Screen Fundamentals: A Special Three-Part Series on Screens

Part One: Choosing the Right Frame. Good screen printing begins with good screens Û and that all begins with a good frame.

From simple one-color logos for local businesses to dazzling, high-end process designs for industry competitions, every screen printing job has at its foundation one thing in common: the screen. It is the fundamental tool in every printer's arsenal, the starting point for all types of printing.

Given the importance of the screen, it's a good idea to have a thorough understanding of the frame and mesh choices available, as well as how to inventory and handle the screen once it's in your shop. The first, and perhaps most important, component of a screen is the frame. Printers have three choices for framesÛstatic wood, static aluminum, and retensionable (also made of metal) Û each with its own advantages and disadvantages.

Got Wood?
Many new printers choose wood frames Û often considered an entry-level choice for printing Û because they're the least expensive way to build a large inventory of screens. They can get the job done, even for high-quantity, demanding orders. In fact, plenty of screen printers turn out millions of shirts and make hefty profits every year using wood screens. They often win industry awards for printing, too, proving that wood can do the job.

In fact, for printers doing lots of repeat, one-color jobs for local businesses, wood really makes a lot of sense, because you can Ïlibrary,Ó or archive, the screens. With minimal capital investment, you can image the artwork on a screen, do the job, and then store it on a shelf until the customer reorders. Of course to library screens, you must have sufficient space.

That said, you're not going to want to library jobs with multiple colors, because the screen's dimensions will change over time and cause major registration headaches. As the temperature changes in your shop and humidity increases or decreases, wood tends to warp and the mesh loses tension. (In some rare circumstances, a wood screen's tension may actually increase because of the way the wood warps, causing the mesh to stretch and the image to distort.)

Put another way, when you have jobs with tight registration or intricate artwork with two or three colors, there's a chance that it won't register when you pull the screens from your library. For instance, let's say you have a job with two screens, each at 20 Newtons. If one of the screens loses tension and slips to 15 Newtons, you can no longer register that job.

Also, it's important to note that librarying really only makes sense for wood screens. If your shop uses more expensive, retensionable screens, you don't want those valuable tools sitting on a shelf, gathering dust. I have found however, the old wood screens make great dust covers for retensionable screens.

Wood Options
Manual screens are usually 18Ó x 20Ó or 20Ó x 24Ó, while automatics start at about 23Ó x 31Ó. (Yes, you can use manual screens on an automatic press although it's not commonly done.) Depending on their size, you can purchase wood frames with mesh already stretched and glued for about $11 each, or without mesh for about $8 or $9, depending on the screen's size.

Given the minimal difference in price, you're probably better off buying wood with mesh included Û but either way, it's always more cost-effective to purchase them a dozen or a case at a time. (It's possible to make your own wooden frames from scratch, using wood from a lumber yard. However, you might want to ask yourself if woodworking is really a good use of your time.)

Also, look for kiln-dried frames, which ensure that there's minimal moisture. (Moisture can cause instability in the screen, which will lead to printing problems.) You also want frames with mitered, glued corners, and the wood should be sealed with a polyurethane varnish to prevent moisture absorption.

That's So Warped
When a screen is first made, it should be perfectly flat, or Ïplaner.Ó Unfortunately, as wood frames age, they tend to rack or warp Û much more so than metal frames. That's because wood tends to absorb moisture over time. In severe cases, you'll see wood screens that look like potato chips because they're curling.

The best way to see if your screen is racked is to find a flat surface Û usually the exposure unit glass. You should be able to push on all four corners individually without any corners lifting. If you push a corner and the opposite one lifts, it's starting to warp. At this point, the screen is only suitable for one-color printing.

Unfortunately, there's really no way to ÏunwarpÓ a wooden frame, although it's theoretically possible to put a weight on it and apply heat. Ultimately, though, warping is just a fact of life with wooden screens.

Still, wood frames are a good choice for printers looking to get lots of screens for a minimal investment, who do a lot of one-color work, and for those looking to build a library of screens that they'll maintain over time.

Metal Mania
Metal frames, which are usually made from aluminum or an aluminum alloy, have a clear advantage over wood: Their metal corners are cut, mitered and welded together in a jig that keeps them perfectly flat and straight.

Like wood frames, metal ones are static, meaning they can't be retensioned once you've put on the mesh. And as with wood frames, a static metal screen's mesh loses tension as it ages.

Years ago, metal frames cost three times as much as wooden frames, but prices have fallen dramatically, putting them more in the range of twice the cost. Of course, keep in mind that cheaper wooden frames will eventually be tossed in the trash as they lose tension and warp. (While you could have a wooden frame re-meshed, it's generally cheaper and more efficient to buy a new one.) With a metal frame, you're not going to have issues with warping, and you can reattach mesh more cost effectively so it has a longer life.

The Best Value Option
A retensionable frame, also made from metal, is the ideal choice in many respects. When the mesh loses tension, you simply retension it. Retensionable frames come in two flavors.

The first type, a roller frame, takes up the excess mesh and rolls it around the frame, which stays the same size. With this type of retensionable system, all frames are the same size.

The second type uses a system of screws to increase tension by making the screen slightly larger. If you start out with a 23-inch screen, you might have a 23? -inch screen after retensioning. Still, this systemÛlike a roller frameÛallows for accurate, efficient registration.

Generally, retensionable frames initially cost about three to five times as much as wooden ones. Over time, you'll see cost savings, though, because you don't toss a retensionable frame into the trash when mesh loses tension; you retension it. Therefore, your investment in retensionable screens is one of the best values in your shop.

By Greg Kitson
U.S. Screen Print & Inkjet Technology - 1901 E. 5th Street - Tempe, AZ USA 85281