Flexographic printing principle:
The transfer of the image in the flexographic printing process is accomplished by a flexible photosensitive resin plate obtained by a photochemical reaction. The ink is in an ink storage tank, and a certain volume of ink is transported to the printing plate through an anilox transfer roller. The anilox ink transfer roller is composed of chromium alloy, and the surface is coated with a ceramic wear-resistant layer or an ion-sprayed ceramic wear-resistant layer. The amount of ink transferred is determined by the number of lines on the surface of the ink transfer roller. Coarse anilox rollers have about 120 lines per inch on the surface, while transfer rollers used to print fine patterns can reach 500-900 lines per inch.
Technical requirements for flexographic printing inks:
◆ Relatively low viscosity: the viscosity of standard UV flexo printing ink is 0.15-1.5Pa.s at a shear rate of 2500s-1, and the printing speed is determined by the construction conditions (in the open air or in a 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, so the printed pattern will have some discontinuous ink accumulation smears. If the viscosity of the ink is very low, the problem is less. However, due to the small amount of ink in flexographic printing itself, the transfer of ink will be affected. The curing speed requirements of UV flexo inks will also affect the viscosity of the ink, because most products with high functional groups are very viscous. In order to increase the curing speed of the ink without affecting the viscosity, the amount of photoinitiator must be increased, which at the same time affects the odor and cost of the ink. So formulators must find the optimum combination of ink viscosity and other properties. Compared with water-based flexo inks, UV flexo printing inks have higher viscosity than water-based flexo printing inks, so the use of UV flexo printing inks can achieve a printing effect closer to offset printing and gravure printing.
◆Pigment wettability and color strength: Flexo printing inks require fairly high pigment content, often 15% to 20%. When the pore density on the surface of the anilox roller is very low, very high pigment content can cause some problems. Many small discontinuous print dots appear. The viscosity is closely related to the wetting of the pigment; if the wetting of the pigment is not good, the characteristics of Newtonian fluid will not be reflected, and the ink will have corresponding flow difficulties. Also, because the pigments are not well dispersed, the color strength will also be weaker.
◆Rheology: ink with good fluidity will have good gloss. Newtonian fluids have the characteristic that the viscosity does not change with the shear rate, which not only determines whether the ink can be transported well, but also is equally important for the transfer of the ink on the ink roller. Air bubbles extruded as the ink passes through the anilox rolls into the remaining ink. Excessive air bubbles in this way can shift the Newtonian rheology of the ink and cause printing failures.
◆Curing speed: The printing speed commonly used at present is 70m-150m/min, so the requirement for ink curing speed is increased.
◆Adhesion on various substrates: Since UV flexo ink is mainly used to print various very soft labels and packaging, the ink must be used on various soft substrates such as: PP, PE, PET, PVC, and Good adhesion on PA.
◆Other requirements: The ink should have scratch resistance, chemical resistance and bacteria resistance, flexibility, and low odor.
Common manufacturing methods for UV flexo inks:
Polyester acrylate or epoxy acrylate and a certain content of pigment are ground together on a three-roller machine. During the grinding process, some active monomers and some dispersing aids will be added to make a color paste.
The ink is made by diluting the color paste with a low-viscosity mixture containing reactive monomers, initiators and leveling agents.
Application examples of wax for flexographic printing inks:
Urethane acrylate 24%
10% of polyester acrylate
Magenta Flexo Ink
For magenta pigment 57:1, the best rheological behavior has to be obtained by mixing with polyester acrylate and grinding. Polyurethane acrylates, due to their high polarity, can cause inks to be too viscous at low shear rates.
Black Flexo Ink
For black flexo printing ink, in order to improve the curing speed of the ink, epoxy acrylate can be appropriately used instead of urethane acrylate in the formulation. Using oxidized carbon black pigments dispersed in epoxy acrylates will have different results. Frequently used epoxy acrylic resins are epoxy resins based on bisphenol A type.
Effects of reactive monomers and dispersants:
If the viscosity reducing components in the system are not selected properly, the proper pigment content will not achieve good results. Viscosity reducing components include reactive monomers used to adjust viscosity, and of course monomers also play a role in coordinating other aspects of performance. In addition, photoinitiators and auxiliaries are also essential. When the dispersant is added in the grinding stage of the pigment, the rheology of the ink tends to be Newtonian fluid, and the optical density also becomes higher.
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