Common Plastic Shrinkage & Injection Molding Defects: Causes & Solutions

Understanding plastic behavior is key to high-quality injection molding. Here’s a clear guide to common defects — especially shrinkage, sink marks, short shots, flash, silver streaks, burn marks, flow lines, and weld lines — with practical solutions.

1. Plastic Shrinkage (Volume Contraction)

• All plastics shrink as they cool. This occurs in three stages:Before gate freeze (during packing)

• Cooling phase

• After ejection

Types of Shrinkage:

• Phase change shrinkage (crystallization)

• Orientation shrinkage (molecular alignment)

• Compression & elastic recovery

2. Sink Marks & Voids (Internal Shrinkage)

Cause:

Thick sections cool slower than surrounding areas, causing inward shrinkage (dimpling) or internal voids.

✅ Solutions:

• Optimize gate size/location (larger or closer to thick areas)

• Use multi-stage packing to maintain pressure

• Improve cooling uniformity in thick zones

• Smooth transitions (e.g., around ribs) to reduce localized thickness

• Raise melt & mold temperature slightly for better packing

• Add venting to avoid trapped gas affecting packing

Tip: Sink ≠ void — sink is surface depression; void is internal air pocket.

3. Short Shot (Incomplete Filling)

Cause: Melt doesn’t fully fill the cavity due to poor flow or insufficient pressure.

✅ Solutions:

• Increase injection speed/pressure

• Raise melt & mold temperature

• Add more gates or optimize layout (especially for multi-cavity molds)

• Enlarge runners, gates, cold slug wells

• Improve mold venting

• Check material drying — wet plastic reduces flow

• Reduce flow length; avoid narrow passages

4. Flash (Burrs / Parting Line Leakage)

Cause: Plastic escapes between mold halves or inserts.

✅ Solutions:

• Increase clamping force

• Check mold alignment: worn guide pins, damaged plates, bent tie bars

• Clean parting line — remove dust, oil, or debris

• Ensure nozzle is properly seated

• Avoid excessive injection speed/pressure or melt temperature

• Reduce cavity projection area if possible

• Use shallower venting depth (< 0.03 mm typical)

5. Silver Streaks (Splay Marks)

Cause:

Moisture, air, or degraded gas trapped in melt, stretched during flow.

✅ Solutions:

• Dry material thoroughly before use

• Lower melt temperature to prevent degradation

• Reduce screw speed and increase back pressure to release trapped air

• Clean nozzle, barrel, mold surfaces

• Avoid cold feed entering cavity — check cold slug well

• Slow down initial injection speed

6. Burn Marks

Cause:

Trapped air compresses and overheats, burning the plastic (common at end-of-fill).

✅ Solutions:

• Improve mold venting, especially at last-to-fill areas

• Use multi-stage injection — slow down at the end

• Apply vacuum venting for deep cavities

• Clean clogged vent slots or inserts

• Avoid overly narrow gates that cause shear heating

7. Flow Lines (Waves or Ripple Marks)

Cause:

Cooler melt front meets newer melt, creating visible ripples.

✅ Solutions:

• Increase mold & melt temperature

• Raise injection speed to maintain melt temperature

• Polish gate and runner surfaces

• Eliminate cold material from nozzle tip

• Optimize gate design to prevent jetting

8. Weld Lines (Knit Lines) & Jetting (Snake Marks)

Cause:

Flow fronts rejoin (weld lines), or fast jet of melt creates snaking trails (jetting).

✅ Solutions:

• Redesign gate position to move weld lines to non-critical areas

• Add overflow tabs (waste traps) to shift weld lines

• Increase melt & mold temperature for better fusion

• Raise injection speed/pressure

• Use overlapping gate, tab gate, or fan gate to prevent jetting

• Slow initial injection speed through small gates

• Improve venting at weld zones

✅ Summary: Prevention Over Correction

Pro Tip: Always start with proper material drying, balanced gate design, and adequate venting — they prevent 80% of common defects.