Designing Undercuts Injection Molds Undercuts are where mold design gets interesting.
Our custom injection molding services include expert mold design for complex parts. A feature that’s impossible to eject straight out requires slides, lifters, or other mechanical solutions. I’ve seen elegant undercut designs that work flawlessly for millions of shots, and I’ve seen undercut solutions that are constant maintenance headaches. Here’s how to design undercuts that work.
Understanding Undercuts
Definition An undercut is any feature that prevents the part from being ejected straight out of the mold cavity.
Types of Undercuts
| Type | Description | Typical Solution |
|---|---|---|
| Peripheral | Around the outside | Slides or stripper plate |
| Internal hole | Hole not on parting line | Core pulls or lifters |
| Interlocking | Has locking part | Complex slides |
| Undercut rib | Rib on interior wall | Lifting cam or lifter |
Undercut Classification
| Class | Complexity | Solution | Cost Multiplier |
|---|---|---|---|
| Class 1 | Simple | Standard slides | 1.2-1.3× |
| Class 2 | Moderate | Complex slides | 1.3-1.5× |
| Class 3 | Complex | Multi-axis slides | 1.5-2.0× |
| Class 4 | Very complex | Special mechanisms | 2.0-3.0× |
Slide Systems
Basic Slide Components
| Component | Function |
|---|---|
| Slide block | Carries the cam/cavity insert |
| Cam/heel | Provides angled surface for action |
| Angle pin | Drives the slide |
| Wear plate | Provides sliding surface |
| Return spring | Returns slide on closing |
Slide Drive Methods
| Method | Description | Stroke | Speed |
|---|---|---|---|
| Angle pin | Cylindrical pin on moving half | Up to 0.5“ | Moderate |
| Cam pin | Flat cam surface | Up to 1.0“ | Fast |
| Hydraulic cylinder | Hydraulic-powered | Any | Fast, controlled |
| Pneumatic | Air-powered | Any | Fast, less force |
| Motorized | Servo-driven | Any | Precise, programmable |
Slide Stroke Requirements
| Undercut Depth | Slide Angle | Stroke Calculation |
|---|---|---|
| 0.125“ (3mm) | 15° | Stroke = Depth / sin(angle) = 0.48“ |
| 0.250“ (6mm) | 20° | Stroke = 0.73“ |
| 0.500“ (13mm) | 20° | Stroke = 1.46“ |
| 0.750“ (19mm) | 25° | Stroke = 1.77“ |
Slide Angle Guidelines
| Guideline | Recommendation | Reason |
|---|---|---|
| Maximum angle | 25° | Prevents binding, excessive stroke |
| Preferred angle | 15-20° | Good balance |
| Minimum angle | 10° | Stroke becomes excessive |
| Stroke calculation | Stroke = d / sin(θ) | Design formula |
Slide Size Guidelines
| Factor | Guideline | Notes |
|---|---|---|
| Slide length | 3-4× stroke | Stability |
| Slide width | 2-3× height | Rigidity |
| Travel check | Verify clearance | Prevent interference |
| Guidance | Full-length guidance | Accuracy |
Lifter Systems
Lifter Types
| Type | Application | Mechanism |
|---|---|---|
| Angled lifter | Internal undercuts | Angled movement |
| Cam lifter | Complex undercuts | Controlled path |
| Roller lifter | High-speed operation | Low friction |
| Hydraulic lifter | Large movements | Powerful |
| Pneumatic lifter | Small movements | Simple |
Lifter Stroke Calculation
| Geometry | Formula | Example |
|---|---|---|
| Angled lifter | Stroke = d / sin(θ) | d=0.25“, θ=15° → 0.97“ |
| Vertical lift | Stroke = d | d=0.25“ → 0.25“ |
| Compound | Vector calculation | Depends on angles |
Lifter Design Guidelines
| Guideline | Value | Reason |
|---|---|---|
| Minimum angle | 10° | Adequate lift |
| Maximum angle | 25° | Prevent binding |
| Stroke clearance | +25% minimum | Safety margin |
| Return method | Spring or gravity | Ensure return |
| Guidance | Full length | Accuracy |
Alternative Undercut Solutions
Alternative Methods
| Method | Application | Pros | Cons |
|---|---|---|---|
| Stripper plate | Peripheral undercuts | Simple, fast | Large plate needed |
| Gerotor | Internal gears | Complex shapes | Limited sizes |
| Collapsible core | Internal undercuts | No slides | Expensive, limited |
| Threaded core | Screw threads | Accurate threads | Slow cycle |
| Unscrewing | Screw caps | Standard threads | Complex, slow |
Stripper Plate Design
| Guideline | Value | Notes |
|---|---|---|
| Plate thickness | 1.5-2× stroke | Rigidity |
| Travel | Stroke + 0.5“ minimum | Clearance |
| Force | Calculate based on area | Adequate cylinder |
| Speed | Controlled | Prevent part damage |
Collapsible Core Applications
| Application | Core Diameter | Collapse Method |
|---|---|---|
| Bottle necks | 10-50mm | Wedge/finger |
| Internal threads | 10-30mm | Segmented |
| Complex IDs | Variable | Custom mechanism |
Undercut Design Guidelines
General Principles
| Principle | Recommendation |
