Draft angles are needed so that a plastic part can be released from the mold without distortion or damage. The high pressures of injection molding force the plastic to touch all the surfaces of a mold’s cores and cavities. The cavity becomes so tightly packed that it is often difficult to remove the part. Sometimes, shrinkage will actually make it easier to take the part out of the mold, but in other cases, shrinkage will cause the part to stick to the mold’s cores. These natural occurrences call for draft angles.

No single draft angle is suitable for all parts. Each individual part requires a unique specification. Large parts call for more draft than small parts. Thin-walled parts that undergo high-pressure injection molding need more draft than parts that are subjected to lower-pressure molding. When calculating appropriate draft angles, the plastic material’s shrinkage and physical properties are also considerations. Sizeable draft angles and smooth polish should be used for parts molded in strong, stiff, abrasive, and gluey materials. Smaller draft angles can be utilized on soft, malleable, and slippery plastics.

From a cost and manufacturability viewpoint, the ideal draft angle is the largest angle that will not lessen the customer’s satisfaction with the product. The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are the authority on what is the lowest acceptable draft.

I hope this gives you some ideas for your next molding project. If you have any questions regarding this or any other injection molding information, please feel free to contact me at (770) 901-3200.