Injection molding design: Produces side action without biting edges

The design of injection molding has clear rules: add drawing die, no biting edge, round edge, clear parting line, wall should be even and not too thick.

Sharp edges require extra processing costs and time; Wall thickness changes can leave unsightly indentation and edge biting, although can be on the mold side action, but increase the cost and cycle time.

Injection mold

Basic injection molding involves two half dies joined together, plastic being heated and pressed into a cavity between the two half dies, and the half dies separating to release the part from the die. The last step is what makes it difficult to shape the edge in the part. The bite edge is essentially invisible from the top or bottom of the part surface.

If you look at the part cross section below, you can see that most surfaces are easily formed by the upper or lower half of the mold, but the small shelf on the right side can 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. In injection molding, however, mold parts are designed to produce hundreds of thousands of pieces. As a result, each mold part needs to be easily separated from the mold when opened, and these bite edges provide a special design for manufacturing challenges.

If your design requires biting edges, is this a rule that can be bent? Yes, that's where the side action comes into the picture.

The side action of an edging tool

Edge biting is not a new problem and a solution has been developed. Instead of just bringing the two halves of the tool together to form the part, another part (or parts, as needed) is created to be moved in from the side, allowing for the formation of surfaces that would otherwise not have been formed, while still allowing the part to be easily released from the mold.

This makes more sense if you look at the way the parts above are molded. To create the shelf, the lower half of the die will have a side action that will move vertically with the bottom die piece and also horizontally as part of the forming cycle. When the mold is closed, this side action forms part of the mold cavity, but when the mold is opened, it slides away from the part, allowing the part to be easily removed from the mold.

While ingenious and capable of producing truly amazing parts that otherwise would not have been moulded, the side action does have drawbacks. Designing laterally acting molds requires additional tooling engineering, dealing 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 tooling tools. All of this adds significantly to the cost of tooling, which requires auxiliary operation.

How do you determine if your part needs to be assisted? With experience, engineers who often deal with injection molding can quickly analyze designs.

Alternative to side action: Avoid biting

The most common solution for edging, and the resulting increased tooling costs and side action delivery cycles, is to cut the material below the edging. In the image below, you can see how the groove on the side of the die allows the clapper to form without any biting edges, and how the hinge barrel can be formed without the need for side action.

Another possible solution is to split the parts. Molding the part into a single unit with multiple side actions, consider molding the design into several smaller parts and ultrasonic welding together after molding. While this can also increase unit costs and tool costs, it is often worth exploring and referencing it as a manufacturing option, especially if your geometry is very complex (like the golf training tool below), or if your part needs to include a volume.

The edge of the design

With more than a century of continuous improvement in injection molding technology, mold design rules are rarely absolute. However, deviations from standard DFM rules do increase the cost of tools and per unit, as does the side action that creates a biting edge on a part.