Why Internal Undercuts Are a Serious Problem in Injection Molding

Let’s be honest: internal undercuts are a nightmare for mold designers. Unlike external features, they can’t be handled with standard sliders.

If the geometry isn’t designed correctly, the consequences are serious:

• Parts stuck permanently on the core
• Surface damage or scuffing
• Lifter rod breakage
• Costly production downtime

When dealing with internal clips or bosses, the mold lifter becomes the most reliable mechanical solution.

How a Mold Lifter Works: The “Side-Step” Principle

A mold lifter doesn’t move like a traditional slider. Instead, it is driven by the ejector system.

During the ejection cycle:

• The lifter moves upward with the ejector plate
• At the same time, it follows an angled path
• This creates a combined vertical + lateral motion

This lateral movement—often called the “side-step”—is what releases the undercut.

Key Design Rule

Always allow at least 2 mm of extra travel beyond the undercut depth.

Anything less increases the risk of:

• Drag marks
• Surface scratches
• Incomplete release

The Most Common Mistake: Incorrect Lifter Angle

One of the biggest design errors is pushing the lifter angle too steep to save space.

Recommended Lifter Angle

• Optimal range: 5° – 11°

Risk Zone

• Above 15°: High risk of binding and failure

At steeper angles:

• Lateral force increases significantly
• Vertical motion becomes restricted
• The lifter may jam or the rod may buckle

When Steep Angles Are Unavoidable

If your design forces a larger angle, consider:

• Hinged lifter base
• T-slot lifter design

These help reduce stress on the lifter rod and improve durability.

Material Selection: Prevent Galling and Wear

Lifter systems operate under:

• High pressure
• High temperature
• Continuous metal-to-metal contact

Choosing the wrong material can lead to galling, where surfaces weld together and fail.

Recommended Materials

• H13 tool steel
• 718H pre-hardened steel

Hardness Guideline

• Target: 50–54 HRC
• Keep the lifter slightly harder than the core

High-Performance Option

For faster cycle times:

• Use Beryllium Copper inserts

Benefits:

• Faster heat dissipation
• Reduced cooling time
• Increased production efficiency

3 Expert Tips for a Bulletproof Lifter Design

1. Add a “Tail” Guide

Always support the bottom of the lifter rod with a guide block.

Without support:

• The rod vibrates
• Vibration leads to flash and wear

2. Use Oil Grooves

Friction is the enemy of any moving component.

Best practice:

• Add lubrication grooves to sliding surfaces

Exception:

• Medical molds → use DLC coating instead of oil

3. Control the Parting Line (PL) Fit

The lifter shut-off surface must match the core perfectly.

Even a 0.01 mm gap can cause:

• Visible witness lines
• Poor surface quality

Lifter vs Slider: Which One Should You Use?

Choosing between a lifter and a slider depends entirely on undercut location.

Use a Slider When:

• The undercut is external
• There is enough space for side action

Use a Lifter When:

• The undercut is internal
• Space is limited
• Hydraulic systems are not practical

Final Thoughts

A mold lifter is one of the most efficient solutions for internal undercuts—but only when designed correctly.

By controlling:

• Angle
• Travel clearance
• Material selection
• Structural support

You can turn a high-risk feature into a reliable, repeatable mechanism that keeps your production line running smoothly.

Need help optimizing your mold design or solving complex undercuts?
Visit www.xinkeymould.com to get expert support and improve your tooling performance.