Discover how UV inks differ from other formulations and what it takes to print and cure them successfully.
For UV inks to cure properly, they must not only be exposed to the right wavelengths of UV energy, but also to the right amount of energy delivered at the correct intensity. The amount of energy arriving at the surface of the printed substrate is called the dose and is measured in millijoules (mJ/sq cm). The dose of energy a UV print receives is affected by the conveyor speed and number of lamps in the curing unit, as well as the number of times that it is exposed to the curing environment. The intensity of energy emitted by curing lamps is known as irradiance and is measured in watts or milliwatts (w/sq cm or mw/sq cm. Irradiance is directly related to electrical power, lamp condition, and the geometry of the reflector that directs and focuses the lamp output. Irradiance does not vary with exposure time.
The depth of cure achieved in the ink film is directly influenced by the irradiance level of the lamp. Delivering higher, more intense energy at the surface of the ink will allow more energy to penetrate the thickness of the ink film.
In curing units, the lamp and reflector assembly that delivers the UV light is call an irradiator. The lamps used in curing units generally consist of mercury vapor contained in a transparent quartz envelope. The lamps are powered in one of two ways. One type has electrodes at each end. When electrical current is applied, an arc is formed within the lamp, causing it to emit UV energy. The other type of lamp is electrodeless and ignites the gas with microwaves. In this system, a microwave-generating unit is placed above the irradiator and the microwaves are directed to the lamp by waveguides.
The medium-pressure mercury lamp is the most common lamp used for curing UV screen-printing inks. Argon gas and an exact amount of mercury (to create the right pressure when vaporized) are captured in a tube of fused silica. As Figure 1A shows, vaporized mercury emits ultraviolet energy at certain wavelengths so that when UV inks with the proper photoinitiators are exposed, cross-linking (curing) results.
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