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Selecting the Right Glass for Exposure Units

(July 2007) posted on Fri Jul 06, 2007

Replacing glass is a part of life for those who use exposure units. Coudray examines the ways in which various types of glass affect exposure efficiency.

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By Mark A. Coudray

Also beware of architectural glass. When you put glass in your house, the two things you want to restrict are UV light and infrared heat. Low E glass, which is designed to save energy, has a special coating that can restrict up to 94% of UV light from passing. You definitely do not want Low E glass. In fact, you do not want any type of tinted, bronzed, or grayed glass—all of these restrict UV transmission to levels of less than 35%.

Conventional window float glass appears clear, but it actually has a very slight green cast to it and is visibly green when you look at the edge of the sheet. This green cast is imparted by the pres¬ence of small amounts of iron and chro¬mium oxides in common silica. These oxides result in UV transmission of 58% for a 3/8-in.-thick sheet of glass. Put another way, this glass absorbs 42% of the UV light that hits it.

All of this information led me to the conclusion that I should use tem¬pered float glass in my exposure unit. Finally, I had to identify a glass that would accommodate the transmission of UV light in the 350- to 420-nm range, which is the area of the electromag¬netic spectrum that enacts the chemical crosslinking in emulsions during screen exposure.

I consulted numerous data sheets and was surprised to find that 3⁄8-in. float glass can allow a UV-transmission range of 35%-90%. The implications here are huge. The more UV transmit¬ted, the shorter our exposures, and the more screens we can process per day. Decreased exposure times also prolong bulb life. My search guided me to low-iron glass.

Low-iron Glass 

Low-iron glass contains approximately one tenth the iron content of common float glass. Each company has its own trade name for low-iron, or iron-free, glass. Examples include Starphire, Ever¬clear, UltraClear, and UltraWhite. When viewed on edge, low-iron glass has virtually no color cast. The important aspect of this type of glass is its ability to transmit UV light in the range of 81-88%, depending on the brand. You must obtain the specification sheet from the manufacturer in order to find out how much UV light the glass can transmit.

Low-iron glass can be 25-50% more expensive than conventional float glass. Putting the UV element into the equa¬tion from my previously outlined prob¬lem yields an installed replacement cost of $650 for low-iron glass vs. $400 for conventional glass. The difference of $250 over the four-year life of the glass is approximately $62.50 per year. Our exposure bulbs run $195 each, and we use two per year. With 25% higher UV transmission, my exposures are now 25% faster, and my bulbs last 25% lon¬ger. We make about 20 screens per day using conventional film, and another 60 screens per day using a computer-to-screen system. For the exposure unit in question, the low-iron glass means five more screens per day, or 1250 addi¬tional exposures per year. At the same time, we save about $100 per year in extended bulb life, which is more than enough to make up the higher cost of the glass.

The bottom line is in the vastly increased productivity in the number of screens we can expose. As a textile printer, I am normally able to charge $15.00 per screen. The additional 1250 screens per year translates into $18,750 of added revenue. Granted, not all of that is profit, but the implications are significant. If you charge more than $15 per frame, you are clearly money ahead. A few hours of research and the right questions asked can mean more profit and longer bulb life. At the same time, one poor decision to save a few bucks on replacement glass can have a major impact on your production, especially when you replace low-iron glass with conventional float glass.

Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Specialty Graphic Imaging Association Int’l (SGIA) and as chairman of the Academy of Screen¬printing Technology. Coudray has authored more than 250 papers and articles over the last 20 years, and he received the SGIA’s Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine. He can be reached via e-mail at



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