Screenmaking:The Gateway to Image Quality
Coudray identifies environmental factors and processing concerns that you need to control in order to produce reliable, high-quality printing screens.
Screenmaking: The Gateway to Image Quality
It is not uncommon for me to receive phone calls from screen printers who are at the end of their rope. They appear to have all the right tools and all the right supplies. They use good chemicals and emulsions. But for some reason, they just can’t seem to get the results they are looking for--image quality doesn’t measure up to their expectations. They say their exposed screens lack sharpness and edge definition and are full of moiré. These common problems most often originate during screenmaking, so this month I’ll describe some ways you can control key aspects of the process, manage your shop’s environment, and explain how you can--in a matter of hours--dramatically improve the quality of your image output.
Back to basics
Following some standard procedures and adding some internal check points will make a majority of the ills vanish. Let’s start with emulsion. Printers who want to produce decent work select quality dual-cure or photopolymer emulsion. The emulsion’s solids content should be more than 35%, which is adequate for good to excellent edge definition. For the photo emulsion to have a fighting chance at maintaining fine halftone dots, or very fine lines, it must be capable of resolving them. This is a problem if the emulsion is too old. If you use a dual-cure emulsion, count on a maximum usable time of four weeks from the time you sensitize it. For the sake of perspective, photopolymer emulsions remain usable almost a year from the time they’re sensitized.
The same concept applies to screens. Once you coat a screen, you have a maximum of one week before it will no longer resolve those 3-4% dots. As time passes, the coated screen experiences a phenomenon known as dark reaction. This is common to almost all photo-grade materials. The term refers to the tendency of photosensitive materials to essentially expose themselves over extended periods of time, even if no addtional light sources are present. So whatever emulsion type you use, get in the habit of recording the date when you sensitized the emulsion. Also mark screens with a coating time and date. This can be very helpful in tracking down problems related to resolution.
Heat and humidity
Certain environmental conditions in your shop can affect the useful life of a coated screen. Heat and humidity have the greatest influence on this time frame. But when you combine these factors with the amount of fine detail you want to maintain in an image, you may find that you only have a short time in which to put the screen to use. At the lower end of what can be printed, which is determined by the mesh threads blocking the openings, the time can be measured in hours. But with halftones above 100 lpi, these conditions can greatly shorten the life of the screen.
The addition of heat can speed up most chemical reactions in a screen-printing shop. This is certainly the case with photo emulsions. When the temperature rises above about 85°F (29.4°C), the sensitizer becomes very active and looks for something to grab onto. This could be either a water molecule or an oxygen-hydrogen (OH) group in the emulsion resin. In either case, that sensitizer molecule no longer is available for the photo-imaging process, because it has reacted with something that interferes with image resolution. And if the sensitizer has reacted with a resin molecule, it no longer will wash out; it becomes insoluble. Essentially, detail is lost when the emulsion prematurely exposes itself.
A lack of proper air flow across coated screens usually is to blame for excess humidity. It is crucial to keep large volumes of air moving across the emulsion’s surface, because water evaporates from the emulsion as it dries. The water vapor is heavier than air, so the evaporating moisture quickly forms a saturated laminar layer on the top side of the screen that effectively blocks further evaporation. On the bottom side of the screen, the heavier water vapor sinks into the laminar layer of the screen below and makes it even harder for evaporating moisture to escape.
The remedy is simple. Circulating air over the screen surface can easily be accomplished with some 20-in. box fans. They cost less than $20.00 each, and they’re highly effective. Equip each fan with a 20-in. furnace filter on the intake side to minimize the amount of particulates in the large volumes of air you create. This method allows your screens to dry quickly and completely, but I have a word of caution: The coated screens will only dry to the room’s level of ambient humidity. Keep the relative humidity between 40-60% for best results. You can measure both temperature and humidity with an inexpensive instrument from Radio Shack. Models that measure both minimum and maximum temperature and humidity over a 24-hr period are available for less than $30.00.
The shop environment’s influence on the quality and usefulness of coated screens only intensifies if the light in the coating area is not controlled; however, I believe that most screen printers understand the fact that light exposes screens. Whether it’s coming in from an open doorway or a poorly filtered source, uncontrolled light affects even the best-prepared screens. But this isn’t the problem. The issue is the quality of the light present in the area.
