Burnt / Black Speck

Burnt black spots appear on the surface or inside the part. The burnt area shows obvious carbonization characteristics — the color ranges from dark brown to full black, the surface is rough and powdery, and carbon powder falls off with a light touch. Burning is often accompanied by a pungent burnt smell, confirming the breakage and degradation of the material molecular chains. In the '2079851475616104452 workshop, the occurrence of burning is usually accompanied by a "popping" sound (release of adiabatic compression of gas), and skilled operators can predict it by sound. The mold cavity near the burnt spot is also often contaminated by carbon particles and requires shutdown for cleaning.

1. Mold design factors: poor mold venting causing adiabatic compression ignition of gas, excessive barrel temperature, screw shear overheating, material retention and degradation.

1. Process parameter optimization and adjustment: improve mold venting, lower barrel temperature and screw speed, reduce residence time, clean dead corners. 2. Systematic troubleshooting method: troubleshoot item by item in the order of "parameters -> material -> mold -> equipment". First adjust the parameters that are easiest to change, such as injection pressure and temperature, and observe the effect; if there is no improvement, check the drying and fluidity of the raw material; further check mold venting and gate design; finally check the mechanical condition of the equipment (check ring, screw). Change only one variable at a time to determine the root cause. 3. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluation. For recurring problems, use fishbone diagram or 5Why analysis to trace the root cause, and fix the countermeasures as PFMEA control points.

The fundamental mechanism of burning is that the local temperature exceeds the thermal decomposition temperature of the material. Solution priority: first improve mold venting (eliminate adiabatic compression), second control melt temperature and reduce residence time (prevent degradation in the barrel), third adjust injection speed (control compression heating at the filling end).