undercuts in injection molding

Learn about the Undercuts in Injection Molding Design for Custom Parts

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Undercut in injection molding is a design that allows the creation of parts with complex shapes. However, these complex designs often present a challenge – molded parts with undercuts are difficult to remove from the mold. The undercuts also increase the complexity and associated costs of a project.


In this article, We will explore what undercuts are, why they are essential, and how to effectively use them in the production of injection-molded parts. Let’s begin to understand the undercuts in injection molding.

What Are Undercuts in Injection Molding?

In the context of injection molding, undercuts refer to features or geometries on a molded part that deviates from the traditional straight-line ejection. These features create indentations or protrusions that make it challenging to remove the part from the mold without causing damage. They introduce complexity to the ejection process, requiring special design considerations and mechanisms within the mold to successfully produce and release such parts.

When is Undercut Injection Molding Required?

Many part designs require undercuts. Here are a few design purposes that require undercut molding.

Vertical Threads

Vertical threads are a 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.

Custom Inserts

By using the undercut injection molding process, you can customize inserts that fit into larger components. Machinists frequently turn to these inserts in applications like medical devices or situations demanding precise fits. Additionally, undercut inserts offer an alternative to traditional fasteners such as screws or rivets when joining two pieces together.

Side Holes

Undercut injection molding excels at creating side holes, that are often difficult to achieve through traditional manufacturing methods. Machinists commonly use side holes for mounting components or linking them to other equipment. or use them for adding a design element or venting heat inside the products.

Barb Fittings

In the case of barb fittings, the male components are undercut to form a lip, ensuring a secure hold on the female mating part.

How to Achieve Undercut Success ly in Injection Molded Parts?

Dealing with undercuts is complex. However, there are ways to deal with them successfully by improving your mold design and optimizing your operations. Here are 5 effective ways to deal with undercut molding designs.

Adjust the Parting Lines

Moving the mold’s parting line to cover an undercut is often the simplest way to deal with undercuts. This approach is based on splitting the feature in half with the parting line, making it easier for the operator to remove the component from the mold without requiring an undercut. This technique allows machinists to zigzag the parting line to align with various features, eliminating the necessity for undercuts. However, it’s important to note that there are limitations to this method, which are influenced by the geometry and the way plastic flows.


What is Parting lins? How to deal with parting line? Click to learn more details. 

Utilize Side-Actions

In a situation where the undercut is absolutely necessary, a side-action feature can help you maintain the functionality of your part. In the simplest terms, a side-action core is an insert that slides out of the part during ejection. When the molten material is ejected in, it is not able to fill up the volume taken up by this insert. When the molding process is complete, the insert slides out, leaving an undercut behind. However, side action features can induce a lot of tooling costs and increase the complexity of the work. Side actions cannot be used when molding softer plastics. Material flow properties and deformability needs are taken into consideration for such an expensive process.

How to design 

Incorporate Sliding Shutoffs

Sliding shutoffs or telescoping shutoffs are often used to create clip- and hook-style mechanisms. These are commonly used for locking together the two halves of a molded product, and in many cases can eliminate the need for side-actions, inserts, and bumpoffs that add more complexity to a mold. The “telescope” is machined into one half of the mold and extends into the opposite side during mold operation, “shutting off” certain part features. Shutoffs offer a way to simplify mold design and reduce product costs. Just be sure the part and mold have a minimum of 3 degrees draft vertically.

Use Bump Offs

Bumpoffs are an easy way to mold lens covers, container caps, and similar parts with functions that call for them to snap into place. Rather than use a side-action cam, an insert is machined so the undercut can be applied and bolted into the mold where a pocket matches the insert dimensions. During ejection, we rely on the plastic material to compress releasing the material from the undercut. There are two considerations about using bump-offs. First, the part must be elastic enough not to break when the operator bends it. Also, the operator must ensure that the lead angle ranges from 30 to 40 degrees.

Choose Hand-loaded Inserts

In this instance, mold inserts might be just the ticket. A machined piece of metal is hand-loaded into the mold cavity, thus preventing plastic from flowing into these areas. Once the molding cycle is complete, the inserts are ejected along with the part, whereupon an operator picks them out for reuse on the next part. This is a manual process, which means the lead time would be longer.

Secondary Operation

Secondary operations don’t need to change mold design or its features but can also support the use of injection molding undercuts. For example, if we want to create a hole in a part. we don’t need to use sliding shut-offs, just put a finished component into a drill press or milling machine instead. Sometimes, a secondary operation is the most cost-effective solution for prototyping or very low production quantities.

Application of Undercuts in Injection Molding

Undercut injection molding is widely used in many industries. Many parts are plastic injection molded with undercut features that can be easily assembled by non-technical users.

Medical Plastic Parts:

Catheters: Undercuts are used to create intricate features for secure connections and functionalities in medical catheters.

Consumer Electronics:

Smartphone Cases: Undercuts can provide secure attachment points for buttons or internal components.


Housings: Enclosures for electronic devices may use undercuts for assembly and venting.

The Main Challenges of Undercuts in Injection Molding

Difficult Part Ejection:

Undercuts create complex shapes that make it difficult for the molded part to be released from the mold in a straightforward manner.

Complex Mold Design:

The design and construction of molds with undercuts are more intricate and require careful consideration of sliding mechanisms, inserts, and other features to accommodate the undercuts.


Utilizing undercuts in your design process can be motivated by various reasons. When designing parts, one of the primary considerations is understanding the moldability of your components for injection molding. Whether you’re working with a straightforward, straight-wall design or one that incorporates undercuts, the key is to construct the geometry with precise specifications. This precision ensures that the plastic flows correctly during the molding process, ultimately resulting in parts with minimal defects.


The undercut plastic injection molding process is a series of complications and intricate requirements that only the right manufacturing partner can help you achieve. Zhongde has advanced equipment and an experienced team to meet your requirements at the best price. We can help you optimize your part design to minimize complexities and help you find the best ways for your projects.

Choose A Reliable Manufacturer for Your New Project!

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