UV curing is a process in which ultraviolet light and visible light is used to initiate a photochemical reaction that generates a crosslinked network of polymers. UV Curing is adaptable to printing, coating, decorating, stereolithography and assembling of a variety of products and materials owing to some of its key attributes, it is: a low temperature process, a high speed process, and a solventless process—cure is by polymerization rather than by evaporation. Originally introduced in the 1960s this technology has streamlined and increased automation in many industries in the manufacturing sector.
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Applications
UV curing is used whenever there is a need for curing and drying of inks, adhesives and coatings. UV-cured adhesive has become a high-speed replacement for two-part adhesives, eliminating the need for solvent removal, ratio mixing and potential life concern. It is used in the screen printing process where the UV curing systems are used to cure screen-printed products, which range from T-shirts to 3d and cylindrical parts. It is used in fine instrument finishing (guitars, violins, ukuleles, etc.), pool cue manufacturing and other wood craft industries. Printing with UV curable inks provides the ability to print on a very wide variety of substrates such as plastics, paper, canvas, glass, metal, foam boards, tile, films, and many other materials.
Other industries that take advantage of UV curing include medicine, automobiles, cosmetics (for example artificial fingernails and gel nail polish, food, science, education and art. This curable ink has efficiently met the requirements of the publication sector on variety of paper and board.
Advantages of UV curing
The primary advantage of curing finishes and inks with ultraviolet lie in the speed at which the final product can be readied for shipping. In addition to speeding up production, this also can reduce flaws and errors, as the amount of time that dust, flies or any airborne object has to settle upon the object is reduced. This can increase the quality of the finished item, and allow for greater consistency.
The other obvious benefit is that manufacturers can devote less space to finishing items, since they don't have to wait for them to dry. This creates an efficiency that ripples through the entire manufacturing process.
Types of UV curing
Mercury vapor lamps are the industry standard for curing products with ultraviolet light. The bulbs work by high voltage passing through, vaporizing the mercury. An arc is created within the mercury which emits a spectral output in the UV region of the light spectrum. The light intensity occurs in the 240 nm-270 nm and 350-380-nm. This intense spectrum of light is what causes the rapid curing of the different applications being used.
In the last few years an emerging type of UV curing technology called UV LED curing has entered the marketplace. This technology is growing rapidly in popularity and has many advantages over mercury based lamps although is not the right fit for every application
Fluorescent lamps made specifically for UV curing are also available. These have the ability to dial in to specific frequencies at a lower price point as fluorescent lamps are an established technology and the spectrum is easily controlled by the type of phosphor used. They can produce frequencies that LEDs and mercury vapor lamps can not, including multiple frequencies. They are somewhat less efficient than LEDs or Mercury vapor but cost a fraction of the price of the other systems. The allow for curing all around an item by using multiple tubes and off the shelf ballast systems.
Mercury vapor lamp (H type)
The mercury lamp has an output in the short wave UV range between 220 and 320 nm (nanometers) and a spike of energy in the longwave range at 365 nm. The H lamp is a good choice for clear coatings and thin ink layers and produces hard surface cures and high gloss finishes.
Mercury vapor lamp with iron additive (D type)
The addition of iron to the lamp yields a strong output in the longwave range between 350 and 400 nm while the mercury component maintains good output in the short wavelength range. The D lamp is a good choice for curing heavily pigmented inks, adhesives, and thick laydowns of clear materials.
Mercury vapor lamp with gallium additive (V type)
The addition of gallium to the lamp yields a strong output in the longwave range between 400 and 450 nm. This makes the V lamp a good choice for curing white pigmented inks and base coats containing titanium dioxide which blocks the most shortwave UV.
Fluorescent lamps
Fluorescent lamps are used for UV curing in a number of applications. In particular, these are used where the excessive heat of mercury vapor is undesirable, or when an item needs more than a single source of light and instead the item needs to be surrounded by light, such as musical instruments. Fluorescent lamps can be created that produce ultraviolet anywhere within the UVA/UVB spectrum. Additionally, lamps that have multiple peaks are possible, allowing a wider variety of photoinitiators to be used. While fluorescent lamps are less efficient at producing UV than mercury vapor, newer initiators require less total energy, offsetting this disadvantage. Fluorescent lamps in a wide variety of sizes and wattages are available.
LEDs
UV LED devices are capable of emitting a narrow spectrum of radiation (+/- 10 nm), while mercury lamps have a broader spectral distribution. Fluorescent ultraviolet lamps can be fairly narrow, although not as narrow as LEDs.
LEDs are much more expensive but last up to 10 times longer, and like fluorescent tubes, can be cycled on and off frequently as they require no startup or cool down period. While they can't produce the same spectrum as mercury vapor or fluorescent tubes, photoinitiators can be formulated to work with them easily.