Die flow marks (flow lines)

Linear textures or streaks appear on the product surface along the extrusion direction. When this defect appears in extrusion molded products, it is usually closely related to process links such as die design, screw plastication state, cooling and sizing system, or haul-off stability. Persistence of the defect will lead to out-of-tolerance product dimensions, unqualified appearance, or decreased mechanical properties, increasing scrap rate and rework costs. Timely diagnosis and troubleshooting from both extrusion process parameters and equipment status are basic requirements for ensuring stable continuous production.

1. Mold/tooling factors: Burrs or micro-cracks in the die flow channel. 2. Mold/tooling factors: Uneven material buildup on the die inner wall. 3. Comprehensive factors: Insufficient melt recombination after the breaker plate or screen. 4. Comprehensive factors: Spider leg marks not eliminated. 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: Fine-polish the die flow channel. 2. Mold/tooling improvement and maintenance: Regularly clean the die and check the inner wall. 3. Equipment inspection and system maintenance: Appropriately increase the number of screen layers to increase back pressure and promote melt recombination. 4. Material/formulation optimization: Optimize the torpedo/spider design. 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 die flow marks (flow lines) 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.