Burning a Great Screen Part 3
Emulsions & Screens:
The last factor in the burn time equation is the screen and emulsion combo. Each emulsion and screen combo is going to have a slightly different burn time. I’m not going to say that one screen is better than another or this emulsion is better than that emulsion. All the variations in emulsions and screens are too extensive to cover in this article, but with some reference points, you’ll be able to know more about your own screen and emulsion combinations.
In the world of emulsions, there are a vast majority of options. All emulsions fall into one of three categories. There are solvent resistant emulsions for use with solvent based inks; water resistant emulsions for use with water based inks; and dual cure inks that are designed to use with both solvent and water based inks. The original formulation of direct emulsions are the diazo emulsions. The diazo emulsions are a 2 part emulsion where there is an emulsion and then a diazo additive. When the diazo is added to the emulsion, it makes the emulsion sensitive to light. Now, emulsions are going to be presensitized and ready to go, with no premixing needed.</span>
With all emulsions, a smooth even coat will provide the best results when burning a screen. If there are some areas that are thicker than others, it will be hard to get a consistent burn time. The thickness of the coating affects how long the screen has to burn so if there are different thicknesses across a screen, it will be difficult to get a good consistent image.
A good clean screen is also important. If there is any dust or lint in the screen, emulsion tends to gather around those particles. Because thicker emulsion requires a longer burn time, the emulsion that accumulated around the lint will not cure with the rest of the emulsion; therefore, you’ll develop pinholes in your screen. A good practice is to always use a good degreaser before coating your screens so that you’ll have a nice clean screen.
Something else that will affect your burn time is the mesh count. There are actually a couple of different factors that go into higher mesh count burn times. Higher mesh counts are used with finer detail prints and half tones, and are usually yellow; however, different colors can be used. The colored mesh is used to absorb light. The while mesh causes light to spread, which can cause problems with finer details. However, because the finer mesh has less space between threads, less emulsion is needed to span the open spaces in the mesh, which allow for thinner layers of emulsion. The thinner layer of emulsion requires less exposure time to cure, but the colored mesh absorbs enough light that the exposure time actually increases for finer mesh screens.