Abnormal extrusion swell

After exiting the die, the melt swells (die swell) beyond or below the normal range, causing the correspondence between actual product dimensions and die design dimensions to become inaccurate. Extrusion swell (Die Swell) is the macroscopic manifestation of elastic recovery of polymer melt — the melt is compressed and sheared inside the die and expands due to elastic recovery after exiting. Abnormal swell ratio makes die dimension design unable to accurately predict product dimensions, increasing the difficulty of die trials and adjustment.

1. Mold/tooling factors: Die temperature changes cause melt elasticity changes. 2. Material/formulation factors: Changes in material molecular weight or molecular weight distribution. 3. Process parameter factors: Draw ratio changes cause actual swell ratio changes. 4. Comprehensive factors: Shear rate changes. 5. Cooling and sizing factors: The cooling rate and uniformity of extruded products determine crystallinity, internal stress level, and dimensional stability. Uneven cooling causes pipe ovality, profile deformation, and internal stress concentration; the cooling water circuit layout and water temperature control must be optimized.

1. Process parameter optimization and adjustment: Stabilize die temperature and melt temperature. 2. Mold/tooling improvement and maintenance: Check batch consistency of incoming material melt index. 3. Equipment inspection and system maintenance: Adjust die dimensions and draw ratio according to swell ratio. 4. Material/formulation optimization: Avoid large drift of the operating point due to process changes. 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.

This problem is a typical manifestation of abnormal extrusion swell in extrusion molding. The core of the solution lies in eliminating the root cause of the defect through systematic optimization of process parameters (temperature, speed, pressure) and precision correction of the die/sizing system. It is recommended to start with parameter adjustment and gradually go deeper into the equipment and tooling levels.