How do you spell relief?
There are various ways to manage
substrate temperature. Some "solutions" to controlling heat are
needlessly expensive. Oftentimes the least expensive approach yields
the best results; I'll call them the ten commandments of curing heat
1. Reduce Infrared emissions from the
Don't waste your money on a 600 watt per inch lamp controlling a
process requiring 200 watts per inch. It will be bigger, run hotter and
wastes energy. Your goal here is to maximize UV output. Powering down a
lamp designed for higher wattages is inefficient and prone to
2. Speed up the process.
May sound silly but if your process can accommodate higher speeds,
you're home free without costing anything.
3. Choose a lamp with the highest arc
temperature for a given wattage.
Effective arc temperature is an indicator of level of UV output. One
can have two identical wattage UV lamps derived from two sets of
voltage/current relationships with different UV outputs. Your lamp
supplier can provide arc temperature information.
4. Choose a lamp with the lowest
In general, as power increases, the lamp's arc temperature increases
favoring near UV over visible. Conversely as mercury pressure
increases, the arc temperature decreases lowering UV output. For the
same lamp operating voltage keep the lamp's diameter small which
reduces mercury pressure and increases UV output. Your lamp supplier
can tell you a specific lamp's operating pressure.
5. Keep the diameter small to maximize UV
and minimize IR!
The amount of infrared generated by the lamp is directly related to its
surface area. A smaller diameter bulb increases UV output; more
importantly, emits significantly less infrared energy. You may have
heard some people talk of heavy duty (HD) lamp construction. Normally
these lamps have thicker quartz walls and greater diameters. If you are
looking from a heat management standpoint, HD construction may be the
wrong way to go. A word of caution:
there are many factors to
take into consideration prior to drastically altering lamp diameter.
This is especially true in screen printing with its longer arc lamps.
The curing process does not happen in a vacuum. One must act in concert
with the equipment supplier to insure cooling is adequate prior to
underwriting a lamp design change.
6. Slightly defocus the lamp.
This will spread infrared over a greater area.
7. Cool the substrate.
Forcing air flow directly on the substrate can be a simple, low cost
solution. Normally this occurs immediately following the lamp assembly.
8. Keep the UV lamp and reflector clean.
Most UV lamps are air cooled hence they must live with whatever air is
provided. Often shop contaminants will blow over the lamp literally
baking onto the surface. This reduces UV energy and may cause bulb
overheating. A small investment in cleaner cooling air will go a
long way reducing infrared output.
Bulbs and reflectors should be
inspected every 200 hours of operation. If lamp has external
contamination or reflector surface is dull, clean immediately! Use a
lint free cloth with Windex or Simple Green to clean. Don't waste your
money on special UV lamp cleaners as they have dubious value! If
solvents are permitted, use isopropyl alcohol. For extreme cases use a
mild abrasive such as Soft Scrub to clean the UV lamp. Be sure to rinse
any residue off the glass before reinstalling lamp. Always allow
lamp to cool and disconnect all power prior to any cleaning.
9. Install a quartz barrier plate.
This is a relatively straight forward technique. A quartz plate is
inserted between the lamp and substrate. Technically the plate does not
absorb infrared, however it reradiates at a lower temperature than the
lamp. Plate can be repeatedly cleaned. While not an optical filter, it
allows copious amounts of cooling air to be impinged on the substrate
without fear of overcooling the lamp. The more air the cooler the
10. Block IR transmission.
The dollars really begin to fly with this technique! This strategy
places optical filters between lamp and substrate. The first step up
the transmission blocking ladder is a hot mirror. A thin vacuum
deposited metal film is applied to a quartz plate reflecting IR back to
the lamp and transmitting UV. Plate has a finite life and can not be
cleaned without damaging the film. A hot mirror is normally coupled
with special IR transmitting UV reflecting reflector called a cold
mirror. This too can't be cleaned and cost can be prohibitive. Finally
at the top of the ladder are water filters. Elegant in design but
difficult to maintain, these filters involve passing water between the
lamp and substrate. The water absorbs IR while passing UV. The heated
water is transferred away from the process to a chiller via a
As in life, if one
follows the Ten Commandments they will see the light. In this case,
following these commandments will result is abundant UV light with