How to Define CTQ Parameters for Molding

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.

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

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

Step 2: Function Analysis

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

CTQ Classification Matrix Risk

CTQ Documentation

CTQ Register CTQ

CTQ Specification

Measurement Strategy for CTQs

Measurement Method Selection CTQ

Sample Size Determination CTQ

SPC Requirements CTQ LevelSPC Required?

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

Control Methods

Incoming Control

In-Process Control

Final Inspection

Inspection Planning CTQ

Control Plan Integration

Control Plan Elements

Control Plan Example

• Injection Part weight25.0g ±0.5g Scale115 min I-MR chart20
• Injection Width50.0mm ±0.2mm CMM5 Hourly X-bar/R25
• Ejection Flash None Visual100% Continuous Stop if flash30
• Trim Gate vestige<0.5mm Gage5 Hourly Go/No-go

CTQ Validation

Risk-Based Validation CTQ

Validation Methods

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.