink rate of a Polypropylene (PP) can be from 0.008 to 0.0035 in/in. Materials with a semi-crystalline structure have very high shrink rates.
