Mastering Undercuts in Injection Molding: What You Need to Know

Injection molding is one of the most versatile and widely used processes in the plastics manufacturing industry. From medical devices to consumer goods, it enables the high-volume production of complex parts with exceptional precision. But not all designs are straightforward.

One of the most common challenges molders face is dealing with undercuts—features in a part’s geometry that prevent it from being ejected directly from the mold. Left unaddressed, undercuts can cause broken parts, stuck molds, extended cycle times, and costly rework.

This post is the first installment in our 4-part series, Mastering Undercuts in Injection Molding, where we’ll explore how to identify and solve these challenges step by step:

1. What Are Undercuts in Injection Molding? (you’re here)
2. Solving External Undercuts (Parting Lines &Cam/Slide Systems)
3. Solving Internal Undercuts (Lifters, Unscrewing Molds & Collapsible Cores)
4. Draft Angles & Best Practices for Complex Part Design

By the end of this series, you’ll have a practical toolkit to approach undercuts confidently—whether you’re designing parts, building molds, or troubleshooting production.

What Are Undercuts in Injection Molding?

An undercut is any protrusion, recess, or feature that interferes with the straightforward removal of a molded part from its cavity. Think of it as a “hook” that locks the part in place.

When the mold opens, parts are usually pushed out along the direction of the parting line. If a feature extends sideways, inward, or backward, it prevents the part from ejecting smoothly. That’s when specialized mold design strategies are required.

Common Features That Create Undercuts

• Threads – Internal or external threads on closures, caps, or medical connectors
• Holes and slots – Side holes in housings or recessed fastener points
• Hooks and clips – Snap-fit connections inelectronics or automotive parts
• Textured surfaces – Deep textures that grip the mold surface like Velcro
• Snap assemblies – Plastic towers or ridges that interlock with other components

These features often improve the functionality, durability, or aesthetics of a product, but they complicate the molding process.

Types of Undercuts: Internal vs. External

Undercuts are generally grouped into two categories:

1.) Internal Undercuts

• Found inside the part’s geometry
• Example: an internal thread in a medical IV connector or bottle cap
• Challenge: hidden from the outside, requiring specialized mechanisms like lifters, unscrewing molds, or collapsible cores

2.) External Undercuts

• Located on the outer surface of the part
• Example: hooks, clips, or side features on housings or fasteners
• Challenge: protrude beyond the straight ejection path and often require slides or cams

Why Undercuts Matter in Part Design

Ignoring undercuts in the design phase can lead to major headaches during production:

1. Ejection Problems
   Parts become stuck inside the mold, delaying cycles and straining the ejector systems.
2. Increased Scrap Rates
   Features like clips or threads may break during removal, which can increase reject levels.
3. Cycle Time Extensions
   Manual intervention for tricky features slows down overall throughput.
4. Tooling Complexity and Cost
   Special mechanisms (cams, lifters, unscrewing systems) add time and expense to tool builds.
5. Quality Issues
   Stress marks, warpage, or broken features occur if undercuts aren’t addressed correctly.

For processors and engineers, recognizing undercuts early enables more brilliant mold design and fewer downstream issues.

Real-World Examples of Undercuts in Injection Molding

Undercuts appear in many industries and everyday products:

• Medical Industry: IV connectors and threaded fittings
• Automotive: Clips, hooks, and interior fasteners
• Consumer Electronics: Snap-fit housings for remotes, chargers, or controllers
• Packaging: Tamper-evident closures with recessed or side details
• Plumbing: Fittings and couplings with internal recesses or threads

Each requires thoughtful tooling to strike a balance between performance and efficient production.

Proactive Design: Reducing Undercuts Before They Happen

While tooling solutions exist for almost any undercut, the best strategy is to design smarter upfront:

• Evaluate feature necessity. Is that slot, clip, or hook essential?
• Adjust geometry early. Minor tweaks can eliminate the need for slides or cams.
• Add proper draft angles. Even complex features eject more easily with sufficient draft (we’ll cover this in Part 4).
• Simulate production. CAD tools can help detect problem areas before molds are cut.
• Collaborate early. Designers and mold makers working together can often find simpler solutions.

Conclusion

Undercuts are an unavoidable part of injection molding, but they don’t have to become production bottlenecks. By understanding the types of undercuts and planning for them early, processors and engineers can reduce scrap, simplify tooling, and improve cycle times.

Key Takeaway: Undercuts complicate injection molding by preventing the straightforward ejection of parts. Recognizing and planning for internal versus external undercuts early in the design process saves time, money, and frustration.

In Part 2, we’ll explore practical solutions for external undercuts, including how parting line adjustments and cam/slide systems can improve ejection and part quality.

Are undercuts giving you trouble in production? Our team has helped hundreds of processors improve cycle times and reduce scrap. Contact us with your questions—we’re here to help.