Sure, emulsions come with the manufacturer’s recommended exposure times, but with so many elements that can affect exposure, those recommended times are really only a starting point.
You could work on a system of trial and error, estimating your exposure times and hoping that your stencil doesn’t wash down the drain of your washout station and that it holds up through your entire press run. Or, you can opt to run a simple test that will help you hone your exposure times so you can be sure that you’ll get a solid stencil every time. Here are three tests you can run to optimize your screen exposure.
You can purchase Stouffer 21 Step Transmission Grayscale strips to test your unit’s exposure times. These film positive strips feature 21 different shades of gray into black to block varying amounts of light during exposure. You simply place the strip against your screen during exposure and then washout the portion of the screen that was under the strip to give you an idea of how thorough your screen’s exposure was. Generally, you want any portion of the screen that was beyond step 7 to be fully exposed and withstand washout. If step 7 or up washes out partially or fully, you need to increase your exposure times.
You can even figure out how long to increase exposure by by using an algorithm: Each exposure step has a value of 1.414 exposures, so for every step that washed out that should not have, you multiple by the 1.414 to determine how many times your exposure should be increased. For example, if you need to go one step further on the scale, you would increase your exposure time by 1.414 times. If you need to go two steps, you would multiply the 1.414 by 1.414. As the product of 1.414 multiplied by 1.414 is 1.999, you know you need to double your exposure times.
While Stouffer strips can last exposure test after exposure test, you also can download similar exposure tests if you prefer to print your own. Downloading and printing your own tests can carry a benefit in that you’re using your printer and your inks so that the opacity of your positives will be a better match for the opacity of the tests. Using downloadable tests is similar to the process of using the Stouffer strips: You print the downloaded grayscale onto a film positive and use it to expose a portion of a stencil. The test you choose to download should give you information on how the test correlates to stencil strength and how to increase exposure to move up and down the test’s scale.
One final way to test the strength of your exposure unit and determine your exposure times is with a DIY test. To do this, you take an emulsion-coated screen and a completely opaque piece of paper of film and expose different portions of the screen for different lengths of time. First, you place the paper on the screen with about 1-inch of the screen uncovered and you expose the screen for one minute. Then, you move the paper downward to uncover another inch and you expose the screen for another minute. You continue on with this process until you’ve exposed the screen about eight times. When you’re done, you will have gradient sections of screen that have been exposed for between 1 and 8 minutes. Washout the screen and see which sections held; you’ll be able to determine what range of time is ideal for exposing your screens.
While testing your exposure unit and exposure times can be one more thing on your to-do list, taking the time to determine the best exposure time for your unit and your preferred emulsion ultimately will lower your frustration. You’ll find that your screens regularly have well-exposed stencils that don’t wash out and that hold throughout your press runs. This can be so valuable, in fact, that some printers will place a test strip on every screen to check their stencil’s strength before every press run. Even if you choose not to test every screen, you should periodically be testing your exposure times, as your exposure unit’s power and bulbs will lose strength over time, lengthening your ideal exposure times. Knowing your exposure times will optimize the exposure process, increasing the quality of your prints and decreasing production time and waste.
Want some more tips on how to properly expose your screen printing screens? Check out these blogs: