Burning a Great Screen Part 2

September 12, 2017 | Posted by Franmar

Continuing our discussion of  “Why doesn’t my screen image come out right?”, with our goal of getting to  “My screens are great every time!”, in this series of blog posts over the next few weeks, we are covering the main factors and many variables that can affect exposure times.This time we are covering The Light Table,


Light Table:

The light table is the second most important variable in the equation.  Depending on whether screen printing is a hobby, second job, or full-time profession will greatly determine the type of light table you are using.  Ultimately, all types of light tables get to the same goal, curing the emulsion.  In order to cure the emulsion, it has to be exposed to UV light.  Most every light source emits some amount of UV light.  Yellow light bulbs, also called bug bulbs, emit very low levels of UV light, which makes them ideal when applying emulsion because the emulsion can be exposed to the light for long periods of time without curing.


Light tables can vary from the sun to a work light hung on the wall to a several thousand dollar vacuum framed, metal halide, digitally controlled exposure unit.  Each step up in a light table addresses one of the four factors that go into light table quality.

- Light consistency

- Exposure time needed

- Image closeness

- Consistent exposure time


I’ll address each of these four factors and the role they play in producing a better quality screen image.  


Light consistency is a very important factor in trying to find that perfect screen image. Even though the sun is the greatest source of UV light, it is not always consistent.  The amount of UV light can vary depending on time of day, time of year, and cloud cover.  For somebody just starting out in the business, the sun may be a perfectly fine source for exposing screens; however, for somebody who is burning dozens of screens per week, the sun will not work very well for them because they cannot get repeatable consistency from the sun.  


Light consistency also factors in with the age of lamps.  With all light sources, the light they produce over time can vary with age.  If you are consistently burning your screens at a certain time, and all of a sudden you have to significantly increase your burn time to get the same results, your lamps could be going bad.  Even if lights are not visibly dimmer, they may not be producing as much UV light, which is the light you really need.  A fresh set of lamps will bring consistency back to your light table, which will bring back consistent burn times.  


Exposure time is the one factor that most everybody associates with image quality.  Many think that if the exposure time is just right, the screen image will come out perfect every time, but it is really just one piece of the puzzle.  Exposure times are dependent on your light source.  It can vary anywhere from a few seconds to several minutes.  The greatest effect the length of time has on the screen image is the longer the screen is exposed, the more opportunities light has to get under the artwork.


There are a couple of different ways light can get under the artwork.  The first way is if the light goes straight through the artwork on the transparency.  This is more common with laser printer transparencies.  Often times, the toner is not dark enough to stop the light.  The toner darkeners do help with this, but it is often times not enough.  With an inkjet, you can have a slightly different problem.  Because the inkjet works by putting drops of ink next to each other, if too little ink is applied to the transparency, light can go in between the dots of ink.  This causes small bits of emulsion to cure under the artwork.


Another factor of different light tables is their ability to keep the artwork as close to the screen as possible.  This cuts down on light refraction.  Refracting light is light that slightly “bends” when it hits a transparent surface.  The best way to see this is to put a straw into a clear glass filled with water, and then look at it from the side of the glass.  The straw in the water does not line up with the straw outside the water.  The light “bends” when it hits the water.  The same thing happens when light hits a transparency.  The light can bend around the artwork and cause edges to not be completely straight.  


Vacuum framed light tables are used to alleviate the refraction problem.  If you set a screen down on a table, there is a small distance between the screen and the table.  Even with the transparency taped onto the screen, a small gap can form between the two.  The larger the distance between the two, the more that light can bend around the artwork.  The vacuum system on light tables presses the artwork and screen solidly onto the glass top so there is no gap between the artwork and screen. While vacuum systems are a great tool, knowing what they do is more powerful because you can adapt whatever system you are using to eliminate those gaps between the screen and the artwork.


Lastly, a precise timer on a light table is a great asset.  While is it good for all light sources, it is especially needed when dealing with metal halide light sources.  Metal halide sources are powerful lights for curing emulsions.  They produce so much UV that it only takes a few seconds to burn a screen.  Because they only take a few seconds, an accurate timer is essential to find that fine line between underburning and perfection.


With other light sources, a good digital timer is still a great tool to have.  With a digital timer, you can easily burn a screen at a consistent time and once you’ve found that perfect burn, you’ll want to keep repeating that time.  Anybody who has spent time in a kitchen knows that a mechanical timer does not have the precision of a digital timer.


Continuing our discussion of how to consistently get great screen images, in this special series of blog posts, we are covering the main factors and many variables that can affect exposure times.   and next, we will discuss steps for Determining Optimum Burn Time and also Underexposed, Overexposed, or Just Right.