Flexo printing is a branch of letterpress printing. Flexographic printing is similar to letterpress printing in terms of printing principle. It differs from traditional letterpress printing in the composition and imprinting process of printing plates and inks. Figure 1 shows the principle of flexographic printing. A brief introduction.

The transfer of the image during the printing process is accomplished by a photo-responsive, flexible printing plate or a patterned rubber stick.

In an ink reservoir, the ink is delivered to the printing plate by an anilox. The anilox roller consists of a chrome alloy and is coated with a ceramic wear layer or an ion-coated ceramic wear layer. The amount of ink delivered is determined by the number of lines on the transfer roller surface. The surface of the thick anilox has about 120 lines per inch, while the transfer roller for printing fine patterns can reach 500-900 lines per inch.

Technical requirements for flexo printing inks

High quality inks have many important parameters, the most important being good pigment wetting. Like high-viscosity systems such as offset inks, flocculation of pigments does not pose a problem. However, for flexo inks, because of the very low viscosity, the pigments are easily flocculated or even settled. This means that resins suitable for offset inks may not be suitable for flexo inks.

Of course, pigment wetting is not the only factor to be considered. It is also difficult to achieve good rheology, reactivity, adhesion, and resistance to bacteria at the same time.

â—† Relatively low viscosity: The viscosity of the standard UV flexographic ink at the shear rate of 2500s-1 is 0.15-1.5Pa.s, and the printing speed is determined by the conditions of the construction (in the open or closed room), and It also determines the viscosity of the final ink used. If the viscosity is too high, the ink will accumulate between the anilox roller and the doctor blade. As a result, the printed pattern will have some discontinuous ink deposits. If the viscosity of the ink is low, the problem will be less. However, since the overprint amount of the flexographic printing itself is small, the ink transfer may be affected.

The reactivity requirements of the ink also affect the viscosity of the ink because most high-functionality products are very viscous. In order to increase the reactivity of the ink without affecting the viscosity, the amount of photoinitiator must be increased, but this also affects the cost of the odor and the ink. Formulators must therefore find the best combination of ink viscosity and other properties.

U V flexo inks have higher viscosity than solvent and water based flexo inks, so using UV flexo inks can achieve closer offset and gravure printing than solvent and water based flexo inks. In some cases UV flexo inks can reach a viscosity of 2.5 Pa.s.

â—† Pigment Wettability and Color Intensity: Flexo inks require a relatively high pigment content, often from 15% to 20%, which makes it possible to print thickest ink film no more than 1 micron. This is important for printed materials and printing prices per square meter. With very low pore densities on the surface of the anilox roller, a very high pigment content can result in many tiny, discontinuous printed dots.

The viscosity has a close relationship with the quality of the pigment wetting. If the pigment does not wet well, the properties of the Newtonian fluid will not be reflected, and the ink will also have a corresponding flow difficulty. In addition, because the pigment is not dispersed well, the color strength also weakens.

â—† rheology: ink with good fluidity, gloss will be very good. Newtonian fluids have the characteristic that the viscosity does not change with the shear rate, which not only determines whether the ink delivery is good, but also is important for the transfer of the ink on the ink roller. The air bubbles that have been pushed out when the ink passes through the anilox roll will enter the remaining ink. As a result, excessive bubbles can cause Newtonian rheology of the ink to shift and cause printing failures.

â—† Reactivity: At present, the commonly used printing speed is 350m/min, so the requirement for the ink curing speed is increased.

â—† Adhesion on various substrates: Since UV flexo inks are mainly used to print a variety of very soft labels and packaging, the ink must be on various soft substrates such as: PP, PE, PET, PVC, and PA has good adhesion.

â—† Other requirements: The ink must have scratch resistance, chemical resistance and antibacterial properties. Ink manufacturers are often faced with a difficult problem in developing new ink types, which is how to still have good adhesion after the coating is eroded by bacteria. In addition, regulatory requirements must also be considered, especially in food packaging applications.

Selected substances and their characteristics

UV flexo inks are usually manufactured using a two-step method: the resin is ground on a three roll machine with a certain amount of pigment. During the grinding process, some monomers are often added and sometimes some dispersing aids are added to make the color. Pulp. The paste is then diluted with a low viscosity mixture containing monomer, initiator and leveling agent.

Grinding process often uses polyester resin or modified epoxy resin as a grinding base (see Table 1). Also shown are some particularly effective monomers used during the grinding or dilution stage (see Table 1).

Experimental results

â–² The effect of grinding resin: The ink extraction equipment has a great influence on the dispersion of pigments. The monomer and the pigment may be mixed with a ball mill for grinding, or a three roll mill may be used for grinding the mixture of the resin and the pigment.

The quality of the grinding effect depends largely on the type and fineness of the pigment. There is no chemical change during pigment grinding.

Magenta flexo ink

It is sometimes difficult to judge the quality of ink by simply changing the relationship between viscosity and shear rate. However, with the Thinning Index (SI), it is easy to find which ink has the best printability.

For the red pigment 57:1, it must be mixed with a polyester acrylate such as EbecrylTM 450, EbecrylTM 657, or EbecrylTM 870 for best rheological behavior. Epoxy resins have high polarities and therefore cause the ink to have excessive viscosity at low shear rates.

Epoxy acrylate-based inks have a lower gloss on paper than polyester acrylates. However, this is not to say that epoxy acrylates cannot be used in such inks, and epoxy acrylates can increase the curing speed of inks, and the low price does not increase the cost. In this case, wetting of the pigment can be ensured by adding other components.

Black flexo ink

For black flexo inks, in order to increase the reactivity of the inks, epoxy resin can be used instead of polyester resin in the formulation. The use of oxidized carbon black pigments has different effects when dispersed in epoxy resins. Frequently used epoxy acrylic resins are based on bisphenol A type epoxy resins. EbecrylTM 3702 is a fatty acid modified epoxy resin. It is a very good pigment wetting material.

EbecrylTM 600 is not good in terms of pigment wettability, making the ink fluid poor so that it cannot be pumped. In fact, the fluidity of the ink is not good. After diluting with DPGDA, the decrease in optical density and the decrease in coloring power all indicate that the pigment flocculates. EbecrylTM 3701 provides good rheological properties. Although the optical density and transparency of the inks using EbecrylTM 3701 are good, the gloss on various substrates is very low.

â–² Effects of monomer and dispersant: If the viscosity-reducing component of the system is poorly selected, proper pigment content will not achieve good results. The viscosity-reducing component includes a monomer for adjusting the viscosity, and of course, the monomer also plays a role in coordinating other properties. In addition, photoinitiators and auxiliaries are also essential.

By investigating the magenta pigment 4BY with the same pigment content diluted with the low-viscosity acrylates EbecrylTM 40 and EbecrylTM 160, respectively, it was found that all inks were thixotropic without the dispersant, but were based on EbecrylTM 160. The printability of the ink is better, as is the optical density, gloss, and transparency.

When the dispersing aid is added in the grinding stage of the pigment, the Newtonian fluid characteristics of the ink become apparent and the optical density also becomes higher. Experiments show that the effect of the dispersing aid with EbecrylTM 160 is much better than that of EbecrylTM 40. The same is true for alkoxylated neopentyl glycol derivatives. Although auxiliaries do not solve the problems as much as we would like to, there are sometimes opposite results. However, using EbecrylTM 160 inks, the gloss on paper is relatively better.

Source: China Ink Network