Uneven cooling / thermal stress

After cooling, the product develops internal stress due to local cooling rate differences, causing deformation or decreased mechanical properties. 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: Unreasonable design of cooling water tank or cooling tooling causes uneven cooling medium distribution. 2. Cooling/environmental factors: Cooling water inlet temperature/flow unstable. 3. Cooling/environmental factors: Large product cross-section thickness variation without targeted cooling design. 4. Process parameter factors: Improper haul-off tension aggravates internal stress generation. 5. 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: Optimize cooling design (zoned temperature control, spray angle adjustment). 2. Mold/tooling improvement and maintenance: Stabilize cooling water temperature and flow. 3. Equipment inspection and system maintenance: For thick-walled products, appropriately extend cooling time and reduce cooling rate. 4. Material/formulation optimization: Perform annealing treatment to eliminate internal stress. 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 uneven cooling / thermal stress 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.