Plastic Injection Molding Defects — Causes and Solutions

Common Injection Molding Defects

Injection molding defects can affect part appearance, function, and cost. Understanding root causes enables quick troubleshooting and prevention.

1. Sink Marks

Appearance: Depressions on part surface, typically near thick sections.

Causes:

• Excessive wall thickness variation
• Ribs too thick relative to wall
• Insufficient cooling time
• High packing pressure needed

Solutions:

• Reduce rib thickness to 50-70% of wall
• Increase cooling time
• Optimize packing pressure and time
• Use gas-assist molding for thick sections

2. Flash

Appearance: Thin excess plastic at parting lines or around features.

Causes:

• Insufficient clamp force
• Worn mold components
• Excessive injection pressure
• Poor mold alignment

Solutions:

• Increase clamp tonnage
• Repair or replace worn components
• Reduce injection pressure
• Check mold alignment and guide pins

3. Short Shots

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Appearance: Incomplete parts with missing features.

Causes:

• Insufficient material volume
• Blocked vents
• Low injection pressure or speed
• Material freezing in thin sections

Solutions:

• Increase shot size
• Clean or add vents
• Increase injection speed
• Raise mold temperature

4. Warpage

Appearance: Part distortion after ejection.

Causes:

• Non-uniform cooling
• Asymmetric part design
• Incorrect ejection timing
• Material shrinkage variation

Solutions:

• Optimize cooling channel design
• Balance wall thickness
• Adjust ejection timing
• Use material with lower shrinkage

5. Weld Lines

Appearance: Visible lines where flow fronts meet.

Causes:

• Multiple gates or flow fronts
• Holes or inserts creating flow splits
• Low injection speed
• Cold material at meeting point

Solutions:

• Move weld lines to non-critical areas
• Increase injection speed
• Raise mold temperature
• Add overflow wells at weld locations

Defect Prevention Checklist

• Design parts with uniform wall thickness
• Include adequate draft angles
• Provide proper venting
• Optimize gate location
• Ensure balanced cooling

Schlussfolgerung

Most injection molding defects stem from design, processing, or mold issues. Early detection and systematic troubleshooting minimize production downtime and scrap rates.

Related Resources

• Engineering Plastic Cost Factors
• PEEK vs PEI Comparison
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Related Resources

• Engineering Plastic Cost Factors
• PEEK vs PEI Comparison
• Flame Retardant Plastics
• Food Grade Plastics Guide
• Nylon Moisture Treatment

Related Resources

• Engineering Plastic Cost Factors
• PEEK vs PEI Comparison
• Flame Retardant Plastics
• Food Grade Plastics Guide
• Nylon Moisture Treatment

FAQ

What causes sink marks in injection molding?

Sink marks occur when thick sections shrink more than surrounding areas during cooling. Use uniform walls and proper rib design.

How do I prevent flash?

Ensure adequate clamp force, maintain mold condition, and optimize injection pressure for the material.

What is a short shot defect?

Short shots are incomplete parts caused by insufficient material, blocked vents, or material freezing before cavity fills.

Why do parts warp after molding?

Warpage results from non-uniform cooling and shrinkage. Balance cooling and wall thickness to minimize.

Can weld lines be eliminated?

Weld lines cannot always be eliminated but can be positioned in non-critical areas and minimized through processing optimization.