Pipe bending/not straight

The pipe shows obvious bending, snake-shaped or S-shaped deformation in the longitudinal direction, the straightness is out of tolerance, and it cannot meet installation and use requirements. Bent pipes produce misalignment during butt welding, seriously affecting the sealing of the pipeline system. This defect is mostly caused by the deviation of the cooling system from the extrusion axis or asymmetric circumferential cooling, and is particularly prominent in thin-wall large-diameter pipes.

1. Mold/tooling factors: the cooling water tank and the die are not on the same axis. 2. Cooling/environmental factors: inconsistent circumferential cooling rate of the pipe. 3. Process parameter factors: uneven or skewed traction roll pressure. 4. Mold/tooling factors: uneven die temperature distribution. 5. Comprehensive factors: insufficient support after leaving the die.

1. Process parameter optimization and adjustment: adjust the coaxiality of the water tank and the die. 2. Mold/tooling improvement and maintenance: optimize the cooling water ring or spray layout to make it circumferentially uniform. 3. Equipment inspection and system maintenance: calibrate the parallelism of the traction rolls. 4. Material/formulation optimization: check whether the die heating band is damaged. 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 pipe bending is that longitudinal cooling asymmetry or axis deviation causes inconsistent shrinkage rates on each side. The core of the solution is to ensure that the cooling system is coaxial with the extrusion axis and that circumferential cooling is uniform, while eliminating traction deflection.