Heat Management

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Heat Management of UV lamps … staying cool under fire

 

Born under intense flame, forged under hydrogen and nurtured in the 1900 degree glow of a vacuum furnace, no wonder UV curing lamps operate the only way they know hot!

So how does something raised in Dante’s Inferno enter into a manufacturing process that can’t lose its cool? Often, extreme heat is the toughest problem to solve. Conversely, heat can improve cure, assists leveling of screen inks and encourages ink adhesion. The objective is to manage heat not abolish it.

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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 management:

1. Reduce Infrared emissions from the source.

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 overcooling.

 

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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 operating pressure.

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.

 

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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 process.

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 recirculation loop.

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 minimum heat.