May 2008 Design Tip

Put Your Parts on a Diet

They say "Less is More," and while that may be a dubious premise in some areas — your next salary review, for example — it is often true of moulded plastic parts. We've written in prior Design Tips about the consequences of overly thick features. As liquid resin cools, it can lead to all sorts of problems — sink at the part surface, hidden voids within the feature, or warp as areas of different thickness solidify and shrink at different rates, resulting in undesirable bending. In addition, unnecessary thickness can throw off part dimensions, reduce strength and necessitate post-process machining.

In the past, we've suggested that large features can be "cored," hollowed out from the bottom, leaving the original surface shape while reducing wall thickness. Think of a solid square cube being turned into a hollow box. If the part doesn't have a "back side" from which coring can be done, there's always the option of splitting the part in two, moulding two hollow halves and then joining them; but that can be complicated and expensive.

What we're suggesting here is a form of coring done from the outside. As materials get stronger, you can see this technique being used in many places. The holes in breeze blocks reduce their weight and the cost of concrete without significantly reducing their effective strength. Sporting equipment made of high-tech materials — knives and bicycle parts for example — are increasingly sporting cutouts of various shapes and sizes that leave the part strong enough for its intended use but far lighter than a solid part would be. A common term for this is "skeletonising."

Our example from the plastics world is a screwdriver handle. The shape shown in Figure 1 is designed to fit the human hand, but in doing so, utilises far more resin than is necessary to transmit the force exerted by the hand to the shaft of the screwdriver. Coring out the handle in the traditional sense, besides complicating manufacturing, would defeat the handle's purpose by eliminating material along its central axis, where it connects to the metal shaft.

Fig. 1: Solid screwdriver handle

Fig. 1: Solid screwdriver handle

A more practical solution is "external coring" as shown in Figure 2. Here we significantly reduce the amount of material required, while producing a design that can still be made in a two-part mould. The overly thick shape has been converted in two ways. The first is a series of relatively thin walls, creating a rib cage structure. The second is to core out the mass with a number of pockets. Both examples retain the shape of the handle while reducing excess material and the potential for injection moulding process issues. We may also improve the user's grip through the addition of the ridges.

Fig. 2: Externally cored screwdriver handle

Fig. 2: Externally cored screwdriver handle

This kind of external coring can be easily accomplished in most 3D CAD programs. Of course, the surfaces of the individual walls now have to be drafted in the direction of mould opening to facilitate ejection, but this is also easy using 3D CAD. And, last-but-not-least, your part now has a cool, modern look.