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Troubleshooting Common Issues in Injection Molding

Injection molding is a versatile manufacturing process, but defects can arise due to material properties, tooling design, or process parameters.

A Beginner-friendly Guide to Handling Defects

Running large-scale production processes can be challenging. We share our experience in identifying common defects in injection molding, their causes, and practical solutions to ensure high-quality parts and efficient production.

For Plastic Injection Manufacturing

Most Common Defects

A. Silver Streaks
Description:
Silvery lines appear on the part surface, often due to moisture in the raw material. This defect is most common in hygroscopic materials like nylon.
Causes:
  1. Excess moisture in raw material
  2. Mold cooling system leaks introducing moisture
  3. Improper mold temperature
Solutions:
  1. Dry raw material to a moisture content of 0.02%
  2. Use dehumidifier systems for thorough drying
  3. Inspect and repair mold cooling system leaks
  4. Maintain proper mold temperature
B. Burn Marks (Diesel Effect)
Description:
Black or brown marks caused by trapped air being superheated (up to 600–800°C) during injection.
Causes:
  1. Insufficient venting
  2. High injection speed creating air compression
Solutions:
  1. Add proper venting through parting line vents, ejector pin vents, or porous venting pins
  2. Avoid reducing injection speed, as it can compromise part quality. Instead, optimize venting
C. Weld Lines
Description:
Visible lines where two flow fronts meet, creating potential weak points.
Causes:
  1. Poor flow front angle
  2. Low mold temperature or high pressure drop
  3. Insufficient part wall thickness
Solutions:
  1. Optimize flow front meeting angles to ~135°
  2. Increase mold temperature to recommended levels
  3. Increase gate size to reduce pressure drop
  4. Ensure proper wall thickness for material
  5. Consider using flow leaders or relocating gates
  6. While weld lines cannot be completely eliminated, they can be minimized with these adjustments
D. Ejector Pin Marks
Description:
Visible marks left on parts due to inadequate ejection mechanisms.
Causes:
  1. Insufficient ejector pins
  2. Poor draft angles or surface finish
  3. Inadequate cooling
Solutions:
  1. Add sufficient ejector pins proportionate to part area
  2. Include proper draft angles to facilitate smooth ejection
  3. Polish surfaces in the ejection direction
  4. Implement proper cooling lines
  5. Use block or stripper plate ejection for large or delicate parts
  6. Size ejector pins appropriately for the load
E. Gate Blush
Description:
White or gray marks around the gate area, typically 3–5mm wide.
Causes:
  1. Material shearing in the gate area
Solutions:
  1. Optimize gate design, such as using a submarine gate with higher volume
  2. Calculate and maintain proper shear rates
  3. Maintain recommended mold and melt temperatures
  4. Avoid quick process fixes that compromise quality
F. Record Grooves/Finger Marks
Description:
Circular grooves or marks caused by inconsistent material flow.
Causes:
  1. Pressure drops in the feed system
  2. Improper gate design or wall thickness
Solutions:
  1. Optimize gate size and design for uniform flow
  2. Maintain consistent injection pressure
  3. Ensure adequate wall thickness to avoid weak spots
  4. Control mold temperature to ensure even flow
G. Molded-In Stress
Description:
Internal stress within the part, leading to warping or cracking.
Causes:
  1. Slow filling causing material cooling before full cavity fill
  2. Overpacking or excessive mold pressure
  3. Cold mold temperatures or sharp corners
  4. Improper ejection methods
Solutions:
  1. Fill quickly to maintain melt temperature
  2. Avoid overpacking by adjusting packing pressure and time
  3. Maintain mold temperature within material recommendations
  4. Use proper ejection methods to avoid deformation
  5. Add radii to sharp corners to reduce stress concentration
  6. Preheat inserts to match mold temperature
H. Nozzle Effects
Description:
Issues related to nozzle configuration and maintenance.
Causes:
  1. Improper nozzle diameter matching
  2. Inadequate heater coverage
  3. Poor temperature control
  4. Rough internal surfaces
  5. Insufficient maintenance
Solutions:
  1. Match nozzle diameter to the machine head
  2. Ensure proper heater coverage (80–90% of the nozzle's surface area)
  3. Use independent temperature control for precision
  4. Maintain smooth internal surface finishes
  5. Regular maintenance to prevent clogging or wear
  6. Understand material specifications to match equipment settings
  7. Pre-dry materials thoroughly to avoid moisture defects
  8. Use equipment sized appropriately for the part and material
  9. Implement proper tool design with adequate runners and venting
  10. Maintain all equipment regularly
  11. Select a screw design compatible with the material's characteristics

Why Choose a Johor Bahru-Based Manufacturer like Ichiplas?

Johor Bahru is a hub for high-quality and cost-effective plastic injection molding services. Local manufacturers leverage advanced technologies and proximity to key markets, ensuring timely delivery and competitive pricing. With expertise in handling diverse materials, manufacturers in Johor Bahru are equipped to meet both small and large-scale production needs.

Troubleshooting injection molding issues requires a combination of understanding material behavior, proper tool design, and precise process control. By addressing common defects like silver streaks, burn marks, and weld lines with the solutions outlined here, manufacturers like Ichiplas can ensure consistent part quality and efficient production.

Contact us today to discuss your plastic injection molding needs, or if you are thinking of moving your manufacturing base to Malaysia!

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