Quality Control in Injection Molding: What Buyers Should Check
In This Guide
Incoming Material Inspection
QC starts before molding. The supplier should verify resin grade, color, and moisture content before processing. Confirm the supplier documents resin lot numbers and retains material certificates for traceability.
In-Process Quality Checks
During molding, operators monitor cycle parameters and inspect parts at set intervals. Key in-process checks include:
- Visual inspection for flash, short shots, and surface defects
- Dimensional check on critical features every 50–100 shots
- Weight monitoring to detect material variation
- Color verification against approved sample
In-process quality inspection — critical feature verification
First Article Inspection (FAI)
FAI is a full dimensional report on the first parts from a production run. It confirms the mold is producing parts within specification. Require FAI approval before the run continues — this catches mold wear, setup errors, and material issues early.
AQL Sampling for Mass Production
Acceptable Quality Level (AQL) sampling determines how many parts to inspect per batch. A common standard is ANSI/ASQ Z1.4 (formerly MIL-STD-105E). Specify AQL levels in your purchase order:
| Inspection Level | AQL | Application |
|---|---|---|
| General II | 0.65 critical, 1.0 major, 2.5 minor | Standard consumer products |
| General II | 0.4 critical, 0.65 major, 1.5 minor | Automotive, medical |
| Special S-4 | 0.65 major | Destructive or costly testing |
Common Defects and Prevention
Understanding common defects helps you set realistic quality expectations and identify root causes:
| Defect | Cause | Prevention |
|---|---|---|
| Short shot | Insufficient material fill | Increase injection pressure, vent mold |
| Flash | Excessive clamping or worn mold | Repair parting line, adjust clamping |
| Sink mark | Thick section cooling unevenly | Reduce wall thickness, add cooling |
| Weld line | Material flow meeting from two directions | Adjust gate location, raise temp |
| Warp | Uneven cooling and shrinkage | Uniform wall thickness, cooling design |
Optical measurement — precision dimensional verification
Pre-Shipment Inspection
Before shipment, the supplier should conduct a final inspection covering appearance, dimensions, function, and packaging. Request an inspection report with photos of packed goods. For high-value shipments, consider third-party pre-shipment inspection.
Documentation to Require
Each shipment should include quality documentation. At minimum:
- Dimensional inspection report (with CMM data for critical features)
- Material certificate or COA
- AQL sampling result
- Packing list and photos
- Defect log and corrective actions (if any)
Hengmei Plastic Products performs optical measurement, quality inspection, and parts finishing in-house. Confirm specific QC documentation requirements during quoting.
Frequently Asked Questions
What AQL should I use for plastic parts?
For general consumer products, AQL 2.5 for minor defects, 1.0 for major, and 0.65 for critical. For automotive or medical, tighten to 1.5/0.65/0.4. Confirm with your quality team based on product risk.
How often should dimensions be checked during production?
Critical dimensions should be checked every 50 to 100 shots. Less critical features can be checked hourly. Confirm the inspection frequency in your quality plan.
What is the difference between FAI and pre-shipment inspection?
FAI verifies the first parts of a run meet specification. Pre-shipment inspection samples the finished batch for acceptance. Both are needed — FAI prevents a bad run, PSI prevents shipping defects.
Should I require CMM measurement?
For parts with tight tolerances (±0.05 mm or tighter), CMM measurement is recommended. For general-tolerance parts, calipers and gauges may suffice. Specify measurement method in your quality plan.
Key Takeaways
Effective QC spans material inspection, in-process checks, FAI, AQL sampling, and pre-shipment inspection. Require documented reports at each stage, understand common defects, and set AQL levels appropriate to your product risk. A supplier with in-house inspection capability delivers more consistent quality.
Discuss Your Quality Requirements
Share your QC standards and documentation needs for a quality plan.
To get an accurate quote, include these details in your inquiry:
- Product type and tolerance
- AQL requirements
- Inspection documentation
- Critical dimensions
- Material certificates needed
- Third-party inspection
Send your specifications to Hengmei Plastic Products and receive a tailored quotation.
