Screenprinting Today: The Basics

EXPOSURE

© 2000 Andy MacDougall.

All rights reserved.
Reprinted with permission

Exposure is the process of transferring the positive image on the film to the stencil, and includes the washout of the image. A UV light from a light source is directed through the film and reacts with diazo or photopolymer elements contained in the photo-stencil emulsion. This causes a chemical cross-linking and hardening of the exposed areas of the stencil. When washed with a spray of water, these hardened areas stay adhered to the mesh, and the areas blocked by the opaque image on the film, which has shielded the underlying stencil from the UV light, wash out with a light spray of water. The end result should leave an image identical to the film positive image as open areas in the mesh, surrounded by a layer of hardened stencil adhered to the mesh.

Components of the exposure process are a light source, a vacuum frame, and a washout booth. An exposure calculator, a light integrator or a timing device, plus hot and cold water are also required. Indirect stencils require a developing tray and peroxide developer.

LIGHT SOURCE.

The light source supplies UV spectrum light rays in the 330-440 nanometer range. (as opposed to visible spectrum light) This causes the photosensitised elements contained in the emulsion to react. Light sources range from photofloods and blacklight tubes, which give off lower amounts of UV light (and take longer to expose) through carbon arc (not used much due to fumes) to enclosed exotic bulbs such as metal halide, which use a transformer to jump the electricity wattage (3k, 5k, 7k, etc.) and ignite the gasses contained in the bulbs. Other light sources include the sun, and direct projection devices. The light sources can be run on a timer for a predetermined time at a specific distance. More accurate is a light integrator, which reads the exact amount of specifically filtered UV light reaching the screen over the course of the exposure.

Larger lighting units with exotic bulbs require warm-up periods, and come equipped with shutter systems to start and end the exposures. It is common with these types of systems to turn them on and do all of a shop's exposures over a short period of time, instead of turning the lamp on and off over the day. The bulbs on the bigger units have short service lives and are expensive to replace.

Some light sources are stand-alone units, able to be moved closer or farther depending on the size of the exposed piece. Others are mounted to the vacuum frame as a one piece unit.

VACUUM FRAME.

This consists of a piece of untempered glass held in a hinged opening frame with a sealed flexible rubber blanket on top, hooked by a flexible hose to a vacuum motor. When the top with the rubber blanket is closed and the vacuum motor is turned on, it presses the film positive in direct contact with the stencil, and holds it tight to the glass. The UV light source is then aimed at the glass, passing through it and the transparent areas of the film, exposing the stencil.

Vacuum frames come as stand alone units mounted on a wall, or on wheels. For sizes up to about 4' x 6', some manufacturers make combo units with the exposure lamp(s) built in. Glass in the exposure frame must always be kept clean and free of dust or dirt, which will show up as unwanted openings on stencils.

WASHOUT BOOTH.

The washout booth is used for both the exposure and reclaiming of screens. It is constructed to contain water overspray, hold the screen at a comfortable working height, resist chemicals, and should have a backlit translucent panel to allow close inspection of the screen mesh during washout and reclaiming. The booth needs hot and cold water, an adjustable spray nozzle, and should have a pressure washer for reclaiming.

MAKING AN EXPOSURE

Before proceeding with an exposure, it is of utmost importance to ensure the following:

• The screen and stencil are dry and cured.
• The glass of the exposure unit, the screen, and the film positive are free of dirt or dust.
• The light source, washout booth, and developing tray (for indirect) are ready for use.

1. The first step is to orient the film to the screen, so that it can be easily printed. In some cases, the image will only fit in a certain position on the press, or needs to be angled, or needs specific clearances. These all need to be considered, and the film positioned accordingly. The positive can be taped to the screen, the glass, or laid in position on the glass. Ensure that the film is EMULSION SIDE UP, RIGHT READING.
   
2. Next, lay the screen over the film, ensuring positioning is OK,and the screen frame does not obstruct the lid and blanket of the vacuum. Indirect stencils are placed emulsion side up. Most units have a cord to collect air to the vacuum pump, and this should be placed along the inside of the frame. Do not let this cover any image areas of the positive, as it can affect the final stencil image, and don't let it hang out to close to the sides of the vacuum frame or it will break the vacuum seal.

   A Quick Note on handling unexposed screens:Once screens have been coated, they should be stored in a completely light safe environment. They will last weeks, as long as they are not exposed to long periods of light leakage. Having understood this point, the exposing area, i.e. the vacuum frame, as long as it is not bathed in direct sunlight, does not have to be light safe. Some shops go to elaborate precautions here, but if they were to take a light integrator and set it up to record the light dosage a screen receives under regular indoor light, they would notice it is almost nil. Photostencils are not like photographic film, they aren't that sensitive to visible spectrum light rays. So as long as the screen is processed in a short time, there's no need to 'expose yourself in the dark'. The reason I mention this is many screen shops could save themselves a ton of touch-up time if they had some lights on so they could see dust and dirt and screw-ups before they start to burn the screen. Don't rush, do it right!!!! 10 seconds to clean the glass, or 10 minutes to fill holes. Take your pick.

