March '15

For the Business of Apparel Decorating

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M any of us have fought these minute-by-minute battles: mottled, inconsistent cov- erage; screen stretch; poor matte down, penetration, and opacity; and a rough surface printed at a speed gauged with a sundial. The client dictates the image, and we don't make the ink and are almost al- ways stuck with the shirt, so the screen is our greatest opportunity to optimize the process. T-shirt screen printing is wrought with barriers to success. The frames are too small, the mesh is low volume with misguided stencils, the inks are too tacky, the blades don't fit the mesh or inks, the substrate is textured and mostly air space, and we have temperature swings on press of 50 degrees F. Nonetheless, screen printing is won by those who can use the stencil, blade, and necessary ink additives to create compatibility between the mesh, ink, image, and substrate—all upon com- mand and in short order. INKS, FRAMES, AND MESH We use a blade at high speed to in- crease the fluid pressure in the ink, which causes a viscosity drop in a shear-thinning color that flows into the mesh. Then, the blade forms a seal on the top of the mesh, directing flow toward the garment. The fluidized ink reaches the garment with greater affinity for the knit- ted or woven construction than to the screen and gets caught up in the knit and releases the mesh. Because of our selection of incom- patible tools (See Figure 1), we traditionally can't fill more than about one-third of the mesh and stencil. Over the years, we've watched the frame du jour evolve—and devolve—from wood to metal to roller and most recently to pre- stretched panels. Here's a cursory review. 1. If you're using wood, I hope printing is only your hobby. 2. If your stretched metal frame bows in- ward, you might as well use wood. 3. If you use a roller for retention, it leads to moiré and achieves higher than 27N/ cm². You must like using a Goodyear to squeeze window putty through a silk scarf onto an escalator. Reconsider your tool and use a sensible tension level. 4. Let's say you're the batter in a baseball game. A prestretched panel is like the pitcher and umpire controlling your at-bats—it's not a profitable move. Before picking a mesh, determine what the mesh does and does not do. Keep in A1. GARMENT Per customer & margin A2. IMAGE Per customer & market A3. INK Unspecified / Ever-Changing C2. STENCIL Define edges, limit ink volume C1. BLADES Thin ink, establish ink volume, complete image shape @ max speed B1. SCREEN MESH 1. Establish max ink deposit 2. Allow ink passage 3. Hold onto stencil 4. Help maintain image shape C3. ADDITIVES Fix ink to suit; mesh, garment, image Filter or Flourish? Optimize the screen printing process for better results B Y J O E C L A R K E Joe Clarke has spent the past 45 years in the lab and engineering department in prepress and on press as a research and development and technical researcher as well as a manager of screen print production. He has received a number of print-related patents and is a mem- ber of The Academy of Screen and Digital Printing Technologies and a Specialty Graphic Im- aging Association fellow. Clarke has presented hundreds of papers, written a couple books, and published more than 600 technical and management articles for which he has earned numerous industry awards. Currently, he is president of Clarke Product Renovation, a Chi- cago-based corporation that brings product and process technology to the screen printing industry. He contributes feature articles on textile screen printing exclusively to Printwear. Reach him at Figure 1. With incompatible tools, more than about one-third of the mesh and stencil can't be filled. 8 4 | PRINTWEAR M A RC H 20 1 5

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