January 2008 Design Tip

The Plane Truth about Rotational Draft

Let's say you're designing a plastic part with rotational symmetry; for simplicity, we'll make it a dowel. In your CAD software you would create the shape of half the cross-section — in this case, a rectangle (see Figure 1) — and rotate that shape through 360° to create the solid. So far, so good! However, knowing that your part is going to be injection moulded and that the parting line of the mould will run along the length of your dowel, you realise that, unless you do something, the end faces of the dowel will be parallel to the direction of mould opening. In other words, those ends need to be drafted. There are two ways to draft those ends, one of which works better than the other.

Fig. 1

Fig. 1: Undrafted Dowel Half Cross Section

The problematic method is also the most obvious: when laying out the cross section, you tilt the ends slightly (See Figure 2).

Fig. 2

Fig. 2: Pre-drafted Dowel Half Cross Section

This way, when you rotate the shape it makes the ends of the finished design shallow cones instead of flat disks (See Figures 3a and 3b). This is "rotational drafting", and it combines the drafting step with the rotation that creates the 3D shape.

Fig. 3a

Fig. 3a: Part Design Resulting from Rotational Draft Method

Fig. 3b

Fig. 3b: Part Design Resulting from Rotational Draft Method

The preferred method is "planar drafting" and it is a separate step from the rotation that creates the basic part. In this case, each half of the end surface is drafted separately in a plane angled away from the parting line (See Figures 4a and 4b). The key difference between these two approaches is what happens as the drafted surface approaches the parting line.

Fig. 4a

Fig. 4a: Part Design Resulting from Planar Draft Method

Fig. 4b

Fig. 4b: Part Design Resulting from Planar Draft Method

Why is this an important topic for rapid injection moulding? Protomold's 3-axis milling process plunges in the z-axis only. This makes a planar draft a simpler, more reliable cut than a rotational draft, due to the latter's varying cut angle. For this reason, the ProtoQuote design analysis will show these rotational draft issues as required changes and ask for increased draft, wall thickness or both. The easiest fix is to replace the rotational draft with a planar draft.

Sometimes a flat world is just a lot easier to navigate.