September '15

For the Business of Apparel Decorating

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Page 93 of 120

2 0 1 5 S E P T E M B E R P R I N T W E A R || 87 SAVE THE SQUEEGEE FOR THE WINDSHIELD Everybody knows we can't print white fast, but we need a soft blade with 11-million metric tons of pressure running speeds in order to get coverage. Now that we have a sensible mesh we can use a blade to shear the ink rather than squeeze the ink. Try to remember it's not the mechanical pressure which causes the ink to transfer, it is the increase in fluid pressure which is caused by the shape formed between the blade, the mesh, and blade speed. Once the "windshield squeegee" buck- les there is only one point where the flu- id pressure, which is required to transfer the white, and the fluid volume that is required to cover the shirt, are balanced. The two must balance to transfer the right amount of flaw-free ink. Excessive fluid volume with a white, due to an imbalance in pressure and volume is not likely but ex- cessive fluid pressure on white causes dry flaws such as pinholes, streaks, voids, tex- ture, and roughness. Lower mesh counts require either high- er print speeds and/or larger blade edges to form a seal between the mesh and the blade. A tight seal causes the white to transfer. The best blades run at minimum angles with minimum pressure but are sufficiently compressible to still make the seal on the coarse mesh. This blade config- uration in conjunction with the mesh up- grade will allow us to run faster, and speed is the key to quality. Just how fast? The limit to stroke speed is the fill rate of the mesh divided by the tack level of the ink, divided by the compressive force of the blade. Rather than worry about how to do the math, simply build a fiducial image like the one shown above. The speed-bar is based on the fact that when the blade exceeds the speed-limit the first parts of the image to suffer are the tiniest positive elements, or highlight dots. Before we panic and won- der how in the world we would image a halftone dot on a coarse mesh, please re- member that there is no mathematic rela- tionship between line count and halftone count. If we have a high fill-rate mesh with a low EOM, low RzS1, and low RzS2, a high shear blade, and shear thin- ning ink, we can print remarkably fine dots on a coarse mesh. Notwithstand- ing, no matter the quality of the rig, we can use a speed-bar to verify the optimal speed, and it is always best to push the speed of the white to the limit for the best coverage and opacity. A shear-thinning white will allow a speed of 40"/second with impeccable quality. WHY A TOASTER? We worry about dye sublimation and mi- gration, yet our "pre-dryer" conditions are tragically similar to a toaster. We heat the platen more than the shirt ink so the garment and white are super-heated from both sides before they enter the dryer. Ironically, the energy output of virtually all flash units is set to work on the plat- en surface; too hot for the resin and not hot enough for the plasticizer. Its better to remove the heat-sink and then set the flash to work on the ink, not on the platen surface. Truth be known, not all plastisol inks, even when properly cured, or over-cured for that matter, will take the abuse required from the image on performance wear. This may be an ink issue and if so, cannot be resolved in our ink room, on-press or in the oven. Casting the constraints aside, we prefer an ink with a low yield stress, a high abso- lute viscosity, a low plastic viscosity, surface tension to whistle through the mesh and wet the yarns, the lowest tack level possible for a polymeric fluid, a fluid momentum which is predicated on top press speed, and a recovery rate which prevents penetration. But the moral to the story is, in order to be profitable we must at least consider our core components, and if they resemble the images on page 83, we may want to consid- er an upgrade. Such an upgrade just might lead to the perfect paradox: if our work is higher quality, longer lasting, and delivered as promised, we may actually be able to get a pretty penny more per print. Determine how fast you can run your blade by measuring fill rate.

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