Inaccurate Online Wet Film Thickness Measurement at the Coating Head

Large errors in online wet film thickness detection cause feedback control failure. When this defect appears in coating-formed products, it is usually closely related to the coating head precision, the rheological properties of the coating liquid, the substrate surface condition, or the curing process conditions. The distribution characteristics of coating defects (transverse/longitudinal/random) provide important clues for diagnosing coating head and curing system problems. Continuous occurrence will cause the functional performance of the coating to fall below standard, and coordinated troubleshooting is required from three aspects: coating liquid formulation, coating equipment, and curing process.

1. Comprehensive factor: the sensor model is not suitable for high-volatility solvent environments (e.g., infrared sensors are interfered by solvent vapor). 2. Comprehensive factor: mechanical vibration exists at the sensor installation position. 3. Material/formulation factor: changes in substrate color and light transmittance interfere with optical thickness measurement. 4. Curing process factor: the curing temperature and time determine the completion degree of the coating crosslinking reaction — insufficient temperature or time leads to incomplete curing (tacky, low hardness, poor solvent resistance), while excessive temperature or time leads to over-curing (embrittlement, yellowing, cracking), which requires precise matching to the curing window of the coating liquid system.

1. Process parameter optimization and adjustment: select a thickness measurement technology that is not interfered by solvent vapor. 2. Mold/tooling improvement and maintenance: isolate the mechanical vibration source at the sensor installation location. 3. Equipment inspection and system maintenance: calibrate the instrument under the same substrate conditions. 4. Systematic troubleshooting method: troubleshooting of coating problems follows the sequence of "substrate → coating liquid → coating head → curing → environment". First check the substrate surface tension (corona treatment effect) and cleanliness, then check the viscosity, solid content and degassing state of the coating liquid, next check the coating head gap and cleanliness, further verify the curing temperature and time, and finally evaluate the influence of ambient temperature, humidity and cleanliness. 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 the 5Why analysis method to trace the root cause, and solidify the 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 inaccurate online wet film thickness measurement at the coating head in coating forming. The core of the solution lies in ensuring the substrate surface condition, optimizing the rheological properties of the coating liquid, and precisely controlling the curing process conditions. It is recommended to start from the two aspects of substrate and coating liquid, and gradually troubleshoot to the coating equipment and curing system.