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| Posted on 12/13/07 at 12:58 PM | |
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Try slowing down your coating speed. Under a microscope a fast coat reveals bubbles behind the mesh knuckles which can turn into pin holes. Also try using an emulsion coater with a thick and thin edge. Use the thick edge in a 1/2 (1 being the print side, 2 being on the squeegee side) on meshes below 160. Use the sharper side on meshes above 160. If your coater has a pointed end cap tilt the coater until the flat part of the end cap is in complete contact with the mesh. Press firmly into the mesh and coat slowly. The higher viscosity of this emulsion requires slowing down your coating speed to avoid bubbles. If you still have pin holes check the age of your bulb in the exposure unit. A 5kw lamp exposing 10-20 screens a day can become weak after a year even though it is still bright. If you use an integrator use "Units" not seconds. This will insure you are exposing the screen with the same amount of light, not the same amount of time the light is on. An aging lamp exposed using seconds will give weaker and weaker exposures as time goes by. If you are exposing on a fluorescent tube exposure system you may need to increase your time considerably. The emulsion should have no scum on the inside during development, if the emulsion is melting on the inside after exposure and during development you are way underexposed. To determine proper exposure without an exposure calculator mark the bottom of your screen in 12 1 inch increments. Tape a highly detailed positive firmly onto your screen, (lots of tape). Double your normal exposure time and divide by the number of marks you made, the more the better. Example on a 5kw a 180 second exposure, (or you can use units) times 2 equals 360 seconds divided by 12 equals 30 seconds. Mark the first row with a marker 360, the next 330, the next 300 and so on til all twelve 1 inch rows have a time marked next to them. 360, 330, 300, 270, 240 and so on. Now place a piece of rubylith or thin cardboard big enough to protect all the remaining emulsion on the first row and expose for 30 seconds. Re-position the ruby or cardboard on row two expose for thirty seconds again and so on until you have made twelve 30 second exposures. Now develop screen. Use a fan spray pressure washer to develop, You will find a band with no scum on the inside and a decent exposure of copy on one of the rows. Take that time as a good exposure for the mesh you are exposing on. Times or unit exposure will vary by mesh and mesh color. Repeat for all mesh counts used. You can further fine tune exposure. If 210 seconds (or whatever time worked for you)shows a good exposure do the same test again by adding and subtracting 10 seconds from row 6 (middle row) so you have an new test running from 150 seconds to 270 seconds. Try this on a 65 line gradation running from 5% to 95% dot left to right on the screen with your twelve steps running perpendicular to the gradation. This will further fine tune your exposure so that one row shows the entire gradation and all dots exposing well. The use of a 50X microscope or strong loupe will allow you to see undercutting and edge definition in the 5% and 95% dots. Use a pressure washer, Murakami emulsion is tough as nails, if a fan spray pressure washer at 18 inches is blowing out the emulsion your lamp is too weak to expose well. Want to know how strong the emusion really is, throw a blank screen out in the noon day sun on a bright summer day and see how long it takes you to blast it out on fan or pin point spray. ____________________ Alan Buffington | ||
  
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