April 2005 Design Tip

A new slant on draft

Imagine a piston in the cylinder of an engine. It seems like a tight fit, yet the parts slide smoothly with no damage. In fact, there are several reasons that a piston moves so smoothly: first, there is space between it and the cylinder; second, both have hard, polished surfaces; and third, the surfaces are liberally lubricated. Eliminate the space or the lubricant and engine damage is guaranteed.

The same thing can happen when you pull a straight-sided plastic part out of a straight-sided mould, because there is neither space nor lubricant, and plastic isn't very hard. To prevent damage or drag marks on the part, surfaces that are parallel to the line of mould opening may have to be drafted - angled away from the line along which the part will be ejected (see figure 1). This causes the part wall and mould wall to move apart during ejection.

Figure 1:

Your CAD package will not tell you which surfaces should be drafted, but it may indicate which surfaces are drafted (or reverse-drafted). When you submit your design for a quote, however, Protomold's online ProtoQuote software identifies, in purple, yellow, or orange, surfaces that need additional draft. Arrows show the direction in which the feature should be drafted (see figure 2).

Figure 2:

Shallow, smooth-sided features may not require draft, but deep ribs, holes, inside cores, and multiple standing features usually do. Textured surfaces have a particular tendency to stick to mould walls, so light (T1) texture requires 3 degrees of draft, and heavy texture (T2) requires at least five.

In figure 3, a protruding arm of the bottom mould half (the telescoping shutoff) that forms the face and hook of the clip must be drafted by 3 degrees both to protect the part and to prevent mould wear due to rubbing of the shutoff against the top mould half.

Figure 3:

Finally, proper drafting allows the cutting of deeper mould geometries at lower cost. A good rule of thumb in parts design, according to Protomold's Kevin Crystal is, "When in doubt, draft it."