Extruded pipe surface sink marks

After pipe cooling, longitudinal or local depressions appear on the surface. 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. Cooling/environmental factors: At uneven wall thickness, the thick-wall part cools and shrinks more than the thin-wall part. 2. Cooling/environmental factors: Excessive cooling intensity forms shrinkage on the surface. 3. Mold/tooling factors: Excessive vacuum in the sizing section causes the pipe to be over-stretched. 4. Die design factors: The geometry of the die flow channel, the land length, and the outlet dimensions directly affect melt flow stability and product dimensional accuracy. Unreasonable flow channel design (presence of dead corners, excessively large convergence angle) is an important root cause of melt fracture, material buildup, and flow marks, and must be addressed starting from optimization of the die structure.

1. Process parameter optimization and adjustment: Improve wall thickness uniformity. 2. Mold/tooling improvement and maintenance: Appropriately reduce cooling intensity. 3. Equipment inspection and system maintenance: Reduce vacuum to an appropriate range. 4. Material/formulation optimization: Adjust die dimensions so the pipe has appropriate internal stress when entering the sizing section. 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 extruded pipe surface sink marks 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.