Plane Truths about Rotational Draft

Protomold suggests a better way to draft those ends.

by DE Editors | Published February 28, 2008

To design a plastic part with rotational symmetry — using the example of a dowel for simplicity’s sake — you create the shape of half the cross-section, in this case a rectangle, with CAD software and rotate that shape through 360 degrees to create the solid.

Knowing that the part will be injection molded and that the parting line of the mold will run along the length of the dowel, the end faces of the dowel will be parallel to the direction of mold opening unless those ends are drafted. There are two ways to draft those ends, one of which works better than the other.

The problematic method is also the most obvious: when laying out the cross-section, tilt the ends slightly. This way, when the shape is rotated, it makes the ends of the finished design shallow cones instead of flat disks. This “rotational drafting” combines the drafting step with the rotation that creates the 3D shape.

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. The key difference between these two approaches is illustrated by what happens as the drafted surface approaches the parting line.

Why is this an important topic for the rapid injection mold market? Protomold’s (Maple Plain, MN; protomold.com) 3-axis milling process plunges only in the z-axis, making 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 shows these rotational draft issues as “required changes” and will 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.

Protomold is a high-tech, entrepreneurial plastic injection molding company located in Maple Plain, MN, just 30 minutes west of downtown Minneapolis. Visit the company online at protomold.com.

Sources: Press materials received from the company and additional information gleaned from the company’s website.

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