September Design Tip

Sometimes You Need to be Square

Much of today's design aesthetic seems to be about swooping curves. Nevertheless, sometimes you just need a nice square corner. When it comes to moulded plastic parts, inside corners are easy. The inside corner of a part is formed by the outside corner of a mould, and the mould is formed by machine tools, which are great at cutting straight lines. Where two of those lines cross, you get an outside corner that is as sharp as the material will allow. If you want a sharp outside corner on a part, however, that's another matter. (Click here to review outside and inside corners further.)

The problem is that you'll need to cut a corresponding inside corner in the mould, and the lines that define that corner can't cross, they must simply meet. Since cuts are typically made by a rotating mill, the corner cannot be sharper than the radius of the cutter. Large diameter cutters produce larger radius inside corners than thinner cutters, and deeper cuts generally require thicker cutters. As a result, milled inside corners of moulds will all have rounded radii (see figure 1), and deeper "square" holes will have more rounded corners. The rounded inside corners of the mould will produce rounded outside corners of the resulting part.

Fig. 1 � Cavity in the mould with rounded inside corners

Fig. 1 - Cavity in the mould with rounded inside corners

If you think you need a square outside corner on a part, there are ways to do it, but you first might want to consider whether you actually need one. Rounded corners are aesthetically pleasing and can reduce the likelihood of warp and internal stress as moulded parts cool. And if you are forming a square peg simply to fit into a square hole, the corners of the peg don't actually have to be sharp. The flat sides of the peg will fit against the sides of the corresponding hole in your assembly and keep the mating parts from wobbling or rotating even if the corners of the peg don't fully fill the hole's square corners.

If, however, you actually need moulded, square, outside corners on your part, there is a way to produce them. In many cases, we can use a selective EDM (electro-discharge machining) process to produce sharp corners in the mould, which in turn produces sharp corners on the resulting part. This process uses CNC machining (which, as we said earlier, is good at producing outside corners) to produce a graphite electrode in the desired "sharp outside corner" shape. The electrode is then used to cut a sharp inside corner in the mould, and the mould produces parts with sharp outside corners.

In summary, if you think you need square outside corners on a moulded part, first consider whether somewhat rounded corners will do. Keep in mind that Protomold's process can produce "sharp-ish" corners, the radius of which will vary with the depth of the required cut in the mould. In figure 2, the portion of the corner shown in gold has a radius of 0.4mm. The deeper cut, shown in red, has a radius of 0.6mm. Finally, if you need sharper corners than can be milled by our normal process, talk to a customer service engineer to see whether EDM is the answer.

Fig. 2 � Square cavity with small radius from Protomold's milling process.

Fig. 2 - Square cavity with small radius from Protomold's milling process.

If you need corners that are even squarer, you can request Protomold's EDM process. Contact your account manager for more details.