Understanding Ctq Injection Molding CTQ,

Critical-to-Quality. The dimensions that matter. The characteristics that, if wrong, will get your parts rejected or cause field failures. I’ve seen CTQ lists that included everything. I’ve seen CTQ lists that missed the critical dimension. Here’s how to identify and control what matters.

Key Takeaways

| Aspect | Key Information |

Understanding CTQ

Definition CTQ characteristics are product requirements that are critical to quality, safety, or regulatory compliance. Failure to meet these requirements results in unacceptable risk.

CTQ Categories CategoryDefinitionExamplesSafetyFailure causes harmLoad-bearing hasRegulatoryLegal requirementsMedical device hasFunctionPart won’t workFit, assembly, performanceCosmeticCustomer rejectsVisible surfacesAssemblyWon’t assembleInterference fits

CTQ vs. Non-CTQ CharacteristicCTQ?

ReasonPart fits assemblyYesFunctionScrew boss holds fastenerYesFunctionSnap-fit flexes correctlyYesFunctionNon-visible rib strengthMaybeDepends on loadOverall lengthYesAssembly referenceHole diameter (fit)YesAssemblyHole diameter (not critical)NoReference onlyCosmetic surface finishYesAppearanceHidden surface finishNoNot visible

CTQ Identification Process

Step 1: Drawing Review ActivityOutputReview all dimensionsComplete dimension listIdentify GD&T symbolsTolerance requirementsNote special notesCritical requirementsIdentify regulatory featuresCompliance requirements

Step 2: Function Analysis QuestionAnswerCTQ?

Does this feature fit with another? Yes/NoIf yes → CTQDoes this feature support load? Yes/NoIf yes → CTQIs this visible to customer? Yes/NoIf yes → CTQDoes failure cause hazard? Yes/NoIf yes → CTQIs this a reference dimension? Yes/NoIf yes → Not CTQ

Step 3: Risk Assessment FactorHigh RiskMedium RiskLow RiskFunction impactWon’t workDegradedNo impactSafety impactDangerousMinorNoneCustomer visibilityVisibleSometimesHiddenAssembly impactWon’t assembleDifficultEasyRegulationRequiredRecommendedNone

CTQ Classification Matrix Risk LevelFunctionSafetyVisibilityAssemblyRegulationResultHighHighHighHighHighHigh★ CTQHighHighMediumMediumMediumLow★ CTQMediumMediumLowHighMediumLowMajorLowLowLowLowLowLowMinor

CTQ Documentation

CTQ Register CTQ #FeatureSpecificationToleranceMeasurementMethodFreqCTQ-001Overall width50.00mm±0.20CMM100%CTQ-002Boss diameter12.50mm±0.05Gage100%CTQ-003Hole position25.00mm±0.10CMMSampleCTQ-004Surface finishRa 0.8MaxProfilometerSample

CTQ Specification FieldInformationCTQ NumberUnique identifierFeature nameDescriptive nameDrawing referenceDimension numberSpecificationNominal valueToleranceUpper/lower limitsMeasurement methodCMM, gage, visualSample sizeHow manyFrequencyHow oftenControl methodSPC, attribute, etc. ResponsibleWho measures

Measurement Strategy for CTQs

Measurement Method Selection CTQ TypeMethodAccuracySpeedPrecision fitCMMExcellentModeratePosition GD&TCMMExcellentModerateFunctional fitFunctional gageVery goodFastSurface finishProfilometerExcellentModerateVisual cosmeticVisual inspectionSubjectiveFast

Sample Size Determination CTQ TypeCriticalitySample SizeFrequency★ SafetyHighest100%Every part★ RegulatoryHighest100%Every part★ FunctionHigh100% or 5/hourHourlyMajorMedium5/hourHourlyMinorLow1-2/hourPeriodic

SPC Requirements CTQ LevelSPC Required?

Chart Type★ CTQYesX-bar/R or I-MRMajorYesI-MR for weightMinorNoMonitoring only

Control Methods

Incoming Control ControlMethodAcceptanceMaterial certsCOA reviewVerify specIncoming inspectionKey testsSampleSupplier monitoringTrend analysisOngoing

In-Process Control ControlMethodReactionProcess monitoringSPC chartsInvestigate signalsFirst/last pieceFull inspectionHold if failsPeriodic checkCTQ measurementAdjust if driftingStatistical inspectionSampleQuarantine if fails

Final Inspection ControlMethodCoverageCTQ verificationPer control plan100% or sampleVisual inspectionStandard100%Functional testAssemblySample or 100%

Inspection Planning CTQ #FeatureMethodEquipmentFrequencyCTQ-001WidthCMM #1HourlyCTQ-002Boss IDGo/No-goRing gageEvery 5 partsCTQ-003PositionCMM #1SampleCTQ-004FinishVisualMaster100%

Control Plan Integration

Control Plan Elements ElementContentsOperationManufacturing stepCharacteristicWhat is being controlledSpecificationCTQ requirementsEvaluation methodMeasurement techniqueSample sizeHow manyFrequencyHow oftenControl methodSPC, inspection, etc. Reaction planWhat if out of spec

Control Plan Example OperationCharacteristicSpecMethodSampleFreqControl20

• InjectionPart weight25.0g ±0.5gScale115 minI-MR chart20
• InjectionWidth50.0mm ±0.2mmCMM5HourlyX-bar/R25
• EjectionFlashNoneVisual100%ContinuousStop if flash30
• TrimGate vestige<0.5mmGage5HourlyGo/No-go

CTQ Validation

Risk-Based Validation CTQ CategoryValidation ScopeDocumentationSafetyFull FAICompleteRegulatoryFull FAICompleteCritical functionFull FAICompleteStandard functionPartial FAIKey dimensionsCosmeticAAI onlyAppearance

Validation Methods MethodUseConfidenceFull dimensionalCTQ listHighCapability studyCTQ parametersHighDesign verificationCritical featuresHighProduction validationActual conditionsVerified

Common CTQ Mistakes

Mistake 1: Too Many CTQs Problem: Listing every dimension as CTQ dilutes focus. Solution: Classify properly. Only true CTQs get full control.

Mistake 2: Missing Critical Dimensions Problem: The one dimension that matters isn’t on the list. Solution: Thorough function analysis. Include failure mode review.

Mistake 3: Wrong Measurement Method Problem: Using slow method on high-frequency CTQ. Solution: Match method to production needs. Functional gages for 100%.

Mistake 4: No Reaction Plan Problem: CTQ found out of spec but no plan to respond. Solution: Document reaction in control plan. Train operators.

Mistake 5: Forgetting Cosmetics Problem: Focusing only on dimensions, ignoring appearance. Solution: Include cosmetic CTQs. Visual standards, AAI.

CTQ Checklist

Identification Drawing reviewed completely All dimensions listed Function analysis completed Safety requirements identified Regulatory requirements identified Customer requirements incorporated CTQ classification complete CTQ register created

Implementation Measurement methods selected Equipment calibrated Control plan updated SPC charts created (if applicable) Sampling plan defined Reaction plan documented Operators trained First articles verified

Validation CTQs measured on production parts Results within specification Capability calculated (if applicable) Control methods verified Documentation complete Customer submitted (if required)

The Bottom Line CTQ is about focus,identifying what matters and controlling it rigorously. Too many CTQs means nothing gets proper attention. Too few means critical things get missed. The drawing tells you what’s specified. The function analysis tells you what’s critical. The risk assessment tells you how to classify. Don’t call everything CTQ. Don’t miss the critical dimensions. That’s how you build quality into the process instead of inspecting it in.