Melt fracture/sharkskin

The extrudate surface shows periodic transverse cracks or sharkskin-like rough texture, with obvious unevenness to the touch, and the appearance presents regular or irregular scale-like patterns. Melt fracture is a typical flow instability phenomenon in extrusion processing, occurring when the shear stress of the melt at the die wall exceeds the critical value. This defect ranges from slight surface roughness to severe periodic fracture, seriously affecting the product appearance and mechanical properties.

1. Mold/tooling factors: the shear stress of the melt in the die exceeds the critical value. 2. Mold/tooling factors: die temperature too low causing melt viscosity too high. 3. Process parameter factors: extrusion speed too fast. 4. Mold/tooling factors: unreasonable die runner design or presence of dead corners. 5. Cooling and sizing factor: The cooling rate and uniformity of the extruded product determine the crystallinity, internal stress level and dimensional stability. Uneven cooling causes oval pipes, profile deformation and internal stress concentration; the cooling water channel layout and water temperature control need to be optimized.

1. Process parameter optimization and adjustment: raise the die temperature to reduce melt viscosity. 2. Mold/tooling improvement and maintenance: lower the extrusion speed. 3. Equipment inspection and system maintenance: optimize the die runner geometry. 4. Material/formulation optimization: use polymer processing aids (PPA) such as fluoroelastomers to coat the die surface. 6. Preventive management: Establish standardized process parameter cards and quality control limits for each set of 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 the 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 melt fracture is that the die wall shear stress exceeds the critical shear stress of the polymer melt. The solution idea is 'reduce shear + modify interface': lower the extrusion speed or raise the die temperature to reduce the shear rate, and use PPA processing aids to improve the slip behavior at the die wall.