Uneven transverse stretching

In the transverse stretching process, local thin areas or thick areas appear on the film surface. 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. Equipment factors: Temperature distribution in the tenter oven uneven along the transverse direction. 2. Comprehensive factors: Uneven clamping force of stretching clips/chains causes local slipping. 3. Comprehensive factors: The film web already has thickness variation before entering the stretching zone. 4. 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. 5. Feed uniformity factors: The temperature uniformity of the calender feed and the shape stability of the material bank directly affect the uniformity of feeding into the roll nip. A material bank that is too narrow, skewed, or unevenly rolling all cause transverse thickness abnormalities in the product; the temperature and flow rate of the banbury or extrusion feed must be kept stable.

1. Process parameter optimization and adjustment: Balance air temperature and air speed distribution in the tenter oven. 2. Mold/tooling improvement and maintenance: Check the clamping state and tension of stretching clips/chains. 3. Equipment inspection and system maintenance: Strengthen thickness uniformity control in the front-stage calendering. 4. Systematic troubleshooting method: Troubleshooting of calendering problems follows the sequence "roll gap → roll temperature → feed → tension → cooling". First check and precisely adjust the roll gap crown and cross angle, then confirm the uniformity of each roll's temperature distribution, next check the feed bank shape and rolling behavior, further adjust the winding tension control parameters, and finally optimize the cooling roll temperature and contact method. Change only one variable at a time to confirm the effect. 5. 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 transverse stretching 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.