Identifying and troubleshooting plastic injection molding defects is essential for maintaining product quality, reducing waste, and optimizing production efficiency. Here, we discuss 9 common injection molding defects and provide practical solutions for addressing them. Of course, the most accurate way to predict and determine injection molded defects is through the use of cavity pressure sensors and a process control system, but here are some techniques to try regardless of your available equipment.

*Please note, these are not exhaustive lists, as there are countless things that could cause any one of these, but it’s a starting point.

1. Sink Marks

Sink marks are depressions or indentations on the surface of molded parts, typically occurring in thicker sections.

Causes:

• Insufficient cooling time
• Inadequate packing pressure
• Improper gate design
• Poor part design

Solutions:

• Increase cooling time to ensure complete solidification.
• Adjust packing pressure to ensure material fills voids during cooling.
• Redesign the gate to improve material flow and pressure distribution.
• Change part design.

2. Warping  

Warping is the distortion or bending of a part, leading to an uneven or twisted shape.

Causes:

• Uneven cooling rates
• Mold temperature imbalance
• Material shrinkage
• Non-uniform part thickness

Solutions:

• Ensure uniform cooling by optimizing the cooling channel design.
• Balance mold temperatures to prevent differential cooling.
• Use materials with lower shrinkage rates or adjust processing parameters to control shrinkage.

3. Flash  

Flash is excess material that extends from the edges of a part, often along the parting line of the mold.

Causes:

• Filling the mold too full during fill
• Worn or damaged mold parts
• Improper clamping force

Solutions:

• Decouple the process to ensure the mold is not being over-filled during filling.
• Regularly maintain and inspect molds to prevent wear and damage.
• Adjust clamping force to ensure the mold halves are properly aligned and sealed.

4. Short Shots  

Short shots occur when the mold cavity is not completely filled, resulting in incomplete parts.

Causes:

• Insufficient material supply
• Low injection pressure or speed
• Blockages in the gate or runner

Solutions:

• Ensure adequate material supply and proper feeding.
• Increase injection pressure and speed to improve flow.
• Clear any blockages in the gate or runner and ensure they are properly sized.

5. Weld Lines  

Weld lines are visible lines on the surface of parts where two flow fronts meet and fail to bond properly.

Causes:

• Low melt temperature
• Slow injection speed
• Poor venting in the mold
• Gate location and part design

Solutions:

• Increase melt temperature to improve flow front bonding.
• Increase injection speed to reduce cooling before meeting.
• Improve mold venting to allow gases to escape and prevent weak bonds.

6. Air Traps  

Air traps are pockets of air that get trapped within the part, leading to voids or bubbles.

Causes:

• Inadequate venting in the mold
• High injection speed
• Complex part geometry

Solutions:

• Improve mold venting to allow air to escape during injection.
• Reduce injection speed to prevent air entrapment.
• Redesign part geometry to minimize areas where air can become trapped.

7. Burn Marks  

Burn marks are dark discolorations or blackened areas on the part, typically caused by trapped gases.

Causes:

• High injection speed
• Excessive melt temperature
• Poor venting in the mold

Solutions:

• Reduce injection speed to prevent overheating.
• Lower melt temperature to avoid material degradation.
• Enhance mold venting to release trapped gases and prevent burning.

8. Surface Delamination  

Surface delamination is the separation of the surface layer from the underlying material, often appearing as flakes or layers.

Causes:

• Contaminated material
• Incompatible material blends
• Improper material handling

Solutions:

• Ensure material purity and proper handling to avoid contamination.
• Use compatible material blends and verify supplier quality.
• Implement thorough material inspection and handling procedures.

9. Voids  

Voids are internal pockets caused by the material shrinking away from itself, leading to structural weaknesses.

Causes:

• Insufficient packing pressure
• Uneven cooling
• High melt temperature

Solutions:

• Increase packing pressure to eliminate voids during solidification.
• Ensure uniform cooling to prevent differential shrinkage.
• Lower melt temperature to reduce gas formation within the material.

Conclusion  

Troubleshooting defects in injection molded parts requires a comprehensive understanding of the process and careful analysis of potential causes. By systematically identifying and addressing the root causes of defects, you can enhance product quality, reduce waste, and improve overall production efficiency. Taking RJG’s Fundamentals of Systematic Molding course equips you with the knowledge and skills to methodically approach troubleshooting, ensuring more effective problem-solving and long-term process stability. Implementing regular maintenance, optimizing process parameters, and leveraging advanced tools, like CoPilot, are essential strategies for achieving consistent, high-quality injection molded parts.