In high-volume plastic injection molding, the most common headache is molding defects: sink marks, flash, short shots, warpage, weld lines, silver streaks, burn marks, and more. These problems rarely have a single cause — they result from the interaction of material, mold, and machine parameters. This guide breaks down the eight most common injection defects, their causes, and remedies from a shop-floor, actionable perspective, helping SQE, process, and QA engineers locate problems quickly and reduce defect rates.
Reading tip: This is an advanced companion to the Plastic Injection Molding Complete Process Guide. If you are not yet familiar with the six injection stages and material selection, read that first — the troubleshooting here will make more sense.
01The Basic Troubleshooting Mindset: Classify First
When you hit a defective part, don't rush to change parameters. An effective troubleshooting sequence:
- Identify the defect type — Photograph and mark the location; confirm whether it is sink, flash, or something else.
- Determine the cause category — Material (moisture, lot), mold (wear, venting), or parameters (temperature, pressure, speed)?
- Adjust one variable at a time — Change only one parameter per trial and record results, so cause and effect stay clear.
- Return to first-article check — After adjusting, re-run first-article inspection and confirm stability before resuming production.
With the three principles "classify first, single variable, keep records," most issues converge within a few shots.
02Eight Common Defects and Remedies
1. Sink Mark
Symptom: Surface depressions, common at thick sections, ribs, or behind bosses.
Main causes: Excess wall thickness with uneven cooling, low holding pressure, high melt temperature.
Remedies: Increase holding pressure and time, lower melt temp, optimize wall thickness (rib at 50–60% of main wall), extend cooling.
2. Short Shot
Symptom: Cavity not filled; missing material, incomplete edges.
Main causes: Low injection pressure/speed, low melt temp, undersized runner/gate, poor venting.
Remedies: Raise injection pressure and speed, raise melt temp, check gate size, improve mold venting.
3. Flash
Symptom: Thin film of excess plastic at the parting line or ejector pins.
Main causes: Insufficient clamping force, high melt temp/pressure, parting-line wear or debris.
Remedies: Increase clamping force, lower injection pressure and melt temp, inspect and repair the parting line.
4. Warpage
Symptom: Part deviates from design shape; surfaces not flat, edges lifting.
Main causes: Uneven cooling, residual internal stress, unbalanced ejection, large wall variation.
Remedies: Balance mold cooling channels, adjust ejection position and speed, improve wall uniformity, extend cooling appropriately.
5. Weld Line
Symptom: A fine line where two or more flow fronts meet, affecting appearance and strength.
Main causes: Multiple gates or flow splitting around holes, low melt temp.
Remedies: Adjust gate location to move the weld line off cosmetic surfaces, raise melt temp and speed, improve venting.
6. Silver Streak
Symptom: Silvery, radiating streaks on the surface.
Main causes: Moist resin, melt-temp degradation, poor venting.
Remedies: Dry the resin thoroughly (especially hygroscopic materials), lower melt temp, improve venting. Drying conditions for materials such as HIPS must be properly controlled.
7. Burn Mark
Symptom: Charring at the part end or poorly vented areas.
Main causes: Trapped-gas overheating from poor venting (diesel effect), high melt temp, excessive injection speed.
Remedies: Improve mold venting, lower injection speed and melt temp, clear runner residue.
8. Flow Mark
Symptom: Wavy or concentric marks near the gate.
Main causes: Low melt or mold temp, slow injection speed, poor gate design.
Remedies: Raise melt and mold temp, adjust injection speed, optimize the gate.
03Defect–Parameter Quick Reference
The table below summarizes the "priority adjustment direction" for common defects for quick shop-floor judgment:
| Defect | Melt Temp | Inj. Pressure | Inj. Speed | Holding | Mold Temp | Other key |
|---|---|---|---|---|---|---|
| Sink | ↓ | — | — | ↑ | — | Optimize wall |
| Short shot | ↑ | ↑ | ↑ | — | ↑ | Check venting |
| Flash | ↓ | ↓ | — | ↓ | — | Raise clamping |
| Warpage | — | — | — | — | balance | Cooling channels |
| Weld line | ↑ | — | ↑ | — | ↑ | Adjust gate |
| Silver streak | ↓ | — | — | — | — | Dry resin |
| Burn | ↓ | — | ↓ | — | — | Improve venting |
| Flow mark | ↑ | — | ↑ | — | ↑ | Optimize gate |
Note: ↑ = try raising, ↓ = try lowering, — = usually not the main factor. Always verify with single-variable trials.
04Stopping Defects Before Production
The most effective "troubleshooting" is preventing defects from happening. Preventive measures:
- Material management — Dry hygroscopic materials properly and control lots to avoid silver streaks and strength issues.
- Mold maintenance — Regularly check the parting line, ejector pins, and venting; address wear early to prevent flash.
- First-article and patrol checks — Run first-article after mold changes/setup, and patrol-inspect during production to catch issues early.
- Parameter standardization — Build standard parameter sheets per product to reduce variation from experience-based tweaking.
Guann-Ming Industrial uses 36 years of plastic-reel manufacturing experience and an ISO 9001 quality system to stop defects before shipment through standardized processes and inspection, ensuring stable, consistent batch quality for SMD reels.
05Defect Control and SMD Reel Quality
For SMD carrier reels, dimensional stability (outer diameter, center bore, flange flatness) directly affects mounting smoothness and tape winding. Uncontrolled warpage or sink can cause reel wobble or difficulty fitting the axle. Controlling injection defects is, in essence, controlling reel reliability.
G.M brand reels use HIPS as the primary material, with selectable anti-static / conductive / insulative properties, are compatible with EIA-481, and are applicable to SMT volume production and the component-packaging needs of OSAT, Fabless, and IDM customers. For full specs and OEM service, see Carrier Reels — Flagship Products and Injection OEM Service.
06Conclusion
Injection defects are usually the result of material, mold, and parameter interaction; the key to troubleshooting is "classify first, single variable, keep records." Building preventive material management, mold maintenance, and parameter standardization keeps defect rates to a minimum. If you face quality bottlenecks in reel production or injection OEM, contact the Guann-Ming team for a full assessment and recommendations.