Draft on Injection Molded Parts

Draft angles are used in the draw direction on plastic parts to aid ejecting the part from the mold. On the part in Figure 1, because of the draft angle, after initial breakaway, the part will no longer contact the mold. This greatly reduces ejection force, part scuffing, and the ability to eject the part. In the case of a cup shaped part, the draft on the cup's sides allows air to get inside the cup shortly after the cup has started to be ejected; otherwise, a substantial vacuum would build up.

Draft is important for all deep draws, including ribs and bosses. The rib in Figure 2 shows an example of how a rib might be drafted to facilitate ejection.

1/2° draft per side for ribs is common. In general, the larger the draft angle, the easier the ejection, but on long draws, large angles will significantly change the shape of the part. Draft angles generally range from 1/2° to 2°.