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The Advantages of Printing Over Wet Underbases

(November 2002) posted on Mon Dec 02, 2002

When printing on both light and dark garments, wet underbases offer a simple, versatile way to improve your efficiency and the quality of your printing.


By Rick Davis

All printers have to cope with lint and other mill scrap on the garments they print. The degree of lint contamination varies based on the type of fabric and knit construction of the garment. The worst offenders are 100%-cotton T-shirts and fleece. Whether you are printing with water-based inks or plastisols, lint will have a natural attraction to the wet underside of the screen. Symptoms of this problem vary. Lint and plasticizer might begin building up in the ink on the back of your screen. Individual lint particles might even block out areas of the image. Whatever the symptom, we all know the result: The operator will eventually have to stop the press (in most cases, multiple times) in order to wipe off the backs of the screens to rid them of the lint. You lose money each time the press is stopped. Your average printing speed immediately decreases and your overall productivity suffers. Textile-printing facilities all over the world have attempted to rid themselves of this hindrance in a number of ways. Some have tried in-line adhesive rollers that pass over the garment before the first color is printed. This works well for about six to ten garments, but then you have to stop the press and refresh the adhesive roller, which is now coated with a thin layer of lint. Other facilities have tried to burn off residual lint. This is a good concept, except that you can begin scorching shirts with even a slight variation in the process. Combating lint buildup is probably the greatest benefit of using a wet underbase. The underbase screen will cover or pick off any residual lint on the surface of the fabric so you won't have to worry about voids in the image area on any of the colors. You want to apply this underbase to any image area being printed, so the film positive for this screen will simply be a composite of the film positives for subsequent colors. Such composites are easily produced in all computer art programs. You need to know two tricks to get the best results with wet underbases. First, print them through a finer mesh count than you are using for any other color in the run. For example, if you print an eight-color graphic and the finest mesh you use is 230 threads/in., you could use a 305 threads/in. mesh for the wet underbase. You might be wondering why you wouldn't need to flash the underbase to avoid smearing the colors. Because the volume of ink you deposit onto the wet underbase is greater than the ink volume of the underbase itself, the overprinted colors will lay smoothly and evenly. If you attempt to print a wet color onto a wet underbase through the same mesh counts, however, you will cause mottling. The second trick is to use a clear underbase. Although the underbase screen will lift residual lint from the surface of the garments just as a white underbase or first color of the job would, you won't see any voids that might be present on the underbase because the ink is clear. Your press operator may still need to wipe down this screen occasionally, but much less often--perhaps once every 100 dozen shirts, instead of every 100 shirts. That's a big difference in productivity. Note that this technique won't change your graphic or have any negative repercussions. Because you use such a fine mesh for the underbase and you don't flash it, the thin ink film won't add any additional hand to the garment. The print will be just as soft as if you hadn't printed the underbase. Another advantage is that the underbase will also help matte down the fibers of the garment to produce a smooth finish in the print. Dark garments The benefits of using a wet underbase are magnified when printing on dark garments. Many graphics on darks are produced using a flashed white underbase to allow the overprinted colors to "pop" off the garment. Unfortunately, white underbases are notoriously prone to lint blockage due to the adhesive characteristics of the white ink. A clear wet underbase won't take the place of the flashed white one, but it will eliminate any concerns that the white screen will become fouled with lint while providing all of the other advantages described earlier. Again, be certain that the wet underbase is printed through a finer mesh count than any other color in the job so that the overprinted colors print properly and give the final print a smooth finish. Beause hand is always a concern when printing on a dark shirt, use as fine a mesh count as possible. You can take the wet clear underbase into consideration as you prepare the separations for the graphic to achieve different effects. Instead of preparing the white screen with 100% image coverage, you can leave open areas in the white underbase so the desired color will print directly onto the clear ink. The result is a muted shade of that color. Many printers accomplish the same graphic effects by printing on and off of the white underbase, and this technique is no different--except that it eliminates the lint problems you would otherwise have to address. Another advantage of using clear wet underbases on dark shirts--especially on synthetic fabrics such as 50/50 cotton/polyester blends--is the ability to control dye migration. You can use a bleed-resistant clear ink for the wet underbase, getting all of the lint-reduction advantages of this technique while having an extra insurance policy (in conjunction with a bleed-resistant flashed white) against potential dye migration. Wet is a safe bet Wet underbase printing can increase your productivity (and therefore your profitability) without the additional hand that is associated with adding an underbase screen. Once you begin using it, you'll quickly see how this technique will improve your printing and open up additional possibilities in designing the graphic.


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