Spiral Marks on the Inner Wall of Pipes

Spiral texture appears on the inner wall of the pipe. When this defect occurs in extrusion molded products, it is usually closely related to process links such as die design, screw plasticizing state, cooling and sizing system, or haul-off stability. If the defect persists, it will cause product dimensions to be out of tolerance, unqualified appearance, or decreased mechanical properties, increasing scrap rate and rework costs. Timely diagnosis and troubleshooting from both extrusion process parameters and equipment status is a basic requirement for ensuring stable continuous production.

1. Comprehensive factors: spiral flow traces of the melt behind the core mold support not eliminated, or unstable mandrel rotation. 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: Optimize the core mold support design and increase the length of the straight section. 2. Mold/tooling improvement and maintenance: Check the core mold fixing structure to eliminate the possibility of loosening. 3. 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. 4. 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 spiral marks on the inner wall of pipes 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 level.