|---|---|
| Minimize undercuts | Eliminate if possible |
| Simplify solutions | Standard slides preferred |
| Consider manufacturing | Design for easy machining |
| Consider maintenance | Access for repair |
Design Checklist
| Item |
|---|
| Undercuts identified in design review |
| Solution type selected |
| Stroke calculated |
| Mechanism fits in mold space |
| Angle within guidelines |
| Return mechanism designed |
| Wear surfaces addressed |
| Maintenance access provided |
Feature Relocation Before adding a slide or lifter, consider:
| Alternative | When It Works |
|---|---|
| Move to parting line | Feature can be on parting surface |
| Change part orientation | Different eject direction |
| Modify geometry | Eliminate if not critical |
| Use snap-fit | Replace rigid feature |
Cost Comparison
| Solution | Relative Cost | Cycle Impact |
|---|---|---|
| Parting line feature | 1.0× | None |
| Standard slide | 1.3-1.5× | +1-3 seconds |
| Complex slide | 1.5-2.0× | +2-5 seconds |
| Lifting cam | 1.4-1.6× | +1-2 seconds |
| Collapsible core | 2.0-3.0× | +3-10 seconds |
Special Undercut Applications
External Threads
| Solution | Application | Cost | Cycle Time |
|---|---|---|---|
| Mitered cut | External threads | Moderate | Standard |
| Stripper plate | Simple threads | Low | Slower |
| Thread inserts | All threads | Variable | Standard |
| Post-mold tapping | Standard threads | Low | N/A |
Internal Undercuts
| Solution | Application | Limitations |
|---|---|---|
| Core pull | Straight IDs | Limited depth |
| Angled lifter | Off-axis holes | Stroke limits |
| Collapsible core | Complex IDs | Size limits |
| Hand load | Prototype/low vol | Manual operation |
Multiple Undercuts
| Challenge | Solution | Notes |
|---|---|---|
| Multiple directions | Multi-axis slides | Complex, expensive |
| Sequential timing | Hydraulic sequencing | Additional cost |
| Symmetric features | Symmetric slides | Coordinated motion |
Maintenance Considerations
Wear Points
| Component | Wear Mechanism | Replacement Interval |
|---|---|---|
| Wear plates | Sliding friction | 100K-500K shots |
| Angle pins | Impact friction | 100K-300K shots |
| Cam surfaces | Sliding friction | 100K-300K shots |
| Lifter guides | Sliding friction | 100K-300K shots |
Maintenance Access
| Design Element | Access Requirement |
|---|---|
| Wear plates | Easy removal/replacement |
| Angle pins | Easy access |
| Return springs | Check/replacement access |
| Hydraulic/pneumatic | Service access |
| Adjustment points | Clear access |
Troubleshooting Guide
| Problem | Likely Cause | Solution |
|---|---|---|
| Slide sticking | Wear, alignment | Check/repair alignment |
| Incomplete retraction | Spring failure | Replace spring |
| Wear marks on part | Wear plate worn | Replace wear plate |
| Part damage | Timing, force | Adjust timing/force |
| Premature wear | Lack of lubrication | Add lubrication |
Design Optimization
Design for Manufacturability
| Guideline | Recommendation |
|---|---|
| Undercut location | Accessible for machining |
| Slide clearance | Adequate clearance for movement |
| Wear surfaces | Hardened steel inserts |
| Standard components | Use catalog items |
Cost Reduction Strategies
| Strategy | Potential Savings | Implementation |
|---|---|---|
| Eliminate undercuts | 20-30% | Design review |
| Simplify slides | 10-20% | Standardize |
| Combine functions | 5-15% | Redesign |
| Use standard parts | 5-10% | Catalog components |
Decision Framework
Undercut Solution Selection
| Question | Answer | Recommended Solution |
|---|---|---|
| Peripheral? | Yes | Slide or stripper |
| Depth <0.125“? | Yes | Standard slide |
| Depth 0.125-0.25“? | Yes | Angle pin slide |
| Depth >0.25“? | Yes | Hydraulic slide |
| Internal feature? | Yes | Lifter or core pull |
| Thread needed? | Yes | Mitered or unscrewing |
Tool Cost Impact
| Solution | Cost Multiplier | Best For |
|---|---|---|
| No undercuts | 1.0× | Simplest |
| 1-2 simple slides | 1.2-1.3× | Most projects |
| 3-4 complex slides | 1.4-1.6× | Moderate complexity |
| Multi-axis slides | 1.8-2.5× | Complex parts |
| Collapsible core | 2.0-3.0× | Specific applications |
The Bottom Line Undercuts are sometimes necessary—but they’re never free.
Each slide adds cost, complexity, maintenance, and cycle time. The design review is where you catch unnecessary undercuts. The solution selection is where you pick the right approach. And the design phase is where you make it work reliably. Don’t add undercuts you don’t need. Pick the simplest solution that works. Design for maintenance and wear. That’s how you build molds that run for a million shots without problems.