Excessive plastication / scorch

The product yellows, color darkens, mechanical properties decline, and even scorched particles are produced; carbonized black spots may appear on the surface. Excessive plastication (scorch) is the result of thermo-oxidative degradation of material caused by excessive banburying or roll temperature. Over-plasticated products show significantly reduced tensile strength and elongation at break; severely scorched products completely lose use value. This defect is especially common and dangerous in PVC calendering.

1. Operational factors: Banbury or extrusion feed temperature too high or time too long. 2. Mold/tooling factors: Roll temperature too high causes material staying on roll surface to overheat. 3. Material/formulation factors: Insufficient heat stabilizer in the formulation. 4. Roll gap adjustment factors: Crown and cross-angle compensation of calender rolls is the core means of controlling the transverse thickness distribution of the film. Crown design is based on roll deflection calculations; if it does not match the actual process load, it will cause abnormal transverse distribution of film thickness — thicker in the middle and thinner at both ends, or an inverse bow-shaped distribution.

1. Process parameter optimization and adjustment: Reduce feed temperature and roll temperature. 2. Mold/tooling improvement and maintenance: Shorten effective residence time of material in high-temperature zones. 3. Equipment inspection and system maintenance: Increase heat stabilizer dosage. 4. Material/formulation optimization: If necessary, perform local heat dissipation in the calender roll space. 6. Preventive management: Establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or 5Why analysis to trace the root cause, and solidify countermeasures as PFMEA control points. Establish a regular equipment maintenance system and implement life management for key tooling such as dies/spinnerets/calender rolls/coating heads.

The essence of excessive plastication (scorch) is thermo-oxidative degradation of material at high temperature. The core of the solution is similar to extrusion degradation — "three controls": control banburying and roll temperature not to exceed limits, shorten material residence time in high-temperature zones, and increase heat stabilizer dosage to protect heat-sensitive materials such as PVC.