September 2007 Design Tip

The inside scoop on outside threads

As every engineer worth his or her sodium chloride knows, one of the most fundamental mechanical devices is the inclined plane, and a spirally-threaded cylinder or screw may be its most commonly used form.

If you are a plastic part designer, you've probably incorporated threads into a design or will in the future. We'd like to pass along some suggestions concerning the geometry of outside threads and the limitations of the rapid injection moulding process to keep in mind when the time comes.

Most threads have undercut areas. It's just a fact of the geometry as the surfaces of the screw wrap around, regardless of the orientation of the screw body. There are several ways to deal with these undercuts.

The first method is driven by the primary rule of engineering: KISS: keep it super simple. Fortunately, for some threads, we can ignore the undercuts, machine what we can, and get a functional thread. For example, Figure 1 illustrates a thread design that cannot be machined exactly as it's designed. The blue faces are assigned to the b-side of the mould, and the green faces are assigned to the a-side of the mould. The thread faces were split at vertical draft. Unfortunately, some faces overhang others (shown in red as undercut faces), creating a mould that (even if we could machine it) will interlock and can't open.

Fig. 1

Fig. 1

Figure 2 shows the solution. We split the screw at a horizontal plane that passes through the axis. Faces above the plane are b-side, faces below are a-side regardless of whether they have reverse draft (shown by the dark blue faces). When we design a mould for this thread and machine it, we will leave a little extra metal at the undercuts. When we mould parts, there will be a little plastic missing in these areas. The threads will be a little thinner than the CAD model in those areas, but in most cases you can't tell the difference without a close examination.

Fig. 2

Fig. 2

If the first approach doesn't work, perhaps with an acme thread or on a large screw, a second method would be to modify your design to eliminate the undercut areas from your thread. We call this a "half-thread" design. It involves cutting the threads off the sides of your screw (see figure 3). The disadvantages of this are additional design, less thread strength and intermittent threads which might be difficult to screw in.

Fig. 3

Fig. 3: Half threaded part — no undercuts

Lastly, if you need the full thread, the way to go might be to use cams. With a cam (side action) on each side of the part, the undercuts can be pulled and you get the full strength of the thread. Figures 4 and 5 illustrate this approach. Disadvantages of this method include four parting lines instead of two on your thread and the additional cost for the mould.

Fig. 4

Fig. 4: Using side actions (cams) to produce undercuts

Fig. 5

Fig. 5: Faces assigned on thread using side actions

Can we do outside threads? You bet! We have a whole toolbox of methods for creating external threads on your plastic part. To learn more, submit a 3D CAD model for a quote or call Protomold at 763-479-3680.