July 2003 Design Tip

Draft your way to a better part!

Figure 1 - Draft definition.

As illustrated in Figure 1, draft is the angle between the direction of ejection of a part from the mold and the surface of the part. Its traditional function is to facilitate the removal of the part from the mold, but in Protomold's Rapid Injection Moulding process, it also permits deeper geometries to be milled while reducing cost and insuring trouble free moulding.

Draft is not always necessary and may be unimportant in shallow features. However, when rib-like features are deeper than 3x their width, draft of at least 0.5 degrees may be required. And drafts of 1.0 degrees and larger may result in lower costs with the Rapid Injection Moulding process.

It is important that textured surfaces have adequate draft to prevent the part from sticking in the mold and to prevent "drag" marks. Protomold recommends that highly textured surfaces (T-1) have a minimum of 2-3 degrees of draft and that more heavily textured surfaces (T-2) have a minimum of 5 degrees of draft.

Figure 2 - Draft can also improve mold life.

Another example of the importance of draft in the manufacturability of your design is illustrated in Figure 2. In this case the draft not only helps the part eject from the mold more easily, but it also minimizes the amount of sliding required between the mold's telescoping shutoff surfaces. A minimum of 3.0 degrees of draft is required for telescoping shutoff features.