October 2008 Design Tip

Cams: They're Not Just for Undercuts

By moving in directions perpendicular to the direction of mould opening, side action cams allow the production of parts with "undercuts" that could not be successfully made in two-part, straight-pull moulds. But there are parts without undercuts that can also benefit from the use of cams. The Protomold process (vertical milling) requires increased wall thickness and draft as the part depth increases. Using cams, we can reduce the need for draft and wall thickness is some instances.

Imagine, for example, a thimble, essentially a cup with tapered sides (see figure 1). The obvious way to mould such a part would be in a two-part mould in which the outside is formed by the A-side mould half and the inside core is formed by the B-side mould half (see figure 2). Resin would be injected through a tab gate placed along the parting line at the rim of the cup. If the walls of the part were thick enough, this might work. In a thin-walled design, however, there could be problems.

Fig. 1

Fig. 1

Fig. 2

Fig. 2

First, in a thin-walled design, the resin could cool quickly enough to result in a "short shot," that is, incomplete filling of the cavity. Obviously, this would not be acceptable. Even if the cavity did fill completely there would be two flow fronts meeting on the side of the core opposite of the gate creating knit lines that could substantially weaken the resulting part. One possible solution would be to use a hot-tip gate and inject resin at the closed end of the cylinder—the bottom of the cup or top of the thimble. This may not be possible due to the height of the part, resin compatibility with the hot tip or other unforeseen issues. A better solution might be to lay the design on its side and use a cam to create the core (see figure 3).

Fig. 3

Fig. 3

In this case, the core of the thimble is formed by a retractable cam while the outside of the part is formed by two straight pull mould halves. As shown in the diagram, the parting line runs across the closed end of the cylinder and down the sides. For the sake of symmetry, a tab gate is located along the parting line at the closed end of the cylinder. Resin is injected through the gate and the resin flows down the length of the cavity uniformly.

So how do you identify parts that can benefit from the use of cams? Tall, thin parts with a core, like our thimble, are prime candidates. Another candidate might be a part that, in a straight-pull mould, would require more draft than the designer is able to provide. In that case, side-action cams may completely eliminate the need for draft as the outside face is pulled perpendicular to the part instead of being pushed or pulled from the mould. And, finally, cams can allow production of parts with texture on faces parallel to the direction of mould opening. In simple straight-pull moulds, such texture acts, essentially, as a field of undercuts that can prevent the clean ejection of parts. If there is a design need and we can get a cam on that face, why not? It just may be the face that makes your part.

Basically, cams aren't just for undercuts anymore. Need a flat face to bolt up against a mating part? How about texture? How about your company's logo or part numbers? Adding a cam to the mould may be just what you need.