During the mass production of solvent-free lamination, the finished films often exhibit cloud-like blemishes, block-like color differences, left-right asymmetry, and uneven brightness and darkness, mainly manifested as inconsistent light transmission on the film surface, dulling of the base color, and chaotic textures. These problems cannot be self-repaired after maturation and are prone to causing degradation in the appearance of transparent films and light-colored color-printed films, leading to customer rejection. They are the most common difficult defects in solvent-free lamination production, mainly caused by uneven coating, abnormal rheology of the adhesive flow, and equipment accuracy deviations.
The lack of uniform coating is the core cause of these defects. The solvent-free polyurethane laminating adhesive has a 100% solid content, high viscosity, and relatively weak temperature stability. If the coating roller and adhesive application temperature are too low, the resistance to the spread of the adhesive flow increases, resulting in uneven microscopic coating thickness, and after curing, cloud-like blemishes will form. At the same time, uneven mixing of the adhesive and instantaneous fluctuations in dynamic ratio will cause inconsistent cross-sectional gelation rates of the adhesive layer in the roll, directly leading to color differences between the left and right sides and the front and back surfaces of the film.
Improper equipment accuracy and process control will further aggravate the blemishing problem. Imbalance in the left and right pressures of the composite pressing rollers, eccentricity of the roller surface, and uneven wear will cause differences in the thickness of the adhesive layer in different areas, resulting in regular patterns of light and dark lines after lamination. Excessive speed of the equipment, frequent fluctuations in tension, and micro-jitter of the film surface will lead to insufficient microscopic adhesion and induce irregular blemishing. In addition, unstable temperature and humidity in the workshop, accumulation of adhesive skin on the roller, and foreign matter and impurities adhered to the roller surface will also cause local cloud-like blemishing and chaotic color differences.
On-site rectification requires synchronous optimization from multiple dimensions such as adhesive, equipment, and process, precise control of the operating temperature, improvement of the adhesive flow and spread effect, and ensuring uniform and consistent coating. Regular calibration of the mixing ratio of the adhesive, verification of the parallelism and pressure balance of the rollers, matching reasonable speed and stable tension, and eliminating film surface jitter and offset. Timely cleaning of the adhesive skin on the roller surface and dirt on the roller, and strict control of the production environment in the workshop. By stabilizing the coating conditions and equipment accuracy throughout, the problems of blemishing, color differences, and front-back surface phenomena of the composite films can be completely solved, continuously ensuring the transparent appearance and uniform quality of the finished products.
