Uneven shrinkage rate

Calendered products show large shrinkage rate variation in different directions or different batches. When this defect appears in calender molded products, it is usually closely related to process links such as roll gap adjustment, roll temperature distribution, feed uniformity, or winding tension control. Defects in calendered products often have transverse distribution characteristics, directly associated with roll surface condition and material bank behavior. Continuous occurrence will affect the film's appearance, thickness uniformity, and subsequent processing performance; troubleshooting must start from both roll system adjustment and the feed system.

1. Process parameter factors: Roll speed ratio and draw ratio settings cause molecular orientation degree changes. 2. Cooling/environmental factors: Uneven cooling rate. 3. Material/formulation factors: Batch differences in raw material shrinkage rate. 4. Mold/tooling factors: Downstream stretching or heat setting temperature changes. 5. Roll gap adjustment factors: Crown and cross-angle compensation of calender rolls is the core means of controlling the transverse thickness distribution of the film. Crown design is based on roll deflection calculations; if it does not match the actual process load, it will cause abnormal transverse distribution of film thickness — thicker in the middle and thinner at both ends, or an inverse bow-shaped distribution.

1. Process parameter optimization and adjustment: Stabilize speed ratio and draw ratio parameters. 2. Mold/tooling improvement and maintenance: Uniformly control the cooling process and temperature. 3. Equipment inspection and system maintenance: Monitor incoming material shrinkage rate. 4. Material/formulation optimization: Stabilize downstream process conditions. 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 shrinkage rate in calender molding. The core of the solution lies in precise adjustment of roll gap compensation, uniform control of roll temperature distribution, and assurance of feed system stability. It is recommended to start troubleshooting from roll system parameters and gradually go deeper into the equipment and formulation levels.