Causes and Improvement Solutions for Burn Marks and Yellow Discoloration on Injection Molded Products

Burn marks, yellow discoloration and black streaks are common surface defects during injection molding. In severe cases, these problems cause material embrittlement and reduce mechanical performance. Such defects not only damage the appearance of finished products, but also lead to potential failure risks during service. The root causes mainly cover thermal degradation of molten plastic, adiabatic heating caused by trapped air inside mold cavities, unreasonable mold structure and improper processing parameters. Operators need to identify different types of defects accurately and implement targeted rectification.

1. Thermal Degradation of Melt and Corresponding Improvement Measures

Yellowing and scorching caused by melt degradation usually appear near gates or along melt flow paths, accompanied by continuous black spots. Excessively high barrel temperature is a typical trigger. When raw materials stay in a high-temperature environment for a long time, polymer molecular chains break and carbonize. Dead zones inside nozzles and hot runners easily trap stagnant melt, forming carbon deposits that contaminate subsequent molded parts. High screw rotation speed and rapid injection generate extra shear heat, further lifting local melt temperature and accelerating degradation.

For parameter adjustment, lower the set temperature of each barrel zone and avoid operating materials near their thermal decomposition limit. Reduce screw speed, injection velocity and back pressure to cut shear heating. If production pauses for more than 30 minutes, decrease barrel temperature or empty residual melt inside the barrel to prevent long-term thermal aging. Regularly clean carbon accumulation on screws, nozzles and hot runners to eliminate stagnant zones. Complete purging before switching different plastic materials to avoid cross-contamination.

2. Adiabatic Heating from Trapped Air and Optimization Methods

During filling, air trapped at melt flow ends and weld lines gets compressed quickly. The sharp temperature rise caused by adiabatic compression exceeds the ignition point of plastic, directly scorching product surfaces. This type of burn mark rarely disappears after adjusting barrel temperature, which serves as an important judgment standard.

The core solution is optimizing mold venting. Add or clean vent slots at weld lines and cavity terminals, and control vent depth properly to exhaust air without generating flash. Moderately reduce injection speed to extend time for air evacuation. Adjust gate layout to prevent air from being enclosed in narrow zones. If product structure allows, add overflow wells at defect positions to guide trapped air out together with a small amount of melt.

3. Mold Structural Defects Leading to Scorch and Yellowing

Sharp corners, narrow runners and small clearances inside molds increase melt flow resistance and produce extra friction heat. Malfunctioning heating bands and offset temperature sensors cause local temperature out of control. Vent channels at parting lines and inserts get blocked rapidly by plastic residues during continuous production. For hot runner molds, unbalanced temperature between individual nozzles leads to local overheating.

Optimization measures include rounding sharp runner corners to reduce flow resistance. Calibrate temperature control systems and replace damaged thermocouples and heating components. Disassemble and clean hot runners periodically. Check matching clearance of ejector pins and inserts to avoid accumulating decomposition gas. Balance temperature for all hot nozzles to stop continuous overheating of melt.

4. Improper Raw Material Management Triggering Discoloration

High proportion of regrind raises risks of yellowing, since recycled plastics experience repeated thermal cycles and tend to degrade easily. Over-drying raw materials at excessive temperature causes premature resin aging. Some additives with insufficient heat resistance decompose under high temperature and discolor finished goods.

Control the mixing ratio of clean regrind strictly. Follow standardized drying temperature and duration to avoid over-drying. Select raw materials and additives with matched heat resistance. Separate production machines for light-colored and dark-colored products to avoid cross-contamination by carbonized impurities.

Summary

Burn marks and yellow discoloration fall into two main categories: thermal degradation of melt and scorching caused by trapped air. Similar in appearance, they require completely different improvement ideas. If defects concentrate around gates and flow paths, prioritize checking temperature, shear heat and carbon deposits. If defects gather at flow terminals and weld seams, focus on mold venting and injection speed. Follow the troubleshooting sequence: adjust process parameters first, then modify mold structure, and finally standardize raw material control. Stabilizing melt temperature, minimizing shear action and guaranteeing smooth air exhaust can effectively reduce such defects, stabilizing surface quality and mechanical performance of molded parts.