Ways to Use Undercuts Successfully in Injection Molding Design

injection molding undercuts

Undercut injection molding is a process used to create parts with complex shapes and features that prevent removing the part from the mold without any damage. Undercut features in injection molding generally increase any design’s overall complexity and associated costs. For these reasons, it is better to avoid them whenever possible.

In this article, we will explore in detail when will undercuts occur in injection molding and various methods used to create undercuts.

what are undercuts

What Situations Make Undercuts Injection Molding Necessary?

There are many scenarios where undercut injection molding becomes necessary, but here are some common ones.

1. Custom Inserts

Using the undercut injection molding process, you can create custom inserts that fit into larger parts. Machinists often use this insert type in medical devices or products that need an exact fit. You can also use undercut inserts as an alternative to screws or rivets when attaching two pieces.

2. Side Holes

Undercut injection molding can create side holes that are difficult to produce using traditional methods. Machinists often use side holes for mounting components or attaching them to other pieces of equipment. You can also use them for aesthetic purposes, such as adding a design element or venting heat inside the product.

3. Vertical Threads

Vertical threads are another common use for undercut injection molding. These threads are commonly found on bolts and screws, but you can also use them as part of an assembly where multiple parts need to thread together to work properly together as one unit.

4. Barb Fittings

In the case of barb fittings, the male parts need to be undercut to form a lip, holding the female mating part of the barb fitting.

5. Interlocking Features

If you are designing an interlocking feature, such as a tongue and groove, you must undercut one side of the tongue so you can mate the side with the groove on another.

Challenges Faced With Undercuts in Injection Molding

The major challenges of injection molding undercut result from the design, choice of materials, and aesthetic requirements of the machined part.

undercuts vs. no undercuts

1. Lack of Draft

The more complicated the arrangement is, the more issues you’ll find with the undercut design. Everything begins there. It would be best to control draft angles, cavity angles, and other complex angles for the greatest performance. To reduce the possibility of any ejection process difficulties, you must make the coating on the hand load of the proper substance.

2. Difficulty in Pulling Materials from Mold

It might be difficult to include undercut characteristics in some materials. For example, some materials, including plastics with glass filling, are more challenging to remove from the mold. According to the general rule of thumb, the harder the material is, the more difficulties you’ll have in extracting it. Therefore, use as much draft as possible while employing these tougher materials. Additionally, the solution you may employ to maximize your undercut molding depends on the material’s general flexibility and elasticity.

3. Aesthetic Consideration

The project’s aesthetic demands may provide another significant obstacle. For example, the undercut design option you can employ will have certain restrictions if you work with a component where you don’t want a noticeable parting line on injection molded parts or any other aesthetic concern. You’ll consequently have to cope with extra difficulties.

Different Ways to Use Undercut Successfully in Molded Parts

Success with undercuts necessitates small mold alterations and a high level of competence. Some of the designs that can assist minimize mold flaws and wear include the following.

1. Use Hand-Loaded Inserts

Machinists employ hand-loaded inserts when it is necessary to include an undercut in machining for challenging features like a lip with a sharp angle. They also come in handy when an additional feature like an awkwardly placed hole makes it impossible to use a standard solution such as a bumpoff.

To make use of a hand-loaded insert, the operator inserts one or more metal pieces into the mold before injecting the plastic. The number and position of the pieces depend on how many undercuts the machined piece needs. These metal pieces effectively create a cavity as they prevent the plastic from flowing into the spaces they occupy.

Once the operator opens the mold after the item has been created, they have to recover the hand-loaded insert physically. The disadvantage is that it increases the injection molding cost and cycle time as the operator must load and unload each cavity individually.

part lines in undercut injection molding

2. Part Lines Placement

Moving the mold’s parting line to overlap an undercut is sometimes the simplest way to deal with undercuts. The rationale is that when the feature is split in half by the parting line, the operator can release the component from the mold without needing an undercut. Machinists can zigzag the parting line using his method to align with different features, eliminating the need for undercuts. There are, however, limitations to this method. These range from the geometry and the flow properties of the plastic.


3. Insert Bumpoffs

Bump-offs are a good choice when working with flexible and elastic fabrics. The process only differs from the standard injection molding process by adding a single insert. The operator removes this insert first after the completion of the procedure. The vacuum it leaves behind gives the component some ‘wiggle room.’ Once removes the insert, they can take the part out of the mold. The “wiggle room” allows the machinist to bend the part slightly while taking it out of the mold, even with the presence of the undercut.

While bump-offs may appear to be relatively straightforward from the outside, using them necessitates careful consideration of several things. First, the part must be elastic enough not to break when the machine operator bends it. Also, the operator must ensure that the lead angle ranges from 30 to 40 degrees.

side actions in undercutting

4. Side Actions

In cases where having an undercut is unavoidable, a side-action feature can help to keep the part functional. A side-action core is an insert that is introduced before the plastic is injected into the mold. When the operator injects the material into the mold, it cannot fill the volume occupied by this insert. The machinist then slides the insert out after the completion of the molding.

However, unlike the case with bump-offs where the machinist removes the insert before the part, they remove the side action cores after taking out the part. Side actions also work best with rigid materials that do not adhere easily to the surface of the mold. One disadvantage of side actions is that each insert has to be specifically designed, making the procedure more complex.

shutoffs in injection molding undercuts

5. Make Use of Shutoffs

Machinists use sliding shutoffs when a feature, such as a hook sticking out from the side of a molding, cannot be easily made by another method. You insert a sliding cutoff through the hole in the main component’s wall to produce the undercut and the hook. The other half of the mold will create the remaining hook feature.

The problem with sliding shutoffs is that they must be exceedingly tight. This is because you cannot allow the plastic to develop past the feature’s shape if you need to use them as two mating sections of a tool. If not, it will cause substantial friction whenever you open and shut the tool.

In addition, you must draft each of those surfaces by about three degrees to prevent mold damage, which would quickly result in molded parts with an unacceptable finish. This is necessary because full metal-to-metal contact won’t occur until the mold is fully closed and a mechanical seal forms between the two faces.

Applications of Undercut in Injection Molding

Undercut injection molding is widely used in numerous sectors due to its capability to turn out products with undercut characteristics. We have selected a handful of these industries and explained them below.

Consumer Electronics

In this application, we use other methods to form features such as buttons and flanges that are difficult to achieve. Undercuts are also used to create cavities for electronic components in consumer electronics like mobile phones and laptops.

Medical Devices

Using other methods like injection molding or machining, you can use undercuts to create complex geometries that you cannot easily manufacture. In medical devices like syringes and catheters, undercuts create grooves along their length, allowing them to function properly.

WayKen Helps You Solve the Undercutting Problem Effectively

The undercut injection molding process can be complicated and requires a lot of experience to execute properly. At WayKen, no matter undercut injection molding or undercut in machining, we can handle all your undercuts requirements and deliver the best results consistently.

WayKen also provides high-quality rapid tooling and prototype injection molding services designed to get your product developed quickly and efficiently. Contact us for an instant quote and get DFM to help you optimize your part design and minimize any complexities.


There are numerous reasons for utilizing undercuts in your design process. First, identifying your components’ moldability is vital when designing any part for Injection Molding. Whether a straight wall design or an undercut, you must build the geometry to precise specifications to allow the plastic to flow properly and produce a part with minimal defects.


What are undercuts in injection molding?

An undercut in molding is a depression or protrusion in a form that prevents it from being withdrawn from a one-piece mold. Undercuts on molded components prevent the part from being discharged straight from an injection molding machine. They can be either internal or external undercuts, with external undercuts on the outside of the component and interior undercuts on the inside.

What is the undercut process?

When extracting the component from the mold, you need some mechanisms to adjust the portion of the mold that touches the undercut to take part from the mold since the part is not directly detachable without the device. This is the Undercut procedure.

What is a “parting line” in injection molding?

The parting line denotes the direction of the mold’s line of the draw or the point at which the machinist separates the two halves of the mold without causing harm to the part. The separating line is usually down the center of the part. However, this changes depending on the part shape.

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