Learn ways to increase your production while reducing cost, heat, and wasted energy.
When you print with the ink prewarmed and the platens hot, the radiating heat from the platen will almost instantly increase the ink film temperature to the platen temperature. This means that to gel the ink, you only need to raise the ink film by another 40 degrees F. It’s an insignificant amount and you can do it with a very minor temperature bump versus the heat blast common in so many shops.
The final component is to synchronize the flash cycle to the press index. Quartz flashes run at a reduced temperature between cycles. This way, a minimum surge of power will get the lamp up to peak emitting temperature in less than half a second. Set your flash cycle to start as soon as the press begins to index. Reduce the flash time progressively until the peak of the flash occurs just before the platen lands in the next station. (See Figure 8.)
This means the flash will be hottest before the platen is under it. The flash will power down as the platen is locking down under the elements, and the 40-degree F boost you need occurs during the cooldown cycle of the flash. The beauty of this method is that you cannot over flash.
As the now-gelled ink rests under the cooling elements, the surface temperature will remain above 145 degrees F. (See Figure 9.) The printed ink film will stiffen as the press indexes into the next color, getting it closer to being properly gelled and reducing its tendency to split.
No cooling station is required, but you will benefit from rolling the gelled underbase ink film in order to create the smoothest possible surface platform for the overprint colors. A number of post-flash rollers are available. Some are heated (which is desirable) and others aren’t; all work well.
Mesh selection for the overprint colors is very important to the success of this approach. You want to print colors that will be applied to the underbase through the finest mesh possible. Normally, I use a 305/34 mesh as a compromise between wash durability and the thinnest ink film possible. It takes much more effort to split a thin ink film than a thick one, and it takes even more effort when the film is partially gelled. Ink deposited through a mesh with a 40-, 48-, or 55-micron thread diameter is very easy to split. Ideally, we could use a mesh with a 27- or 31-micron thread, but with today’s ink technology, such a very thin ink film would have poor wash durability and the print would fade sooner than it normally would.
The print sequence of the overprint colors helps as well. Starting with the color with the least area and printing the one with the largest overprint area last will minimize ink transfer. The tackiness or stickiness of the ink also comes into play. Some pigments are stickier than others. Printing small areas of high-tack ink will make it more difficult to transfer the ink film.
We have covered quite a lot of ground in this article. Having a clear understanding of the flash temperature and the differences between flash units is the starting point. Investing in fast, medium-wave IR quartz flash units is well worth it. They are extremely controllable and will allow you to progressively reduce the flash duration and temperature during the run. If your press has surface sensing thermo probes, as in Figure 2, this will happen automatically.
Preheating your inks and platens and balancing your system to control heat buildup will ensure the highest possible production with the least energy usage. The chart in Figure 10 can help you assess what eliminating those unnecessary seconds of flash delays from your print sequence can mean to your production. If your system is balanced, you can completely gel the ink without having to delay the print cycle at all.
Read more from Screen Printing's August/September 2017 issue.
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