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The Mysteries and Myths of UV Curing

(March 2002) posted on Wed Mar 13, 2002

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


By Bea Purcell

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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 2A shows, vaporized mercury emits ultraviolet energy at certain wavelengths so that when UV inks with the proper photoinitiators are exposed, cross-linking (curing) results.

Research has revealed that the addition of compounds called metal halides to the mercury enhances the spectral output of the lamp. This enhanced output is advantageous for curing opaque ink systems and those containing the white pigment titanium dioxide (TiO2). Two such "doped" lamps include the iron and gallium varieties (see Figures 2B and 2C). Most curing units offered today are designed to allow lamps to be changed in a matter of minutes.
 

One factor that has the greatest impact on the curing efficiency of the lamp is the amount of electrical current that flows through it during operation. According to Ohm's Law, volts x amps = watts. However, to calculate the actual power that the curing unit is receiving, watts must be multiplied by the "power factor" for the particular area (this factor can be supplied by the local electric company), and divided by the length of the lamp. The result is the exact power that the lamp is receiving, provided that there are no power fluctuations.

Curing units employ reflectors with different physical geometries to focus or scatter the lamp output across the print. The specific configuration of the reflector influences how the ink film is cured. Three main reflector geometries are used today (Figure 3):

• elliptical, which concentrates energy at the print surface with a narrow focal width

• parabolic, which reflects energy at a slightly wider focal width


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