Mastering Undercuts in Injection Molding: Solving External Undercuts

In Part 1 of our series, we introduced undercuts in injection molding, explained the difference between internal and external undercuts, and discussed why they matter in complex part design.

This post is the second installment in our 4-part series, Mastering Undercuts in Injection Molding, where we’re breaking down practical solutions step by step:

Part 1: What Are Undercuts in Injection Molding?

Part 2: Solving External Undercuts (Parting Lines & Cam/Slide Systems) (you’re here)

Part 3: Solving Internal Undercuts (Lifters, Unscrewing Molds & Collapsible Cores)

Part 4: Draft Angles & Best Practices for Complex Part Design

In this article, we’ll focus on external undercuts—features like clips, hooks, or snap assemblies that appear on the outside of a part. These are extremely common in industries like automotive, electronics, and packaging. We’ll explore two proven solutions: parting line adjustments and cam/slide systems.

Why External Undercuts Are a Challenge

External undercuts are found on the outside surface of a molded part. Unlike internal undercuts, they’re visible—but that doesn’t make them easier to deal with.

If you’ve ever worked with parts that include:

• Snap-fit assemblies for consumer products
• Side clips for automotive interiors
• Tamper-evident features in closures
• External hooks or protrusions for assembly

…then you’ve encountered external undercuts.

The challenge is simple: when the mold opens, these features block the part’s ejection path. Without a solution, the part either gets stuck in the mold or the feature breaks off, leading to scrap.

Solution 1: Using Parting Lines to Eliminate External Undercuts

A parting line is where the two halves of the mold meet. With clever design, the parting line can sometimes be positioned in such a way that the undercut is split, allowing the part to be ejected without the need for additional tooling.

How It Works

• By shifting the parting line to intersect the undercut, the mold can release the feature naturally.
• This method essentially “splits” the geometry across the mold halves, freeing the part on opening.

Advantages

• Cost-Effective: Avoids the expense of additional mechanisms.
• Simple to Maintain: With fewer moving parts, there is less wear and tear.
• Efficient: No added cycle time from cams or slides.

Limitations

• Aesthetic Impact: The parting line may leave visible marks, which could be an issue for cosmetic parts.
• Geometry Restrictions: Not all features can be split by a parting line; complex or deep undercuts usually require alternative solutions.

Example Applications

• Packaging parts where parting lines aren’t visually critical
• Industrial housings where function outweighs appearance
• Clips and side features that can be aligned with mold separation

Solution 2: Cam/Slide Systems for Complex External Features

When the parting line can’t eliminate the undercut, cam/slide systems step in. These are one of the most widely used solutions for external undercuts.

How They Work

• Slides move into position as the mold closes, guided by angled pins.
• Once in place, they form the undercut feature during the molding process.
• As the mold opens, the slides retract, pulling away from the undercut before ejection.

This ensures the part can be released without breaking clips or hooks.

Advantages of Cam/Slide Systems

• Handles Complex Features: Perfect for snap assemblies, external holes, or side details.
• Fully Automated: The process is built into the mold’s opening and closing cycle.
• High Repeatability: Ensures consistent part quality in high-volume production.

Considerations

• Cost: Adding slides increases mold cost and complexity.
• Maintenance: Slides require lubrication, inspection, and precise alignment to avoid failures.
• Cycle Time: In some cases, the added movement increases cycle time.

Example Applications

• Automotive: Dashboard clips, air vent assemblies, or cable housings
• Consumer Electronics: Side hooks or snap-fit connectors in housings
• Closures: Tamper-evident packaging with side locking features

Comparing Parting Lines vs. Cam/Slide Systems

Best Practices for External Undercuts

• Evaluate early in design: The earlier you decide between a parting line adjustment or slides, the more cost-effective your mold will be.
• Prioritize simplicity: If a parting line solution works, it’s almost always better than adding slides.
• Balance appearance vs. cost: For cosmetic parts, slides may be worth the added expense to avoid visible parting lines.
• Maintenance plan: If using slides, build a maintenance schedule to keep them running smoothly.

Conclusion

External undercuts are common in injection molding, especially in parts that require clips, hooks, or snap features. Fortunately, there are reliable ways to solve them.

Parting line adjustments offer a simple and cost-effective solution, while cam/slide systems handle more complex geometries with precision. By understanding these options, molders and designers can improve ejection, reduce scrap, and maintain part quality.

Key Takeaway: For external undercuts, always explore parting line adjustments first for simplicity and ease of use. When the geometry demands more, cam/slide systems are the industry standard for reliable results.

In Part 3, we’ll turn to internal undercuts—looking at how lifters, unscrewing molds, and collapsible cores allow molders to produce complex internal features without compromising efficiency.

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.