Coudray walks through screen and stencil preparation, identifiying techniques for producing accurate and durable screens that will last beyond 100,000 impressions.
Technique number two is a modification of the first. After we washout and dry the screen, we recoat the inside and outside using a two-over-two, wet-on-wet coating approach. This will fill in all of the open mesh that we just washed out and it will apply a thin layer of emulsion on the squeegee and print side of the screen. We then dry as we normally would.
Now it is time for our second exposure. For the second exposure we will have had the art department make a solid mask of the image area that is exactly 1/16 in. (1.5 mm) bigger than the original image. It will be solid black. We register this mask to the screen and make a second exposure at 300% of the original exposure, then wash out the screen.
When the washout is complete, we will have a double thick emulsion over all the nonimage areas of the screen. All pinholes and weak spots will have received a double reinforcement of emulsion. And the longer exposure will assure that all available sensitizer will cross link with available resin.
Taping the screen is important as well. It's wise to pay the extra price and use the pressure-sensitive tapes designed for this purpose. These tapes are usually 2 or 3 in. wide and will cost anywhere from $6-15 per roll. As expensive as they are, the adhesive is very aggressive and will not break down from constant immersion in the ink. We run two parallel strips of tape down the length of the frame 1/2 in. inside of the squeegee and floodbar path. This will form a friction track that the squeegee and floodbar can run on. The tips of the floodbar and squeegee are the areas where the highest abrasion occurs. By having the squeegee and flood run on the tape, we greatly reduce the friction on the mesh.
We continue the taping by duplicating the squeegee/flood track on the print side of the screen. If either the squeegee or flood were to work though the emulsion on the inside of the frame, this layer of tape will delay the emulsion breakdown.
When taping the screen, we should avoid applying tape to the areas of the screen where the squeegee and floodbar change over. This is a real problem area in the screen because tremendous, aggressive friction is applied in exactly the same spot over and over. If we put tape down to minimize this (a logical approach), we will quickly discover that the tape will be pulled into the image area by the squeegee. It is better to use extra emulsion, as described previously, to combat this problem. The friction tracks are not subject to this problem because the length of tape is perpendicular to the squeegee/floodbar motion.
Register marks can also be a problem for textile printers. Once the job is set up, the marks are usually taped out. For long runs, this is unacceptable. A better approach is to use water-soluble blockout. We card it over the marks, let them dry, and then tape the print side of the screen. If we need to reregister, we simply remove the tape, wipe out the marks with a damp cloth, and make our test print. If we do not follow this approach, ink will find a channel under the tape, working itself out onto the substrate. This may take quite some time, but it will eventually happen.
As aggressive as these methods sound, screens created using these techniques are no more difficult to reclaim than normal screens. The combination of higher-emulsion coating and complete exposure, coupled with minimum squeegee pressure and off-contact, will allow for screens that can stand up to tens of thousands of impressions without even breathing hard.
Long runs pose many special challenges. When screens begin to breakdown and eventually rip on press, the economic consequences are huge. This is multiplied when we are using a multicolor press and screens fail at faster rates. But by using the methods outlined here, not only will screens last much longer, but the stencils will perform at optimal levels throughout the entire run.
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