Flexographic technology printing entails directly applying ink onto a specialized plate, which transfers the ink to the substrate. This printing plate is crafted from a cured or hardened rubber layer, and carefully calibrated for the appropriate thickness. The rubber-coated printing cylinder is then equipped with ink using a flexible roller or paper web.
After inking, the image on the printing plate is pressed onto the substrate, typically accomplished through a printing press. This process is widely employed to imprint designs onto various materials. The popular methods for curing or drying Flexo printing inks include the following:
1. Osmotic Drying
This drying is mainly accomplished by the absorption of paper and the penetration of ink. Its mechanism can be explained this way: the pore size is tiny since paper is a porous substance interwoven with fibers. When the ink is transferred to the paper, the binder in the ink begins to be absorbed due to the capillary action of the paper fiber voids.
During the absorption process, the composition and rheological properties of the ink gradually change, the liquid component of the ink gradually decreases, the cohesion of the pigment particles gradually increases, and the ink gradually loses its fluid properties and becomes solid.
2. Oxidative Polymerization
This dry form uses oxidative polymerization to convert the ink from liquid to solid. The mechanism is that the ink’s dry oil contains unsaturated double bonds and polymerizes into a polymer film through a peroxide bridge.
During the drying induction period, the dry oil absorbs less oxygen due to the trace amount of phospholipid organic antioxidants, and the oxidative polymerization reaction is prolonged. When the antioxidant is destroyed, the dry oil absorbs oxygen so that the methylene adjacent to the double bond reacts with oxygen to generate hydrogen peroxide.
Because hydrogen peroxide is very unstable, it will be separated into two free radicals under light, one of which is carboxyl-OH. Due to the active chemical nature of the free radical, when it attacks another oil molecule, a new free radical will be generated, and the original free radical will develop a stable compound.
As a result, the small molecules of the oil are polymerized into large molecules until there are no double bonds. The ink becomes a dry and solid film with a polymer network structure.
3. Thermal Polymerization Drying
After printing, the ink must be heated to undergo a polycondensation reaction and solidify. Heat-curable inks are mixed with resin and curing agent and heated after printing to make the resin react to form an ink film.
This ink is called heat-polymerized ink. Its advantages are one liquid type, no solvent, stable viscosity before heating, good adhesion fastness, and resistance to printed matter; the disadvantage is high energy consumption.
The curing speed of this ink depends on the content of the catalyst and the heating temperature. The ink must be cured at over 150 degrees when no catalyst is added.
After adding a catalyst, it can be cured at a medium temperature, but the catalyst content is significant, which is not convenient for preserving the ink. The hardened ink film of this ink has excellent physical properties and is mainly used for metal and plastic printing.
4. Filter Drying
The method of combining osmotic drying and gel drying is called filter drying. When printing the coating with quick-drying ink, the low-viscosity petroleum solvent in the binder is quickly absorbed into the layer through the capillary action of the coating. The resin component that swells into a gel contains the pigment particles, stays on the paper, and the ink is cured.
4. Wet-solid Drying
Print on paper with wet-fix ink, and blow in water vapor. The ink absorbs the moisture in the air or the form. At the same time, the resin dissolved in alcohol precipitates and precipitates. The ink starts to dry, and the moisture is finally drained from the inside of the paper due to evaporation. Particular attention should be paid to selecting the appropriate ink roller when using this ink.
5. UV Ink Drying
UV ink drying is a crucial process in flexographic printing, involving using ultraviolet (UV) radiation to quickly and efficiently cure the ink. UV inks contain photoinitiators that remain inert until exposed to UV light. When the ink is applied to the substrate during printing, it remains liquid.
As the freshly printed ink passes under UV curing lamps, the intense UV light activates the photoinitiators, triggering a rapid polymerization process. This causes the ink to solidify and adhere to the substrate almost instantaneously, allowing for sharp, vibrant, and durable prints.
In conclusion, selecting the proper drying or curing method for flexographic printing inks is pivotal in achieving high-quality and efficient prints. Whether opting for traditional drying techniques or embracing the advantages of UV curing, understanding the unique characteristics of each method is essential in maximizing print performance and meeting the diverse needs of the printing industry.