Analysis and Enhancement of Pros and Cons in Curtain Coating

1. Drawbacks of Conventional Coating Methods: Traditional coating techniques, such as scraper and rod coating, exhibit certain disadvantages. Scraper and rod coating exert static pressure on the paper web, leading to potential scratching or cracking of the coated product. Additionally, limitations in low viscosity coating during the process result in increased coating penetration into the original paper, elevating the coating amount. When the paper machine speed exceeds 1000 m/min, turbulent material pool conditions may cause glue spray, necessitating increased pressure in the pressure zone to maintain consistent glue amounts. However, this can elevate the risk of paper breakage.
2. Advantages of Curtain Coating: Examining the principles of curtain coating reveals a non-contact coating method that avoids imposing mechanical stress on the base paper. This minimizes paper web breaks and ensures excellent coating coverage, offering opportunities for improved production efficiency and reduced coating amounts. Curtain coating essentially replicates the coating process, forming a liquid film upon contact with the paper. The film maintains uniform longitudinal and horizontal coating thickness, free from scratches, stripes, or orange peel lines, resulting in good opacity. Coating amount adjustment is easily achieved through the coating pump speed, leading to low operating costs, high production efficiency, and the absence of paint splashing or smoking. Furthermore, compared to other coating methods, curtain coating boasts a wide operating window, with experiments confirming its stable operation at high speeds.
3. Shortcomings of Curtain Coating: While curtain coating outperforms blade coating, bar coating, and air knife coating, challenges persist at high speeds. Stable operation requires specific coating characteristics, such as sufficient ductility, no bubbles, and an appropriate viscosity range. The coating machine demands optimized geometry, precise processing accuracy for the slit nozzle lip plate, a reliable feeding system, and stable material selection. Compensation for coating surface flatness and smoothness involves optimizing the coating formula and finishing process. The sensitivity of the free-falling liquid curtain to turbulent behavior can result in irregular paint flow. Maintaining an adequate flow rate is crucial to prevent curtain breakage. However, the primary issue with curtain coating lies in the air film and bubbles between the coating and paper web, causing uneven coverage and unstable coater operation at high speeds.
4. Solutions to Shortcomings: While various problems are easily resolved, this paper focuses on addressing the issue of air film and bubbles. This has been a persistent concern in curtain coating, prompting extensive research. Solutions include incorporating a converter device, air scraping device, and enhancing paper web smoothness. For printing paper, three methods can be employed: (1) developing a special coating for curtain coating; (2) improving the coater with a degassing device; (3) selecting parameters that control the coating process, such as curtain height and descent speed. Recent research indicates that careful selection of coating formulation and debubbling device can achieve stable curtain coating operation at high speeds.