Degradation Bands of the Melt in the Static Mixer

The product shows overall or local yellowing and color darkening; in severe cases, burnt black spots are visible, accompanied by a pungent odor. Degradation is the process in which polymer molecular chains undergo breakage and chemical changes under high temperature, high shear, or long residence conditions. PVC degradation releases HCl that corrodes equipment; PE/PP degradation produces oxidized chromophoric groups that cause product yellowing. Degradation not only affects appearance but also substantially reduces mechanical properties—impact strength can drop to below 50% of normal values.

1. Material/formulation factors: flow dead zones exist in the mixer, causing material to be retained and degraded for a long time, and intermittently carried out with output fluctuations. 2. Die design factors: The flow channel geometry, straight section length, and outlet size of the die directly affect melt flow stability and product dimensional accuracy. Unreasonable flow channel design (presence of dead corners, excessively large contraction angle) is an important root of melt fracture, material buildup, and flow marks, and needs to be addressed starting from die structure optimization. 3. Cooling and sizing factors: The cooling rate and uniformity of extruded products determine crystallinity, internal stress level, and dimensional stability. Uneven cooling leads to pipe ovality, profile deformation, and internal stress concentration, requiring optimization of cooling water channel layout and water temperature control.

1. Process parameter optimization and adjustment: Replace with a static mixer model with smoother flow channels or redesign it. 2. Systematic troubleshooting method: Troubleshooting of extrusion problems follows the order of "temperature → pressure → speed → die → cooling". First check and stabilize the temperature control of each section, then observe head pressure fluctuations to judge the feeding and screen condition, then verify the matching relationship between screw speed and haul-off speed, further check the condition of the die flow channel and sizing sleeve, and finally optimize the cooling water channels and water temperature distribution. Change only one variable at a time to confirm the effect. 3. 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 degradation yellowing is that the polymer molecular chains undergo chain scission and chemical changes under high-temperature and high-shear conditions. The core of the solution is "three controls": control the processing temperature within the material's recommended range, control the residence time to avoid excessively long retention, and control the amount of heat stabilizers and antioxidants in the formulation to fully protect the molecular chains.