10 Common Injection Molding Defects and How to Avoid Them
Discover the 10 most common injection molding defects including flash, sink marks, voids, weld lines, and warping. Learn the causes and practical solutions for each.
Introduction to Injection Molding Quality
Producing high-quality injection molded parts requires careful control of many variables. Even with advanced equipment and experienced operators, defects can occur. Understanding the root causes of common molding defects is the first step toward prevention and consistent quality.
In this article, we examine the 10 most common injection molding defects, their causes, and proven strategies to eliminate them from your production.
1. Flash (Burrs)
Description: Excess material that forms along the parting line of the molded part, appearing as thin, unwanted ridges or fins of plastic.
Causes:
Insufficient clamping force — the mold is pushed open by injection pressure
Worn or damaged mold surfaces at the parting line
Excessive injection speed or pressure
Poor mold alignment or dirty parting surfaces
Solutions:
Increase clamping force or verify tonnage is appropriate for the projected area
Inspect and repair mold parting surfaces
Optimize injection speed and pressure profiles
Ensure proper mold alignment and clean parting lines
2. Sink Marks
Description: Small depressions or dimples on the surface of the part, typically occurring in thicker sections where material shrinks inward.
Causes:
Uneven material shrinkage in thicker wall sections
Insufficient pack or hold pressure
Insufficient cooling time
Excessive melt temperature
Solutions:
Redesign part with uniform wall thickness where possible
Increase pack pressure and hold time
Extend cooling time
Reduce melt temperature and mold temperature
3. Voids and Bubbles
Description: Air pockets or voids trapped inside the plastic part, which can be visible through transparent materials or detected by part weight and density checks.
Causes:
Trapped air that cannot escape through vents
Insufficient venting in the mold
Too fast injection speed trapping air
Moisture in the material causing gas formation
Solutions:
Add or enlarge vents at the last-to-fill areas
Reduce injection speed to allow air to escape
Pre-dry hygroscopic materials (nylon, PC, PET) before molding
Optimize gate location to push air toward vented areas
4. Weld Lines (Knit Lines)
Description: Visible lines or marks on the part surface where two separate melt fronts meet. Weld lines can be cosmetic issues or can reduce structural strength.
Causes:
Multiple gates causing flow fronts to meet
Holes, inserts, or features that split the melt flow
Low melt temperature causing poor fusion
Low injection pressure or speed
Solutions:
Increase melt temperature and mold temperature
Increase injection speed to maintain front temperature
Optimize gate location to move weld lines to non-critical areas
Use overflow wells to move the weld line outside the part
5. Warping
Description: Distortion or twisting of the part from its intended shape, caused by non-uniform shrinkage in different areas of the part.
Causes:
Uneven cooling rates across the part
Non-uniform wall thickness causing differential shrinkage
Improper part design with high aspect ratios
Incorrect processing parameters (temperature, pressure, cooling)
Solutions:
Design parts with uniform wall thickness and symmetric features
Optimize cooling system for uniform heat dissipation
Reduce melt temperature and mold temperature gradually
Increase packing pressure and hold time
Consider annealing post-process for residual stress relief
6. Short Shots
Description: The mold cavity is not completely filled, resulting in an incomplete or partially formed part.
Causes:
Insufficient material injected
Melt temperature too low (high viscosity)
Injection pressure or speed too low
Restricted flow due to small gates or runners
Insufficient venting causing air traps
Solutions:
Verify shot size is adequate for the part volume
Increase melt temperature for better flow
Increase injection pressure and speed
Enlarge gate or runner size if flow is restricted
Check and improve venting to eliminate air traps
7. Surface Gloss Differences
Description: Variations in surface gloss or shine across different areas of the part, creating an inconsistent appearance.
Causes:
Uneven mold temperature across cavity surfaces
Variations in pressure or packing density
Differences in wall thickness affecting shrink rate
Poor mold surface polish or inconsistent texture
Solutions:
Ensure uniform mold cooling/heating across the cavity
Optimize packing pressure and hold time
Redesign for more uniform wall thickness
Verify mold surface finish is consistent across all cavity surfaces
8. Silver Streaks
Description: Shiny silver-colored streaks or lines on the part surface, typically running in the direction of material flow.
Causes:
Moisture in the plastic resin (most common cause)
Excessive shear heating from high injection speed
Degraded material from high barrel temperatures
Trapped volatiles in the melt
Solutions:
Properly dry hygroscopic materials before processing (follow manufacturer specs)
Reduce injection speed to minimize shear
Lower barrel temperatures and reduce residence time
Use proper hopper drying and handling to prevent moisture reabsorption
9. Burn Marks / Black Spots
Description: Dark brown or black discoloration on the part surface, appearing as spots, streaks, or charred material.
Causes:
Material degradation from excessive heat or long residence time
Dead spots in the barrel or nozzle where material burns
Compression of trapped air causing diesel effect (adiabatic heating)
Contaminated or dirty resin
Solutions:
Reduce melt temperature and residence time in the barrel
Improve venting to prevent trapped air compression
Reduce injection speed in the final fill stage
Purge the barrel regularly and verify material cleanliness
Inspect screw and barrel for wear or dead spots
10. Ejector Marks
Description: Visible marks, indentations, or whitening on the part surface where ejector pins push the part out of the mold.
Causes:
Part not sufficiently cooled before ejection
Insufficient draft angle on the part walls
Ejector pins too small or poorly positioned
Excessive ejection force
Solutions:
Increase cooling time to ensure part rigidity before ejection
Add or increase draft angles on vertical surfaces
Increase ejector pin size or add more pins to distribute force
Position ejectors in less visible areas or on the B side of the part
Polish core surfaces to reduce ejection friction
Quality Control Best Practices
An ounce of prevention is worth a pound of cure — especially in injection molding, where correcting a defect in production is far more costly than getting the design and process right from the start.
To maintain consistent quality, implement these practices:
Establish and document standard processing parameters for each part
Perform first-article inspection (FAI) at the start of every production run
Implement in-process quality checks at regular intervals
Use statistical process control (SPC) to monitor key dimensions and features
Maintain proper mold maintenance schedules
Train operators and technicians on defect recognition and troubleshooting
Need help troubleshooting a persistent molding defect? Our engineering team has decades of experience in process optimization and mold modification. Contact us today for a technical review of your application.