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Prepress & Screen Making



Until computer-to-screen (CTS) imaging technology becomes prevalent and CTS equipment prices begin falling, the majority of us will continue to rely on the vacuum-frame exposure process when producing stencils for high-resolution printing. Stencil exposure on a vacuum table is a critical step in the prepress workflow, and one that is almost universally neglected. This month, I would like to spend some well-deserved time outlining important factors to be aware of when working with vacuum-frame exposure units.

Until computer-to-screen (CTS) imaging technology becomes prevalent and CTS equipment prices begin falling, the majority of us will continue to rely on the vacuum-frame exposure process when producing stencils for high-resolution printing. Stencil exposure on a vacuum table is a critical step in the prepress workflow, and one that is almost universally neglected. This month, I would like to spend some well-deserved time outlining important factors to be aware of when working with vacuum-frame exposure units. In particular, I would like to focus on issues surrounding the condition and maintenance of the glass exposure surface.


The primary factors we’re concerned with in regard to the glass exposure table are its cleanliness and surface condition. Cleanliness is the first and most obvious issue to consider. Only a clean glass surface will allow us to fully expose stencil images without pinholes or other defects. When pinholes occur, we not only risk material waste, we lose production time while we stop the presses to make stencil repairs.


Dirt on the vacuum glass can be a real problem, since there are so many sources of airborne debris and so many ways it can find its way to our exposure units. Making the matter even more difficult is the fact that the constant release of high-intensity UV light from the units facilitates the development of static charges on the glass surface. This static can draw dirt out of the air and onto the glass like a magnet.


Keep it clean for pristine screens


How clean is clean enough for exposure glass? It might sound like a ridiculous question, but judging from the condition of exposure units in hundreds of shops I have visited over the years, it is one that needs to be addressed. The first thing to note is that "clean" is a relative term that depends on the operation to which it is being applied.


I once visited a printing operation that was renowned for screen printing extremely high-resolution halftones (150 lines/in. and up). When I arrived at the plant, I expected to see printers and prepress technicians wearing white lab coats and performing their tasks in sterile, hospital-like conditions. As it turns out, my expectations were contrary to what I found, especially in the screenmaking area.


The screen department was one of the most grungy that I had ever seen, with a thick coating of old emulsion, ink, and who knows what else covering the floor. I was shocked–it was as if I had just been told that there was no Santa Claus! When I commented on the conditions, the screen technicians laughed. They replied that the gooey, sticky floor not only captured floating dust, but it actually prevented foreign material from entering the air. It was several more years before I saw my first sticky mats at the entrance to a cleanroom in another printing operation, a high-tech version of what I had already experienced in a less sophisticated form.


Controlling dust and contamination can be a full-time job. Oddly, the situation is the worst in brand new facilities, where everything is so new and shiny that nothing will stick to it. Add to this the fact that new construction generates huge amounts of dust and dirt ranging from drywall powder and sawdust to lint, carpet fiber, and you name it. All of this debris floats through the air looking for something to land on. In the screenroom, if this dust doesn’t settle on our freshly emulsion-coated screens, you can bet it will find its way to the glass of our vacuum frames.


Fortunately, we can take several steps to minimize the amount of airborne contamination we have to deal with. We begin with the air handling. If we have air conditioning and heating ducts, we should install HEPA filters in the heating and cooling system. These filters are more expensive than conventional filters, but they will capture 99.99% of visible dirt. Almost anything sucked into the air returns will be filtered out, including outside dirt or duct dirt that may have become dislodged. The filters can be purchased from virtually any major home-improvement center or hardware store.


Filtration does not stop here, however. If we are using a box fan to create air movement in our coating area to aid in drying of screens, we should tape a 20 x 20-in. furnace filter to the intake side. This will trap most of the debris that is pulled through the fan, eliminating dirt that might otherwise become trapped in the wet emulsion on your screens or drift onto your exposure unit.


Dirt is constantly being walked into the screen-exposure area. It is on our shoes, clothes, skin, and hair, not to mention on frames that have just been stretched or reclaimed. Sweeping the floor is not the best approach for maintaining cleanliness while screenmaking activities are going on, since sweeping just agitates and throws the dirt back into the air. For cleaning during production times, it’s better to use a wet mop. I recommend keeping a mop and mop bucket in the exposure area and using it to do a quick damp mop of the floor 2-3 times per day. This will eliminate almost all of the dirt that is on the floor and keep it away from critical areas. A good habit to get in is to quickly mop whenever a screen is being exposed.


