September 2006 Design Tip

Deep Thoughts

At Protomold, all parts must fit in a 355mm x 190mm x 76mm box. But there is one other size you should remember in designing parts for the Protomold rapid injection moulding process. Our cutters extend 25.4mm, 38.1mm or 50.8mm from the collet attaching them to the milling machine. There's sort of a sliding scale, based on the diameter of the cutter and the draft of the wall being cut. That means that cuts into a mould that are near a wall are limited to 25.4mm, 38.1mm or 50.8mm in depth. Depending on your design, that limitation may or may not be a problem. If it is an issue in your design, there are several ways you might be able to resolve it.

Figure 1

Figure 2

The reason for the limitation is simple. Figure 1 shows a cut that cannot be made due to the length limitation. As you can see, the "shoulder" of the collet and the shoulder of the mould being cut collide, which limits the reach of the cutter.

Figure 2 shows one possible solution to the problem. Here, the straight side of the cut into the mould has been stepped to allow the collet to reach farther into the cut without hanging up. The resulting feature on the part will look like a layer cake instead of a straight-sided cylinder but that may not adversely affect the finished design.

Figure 3

Figure 4

Figure 3 shows still another way of solving the problem. Here, the side of the cut has been slanted rather than stepped. As in the stepped version, the change in wall geometry allows the collet and cutter to reach into the cut, eliminating the problem and extending the downward reach of the cutter. This can be applied to instrument housings and enclosures by drafting the walls inside and out, resulting in geometry that can be designed and milled deeper.

Obviously, some shapes cannot be slanted or stepped enough to accommodate the collet of a milling machine (although any “straight” cut will have to be drafted somewhat to allow clean ejection from the mould). One such example is a narrow rib that stands taller than the 25.4mm, 38.1mm or 50.8mm that corresponds to the wall thickness. (See Figure 4.) Thickening and/or adding draft to the rib enables the use of a longer cutter, but there are limits to the amount of draft which can be added before the base of the rib becomes too thick. Often �� or 1� of draft is a good compromise.

Figure 5

Figure 6

Finally, in some cases, it may be possible to resolve the issue by moving the parting line of the mould. Figure 5 shows a cross section of a mould for a 76.2mm long tube. Since the parting line is placed at one end of the tube, the cutter would need to reach 76.2mm into the cut for the thin wall of the tube, which it cannot do.

Figure 6 shows the solution. Here, the parting line has been moved to a point midway between the two ends of the tube. In this case, the cutter only has to reach 38.1mm into the cut in each mould half, which it can do. Problem solved!

So next time you need a “straight-sided” cut deeper than 38.1mm, consider:

  1. Slanting the sides
  2. Stepping the sides, or
  3. Moving the parting line to reduce the depth of the cut by dividing it between the two mould halves.