This happens to be the trickiest part of the screen printing process due to the number of variables that can affect the exposure.
After gaining more experience, I realized I had a lot of trouble because I kept changing the variables (i.e. different emulsion types/brands, using vellum vs. transparency film, changing printers with different black ink deposits on the film, changing exposure light sources, artwork that was very detailed versus solid areas, changing the ink types, changing the substrate, etc). My advice is to get solid with a single process first, then learn as you change one variable at a time. The hop-skip-jump method will only lead to frustration and lot’s of wasted time and materials. Trust me, I learned the hard way.
Be patient, be diligent and persevere! Once you get the process down, your results will remain consistent as long as you follow the same steps and your equipment/materials stay intact.
There are many ways to expose a screen. Any light source containing UV light between 350 and 420 nanometers can expose a screen. Some people even use sunlight (which happens to contain a fair amount of UV radiation). This post will assume you have a bottom-up light source already built (i.e. home-made exposure unit or professional unit).
Step 1 - In light-safe conditions, p lace your film positive(s) right side up onto the glass of your exposure unit. The film positives should appear exactly as you intend them to be printed (not reversed). I tend to place my artwork directly in the middle and away for the very edges of the exposure unit.
Step 2 – Place your coated screen (completely dry) print side down onto the film positives. Be sure to center the film positives, leaving at least 2-3 inches of room around the designs. Next, place a piece of foam (cut to the inside dimensions of the screen frame) inside the screen frame. If you want to be extra careful about light diffusion, you can fit a black t-shirt over the foam before placing it into the screen. The film positive should be in between the glass and the screen mesh after this step.
Step 3 - On top of this, add some weighted books or anything that will distribute a even amount of pressure and ensure that the artwork and emulsion have great contact with the glass.
Step 4 - Turn off all other lights and then turn on the exposure unit (or if it has a built in timer, set it to the correct amount). Depending on your emulsion type, mesh count, UV tube wattage and artwork, exposure times may vary. You may have to do this step a few times to determine the exact exposure time.
Using Ulano Dual Cure Direct Emulsion, a 30” x 40” exposure unit with 8 UV tubes (FL30T8/BL350), I typically expose for 2 – 3 minutes using this method for great results.
Step 5 – After the screen has been exposed, quickly move to the washout area. Use a garden hose and cold water to spray both sides of the screen (this stops the chemical reaction). Continue spraying the screen with water with medium pressure. After some time you will notice the screen starting to “open up”. Once this happens use the hose to gently spray the design areas. Do not stop until all the areas have opened up completely. Do not use a pressure washer to do this step as it may actually remove parts of the emulsion that make up your design.
Step 6 – Throughout the process, hold up the screen to a light in order to see if the screen still has bits of emulsion that still need to be removed. Once you are satisfied you can leave the screen to dry. Also, for thinner lines and detailed work, you may need to be very gentle with the washout process and may actually require less exposure time. In these instances, I would consider drying the screen and then re-exposing to “harden” the emulsion.
Step 7 – Ready to be taped!