There are two steps you should take to regain control. First, all fluorescent bulbs should be gold or yellow in color. Second, these bulbs should be encased with gold or yellow UV sleeves. This may seem like overkill, but there is a good reason to take this extra precaution. These sleeves--available from graphic-arts suppliers--do a very good job of sealing the ends of the bulbs and filtering out any stray UV light. Simply having a colored tube is not good enough, because the internal coatings do not hold up that well. It is very common for the ends of these tubes to leak unfiltered light. The amount of yellow coating on the bulb overpowers your eye and prevents you from detecting any leakage from the length of the bulb. This is where the sleeves come in.
The sleeves have special UV inhibitors built into the plastic, usually polycarbonate, and they have end caps that completely enclose the fluorescent tubes. There also is an added safety advantage to the sleeves. Polycarbonate is a very tough plastic, so it takes a great deal of force to damage it. If you accidentally hit a bulb and break it, the sleeve will retain the broken glass. The sleeves cost between $10-20, but the good news is they never wear out.
Once you have environmental and lighting conditions figured out, the next step is screen coating. Unless you’re coating large quantities of screens, or are producing fineline halftones, you do not need an automatic coater. As long as you dry the screens horizontally, you can hand-coat screens for halftones up to 100 lpi.
When I consider the coating sequence, I look for evenness of the coating and sufficient emulsion over mesh (EOM) to form a good edge profile. EOM varies depending on level of detail, thread diameter, and ink type. I normally consider 8 microns the minimum. As you increase the EOM, you can run into problems printing highlight dots. Regardless, it is much more common to have an EOM that is too thin than too thick. An electronic thickness gauge is very helpful in measuring emulsion thickness.
To maximize your coating, I recommend using a dull (.5-1-mm radius) coater-edge profile. Use a slow stroke with even pressure. Coat the print side first with two passes, wet-on-wet. Immediately turn the screen and coat with two more wet-on-wet passes from the squeegee side. Dry with the print side down. Measure the EOM. If it is too thin, increase the number of passes on the squeegee side until you reach the level desired. The EOM is determined by the viscosity and solids content of the emulsion. Each brand is different, so test accordingly. Most of the time, you will need to apply between two and four coats to the inside of the screen. I know this sounds incredibly basic, but you would not believe how many different techniques are out there. They may work, but I know this method to be reliable. The important thing is to measure. When you take a bunch of measurements, you begin to see patterns. These patterns help you settle on the best practices necessary to obtain the ideal stencil thickness for your shop.
The last step is exposure. While calculating exposure time is relatively easy, it’s hard to figure out why the practice of using metal-halide bulbs until they fail to light is so common. This is a big mistake. Metal-halide bulbs generally are good for about 1000 hr of operation. This is about 6 mo in a busy shop. Lighting engineers have told me that a bulb loses 2-4 hr of life every time it’s lit. The more times you light the bulb, the faster it dies.
A bulb’s UV output drops quickly as it ages. The dropoff is invisible to the human eye, so it must be measured--or approximated. If you use an integrator, check the length of the exposure. When the output reaches 2/3-1/2 the exposure of a new bulb, it is time for replacement. I can’t tell you how many times I’ve been in shops where they have never changed the bulb. They figure that if it is still giving off light, it must be working. For coarse work, this may be acceptable. But replace the bulb every 6 mo if you plan to print any kind of halftone work. As you should with the emulsion and coated screens, also date the bulb when you install it.
Focus on the front end
The solutions I’ve described here are big, easy, and quick. They should help you overcome a majority of the quality problems in your screen room. You can get as specific as you want or need with these adjustments, but you just can’t ignore the theme that’s common to the all tips I’ve presented: measure and document. I know I say it a lot, but paying attention to the process, as well as the product, is the key to improving your work.
Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Screenprinting and Graphic Imaging Association Int'l (SGIA) and as chairman of the Academy of Screen Printing Technology. Coudray has authored more than 70 papers and articles over the last 20 years, and he received the SGIA Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine.