Warp is caused by a variation in shrinkage. As molten plastic cools within a mold, the spaces between molecular chains become smaller and more compact.

Semi-crystalline materials tend to warp more because the chains bunch up and create crystals causing shrinkage. If the part is not properly designed for shrinkage, it can result in excessive warping. One way to help prevent this is to keep a nominal wall throughout the part. If there is not a nominal wall throughout the part, the thinner sections of the part will cool faster than thicker parts. Cooling lines would need to be strategically placed to create consistent cooling in the part.

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Mold temperature can also affect warpage. If one half of the mold is hotter, the material tends to pull away from the hotter side. Warpage is also caused by the way material flows into the mold. The greater the difference is in flow vs. transverse to flow shrinkage, the greater the stresses, and greater the amount of warp. Another factor that affects warp is the addition of fibers and additives.