March 2007 Design Tip

The Orphan Fillet

Fillets, in certain geometries, can be a problem. Take, for example, the part shown in Figure 1. As produced by the designer's CAD package, the green faces of this part (including the left and far sides that don't show in the diagram) are created by the A-side mould and drafted toward the A-side. In other words, they taper slightly toward the bottom of the diagram. The surfaces shown in blue are created by the B-side of the mould and are drafted toward the B-side, which means they taper toward the top of the diagram. The problem is with the fillet.

Fig. 1

Fig. 1

Figure 1 actually shows the 3D CAD diagram as evaluated by Protomold's ProtoQuote® quoting and analysis software. The area noted in red, the fillet, is where ProtoQuote® has identified a problem. The customer's CAD software, recognizing that the fillet connects to both an A-side face and a B-side face has tried, unsuccessfully, to resolve the conflict between two opposite draft directions. The reason it has been unsuccessful is that, in reality, this fillet can not be part of either side. In other words, this feature is an orphan.

To be clear, this is a fillet which connects an A-side drafted face to a B-side drafted face, and is over (or could be under) a flat surface, which creates undercut geometry.

If this fillet were created by the B-side of the mould, it would have to taper in the same direction as the adjoining blue face, that is, toward the top of the part. The problem is that the adjoining green face, which is part of the A-side, is tapering in the opposite direction, toward the bottom on the part. The result would be a misalignment—a step—along the line where the fillet (red) meets the A-side face (green).

If, on the other hand, this fillet were created by the A-side of the mould, there would be a problem in the area that appears, in Figure 1, as a small red triangle on the foot of the part at the base of the vertical tower. The part of the A-side of the mould that created the fillet would trap the plastic part under it at that triangle when the mould opens. (Figure 2 shows the A-side mould itself and in red the projecting feature that would trap the part.)

Fig. 2

Fig. 2

In Figure 1, Protomold's analysis software has attempted, unsuccessfully, to resolve the conflict by dividing the fillet between the two mould halves. The red half has been assigned to the A-side, the blue half assigned to the B-side. The bright blue lines indicate the undercut area. Unfortunately, the problem of mould entrapment remains, as can be seen in both Figure 1 and Figure 2. Figure 2 also shows a secondary problem. The area of the mould that is supposed to create part of the fillet comes to a "razor" edge. Such an edge would be subject to extreme wear and, as a result, allow the formation of undesirable flash.

There are three possible solutions to the problem of the orphan fillet:

  1. The designer could redesign the part so that everything was drafted toward the top. In that case, the entire part, fillets included, could be moulded in the B-side mould, with the A-side just forming the base of the part.
  2. The designer could avoid vertical fillets that connect A-side and B-side drafted faces. This would prevent the problem in the first place.
  3. This part could be manufactured as designed with the addition of a side-action cam. Protomold can include up to four such cams in a mould, but this would increase the cost of the mould.