The Darkroom Process: Film, Emulsion Density, Exposing, and Washout
After coating a screen, it's time to expose and washout the screen. Dialing in screen exposure is no easy feat. In the latest videos, screen printing expert Colin Huggins goes over the tools you need to create optimal and to make solid, exposed screens. He'll also address nuances within the exposing and washout process and share ways to improve your darkroom procedures.
Many types of film output printers exist. The most common is an inkjet printer. Inkjet film has one side that's coated, and one side that isn't coated. The coating is important because it holds onto the ink from the printer. The coating also helps maintain the density and shape of the image.
When you print on film, the printer is using one of two inks. The ink could be UV-blocking (Epson printers use this ink) or it's a dye ink that's meant to completely stop all light. During his time at Ryonet, Colin has been using an Epson T3270 printer with UV-blocking ink and has had no issues with high-detailed prints or halftones.
Why do you need RIP software with an inkjet printer? The software allows you to have a droplet density that you can control. Thin or thick, you can dictate the density of the ink to fit the needs of the design.
Every printer should have a 21-Step Grayscale Calculator in their shop. This calculator will inform whether or not your screen is over-exposed, under-exposed, or perfectly exposed. It's small, so it'll easily fit on the screen without interfering with the design.
You could also use a Droplet Density Test. After exposing the screen, the Droplet Density Test will show the minimum droplet weight for a good stencil.
Lastly, you could use a Grayscale Halftone Test. The test will demonstrate the halftones the screen will be able to capture.
EMULSION OVER MESH (EOM)
In the video, Colin uses a device that can measure the thickness of the mesh and emulsion. This device isn't necessary for a printer to have. Colin has the device to show the differences of the layers of emulsion between thin thread and standard thread. In the video, he exhibits that thin thread holds more emulsion than standard thread; therefore, you'll need to expose thin thread screens longer than standard thread.
VACUUM VS. COMPRESSION EXPOSURE UNITS
The purpose of the vacuum is to seal the film onto the screen. The vacuum leaves no room for the light to wrap around the stencil, which creates soft, fuzzy edges. If your budget can handle it, get an exposure unit with a vacuum.
If you're using an exposure unit with compression, put as much weight as possible on top of the lid where the cushion is pressing against the screen. The more weight you put on the unit, the more contact you'll create between the film and the screen.
WHAT YOU'LL SEE IN THE WASHOUT BOOTH
After exposing the screens, Colin placed the screens in a dunk tank filled with water to loosen the areas the film blocked out (the image). Exposed screens can sit in a dunk tank from 30 seconds to a minute before you can wash them out. If the screen has been cured fully, screens can stay in the tank overnight.
You'll start to see signs whether or not the screen was properly exposed. If the emulsion looks milky, that means the emulsion did not crosslink well, so the screen is underexposed. It's important to note that you will have a tiny layer of emulsion that's underexposed (unless you purposely over-exposed it), and that's okay. When you rinse the screen with a hose, you'll see that thin layer come off.
Insider's Tip: When you're washing out the screen on the squeegee side, you don't want to use any sort of pressure. Remember, the emulsion is thinner on the squeegee side, so you don't want to blast any of your details out. On the shirt side, you may use a pressure washer, but do not get too close because you may wash out some of the image or emulsion.
Your exposure calculator comes into play while you're washing out the screen. If you're able to wash the calculator out to step seven, the screen has been exposed perfectly. If you're able to wash out past the seven, the screen is under-exposed. Not being able to wash out to step seven means the screen is over-exposed.
Tune in next week when Colin will look at the details of his stencil through a microscope to see how well it was captured. The video will be live on YouTube, be sure to subscribe to our channel and turn on notifications so you don't miss it!