Metal halide lamp application notes
In
the electromagnetic spectrum, UV light is just below visible light. The
wavelength of UV rays is shorter than the violet end of the visible spectrum but
longer than the X-ray. UV energy is
measured in nanometers (nm). Of particular interest to the UV curing process is
the range 240 nm to 440 nm. This area of electromagnetic radiation is further
divided into several broad classifications: UVA, UVB, UVC and UVV. Of
these UVC reflects the shortest wavelength area, i.e. lowest nanometer
measurement. Mercury
vapor lamps produce copious amounts of UVC. They are the workhorse of the
industrial curing industry. Customarily they are used to cure press varnish,
inkjet and screen inks. Lamps are inexpensive and dependable. When
one transitions from varnish to inks, one soon discovers that longer
wavelengths are required for thorough cure. In other words certain inks behave
differently to UV light. In an attempt to provide output over a broader
spectral area, metal halides are added to mercury lamps. Black and white inks
cure at higher wavelengths. Black inks benefit from iron iodide lamps with
their increased output between 350 to 390 nanometers. Likewise
colors such as whites and metallics are more difficult to cure due to the
pigment, titanium dioxide or aluminum flake either absorbing or reflecting the
UV. These benefit from using gallium
iodide UV curing lamp. The
obvious question is why aren’t all UV systems produced using metal halide
lamps? At
TCS Technologies we believe in providing useful use information free of sales
hype. On a practical basis metal halide lamps especially iron iodide lamps
are fraught with operating issues. They are hard to start, must be run only at
high power and are extremely easy to overcool. Metal halide lamps have a strict temperature
profile and are not intended to be dimmed i.e. operated at reduced power. These
lamps are designed, electrode chosen and bulbs shaped based on a certain power.
Lamp output will not remain stable unless operated at high power level. UV
equipment designers seem to overlook this requirement and attempt to operate
these lamps as if they were pure mercury vapor. Customers just want to cure.
They could care less about the fine details. Often times they are misled by UV
equipment companies that one can “just interchange lamps” (metal halide for
mercury) and their problems will go away.
Let me assure you it does not matter how efficient the cure, if the lamp
will not light or gets dim each time one closes a shutter. Gallium iodide lamps have far fewer quirks
and are available over a wide range of arc lengths. Iron iodide lamps have far
more issues. If you must use iron lamps, keep arc length to under 10-inches.
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