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The Mysteries and Myths of UV Curing

(December 2006) posted on Wed Dec 27, 2006

Discover how UV inks differ from other formulations and what it takes to print and cure them successfully.

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By Bea Purcell

The electromagnetic spectrum exists as waves of electric and magnetic "particles" that travel through air, vacuum, and other substances. Electromagnetic waves are characterized by their frequency (number of waves per second) and associated wavelength (distance from wave crest to wave crest). Frequency is measured in gigahertz (GHz), and 1 GHz equals 1 billion cycles/sec. Wavelength is measured in nanometers (nm), and 1 nm equals 1 billionth of a meter. The electromagnetic spectrum (Figure 1) is a continuum of electromagnetic waves, all of which, with the exception of visible light, cannot be seen with the naked eye.

In the near-visible range of the spectrum, we find the range of wavelengths identified as UV. The UV range occurs at approximately 10-400 nm. The photoiniators used in UV inks typically react to specific wavelengths within the 200- to 400-nm range. However, the wavelengths that drive the curing reaction vary for different ink systems depending on what applications the inks are designed for. This is why UV-curing systems support different lamp types that deliver specific frequencies of UV energy. We'll discuss curing lamps in more detail later in this section.

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 (such as in the case of multicolor UV prints).

The intensity of energy emitted by curing lamps is known as irradiance and is measured in watts (or milliwatts)/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.


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