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Prepress & Screen Making



Most of us follow the same rigid schedule every morning during the work week. My schedule usually unfolds in this manner. I get up, stumble to the kitchen, wait patiently for the kettle to boil, make some very strong tea (I’m British), turn on the radio to catch the news, check my e-mail, and then head off to take a shower. I turn on the hot water, make an educated guess about how many turns of the cold faucet will produce the right temperature, and I hop in.

Some days I discover that my faucet-turning skills, honed to perfection over a lifetime of showering, deliver the perfect bathing experience, but that’s rare. More often, I find that I am standing in either a freezing torrent that threatens to send me to the hospital with acute hypothermia, or I am scalded within an inch of my life. I make no adjustments, however, since I know that I can spend much of the rest of my morning wrapping myself in heated blankets to bring my core temperature back to normal or smearing medication on the blisters I’ve raised.

My wife, I have observed, has no such problem because she has worked out a system where, with her hand, she tests the temperature of the water and makes calculations based on how hot or cold she thinks the temperature is. If it’s too hot, she increases the amount of cold water until she finds just the right balance and vice versa if it’s too cold.

I would adopt this method, of course, if I weren’t always in a great rush in the morning. If I add the hand-testing method into my daily regimen, I will lose valuable minutes and find myself arriving late for work. If I arrive late for work, then I won’t have time to find my exposure-calculator kit and attach it to the first screen of the day before loading it on the exposure unit. I know that if I don’t do this every morning, then I run the risk of underexposing or overexposing my screens, and I will spend the rest of the day dealing with the resulting problems. Come to think of it, not doing that test would be almost as stupid as climbing into the shower in the morning without first testing the water temperature, wouldn’t it?


Forming good habits in screen exposure

Our recent journey through the screenmaking process has finally brought us to the point of exposure. Our screens are now stretched with the correct mesh count, tensioned to the recommended levels, and allowed to relax and be retensioned until they become stable. The mesh is gently abraded and degreased and the emulsion applied in a manner that gives us a repeatable and accurate thickness. And it’s thoroughly dried, leaving no moisture to cause pinholes during washout. The graphics professionals check and recheck the artwork, and we are finally ready to expose the stencil. It’s then time to learn a little bit about what goes on when we draw down the vacuum blanket and turn on the very powerful UV lamp.

I only have a little more than a thousand words allotted to my column, so I will not explain the ins and outs of photochemistry this month. It’s actually not that complicated, and I am sure that your emulsion supplier will be more than happy to discuss the basics with you. I also know that your supplier has detailed instruction sheets that you should follow to the letter, and he or she should offer on-site training. Check past issues of this magazine for great articles on this subject—the ones you skipped in the past because they sounded too complicated. Take the time to understand what you are trying to achieve with the process, and you will soon be exposing to perfection. But first, make sure you have an exposure unit that is maintained well enough to expose a stencil perfectly every time you load in one of your perfectly prepped screens.

You should take some basic precautions every day to ensure a perfect stencil, regardless of whether you have a small tabletop exposure unit, a vacuum frame with a separate light source, or an integrated, all-in-one, wall-to-ceiling exposure unit. Some are obvious. Keep the glass clean and scratch free, and minimize dust in the workspace. Always use anti-static cloths and brushes after you finish cleaning the exposure unit to avoid attracting dust in the air onto the glass. Check the blanket every day for damage, and make sure that the vacuum is complete when it draws down prior to exposure. Now let’s look at some less obvious precautions and gain control over some variables that we regularly miss in the average exposure.

Purchase an inexpensive exposure calculator and learn how to use it properly. Ask your emulsion supplier to make sure you really know how to do the simple calculations necessary. You might even spend some money on a UV puck, which you can use routinely to verify that the correct amount of UV light is hitting the screen and the wavelength is correct for your particular emulsion type. Once you get a handle on these simple tests, you will be amazed to see how much variation there is in the ex-posure unit and that it cost you most of last year’s profits.

There are several reasons for variation in the exposure unit. The moment you expose a screen, you put the light source through some extreme conditions. The bulb shifts from room temperature to something approaching the temperature of the nose cone of a re-entering space capsule and then back to room temperature in a short period of time. It does this by being blasted with unimaginable amounts of high-voltage electricity, and every time you turn the light on, a little more incremental deterioration occurs in the quality and strength of the light source. Over time, you will need to adjust your exposure times to compensate for this.

It will happen slowly over weeks, right? Wrong! I have seen brand new bulbs die within hours of installation, and I knew it because the results showed up immediately in an exposure test. Without the test, everything appeared perfectly normal. The light was turned on, the screen washed out, and then promptly broke down on the press. One of my customers found that he could never settle on a workable exposure time. He replaced bulbs and sent back defective units until he discovered that his air conditioner was draining so much power that it was causing brown outs in the screen room. All of his screen problems disappeared after electricians put in a separate circuit for the floor unit that he used. He’d have never known about the problem had he not been testing exposure. If you are underexposing or overexposing your screens, you are probably experiencing the following problems in your shop right now:

Pinholes I’m sure you’re blaming pinholes on the person who is supposed to clean the glass before every exposure.

Breakdown of the emulsion on the press You’ll blame the screen-prep person and accuse him of not abrading the screen properly or not adequately degreasing it.

Emulsion residue in the open areas of the mesh after washout This is unavoidable; nevertheless, you’ll blame the hapless person who you asked to perform the supposedly simple task of washing out the exposed screen.

Image is not crisp You’ll attribute this problem to the artwork and blame the opacity of the film or the skills of the poor fellow who produced it.

I could go on, but I think you get my point. Here’s your chance to be a hero by clearing the names of the employees who’ve faced false accusations and finally place the blame fairly and squarely where it should be placed. Find the exposure calculator, and start using it every day. Meanwhile, I’m going to stop typing now and have a doctor look at my blisters.


Gordon Roberts has a history in screen-printing production management that spans more than 25 years. He has held supervisory positions in shops that represent a broad spectrum of application areas and markets, including printed electronics, apparel, signage, and retail graphics. Roberts has presented training courses on the basics of screen-printing production and on shop management for the Screentech Institute and is presently a consultant for the screen industry. He can be reached at [email protected]


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