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Control Points for Quality Screens, Part 2

(May 2001) posted on Tue Nov 06, 2001

David discusses five more factors that influence screenmaking success.

By Rick Davis

Your printed result is only as good as the screen used to print it, so it pays to focus on quality screenmaking. Last time, we looked at five of the top ten variables to control in order to achieve quality screens: mesh selection, screen frames and tension, mesh degreasing, manual pressure washers, and the screen-coating and drying process. This month we will look at the rest of my top ten. 6. A clean working environment Regardless of the size of your shop, cleanliness in the screenroom is likely to be the most neglected aspect of production. Even if cleanliness isn't neglected, it's still very difficult to maintain the dust-free, lint-free environment you need, especially when the screenroom is often so close to the production area. You want to produce screens and get them quickly into production, so on one hand, having your screenmaking area set up conveniently near to the pressroom makes sense. On the other hand, vast amounts of dust and lint invariably end up on your screens. It's impractical to move the department far enough away to escape all the lint and dust, so you must take steps to keep contaminants from getting into the screenmaking department. The portal for all this dirt is, of course, the doorway. A number of different tricks are typically used to stop the flow of lint and dust through the door to the screenmaking area. Some printers use wool blankets to shield the area from dirt, as well as light. Others put up plastic sheeting, and I have even seen rotating, light-proof darkroom doors placed at the entrance to the screenmaking department. The old wool blanket is a poor approach because it doesn't filter out enough light, plus it actually attracts dust and lint. When someone enters the room by throwing back the blanket, contaminants fly everywhere, and a lot of them land on your coated screens. Dust and lint that become embedded in your emulsion coating will result in pinholes, either when you wash out the stencil or while you are printing. Sooner or later, the weak spot caused by lint on the stencil will cause trouble. When pinholes appear, you either end up wasting time touching up screens before they go to press, or you waste time at the press by filling in the pinholes. Either way, you are wasting time. Screen printers are very creative, though, and some have devised a way to "solve" this problem. First, a layer of emulsion is "carded" all over the non-image areas of the stencil. Then the screen is re-exposed. Finally, tape is applied to the non-image areas of the print side of the screen. That is an awful lot of effort when you consider that keeping dust and lint out of the screenroom can prevent the problem in the first place. If you want to avoid the cost of a darkroom door, install a standard door. Avoid any type of hanging "curtain." To make sure the door isn't left open to the production area, you might also install an automatic closure device on it. In addition, you may consider applying sticky paper on the floor at doorways to remove dirt from shoes upon entering the room. To help maintain a clean screenmaking environment, control the temperature and humidity in the screenroom. Central air-conditioning and air-filtration equipment are very effective. Window air conditioners are not very efficient as they tend to fill a room with cold, damp air that compromises the screen-drying process. Keeping the humidity at a level that will keep dust down without slowing down screen-drying time might require a humidifier for dry months and a dehumidifier during the summer. 7. Vacuum-table maintenance Dust and scratches on the vacuum-frame glass can easily appear as defects in your stencil. One of the easiest ways to avoid this type of problem is to keep the glass clean and exercise care when placing items on the glass. Clean the glass on both sides first thing in the morning and then throughout the day as needed. Use a soft cloth and a good quality glass-cleaning product. If you use a window squeegee, make sure it has a clean edge that is free of defects. The second precaution is to use care whenever you lay a screen onto the glass. Metal screen frames can easily scratch the glass, especially if you slide them around while positioning them for exposure. Once a scratch is introduced to the surface of the glass, it will be reproduced on any stencil that is laid over that particular area of the glass. Although the scratch may not appear on the surface of the screen once it is exposed and developed, the "shadow" of that scratch is still there and will act as a weakened area of the screen with the greatest potential for breakdown on the press. 8. Exposure meter/light source Light integrators compensate for the aging of your exposure lamps and power fluctuations during their operation. They ensure that your stencil receives the dose of light you intended it to get. Shops that do not have light integrators sometimes run regular exposure tests to determine how long to expose their screens. Unfortunately, many facilities fall out of the habit of using their light integrators or doing exposure tests. They are literally taking a "shot in the dark" when it comes time to expose screens and often end up under-exposing the stencils. This leads to all sorts of problems in production, from pinholes and stencil breakdown to difficulties in reclaiming the screen. 9. Touch up If you have a well-run screenmaking operation, you should never have to spend much time touching up screens. When a screen is properly coated and exposed, the touch-up process should consist of nothing more than using a small paintbrush to fix pinholes. By the way, you should not have more than ten pinholes on the average size garment-printing screen. Once you patched the pinholes, all you need to do is tape the inside perimeter of the screen to prevent ink leakage under the frame. Now you're good to go. There should be no need to re-coat the print side of the screen with emulsion, re-expose, or tape the entire print side of the screen for endurance purposes. I have seen printing facilities where I wished I had their tape account alone because they typically use half a roll of tape on a single screen! 10. Proper press settings Proper press settings are an aspect of the process that few equate to screen quality or performance. However, press settings play a crucial role in how the screen performs on the press and also affect the life of your stencil and mesh. Excessive squeegee pressure, floodbar pressure, and off-contact distance will put undue stress on your mesh and your stencil, and these excesses don't necessarily give you a better looking print. When printers decide to improve screen quality with higher tensions, sometimes they forget to decrease the off-contact distance. You must reduce this setting as screen tension increases. A higher tension screen won't need more than 1/32-1/16 in. (0.79-1.59 mm) of off-contact. All screens should be at a uniform distance from the platen surface, and each platen should be checked to ensure that it is parallel to each printing head. Several instruments can be used to determine the off-contact height of your screens. A very simple method uses coins, while a more sophisticated method uses dial indicators to set the distance as low as 0.01 in. (0.254 mm), assuming that the platens are perfectly flat. Conclusion Making good screens is not rocket science. It isn't difficult or outrageously expensive, though sometimes, a screenmaker's shortcuts can make it very time consuming and costly. I can almost guarantee that focusing attention on these ten areas will result in more consistent and predictable results, better image quality, and longer lasting screens. Those improvements add up to cost savings as well.


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