January 2007 Design Tip

Fun with Cams

In November's design tip, we used the example of a house-shaped box with a "mousehole" doorway (See figure 1). The outside of the house was formed by the A-side of a simple straight-pull mould; the inside was formed by the B-side of the mould. A shutoff, a raised pad on the surface of the B-side mould, formed the doorway. This month we're going to complicate the process by turning the door shown in figure 1 into a window (See figure 2). By adding material below the bottom of the feature, we've created an undercut feature that cannot be produced in a two-part mould.

Figure 1

Whereas the shutoff that created the doorway in figure 1 can exit the doorway through the open bottom of the feature when the mould opens, a mould feature used to form the window in figure 2 would be trapped when we try to open the mould. The solution is to create a third mould part that moves perpendicular to the direction of mould opening (or parallel to the plane of the mould's parting line). This "side-action cam" fills the space that will become the window. When a side action is used, mould opening drives the cam out sideways as the two primary halves of the mould open, after which the part is ejected. Sometimes Protomold will add other faces to the cam to eliminate parting lines on a critical face. We have done this with the whole front of the house to prevent parting lines below the door/window. You can discuss this with your Protomold customer service engineer.

Figure 2

While a wide variety of parts can be produced in straight pull moulds, side actions literally open up whole new dimensions in mould design. One of the most common applications is the production of through-holes, of which the window mentioned above is an example. Producing a through-hole in the process of moulding saves the time and cost of a separate operation after the part has been moulded. In a straight pull mould, through-holes can be made in the direction of pull. They can also be made in other directions using sliding shutoffs, which work well for some applications, such as the dormer window in the house. See our tip at: Creating Through-Holes. When sliding shutoffs aren't appropriate, side action cams can create holes and other features in other directions as long as the direction of cam travel is perpendicular to the direction of mould opening and the feature is on the outside of the part.

Figure 3 shows a part with several features that could only be made using side actions. The pink circular hole is similar to the house window in figure 2. The purple rectangular indentation can be thought of as a hole that doesn't go all the way through the wall. But like a hole, it would be an unmouldable undercut in a straight-pull, two part mould. The side-action cam, however, is well out of the way before the part is ejected.

Figure 3

In all of the previous examples, cams are used to create small features on a larger part, but this is not the only way they can be used. The part shown in figure 4 uses cams to create the entire circumference of the part, while the indented top and sides are formed by the A- and B-side primary mould halves. Arguably the entire part could be rotated 90 degrees making the sides in the diagram with the A and B mould halves and using side actions to create what are shown as the top and bottom.

Figure 4

In short, now that we've added side action cams to our mould-making tool kit, Protomold is not just for simple parts anymore. Here are some guidelines:

  • We can build up to four separate side actions into a single mould.
  • While they must all be in planes parallel to the plane of the primary mould parting line, they need not all be in the same plane.
  • Side actions can be used to produce features on the outside of a part but not (yet) on the inside.
  • Like primary mould sections, side actions may require drafting. This was discussed in the July 2006 design tip.

If you have any questions regarding the application of side actions to your parts, feel free to contact us.