1. Definition of scraper coating
Squeegee coating is a precision wet coating technology that is widely used in the process of uniformly coating liquid coatings on the surface of flexible substrates. This technology scrapes away pastes, adhesives, or functional coatings that are pre-applied or over-present on the substrate to a predetermined thickness through a precision squeegee element, resulting in a continuous, uniform, and precisely thicknessed functional coating on the substrate's surface. The core of scraper coating lies in using the precise gap between the scraper and the substrate to control the final thickness of the coating, while using the shearing action of the scraper to smooth the coating surface. According to the different forms of scrapers, they are mainly divided into comma scrapers, stainless steel sheet scrapers, steel wire scrapers and airflow scrapers. Among them, comma scrapers dominate the coating operations of high-viscosity and high-solids coatings due to their high strength and excellent coating accuracy.
2. The working principle of scraper coating
The basic principle of scraper coating is based on the physical process of accurate metering and shear leveling of fluids. In a typical operating process, the substrate (such as paper, film or metal foil) is driven by a conveyor system and first passes through the coating feeder to carry excess paint on the surface of the substrate. The substrate then carries the paint through a narrow gap between a precision scraper and a support roller (back roller). At this point, the edge of the scraper acts like a precision weir that blocks and scrapes excess paint over the gap thickness back into the trough, while the paint remaining on the substrate forms a wet film of the desired thickness. For systems with comma scrapers, the scrapers are usually fixed and do not rotate, relying on their high rigidity edge and fine-tuning mechanism to achieve precise control of the amount of coating. In the anilox roller coating system, the scraper is used to scrape off the excess paint outside the anilox roller cavity, so that the paint remains only in the cavity and is transferred to the substrate by contact pressure. The smooth running of the entire process depends on the rheological properties of the coating, the flatness of the substrate, and the dynamic balance between the squeegee parameters.
3. Measurement and evaluation method of scraper coating quality
The evaluation of the coating effect of the scraper is a multi-dimensional process that involves the measurement of the physical parameters of the coating, the apparent quality and the mechanical properties.
Measurement of coating thickness:This is the most direct quantitative evaluation indicator. In-situ in-line laser thickness measurement is often used in production to monitor the thickness of the coating and its transverse and longitudinal uniformity in real time. Laboratory offline measurements can be calculated by precision thickness gauges or by sampling and weighing according to standard methods. For example, for aluminum foil coating, the dry film coating amount can be accurately calculated by measuring the quality difference per unit area of the sample before and after coating, and considering the solid content, and the calculation formula can be expressed as: coating amount (dry film) = (sample mass after drying - original substrate quality) / sample area.
Coating Appearance and Mechanical Properties Inspection:The surface quality of the coating is visually or microscopically inspected to assess the presence of scratches, vertical stripes, lateral lines, thick edges, or punctate defects (such as porosity, shrinkage), etc. The adhesion performance of the coating is often carried out by using a hundred grid knife (paint film graver) for cross-cutting test, combined with special tape tearing, by evaluating the degree of peeling off the coating grid from the substrate.
Precision inspection of the scraper itself:As a key component, the blade straightness, edge angle accuracy, roughness and wear resistance of the scraper need to be tested by high-precision instruments such as laser interferometer and contour projector, which directly determine the long-term stability of coating quality.
4. The main factors affecting the quality of scraper coating
Scraper coating is a complex process with multivariable interactions, and the final effect is subject to the following aspects:
Squeegee parameters:The pressure, angle, sharpness, flatness and wear state of the scraper are the core influencing factors. Too much or too little pressure can lead to a deviation in the amount of coating. The angle of contact between the scraper and the roll surface (usually between 15° and 30°) determines the scraping effect and the speed of edge wear. The straightness error of the scraper edge will be directly replicated on the coating, resulting in uneven thickness.
Coating characteristics:The viscosity, solids content, surface tension and rheological behavior of the coating are crucial. High viscosity coatings require higher strength scrapers (such as comma scrapers). The rheology and water retention of the coating directly affect its performance during high-speed scraping, and if the viscoelasticity is too high, it may lead to problems such as scraper marks or material turning. The surface tension of the coating is related to its spread on the substrate and the suppression of edge effects.
Substrate and equipment status:The thickness uniformity and tension stability of the substrate, as well as the processing accuracy and smoothness of the coating roller and back roller, together constitute the mechanical basis of the coating process. Any mechanical vibration or belt slippage can create lateral lines or uneven coating on the coating.
5. The main application areas of scraper coating
With its wide applicability and increasing coating accuracy, scraper coating technology has penetrated many high-end manufacturing sectors. In the new energy industry, it is one of the key processes in the manufacturing of lithium battery electrodes, especially the comma scraper coating process, which is widely used to coat positive and negative electrode active pastes on copper foil and aluminum foil. In the printing, packaging and paper industry, squeegee coating is used to produce coated paper and paperboard to improve their surface smoothness and printability. In electronics and optics, this technology is used to manufacture flexible substrates, optical films, and thermal transfer ribbons. In addition, in the production of pharmaceutical packaging materials, squeegee coating is used for adhesive coating of aluminum foil to achieve heat sealing with polyvinyl chloride (PVC) rigid sheets. At the same time, it is also one of the basic processes for the production of various adhesive tapes and protective film products.
6. Summary
Scraper coating, as a long-standing yet dynamic precision coating technology, plays an indispensable role in modern industry. It enables precise control of coating thickness and uniformity through a seemingly simple scraping action, with a delicate balance between scraper, coating, substrate and equipment. From the earliest paper coating to today's highly consistent lithium battery electrode and optical film coating, squeegee coating technology is evolving towards higher speed, higher solid content, and higher precision. Understanding its definition, principles, influencing factors and evaluation system not only helps optimize existing production processes and solve practical defects such as scratches and thick edges, but also serves as the basis for promoting the development of new materials and processes. With the continuous improvement of coating functionality and miniaturization requirements, the in-depth study of scraper coating mechanism and the innovation of control technology will continue to be important topics of common concern to industry and academia.