With the addition of the newly published GANA "General Guidelines for Screen Printing on Flat Glass", we thought it was time to start talking in more depth about common screen printing issues on flat glass. It's simply not easy, flat glass printing. You have a roughly flat surface. No pores or rough surfaces for the ink to grab onto; and don't forget the glass has a high surface tension and issues with static electricity. Kinda makes you wish you were printing t-shirts, no? There is a degree of art that goes into screen printing on glass, it's not just an easy task you can throw a manual at a guy or gal and say, get to town. No magic computer screen to plug numbers and get a good output. It takes experience and understanding. It's simply a difficult task but not impossible and not hard to do if given some time to be perfect.
The GANA guide is fantastic. It's part definitions and part "tips & tricks." In this little gem, you will find some great bits of advice that came from simply making mistakes. However, you need to go a bit further. You have three big friends in the process with you; the screen manufacturer, the fabric manufacturer, and the ink manufacturer. Each of these will have recommendations for you, things you really ought to listen to like: the distance the screen should be from the glass for printing, the recommended screen tension per your fabric and frame, and from the ink maker you get key info on your ambient conditions for printing. Honestly, most of these guys have done the work for you.
My thought is this thread, screen printing on glass, will become a series of posts about the best-practice for doing it. But, I want to leave you with some advice and tips to use. One of the most knowledgeable consultants in the industry is Mike Young with imagetek Consulting International. He offers some sage advice in his presentation on the "Top-5 Secrets to Screen Print Architectural Glass Successfully."
1. Follow proper "Image-to-frame ratio"
From small to large format, an image-to-frame ratio should be between 30% and 70%. If demanding quality isn't a factor, you can operate in the 50% to 70% range. If you have somewhat of a demand on quality, go for 40% to 70%. And if you have to control your
deposit and have minimal distortion, make sure you are in the 30% to 45% range. Take a look at the illustrations from Mike's presentation, it really illustrates what I'm saying and how distorted the image can get if you operate too close to the edge.
2. Pick the right fabric for the job.
When we talk about screen tension next, you will see that squeegee speed and pressure combine to drop your tension over time, resulting in a rejected print. So, what does the fabric have to do with it? Take for example, two screen fabric types, both 77 mesh but one with a thread size of 55 um and the other of 48 um. The first gives an open area of 28% while the latter gives 36% or 29% more open area for ink to deposit through. The result is the second fabric not only gives more exposure for ink transfer but a better uniform deposit, and less squeegee pressure needed.
3. Keep on top of your screen tension.
Do you have a tension meter in your kit near the screen print machine? This really should be the number one issue, dropping screen tension. In the glass industry, I bet this would hold to be one of the top, if not the top issue on the list. Reason is, we screen print the same product for long periods of time and have set up our machines to work with that product and specific screens for long periods of time. Most screen printers; for plastics, fabrics, etc, have to deal with differing inks all the time. They are used to tooling the machine to work for the ink or desired output. Know what the recommended screen tension is for your screens? If you don't you gotta find out. It's the difference between quality repeated prints and streaks, lines, and light/dark spots.
4. Ensure your squeegee is of proper length.
Which do you think is better? A squeegee that fits the screen or one that fits the image? Well, when you have a squeegee that is wider than your image you actually get an elongation of the image as the squeegee is tugging on the sides of the fabric. The closer to the edge you get the more tugging due to tension. A well deposited image is one that had the squeegee just bigger than the image printing. Look at the illustration below to see what I mean.
The flood bar or flood coater is the part of the squeegee arm that reloads the screen full of ink after a print is made. To ensure proper loading is done, always make sure you are back flooding right after a print, and that you do not have a back flood bar that is too big for your screen. Just as in a too large a squeegee can distort your image, a flood bar acting on the same physics will load your screen wrong and also result in a poor print. Look at the illustration below.
So, those are some of the high profile tips you should be considering each time you fire up the screen print machine. In the next few posts, we will cover a myriad of other issues you might encounter, from proper material preparation to how you set up your ambient conditions for printing. This might even turn into a downloadable e-book, you never know.