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 custom 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.
Common types of undercuts include external undercuts, internal undercuts, and side-wall undercuts, each requiring different mold design strategies to achieve proper release.
When is Undercut Injection Molding Required？
Undercuts are generally avoided in injection molding because they complicate tooling and increase cost. However, in some cases, they are essential to achieve the required function or performance of a part.
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 lines? 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.
Use Lifters for Internal Undercuts
Lifters are commonly used to release internal undercuts that cannot be removed through a simple straight ejection process. Unlike side-actions, which typically move horizontally to release external or side-wall features, lifters move at an angle together with the ejector system to release internal features during part ejection.
During ejection, the lifter slides away from the undercut area, allowing the molded part to separate from the core without damage. This mechanism is often used for features such as internal hooks, clips, and locking structures.
For more information about lifters, explore Comprehensive Understanding Injection Molding Lifter.
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.
Consumer Electronics
Undercuts are commonly used in phone chargers, remote controls, and device housings to create snap-fit covers and internal locking features. For example, battery compartments often rely on small internal undercuts so the cover clicks firmly into place without screws.
Automotive Components
Car interior and exterior parts often rely on undercuts for clips, fasteners, and trim retention. For example, dashboard panels and door trims use hidden locking features so they stay firmly in place even under vibration and long-term use.
Household Appliances
Products like vacuum cleaners, coffee machines, and kitchen tools often use undercuts for latch systems and removable covers. This allows users to open, clean, or replace parts easily while keeping everything firmly locked during use.
Industrial Equipment
In industrial enclosures and machinery covers, undercuts are used to improve structural locking and vibration resistance. They help panels stay in place under repeated use or harsh working conditions without loosening over time.
Challenges in Undercut Injection Molding
Undercuts can add real functional value to a plastic part, but they also introduce a set of manufacturing challenges that directly affect mold design, cost, and production stability.
Increased Mold Complexity
Undercuts usually require side actions, lifters, or collapsible cores in the mold. These mechanisms make the tooling more complex and reduce overall robustness compared to standard straight-pull designs.
Higher Tooling Cost
Adding moving components to release undercuts directly increases mold manufacturing cost. The more undercuts a part has, the more expensive and time-consuming the tool becomes to design and build.
Longer Cycle Time
Side actions and lifters need time to move in and out during each molding cycle. This adds mechanical steps to the process, which can slightly increase cycle time and reduce production efficiency.
Higher Maintenance Requirements
Moving components in molds are subject to wear over time. Undercut mechanisms require regular maintenance to ensure alignment, smooth operation, and consistent part quality.
Risk of Flash or Defects
If the undercut mechanism is not precisely aligned, it can lead to issues such as flash, incomplete filling, or part sticking, especially in high-volume production.
Reduced Mold Lifespan in Complex Tools
Compared to simpler molds, tools with multiple undercut features experience higher mechanical stress, which can shorten service life if not properly designed and maintained.
Conclusion
Utilizing undercuts in your design part 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.
At Zhongde, we help customers evaluate whether undercuts are necessary and optimize the tooling solution when they are. Our custom injection molding service supports everything from design review to mold making and production, ensuring your parts are both functional and cost-efficient.
