Every plastic injection mold and parts manufacturer strives to produce flawless molded parts. However, like all manufacturing processes, defects can occur. One common issue in precision injection molding is burn mark defects . Understanding what causes these defects and how to prevent them is crucial for producing high-quality plastic parts and maintaining customer satisfaction.
Burn marks appear as dark reddish-brown or black streaks on the surface of molded parts. They result from trapped air in the mold cavity that becomes highly pressurized and superheated during the injection process. This heat causes localized combustion of the polymer, leaving carbonized residues. Burn marks, sometimes referred to as the “diesel effect,” affect both cosmetic appearance and mechanical integrity.
Burn marks are particularly problematic on thin-walled parts, sharp corners, or areas near gates. If not addressed, these defects can reduce surface quality, compromise part performance, and increase rejection rates. For manufacturers, preventing burn marks is essential for achieving consistent, precision plastic injection molded parts .
A primary cause of burn marks is trapped air due to inadequate mold venting. Air pockets in blind holes, tight corners, or complex geometries become compressed during injection. Without proper escape routes, the compressed air superheats and scorches the plastic. Optimizing mold design with well-placed vents and rounded corners helps prevent this issue. Retrofitting vents is possible but can be costly, so proactive mold design is preferred.
Injection molding machines can also contribute to burn marks. Excessive melt temperature and high injection speed create conditions for superheating and air compression. High-speed or high-pressure filling can lead to gas pockets and localized combustion. Properly adjusting machine parameters—such as injection speed, pressure, and melt temperature—is critical for reducing burn mark defects.
The type and handling of plastic resin influence burn mark formation. Using multiple resin types without thorough purging can leave residual material that ignites under high pressure. Additionally, plastics containing moisture may generate steam during injection, leading to localized air expansion and scorching. Selecting heat-stable polymers and ensuring proper drying are essential preventive measures.
Adequate venting is the most effective way to prevent trapped air from causing burn marks. Designers should include vents in all potential air-trapping areas and regularly inspect mold channels to ensure they remain clear.
Controlling injection speed, melt temperature, and pressure reduces localized overheating. Employing multi-stage injection or segmented packing can improve flow and decrease air compression in cavities.
Proper drying, purging, and material selection are essential. Heat-sensitive plastics should be carefully processed, and moisture content monitored to prevent steam formation that contributes to burn marks.
Rounded corners, uniform wall thickness, and optimized gate locations help minimize areas where air can be trapped. Early collaboration between designers and mold makers ensures that geometry and venting are suitable for high-quality production.
Routine inspection and cleaning of molds, vents, and gates help identify potential issues before defects occur. Continuous monitoring of production parameters allows early detection of burn mark risk, reducing waste and improving efficiency.
For industries such as automotive, electronics, and medical devices, controlling burn marks is essential for both aesthetics and function. Prototyping and simulation tools, including CAD flow analysis, can predict potential problem areas. Working with experienced precision injection molding suppliers ensures that preventive measures are applied from the design stage through full-scale production.
RM1223, 12F, No.1, Fuji Bldg, No. 6018 Longgang RD. , Longgang Dist., Shenzhen,China.
Tel: 0086-755-89618186
Contact: Paul Hu 0086-18675501028
LEAVE MESSAGE
