Longitudinal thickness fluctuation

Film thickness fluctuates periodically or randomly along the calendering direction. 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: Unstable roll speed or fluctuating roll speed ratio (friction ratio). 2. Process parameter factors: Feed extrusion output fluctuation. 3. Mold/tooling factors: Periodic roll temperature changes. 4. Process parameter factors: Stripping tension and winding tension fluctuation. 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 roll speed motors and transmission system. 2. Mold/tooling improvement and maintenance: Ensure stable feed extrusion output. 3. Equipment inspection and system maintenance: Improve roll temperature control accuracy. 4. Material/formulation optimization: Stabilize the winding tension control system. 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 longitudinal thickness fluctuation 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.