3. Turn the vacuum pump on, and once the blanket has sucked down, flip the exposure unit to face the light source. Before the light source is turned on or the shutter opened, do a final inspection. If positives have shifted, or dust or dirt is visible in the image, fix the problem before proceeding.
4. At this point, turn on the light source. Some units and bulbs require pre-heating, and are equipped with shutter systems for the actual exposure. The timer or light integrator is turned on at the start of the exposure, and shuts the lamp off or closes the shutter at the end.
5. Once the exposure is complete, remove the screen from the exposure frame and place it in the washout booth. Using cold or lukewarm water, spray the stencil from both sides, and then position the screen with the top (squeegee side) facing you. Allow the water to soften the stencil for a minute, and then proceed to wash out the image, using a light spray and working over the entire area of the stencil, not just the image. Use the back light to determine when all the emulsion has washed out of the image area, and then continue to wash for another few minutes. Wash from the back of the screen as well. It is important to remove ALL unexposed stencil material, and this includes a thin film that covers the area outside the image. In most cases, water will not damage a properly exposed stencil, and it is better to wash longer than to stop prematurely.
6. The stencil is very soft at this point, and should not be scrubbed or rubbed. A light blotting with clean absorbent paper will remove excess moisture, and the screen can then be force dried using a fan or heater.

   One more thing --Even though I caution you regarding rubbing the screen, If you have any blockages or parts that didn't washout completely, especially from weak film positives, you can GENTLY rub at stencil material you want to remove. Sometimes this is all it takes to get little bits to dislodge and open up an area. Do this from the bottom side of the screen. If a blockage won't come out, and you know it was from a pinhole or spot on the positive, wait until the screen is dry and then use a small amount of emulsion remover on a cue-tip or tip of a rag to remove the offending spot.

7. backside of the screen. (The reason you put it on the bottom side is so it won't wear nicks in your squeegee) Solvent-base inks require a water-soluble blockout, and waterbased inks require a lacquer blockout, or emulsion, or special water-resistant block. The emulsion blockout needs to be dried in the dark and re-exposed for it to resist the waterbase inks. Don't put block out on very thick, whatever you use.
   
8. It doesn't hurt to re-expose the screen after all these steps, just to ensure everything is locked in. The sun makes for an easy and quick re-exposure.

TIPS &TIDBITS

Exposure distance from the light source, to ensure even light distribution, should be 1.5 times the diagonal of the stencil area. The further away the lamp, the more light required, so exposures need to take this into account. The main advantage to using light integrators instead of timing devices to measure exposure is that they measure the amount of light hitting the stencil, and automatically compensate for different distance or variations in light source intensity. When using a timer, it is necessary to test expose from specific distances using an exposure calculator, and then use these test results to determine different times for different distances. Also note that point light sources expose the stencil quicker directly in front of the bulb, with the areas to the outside taking longer to expose.

Under-exposure, or not enough light, results in thin stencils, pinholes, delamination of the stencil during printing, and breakdowns on the press. Stencils are also harder to reclaim if they are underexposed. Remember, the light is working its way through the emulsion layer from the back facing the light, through to the top side of the screen. This is why underexposed screens are sticky or give off unexposed emulsion scum or slime when rubbed or blotted on the squeegee side of the screen. This is also why you have to keep washing the whole area of the stencil even after it looks like it's finished. The last bits of unexposed emulsion will dry on the top, and then when you start printing they will rewet and fill in your stencil. Then you start swearing, and the whole thing goes downhill from there --They call that scumming, or @$^%^&^##!!!! in the universal screenprinter's language.

Over-exposure will result in fine detail filling in, and poor washout. In some cases, light will actually penetrate the opaque film images and they will not wash out.

An exposure calculator has a range of filtered images on one sheet of film, and will allow you to determine the optimum exposure to give you the best detail and a properly exposed stencil. Different stencil types require different amounts of light for proper exposure, and this is determined using an exposure calculator.

How to use an exposure calculator:Purchase or borrow one from your supplier. Autotype makes a good one that is easy to use.

Simply make an exposure using the calculator film and your best guess at time or light units. (Timer or Light integrator) After washout, closely inspect the images. Each one is filtered and has a multiplier. Use the multiplier from the best image on the time you used to arrive at the corrected exposure time. Make another exposure with this time, and it should be the 1:1 image that is the best looking. You can then adjust your exposure up or down depending on other considerations. Once you've used it to determine the right time or light units, and then expose under the same conditions (light, photostencil, mesh count & colour, coating, etc) you can use the same settings as long as you have a light integrator or fixed distance.

One other variable that will change times for an exposure is the thickness of the stencil. When using indirect or capillary films, this is not a problem because the film thickness is constant. When using direct emulsion, the number of coats can vary depending on the type of mesh or the type of print job. A direct emulsion stencil will require more exposure time as it increases in thickness.

