Undercuts Design in plastic injection parts

An undercuts is an indentation or projection on a plastic injection part usually means that the core moving direction is not parallel to the draw direction of mold, so that the molded plastic injection part makes ejection from the sample two- part mold almost impossible. Because of the movement way they are oriented, some feature is introduced to injection mold design. The cores or protrusions, which used to form these indentation, referred to as undercuts, are typically used in articles such as enclosures to provide a snap fit to assemble plastic injection parts.

Internal undercuts can be molded by using two separate core pins, as shown in Fig 1. This is a very practical method to use, but flash will sometimes occur where the two core pins meet.

Fig 1 An internal undercut can be molded by using two separate core pins

    Fig 2 shows different types of undercuts, which are frequently necessary in a molded plastic part design. It can be classified as an undercut on one side wall, internal undercut, external undercut, and circular undercut of a part.

There is a very fine distinction between undesirable and impossible undercuts. Fig 2a is an internal undercuts which can be formed using movable cores that are low efficiency and expensive, and that should be avoided. A more efficiency method to molding an internal undercut is redesign the products as shown in Fig 2b.

External circular undercuts are loaded in the outside contours of the part as shown in Fig 2c. A more efficiency method to molding an external undercut is to use two retractable ejector wedges. The other less expensive method is forming by grooving the part in a lathe with a properly designed cutting tool. Side wall undercuts as shown in Fig 2d must be molded by retractable forming core or wedge which is quite expensive.

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Fig 2 Different types of undercuts in molded plastic injection parts