Coating is the process of applying a clear, transparent coating to the surface of printed materials after printing is complete, resulting in a shiny film layer. During the coating process, the even spread of the coating on the printed surface enhances the smoothness, not only improving the surface luster of the printed materials but also improving their surface functionality. It strengthens the ink's resistance to light, adds heat and moisture resistance to the ink layer, and serves to protect and beautify the printed graphics, making it widely used in the surface processing of printed materials such as book covers, packaging decoration, albums, large-scale decorations, and poster prints.
There are numerous methods for applying finishes, which can be categorized by the type of coating material into oxidizing polymer finishes, solvent-evaporation finishes, thermal curing finishes, and light curing finishes. Additionally, finishes can be classified based on the application method as spray/brush coating, printing coating, and centralized machine coating. In terms of equipment, there are standalone coaters and coaters integrated with printing machines. Furthermore, finishes can also be differentiated by the product they are applied to, such as full surface coating, spot coating, matte finishes, and artistic coatings.
Regardless of the technique or material used for varnishing, the quality of the varnish is essential to achieve the desired outcome. From a material perspective, secondary factors affecting the varnishing quality include the printing paper, inks, and varnish coatings having good varnishing compatibility.
Printing Paper. The smoothness and surface absorbency of the paper significantly affect the gloss coating, impacting the flowability of the coating and the formation of a high-smoothness, glossy film on the printed surface. Higher smoothness of the paper allows for better contact between the paper surface and the coating layer, resulting in easier flow and a higher smoothness film surface. Strong surface absorbency of the paper causes the paper fibers to absorb the coating too quickly during the flow process, leading to reduced coating fluidity and difficulty in achieving an even coating. Conversely, weaker surface absorbency means the paper fibers absorb the coating more slowly, making it harder for the coating to form a film during the flow process and difficult to achieve a high-smoothness glossy film. Therefore, different types of paper should be matched with suitable coatings, adjusting process parameters such as the gloss coating flow time and drying temperature.
Printing Ink. The particle size of the ink affects the wettability of the ink layer, thereby influencing the leveling and brightness of the coating. If the ink particles are large, the printed surface will necessarily be rough, difficult to wet, and not easy to level.
Topcoat Paint. Topcoat paint is also a secondary factor affecting the quality of the finish. It must not only possess properties such as colorlessness, odorlessness, toxicity-free, strong luster, resistance to chemicals, quick drying, but also good transparency, resistance to discoloration, and excellent flexibility, heat resistance, leveling, and adhesion. The viscosity of the topcoat paint has a significant impact on the finish quality, followed by surface tension and the volatility of the solvent. Solvents in the paint gradually decrease due to evaporation, penetration, and diffusion, leading to an increase in viscosity. The driving force behind leveling is the surface tension of the paint, which smooths out the marked surface into a smooth one by trying to contract the coating into a smaller surface shape. Therefore, the surface tension of the paint has a considerable effect on leveling.