You can also use quick wipes made for cleaning floors. Several brands are available and they are useful for picking up loose dirt, hair, and dust. In the beginning, controlling dirt and dust is a big job. But by applying regular cleaning practices, the work will diminish over time.


Some amount of dust will inevitably find its way to our exposure units regardless of the precautions we take. So to avoid the risk of stencil problems due to dust, we must clean the surface of the vacuum-table glass before every exposure. In the long run, this approach will have the least impact in terms of time required and the greatest impact on stencil quality.


Among other benefits, cleaning the glass before every exposure will prevent dirt, adhesive, and emulsion from building up and hardening (emulsion and adhesive will harden on exposure to UV). The process I use involves three easy steps. First, use a razor blade to gently scrape off anything that is big and stuck, including emulsion, tape, dried ink, wet ink, and frame adhesive. The second step is to use film cleaner to dissolve anything stuck to the surface that is solvent soluble. Finally, apply a quick shot of high-grade window cleaner to remove any remaining streaks or dirt missed in the first two passes. This whole sequence should take no longer than about two minutes.


If you print large-format work and use extremely large screens (6 x 8 ft or larger), consider purchasing professional window-washer tools. These consist of professional grade, streak-free cleaner, a lambs-wool bar for scrubbing, and a high-grade window squeegee for drying. The whole set will run less than $20.00 and will also allow you to clean the exposure glass in less than two minutes.


You can also take preventive steps to reduce your exposure-glass maintenance time. Modern Solutions (608-222-2022) specializes in dust reduction in prepress areas. They make products like IMPRESS, which are used to coat the glass and repel airborne dust and dirt. The glass is coated once in the morning and then simply wiped with a special wand between exposures. The results are impressive.


Consumer products like RainX may also be used. This product is available nationally through automotive stores and is used to coat your windshield to prevent road dirt from adhering. Although this application involves vacuum-frame glass rather than an auto windshield, the results are the same and the product works quite well.


When you keep track of what kinds of dirt typically appear on exposure glass, it becomes easy to find ways of getting rid of the sources. Again, the main culprits are tape, blockout-tape adhesive, frame adhesive, old ink, and new and old emulsion. Tape adhesive comes from the frame, blockout tape, or tape used to position the positive. The adhesive often will ooze out onto the glass when vacuum pressure is applied during stencil exposure. If the tape is temporary, there is not a lot you can do. If it is permanent blockout tape on the mesh, you can seal the edges to the mesh to prevent bleedout. This is usually done through the use of a frame sealing compound.


When it comes to tape adhesive, one of the most annoying situations is the adhesive that sticks to the blanket, and is subsequently transferred to the glass. The same situation happens with ink and emulsion. To combat this, I have seen printers cover the printing frame with 6-10 mil black polyethylene plastic before the vacuum blanket is lowered and vacuum pressure applied. This plastic, typically used as garden groundcover, is available at just about any hardware store. It acts as an insulator between the glass and the blanket. Hardly anything will stick to the plastic. And as the plastic ages, we simply cut a new piece and replace. This is an excellent–and inexpensive–way of keeping the vacuum blanket clean and in good condition.


Another very helpful approach is to protect all areas of the vacuum glass that are not being used for exposure. Most printers work with only one or two standard frame sizes. In such situations, there’s an easy way to add years to the life of the exposure-table glass. Simply take long strips of clear polyester film (0.004-0.007 in. thick) and tape the strips to the glass where the printing frame edges will rest. This way you can slide your frames on the polyester instead of the glass. Sliding metal frames on the glass is a sure way to scratch it. And once scratched, the glass will become much harder to clean. In the worst cases, large scratches can impact exposure and make it necessary to completely replace the glass.




Clean vacuum-frame glass is about more than just achieving good exposure. When the glass is dirty, most printers will simply increase their exposure to burn out the dirt. This may be fine if we have a short-run job with coarse detail. But if there are fine lines or halftones, we cannot use this option.


Increasing the exposure will close up fine lines and plug small halftone dots. Worse, the particles of dirt on the glass will create weak spots in the screen. Invariably, these spots lead to stencil breakdown on the press. I know of few things more frustrating than constantly having to stop the press to patch a problem pinhole. It seems the more you patch, the more that show up. But when you account for the reduced productivity over the coarse of a shift that results from these frequent pinhole breaks, the extra time it takes to clean the glass will seem insignificant.





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