Designing Undercuts Injection Molds Undercuts are where mold design gets interesting. 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.

Key Takeaways

| Aspect | Key Information |

Understanding Undercuts

Definition An undercut is any feature that prevents the part from being ejected straight out of the mold cavity.

Types of Undercuts TypeDescriptionTypical SolutionPeripheralAround the outsideSlides or stripper plateInternal holeHole not on parting lineCore pulls or liftersInterlockinghas locking partComplex slidesUndercut ribRib on interior wallLifting cam or lifter

Undercut Classification ClassComplexitySolutionCost MultiplierClass 1SimpleStandard slides1.2-1.3×Class 2ModerateComplex slides1.3-1.5×Class 3ComplexMulti-axis slides1.5-2.0×Class 4Very complexSpecial mechanisms2.0-3.0×

Slide Systems

Basic Slide Components ComponentFunctionSlide blockCarries the cam/cavity insertCam/heelProvides angled surface for actionAngle pinDrives the slideWear plateProvides sliding surfaceReturn springReturns slide on closing

Slide Drive Methods

| Method | Description | Stroke | Speed |

Slide Stroke Requirements Undercut DepthSlide AngleStroke Calculation0.125” (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 GuidelineRecommendationReasonMaximum angle25°Prevents binding, excessive strokePreferred angle15-20°Good balanceMinimum angle10°Stroke becomes excessiveStroke calculationStroke = d / sin(θ)Design formula

Slide Size Guidelines FactorGuidelineNotesSlide length3-4× strokeStabilitySlide width2-3× heightRigidityTravel checkVerify clearancePrevent interferenceGuidanceFull-length guidanceAccuracy

Lifter Systems

Lifter Types TypeApplicationMechanismAngled lifterInternal undercutsAngled movementCam lifterComplex undercutsControlled pathRoller lifterHigh-speed operationLow frictionHydraulic lifterLarge movementsPowerfulPneumatic lifterSmall movementsSimple

Lifter Stroke Calculation GeometryFormulaExampleAngled lifterStroke = d / sin(θ)d=0.25”, θ=15° → 0.97”Vertical liftStroke = dd=0.25” → 0.25”CompoundVector calculationDepends on angles

Lifter Design Guidelines GuidelineValueReasonMinimum angle10°Adequate liftMaximum angle25°Prevent bindingStroke clearance+25% minimumSafety marginReturn methodSpring or gravityEnsure returnGuidanceFull lengthAccuracy

Alternative Undercut Solutions

Alternative Methods MethodApplicationProsConsStripper platePeripheral undercutsSimple, fastLarge plate neededGerotorInternal gearsComplex shapesLimited sizesCollapsible coreInternal undercutsNo slidesExpensive, limitedThreaded coreScrew threadsAccurate threadsSlow cycleUnscrewingScrew capsStandard threadsComplex, slow

Stripper Plate Design GuidelineValueNotesPlate thickness1.5-2× strokeRigidityTravelStroke + 0.5” minimumClearanceForceCalculate based on areaAdequate cylinderSpeedControlledPrevent part damage

Collapsible Core Applications ApplicationCore DiameterCollapse MethodBottle necks10-50mmWedge/fingerInternal threads10-30mmSegmentedComplex IDsVariableCustom mechanism

Undercut Design Guidelines

General Principles PrincipleRecommendationMinimize undercutsEliminate if possibleSimplify solutionsStandard slides preferredConsider manufacturingDesign for easy machiningConsider maintenanceAccess for repair

Design Checklist 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: AlternativeWhen It WorksMove to parting lineFeature can be on parting surfaceChange part orientationDifferent eject directionModify geometryEliminate if not criticalUse snap-fitReplace rigid feature

Cost Comparison SolutionRelative CostCycle ImpactParting line feature1.0×NoneStandard slide1.3-1.5×+1-3 secondsComplex slide1.5-2.0×+2-5 secondsLifting cam1.4-1.6×+1-2 secondsCollapsible core2.0-3.0×+3-10 seconds

Special Undercut Applications

External Threads SolutionApplicationCostCycle TimeMitered cutExternal threadsModerateStandardStripper plateSimple threadsLowSlowerThread insertsAll threadsVariableStandardPost-mold tappingStandard threadsLowN/A

Internal Undercuts SolutionApplicationLimitationsCore pullStraight IDsLimited depthAngled lifterOff-axis holesStroke limitsCollapsible coreComplex IDsSize limitsHand loadPrototype/low volManual operation

Multiple Undercuts ChallengeSolutionNotesMultiple directionsMulti-axis slidesComplex, expensiveSequential timingHydraulic sequencingAdditional costSymmetric featuresSymmetric slidesCoordinated motion

Maintenance Considerations

Wear Points ComponentWear MechanismReplacement IntervalWear platesSliding friction100K-500K shotsAngle pinsImpact friction100K-300K shotsCam surfacesSliding friction100K-300K shotsLifter guidesSliding friction100K-300K shots

Maintenance Access Design ElementAccess RequirementWear platesEasy removal/replacementAngle pinsEasy accessReturn springsCheck/replacement accessHydraulic/pneumaticService accessAdjustment pointsClear access

Troubleshooting Guide ProblemLikely CauseSolutionSlide stickingWear, alignmentCheck/repair alignmentIncomplete retractionSpring failureReplace springWear marks on partWear plate wornReplace wear platePart damageTiming, forceAdjust timing/forcePremature wearLack of lubricationAdd lubrication

Design Optimization

Design for Manufacturability GuidelineRecommendationUndercut locationAccessible for machiningSlide clearanceAdequate clearance for movementWear surfacesHardened steel insertsStandard componentsUse catalog items

Cost Reduction Strategies StrategyPotential SavingsImplementationEliminate undercuts20-30%Design reviewSimplify slides10-20%StandardizeCombine functions5-15%RedesignUse standard parts5-10%Catalog components

Decision Framework

Undercut Solution Selection QuestionAnswerRecommended SolutionPeripheral?

YesSlide or stripperDepth <0.125”? YesStandard slideDepth 0.125-0.25”? YesAngle pin slideDepth >0.25”? YesHydraulic slideInternal feature? YesLifter or core pullThread needed? YesMitered or unscrewing

Tool Cost Impact SolutionCost MultiplierBest ForNo undercuts1.0×Simplest1-2 simple slides1.2-1.3×Most projects3-4 complex slides1.4-1.6×Moderate complexityMulti-axis slides1.8-2.5×Complex partsCollapsible core2.0-3.0×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.