Burn Marks and Blisters on Injection Molded Parts: Causes and Solutions

1. Burn Marks Caused by Melt Fracture

1.1 Core Causes

• Melt fracture occurs when molten plastic is injected into large-volume cavities at high speed/pressure, leading to transverse surface cracks. The rough fractured areas are embedded in the part’s surface, forming blisters.

• The phenomenon is more severe when a small amount of melt is injected into a large cavity.

• Root cause: The elastic behavior of polymer melts. In the barrel, melt near the wall experiences high friction, stress, and slow flow; when exiting the nozzle, wall stress disappears, and the central melt accelerates sharply. Excessive injection speed causes stress to exceed the melt’s strain capacity, triggering fracture.

• Additional factor: Abrupt changes in runner shape (e.g., diameter shrinkage/expansion, dead ends) trap and circulate melt, increasing shear deformation. When mixed with normal flow, inconsistent deformation recovery causes fracture.

1.2 Optimization Measures

• Control injection speed to avoid excessive stress changes that induce fracture.

• Optimize runner design: Reduce abrupt shape changes to minimize melt stagnation and circulation.

• Ensure smooth transitions in the mold’s flow path to maintain uniform melt flow.

2. Burn Marks Caused by Improper Molding Condition Control

2.1 Core Causes

• Injection Speed: Laminar flow (low speed) produces smooth surfaces, while turbulent flow (excessive speed) causes blisters and internal air holes.

• Melt Temperature: Overheating leads to thermal decomposition and carbonization of the plastic, forming burn marks.

• Excessive Shear Heat: High screw speed or back pressure generates excess friction heat in the barrel, accelerating melt degradation.

2.2 Optimization Measures

• Adjust injection speed to maintain laminar flow during mold filling.

• Control screw speed and back pressure to reduce frictional heat generation.

• Optimize barrel temperature: Increase the feed zone temperature appropriately, especially for low-lubricity raw materials.

• Avoid overheating the melt by matching temperature parameters to the plastic’s thermal stability.

3. Burn Marks Caused by Mold Failures

3.1 Core Causes

• Poor Ventilation: Blocked, insufficient, or improperly positioned vent holes trap air in the cavity. Adiabatic compression of trapped air generates high temperatures, causing resin decomposition and burn marks.

• Inappropriate Gate Design: Poorly designed gates disrupt melt flow and ventilation, increasing the risk of burn marks.

• Excessive Mold Release Agent: Overuse of mold release agent affects surface finish and may cause carbonization at high temperatures.

3.2 Optimization Measures

• Clean blocked vent holes regularly and optimize vent layout/size to ensure smooth air escape.

• Improve gate design to facilitate melt flow and ventilation; maintain a smooth cavity surface.

• Control mold release agent usage: Apply only the minimum amount needed to avoid residue and carbonization.

4. Burn Marks Caused by Substandard Raw Materials

4.1 Core Causes

• Excessive Moisture or Volatiles: High moisture/volatile content in raw materials vaporizes at high temperatures, forming bubbles and burn marks.

• Excessively High Melt Flow Index (MFI): Plastics with high MFI have poor thermal stability, making them prone to decomposition.

• Improper Lubricant Usage: Excessive or incompatible lubricants cause carbonization during molding.

4.2 Optimization Measures

• Pre-dry raw materials using a hopper dryer to reduce moisture and volatile content.

• Select resins with an appropriate MFI (prefer lower MFI for better thermal stability).

• Control lubricant dosage strictly; choose lubricants compatible with the base plastic.

5. Conclusion