Draft angle

Draft angle (often just draft, or mold draft) is a slight taper of a part's walls relative to the direction in which the part is removed from the mold. A wall parallel to the opening direction would rub against the mold along its entire length. With draft, the part breaks free of the mold on the very first movement of the ejector pins and is not damaged. Draft should not be confused with a chamfer on an edge or a weld bevel, which are also angled faces but serve different purposes.

Draft is needed by every process in which a part takes shape in a mold: plastic injection molding, high-pressure and gravity die casting, sand casting, closed-die forging, thermoforming and composite molding. As the material cools, it shrinks: it pulls away from the outer walls of the mold but shrinks onto cores and internal protrusions. Inside surfaces therefore need more draft than outside ones.

Typical values:

For injection molded plastics, the minimum is around 0.5° on polished walls, and 1° to 2° is the usual design value. Textured surfaces need draft according to the texture depth, roughly 1° to 1.5° for every 0.025 mm (0.001 in) of depth. Aluminum die castings typically get 0.5° to 1° on outside and 1° to 2° on inside surfaces, while sand castings and forgings usually need more. In thermoforming, a male (positive) mold, onto which the plastic shrinks, needs more draft than a female (negative) mold.

Draft changes dimensions. On a 50 mm high wall, a 1° draft changes the size by about 0.87 mm on each side, so the width of a box changes by 1.75 mm. CAD systems offer a draft analysis that color-codes faces with insufficient draft as well as undercuts.

Plan draft from the very start of the design of any part that will be produced in a mold, even if the first prototypes are 3D printed or machined. Adding draft to a finished model later changes wall thicknesses and the position of mating faces, and it often triggers a chain of further changes.

Draft always relates to the parting line and the pull direction, which are best defined together with the toolmaker or foundry. Walls without draft are possible only in exceptional cases, for example on side actions moving perpendicular to the opening direction or on very low walls, and they usually make the mold more expensive.

The most common mistake is a part designed without draft because it works in the CAD model and as a 3D-printed prototype. In the mold, the part then sticks, the ejector pins deform it, drag marks appear on cosmetic surfaces, or an expensive mold modification is needed. Another mistake is draft that runs the wrong way relative to the parting line.

On the drawing, define where a dimension applies (at the parting line, at the bottom or at mid-height), otherwise the toolmaker will interpret it their own way. Take draft into account for screw holes and holes for inserts, for assembly clearances and for faces that must seat parallel. If a face must not have draft, for example because of a fit, plan to machine it afterward. For textured surfaces, confirm the required draft with the texture supplier before you finalize the model.