UV printing is a relatively new printing technique that has gained popularity in recent years. It involves curing inks containing photosensitive agents on the surface of printed materials using ultraviolet light, creating a highly glossy and artistic coating with a sense of luster. The surface is raised and elastic, effectively enhancing the subtle details and outlines of the graphics.

UV printing primarily consists of two process flows:

Coating UV Oil.

Printing itself is a coating process, where relief printing, intaglio printing, and flat printing all apply ink to the substrate. UV varnish can also be coated onto the substrate, but it has a much lower viscosity than ink, requiring the substrate surface to be dense and non-absorbent to the varnish.

2. Solidify.

UV coating is applied to the substrate, and after a period of leveling, it enters the UV radiation zone for curing. The curing process is a chemical cross-linking process, where unsaturated carbon-carbon double bonds (in liquid form) in the prepolymers and monomers absorb energy transferred by the photoinitiator and cross-link to form a network structure of molecules (polymers). However, curing is affected by the thickness of the UV coating, the duration, and the intensity of the UV radiation. The relationship between the transmission of radiation energy and the thickness of the UV coating is exponentially decreasing.

The coating layer thickness is typically only 3-5μm, extremely thin, with the top and bottom layer varnishes curing together. However, when the coating is thicker (such as when using a screen printing or roller coating machine), the top layer may cure while the bottom layer remains uncured. The curing process varies over time, with many initiators having a reaction time of 0.1-0.3 seconds, but some residual initiators (unreacted) may continue to react for 6-24 hours, known as post-polymerization reactions.

If the initial light exposure is excessive, the UV oil will become increasingly brittle due to post-curing reactions, thus making it difficult to overexpose during UV coating. When using a UV curing machine, it's typically recommended to start with one light and adjust the distance between the light tube and the coated item (usually 5-15 cm). If one light does not cure it, additional exposure is needed. The curing process is affected by the intensity of UV radiation; as the UV intensity per unit area increases, the curing rate accelerates, but not linearly, rather by multiples.