May 2007 Design Tip

When you really need to dodge the draft

We've spent so much time reminding designers that parts must be drafted to facilitate ejection from moulds that it seems strange to talk about how to avoid having to draft parts, but it can sometimes be done when absolutely necessary. Keep in mind, however, that drafting is still the key to simplicity of design, ease of moulding, and cost control. As we've said in previous Design Tips, when a surface is parallel to the direction of mould opening, it should be slightly tapered toward the mould; otherwise the mould surface will drag across the surface as the mould opens, damaging the surface. Drafting causes the part face to move away from the mould face as the part is ejected, preventing damage. The slight change in face angle usually makes no difference in either the functionality or appearance of the part. But what if it does?

Probably the most common reason not to draft a surface is to make it fit with other parts of a finished product. Figure 1 is a bracket which bolts to a machine. If the mating face is drafted, the top face tilts at an angle that is unacceptable for this application

Fig. 1

Fig. 1

For a specific requirement, like this one, Protomold can incorporate cam-driven side actions into a mould. These are typically used to create undercuts that could not be moulded in a simple two-part mould. But, because cams move perpendicular to the direction of primary mould opening they can also be used to produce surfaces that are undrafted in relation to the A- and B-side mould halves.

Imagine a part with a surface parallel to the direction of mould opening. Let's assume that we cannot draft the problem surface and must find some other way to protect it during ejection (see figure 1). Protomold would normally require draft on this face as shown in figure 2.

Fig. 2

Fig. 2

If the surface cannot be drafted, so as to move away from the mould as the part is ejected, an alternate solution is to have the mould move away from the surface. This is achieved using a side-action cam (pink face in figure 3).

Fig. 3

Fig. 3

In essence, a side action works like the moving wall in the Death Star trash compactor in Star Wars. As the A- and B-side mould halves open or close along the X-axis, the cam (or cams) move along the Y/Z axes. During ejection, the cam will withdraw leaving no mould wall next to the problem face to cause problems as the part is ejected.

Aside from undrafted faces and the obvious undercut features, there are several other applications for side actions. Raised lettering on a face parallel to the direction of mould opening presents a problem even if the face is drafted; side actions solve that problem (pink face in figure 4).

Fig. 4

Fig. 4

Similarly, texture on a low-draft face, which might not be reproducible in a straight-pull mould, can be produced in a mould with side actions.

There is additional cost for each side action, and there may be some flash between the side action face and the rest of the mould. Therefore they should be considered an option with tradeoffs, not a panacea for all undercuts or zero-draft faces.

One more application is the production of decal recesses. These are shallow undercuts, but they can simplify the placement of decals and, if they fall in faces that are parallel to mould opening, are made possible using side actions. ProtoQuote® now points out areas that can be produced using side actions, giving users the option of redesigning their parts for standard straight-pull moulds or using this more advanced capability.