Plastic injection molding offers many benefits. In large runs, molding is much cheaper than machining the parts individually, and the manufacturing speed is much faster. However, when designing a part that will be molded, there are some special considerations that need to be applied.
Because plastic molded parts are injected into mold cavities (voids) and over cores (standing steel), they require draft in order to release from the mold. Draft is the amount of taper on the vertical walls of the plastic part. Without draft, a part will either not eject from the mold, or sustain damage during ejection. Typically, draft angles between 1° and 2° are required, but can vary depending on part restrictions and specifications.
Whenever possible, plastic parts should be designed with a nominal or consistent wall thickness. All injection molded parts shrink as they cool, and differences in the thickness of the part will cause the part to shrink at different rates. Thicker areas of the part will shrink more than thinner areas. One result can be deformation of the part. Another is sink marks, areas of the part that are "dimpled" due to excessive shrinkage. It is often necessary to remove, or "core out" thick areas of the part to avoid these conditions.
A part designed with a nominal wall thickness may not be as strong as a thicker part, so ribs can be used for strength. When utilizing ribs on a part, their thickness should be no more than 70% of the nominal part thickness to avoid sink marks. As with other vertical surfaces on the part, ribs should also have draft.
Because the two halves of an injection mold separate in opposite directions, the features of the plastic part generally need to be able to release in that same direction. Any part features (holes, undercuts, shoulders, etc.) that cannot release in the direction of mold separation will require either redesign or special attention.
For example, the axis of a hole that is formed in the part must be parallel to the direction of mold separation. There are some exceptions to this rule. A hole whose axis is slightly angled from the direction of mold separation could be created with a "split pin" design, where half of the coring required to make the hole is on one side of the mold, and the other half is on the opposite side. Also, mold components called "side actions" can be used, which pulls coring in a direction other than the direction of mold separation. This design feature adds flexibility to the part design, but also increases the price of the mold.
There are other considerations for designing an efficient molded plastic part. Every part design is unique, and the requirements and functions of the part will often make following these considerations a challenge. Crescent Industries' engineers are ready to help with advice and suggestions.
-Written by Kurt Anderson, Design Engineer