Injection molding is one of the most widely used manufacturing processes for plastic parts. However, even with a well-designed mold and stable process, injection molding defects can still occur.
These defects may affect product appearance, dimensional accuracy, and even functional performance.
This article explains the most common injection molding defects and how to fix them using practical engineering methods in both design and production stages.
1. Sink Marks
Sink marks are small surface depressions that usually appear in thick sections such as ribs or bosses.
Causes:
- Uneven wall thickness
- Insufficient packing pressure
- Poor cooling design
How to Fix:
- Improve wall thickness uniformity
- Increase packing pressure and holding time
- Optimize cooling system design
Sink marks are one of the most common injection molding defects and are usually related to part design.
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2. Flash
Flash is thin excess plastic that appears at the parting line or mold gaps.
Causes:
- Low clamping force
- Worn mold surfaces
- Excess injection pressure
How to Fix:
- Increase clamping force
- Repair mold parting line
- Optimize injection pressure
Flash is often a sign of mold wear or process imbalance.
3. Warpage
Warpage refers to deformation or bending of the molded part after cooling.
Causes:
- Uneven cooling
- Internal stress
- Poor gate location
How to Fix:
- Balance cooling system
- Optimize part design
- Adjust gate position
Warpage is one of the most critical injection molding defects because it affects assembly accuracy.
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4. Short Shot
Short shot means the mold cavity is not completely filled.
Causes:
- Low injection pressure
- Poor venting
- Low material temperature
How to Fix:
- Increase injection pressure
- Improve mold venting
- Raise melt temperature
Short shot defects are usually related to flow restriction or insufficient process parameters.
5. Burn Marks
Burn marks appear as black or brown discoloration on the part surface.
Causes:
- Trapped air
- Excessive injection speed
- Poor venting
How to Fix:
- Improve venting design
- Reduce injection speed
- Lower melt temperature if needed
Burn marks often indicate trapped gas inside the cavity.
6. Weld Lines
Weld lines occur when two melt fronts meet but do not fully fuse.
Causes:
- Multiple gate flow
- Low melt temperature
- Poor flow design
How to Fix:
- Increase melt temperature
- Optimize gate location
- Improve injection speed
Weld lines can also reduce mechanical strength in critical areas.
7. Voids (Air Pockets)
Voids are internal empty spaces inside thick sections.
Causes:
- Fast surface cooling
- Insufficient packing
- Thick wall sections
How to Fix:
- Increase holding pressure
- Improve cooling balance
- Optimize wall thickness
8. Jetting
Jetting appears as snake-like flow patterns on the surface.
Causes:
- High injection speed at gate
- Poor gate design
How to Fix:
- Redesign gate location
- Reduce initial injection speed
9. Flow Marks
Flow marks are visible streaks or uneven surface patterns.
Causes:
- Irregular flow speed
- Low mold temperature
How to Fix:
- Increase mold temperature
- Adjust injection speed profile
10. How to Reduce Injection Molding Defects Overall
Most injection molding defects come from four main areas:
- Part design
- Mold design
- Material selection
- Process control
Therefore, solving defects requires a systematic approach rather than adjusting only machine parameters.
CONCLUSION
Injection molding defects are common, but most of them can be prevented or fixed with proper engineering control.
Key methods include:
- Better part design
- Proper mold structure
- Stable process settings
- Correct material selection
By addressing these factors early, manufacturers can significantly improve product quality and reduce production cost.
INTERNAL LINK
👉 Learn more about injection mold solutions here:
https://www.fentormold.com/