5 Methods for Applying Textures to Injection Molding Tools

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When developing parts for plastic injection molding, applying draft (or a taper) to the faces of the part is critical to improving the moldability of your part. Without it, parts run the risk of poor cosmetic finishes, and may bend, break, or warp due to molding stresses caused by the plastic cooling. Equally important, an absence of draft may prevent parts from ejecting from the mold, damaging not only the parts, but possibly the mold itself—a costly and time-consuming detour.

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What is draft in injection molding?

Draft is a taper applied to the faces of the part that prevent them from being parallel to the motion of the mold opening. This keeps the part from being damaged due to the scraping as the part is ejected out of the mold.

Here are five ways that draft can improve part moldability.

Basic Guidelines for Draft on Injection-Molded Parts

The truth is no single draft angle can be applied to all part designs. Factors like wall thickness, material selection, ejection, shrink rates, finish/texture, wall depth, and manufacturing capabilities all come into play. But fear not, there are some simple rules that can be followed that can help.

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When designing a part, apply as much draft angle as possible—a general rule of thumb is 1 degree of draft per 1 inch of cavity depth, but that can change with the aforementioned factors. Try following these general guidelines:

• 0.5 degrees on all vertical faces is strongly advised.
• 1 to 2 degrees works very well in most situations.
• 3 degrees is minimum for a shutoff (metal sliding on metal).
• 3 degrees is required for light texture (PM-T1).
• 5 or more degrees is required for heavy texture (PM-T2).

What can be done when draft may negatively impact part performance? Parts can be designed with 0.5 degrees of draft, or even as little as 0.25 degrees, which is still an improvement over zero draft. Again, the smallest degree of draft that is possible is dependent on material, part geometry, and manufacturer and should be discussed with the manufacturer before finalizing a part with very limited draft.

Many low-volume injection tools are manufactured from aluminum and use CNC machining to mill nearly all of the features in the core and cavities of the mold. With fewer manufactured parts and inserts than a steel production mold, additional draft and wall thickness may be required to produce the same parts with an aluminum mold due to the diameter, length and draft of the end mills that are used to create them. The increase in draft and thickness typically won’t affect the parts, and it may actually improve the performance of eventual production mold

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