Figuring out exposure times is one of the most difficult tasks in screen printing. If you're struggling, you're not alone. The best thing you can do is invest in a 21-Step Exposure Calculator. How does it work? Let's dive in.
WHAT IS AN EXPOSURE CALCULATOR?
What does an exposure calculator measure? It measures the durability of the emulsion layer. Durability means how long the emulsion takes to break down or how long the screen will last on press. If you have a soft 7 when done rinsing out, the screen can see some durability issues during long runs. If you rinse out at a 6 or lower you will see durability issues depending on the ink type used. This is primarily an issue when using aggressive inks like solvent, water base or discharge. If you’re using plastisol ink only it will not become as much of an issue.
Note: An exposure calculator does not measure the level of detail your screen or emulsion can capture.
FACTORS AFFECTING EXPOSURE
The first thing to note is that you're not going to use the same time for all screens. The following factors will affect how long or short you need to adjust the exposure time.
Emulsion stencil thickness: thicker stencils take longer to expose.
Mesh count: lower mesh counts create thicker emulsion layers, which take longer to expose.
Mesh color: yellow mesh takes 30% longer to expose than white mesh.
RELATED: MESH COUNT AND YOU
Type of emulsion: Presensitized emulsions like Baselayr Complete expose quicker than diazo-mixed or dual cure emulsion like Baselayr Long Lasting.
Humidity levels in darkroom: keep the darkroom dry — under 40% humidity — for best exposure, especially if your emulsion requires diazo. Too much humidity in the screen spells exposure trouble.
Dry screens: Make sure your screens are completely dry before exposing. Pick up a hygrometer to monitor humidity and the environment you dry screens in. If the area is too humid, invest in a small dehumidifier.
Thin thread vs. standard thread: because the threads in thin thread screens are smaller in diameter, they create a larger opening between the threads in the screen. Compared to a standard thread screen of the same mesh count, the thin thread screen will take longer to expose because the emulsion layer will be thicker. When using thin thread screens, test exposure times and coating techniques to determine the best emulsion layer for the screen.
HOW TO USE THE 21-STEP EXPOSURE CALCULATOR
It's pretty simple. Place the calculator on the screen, away from the design. Expose the screen.
Wondering how to even pick a time to start? The product page of each emulsion shares a starting time depending on the exposure unit you're using. Remember, it's a starting point. Depending on the other factors mentioned before, you will have to adjust the time to find the perfect time for exposing that particular screen.
Once the screen has been exposed, bring it to the washout booth. Soak the screen in water in a dip tank or gently rinse it with a hose and let the water soak into it before using a pressure washer. Rinse the screen, and take a look at the calculator.
The lucky number you want to hit is seven. If you've hit the seventh mark, it means you're screen is perfectly exposed! If you can wash out past the seventh mark, the screen is under-exposed. If it stops washing out before the seventh mark, the screen is over-exposed.
Off by a few steps? No worries. The exposure calculator has a handy table that shares how you should adjust your time based off what step you had reached. Grab a new screen, make the time adjustments, expose, and bring it back to the washout booth.
LEARN MORE ABOUT MAKING SCREENS
That's all there is to it! The 21-Step Exposure Calculator is an inexpensive tool that helps printers dial in their exposure times much faster without the headache and frustration.