The Common Plastic Injection Molding Defects

Plastic injection molding is one of the most widely used manufacturing processes for producing high-volume, complex, and precise plastic parts. From automotive components to consumer goods, medical devices, and packaging, this process provides efficiency and consistency.

However, even with advanced machinery and high-quality molds, defects can still occur during production. These defects not only compromise product quality but also increase scrap rates, downtime, and costs. Understanding the common plastic injection molding defects—their causes, prevention, and solutions—is crucial for manufacturers aiming for consistent, high-quality output.

This article explores the most common defects in injection molding, providing in-depth explanations and actionable solutions.

 

1. Short Shots

Definition:
A short shot happens when molten plastic does not completely fill the mold cavity, leaving incomplete parts with missing sections.

Causes:

• Insufficient injection pressure.
• Inadequate venting, leading to trapped air.
• Low melt temperature causing poor flowability.
• Improper gate design or small gate size.
• Material with low flow characteristics.

Prevention & Solutions:

• Increase injection pressure and speed.
• Raise melt and mold temperature for better flow.
• Improve venting to release trapped gases.
• Redesign gates and runners to allow smoother material flow.
• Use materials with higher melt flow index (MFI).

2. Flash

Definition:
Flash occurs when molten plastic leaks out of the mold cavity, creating excess thin layers of plastic along parting lines or ejector pin locations.

Causes:

• Worn-out or damaged mold components.
• Excessive injection pressure.
• Insufficient clamping force.
• Misaligned mold halves.

Prevention & Solutions:

• Regularly maintain and repair mold components.
• Optimize injection pressure to avoid excess force.
• Ensure proper mold alignment and sufficient clamping force.
• Use high-precision machining for critical mold areas.

 

3. Sink Marks

Definition:
Plastic injection molding sink marks are small depressions or dents on the surface of a molded part, typically caused by uneven cooling or thick sections.

Causes:

• Excessive wall thickness.
• Insufficient packing pressure during solidification.
• Poor cooling design.
• Material shrinkage.

Prevention & Solutions:

• Maintain uniform wall thickness in part design.
• Increase packing/holding pressure and time.
• Optimize cooling system design to ensure even heat distribution.
• Add ribs or gussets instead of using thick walls.

 

4. Warpage

Definition:
Warpage occurs when parts deform or twist after ejection due to uneven shrinkage or residual stress.

Causes:

• Uneven cooling across part surfaces.
• Poor part design with varying wall thickness.
• High injection pressure or temperature.
• Improper material selection.

Prevention & Solutions:

• Design parts with uniform wall thickness.
• Improve cooling channel placement for uniform heat removal.
• Adjust processing parameters to reduce stress.
• Select materials with lower shrinkage rates.

 

5. Weld Lines

Definition:
Weld lines (also called knit lines) are visible lines formed where two flow fronts of molten plastic meet and fail to bond properly.

Causes:

• Low melt temperature.
• Poor venting causing air entrapment.
• Complex part design requiring multiple flow fronts.
• Insufficient injection speed.

Prevention & Solutions:

• Increase melt and mold temperatures.
• Adjust gate placement to reduce multiple flow fronts.
• Improve venting to allow smooth material flow.
• Increase injection speed for stronger bonding.

 

6. Voids and Bubbles (Blistering)

Definition:
Voids are empty pockets inside the molded part, while bubbles appear as trapped air or gas on the surface.

Causes:

• Insufficient packing pressure.
• Trapped air due to poor venting.
• Moisture in raw material.
• High melt temperature causing gas release.

Prevention & Solutions:

• Increase packing pressure and holding time.
• Dry raw materials thoroughly before processing.
• Improve venting system to release trapped gases.
• Adjust injection temperature to prevent gas formation.

 

7. Burn Marks

Definition:
Burn marks appear as dark brown or black discolorations on the surface of molded parts.

Causes:

• Trapped air or gas compressed at high temperatures.
• High injection speed leading to overheating.
• Insufficient venting.

Prevention & Solutions:

• Improve mold venting to release gases.
• Reduce injection speed and pressure.
• Lower melt temperature to prevent overheating.

