Ultraviolet reactive inks and coatings require a high intensity source of ultraviolet light to initiate a chemical reaction, curing the ink or the coating almost instantaneously.  Ultraviolet light forms a small part of the electromagnetic spectrum, which ranges from radio waves at the long-wave end, to x-rays and gamma rays at the short-wave end. The chart below shows how ultraviolet wavelengths fit into the electromagnetic spectrum.

The Electromagnetic Spectrum

The ultraviolet wavelengths most suitable for curing inks lie between 200 and 400 nanometers.  

There are several types of lamps suitable for generating these wavelengths, the main ones being high-pressure mercury arc lamps, electrodeless lamps and medium pressure mercury arc lamps.  

The high-pressure mercury arc lamp is generally constructed as a capillary type tube and requires a water jacket to maintain correct running temperatures.  These lamps are limited to short lengths only and lamp life is usually less than 1000 hours.  

The electrodeless lamp has, as its name implies, no electrodes.  An arc is established by the generation of micro-waves.  These types of lamps are generally produced in two standards lengths, 6 inch and 10 inch.  

By far, the most widely used is the medium-pressure mercury arc lamp (an MPMA Lamp).  This can be air or water-cooled and can be manufactured in a wide range of lengths.  Single lamps of three and a half meters long are not uncommon, and the working life of MPMA lamps can be expected to be well over 1000 hours.

Typical MPMA Lamp Construction

The main body of the lamp is made from fused silica quartz.  This is a mined material that is cleaned and purified to a level that has a total contamination of less than 50 parts per million (PPM) and OH (water born in solids) of less than 5 p.p.m.  The quartz that we use is typically less than 3 p.p.m. 

Quartz has several properties critical to U.V. Lamp manufacture.  Firstly and most importantly, quartz allows the transmittance of U.V. light throughout the spectral ranges required for U.V. curing.  If you compare quartz to a window, through a window, using normal glass with a higher contamination rate, you would feel heat from the sun, you would see light and colors but you would not get sunburnt. 

If, however, the window were made from quartz you would also get sunburnt.   The second property of quartz is that it withstands high temperatures.  If it were made of glass, it would only get to about 600°C before melting.  Quartz, however, does not start changing form until 1075°C.  The average surface running temperature of the lamp is somewhere between 600 and 800°C. 

The third physical property of quartz is that it has very low co-efficient of expansion or thermal shock, i.e. as the lamp heats up and cools in different areas along its length, it does not cause internal tension resulting in the possibility of a breakage.

The electrode consists of a central tungsten pin, which is surrounded by a tungsten coil.  The tungsten coil is in place to encourage heat dissipation and to contain special emitter materials, which increase electrode life and performance during the lifetime of the lamp.  Tungsten is a very durable metal, can withstand extremely high temperatures and is a good conductor.

Typical Constant Wattage Circuit Schematic

Electronic Power Supplies 
Over recent years electronic power supplies have become more widely available. In comparison to the conventional iron cored ballast designs the main advantages are as follows:

• Compact and light weight
• Unity power factor
• Compatible with mercury and metal halide lamp designs
• Fully variable and stepless power control
• Improved lamp lifetimes due to reduced peak operating and in-rush currents
• Can be balanced on three phase electrical supplies
• No lamp power variation due to any variance in the mains supply voltage

Spectral Output Characteristics 
Primarc electrode and electrodeless lamps can be manufactured with standard mercury and metal halide fills to match the customer’s specific application. 

 





By far, the most common curing lamp is mercury and in almost every application, a standard mercury lamp will provide a sufficient cure.  There are however certain applications where it is advantageous to utilise a spectrally enhanced metal halide lamp.  

Gallium is used more for its long wave curing characteristics, around the 417 nm wavelength region, and is predominantly used in the furniture manufacturing industry where a greater depth of cure is required for resins, lacquers surface coatings etc.  It is also used for curing white coatings on furniture that have a high concentration of titanium oxide, which filters out the shorter wavelengths.  

Lead and iron fills are commonly utilised for ink-jet applications where there are demands for exceptional surface and through cure capabilities.  

Iron is predominantly used in the Far East market and used in the production of PCB, legend and photo-resist printing where the chemistry of the compounds is particularly suited to the frequencies found in these lamps.  

Additive lamps are harder to strike and control and require more accurate temperature cooling than their standard mercury counterparts.  It is essential that the spectral distribution of the lamp is exactly keyed to the photoinitiator distribution within the curable material, i.e. it may be that the mercury lamp will have far superior curing power over that of an iron or gallium lamp given that the compound it is curing is more attuned to these frequencies. The situation works in reverse where we have a compound, ink or surface coating, which is tuned to iron or gallium.  

An iron, gallium, tin, lead or any other halide lamp will have the same overall output power as mercury. However, the spectral distribution will be redistributed, i.e. squashed or emphasized in some places according to the metal halide additive.  Every metal halide lamp needs a mercury component within it for the additive to function in the arc.

Lamp Life
Medium-pressure mercury arc lamps do not normally fail suddenly, as do ordinary household light bulbs. Efficiency declines relatively slowly, until insufficient UV light is being emitted for the lamp to cure effectively. This decline is caused primarily by the deterioration in UV transparency of the quartz jacket, and depends on a number of factors:- lamp cooling efficiency, power rating, current rating of the electrodes, electrode cooling efficiency, contamination of the lamp's external surface (dust etc.) and switching frequency.  

When correctly used, Primarc UV Curing Lamps are guaranteed to produce a high level of curing efficiency.  Standard mercury lamps are guaranteed for at least 1500 hours and, with proper handling, they will still be capable of delivering at least 75% of original output.


Primarc
2 Danforth Drive
Easton, PA  18045
Telephone: 610.829.4240
Fax: 610.829.4260
E-mail: sales@primarcuv.com