The use of UV light as a curing technology has been around for a long time. In the last few years it has become more popular since the technology on the lamps and the materials side has greatly improved. Applications are now on sheetfed, web and wide format inkjet equipment. The major advantages of UV inks are:
1) Press sheets are dry when they come off the press
2) Higher throughput speed than Infra Red drying
3) No Volatile Organic Compounds released in the air
4) Resist smudging and abrasion
5) UV Coatings have a “wet look”
6) Do not have solvents to penetrate uncoated stocks
In order to find out more about UV and how it works I went to an expert who has been working with UV technology for over 20 years, Norm Fitton, President of Anniversary UV. Most printers buy UV systems which may be supplied by the manufacturer of the equipment but made by someone else. Understanding how UV lamps work can improve their performance and save you money.
There are different types of UV lamps for different applications. Low pressure UV lamps may be used for disinfecting purposes, curing nails and dental fillings, or water purification. The type of lamp used in printing applications is usually a medium pressure, linear (straight tubes), mercury vapor arc lamp. Medium pressure UV lamps cure inks and coatings instantly. It is a photochemical not a heat process. It allows the equipment to run at very high speeds for extended periods.
General use light bulbs have a filament. The electricity causes the filament to glow, producing light. Medium pressure UV lamps do not have a filament. They utilize a high voltage charge to ionize a mercury/gas mixture in the lamp creating a plasma that emits UV light. This system requires a high voltage/amperage power supply (typically a magnetic ballast transformer with a high voltage capacitor bank). The ballast is wired in series with the lamp and performs two functions. Initially, the ballast provides a high voltage charge to ‘strike’ or ‘ionize’ the mercury. Then, once the mercury is ionized, the ballast reduces the voltage and amperage required to keep the mercury ionized and emit a stable stream of UV light.
These lamps generate a specific wavelength to cure the inks or coatings. Currently, most of these lamps operate at 300 to 600 watts per inch with some newer systems using lamps that generate up to 1000 watts per inch. So a 30 inch UV bulb may be capable of an output of 30,000 watts. They also operate at very high temperatures (850 to 950 Celsius or 1550 to 1750 Fahrenheit).
This type of UV lamp is made from Quartz. A general glass product would not be able to withstand the high temperatures. An inert gas (usually argon) is pumped into the quartz sleeve and then mercury is added to achieve the proper electrical specification. Iron and gallium are occasionally added to achieve special wavelengths. The tubes are sealed and the correct electrical end-fittings are added to complete the lamp.
These lamps need a powerful cooling system to offset the high operating heat. They are usually air or air and water cooled. They also use reflectors to maximize the ultraviolet light delivered to the substrate. There must be an even flow of air or water across the lamp for proper curing. If lamps run too cool they may not cure the ink or coating. Some systems use outside air for cooling. As the seasons change, depending on your geographic location, you may need to adjust your fan speed or increase/decrease water temperature to maintain proper cooling.
Contamination is another problem that can affect lamp performance. Due to the high heat air contaminants such as spray powder from other presses or dust particles can bake on the lamps creating a haze. This decreases the performance of the lamps. Ideally, even after extended use the quartz should be completely clear.
Here are some things you can do to increase the life and performance of your lamps.
- Make sure lamps are run at the proper operating temperature with an even flow of air or water over the entire tube. Adjust fan speeds by season if necessary to maintain proper temperature. If the system is also water cooled, as a general rule of thumb, the water temperature should be maintained as close to 72 degrees as possible.
- Clean lamps weekly with alcohol on a clean cotton cloth to reduce contamination. Don’t rub too hard.
- Check and change filters in the cooling system regularly to make sure they are not clogged.
- Rotate the lamps a quarter turn in the same direction each week.
Replacement UV lamps can be ordered from the OEM manufacturer or other lamp distributors/manufacturers throughout the US. The quality of the bulb is important and not all bulbs are manufactured to the same set of tolerances. In order to get the best match, if you are ordering from someone other than the OEM manufacturer, here are some tips to get the right bulb:
- Provide a working sample - an unbroken used lamp as long as it will still light- allows supplier to measure physical and electrical signature
- Get a part # (not serial #) plus physical measurements such as total tip to tip length, electrode to electrode length (arc), outer diameter of quartz in mm, end fitting description, to confirm part # is correct
- Determine the exact physical and electrical measurements-
- Total tip to tip length, electrode to electrode length (arc), outer diameter of quartz in mm, and end fitting description
- Lamp operating voltage and amperage (operating electrical signature from ballast transformer to lamp)
Proper maintenance as well as shopping around for sources can help you get the most from these systems.
Thanks to Norm Fitton from Anniversary UV (email@example.com or (610) 838-2784) for help in putting this article together.