UV LED pad printing ink is irradiated by ultraviolet rays, the photosensitizer in the system is excited and decomposed into free radicals, and the free radicals cause the cross-linking reaction to occur rapidly. All the components of the system become solid instantly, so the UV LED pad printing ink is generally a chemical process in which the liquid components react under the action of ultraviolet light to form a macromolecular network. Compared with the drying process of solvent-based inks, the drying of UV LED pad printing inks produces a new type of resin. The matching of this resin and the printing material is very important, and it determines the final adhesion of the ink. The drying of solvent-based inks is basically a sedimentation process. When the solvent evaporates, the resin settles into a solid and dries on the surface of the printing material.
UV absorption of photosensitizer:
The photosensitizer in the UV LED pad printing ink system plays a key role. For example, when benzophenone is irradiated with a wavelength of 310-340nm, it will transform into a triplet state. The triplet state deprives the H atoms in the active amine to form two Free radicals, free radicals can initiate the polymerization and crosslinking reaction of photosensitive resin and reactive diluent monomers. The free radicals further promote the polymerization and cross-linking reaction of the photosensitive resin and the reactive diluent monomer to generate a network-like polymer compound to complete the drying process of the UV ink.
UV LED curing:
The light source for UV LED curing should be able to emit ultraviolet light in the 300-380nm band. The most commonly used in the screen printing and pad printing industry is the mercury lamp. According to the intensity of the light emitted by the mercury lamp, it can be divided into three types: 80W/cm, 120W/cm, and 200W/cm. The higher the intensity, the shorter the curing time required. However, consider the distance between the light source and the substrate, because too close a distance may deform the temperature-sensitive substrate. It is recommended in practice to ensure that the distance between the substrate and the light source is at least 0.5m. Or when designing the curing device, the heat source can be cut off quickly to prevent the temperature rise from affecting the substrate.
