10 Common Injection Molding Defects and How to Fix Them

Defects rarely have a single cause. The fastest troubleshooting method is to ask: is it a material problem, a process problem, a mold problem, or a part-design problem? Work through that order for each defect below.

Figure 1. Chart of six common injection molding defects.

1. Sink Marks

Symptom: Local depressions on the surface, usually above ribs or bosses.

Causes & fixes:

• Insufficient packing pressure/time → increase holding pressure; verify gate seal time
• Thick sections behind the surface → core out ribs/bosses, reduce rib thickness to ≤60% of wall
• High melt or mold temperature → reduce slightly to shorten the pack window

2. Warping

Symptom: Part is twisted or bowed after ejection.

Causes & fixes:

• Differential shrinkage from uneven wall thickness → redesign toward uniform walls
• Unbalanced cooling → balance cooling circuits; check fixed/moving half temperature difference
• Molecular orientation stress → raise melt/mold temperature, reduce injection speed
• Fiber orientation in glass-filled grades → change gate location or add flow leaders

3. Flash

Symptom: Thin plastic escaping at the parting line or around pins.

Causes & fixes:

• Excessive injection/holding pressure → reduce, especially second-stage pressure
• Damaged or contaminated parting surface → clean, inspect, and repair shut-off faces
• Insufficient clamp tonnage → verify project area × cavity pressure vs. available tonnage
• Worn mold → check guide pillars and cavity alignment

4. Short Shot (Incomplete Fill)

Symptom: Part is not completely filled; edges or thin features missing.

Causes & fixes:

• Insufficient shot size or cushion → verify dosing and non-return valve condition
• Air traps in last-to-fill areas → add or enlarge vents; profile speed down at end of fill
• Flow restriction → check gate size, runner balance, and cold slug wells
• Material too viscous → raise melt temperature or switch to an easier-flow grade

5. Weld Lines (Knit Lines)

Symptom: Visible line where two flow fronts meet; possible weak point.

Causes & fixes:

• Cold fronts meeting → raise melt and mold temperature 10–20 °C
• Low pressure at the junction → increase holding pressure; relocate gate to move the weld line
• Weld in a cosmetic/structural area → change gate position, add overflow well, or redesign with a flow splitter

6. Silver Streaks / Splay

Symptom: Silvery streaks radiating from the gate.

Causes & fixes:

• Wet resin → dry to spec (e.g., PC: 120 °C / 4 h; PA: 80 °C / 4 h) and confirm moisture <0.02%
• Excessive melt temperature causing degradation → lower barrel temps, reduce residence time
• Gate/runner too small causing shear splay → enlarge gate, reduce injection speed

7. Burn Marks (Gas Burns / Dieseling)

Symptom: Dark or scorched marks at the end of fill or in deep ribs.

Causes & fixes:

• Trapped compressed air → add vents in last-fill zones; profile speed down before full fill
• Injection too fast into thin sections → multi-stage speed profile
• Contaminated or degraded material → purge, check residence time

8. Flow Marks / Record Grooves

Symptom: Circumferential ripples near the gate.

Causes & fixes:

• Melt front stalling then re-accelerating → increase injection speed for a continuous front
• Low melt/mold temperature → raise both slightly
• Gate too small → enlarge to keep the front moving

9. Voids

Symptom: Hollow bubbles inside thick sections, visible on X-ray or cut samples.

Causes & fixes:

• Shrinkage in thick sections → core out the section, increase pack pressure and time
• Moisture or volatiles → dry material properly
• Use gas-assist or foaming for unavoidably thick geometry

10. Brittle Parts / Degradation

Symptom: Parts crack under normal load, discoloration, or smell.

Causes & fixes:

• Over-drying or wrong drying temperature (especially PA) → follow supplier schedule
• Excessive residence time → size the shot to demand; barrel temperature too high
• Weld line in a stress area → relocate gate; anneal parts where required

The Troubleshooting Mindset

Change one parameter at a time and document every trial. When process changes stop helping, the root cause is usually design or tooling — and no amount of parameter tuning will compensate. If you are fighting a persistent defect, send us the part, material, and process sheet; our engineers diagnose most recurring defects within one review cycle.

Figure 2. Quick reference: the key checks and reference values from this guide.