Quick answer: A critical-to-quality characteristic is a measurable product or process feature whose variation can materially affect safety, regulation, function, assembly or agreed appearance. Derive CTQs from failure consequences, define the measurement state, connect them to mold and process mechanisms, and release controls and reactions proportionate to risk.
Release Decision Matrix
| Requirement | Possible CTQ | Molding mechanism |
|---|---|---|
| Seals after aging | Land size, flatness, surface and leak result | Shrinkage, gate, cooling, parting and material state |
| Snap retains load | Geometry, insertion/retention force and cycles | Weld line, orientation, stress and conditioning |
| Visible appearance | Color, gloss, texture and defect boundary | Flow, temperature, vent, gate and cavity surface |
| Assembly alignment | Functional datum positions and fixture result | Steel, actions, warpage and cavity variation |
Approval Checklist
- Trace CTQs to safety, function, assembly, regulation or appearance.
- Define failure consequence and requirement owner.
- Use functional outputs and only necessary supporting dimensions.
- Specify datum, fixture, conditioning, cavity and measurement method.
- Map each CTQ to steel, gate, cooling, action and material mechanisms.
- Correlate justified process signals through trial evidence.
- Release sampling, records, containment and restart rules.
- Review CTQs and controls after relevant changes or field evidence.
Begin With Use and Failure
Describe user, load, environment, life, assembly and misuse relevant to the part. Ask what fails, who notices and what consequence follows. A dimension becomes critical because it controls a function, not because it has many decimals.
Separate mandatory safety or compliance from performance targets and preferences. Trace each CTQ to an approved requirement source.
Choose Product-Level Characteristics
Prefer functional output where it captures the requirement: leak, torque, force, electrical spacing, optical result or assembly gauge. Add dimensions and process signals that prevent or explain failure. Avoid making every drawing dimension a CTQ.
For appearance, define zones, lighting, distance and boundary samples. Subjective words such as suitable or consistent are not measurable release criteria.
Define Measurement State
Specify datums, method, fixture, resolution, sample location, time, temperature, moisture and assembly state. Plastic dimensions can move after molding. Flexible parts may need both free-state and functional-fixture checks.
Validate the measurement system for the tolerance and decision. Include cavity identity and repeatability. A CTQ cannot control production if the gauge signal is unreliable.
Map CTQs to Mold Mechanisms
Trace each characteristic to cavity/core steel, parting, gate, weld line, vent, cooling, action, ejector and material behavior. Mark steel-safe correction and difficult-to-change conditions. This map improves DFM and T1 planning.
Review cavity strategy. One CTQ may vary through runner balance or local thermal conditions, so multi-cavity approval needs cavity-level evidence.
Link Process Signals Without Confusing Them With CTQs
Fill time, peak pressure, cushion, part weight, mold temperature and coolant flow can predict or contain risk, but they are process controls rather than product requirements. Select signals proven to relate to the CTQ.
Use trials or designed experiments to establish relationships and approved limits. Do not set alarms from one convenient batch and assume causation.
Release the Control and Reaction
Define sample size, frequency, cavity coverage, records, owner and reaction. For failure, state containment boundary, escalation, diagnosis, disposition and restart evidence. Integrate CTQs into drawing, control plan and supplier agreement.
Review after product, resin, mold, machine, gauge or process change and from field feedback. Remove controls that no longer manage risk only through authorized review.
Illustrative CTQ Translation
Illustrative engineering example—not a claimed CKMOLD customer result: A cover requirement says ‘must not rattle.’ The team identifies latch retention force and assembled gap as product CTQs, defines conditioning and gauge, maps one latch to a weld-line risk and correlates part weight plus mold temperature during validation. ‘No rattle’ becomes a testable control system.
Supporting Review Resources
Questions Before Approval
What is a CTQ in injection molding?
It is a measurable characteristic whose variation materially affects an important product requirement and therefore needs controlled evidence.
Are all drawing dimensions CTQs?
No. Prioritize dimensions and functions tied to significant consequences; other dimensions can remain standard specification characteristics.
Can a process parameter be a CTQ?
It is usually a process control linked to a product CTQ. Keep product requirement and process evidence distinct.
Why include cavity in CTQ data?
Cavities can differ through steel, gate, vent, cooling and wear, and pooled data can hide a weak cavity.
Apply the Turning Product Requirements Into Injection-Molding CTQs review to your released design. Share the function, exact resin, annual volume, CTQs, cosmetic limits and launch timing through the CKMOLD project form. If CAD is relevant, send it directly to jerry@ckmold.com; the form does not require an upload.