Get Draft Angles Right in Plastic Part Models

Draft angles are essential for ejecting plastic parts cleanly from molds. Without them, parts stick, warp, or get surface scuffs. The right angle depends on the material, texture depth, and part depth. A shallow texture needs more draft, typically one degree per 0.025 mm of texture depth. For smooth surfaces, start with one degree, but deep ribs or bosses may need two or three degrees.

In CAD, use the draft tool on all vertical faces, including inside and outside walls. Pay special attention to internal features like ribs and bosses. They act as undercuts if left vertical, so add draft to them too. A common mistake is drafting only the outer shell and forgetting the core side.

To verify, run an angle analysis on your model. Look for faces showing zero or negative draft. Adjust the pull direction first, then apply draft. If you see red, fix it. For quick checks, use the rule of thumb: add one degree per 25 mm of depth, but never less than 0.5 degrees.

Finally, coordinate with your mold maker early. They may ask for specific draft values based on their ejection system. Getting this right upfront saves costly mold revisions later. Test a simple protrusion first to confirm ejection before finalizing the whole part.

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