Importance Of Good Emulsion Coverage On The Screen

Recently, we received another inquiry regarding the importance of emulsion coverage on a screen being prepped for screen printing.  International Coatings' Western Regional Sales Manager, Kieth Stevens, was asked to give us a better insight into this important tool: "One of the most overlooked aspects of a screen printing shop is the condition of the emulsion on their screens. When I travel around supporting our ink products throughout the Western United States, one of the first things I like to check are the screens that are ready for sample printing or production. Sometimes I just touch them as I pass by on a tour, but other times if the customer is really open for my opinion, I will hold a screen up to the light to see its quality. Believe it or not, the condition of the screens can really say something about how well the printing company cares about the quality of their prints. There is an absolute correlation between the quality of a print and the tools that the printer is using. Here is how quality emulsion will make a difference: Let’s assume that all the emulsions out there are of high quality. With that said, there are 3 types of emulsion that are readily available to today’s printers. First there is Diazo, the least expensive but also the least sensitive to light thus requiring a longer exposure time. Diazo can be obtained as solvent-resistant or water-resistant emulsions but most printers use the water-resistant Diazo for water base inks.   Photopolymer emulsions, also known as one-pot emulsions, are a high-cost alternative to Diazo.  Photopolymer responds to very quick exposure, has a longer shelf life and is good for most inks other than water-based. Thirdly, there is a hybrid emulsion consisting of both, Diazo and Photopolymer, termed Dual Cure. The Dual Cure is at a medium price point and has a good exposure time with a moderate amount of resistance to water-based inks. Now that I’ve expounded on the basics of emulsion, let’s move to what I really wanted to get at:  One of the most overlooked tools in a shop is the consistency of the emulsion thickness. To make my point easier to understand, think of emulsion as a gasket or seal. Without this proper gasket, straight edges of the design can look like stairs or zig zags, half tone dots can end up looking like stars and you will see a tremendous amount of dot gain (dots touching or running together). Emulsion manufacturers use a tool to measure the exact emulsion thickness and also can provide charts to show what the optimum amount of emulsion for each mesh should be. Having enough emulsion on the screen is imperative. This is not a tool to skimp on in an attempt to cut costs. Feel the thickness on the underside of the screen where the image is; it should be thicker on an open mesh and thinner on a fine mesh.  In any case, there should be something substantial to feel.  If the edge of the emulsion is not distinct, chances are that there is not enough emulsion on the screen and pinholes or premature emulsion breakdown may be taking place. One of the most fascinating things I often see when I visit a print shop is what I lightheartedly refer to as the “Picasso Syndrome.”  What I mean by that is that I observe one of the shop employees in the screen room spending too much time touching up the screen by using a backlight so he can see pinholes that have developed on the screen.  Granted, a number of factors could have contributed to this syndrome, such as the glass or film being dirty or the screens being under exposed, or the possibility that the screen could also not have been properly degreased.  However, the most likely culprit to the Picasso Syndrome is that the screen may not have enough emulsion covering it.   This blog is by no means a complete list of screen room issues that may affect your print quality, but lack of emulsion coverage is a pet peeve of mine.  I hope that I was able to shed some light on what is a very common problem in many print shops. Stay tuned for more of my ramblings." Kieth Stevens