Fish eyes / gel particles

Transparent or translucent hard granular protrusions appear on the product surface, rough to the touch, especially obvious in films and pipes. The essence of gel particles (fish eyes) is ultra-high molecular weight components or cross-linked gel particles in the resin that are not completely melted; they fail to fully fuse with the surrounding melt at extrusion temperature and retain their original particle form. Gel particles not only affect surface finish but also become stress concentration points and fatigue crack initiation points.

1. Material/formulation factors: Ultra-high relative molecular mass particles or cross-linked gels exist in the resin. 2. Comprehensive factors: Insufficient plastication temperature. 3. Material/formulation factors: Non-calendering grade resin mixed into the resin grade. 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 feed temperature to ensure full plastication. 2. Mold/tooling improvement and maintenance: Use calendering-specific resin (narrower molecular weight distribution). 3. Equipment inspection and system maintenance: Install a melt filter to filter out gels. 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.

The essence of gel particles (fish eyes) is ultra-high molecular weight components or cross-linked gels in the resin that are not completely melted. The solution approach is "heat up + filter + change material": increase plastication temperature to ensure full melting, use screens to filter gel particles, and select calendering/extrusion-specific resins with narrower molecular weight distribution.