Deep Thoughts
At Protomold, all parts must fit in a 14” x 7.5” x 4 ” box. But there is one other size you should remember in designing parts for the Protomold rapid injection molding process. Our cutters extend 1.0", 1.5" or 2.0" 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 mold that are near a wall are limited to 1.0", 1.5" or 2.0" 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 mold 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 mold 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 mold). One such example is a narrow rib that stands taller than the 1.0", 1.5" or 2.0" 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 ½° 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 mold. Figure 5 shows a cross section of a mold for a three inch-long tube. Since the parting line is placed at one end of the tube, the cutter would need to reach three inches 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 1.5 inches into the cut in each mold half, which it can do. Problem solved!
So next time you need a “straight-sided” cut deeper than 1.5 inches, 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 mold halves.
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Design Guidelines
As always, you can visit the Protomold Design Guide for helpful Rapid Injection Molding design information.
The difference between high density polyethylene and vinyl is the presence of:
Last month's question/results
According to The Old Farmer’s Almanac, which is true of Legos?
- Legos were originally introduced as a puzzle rather than a building system.
- The name “Lego” comes from the Greek word for brick.
- Early shipments of Legos were rejected by retailers due to a tendency to melt in direct sunlight.
- Six Lego blocks can be fitted together in over 100 million different ways.
- Legos have been made from acrylonitrile butadiene styrene (ABS) since their original introduction.
The correct answer is D. Six Lego blocks can be fitted together in over 100 million different ways. The guesses are represented in the following chart:
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