Exposure issues can create a lot of headaches in your screen printing shop, but fortunately there are some easy steps you can take to prevent them!
Problems with exposure generally manifest during the washout process, through screens that won’t wash out well enough, emulsion that softens too much or images that develop blurred lines or pinholes during the washout process. Knowing how to troubleshoot the most common exposure problems can help you nip these issues in the bud, helping you to avoid a great deal of frustration in your screen printing shop.
Seeing your newly exposed stencils wash down the drain can be extremely frustrating. Fortunately, when stencils wash away, the problem is clear: Your screens haven’t been adequately exposed. The two most common culprits for inadequate exposure are transparent positives or too short exposure times. First, hold your film positive up to the light. Can you see through the black ink of the positive? You need to use a darker ink to create your positives. If you can’t see through the ink, try lengthening your exposure times. It can be helpful to test your emulsion’s exposure time: Using a piece of cardboard, cover a swatch on the screen and expose. Pull the piece of cardboard back and expose for a minute longer. Continue this process until you’ve tested several times, then wash the image away. The point on your screen where the emulsion doesn’t wash away is the ideal exposure time for your emulsion.
When your stencils won’t wash out, you’re having the opposite problem; your emulsion is over exposed. Make sure that your emulsion isn’t being exposed to UV light at some point in your screen-making process. Is UV light sneaking in to your exposure room? Perform the exposure time test mentioned above to be sure that you aren’t over exposing your screens. Finally, check to make sure your positives are being held firmly to the glass of your exposure unit. If UV light is finding its way around the edges of the film positive, your image won’t wash away.
Sometimes your stencil doesn’t wash away, but your emulsion is left with a soft or “gooey” feeling. This could be due to under exposure, but if you’ve followed your usual, tested exposure timeline and this still occurs, you will have to look for other solutions. First, you might have laid down too thick of a coating of emulsion on your screen. If that’s the case, you likely will notice a softness, or gooey texture upon washout, on the squeegee side of your screen, the side that wasn’t exposed to UV light. If your emulsion thickness seems to be up to your standards, you might need to retest your exposure times; in some exposure units, the bulbs will weaken over time, meaning you need to lengthen your exposure time to fully cure your emulsion.
Did your stencil look alright on the screen, but problems emerged during printing in the form of wavy or jagged edges? This can be caused either by softened stencil edges, overly aggressive washout, or using the wrong mesh count. If your stencil edges are soft, you may again be looking at the problem of UV light leaking around the edges of your film positive because the positive isn’t being held tightly to the glass. This can cause a transition in the exposure line, rather than a hard stencil line. Edges also can become softened, or take a sawtooth appearance, if you are being too aggressive when washing out your screens. If you are using a mesh count that is too low for the detail in your print, your emulsion might not be able to span the fibers of the mesh to make a straight line, causing a jagged or wavy edge line.
If you don’t check your screens right after washout, pinholes are another problem that can appear during printing. If the pinholes are minor, you likely can tape up the holes and proceed with your press run. If pinholes are a recurring problem, or appear suddenly in a large degree, you will want to evaluate your exposure process. First, pinholes can be caused by greasy or dirty screens. When reclaiming screens, make sure you are beginning with screens that have been thoroughly cleaned and degreased with the proper screen printing chemicals. If your screens are clean, check your emulsion. Is it old or outdated? Old emulsion can pinhole during the exposure process. Finally, make sure that the glass in your exposure unit is clean. Smudges or pieces of dust might block the UV light of your exposure unit, causing portions of the stencil to wash away, creating pinholes.
Don’t let frustration over exposure problems get the best of you. It’s true that for many exposure problems, you can fudge your process to get a result that’s good enough for printing. But taking the time to pinpoint your exposure problems can save you frustration and time, speeding along your production process because you waste time trying to find ways to make a subpar screen work. The good news is that exposure problems usually can be traced back to a handful of culprits, and checking your exposure times, stencil opacity, screen cleanliness and washout intensity can often save your stencils.