Mould: Undercuts Deisgn (2)

Whenever the axis of a hole is not parallel to the draw of mold, either the forming core or pin that make the hole must be removed from the workpiece before the workpiece is retracted from the mold or the forming core must be ejected with the workpiece together and then be taken from the workpiece, the later method needs a lot of labor.

Fig.2.30 shows different types of undercuts, which are frequently necessary in a molded plastic workpiece design, in molded plastic workpieces. It can be classified as an undercut on one side wall,

internal undercut, extermal undercut, and circular undercut of a workpiece.

Undercuts may be molded by ways of split-mold cavity sections and by means of movable side cores that must be drawn away from the workpiece before the workpiece can be ejected from the mold. Because wedges or pull pins must be used to mold undercuts, flash or parting lines will be evident where the movable mold sections meet or where they meet the fixed mold sections.

There is a very fine distinction between undesirable and impossible undercuts. Fig. 2.30a 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 intemal undercut is redesign the products as shown in Fig. 2.30b

External circular undercuts are located in the outside contours of the workpiece as shown in Fig. 2.30c. A more efficiency method to molding an extemal undercut is to use two retractable ejector wedges. It must be remembered that a flash or parting line will be visible on the workpiece and the mold is quite expensive. The other less expensive method is forming by grooving the workpiece in a lathe with a properly designed cutting tool. Side wall undercuts own In Fig.2.30d must be molded by retractable forming core or wedge which is quite expensive.