Time to read: 4 min

Design for injection molding has clear rules: Add draft. No undercuts. Round edges. Clear parting lines. Uniform walls, not too thick. But learning about advanced plastic manufacturing is like visiting Morpheus’s dojo in The Matrix: Some rules can be bent. Others can be broken. But breaking these rules is costly.

Sharp edges require additional machining expense and time; wall thickness variation can leave unsightly sink marks; and undercuts, while possible, require side actions on the mold that can increase both cost and cycle time. To understand why, we’ll need to visit the dojo.

Injection Molds

As covered in other articles here on Fictiv, basic injection molding consists of two mold halves coming together, plastic being heated and forced into the cavity between those two halves, and the mold halves separating to release the part from the mold. That last step is what makes undercuts in parts difficult to mold.

What is an undercut? Essentially, this is any surface of the part that can’t be seen by looking at it from the top or bottom. (In reality, the “pull direction”—the axis along which the mold opens and closes—is typically horizontal, but conceptually, it’s easier to think of it vertically.)

If you look at the part cross-section below, you can see that most of the surfaces are easily formed by the top or bottom half of the mold, but the small shelf along the right side would cause the part to be stuck with the lower mold half.

side action sample

In other types of casting, like lost wax or sand casting, the molds are disposable. In injection molding, however, the mold parts are designed to produce hundreds of thousands of pieces. Therefore, each mold part needs to easily separate from the mold when it opens, and these undercuts present a special design for manufacture challenge.

If your design requires an undercut, is that a rule that can be bent? Yes, and that’s where side actions enter the picture.

Side Actions in Tools for Undercuts

Because undercuts aren’t a new problem, enterprising engineers have developed a solution. Instead of just two halves of the tool coming together to form the part, another piece (or pieces, as required) is created that moves in from the side, allowing surfaces to be formed that were otherwise impossible, while still allowing the part to be released easily from the molds.

That makes more sense if you take a look at the way the part above would be molded. In order to create that shelf, the bottom half of the mold would have a side action that would move vertically with the bottom mold piece and would also move horizontally, as part of the molding cycle. When the mold is closed, this side action forms part of the mold cavity, but as the mold opens, it slides away from the part, allowing the part to release easily from the mold.

side action sample

side action sample

side action sample

side action sample

While ingenious and capable of producing truly amazing parts that are otherwise impossible to mold, side actions do have drawbacks. Designing a mold tool with side actions requires additional tooling engineering, dealing with the high forces and heating and cooling cycles present in all molding, as well as the additional moving parts. These parts also require additional machining time to produce and assemble the mold tools. All of this significantly increases the cost of molds requiring side actions.

How can you tell if your part will require side actions? With experience, engineers who have dealt frequently with injection molding can quickly analyze a design.

Alternatives to Side Actions: Avoiding Undercuts

The most common solution to undercuts, and the resulting increase in tooling cost and lead time from side actions, is to cut away material below the undercut. In the picture below, you can see how a groove in the side of a molded piece will allow the snap to be formed without any undercuts. In the image below that, you can see how a hinge barrel can be formed without requiring side actions.

snap with and without undercut


Another possible solution is to section your parts. Instead of molding parts as a single unit with multiple side actions, consider having the design molded as several smaller parts and ultrasonically welded together after molding. While this can also increase the per-unit and tooling cost, it’s often worth exploring and quoting this as a manufacturing option, especially when you have either very complex geometry, as with the golf training tool below, or when your parts need to enclose a volume, as with Apple ear pods.

Swing Coach Club sectioned, injection molded partSwing Coach Club separate pieces

Undercuts in Your Design

With more than a century of refinements in injection molding technology, rules of design for molding are rarely absolute. However, veering from the standard DFM rules does increase the cost of the tooling and of each unit, and side actions to produce undercuts on parts are no exception.

You can learn more about Fictiv’s Injection Molding service capabilities here and create your free Fictiv account to get a rapid quote for injection molded parts on-demand with low minimum order quantities.