Are you using the varnishing process correctly?

Labels are a type of printed product used to indicate information related to a product. By being affixed to the outside of product packaging, they can prominently inform consumers of the product's name while effectively distinguishing the product from other similar products and highlighting the brand. To further attract consumers' attention, besides using novel packaging shapes, adding a coating process on top of richly colored label printing can make the original label shinier and provide consumers with a fresh visual experience.

The coating process refers to the application of a colorless, transparent varnish on the surface of the label. Its purpose is to increase the gloss of the printed surface and to provide protection against stains and moisture, as well as to protect the graphics and text. Generally, the coating process can be divided into overall coating, spot coating, and reverse coating. In this article, the author mainly elaborates on these three coating processes, discussing their characteristics and usage considerations.

PART1

Overall Coating

01

Characteristics of Overall Coating

Overall coating, also known as full-surface coating, involves applying a layer of colorless transparent varnish to the entire label surface. After leveling, drying, polishing, and curing, a thin and uniform transparent glossy layer forms on the label surface, enhancing surface smoothness and providing decorative processing to the printed graphics. It should be noted that overall coating differs from lamination. Lamination refers to the process of covering the printed label with a transparent plastic film 0.012–0.02mm thick using heat or pressure from a laminating machine, bonding the label and the plastic film together. Figure 1 shows pre-coated materials that require heat lamination, while Figure 2 shows film materials that require pressure lamination. Because lamination requires the use of plastic film as raw material, from an environmental perspective, coating is more eco-friendly, especially water-based and UV coating. Water-based coating is non-toxic, non-irritating, free of organic volatiles, cost-effective, widely sourced, highly transparent, has good gloss, does not yellow easily under prolonged intense sunlight, and is resistant to abrasion and scratches. UV varnish contains almost no solvents, requires relatively low energy for curing, strongly adheres to ink, strengthens durability against wear, chemicals, and solvents, and maintains stability, allowing cleaning with water or ethanol. UV-coated products are less prone to sticking, can be stacked immediately after curing, saving time for the next processing steps and improving production efficiency.

Figure 1 Pre-coated film material that needs to be heated and laminated

Figure 2 Film materials requiring pressure lamination

02

Considerations for Overall Varnishing

When using an overall varnishing process to increase the gloss of labels, to achieve varnish quality and ensure normal production operation, attention should be paid to the following four aspects: First, control the thickness of the varnish; second, choose the appropriate varnish, as there are many types of varnish. Therefore, when selecting a varnish, it is necessary to consider the different substrates as well as the different varnishing methods to choose different varnishes; third, control the viscosity and surface tension of the varnish. Viscosity affects leveling, drying speed, surface gloss, and other properties, so it is necessary to adjust the varnish viscosity according to different substrates. At the same time, attention should be paid to changes in the surface tension of the varnish, which should be lower than that of the ink layer on the substrate to ensure that the varnish can properly wet, adhere to, and penetrate the surface of the label; fourth, pay attention to the coordination between varnishing speed, coating amount, pressure, and curing equipment power. These factors should be reasonably adjusted according to the substrate surface, wetting, and absorption conditions.

PART 2

Spot Varnishing

01

Characteristics of Spot Varnishing

Spot varnishing refers to applying varnish only on specific prominent areas of a label, making it appear glossier, more vivid, and more three-dimensional compared to the surrounding areas, thereby creating a unique artistic effect. Common spot varnishing effects include spot gloss varnish, spot matte varnish, spot snowflake, spot frosted, spot foaming, spot refractive, and spot wrinkled effects. Spot varnishing can be applied after lamination or directly on printed products, but to further highlight the effect of spot varnishing, it is usually applied after lamination, with matte film lamination being the most common (about 65% of spot varnished products). As shown in Figure 3, this illustrates the spot varnishing effect on labels. Thus, labels treated with spot varnishing have a more textured and three-dimensional surface, with high brightness, transparency, and wear resistance in those areas, producing a strong visual impact.

Figure 3 Display of Local Spot Varnish Effects on Labels

02

Precautions for Local Spot Varnishing

Local spot varnishing can be achieved through screen printing, flexographic printing, and other methods. Since the ink layer in screen printing is thicker compared to offset printing, gravure printing, or relief printing, using screen printing for local spot varnishing will produce a more pronounced effect. However, when using screen printing for local spot varnishing, the following points should be noted: First, the formulation of UV varnish differs between winter and summer, so attention should be paid to the change in season and temperature, and the appropriate varnish should be selected. Typically, the curing content of UV varnish in winter is lower than in summer. Using the winter UV varnish formula in summer may result in incomplete curing, causing issues like sticking, while using the summer UV varnish formula in winter may lead to poor leveling, resulting in bubbles or skinning. Second, UV varnish should be thoroughly stirred before use because it is composed of multiple chemical ingredients that may separate if left unused for a long time, so thorough mixing is essential before application.

PART 3

Reverse Varnishing

01

Characteristics of Reverse Varnishing

Reverse varnishing is a varnishing method that combines local spot varnish and local matte effects to achieve high contrast in specific graphic areas. The reverse varnishing process can be completed in one pass on a printing press or offline.

Reverse varnishing allows the label surface to display both glossy and matte textures, creating a stronger contrast in brightness, resulting in a more effective varnishing effect. It can serve both anti-counterfeiting and aesthetic purposes. In reverse varnishing, a transparent matte base varnish is first printed on the non-glossy areas of the design using the offset method. Then, a full-face top varnish is applied and cured on the label print. The top varnish reacts with the previously printed base varnish to create a matte effect in the non-glossy areas, while areas without base varnish remain glossy. Because non-glossy areas are printed using offset printing, registration is very accurate, ensuring the printing precision of the glossy areas. Figures 4 and 5 show labels produced using reverse varnishing. In Figure 4(b), a magnified local area of the label is shown, with glossy and matte areas marked. In Figure 5, the red printed areas on the label are glossy, while the white and blue areas are matte.

(a)

(b)

Figure 4 Label product using reverse coating process

Figure 5 Label products using reverse varnishing process

02

Precautions for Reverse Varnishing

When using the reverse varnishing process, the following should be noted: First, ensure that the ink used for regular graphic printing on the label product is completely dry or cured; if the ink layer is not dry, the varnish will not cure properly and the contrast effect will not occur. Second, during the design of the file, do not choose small text or small patterns for reverse varnishing; if the font or pattern is too small, it may cause blurring issues. Third, the fineness of the matte texture is related to factors such as the speed of the printing press, printing pressure, varnish thickness, type of varnish, and the smoothness of the substrate surface. If the printing press speed is fast and the varnish layer is thin, the matte texture will be coarser; if the substrate surface is smooth, the matte texture will be very distinct. Overall, reverse varnishing can achieve contrast effects that ordinary varnishing cannot, making the printed effect more exquisite, adding a finishing touch, and enhancing the quality of label printing.

Faced with increasingly strict national requirements for environmental protection and pollution prevention, as well as consumers' preference for bright and novel printed appearances, and in the context of intense market competition, more and more label printing enterprises are beginning to use various post-print finishing processes on labels, especially varnishing. By making small changes in post-print processes, product design can become more diversified to adapt to the new environment, meet new demands, and add stronger visual impact to label products.