UV lamps: Maintenance, Operation and Machine Design
The manufacturer’s recommended maintenance programs for your UV lamp is fine but read between the lines: it’s in their best interest to have you buy more and buy often. On the other hand, waiting too long to change out lamps impacts production and your wallet. So, at what point do you bite the bullet and change out the lamp?
Most lamps used for UV curing are categorized as medium pressure mercury vapor. They typically operate between 200 and 600 watts per inch and have an operating surface temperature of 600 to 800 degrees centigrade. If operated correctly, you should be able to see 1000 hours on these lamps all day long. I say “operated correctly” not only to the end user but to the OEMs as well.
There is a lot of equipment in the field that bends the rules of physics or outright ignores them. I know we are trying to focus on maintenance, but in many cases, the machine manufacturer has made the end user’s job twice as hard as it needs to be due to poor machine design.
Here are a few conditions that can shorten your lamp’s life or lead to unexpected results:
1. Cooling lamps by blowing on them:
If your lamp reflector has holes in it, blowing air on the lamp body to cool it means you have bought a questionable design. These lamps do not do well with hot and cold spots across the arc. Blowing on the lamp introduces areas of contamination and can even cause the plasma arc to become unstable and resonate, thus snaking or curling. You would be much better off using a reflector that has a full length slot in it, exhausting the air out the back. This offers the lamp even cooling across its full length. Also, because you are exhausting the air, you are pulling room temperature air across the ends of the lamp where a maximum temperature of 250 degrees centigrade is a must.
2. Using equipment that forces the use of lamps with very short ribbon seals: There are countless cases where the machine design forces the lamp design to be something less than desirable. If the differential between the arc length and overall length is less than four inches, the lamp design is, in my opinion, in question. Underneath the lamp’s metal or ceramic end, the molybdenum ribbon is connected to either a wire that exits from a ceramic end or is connected internally to a metal end. This area is not protected by a vacuum. If the lamp exceeds 250 degrees C in this area, the molybdenum components will start to oxidize or burn. Obviously, the shorter the ribbon, the higher the potential for excessive exposure to heat in this area.
3. Standby power can be a serious issue: If the lamp is overcooled in low or standby power, the mercury can start to condense. When this condition occurs, the internal pressure falls, the power starts to drop and the electrodes start to emit very poorly, throwing off tungsten. A lamp with very dark ends could be an example of this condition.
OK, that is enough pounding on the machine manufacturers and their designs. Let’s cover some good old common sense for the users.
• Excessive Starts
Avoid excessive cold starts. When a lamp is first started, the internal pressure is low. The electrodes during this time are emitting poorly, throwing or spitting off tungsten into the lamp. This causes points of contamination which can cause premature failure of the lamp. Lamps should typically be started in high power to reduce the length of time they are in the starting mode. Excessive starts cause premature darkening of the ends, resulting in a drop in output, as this darkening eventually migrates into the body of the lamp. For this reason, whenever possible, gather all of the work to be done and perform it on one lamp start.
• Check Reflectors and Cooling System
Check to see that your reflectors are clean and free of distortion from the lamp’s radiated heat. Periodic cleaning and/or replacement of reflectors may be necessary to maintain maximum efficiency of both the lamp and equipment. Lamps running too hot can sag or expand causing a drop in the power and a reduction in UV output. Sometimes lamps sag unnoticeably, so it is always a good idea to rotate them periodically.
• Handle Lamps With Care
To lengthen a lamp’s life, follow the OEM’s installation care instructions carefully. For example, avoid finger oil on the lamp body by using a paper towel or glove. If you do handle the lamp with bare hands, the use of an alcohol wipe is recommended to assure clean surfaces. It is important to ensure that the electrical connections are tight and not corroded. In the case with spring loaded sockets, it is imperative that the sockets are in good shape with proper spring tension and clean contact surfaces. Lamps that are held rigid in their mounts should not be held so tight as to restrict movement as the reflectors and associated metal mounts expand, contract and warp during operation.
• Keep a Spare on the Shelf
Take a lamp or two out of service that you know are in good condition and keep them as spares. You wouldn’t operate other equipment without a backup plan. For rush orders, you will probably pay a premium, and of course you will disrupt your production. Having more than one spare on hand may sound self-serving, but I think it is just smart planning.
If you follow the basic guidelines that I have covered in this article, you will probably stay out of trouble. Remember, common sense covers 90 percent of your needs. I'll give the luck of the draw the remaining 10 percent.