 

8. Jetting

Definition:
Jetting occurs when molten plastic enters the cavity at high velocity, forming snake-like flow patterns before the mold fills properly.

Causes:

• Improper gate location.
• High injection speed.
• Low melt temperature causing poor flow.

Prevention & Solutions:

• Relocate gates to allow smoother material entry.
• Reduce injection speed.
• Increase melt and mold temperature for better flow.

 

9. Delamination

Definition:
Delamination is when thin surface layers of the molded part peel away, reducing strength and appearance.

Causes:

• Contamination in raw materials.
• Incompatible resin blends.
• Excessive mold release agents.
• Poor bonding due to low melt temperature.

Prevention & Solutions:

• Use clean, compatible raw materials.
• Dry materials properly.
• Avoid excess mold release agents.
• Optimize processing temperatures for strong bonding.

 

10. Discoloration

Definition:
Discoloration is an unwanted change in color or streaks on the molded part.

Causes:

• Contamination in material or hopper.
• Excessive residence time in barrel.
• Overheating of resin.
• Improper mixing of colorants.

Prevention & Solutions:

• Clean hopper, barrel, and screw regularly.
• Use color masterbatches compatible with base resin.
• Optimize processing parameters to prevent overheating.
• Avoid long residence times inside the barrel.

 

11. Surface Delays: Flow Lines and Tiger Stripes

Flow Lines: Visible streaks or wave-like patterns caused by cooling during incomplete filling.
Tiger Stripes: Dark lines appearing parallel to flow direction.

Causes:

• Inconsistent injection speed.
• Sudden changes in wall thickness.
• Poor mold temperature control.

Prevention & Solutions:

• Optimize injection speed and pressure.
• Redesign part with smooth transitions.
• Maintain consistent mold temperature.

 

12. Ejector Marks

Definition:
Ejector marks are circular or shiny imprints left on the part by ejector pins.

Causes:

• Excessive ejector force.
• Poor pin placement.
• Insufficient draft angle.

Prevention & Solutions:

• Apply appropriate draft angles for easy ejection.
• Relocate or redesign ejector pins to distribute force.
• Reduce ejection force gradually.

 

13. Splay Marks

Definition:
Splay marks appear as silver streaks or lines on the surface, often mistaken for scratches.

Causes:

• Moisture in raw material.
• Air entrapment during injection.
• Degraded resin due to overheating.

Prevention & Solutions:

• Dry hygroscopic materials (like ABS, Nylon, PET).
• Lower melt temperature to avoid degradation.
• Improve venting and optimize injection parameters.

 

14. Warpage Due to Residual Stress

Even if no visible defects appear immediately, residual stress can cause long-term deformation or cracking.

Causes:

• Uneven cooling.
• High packing pressure.
• Incompatible resin additives.

Prevention & Solutions:

• Optimize cooling design.
• Balance holding pressure and time.
• Use stress-relieving materials.

 

Best Practices to Minimize Injection Molding Defects

1. Part Design Optimization
• Use uniform wall thickness.
• Add draft angles.
• Avoid sharp corners.
2. Mold Design Precision
• Ensure proper gate, runner, and venting design.
• Place cooling channels strategically.
• Maintain high-quality surface finish.
3. Material Handling
• Dry hygroscopic resins before processing.
• Use clean, uncontaminated materials.
• Match additives and colorants with resin type.
4. Process Control
• Optimize injection pressure, speed, and temperature.
• Monitor cooling times and cycles.
• Regularly maintain equipment.
5. Simulation and Testing
• Conduct mold flow analysis before manufacturing.
• Run trial shots to identify issues early.
• Adjust parameters before mass production.

 

Conclusion

Defects in plastic injection molding are common, but they are not inevitable. By understanding the types of injection molding defects, their root causes, and preventive measures, manufacturers can significantly improve product quality, reduce waste, and optimize production efficiency.

From short shots and flash to sink marks, weld lines, warpage, and bubbles, each defect points to specific issues in part design, mold design, material handling, or processing conditions. With proper design practices, advanced mold flow analysis, and strict process control, these defects can be minimized or even eliminated.

A proactive approach not only reduces costs but also ensures higher customer satisfaction and long-term business success.