Surface water marks / cold spots

The product surface shows cloud-like, water-flow-like textures or locally darkened areas, like marks formed by water drops falling on the surface. Cold spots and water marks are traces of the melt failing to fully spread and fuse on the roll surface — low-temperature melt cannot completely replicate the roll surface finish after brief contact with the high-temperature roll surface, leaving traces of incomplete flow. This defect is most obvious in transparent films and light-colored sheets, seriously affecting appearance quality.

1. Mold/tooling factors: Roll temperature too low causes the melt to fail to fully spread on the roll surface. 2. Comprehensive factors: Bank temperature too low or uneven plastication. 3. Process parameter factors: Calendering speed too fast causes untimely bank renewal. 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: Increase calender roll temperature. 2. Mold/tooling improvement and maintenance: Ensure uniform banbury/extrusion feed temperature. 3. Equipment inspection and system maintenance: Appropriately reduce calendering speed or increase bank reserve. 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 surface water marks / cold spots 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.