Drying Direct Emulsions
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Drying Direct Emulsions

Since liquid emulsions have to be dry before they can be exposed, a certain amount of drying time has to be allowed. How much time? Quite often, not enough.

By Bill Stephens

Drying screens properly is not only more important than most screenprinters suspect, but a bit more complicated as well. It's also a critical step that tends to get slighted in the rush to get a screen exposed and onto the press. If your emulsion is not completely dry when you expose it, it simply cannot create a durable stencil.

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  • Water molecules present in the emulsion stop the crosslinking process part way, which means the emulsion never truly becomes hard. This has the unfortunate habit of showing up in the middle of production when the stencil begins to break down. Rushing the drying process may mean having to stop printing later on while you reshoot a replacement screen.

    The first thing to understand about drying is that it's a big job. Remember, the solids content of emulsions normally falls between 50% and 30%, so it follows that an emulsion may be as much as 70% water. That's a lot of liquid to get rid of, and since emulsions dry by evaporation, there's only one place for it to go -- into the air.

    The rate at which screens dry depends on the ability of the surrounding air to absorb water vapor. The term “relative humidity” describes how much water vapor is present in the air compared to how much it would take to make the air saturated at that particular air temperature. In an enclosed space like a drying room or drying cabinet, the air can soon become saturated. As the space fills with coated screens, the relative humidity (the amount of water vapor present in the air) can soon rise to the point where further absorption becomes impossible. At that point, instead of drying, the emulsion may actually begin reabsorbing water from the surrounding air. This means the air in the room has become saturated and if you are going to get your screens to dry you are going to have to either bring in new, unsaturated air or find a way of making the old air hold more water.

    There are a couple of things you can do. The first is to push brand new unsaturated air molecules past the screen. In simple terms: stick a fan in front of it. The fan will provide a continuous stream of unladen air molecules, but it will also pick up any dust in the immediate area and deposit it in the wet emulsion. When you expose the screen every dust particle becomes a tiny positive, keeping the UV light from reaching the emulsion behind it. Consequently that tiny fragment of emulsion never gets hardened and when the screen is washed out you end up with a pinhole where every bit of dust adhered.

    While pinholes are not usually considered quite as serious a problem as stencil breakdown, it's well to remember that sometimes pinholes wind up in places where they can't be fixed with a dab of blockout. They might end up smack in the middle of a fine line or cut into the shape of a halftone dot. As the pinhole count mounts up, so does the time require patching them and that soon begins to eat into the bottom line. It can be a tough call deciding to invest the time in trying to rescue a marginal screen or just giving up and reshooting it. Whichever way you go, those pinholes have been just as deadly to your production schedules as any stencil breakdown.

    A really dirty drying room can cause not one, but dozens of pinholes in a screen. Every pinhole adds extra time to the prepress process, the press sitting idle while somebody squints at a screen and dabs at pinholes with a blockout-loaded brush. The obvious conclusion is that fans have to be used with caution, and only in an area that is perfectly clean, or you may be slowing things down rather than speeding them up.

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    Still there are some other, if more expensive, ways of improving the air's ability to absorb liquid. For instance, raising the temperature will increase the air's ability to hold water. Of course, there are limits to this. You can't keep raising the temperature indefinitely, because at some point, the heat will cause the emulsion to start hardening, UV light or no UV light. Most emulsion manufacturers begin to sound the alarm bells at temperatures a few degrees above 100°F.

    Heat can cause another problem. When the room is too hot the emulsion dries so rapidly, that it doesn't dry the whole way through. Instead, the outside of the coating dries quickly trapping a core of moisture-laden emulsion within. Because the remaining moisture can't escape through the outer emulsion layers, a dried screen may take forever to dry completely. Such screens can also be deceptive because they may feel quite dry to the touch, yet still retain enough moisture to interfere with the crosslinking process.

    The truth is we want our screens to dry quickly, but not too quickly, so we have to be cautious about how much we raise the temperature. A safer solution may be to extract some of the water vapor from the air itself. A dehumidifier does this job very well. Even a yard sale bargain can pull a lot of water out of the air in a screen room. Combine a dehumidifier with a space heater and you have a really effective combination for your drying room.

    To keep screen production in high gear during the hot and humid days of summer, air-conditioning can't be beat. Otherwise, outside conditions may overpower the benefits of a dehumidifier. A modest investment in a combination thermometer/hygrometer will enable you to keep tabs on the microclimate in your drying room. The hygrometer measures humidity and a thermometer can help you keep the heat at the optimum level for absorption and well within safety limits. These inexpensive devices can be picked up for under $50. When you get yours installed, you may be surprised at how the readings fluctuate during the day. Try to maintain a relative humidity of around 40%, plus or minus 10%.

    If you maintain a constant temperature and relative humidity, trial and error will give you some notion of how long your screens will take to dry. This will vary, of course, depending on number of emulsion coats, size of screen, and mesh count. But even if you can perfectly control the conditions in your drying room, you still have no way of telling if your screens are dry other than by looking at them or by touching them. (Dry emulsion usually has a dull appearance.)

    To eliminate uncertainty completely you may want to consider a device offered by Saati (Majestech): the TQM Aqua-check. This is a handheld instrument about the size of a paperback dictionary with a color-coded gauge reading red for wet, orange for borderline, and green for dry. A pair of blunt metal probes, about an inch apart, protrudes from one end. Press the probes against the emulsion, press the "on" button and the needle indicates whether or not your screen is ready to expose.

    Tip: Since the moisture meter only measures the small area of emulsion between the two probes, it's a good idea to take several readings from several different places on a screen just to be sure the entire screen is dry.

    Buying one of these will set you back considerably more than a thermometer/hygrometer. You can expect to pay a few hundred dollars, but if you expose a lot of screens, a moisture meter can be a big boost to productivity. By eliminating guesswork, it not only allows you to send screens to the exposure unit the very minute they are ready to shoot, but helps eliminate problems associated with trying to expose screens that aren't quite dry.

    Setting up a drying area
    Before we start thinking about equipment for the drying area, let's give some thought to the size of the place. Screens should be dried horizontally with the print side down. This allows the emulsion coating to dry evenly and allows it to pass through to the print side of the screen where you need the thickest part of your stencil.

    Of course, this screen position can eat up a lot of space in the coating room and becomes awkward when drying larger screens. Some sort of racking system needs to be designed, ideally one that does not impede airflow around the screens. It can be quite simple. We fastened a narrow wood ledge to the wall of our drying room. This supported one edge of the screen and a sawhorse the same height as the ledge was positioned to support the opposite edge. We could quickly adjust this arrangement to support fairly large screens or a number of small ones if they happened to be of the same size. With really large screens, however, you have little option except to lean them against the wall. The differences between screens dried in the horizontal and vertical positions can be hard to detect in most cases, but in any event make sure you use a horizontal drying position for critical and sensitive print jobs and for screens with open meshes (74 threads per inch and under).

    If you're just thinking about getting into direct emulsions, be sure to incorporate plans for a climate-controlled dust-free drying area in your setup. In the rush to get screens exposed and into production many screenprinters tend to overlook the importance of drying screens properly, and never quite get around to thinking about the drying area until exposure problems turn up. Even then, the blame usually falls on the exposure itself: exposure length, the exposure unit, even the emulsion. Yet, many of these problems can be traced back to improper drying.

    The truth is when using direct emulsions, every part of the process is equally important, and every step can be undermined by sloppy work in the preceding stage. The simple solution is to concentrate on doing every single job well, and then each successive step becomes easier.

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