Formula design principle of flexographic printing ink

Flexographic printing principle:

The transfer of images during flexographic printing is accomplished through a photo-responsive flexographic resin plate. The ink is passed through an anilox roller in an ink tank (ani transports a certain volume of ink onto the printing plate. The anilox roller consists of a chrome alloy with a ceramic wear-resistant layer or ion-coated. In the ceramic wear layer, the amount of ink transferred is determined by the number of lines on the transfer roller surface.The surface of the coarse anilox roller has about 120 lines per inch, and the transfer roller for printing fine patterns can be reached in inches per inch. 500-900 line.

Technical requirements for flexo printing inks:

â—† Relatively low viscosity: The viscosity of standard UV flexographic inks at the shear rate of 2500 s-1 is 0.15-1.5 Pa.s, and the printing speed is determined by the conditions of construction (in open or closed rooms), 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 curing speed requirements of UV flexo inks also affect the viscosity of the inks, because most high-functionality products have high viscosity. In order to increase the curing speed 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. Since UV flexo inks have higher viscosity than aqueous flexo inks, compared with aqueous flexo inks, UV flexo inks can achieve printing effects closer to offset and gravure.

â—†Pigment Wettability and Color Intensity: Flexo inks require a relatively high pigment content, often between 15% and 20%. Very low pigment content results in very low void density on the surface of the anilox roller. A lot of small and discontinuous printing points appear. 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—† Curing speed: At present, the commonly used printing speed is 70m-150m/min, so the requirement for the ink curing speed is increased.

â—†Adhesion on a variety of substrates: Since UV flexo inks are mainly used to print a variety of very soft labels and packaging, the ink must be in a variety of 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, flexibility, and low odor.

Flexo printing ink manufacturing process:

UV flexo ink general manufacturing method:

A\ Polish the polyester acrylate or epoxy acrylate together with a certain amount of pigment on a three roll mill. During the grinding process, some active monomers will be added and some dispersing aids will be added to make the paste.

B\ Dilute the paste with a low viscosity mixture containing reactive monomer, initiator, and leveling agent to make the ink.

Application example of flexo printing inks:

Percentage of material

Pigment 12%

Urethane Acrylate 24%

Polyester acrylate 10%

Thinner 42%
Photoinitiator 10%

Additives 2%

Explanation:

Magenta flexo ink

For the magenta pigment 57:1, it must be ground by mixing with polyester acrylate to get the best rheological behavior. Urethane acrylates are too polar and therefore cause the ink to be too viscous at low shear rates.

Black flexo ink

For black flexo inks, epoxy acrylates may be used instead of urethane acrylates in the formulation in order to increase the curing speed of the ink. The use of oxidized carbon black pigments dispersed in epoxy acrylates has different effects. Frequently used epoxy acrylic resins are based on bisphenol A type epoxy resins.

Effects of active monomers and dispersants:

If the viscosity-reducing component of the system is not good, proper pigment content will not achieve good results. The viscosity-reducing component includes a reactive monomer for adjusting the viscosity, and of course, the monomer also plays a role in coordinating other aspects of performance. In addition, photoinitiators and auxiliaries are also essential. When a dispersant is added in the grinding stage of the pigment, the rheology of the ink tends to Newtonian fluid, and the optical density also becomes higher.

Reprinted from: 21st Century Fine Chemicals Network