So, you’re struggling to dial in your exposure time. Welcome to the club. This is the most common issue we hear about. In fact, questions about exposure time come in from screen printers every day. Multiple times a day. Most of the time we suggest getting a step wedge test to really dial in the exposure time, but it’s not the simplest tool to use. So…here is a straightforward explanation for how to calculate your exposure time using a step-wedge test.
First you’ll need a few things:
- Fully emulsion screen ready to be exposed.
- Thick sheet of black opaque paper, or black trash bag.
- Exposure unit or UV bulb.
- Step wedge test calculator.
Here’s how you can use a step wedge test to calculate your exposure time:
1. Divide your screen into four sections and block off 3/4th of them using the opaque paper or bag.
2. Place the step we starting with the manufacturer’s suggested time, increasing in increments of two minutes per section. Mark the screen and at the top of each section write four minutes, six, eight and 10. Produce a piece of art with type, shapes and even some halftones. Place it in the four-minute section, cover the remaining three sections with the black sheet and expose for four minutes.
After the initial exposure moves your art and sheet to the next section, covering the four-minute section with a safe light sheet as well, you only want the six-minute section exposed to your light source. Expose this section for six minutes and follow this procedure until ALL four sections have been exposed at the times indicated.
Wash out the screen as you normally would, completely wetting both sides the let it sit for a few minutes allowing the emulsion to soften (your washout room should be yellow lit as well) then spray softly again on both sides until you see your image washing out.
IMPORTANT: Always do your final washout from the SHIRT side of the screen as this is the side exposed. To determine the best exposure look for; edge definition and degree of unexposed emulsion on the squeegee side while washing out. A properly exposed screen will have less slime on the inside as the exposure light has burned 75 to 90 percent of the emulsion. Be aware on finer detail you may want to cut back time to maintain the details.
After you have gone through all these parameters, found your time, washed, dried and blocked out your screen and ready to tape dont forget to post-harden your screen for strength and longevity. This is nothing more than exposing the screen on the unit for 10 to 15 minutes or setting it in the sun for five minutes, why? During your initial exposure your only trying to burn the image on a screen and maintain detail, post-hardening will ensure a strong, long lasting screen.
Watch the video below for an in-depth look at how to calculate your exposure time using a step-wedge test.