Burn Marks in Injection Molding：Causes and Preventive Solutions

In plastic injection molding, burn marks are a common yet often overlooked surface defect. These black or dark brown spots or streaks may appear minor but can severely compromise both the aesthetics and structural integrity of molded parts, leading to customer returns and scrap losses. The causes of burn marks are multifaceted, involving mold design, machine condition, processing parameters, and raw material management. Only through systematic analysis and optimization can these defects be effectively prevented and eliminated.

 

Further reading：Sink Mark in Injection Molding: Solutions for High-Quality Production

Further reading：Short Shot in Injection Molding: Causes, Detection & Solutions

 

 

 

Burn marks typically result from trapped air and overheating of the molten polymer. When molten plastic is injected at high speed into the mold cavity, any residual air that is not adequately vented becomes compressed by the high-pressure melt. This compression can cause localized temperatures to exceed 300°C, leading to combustion of trapped air or thermal degradation and carbonization of the polymer, forming visible burn marks.

 

Burn marks frequently occur in the following areas：

• Cavity end-of-fill regions
• Poorly vented corners or dead zones
• Enclosed structures, such as bosses or ribs
• Weld lines where melt fronts converge

 

• Appearance of black or dark brown streaks or spots
• Surface roughness, discoloration, and reduced gloss
• In severe cases, structural embrittlement and part failure

 

 

 

1. Excessive Injection Speed

• Cause：High-speed injection compresses air violently, increasing temperature
• Solution：Implement multi-stage injection with reduced speed and pressure in the final stages

2. Overheating of Melt

• Cause：Excessive barrel temperature, back pressure, or screw speed leads to thermal degradation
• Solution：Adjust barrel temperature, reduce back pressure and screw speed to maintain melt stability

3. Material Degradation in Equipment

• Cause：Residual plastic inside the barrel or on the screw is not properly purged and becomes carbonized
• Solution：Perform regular screw cleaning and barrel maintenance; inspect heaters and thermocouples for proper function

 

1. Inadequate Venting Design

• Cause：Insufficient venting grooves or lack of ejector pin venting
• Solution：Add or enlarge venting channels; optimize venting at parting lines and mold-opening directions

2. Enclosed Mold Geometry

• Cause：Air cannot escape from enclosed features like ribs or bosses
• Solution：Redesign enclosed areas to include gas escape paths

3. Undersized Runners and Gates

• Cause：Limited flow area restricts melt speed and increases air entrapment
• Solution：Enlarge runner and gate cross-sections; consider hot runner systems for improved flow balance

 

1. High Moisture Content in Raw Materials

• Cause：Hygroscopic materials (e.g., PA, PET) not properly dried
• Solution：Dry materials according to supplier specifications to meet the required moisture content

2. Poor Quality Additives

• Cause：Some colorants or additives may thermally degrade, forming carbon residue
• Solution：Use thermally stable masterbatches and additives specifically formulated for injection molding

 

 

 

Inspection Item	Diagnostic Clue	Recommended Action
Injection Speed & Pressure	Burn marks near end-of-fill；overly fast filling	Apply multi-stage injection, reduce speed & pressure gradually
Mold Venting Condition	Burn marks in enclosed or corner areas	Add venting grooves or vented ejector pins
Barrel Temp & Melt Quality	Dark melt color, unstable flow	Lower barrel zone temperatures；adjust screw speed & back pressure
Material Drying	Use of hygroscopic resin without prior drying	Set appropriate drying conditions to prevent vaporization
Runner & Gate Design	Poor flow, slow fill at end of cavity	Enlarge flow paths；optimize runner & gate dimensions

 

 

 

During the mold design phase, mold flow simulation is used to predict trapped air locations and melt behavior. Optimize venting and runner layout before tool fabrication to reduce defect risks.

 

Create a database of optimal injection molding conditions for various materials (e.g., injection temperature, pressure, back pressure). This ensures process repeatability and improved part yield across production batches.

 

Establish preventive maintenance schedules for machines, implement strict incoming material inspection, and control additive quality. Managing these sources proactively minimizes the likelihood of defect generation.

 

 

Burn marks are more than just surface flaws； they signal systemic imbalances in the injection molding process. By adopting a holistic approach that covers mold design, machine settings, material handling, and equipment maintenance, manufacturers can uncover actionable opportunities to elevate product quality and earn long-term customer trust.

 

 

• Group Name: Huarong Group
• Brand: Huarong, Yuhdak, Nanrong
• Service Offerings: Injection Molding Machine, Vertical Injection Molding Machine, Injection Molding Automation
• Tel: +886-6-7956777
• Address: No.21-6, Zhongzhou, Chin An Vil., Xigang Dist., Tainan City 72351, Taiwan
• Official Website: https://www.huarong.com.tw/