Shedding Light on the Coating Process
Discover where mistakes are most likely to happen in the screen-coating process and what your screenroom staff needs to do to avoid them.
Last month, we continued our journey through the often neglected but crucial stencilmaking area of the print shop. This month, we’ll attempt to throw some light on the coating part of the equation. (Please excuse the pun.)
Some of you have done your research, bought a state-ofthe- art coating machine, purchased a new exposure unit at the most recent SGIA show, purpose-built your drying chamber with its own unique dehumidifying units, and sent your staff away for extensive training, furnished generously by your favorite emulsion supplier. If this sounds familiar, I suggest you take this time to read and enjoy your latest financial statement. You will see the savings and the profitability that result from all this time and effort. For the rest of us—and from my observations, we seem to be the majority—read on for a quick wake-up call that will spark new life into your entire screenproduction operation, remove frustration, add hours to your production day, and improve the quality and repeatability of the images you produce.
I am amazed by the number of people I meet who create stencils for a living and have little or no idea about exactly how the process works. You don’t have to be a chemistry genius to create stencils, but I strongly believe that you do need to understand the basics of the stencilmaking process. Take the time to learn what is actually going on at the molecular level when you shine a UV light source on a photochemically coated piece of fabric. Ask your supplier to explain the theory. If it doesn’t make sense to you, make him or her explain it again and again until it does make sense to you. Check back issues of Screen Printing magazine for some great articles written on this subject. Most importantly, don’t assume that the information is too technical for you. It’s actually very simple once you take the time to learn the basics. Let’s move on to some basic truths and see if we can’t make small changes in your stencil-prep area that will yield huge gains for your screenmaking process as a whole.
First basic truth: Never use a screen that is not fully prepared for coating
Create a checklist that covers all the variables that can spell disaster on the press, and check every screen against the list before you release it to be coated. Here are a few of the essential points to cover. (I’m sure you will come up with many more.)
Does it have the correct mesh count for the job? Has it been properly degreased? Is it newly stretched, and if so, has the mesh been work-hardened? Is the tension correct across the entire screen?
If any of the basic criteria listed above are not correct, then the screen will not perform properly and your printers will struggle to produce a decent print. Most problems occur when the screen maker decides to cut corners and send out a screen that is just not quite perfect. Give your screen preparers the professional respect they deserve, and give them the right to refuse to expose a coated screen until everything on their checklist is complete. Initially, it may slow things down, but believe me, you will gain it all back on the press.
Second basic truth: Don’t try to rush the coating process
If you use emulsion that needs to be mixed, follow the manufacturer’s instructions to the letter. If the instructions tell you to let it sit overnight to allow bubbles to work their way out, make sure that you allow time for this. We all know that you can get something to work in an emergency, and the tendency is to start bypassing all of these little rules until you find yourself yelling at the manufacturer’s rep because his emulsion no longer works. If you scoop coat, make sure that the leading edge of your scoop coater is not damaged. If it does get damaged, make sure you replace it immediately. They are expensive, but you should always have a replacement on hand. Never use a coater that doesn’t give you a clean, perfectly flat layer of emulsion.
I am often shocked to find people using equipment that they should have discarded long ago—and living with the resulting problems. Also, I am shocked that employees use equipment in a manner that allows the tools to get so easily damaged. Treat all of your equipment with the respect it deserves.
Many screen printers have made the switch to capillary film, which eliminates most of the problems involved in the scoop-coating process. The down side, of course, is that it’s more expensive. However, it might be worthwhile having someone cost out the savings. Capillary film, when applied properly, will always outperform direct emulsion. The ease of application and the amount of time you save in screenmaking might just make it worthwhile.
Third basic truth: The screen is not ready for exposure until the emulsion is completely dry
I often hear grumbling among screenprep staff about the number of pinholes they have to touch up, and I hear them blame the phenomenon on dust, miniscule scratches on the glass surface of the exposure apparatus, film quality, etc. Here is a fact to ponder: Most pinholes that occur in emulsion-based stencils are due to inadequate drying. If there is any moisture left in the screen when the emulsion is exposed, it will impede the photochemical reaction and wash out when the screen is rinsed out after exposure, resulting in minute pinholes.
Here is another fact to consider: If you use heat in your drying process, it is possible for you to dry the emulsion’s outer layer too quickly, sealing in moisture and causing pinholes. I strongly believe that the best way to dry a screen is through dehumidifying the air surrounding it and by drying it over an extended period of time, preferably overnight. If you think residual moisture might be the source of your pinhole problem, leave a screen overnight in a room with a decent dehumidifier that is capable of bringing the humidity level in the room below 40%. Expose the stencil the next day, and I suspect you will see a great improvement
The whole truth: You cannot cut corners when making stencils—period
Once again, we return to the one truth that seems to be most self-evident and almost always is disregarded in our work environments. If we constantly cut corners and make compromises to get the job done on time, then we are most likely causing the problems that force us to constantly play catch up.
If a screen is not perfect when it leaves for the press, then it will, at best, slow down the print run—and at worst, not perform at all. When this happens, we throw everything into turmoil, quickly re-shoot a screen and, as always, cross our fingers and hope everything will work. It might take a little longer to achieve the kind of results necessary in the stencilmaking area, but quality in this area will allow for a much smoother workflow. The time is always well spent.
In my next column, we’ll look at the problems wrapped up in the highly misunderstood piece of equipment that you use to expose stencils. Before we go there, order a new bulb for your exposure system right now, along with an exposure testing kit. I have a feeling you will need both by then.
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.