Screen Printing Articles Industry Alert Hot Shots Photo Gallery Message Boards Visit Our Advertisers: Clarke Systems Estimate Software International Sign Assoc. Matrix Payment Systems American RENOLIT Corp SGIA Specialty Graphics Imaging Assoc

Direct emulsions are so-called because they are applied directly to the screen. They belong to the group of products known as photostencils -- light-sensitive materials that undergo chemical changes when exposed to light. Because they use a photographic process to transfer artwork to the screen, very complex artwork can be accurately reproduced.

If that weren't enough, direct emulsions also create very durable stencils capable of standing up to a variety of solvents and rugged enough to last through print runs that number into the tens of thousands. And, when the job is over, you can remove the stencil from screen in a reclaiming process that will leave the screen, mesh intact, ready to repeat the whole process. As a result, one screen can be reused many times.

Exposing your own screens is an essential step for anyone serious about screenprinting, but it does require a considerable investment in equipment. At the very least you'll need an exposure unit, a wash tank, a light-free area to dry and store your unexposed screens, a coating trough or scoop coater to apply the emulsion, and a pressure washer and reclaiming chemicals to restore your screens to their pristine state.

Because it is expensive to get set up, the temptation to cut corners is ever-present, but some things you should never go cheap on -- an exposure unit, for instance. You'll always want to buy the best exposure you can afford, and your chief concern should be in getting one big enough to accommodate your screens and with enough output to completely expose them. To help you with your planning let's take a closer look at some key steps in the direct emulsion process.

Wash tanks tend to be one of the dirtiest areas in any busy shop. Backsplash from the pressure washer tends to spray residue from chemicals far and wide, making the surrounding area a bad place to store screens.

Degreasing: An Essential First Step
The first place to focus our attention is the wash tank, because the successful application of any stencil depends on starting with a clean, grease-free screen. This cleaning step is commonly called degreasing, and doing the job properly means using a product specifically designed for that purpose. You can apply the degreasing solution to the screen with a spray bottle. Then, give the screen a thorough scrubbing on both sides with a soft-bristled brush.

Tip: Make sure those bristles are soft. Some brushes can actually leave scratch marks on the mesh.

All screens need to be degreased, even brand-new screens. Mesh picks up all kinds of oils and debris in the manufacturing process. New mesh right off the roll actually feels oily. Don't neglect to degrease screens that have already been degreased but have been sitting around the shop for a while.

For some reason, vast numbers of screenprinters succumb to the temptation to use household cleaning products and any number of other substitutes for professional degreasing products. When you consider that all the subsequent steps of the process depend on this one step, this surely is a false economy. A few messed-up screens cost more in lost time and money than any possible saving.

The next step is to wash the degreaser out of the mesh. If you use a pressure washer, follow up by washing the screen down with a low-pressure spray. The high pressure splashes the residue everywhere, including right back on the screen. Degreaser also tends to get trapped down where the frame meets the mesh.

Tip: By the way, anywhere within splashing distance of the wash tank is a poor place to store screens. Tiny bits of stencil and reclaiming chemicals tend to get scattered in a fairly wide area, and some of that will end up on your screens and become difficult, if not impossible to remove.

Now you need a dirt-free place to dry the degreased screen. The screen has to be perfectly dry before it can be coated with emulsion, but resist the temptation to check for dryness by running a finger over the mesh. You're just replacing some of the oils you so carefully degreased away.

Screens just out of the stretcher heads. Though stretched with brand-new mesh, they must be degreased before being coated with emulsion. Because screens left sitting around can pick up dirt from the environment, put off degreasing until shortly before the screen will be coated and exposed.

In many shops, one screen room serves both to dry degreased screens and ones that have been coated with emulsion. Obviously, this is not ideal, because you're trying to keep the humidity down and a stack of dripping screens is the last thing you need.

Tip: A separate screen room is essential. It can provide a relatively dust-free area to store screens and with the addition of a dehumidifier and some baseboard heating you can even create an ideal environment for drying screens. The lighting should be safelights that protect the screens from premature exposure. Screen emulsions are not nearly as light sensitive as photographic film, so use safelights designed for screenprinting use rather than those intended for a darkroom. In fact ordinary fluorescent lights can be converted into safelights by covering them with inexpensive plastic sleeves available from screen suppliers.

Coating Troughs
When the degreased screen is dry, it's ready to be coated with emulsion. Emulsion is a liquid with a consistency somewhat like heavy cream. You apply it to a clean, dry screen using a coating trough or scoop coater. The coater is a long v- or u-shaped piece of metal with both ends blocked off. It resembles a miniature version of eaves trough. The good news is that with a little practice, almost anyone can learn to use one.

Size matters when choosing a coating trough. You want one that will be an inch or so wider than the artwork, but not so wide that it uses excessive amounts of emulsion or runs into parts of the frame. Remember, the trough has to be small enough to clear the frame when you coat the inside of the screen. Mesh close to frame is relatively inflexible, whereas mesh toward the center of the screen flexes a great deal. If the edges of your coating trough come too close to the frame, you won't be able to maintain good contact along the width of the frame. Your coating will end up thick in the middle and sparse along the outside edges.

Coating troughs have two edges. If you look closely you'll see that one is sharp and the other rounded. Some controversy surrounds which of these two edges are considered the business end. Obviously, the rounded edge will lay down a much thicker coat of emulsion than the sharp edge. Both edges have their uses, which will be covered in a future article.

Coating
Begin by pouring emulsion into the coating trough. It's a good idea to make sure the coater is filled with plenty of emulsion, because the last thing you want is to run out of emulsion with the trough only halfway up a screen. Support the screen in an upright position with the top angled away from you. If you come up with a way to support the bottom of the screen, the top edge of the frame can be leaned against a wall, which will allow you to keep both hands on the coating trough for extra control.

Place the edge of the coating trough against the mesh. Tilt it so that emulsion comes into contact with the mesh, and then slowly move the coater upward while keeping the edge pressed snugly to the screen. Try to keep a steady speed, because coating too quickly can trap air bubbles in the screen and create an uneven coating. When you get to the top, tilt the coater back and lift it away from the screen. You may notice two thin lines of emulsion running up either side of the screen where emulsion has flowed around the end caps. Some screen printers keep a small piece of flexible plastic handy to smooth down drips and tracks that have occurred along the outside edge of the emulsion coating.

Tip: You don't have to cover the entire width of the screen with emulsion. Try to limit your coating to within the approximate area where you'll be placing the artwork. Open mesh outside the emulsion coating can easily be filled with inexpensive blockout after the screen has been exposed and washed out.

When you've finished, some emulsion will be left in the coating trough. This can be scraped back into the emulsion tub with a piece of plastic, if you act before the emulsion begins to dry out. Some people don't like to return the unused emulsion to the original container, but the practice saves so much money, it is certainly worth considering. The emulsion container should then be tightly sealed and the coating trough promptly rinsed out at the wash tank. IF you allow emulsion to dry in the coater, it will be the very devil to clean out. A few minutes now can save a lot of minutes later.

Coating troughs have to be handled carefully. The edges are easy to dent and once dented, virtually impossible to repair. Dented troughs will, forever after, leave a streak through every screen they touch. Very minor nicks can be filed down, but the only solution for a badly dented trough is to cut it down into smaller ones and use it for coating smaller screens.

The number of coats of emulsion you apply depends upon the mesh, the application, the emulsion itself, and which edge of the coating trough you've been using. It also depends a good deal on personal preference. We'll take up this matter next time, when we continue our walk- through of the direct emulsion process and take a closer look at the emulsions themselves.