Mastering Undercuts in Injection Molding: Solving Internal Undercuts

In Part 2 of our series, we looked at solutions for external undercuts, focusing on parting line adjustments and cam/slide systems. These strategies are highly effective for external features, such as clips, hooks, and snap assemblies.

But what happens when the challenge is hidden inside the part?

This post is the third installment in our 4-part series, Mastering Undercuts in Injection Molding:

Part 1: What Are Undercuts in Injection Molding?

Part 2: Solving External Undercuts (Parting Lines & Cam/Slide Systems)

Part 3: Solving Internal Undercuts (Lifters, Unscrewing Molds & Collapsible Cores) (you’re here)

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

Here, we’ll dive into internal undercuts—features that exist inside a molded part, such as threads, recessed clips, or internal snap fits. These are among the most challenging undercuts to address because they can’t be reached from the outside. We’ll explore three proven tooling solutions: lifters, unscrewing molds, and collapsible cores.

Why Internal Undercuts Are More Complex

Unlike external undercuts, which are visible and accessible, internal undercuts are recessed inside the part geometry. That means they can’t be solved with a simple parting line adjustment or an external slide.

Examples of internal undercuts include:

• Internal threads inside bottle caps or IV connectors
• Recessed clip towers for snap assemblies
• Slots, grooves, or hook features within a cavity

Due to their location, these features necessitate tooling components that can move or change shape within the mold to release the part.

Solution 1: Lifters – A Simple Yet Effective Option

Lifters are one of the most widely used solutions for internal undercuts, especially when dealing with recessed snap features or clip towers.

How Lifters Work

• Lifters are mounted within the ejector system of the mold.
• As the mold opens, the ejector system pushes the lifter at an angle or upward.
• The lifter engages the undercut, pushing it free as the part is ejected.

Advantages

• Straightforward design: Less complex than other internal undercut solutions.
• Durability: With proper maintenance, lifters can handle high-volume runs.
• Effective for small features: Ideal for clips, snap assemblies, and minor recesses.

Limitations

• Restricted by geometry: Best for relatively small or shallow undercuts.
• Wear over time: Lifters endure high stress during ejection and require regular maintenance.

Common Applications

• Consumer goods with snap-fit assemblies
• Automotive clip towers
• Packaging closures with recessed locking features

Solution 2: Unscrewing Molds for Internal Threads

When a part requires internal threads, lifters won’t cut it. That’s where unscrewing molds come in.

How Unscrewing Molds Work

• Core pins with threaded details are positioned inside the cavity.
• Once the part cools, these pins rotate and unscrew the molded thread from the part.
• A rack-and-pinion system, powered by gears or a hydraulic motor, typically controls this motion.

Advantages

• Precision: Produces high-quality, accurate threads.
• Repeatability: Reliable for long production runs.
• Durability: Handles high-volume output without damaging delicate features.

Limitations

• Complexity: Much more mechanically complex than lifters.
• Cost: High tooling investment.
• Cycle time: The unscrewing action can slightly extend molding cycles.

Common Applications

• Medical: IV connectors, syringe parts, threaded housings
• Packaging: Bottle caps, closures, threaded lids
• Industrial: Fasteners and connectors with precision threading

Solution 3: Collapsible Cores – A Space-Saving Alternative

A more advanced option for internal undercuts is the collapsible core, designed to release threads or recessed features without requiring a rotation system.

How Collapsible Cores Work

• During molding, the core expands outward to form the undercut or thread.
• As the mold opens, the core collapses inward, shrinking away from the part.
• This allows the part to eject smoothly without damaging internal details.

Advantages

• Simplified cooling: Easier mold cooling compared to unscrewing molds.
• Faster cycles: Ejection doesn’t require rotation, reducing cycle time.
• Compact design: Eliminates bulky rack-and-pinion systems.
• Consistency: Provides repeatable results for high-volume runs.

Limitations

• Cost: Collapsible cores are specialized components that add to the expense.
• Size restrictions: More effective for smaller parts and moderate undercuts.
• Maintenance: Mechanical components must be inspected regularly to ensure optimal performance.

Common Applications

• Caps and closures
• Plumbing fittings
• Small threaded or recessed components

Comparing Lifters, Unscrewing Molds & Collapsible Cores

Best Practices for Internal Undercuts

• Plan early: Internal features are more difficult to modify after the mold is built.
• Choose based on geometry: Threads require unscrewing or collapsible cores; clips and towers are ideal for lifters.
• Balance cost and production volume: For high-volume runs, more expensive solutions like unscrewing molds or collapsible cores may pay off long term.
• Prioritize maintenance: Moving components, such as lifters and cores, require regular lubrication and inspection to prevent breakdowns.

Conclusion

Internal undercuts present some of the toughest challenges in injection molding. Fortunately, solutions like lifters, unscrewing molds, and collapsible cores make it possible to mold complex internal features without sacrificing efficiency or part quality.

Key Takeaway: Select your tooling strategy based on part geometry, production volume, and associated costs. Lifters are ideal for small recesses, unscrewing molds are essential for precision threads, and collapsible cores offer a faster, more compact alternative.

In Part 4, we’ll wrap up this series by covering draft angles and best practices for part design—so you can prevent many undercut issues before they happen.

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.