July 2006 Design Tip

Bad CAD and Side Action Drafting

By now you know that Protomold customers can design their parts with undercuts that are formed using side-action cams. These cams form the undercut in the closed mould and then automatically retract, allowing the finished part to be ejected. The areas formed by such side actions must be accessible from the exterior of the part and the cams must be pulled parallel to parting line—in other words, perpendicular to the direction of mould opening.

We have talked in other Design Tips about the importance of draft on the A and B sides of your part. If your design requires side actions, those must be drafted as well. Drafting a side action can be slightly more complex than drafting a part for removal from a straight pull mould. The reason is simple and can be demonstrated with a somewhat exaggerated example.

Imagine a hollow, baseless plastic pyramid. To make our point (pun intended), the walls are canted inward to meet at the tip, a rather extreme form of drafting. The structure has been cored out for ease of moulding. Now imagine a mould to make this pyramid: A big block with a pyramid shaped cavity is the A-side of our mould. A slightly smaller pyramid is the B-side / inside / or ejector side of our pyramid.

Now imagine a circular hole in one face of the pyramid, a sort of porthole. The hole is perpendicular to the face, is slightly tapered and is fitted with a similarly tapered cork. This cork is the side action cam, and the taper of the cork (and hole) are the draft that allows easy removal of the “cam” without sticking to or scraping the sides of the hole.

It all seems very logical, but there’s a problem. As mentioned earlier, side action cams normally move perpendicular to the “pull line” of the mould. In this case, the pull line of the pyramid mould is straight up through the center of the pyramid, so the pulled cork would move parallel to the ground rather than perpendicular to the pyramid’s slanted wall. As the diagram shows, when the cork is pulled to the side, its lower edge is trapped behind the lower lip of the hole (indicated by red “undercut” lines). So, even with a side action cam, we are left with an undercut. (See Figure 1.)

Figure 1

Fortunately, there is a simple solution. We can simply create and draft the hole perpendicular to the direction of pull, rather than relative to the surface of the part. As Figure 2 clearly shows, the side-action cam can now be pulled straight out to the side without hanging up on the edge of the hole.

Figure 2

In most 3D CAD programs the adjustment is very easy. It’s a matter of reference. CAD programs typically apply draft to a feature or specific face in relation to another face or plane. As our pyramid porthole demonstrates, using the face of the model around the undercut feature as the draft reference can spell trouble. As in this case, you can end up with geometry that can’t be pulled using a side-action parallel to the parting line. The solution is to create a plane perpendicular to the parting line and create the desired feature in relation to that plane. Then you can apply draft in reference to that plane, which will allow your feature to be pulled parallel to the parting line.

What does all of this mean? Simply put, be aware of your draft, it may be the difference between getting your part moulded or going back to the drawing board.