DRAFT

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GUIDELINES:

For parts that are formed into a female mold with a texture allow one degree of draft for every thousandth of an inch of texture depth. With parts formed over a male mold allow three degrees of draft as a minimum.

OVERVIEW:

The need for draft is driven by the coefficient of thermal expansion of the plastic. As the part is held in the mold it is cooled below the set temperature. This temperature change can be anywhere from 100 to 300 degrees depending on the resin. This change in temperature coupled with the coefficient of thermal expansion will cause the part to shrink. Draft also allows for better material distribution by opening up a corner area to allow clearance for a plug assist to push material.

SPECIFICS:

Parts can be molded with little or no draft. However, there is high probability that the part will not release from the mold or will have severe scuffing from any texture that is on the mold. By designing in draft the part is able to release from the mold much sooner in the release cycle. The greater the draft the quicker the release and the lower the risk of part hang-up or texture scuffing.

Draft also "opens" up a corner (two or three sided) and allows for a better draw ratio. The drafted wall also allows for an assist plug to move material down into the mold with less risk of the plug hitting the sidewall.

Almost every plastic molding process requires draft. In thermoforming the advantage of a one sided molding process becomes apparent with draft. In a female mold the material wants to shrink away from the sidewall of the mold. There is no mold "core" to prevent it from doing so. In this respect the shrink of the part actually helps keep the draft requirement to a minimum. On a male mold the part actually shrinks tighter on the mold making the draft requirement greater.

A texture on the mold actually represents a series of undercuts in the mold. The deeper the texture, the greater the undercut and the greater the draft angle that will be required. Because on female molds the part shrinks away from the mold there is less chance the part will scuff during release.

A final note on draft. Be sure that there are no male features on the female mold that will prevent the part from shrinking away from the sidewall. A male section at the bottom of a female mold will be a big problem unless allowance for adequate draft has been made.