Expose Yourself at 330-440 Nanometers!
The artist who wants to make photostencils needs an exposure system, which brings up the issue of what kind of light sources work to expose photoemulsion. All lamps (and light itself) are not created equal. There are incadescent, flourescent, black light tubes, photofloods, mercury vapour, quartz, halogen, metal halide, carbon arc, and probably a whole bunch more.

The bottom line on exposure lamps is that you want UV light in the 330-440 nanometer wavelength, and these different light sources all produce varying amounts. The more powerful the lamp and it's concentration of UV light in this range (commercial screen exposure systems go up to 10 kw) the faster the exposure. Regular flourescent gives off very little of this UV, and the old standby, the photoflood is the same - better than a regular 100-watt light bulb, but not really that strong. That's why exposures take 10-15 minutes and don't wash out very well, or produce strong stencils. Correct exposure is important in screenprinting, less so when using solvent based inks, but critical if you are printing with waterbase.

Economical Solutions
So all these light sources are an economical solution to the problem of paying $2000-$5000 and $400 per bulb for a commercial lamp. (although the smaller table-top models which expose screens up to about 24"x36" can be had for $1800 new or used for under $1000. Five-way Vaculite by Atlas is typical of these types - every T-shirt shop or school seems to have something similar.) The main difference between a bank of flourescents and a point light source will show up in how it exposes the screen. The best exposures are acheived by a point light at a distance of at least 1.5 times the diagonal of the image away from the glass. This produces a sharp stencil with minimal undercutting.

Undercutting?
Draw a line and then draw a point over the centre of the line. Now draw rays from the point to the line. In the centre, the light hits straight on and is the strongest (shortest distance for the UV rays), and this would give you a sharp edge on your stencil. Out by the edges, the light hits at an angle, and depending on this angle, it undercuts the stencil edge. Not a big deal for blocky solid colours, but when you start to print halftones, stochastic, or fine detail, you run into either dot gain, or closing in on the stencil, or soft edges, and this causes problems in printing.

The best solution would be to place the light way back, so the light lines enter almost parallel.The sun is a good example (see below). Unfortunately with indoor artificial sources, the further away the light, the longer the exposure because the strength of the UV rays drops inversely with the distance. The problem with flourescents is they must be close to the glass for full UV power, but they radiate the light in an arc, so you can have some problems with detail, and hot spots, and undercutting because of the angle. The part of the lamp closest to the screen will expose the most - the gaps between the light will be less. The trick is to find the balance point; 3"-4" is probably best.

You have to look at the work you are producing and let that dictate the exposure unit you need. I guess the money becomes part of the equation. I have an old arc lamp from about 1930 that I found in a barn that I use in a pinch, when it's dark and gloomy during the long Canadian winters. Otherwise, it's all solar here in Squeegeeville.

Here's the article on Solar Exposing, first published in Screen & Display Graphics magazine a couple of years ago. I use solar in my studio, you can see images I print and the set-up on the Squeegeeville website . I think it's one of the most amazing tools available for printmakers, and totally underutilized as a viable exposing system in the graphic arts field. Free UV, who can argue with that?

Exposing the myth of solar exposure
SUN SUN SUN, here it comes...

Adios to the Electric Bill!
There are various historians who will affirm that screen printing has been around for a long time, even pre-dating Gutenberg. Photo-screenprinting has a much shorter history, in that it had to wait for Thomas Edison to invent electricity, the light bulb, and the monthly electric bill. Today roughly 99.9 % of commercial screenprinters, regardless of the type of stencil system they use, go along with conventional thinking and pay the power company so they can use their exposing lamps to make screens.

The other .01 percent are the ones we are going to look at today. How do they make exposures?

Two words: "Solar Power!"

Just to be perfectly clear, we're not talking about running a lamp using collector cells on the roof and a converter. We're talking about direct use of the sun's naturally occurring 330-440 nanometer UV rays to perform a task usually reserved for a sophisticated (and expensive) lighting system.

