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