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Common Injection Molding Defects and Practical Solutions for Better Part Quality
Common injection molding defects including bubbles weld lines sink marks flash and burn marks on plastic products

Common Injection Molding Defects and Practical Solutions for Better Part Quality

Injection molding defects can significantly affect product appearance, dimensional stability, strength, and production efficiency. Many molding problems are caused by improper processing conditions, mold design issues, unsuitable materials, or inconsistent cooling.

Understanding the causes of common injection molding defects can help manufacturers improve product quality, reduce scrap rates, and optimize production efficiency.

1. Cracking

Cracking is one of the most common defects in plastic products and is mainly caused by stress deformation. Cracks may result from residual stress, external stress, or environmental stress.

Residual Stress Cracks

Residual stress is often caused by overfilling, demolding problems, or metal inserts.

  • Use multi-point gates, side gates, or fan gates if cracks occur near the sprue.
  • Increase resin temperature to improve melt flow and reduce injection pressure.
  • Increase mold temperature when necessary to reduce stress.
  • Shorten injection and holding time if they are too long.
  • Pay special attention when molding non-crystalline resins such as ABS, PMMA, and AS resin.

External Stress Cracks

External stress cracks are usually caused by poor product design, especially sharp corners and stress concentration areas.

Environmental Stress Cracks

Chemicals, moisture absorption, recycled material, and environmental exposure can reduce plastic performance and lead to cracking.

2. Short Shots and Insufficient Filling

Insufficient filling can be caused by inadequate resin volume, low cavity pressure, poor material flow, or poor venting.

  • Increase injection time and speed.
  • Raise mold temperature and resin temperature.
  • Increase injection pressure.
  • Enlarge gate size and place the gate at the thickest section of the product.
  • Add vent grooves or ejector pin venting.
  • Use lower-viscosity materials if necessary.

3. Wrinkles and Surface Pockmarks

Wrinkles and pockmarks are closely related to insufficient filling and poor melt flow. Solutions are similar to those for short shots.

For materials with poor flow characteristics, such as polyoxymethylene, PMMA, polycarbonate, and polypropylene, proper gate enlargement and longer injection time are especially important.

4. Sink Marks

Sink marks are generally caused by localized shrinkage in thick wall sections.

  • Maintain uniform wall thickness during product design.
  • Reduce the thickness of ribs and bosses.
  • Increase holding pressure and holding time when appropriate.

5. Flash

Flash occurs when molten plastic escapes from the mold parting line or gaps in the tooling.

  • Reduce injection pressure.
  • Lower resin temperature.
  • Use higher-viscosity materials.
  • Reduce mold temperature.
  • Improve mold clamping force and mold fit.
  • Use harder mold steel and reinforce the mold structure if necessary.

6. Weld Lines

Weld lines form when multiple melt fronts meet but do not fully fuse together. They often reduce part strength and affect appearance.

  • Increase resin temperature, mold temperature, injection pressure, and injection speed.
  • Add vent grooves near the weld line area.
  • Reduce the use of release agents.
  • Move the weld line to a less visible area through gate redesign.

7. Burns

Burn marks may be caused by overheated resin, trapped gas, or poor venting.

  • Clean the nozzle, screw, and barrel if resin degradation occurs.
  • Add vent grooves to improve air release.
  • Reduce screw speed, back pressure, and injection speed if overheating occurs.

8. Silver Streaks

Silver streaks are usually caused by moisture in the material.

  • Dry the resin thoroughly before molding.
  • Avoid excessive residence time in the barrel.
  • Avoid mixing incompatible materials such as ABS and polystyrene.

9. Jetting

Jetting appears as snake-like flow marks caused by excessively fast resin flow through the gate.

  • Reduce injection speed.
  • Increase gate size.
  • Increase mold temperature to slow surface cooling.

10. Warpage and Deformation

Warping is often caused by uneven shrinkage, poor cooling, mold release issues, or residual stress.

  • Reduce injection pressure and increase mold temperature consistency.
  • Improve ejector design and draft angle.
  • Optimize cooling channel layout and cooling time.
  • Design products with uniform wall thickness whenever possible.

Crystalline plastics such as nylon, polypropylene, polyethylene, and PET generally show greater shrinkage and deformation than non-crystalline plastics such as ABS and PMMA.

11. Bubbles

Bubbles may be caused by vacuum voids in thick wall sections, gas generation, or poor melt flow.

  • Use proper gate and runner sizes.
  • Increase holding pressure and holding time.
  • Reduce injection speed if necessary.
  • Dry the material thoroughly.
  • Increase mold and resin temperature to improve flow.

12. Whitening

Whitening often occurs in ABS parts and is usually caused by poor demolding.

  • Reduce injection pressure.
  • Increase draft angle.
  • Increase ejector pin quantity or contact area.
  • Reduce mold surface roughness.

Careful mold design, proper processing conditions, suitable material selection, and effective cooling are the key factors in preventing injection molding defects and improving overall product quality.

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