Does It Work?
Does it work? Better than you would ever imagine. Why more people don't use it as an alternative to conventional exposure systems is an even more interesting question. No matter where you live (even up here in the frozen north) the sun's UV rays are constantly available during daylight hours, and last time I checked, the government still hadn't got around to taxing them.

I'm going to propose a little experiment that will prove the concept. Once you've performed the experiment and are satisfied ol' Andy isn't pulling some kind of joke on this highly-educated-and-sophisticated readership, you can finish the rest of the column and find out how a few screenprinters in other parts of the world have adapted Solar Exposing as part of their everyday production process. Who knows, maybe this column will provide some ideas on how solar exposing could work for you.

Solar Exposing Experiment Number 1

To conduct this experiment you will need:

• 1 small screen, coated with a regular diazo or dual cure emulsion (SBQ / photopolymer is too fast)
• 1 old film positive to fit screen, halftones OK, fine lines OK
• 1 can of spray adhesive
• a wristwatch
• a sunny day

The procedure:

1. Spray the emulsion side of the film with a light coat of glue and stick it to the bottom side of the screen. (for now, you need to pretend this is your vacuum frame) Make sure it is making good contact everywhere.

2. From inside a shaded doorway, step directly into the sun's path, and hold the screen with the positive directly facing the sun at a right angle.

3. Time the exposure. Southern States go 30 -45 seconds, Northern States & Canada go 60 -90 seconds. If you live in the tropics, your time will be even faster.

4. Step back inside the building and wash out the image.

Depending on the sun's intensity when you try this, you should end up with a usable stencil. Remember, the experiment is only proving the concept with spray glue and inaccurate timing. Where this gets interesting and the potential becomes more obvious is when you start to use available technology to 'control the variables.'

If you were over or under exposed, and want to try to get a more exact exposure based on light intensity / screen mesh / stencil thickness, do what you would normally do: use an exposure calculator. If you want to avoid the spray glue, use a vacuum frame. If you want to make an accurate exposure based on the amount and intensity of sunlight regardless of cloudy conditions or seasonal variations, use a light integrator. By adapting these three common production tools to solar the inventive screenprinter can produce consistently high-quality stencils.

Let's look at some of the advantages of solar exposition:

•First the obvious one of replacing the need for a metal halide or similar exposing lamp to make an exposure. And it's cheap! For the shop that can't afford a good lamp, this method offers an alternative to low-output photofloods or home-made lamp systems.
• Lessened environmental impact through reduced power consumption.
• Large format screens burn at the same speed as small ones…fast.
•The ultimate point light source. No undercutting of stencil edges.
• Speeds up processing of cheaper diazo or dual cure emulsions.

Graham Bebbington's Story
Each user of solar exposing has his own reasons for choosing it over conventional lamps. One of my favorite stories comes from Australia. Correspondent Graham Bebbington, a retired flexographic platemaker, works when he wants to and keeps himself in "petrol, beer, food, and caravan park fees" by running a mobile screen printing service from his motorhome in eastern Australia. He travels between small towns, printing decals, T-shirts, and other screen printed items, producing his art on a computer, his positives on a laser printer, and his stencils…well, I'll let Graham tell the story.

"Sometimes I pull up to a nice river and park the van under some trees for a couple of days. This gives me a problem in that I do not have access to a 240v power supply to expose my screens, so I use the sun. I coat my screen under the stars at night, and print the next day. I made a home-made vacuum pump using a 12v bike pump. Not excellent, but better than compression only."

Doug Gibson's Story from Hawaii
Doug 'Dragin'Ink' Gibson is one of the top T-shirt printers in Maui. Those are his designs you see in all the shops when you visit. A fascination with screenprinting and some first experiments with photo-emulsion led him to solar way back in the early '80s.

