Introduction
Injection molding defects are one of the most common challenges in plastic manufacturing.
Even with advanced machines and well-designed molds, defects can still occur due to material issues, process parameters, or mold design problems.
In this article, we will explain the most common injection molding defects, their causes, and how to prevent them effectively.
🟦 H2：1. Short Shot (Incomplete Filling)
Short shot occurs when the plastic does not fully fill the mold cavity.
🔍 Causes:
- Low injection pressure
- Insufficient material temperature
- Poor venting design
- Thin wall sections
✅ Solutions:
- Increase injection pressure
- Improve mold venting
- Optimize gate design
- Adjust material temperature
🟩 H2：2. Flash (Excess Material)
Flash appears as thin excess plastic at the parting line.
🔍 Causes:
- Low clamping force
- Worn mold surfaces
- Excessive injection pressure
✅ Solutions:
- Increase clamping force
- Repair mold surfaces
- Optimize injection pressure
🟥 H2：3. Sink Marks
Sink marks are surface depressions caused by uneven cooling.
🔍 Causes:
- Uneven wall thickness
- Low packing pressure
- Insufficient cooling time
✅ Solutions:
- Improve wall thickness design
- Increase packing pressure
- Optimize cooling system
🟨 H2：4. Warpage (Deformation)
Warpage refers to bending or distortion after cooling.
🔍 Causes:
- Uneven cooling
- Internal stress imbalance
- Poor mold design
✅ Solutions:
- Optimize cooling channels
- Improve gate location
- Balance material flow
🟪 H2：5. Burn Marks
Burn marks appear as black or brown discoloration.
🔍 Causes:
- Trapped air
- Poor venting
- High injection speed
✅ Solutions:
- Improve venting system
- Reduce injection speed
- Optimize flow path
🟫 H2：6. Weld Lines
Weld lines occur when two flow fronts meet.
🔍 Causes:
- Multiple gates
- Low melt temperature
- Poor flow design
✅ Solutions:
- Increase melt temperature
- Optimize gate position
- Improve mold design
⬛ H2：7. Air Traps
Air traps happen when air is trapped inside the mold cavity.
🔍 Causes:
- Poor venting
- Complex geometry
- Fast filling speed
✅ Solutions:
- Add vent channels
- Adjust injection speed
- Optimize cavity design
🟧 H2：8. How to Prevent Injection Mold Defects
To reduce defects, follow these key principles:
- Use proper DFM (Design for Manufacturability)
- Optimize mold design before production
- Control injection parameters carefully
- Select correct material
- Use high-quality mold manufacturing
👉 Most defects can be avoided during the design stage.