Changing a bulb. When it comes time to change the bulbs in exposure units, there are a series of steps to take. I had a friend end up in the hospital because he didn't follow procedure. These are not 60-watt household light bulbs.

1. Determine the correct replacement bulb. There are companies who supply replacement bulbs for most units, sometimes cheaper than from the equipment dealer. Check around. It's a good idea to always have a spare, or replace a bulb before it burns out and keep the old one for back up. In a lot of cases, old bulbs don't burn out, they just lose their spectral output, and exposures start to take longer and longer.
   
2. When removing a bulb, ENSURE THE ELECTRICAL SYSTEM IS DISCONNECTED. My friend's lamp went in the middle of production, and he forgot that just because it wasn't working (it had burned out) it WAS STILL ON. Third degree burns when the bulb lit up and exploded!
   
3. When replacing the new bulb, wear cotton gloves. Many bulbs come with a pair. The oils on your hands will affect the glass surface of the bulb, and cause it to break or malfunction when it is heated.
   
4. Be extremely careful you don't damage the bulb when removing it or putting it into the holder.
   
5. Test that it is working properly after replacement. Don't ever look directly at an exposure lamp when it is on. That's like looking at a solar eclipse or a welder!

Careful With that Glass, Eugene The Vacuum frame, because it has a large sheet of untempered glass being used in a situation where the glass gets smacked on a regular basis, has the potential to be the most dangerous piece of equipment in the print shop, short of sticking your hand in a power cutter or climbing under a large automatic press and starting it up. I've seen all three happen, and all three accidents could have killed someone. Be very deliberate in your placement of screens on the glass, keep the edges of plate glass protected, and never rotate or open an exposure unit without checking everything is locked.

RECLAIMING THE STENCIL

Once a stencil has been used in production, the screen must be reclaimed and readied for it's next use.

1. All loose ink should be scraped from the stencil, and using either lacquer thinner or specialized cleaners for solvent based inks, or water & special screen cleaner for waterbase, remove all ink residue. A sponge works great for cleaning waterbase.
   
2. The screen is then placed in the washout booth and soaked with water.
   
3. A stencil remover paste or liquid, which contains enzymes, which break down the stencil is worked into the screen. The stencil will then wash away with water. If you use a powder dissolved in water, a plant sprayer is an excellent way of covering the screen. It can then be rubbed or scrubbed in with a brush.

   A Quick Note on Stencil Removers: these liquids lose their potency if they sit for weeks, so if they are not working and you haven't made a fresh batch, try that first before you start flipping out. Also, don't ever let stencil remover dry on a screen. The stencil will re-harden, and you will never get it off again.

4. A haze remover paste or liquid is then applied with a brush and left on the screen to dissolve any remaining stencil, ink, or ghosting on the mesh. This is rinsed away using a pressure washer or a nozzle on good pressure.

   A Quick Note on Haze Removers -- Hey, they dissolve hardened ink, so think what they can do to your skin --Always wear gloves, eye protection, and cover your exposed skin, because the haze removers in concentrated form will burn you if they get splashed back

5. A degreaser liquid is then scrubbed into the mesh and left for a few minutes to neutralize any residual oils on the screen and ensure adhesion of the next stencil. Do not skip this step, especially with waterbase, or you risk the stencil delaminating or breaking down during the next print job.

NEW TECHNOLOGY

Photopolymer, 'one-pot', or SBQ emulsion, is a recent innovation that combines very fast exposure times and sharp edge definition. They are especially suited to high resolution and large format applications. When the polymer molecules cross link under UV light, they do so in straight lines, which gives a sharper stencil edge than traditional diazo direct emulsions. A hybrid 'dual-cure' combines the advantages of both types and allows water and solvent based inks to be used with one stencil type.

Direct Projection eliminates film positives and a vacuum frame and projects light through a negative and focuses it on the screen.

Toxic Chemicals Most reclaiming solutions are now biodegradable and 'drain-safe' when diluted with water.

The Worst Part of the Whole Process Many shops are installing closed system screen reclaiming and cleaning systems that automatically remove stencils and inks, filter out the solids, recirculate and reuse the chemicals. No muss, no fuss, and the crappiest job in the screen shop is suddenly fun and easy. Only one problem -- they're expensive. You need volume to make this affordable, or a strong commitment to waste management.

Well ladies and gentlemen, start your squeegees !!!! May is coming and that means two things. The Indy 500 and the next installment of SCREENPRINTING TODAY: the Basics. You can't run a racecar without fluids, and you can't screenprint without inks. Book mark this spot for May 1, get out your messiest printing clothes, and we'll get all covered in gooey, sticky, runny, lumpy, smelly, colorful 'INK'.

For further information about workshops and seminars based on SCREENPRINTING TODAY -- contact ANDY MACDOUGALL andy.squeeg@shaw.ca; (250)334-2598. Comments and questions always welcome. Read the Squeegeeville Gazette online.