"I bought some frames and some mesh and a small amount of emulsion and started playing with it all. I was under the impression that I needed a "real" light source to expose my crude screens. At the time I worked in the photography industry and I thought I'd use my slide projector to expose these screens. Ha Ha. After burning screens in my bedroom for literally hours I was very frustrated to say the least. One day - one sunny day - I just walked out and pointed my screen at the sun and IT WORKED! I would put down a very primitive positive on the primitive screen and hold a piece of glass on top and count to 30 ... It worked! Not every time at first, but I saw the potential of it all way back then."

Doug comments, "It's no secret that the sun is the most powerful single point light source known to man and that all these fancy boxes (exposure units) are trying to emulate what we already have."

His equipment? A can of pallet adhesive, and a ladder going up to a hole in his roof, although he recently upgraded his system with a 'space bag', one of those clear vacuum bags they hawk on the shopping channel.

"Now, I know that when people hear this they will cringe," Doug continues, "but that's the way I do it and if you look at some of my work you would probably say NO WAY! but that's the way it is." Just for reference Doug's printing equipment includes an 8/8 and 6/6 manual and a TAS 10/12 auto.

"I do spot colors this way . I do four-color process this way (even 85 lpi halftones with great success). I do simulated process this way. I do index this way. I do it all this way! Oh, except on rainy days or late nights.
Then I have a back up unit!"

How does Nanook of the North Do It?
How about larger format, in areas that aren't as blessed with sunshine as Maui or Australia? Where Doug just cuts a hole in his roof, I live off the 'wet' coast of Canada, at about 50 degrees latitiude (Think Seattle, only more rain and no Starbucks.) My setup includes an exposing bay with a large window oriented to the southwest. A 4' x 6' vacuum frame rolls into this. I use a light integrator which allows me to measure the correct light dosage, even on overcast or cloudy days. I've made some interesting discoveries over the last five years of using this set-up for exposure.

The light wavelength from the sun used for the exposure (330-440 nanometers) only seems to travel in straight lines from the source, with no reflection or defraction. This characteristic allows the handling of an unexposed screen to take place as long as it's not in direct sunlight. Indirect room lighting during set up for the exposure seems to have no noticeable effect on the exposure.

The same straight-line light characteristic also determines how you go about aiming. In a normal exposure with a fixed light source, the plane of the glass, positive, and emulsion on the screen are centered and perpendicular to the lamp. Solar exposing is no different, but due to the movement of the sun in the sky it requires the screen face to be 90 degrees to the sun both vertically and horizontally. This is easily accomplished if you are hand holding a smaller screen, but in larger vacuum frames adjustable angle stops and base castors are required to achieve proper elevation and direction.

Beyond Large
One of the most exciting applications of Solar Exposing is ultra-large format. When making 16' screens, typical lamps, even 10kw monsters, must either be step exposed, or moved back so far that exposures can take 15 to 20 minutes or longer. Omni Promotional, located in Boulder, Colorado, are integrated manufacturers of event tents, inflatable structures, event fencing, banners, and other products covered with large format screened graphics. Ever wonder who makes those miles of logoed race fencing seen at World Cup or Olympic ski races? The inflatable start and end gates from the Iron Man Triatholon? That's Omni Promotional.

Omni's printing department supervisor Randy Rollheiser produces 5-10 large format screens per day using the sun. "We use a light spray adhesive, walk the screens out our bay door, and expose them for 30 to 60 seconds depending on the intensity of the sun." Using a straight diazo emulsion on 155 white mesh, he determines the correct exposure by the changed color of the stencil.

"We are starting to use more UV inks and want to produce more process color work, so one of our next purchases will be a large rotating vacuum frame, with castors and a light integrator," he said. " I've been using this solar method for six years. It works great, but we want to start gaining better control over variables, so that means eliminating the spray glue and getting exact exposure times. The original owner of the company started using the sun because they had no space or money for a large exposing system. We still use it because it works, and we're looking forward to the new equipment making it work even better."

by Andy MacDougall
http://www.squeegeeville.com