Creating the Perfect Screen
The key to consistency in screen printing lies in the screen. Discover how discipline and science can help you fine tune the screenmaking process.
I was very lucky when I first got into this business. I landed a job early on with a company that was riding the exploding electronics boom of the early 1980s, and I cut my screening teeth in the demanding world of printed circuits. While most of my screen-printing contemporaries were still mastering spot colors and rudimentary process printing, a team of engineers was helping me figure out how we could bring our tolerances in line with the demands of the customer, who was always wanted denser circuitry on ever smaller boards. It was a great opportunity that would pay off handsomely—if we could just figure out how to get this highly variable process tamed enough to allow our guys to consistently print to the tolerances our clients demanded.
It soon became obvious that the key to consistency was the screen itself. If we could control the variables in the stencil, then the printing would generally take care of itself. I found myself spending a lot of time with a lot of expensively educated engineers in the murky dampness of the screen-prep area, where we systematically broke down every step of the process in our quest to understand how it all actually worked. In this month’s column, we will continue our journey through what is undoubtedly the most crucial element in any screenprinting process—stencil making—and learn to apply science and discipline to move closer to our goal of creating the everelusive perfect screen.
The frame as the foundation
The first thing we looked at was the frame. At the time, the norm in the circuit-board world was a rigid, aluminum frame with the mesh pretensioned using vacuum clamps and then glued into place. The printers hated using new screens. The image warped during the process, and it was a fight from the first pull to the last to keep the print aligned enough to complete the job. These problems improved a little as the screen hardened off with several uses, but it brought more problems for the printer to solve on the press. Snap-off was compromised, the screens had to be raised off of the substrate, the image began to distort when it was printed, and on and on. We immediately realized the brilliance of the retensionable screen frame and enthusiastically embraced the new technology.
Today, I find it incredible that we achieved even the crude results that we did without correctly tensioned screens. I am even more amazed when I walk into a screen shop now and find people still using those same methods and complaining about the poor quality of their prints. If you can’t place a correctly tensioned screen into the hands of your press operators every time they print a job, then you are building frustration and inefficiency into your process, and you are paying for that in lost production. The cost of retensionable frames is, of course, the reason that many shops choose to bypass them, but it’s a false economy. In today’s marketplace, I am hardpressed to think of a print job that would not benefit from such frames. Remember, a company that chooses to restrict its quality potential will not survive for long.
Now let’s take a look at mesh. Back in the day, we had a few options, and we used trial and error to arrive at the best mesh count for the job. Today, your options are almost overwhelming, and it can seem a little daunting to figure out this part of the puzzle. Remember that mesh is a very expensive item, which means there are a lot of vendors out there who stand to make a lot of money over the long haul selling the stuff to you. Make them work for your business in every way. Get them into your shop and use their knowledge to fine tune your mesh-buying decisions. Let them know that you are talking with all the vendors in an effort to streamline your production process. Find out what they are willing to do to help you out. Keep track of the results, and if things don’t improve, get the next rep in to pitch you his product.
Don’t assume that you know everything you need to know about mesh counts simply by reading the ink supplier’s recommendations. Things change all the time in this business, and you need to be the first one on the block to know about advancements in all of the products you use, especially in such an expensive commodity as mesh.
Somebody from your screen-prep area needs to be at major industry trade events every year, researching new products. I recently spent a few days at a shop and discovered that they were paying a premium for mesh that had to be specially ordered just for them. When I asked them why, they said it was what they had always used. It turns out that the mesh had been all but discontinued because it’d been replaced by an improved and far cheaper version. The company’s rep had done his job of informing them when the new mesh was introduced, but the shop’s resistance to change prevented it from taking advantage of the better product. The result was that the printer paid a premium for old technology. Don’t get set in your ways. Complacency will cost you in the long run.
Now we come to stretching screens. I will not go into great detail, because I’ll end up muddying the waters with generalizations that will help nobody. Choose your screens carefully and then insist upon receiving thorough hands-on training on screen stretching from the company that sells them to you. Listen to everything they tell you, and make sure you implement what they instruct you to do. Buy a good tension meter, and use it on every screen. Don’t coat anything until you are satisfied that the mesh is stable and properly tensioned.
Monitor every part of this process daily, and make sure you don’t let things drift. Use the piece of very expensive ground glass that comes with the tension meter and calibrate it every morning. Go check it right now. You might be surprised. And remember, if you have ever dropped that baby, it probably isn’t working right. Check it against the one your rep carries with him whenever he visits. This is an excellent test of the mesh company’s commitment to its products. If the rep doesn’t have one on hand, then you probably need a new rep.
I remember telling somebody at a trade show 20 years ago that we were screen printing down to tolerances of ±0.002 in. in circuit-board applications, after being told that it was impossible. Now, the best process printers in our business print to those tolerances routinely and repeat-print it hundreds of times through the same screen.
In my next column, we will get into the world of coating screens and discuss the vagaries of the exposure process. Let me leave you with something to think about. Over the years, I have made a good living applying a lot of the troubleshooting techniques and process-control knowledge that I learned from that team of engineers years ago. They had no special interest in screen printing beyond the way that it fit into their production of complex printed circuit boards. They approached problems in the same scientific manner that they approached every other aspect of the process.
When I became a consultant in the screen-printing business, I always built into my contract a nice bonus for times when I could improve my client’s overall productivity by 20% in three months. Thanks to those process engineers, I have never failed to cash that check—and usually well ahead of schedule. What should interest you the most about this is that nearly all of the production increases came about due to the changes made in the screen-prep area. Worth thinking about, isn’t it?
Gordon Roberts has a history in screen-printing production management that spans more than 25 years. He has held supervisory positions in shops that represent a broad spectrum of application areas and markets, including printed electronics, apparel, signage, and retail graphics. Roberts has presented training courses on the basics of screen-printing production and on shop management for the Screentech Institute and is presently a consultant for the screen industry. He can be reached at email@example.com.