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The coating method employed in modern paperboard machines is primarily determined by several factors, including the machine's equipment configuration, operating speed, raw materials, paperboard types and intended applications, as well as investment scale. In practice, a two-stage coating approach-consisting of pre-coating followed by topcoating-is commonly adopted. Pre-coating is typically performed using an adjustable VARI-BAR coater or FILM PRESS equipment; some manufacturers also employ blade (doctor blade) systems directly. After pre-coating, the board must undergo drying before surface coating can be applied. Surface coating may be carried out using adjustable blade coaters, air knives, or COMBI-BLADE systems. Topcoats are generally applied one or two times, depending on the desired quality specifications of the final product.
For instance, JAGENBERG configured the three coating stations of a folding boxboard machine-designed to process 100% recycled waste paper for printing-as follows: (1) Pre-coating is performed using an adjustable blade coater to enhance surface smoothness and ink absorption capacity. (2) The first topcoat is applied via air knife coating to increase coating weight and improve brightness. (3) The second topcoat is applied using a soft blade coater, thereby achieving optimal gloss and printability for the folding boxboard.
In the case of white board used for instruction manuals, invitations, postcards, and similar high-quality printing applications, the raw materials typically consist of bleached chemical pulps such as LBKP and NBKP. Both sides of the board require high levels of smoothness and gloss, necessitating one or two coating applications per side. To meet these requirements, JAGENBERG has equipped a paper machine with four coating heads for white board production. The first and second coating heads are dedicated to pre-coating both sides of the board. These pre-coating heads feature combination blade systems and can alternatively operate with adjustable blades as needed. Two drying cylinders are installed between the pre-coating and topcoating sections to ensure adequate drying. In the topcoating section, two combination blade coating heads are used for applying coatings to the top and bottom surfaces of the board, respectively.
Currently, an increasing number of manufacturers are adopting FILM PRESS technology for pre-coating. This method offers significant advantages, including the ability to achieve higher bulk and thickness while effectively minimizing fiber linting issues associated with conventional blade systems. FILM PRESS is widely used in the production of high-grade coated white board and premium white cardboard. Following pre-coating, two successive combination blade coating stages are typically applied to attain the desired surface properties.
In terms of coating technology, the gap between China and advanced international standards has been steadily narrowing. Coating machines manufactured by several leading Chinese equipment suppliers have gained recognition from domestic customers due to their reliability, performance, and after-sales service. Presently, many paperboard manufacturers in China have successfully implemented domestically developed three-stage blade coating processes. Further advancements in this field are expected in the coming years.
