Understanding Warpage

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781-789-3380 or send an email to mhowards@3DShapes.com

You can easily see how a part warps. Figuring out why can take more work.

As an analyst, it took me a long time to fully comprehend warpage analysis results and be able to explain them without confusing the reader.

Almost anything that is not uniform can contribute to warpage. That includes temperature, pressure, wall thicknesses, melt-front advancement, etc. That means almost anything from the start of design to ejection of the molded part may play a part.

Rigidity of the part and material oppose any tendency to warp. Therefore, a thin flat part made with PP may warp whereas the same part in ABS or with a small return wall around the outside edges may be completely flat. Many times, the geometry of the part plays a bigger part in warpage than anything else. In that case, no matter what is done to improve the process the part has similar warpage.

The part will always be affected by normal material shrinkage. That’s what you are accounting for using a mold shrinkage value when designing the tool. If a part has little molded-in stress and shrinkage is uniform then it will simply get smaller. All parts have molded-in stress and shrinkage is never completely uniform. Based on the part geometry and material, the part may warp or it may have a lot of molded-in stress or it may have a little of both.

Although a single mold shrinkage value is selected during mold design and often work well, shrinkage is not uniform throughout a part. If the value is properly selected then it will end up being close to the average value in the part.

Another point of confusion relates to datums. By default Moldflow shows warpage based on an x, y, z datum that is at an “average” location. If you supply one or more datums based on important features of the part and quality control methods, the results of the analysis will be much easier for you to understand. Warpage results can be reported to match what is important to you, just ask.