Three issues arise when it comes to screens in a print shop. One is having the correct mesh count for the job at hand. Two is having the best tension on the mesh. And three is properly preparing the screen for the production floor.
Mesh Count: A persistent misconception among screen printers is that lower mesh counts are preferable to give a more opaque laydown of ink. If you’re using a 110-mesh screen to print a white under base, you’re laying down too much ink. The reason for this is almost always due to cutting corners, using the white as both an under base, but also as the white ink in the image. By printing the under base through a higher mesh count, a 160 mesh for instance, you can achieve a smooth, thin ink surface. After flashing this base print, the other colors are printed, followed by a highlight white as the last screen to give us a bright white pop to the design.
For day-to-day printing, your mesh counts should start higher than is common in most shops, and should rarely go below a 160 mesh. The results will be thinner, more crisp detailed images, but with all the opacity necessary for a brilliant graphic.
Mesh tension: Most of our screens start life with good tension – in the range of 25 Newtons. The standard measurement of screen tension is measured in Newtons. But many printers continue using screens far beyond reasonable usefulness. Through regular use, screens will lose tension. Again, we’re talking about cutting corners, by leaving screens in service too long.
What happens when a screen has poor tension (10 Newtons or less)? The most common problem is getting good release of the screen mesh from the fabric being printed. Garments are printed “off contact,” meaning the screen mesh only touches the garment along the edge of the squeegee blade. If our mesh is not tight, the ink on our shirt will stick to the mesh. When the screen is lifted, ink will try and hold to the mesh and pull upwards with the screen, leaving a rough surface to our print. A properly tensioned screen will leave a clean, smooth print surface.
Prepping for production: Degreasing (washing the screen as a first step) is an integral part of the screen prep process. We are simply washing away oils and contaminants from the screen mesh before coating with emulsion. The emulsions we use are water based, so any oils or other contaminants on the mesh will cause poor adhesion of the emulsion to the mesh. This means the oils in your own hands as well. Oil and water don’t mix, with the result being pinholes opening in your screen during production. Pinholes are tiny dots of ink that appear throughout the print run when a screen is not degreased or is improperly degreased.
Coating screens with emulsion is a much debated process in the industry. It seems no two operators do it quite the same way. Usually, these variations involved additional coats of emulsion on the screen, and based primarily on creating a Band-Aid to some other issue earlier in the process. Proper pretreating alleviates most of the need for additional coats of emulsion.
Using the sharp side of the scoop coater, a single pass of emulsion on both sides of the screen mesh should suffice for most production jobs. The most important part of the coating process is in drying the emulsion. It’s very important to place the emulsion-coated screens in a rack with the ink side up and the print side down, the same position the screen is loaded in the press for printing. This will cause the emulsion to form a heavy, smooth surface where the screen touches the garment being printed.
Before the screen goes to press, taping the inside will keep ink from leaking onto the garment and will speed cleaning of the mesh and frame after the job is complete. Don’t use masking tape or inexpensive box tape. Masking tape is a paper product that will degrade and foul the ink during production. Low-end box sealing tape tends to leave an adhesive residue behind on the mesh and screen frame. Tape specifically made for screens is your best option.
When it comes to screen printing, the most important component is the screen. If you get the screens right, the rest of the process falls into line.
Terry Combs is a 35-plus-year veteran of the garment-printing industry and has managed production shops large and small across the United States. He has written hundreds of management and technical articles for garment-printing publications and spoken at industry events worldwide. He is currently in sales and training with Equipment Zone, Franklin Lakes, NJ, working from Scottsdale, AZ.