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| Posted on 4/22/08 at 07:30 AM | |
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The situation of flash time and temperature is not as simple as just raising the temp. In physics there is a principle known as Black Body behavior. It simply says that as a black body (usually carbon or graphite) is heated to higher and higher temperatures, it will change color from black, to deep red, cherry red, orange, etc until it eventually reaches blue white. The problem comes in as we increase the temperature to the point where the panel begins to change color. This is above the 900° F point you describe, but not by much. As you cross 900°, less of the heat is available for absorbption. In fact, the flash is no longer color blind. You begin reflecting energy from lighter colors like White while still absorbing energy in the dark colors like the black tshirt. The obvious problem is that you need to increase the dwell time to cure the white, while at the same time running the risk of scorching the black as it heats faster and absorbs more of the emitted energy from the flash. There is a better approach, and it is two fold. First, 900° is the optimum temperature for plastisol. The PVC resins absorb infrared energy in this range most efficiently, meaning close to 1 unit of energy is absorbed for every unit of energy emitted, there is much less loss at this temp. So, the idea is to increase the energy concentration at the surface of the garment. IR panels, and quartz for that matter follow another principle called the Inverse Square Law. Without getting into any confusing math, this simply says that if you double the distance of the flash from the garment surface, you will need 4X the time to reach the same temperature. So, stated another way, if you cut the distance by half, youf flash time will decrease from 10 sec to 2.5 sec. If you actually want to do the math, email me offline and I'll send you the formula and what you have to do. But that is not the whole problem either. One of the main reasons printers have to use such long flash times is due to how the laydown the ink. Basically, they are putting down way too much and are saturating the t shirt material with ink. In order to achieve a gel cure, you have to heat both the garment and the ink to about 200°F before it gels. Think of it this way. How fast does a piece of paper catch on fire when you hold a match under it? Compare this to trying to light a thin piece of wood. Heat transfers to a thin surface layer much, much faster than it does to a thick layer of fabric and ink. On top of this, all that mass retains the heat after the flash and you end up will all kinds of tack problems, sticking, and lifting because of this stored, wasted energy. The solution is to up the mesh tension above 30 N/cm. The higher the tension, the better your control of the ink layer, and the better the net opacity at the surface of the t-shirt. We typically print at 50 - 60 N/cm for our underbases and flash at .5 sec with no cooling station. While we use quartz for more control, we actually gel on the down cycle as the bulbs are cooling. With the higher mesh tension, you can decrease the meshcount significantly without effecting opacity. We typically underbase with 180 - 230 meshes. This further reduced the printed ink film thickness, thus shortening your flash times. This is already a really long post, and for that I appologize, but there is alot going on here. The quick summary is: Cut the distance from flash to print surface and up the tension on the screens while increasing mesh count so you are only printing a thin surface layer of ink and not saturating the shirt. You will end up with brighter prints, softer hand, and much, much short flash times. | ||
  
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