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An Overview of UV Curing

(December 2011) posted on Wed Jan 25, 2012

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


By Screen Printing's Solution Sourcebook

click an image below to view slideshow

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 (Figures 1B and 1C). 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: elliptical, which concentrates energy at the print surface with a narrow focal width; parabolic, which reflects energy at a slightly wider focal width; and multifaceted, which reflects energy at the widest focal width (Figure 2).

One final consideration about curing equipment concerns heat management. High temperature created by the IR energy UV lamps emit is an unfortunate byproduct of the curing process. Preventing this heat from building up and damaging sensitive substrates is a critical concern in UV curing. Systems for heat management in modern curing units include water cooling systems, fans, automatic shutter systems, and dichroic mirrors, which reflect UV but absorb IR energy.

Processing issues
Controlling UV-ink-film thickness in screen printing is of paramount importance. Mesh type, screen-tension level, emulsion thickness, and related screenmaking factors all contribute to the amount of ink deposited on the substrate. These variables have to be controlled to achieve the ink-film thickness recommended by the ink manufacturer. Similarly, press-setup parameters must also be controlled, including squeegee hardness, screen off-contact, and squeegee/floodbar speed and pressure.

The type of substrates you are printing on also influences the results you can expect from UV curing. On polycarbonate substrates, for example, UV energy at shorter wavelengths (below 300 nm) tends to affect the substrate surface. This phenomenon, known as photo-oxidation, appears as a gradual yellowing of the polycarbonate surface that occurs with repeated exposure to the UV energy.

The subtle change in the polycarbonate surface affects ink adhesion, especially when multiple colors are printed. However, the use of gallium lamps, which release UV energy in the shorter wavelength range, has been shown to minimize this photo-oxidation of polycarbonate.Polyester is another material commonly printed with UV. The main factor to keep in mind when working with polyester is that this material needs to be pretreated by flame or corona or coated with some form of primer to achieve good ink adhesion.

The last issue in using UV technology is safety. While UV inks aren’t characterized by dangerous solvents, the monomers they contain can cause allergic reactions in users who are sensitive to them. Monomers used in UV inks are tested for toxicity (skin irritation) and assigned a rating from one to ten, where one is least toxic and ten most toxic. But even with low-toxicity monomers, it is always a good idea to wear protective clothing, including eye and hand protection, when handling UV inks.

Besides UV energy, UV lamps also emit very bright visible light that could be harmful to the eyes if workers are exposed to it frequently or for long periods. This is why it is important to block light from escaping through the curing chamber’s conveyor openings by installing shields.

One other safety issue to keep in mind concerns ozone, which is a very unstable molecule of oxygen. Ozone is generated when an electric discharge passes through air or when oxygen is exposed to high-intensity UV energy. Ozone generated by curing units can lead to respiratory problems for those who work near the curing units, so it’s important to make sure that curing units are well vented to evacuate ozone from the work area.
 


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