Why Material Matters in Defect Diagnosis
In injection molding, the same visual defect can have completely different root causes depending on the plastic you’re running. For example, silver streaks in ABS often come from moisture, while in PC they may signal material degradation from excessive melt temperature.That’s why a material-specific defect guide is essential for anyone working with custom injection molds across varied product lines. Below are the 10 most common injection molding defects, grouped by typical material types, with quick diagnostic tips and corrective actions.
1. Short Shot (Incomplete Fill)
Common in: PP, HDPE, thin-wall ABS, PCCauses by material:
- PP/HDPE: Low melt viscosity + low injection pressure → flow stalls in thin sections.
- ABS: Improper gate location for complex geometry.
- PC: High melt viscosity + low barrel temperature → poor flow.Quick Fix:
- Increase injection pressure (gradually).
- Raise barrel/mold temperature for high-viscosity plastics (PC, ABS).
- Optimize gate location/number for uniform flow.
2. Sink Mark (Depression on Thick Sections)
Common in: PP, PE, glass-filled PA66Causes by material:
- PP/PE: High shrinkage + insufficient holding/pack time.
- Glass-filled PA66: Lower resin shrinkage but filler-rich areas can leave depressions if packing is uneven.Quick Fix:
- Extend holding time until part stops shrinking.
- Increase gate size or add supplemental gates.
- For GF-PA66, ensure uniform packing pressure to avoid filler settling voids.
3. Flash (Burrs at Parting Line)
Common in: Soft PVC, LDPE, flexible TPE overmoldingCauses by material:
- PVC: Low melt strength + high injection pressure → excess flow at weak parting lines.
- LDPE/TPE: Very high flowability → easily overflows small gaps.Quick Fix:
- Lower injection pressure/velocity.
- Improve mold clamping force or repair worn parting surfaces.
- For PVC, ensure barrel temperature is controlled to avoid excess melt fluidity.
4. Weld Lines (Knit Lines)
Common in: ABS, PC, PMMA, glass-filled nylonsCauses by material:
- ABS/PC: Low melt temperature → fronts meet cold and don’t fuse well.
- GF-nylons: Fibers disrupt fusion, creating visible lines.Quick Fix:
- Raise melt and mold temperature to keep flow fronts hot.
- Increase injection speed to push fronts together before cooling.
- For GF materials, optimize gate placement to minimize flow merges.
5. Silver Streaks / Bubbles
Common in: ABS, PC, PA66, PMMACauses by material:
- ABS/PA66: Moisture → vaporizes in melt.
- PC: Overheating → material degrades, releasing gases.
- PMMA: Trapped air + moisture.Quick Fix:
- Dry materials to required moisture level (ABS/PA66: <0.1%, PC: <0.02%).
- Lower melt temperature if degradation suspected.
- Improve venting in trapped-air zones.
6. Burn Marks / Black Specks
Common in: PVC, PC, POMCauses by material:
- PVC: Thermal degradation releases HCl gas → carbonization.
- PC: Excessive shear heat in nozzle/runner.
- POM: Decomposes at high temps → black deposits.Quick Fix:
- Reduce barrel/nozzle temperature.
- Lower injection speed to avoid air compression heating.
- Clean contaminated zones and check screw/check ring wear.
7. Warpage (Twisting)
Common in: PP, HDPE, glass-filled PA66, ABSCauses by material:
- PP/HDPE: High shrinkage anisotropy.
- GF-PA66: Differential shrinkage between flow and transverse directions.
- ABS: Uneven cooling → internal stresses.Quick Fix:
- Optimize cooling channel layout for uniform temperature.
- Increase mold temperature slightly for crystalline plastics (PP, PE).
- For GF materials, orient gates to reduce flow-induced orientation.
8. Jetting (Snake-like Flow Marks)
Common in: PC, PMMA, rigid PVCCauses by material:
- High melt viscosity + improper gate land length → melt shoots into cavity like a jet, cooling prematurely.Quick Fix:
- Increase gate land length (smooth transition).
- Lower injection speed initially, then ramp up.
- Raise mold temperature to reduce rapid surface skin formation.
9. Delamination (Layer Separation)
Common in: ABS/PC blends, multi-material overmolding (TPE over rigid substrate)Causes by material:
- Contamination between incompatible melts or moisture at interface.Quick Fix:
- Ensure proper purging between material changes.
- Dry all materials thoroughly.
- Optimize bonding process parameters (temperature, pressure).
10. Flow Lines (Visible Flow Pattern)
Common in: Transparent materials (PC, PMMA, SAN), PPCauses by material:
- Melt front cooling at different rates → variation in refractive index visible.Quick Fix:
- Increase melt and mold temperature to reduce viscosity difference.
- Optimize injection speed profile (constant speed where possible).
- Polish flow-direction transitions in cavity.
Quick Material-to-Defect Reference Table
| Material | Frequent Defects | Primary Cause | Key Fix |
|---|---|---|---|
| ABS | Short shot, weld lines, silver streaks | Moisture, low temp | Dry, raise temp/speed |
| PC | Silver streaks, burn marks, flow lines | Moisture, overheating | Dry, lower temp |
| PP | Sink mark, warpage, short shot | High shrinkage, low pressure | Hold longer, raise pressure |
| Glass-filled PA66 | Sink mark, weld lines, warpage | Filler settling, orientation | Uniform packing, gate optimization |
| PVC | Flash, burn marks | Thermal degradation | Control temp, reduce pressure |
| PMMA | Bubbles, flow lines | Moisture, trapped air | Dry, improve venting |
Conclusion – Diagnose Faster, Fix Smarter
By linking defects to material behavior, this guide helps you reduce trial-and-error in your injection molding process, especially for custom injection molds running multiple resins. Remember: the more you know about how your material behaves, the quicker you can zero in on the real cause and apply the right fix.Need a material-specific DFM review for your next project? Send us your part data and resin type — we’ll help you preempt defects before mold build.(P.S. Want this table as a printable PDF? Comment “MATERIAL DEFECTS” and we’ll send it over.)
