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| Posted on 5/24/06 at 12:59 PM | |
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It is difficult to predict proper exposure time because there are so many variables with equipment. In the time it takes to predict, you can make a stepped exposure test and know exactly. QX-1 will not have a significant color change that you can judge with like your diazo sensitized emulsion did. The advantage to diazo is that it changes color as the diazo reacts with UV light. When there is no more color change, you have saturated the stencil. You then have to judge if you have OVERexposed the stencil when fine lines begin to close up. Exposing the screens as long as possible assures complete exposure and maximum resistance. Developing with a pressure washer assures uniform washout and opens small details. When any emulsion is dried properly and exposed fully there is no risk in developing with a pressure washer. An exposure calibration test using a commercially available exposure calculator, or a series of stepped exposures using your own artwork, will help you determine where your proper exposure times truly need to be. Fully, or even over exposed screens, contrary to popular belief, actually reclaim easier than under exposed screens. They also leave less ghost haze images, don't break down on press, and result in far fewer pinholes. Tape your positive to the stencil and make a series of measured exposures covering a little of the positive each time you expose. These are the 'steps' of a stepped exposure. Imagine going to the beach for a suntan and covering up a little of your arm every hour. You would get 10 different exposures over 10 hours. You don’t need any special equipment to make an Exposure Step Test, just a coated screen, a typical positive with a variety of line thickness and some Ulano Rubylith™ masking film or something thin, to block UV light. You are going to make 5 or more exposures by blocking portions of the positive as you expose it. The exposure times can be varied if you desire but the goal is two exposures above and below the approximate amount of UV light. Ulano 1-Step Exposure Calculator simulates 5 exposures at one time with a filters. The filters vary the amount of UV light that gets to the stencil in one exposure so you don't have to take the screen in and out of the vacuum frame every time you want to move the mask. The result should be identical to the time consuming manual step test. Older style exposure calculators attached the filters directly to a camera film positive but this usually does't represent your actual conditions, BUT they stop UV light better than laser or inkjet positives. Large calibrated filters like the Ulano One-Step can be placed directly over your laser or ink jet test positive for the best results. 1. Tape your positive to a typical coated screen. You should coat all the screens in each mesh count the same way so the exposure time can be the same. You will not move this positive during this test. 2. Place the coated screen with the positive attached into your vacuum frame and expose for half the time you estimate as the estimated exposure time for this combination of light source and stencil. For example: I think a screen will take 4 minutes to expose, so my initial exposure will be 2 minutes. Remember to use a skinny rope to let air escape from inside the screen. 3. Turn off the vacuum and turn over the screen. 4. Tape a piece of masking film so it covers about 1/5th of the positive. Put it back in the vacuum and expose for 25% of the approximate exposure time. In this case, 1 minute (25% of 4 minutes, you time will vary). For the third exposure, move the mask so it covers 2/5ths of the positive and expose for 25% of the approximate exposure time. For the fourth exposure, move the mask so it covers 3/5ths of the positive and expose for another 25% of the approximate exposure time. For the last exposure, cover 4/5ths of the positive and expose for another 25% of the approximate exposure time. Stencils with diazo sensitizer will have a color change because the yellowish diazo will actually be used up and the stencil will return to the color is was before you sensitized it. For the best durability, look at the section where you don’t see any color change. This means the stencil reached it's saturation point, and more light just doesn't change it any more. Don’t waste time exposing it if it won't change any more. SBQ stencils are harder to judge so you have to look closely for when fine lines start to close up. That is the maximum time for resolution. Wash out the stencil in the usual manner with skin temperature water spray and look for signs of under exposure on the squeegee side of the stencil. It will be sticky and slimy because it didn't get enough UV light to cross-link the molecules so they won't dissolve with water and go down the drain. The core responsability of the screen maker is to able to judge exposure time with tests. It takes lots less time to actually test a screen than type up the instructions. If I was at your shop today, I would have to make a test. If you have more questions, Call Ed Toney (the Voice of Ulano) in Tech Services - 1-800-221-0616. ____________________ Richard Greaves, ASPT Ulano Corp. New York Technical Services 718-943-1338 direct | |
  
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