The design of injection molding has clear rules: add draft, no undercut, round edge, clear parting line, and the wall should be uniform and not too thick.
Sharp edges require additional processing costs and time; Changes in wall thickness will leave unsightly shrinkage marks and undercuts. Although it can act on the side of the mold, it will increase the cost and cycle time.
Injection mould
Basic injection molding consists of two mold halves joined together, the plastic is heated and pressed into the cavity between the two mold halves, and the mold halves are separated to release the parts from the mold. The last step is the reason why the undercut in the part is difficult to form. Undercuts are essentially part surfaces that are not visible from the top or bottom.
If you look at the cross-section of the part below, you can see that most of the surface is easily formed by the upper or lower half of the mold, but the small shelf on the right will cause the part to get stuck with the lower half of the mold.
In other types of casting, such as dewaxing or sand casting, the mold is disposable. However, in injection molding, mold parts are designed to produce hundreds of thousands of pieces. Therefore, each mold part needs to be easily separated from the mold when it is opened, and these undercuts provide a special design for manufacturing challenges.
If your design needs undercut, is this the rule that can be bent? Yes, this is where you enter the picture from the side.
Side effect in undercut tool
Undercut is not a new problem and a solution has been developed. Instead of just joining the two half parts of the tool together to form a part, create another part (or multiple parts, as required) to move in from the side, allowing the formation of a surface that could not have been formed, while still allowing the part to be easily demoulded from the mold.
It makes more sense if you look at the molding method of the above parts. To create this shelf, the lower half of the mold will have a side action that will move vertically with the bottom mold part and horizontally as part of the molding cycle. When the mold is closed, this side action forms part of the mold cavity, but when the mold is opened, it will slide away from the part, so that the part can be easily removed from the mold.
Although it is ingenious and can produce truly amazing parts, otherwise it cannot be formed, the side action does have shortcomings. Designing molds with lateral action requires additional mold engineering to deal with the high forces, heating and cooling cycles, and additional moving parts present in all molds. These parts also require additional processing time to produce and assemble mold tools. All these greatly increase the cost of the molds, which require auxiliary operations.
How do you judge whether your part needs to take auxiliary measures? With experience, engineers who often deal with injection molding can quickly analyze and design.
Alternative to side action: avoid undercut
The most common solution for undercut, and the resulting increased mold cost and lead time for side actions, is to cut the material below the undercut. In the following figure, you can see how the groove on the side of the molded part allows the buckle to be formed without any undercut, and how the hinge barrel can be formed without side action.
Another possible solution is to split the part. The part is molded into a single unit with multiple side effects, and the design is molded into several smaller parts and ultrasonically welded together after molding. Although this also increases unit cost and tool cost, it is usually worth exploring and referring to as a manufacturing option, especially when your geometry is very complex (such as the golf training tool below), or when your part needs to contain a volume.
Undercut in design
With the continuous improvement of injection molding technology for more than a century, the mold design rules are rarely absolute. However, deviating from the standard DFM rules does increase the cost of tools and each unit, and side actions that produce undercuts on parts are no exception.
