Common Injection Moulding Defects: Causes, Prevention & Design Solutions

Injection moulding is a highly repeatable manufacturing process, yet defects can still occur when design, tooling, material behaviour, and processing conditions fall out of alignment.

Understanding these defects — and more importantly, their root causes — allows engineering teams to prevent costly delays, reduce scrap rates, and maintain consistent product quality.

In many cases, the difference between stable production and recurring issues comes down to decisions made long before the first part is moulded.

Why Defects Occur

Plastic behaves differently from metal during manufacturing. As molten polymer flows into a mould, it cools, shrinks, and solidifies — and each phase must be carefully controlled.

Defects typically stem from one of four areas:

• Part design
• Tool engineering
• Material selection
• Processing parameters

Addressing problems at their source is far more effective than correcting them after production begins.

Sink Marks

Sink marks appear as shallow depressions on the surface of a part, usually above thicker internal sections.

Common Causes

• Excessive wall thickness
• Poor rib design
• Insufficient packing pressure
• Uneven cooling

Prevention Strategies

• Maintain uniform walls
• Keep ribs around 50–60% of nominal thickness
• Optimise packing pressure

Warping

Warping occurs when different areas of a part cool at different rates, causing distortion.

Common Causes

• Non-uniform wall thickness
• Asymmetrical geometry
• Material shrink variation

Prevention Strategies

• Design symmetrical parts where possible
• Promote consistent cooling
• Select dimensionally stable materials

Short Shots

A short shot happens when the mould cavity does not completely fill.

Common Causes

• Walls that are too thin
• Injection pressure too low
• Restricted flow paths

Prevention Strategies

• Increase wall thickness where appropriate
• Improve gate placement
• Adjust processing parameters

Weld Lines

Weld lines form where separate flow fronts meet but fail to bond fully.

While sometimes cosmetic, they can weaken structural performance.

Prevention Strategies

• Optimise gate location
• Raise melt temperature
• Simplify flow paths

Flash

Flash appears as thin excess plastic along the parting line.

Common Causes

• Worn tooling
• Excess injection pressure
• Improper clamp force

Prevention Strategies

• Maintain tooling regularly
• Control processing pressure
• Ensure proper mould alignment

Burn Marks

Burn marks typically appear as dark streaks caused by trapped gases overheating.

Prevention Strategies

• Improve venting
• Reduce injection speed
• Lower melt temperature

The Most Effective Way to Prevent Defects

Many production issues originate during the design stage. Engineering parts with manufacturability in mind dramatically reduces risk.

Early collaboration with an experienced injection moulding partner often prevents problems that are expensive to correct later.