Burning a Great Screen Part 4

September 12, 2017 | Posted by Franmar

Continuing our discussion of how to get great screen images, we should be moving away from “Why doesn’t my screen image come out right?” With the last post in this series, hopefully we are all arriving at  “My screens are great every time!”


Determining Optimum Burn Time:

The best way to figure out your ideal burn time is with a step test.  There are companies that sell a kit that walks you through finding your ideal burn time.  The following is another approach you can use to get similar results. Using this approach, you will gradually be testing shorter and shorter exposure times, as you move across the screen. Here’s what you do:


- Make a graphic with 6 or 7 repeated images across the page.  Allow some space between each image.

- Get a thick piece of paper or something else able to block the light from your exposure 

- Estimate expected exposure time; double that, then divide by the number of repeated 

images on your graphic.  This is the time to use for each (and every) image.

- Tape the graphic to the screen

- Tape the heavy paper over the graphic, leaving one of the repeated images exposed to the light.

- Expose for the calculated time for each image.

- Once the time has passed, move the paper over so the second repeated image is also exposed to the light.

- Put back on light table and expose again for the calculated time for each image.

- Repeat this process until all the images have been exposed, with the last images    having the shortest exposure times, and the first images having been exposed multiple times.

- Blow out the screen

- Examine each graphic looking for signs of overexposure, underexposure, or good> clean artwork

- Calculate your exposure time from how many images/increments of exposure time it took to reach the ideal: You will start from the least exposed image, so the last image to be exposed will be counted as Number One. Proceed from there to count the number of images to the best exposed image. If the third image is the best, optimum exposure is three times the calculated time used for each image. </span>


You’ll want to repeat this process for every screen and emulsion combination that you use.  If you find most of the screen is underexposed or overexposed, redo the test so that the time intervals are smaller and centered around the time closest to the proper exposure.  It may take a couple of screens to really narrow in on the proper exposure time.  Once you find those times, write them down somewhere around the light table.  That way you’ll have quick access to the correct exposure time for each emulsion and screen combination that you use.  


- Extreme underexposure: It will be very easy to tell if your screen did not spend enough time on the light table. When you blow the screen out, the emulsion will come off in sheets, and not just where the artwork was. All you can do at this point is get all the emulsion out with a good emulsion remover and start over.


- Extreme overexposure: What you’ll notice is it is nearly impossible to blow out the artwork. Overexposure is a little harder to pinpoint the exact cause. While it is possible to leave a screen in the exposure unit too long, other factors, such as poor artwork, could be the root cause.


- Slight overexposure: When you’re not looking at extremes, it becomes even harder to spot the little variations that indicate a need to change your burn time. As with the extreme overexposure, there could be other factors that are causing the problem; however, if those factors are ruled out, then it comes back to the burn time. When looking at the smaller variations in burn time, the best place to look is around the artwork. In an overexposed screen, you may see a jagged line along the artwork, which is actually small specs of cured emulsion around the artwork that creep into the artwork. You may also notice small specks of cured emulsion in the middle of the artwork area.


- Slight underexposure: Most screen printers tend to underexpose their screens. Some videos will even tell you to re-expose the screen after the artwork is blown out to finish curing the screen, which is not a good practice. A properly burned screen does not need to be put back on the light table after the artwork is blown out. The reason that screen printers underexpose their screens is because a slightly underexposed screen is easier to blow out than a properly exposed screen. There are a few signs that indicate an underexposed screen.


- Similar to the overexposed screen, an underexposed screen may have a jagged edge, but instead of the emulsion creeping in, you’ll see small bits of emulsion blown out where they shouldn’t have blown out. A couple of other signs of an underexposed screen show up when you try to reclaim your screen. The first sign is if you have a hard time breaking up the emulsion with a good emulsion remover such as Franmar’s Strip-e-doo Emulsion Remover.  Strip-e-doo takes about a minute to break down the emulsion on a properly burned screen, and while it’s working on the screen, you’ll see a slight color change and feel a little pull on the pad while scrubbing. With an underexposed screen, there may be no color change and pull or just a very slight color change. If there is a slight change, you still may be able to get the emulsion out with some time and more Strip-e-doo; however, if there is no change in color, the emulsion is locked in.  </span>


Something else that you may see on an underexposed screen is if, when reclaiming a screen, you have stubborn emulsion that outlines your artwork. It may even seem like a ghost image, but it is often too crisp a line to be a ghost image, and it is just the outline of the image, not the whole image. Unfortunately, this emulsion is locked in.


However, with locked in emulsions, our D-Haze Gel can sometimes clean it out. It doesn’t always work, but it’s usually worth a shot. You can put the D-Haze Gel on the locked in emulsion and let it sit overnight. It won’t harm the screen, and because it’s gelled, it will stay on top of the mesh. If it works, you’ve saved a screen.


These are a few tips and tricks that hopefully will help you find that perfect exposure time, every time.