12 Common Injection Molding Defects and Solutions

Injection molding is widely used for manufacturing high-precision plastic parts, but defects can occur due to issues in design, materials, processing, or mold structure. This guide outlines 12 common injection molding defects, their causes, and practical solutions to improve product quality and production efficiency.

1. Shrinkage and Sink Marks

Shrinkage and sink marks are common defects caused by uneven cooling or insufficient material packing.

Causes

Machine-related

• Incorrect nozzle size
• Clamping system issues
• Worn screw or barrel

Mold-related

• Uneven wall thickness
• Poor cooling system design
• Improper gate and runner design

Material-related

• High shrinkage in crystalline plastics
• Insufficient material compensation

Processing-related

• Low injection pressure or short holding time
• Excessive barrel temperature
• Insufficient packing density

Solutions

• Ensure uniform wall thickness
• Optimize cooling system for consistent temperature
• Increase injection pressure and holding time
• Adjust material temperature and flow
• Place gates in thicker sections

2. Flow Marks (Weld Flow Patterns)

Flow marks appear as wavy or ripple-like patterns on the surface.

Causes

• High melt viscosity
• Slow filling speed
• Premature cooling of melt front

Solutions

• Increase barrel and mold temperature
• Increase injection speed and pressure
• Optimize runner and gate design
• Improve mold venting

3. Blisters and Swelling

Blisters usually appear in thick sections or near inserts due to trapped gas expansion.

Causes

• Incomplete cooling
• Internal gas expansion
• Excessive wall thickness

Solutions

• Improve cooling efficiency
• Reduce injection speed
• Increase holding pressure
• Optimize wall thickness design

4. Cracking

Cracks can occur due to internal stress, poor material properties, or improper mold design.

Causes

Processing

• Excessive pressure or speed
• Long holding time
• Low melt temperature

Mold

• Poor ejection balance
• Sharp corners causing stress concentration
• Improper gating system

Material

• High recycled material content
• Moisture contamination

Solutions

• Reduce internal stress through process optimization
• Improve mold polishing and draft angle
• Use annealing to relieve stress
• Avoid sharp corners in design

5. Air Bubbles

Air bubbles appear due to trapped air or insufficient material packing.

Causes

• Low injection pressure
• Poor venting
• Inadequate filling

Solutions

• Increase injection pressure and speed
• Improve mold venting
• Optimize gate location (prefer thick sections)
• Adjust mold and material temperature

6. Warpage

Warpage is caused by uneven shrinkage and internal stress.

Causes

• Non-uniform wall thickness
• Uneven cooling
• Material shrinkage differences

Solutions

• Ensure uniform wall thickness
• Balance cooling system
• Optimize gate layout for symmetrical flow
• Add reinforcing ribs to improve rigidity

7. Color Streaks / Color Lines

Color inconsistency often occurs when using masterbatch.

Causes

• Poor mixing of colorants
• Low melt temperature
• Inadequate shear in barrel

Solutions

• Increase melt temperature
• Increase back pressure
• Improve mixing efficiency
• Optimize gate design

8. Transparency Defects (Silver Streaks / Splay)

Silver streaks appear as shiny lines caused by gas or moisture.

Causes

• Moisture in material
• Gas contamination
• Material degradation

Solutions

• Fully dry plastic material
• Reduce melt temperature
• Improve venting
• Clean runners and nozzles

9. Color Variation

Uneven color distribution across parts.

Causes

• Poor pigment dispersion
• Inconsistent processing conditions
• Uneven cooling

Solutions

• Stabilize processing parameters
• Improve mixing of colorants
• Maintain consistent mold temperature
• Optimize the gating system

10. Poor Surface Gloss

Surface appears dull or uneven.

Causes

• Poor mold surface finish
• Low temperature
• Insufficient pressure

Solutions

• Polish mold cavity
• Increase mold and material temperature
• Improve injection pressure and packing
• Prevent cold material entry

11. Silver Streaks (Gas Marks)

Silver streaks are caused by gas interference inside the melt.

Causes

Machine

• Worn screw/barrel
• Overheating

Mold

• Poor venting
• Runner resistance

Material

• High moisture content
• Contamination

Solutions

• Dry materials thoroughly
• Improve venting system
• Optimize processing parameters
• Reduce screw speed if necessary

12. Weld Lines

Weld lines occur when two melt fronts meet but do not fully fuse.

Causes

• Multiple gates
• Poor venting
• Low melt temperature

Solutions

• Reduce the number of gates or reposition them
• Improve venting near weld areas
• Increase melt and mold temperature
• Maintain uniform wall thickness

Injection molding defects can significantly affect product quality, appearance, and performance. Most defects are caused by a combination of mold design, material selection, and processing parameters.

By identifying the root cause and applying the correct solutions, manufacturers can:

• Improve product consistency
• Reduce defect rates
• Increase production efficiency