Creating Undercuts in Injection Molding

As much as you should avoid using undercuts in injection molding whenever possible, sometimes your part design requires it. Luckily, there are many effective ways to incorporate undercuts into your mold design. Just keep in mind that each method is likely to add to the cost of your mold (sometimes significantly), so it’s still better to avoid undercuts when you can.

What Are Undercuts?

Undercuts are recessions or protrusions in a part that prevent it from ejecting from the mold. Examples of undercuts include screw threads, locking tabs, barb fittings, slots, and holes.

Ways to Create Undercuts

While it’s best to avoid undercuts when you can, there are many ways to create them if your part requires them. Here are some of the most common ways to incorporate undercuts into injection molding designs.

Side Actions

As the name suggests, side actions insert themselves into the mold from the side as it closes. Consider a screwdriver handle. With the curves on the outside of the handle and the hole in the middle, it would be impossible to create with a simple 2-piece mold, so a side action comes in to block off the area of the mold that will eventually be filled with the screwdriver head, then slides out before the mold is opened and the part is ejected.


When dealing with flexible materials, you can sometimes use bump-offs in your part design so the part deforms over the mold during ejection. Also called stripping undercuts, bump-offs require a lead angle of 30° to 45° and must be located away from stiffening features like corners or ribs.

Unscrewing Mold Mechanisms

When dealing with threaded parts, an unscrewing mechanism built into the mold can unscrew the part from the core as a secondary action. Unscrewing mechanisms have many limitations and are among the most complex of all injection molds, so they’re usually considered a long-term investment that will require frequent maintenance.

Telescoping Shutoffs

A telescoping shutoff extends from one side of the mold to the other, “shutting off” certain part features from being filled. Telescoping shutoffs are commonly used to create clip- and hook-style mechanisms. They can be a simple solution for some undercuts, but you must incorporate at least 3 degrees of draft to the part to prevent metal-on-metal rubbing.

Collapsible Cores

Flexing steel collapsible cores work by collapsing radially inward during the normal mold sequence, which eliminates complex coring approaches and secondary operations. That can provide significant cycle time reductions – up to 30% compared to unscrewing mechanisms.

Dovetail Collapsible Cores

Dovetail joints are common in carpentry due to their strength. Translating that design into a Dovetail collapsible core offers a method that’s stronger and easier to repair than a traditional collapsible core.

Parting Lines

Occasionally, the simplest way to deal with an undercut is to move the mold’s parting line to intersect it. While parting lines are traditionally straight, they don’t have to be. They can often zigzag through multiple features to help them eject from the mold. Parting lines do have their limitations, so they aren’t the right solution for every problem, however.

Hand-Loaded Inserts

While a fully automated process is often the preferred method for injection molding, sometimes having an operator inserting machined pieces of metal into the mold cavity before it’s closed is the best way to accomplish the undercut you need. The inserts are ejected with the finished part, then reinserted for the next batch. Hand-loaded inserts can increase the cycle time and cost, but it may be a more cost-efficient option for small batches compared to investing in more complicated molds.


With more than 50 years of experience, Universal Plastic Mold (UPM) knows all the best ways to incorporate undercuts into your design when they can’t be avoided. We’re the injection mold manufacturer of choice for businesses across the US because we can streamline the manufacturing process, saving you money and getting your product to market quicker.To learn more about why you should choose UPM or to get a quote, click here or call 1-888-893-1587.

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