Screen Printing - August/September 2017 - 27

FIGURE 9

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
FIGURE 10
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
When the platens and inks are preheated (FIGURE 9), only a minimal temperature bump
post-flash rollers are available. Some are
from the flash units is needed to gel the ink. FIGURE 10 shows what such a decrease in
heated (which is desirable) and others
flash times can mean to your production.
aren't; all work well.
Mesh selection for the overprint colors
is very important to the success of this approach. You want to
The resin begins to absorb the liquid plasticizer when the temprint colors that will be applied to the underbase through the
perature reaches roughly 180 to 210 degrees F. This means all
finest mesh possible. Normally, I use a 305/34 mesh as a comprowe need to do is reach that temperature in the ink film.
mise between wash durability and the thinnest ink film possible.
Plastisol is thermally sensitive: The colder it is, the stiffer
It takes much more effort to split a thin ink film than a thick one,
the ink and the higher its viscosity. (See Figure 6.) As we heat
and it takes even more effort when the film is partially gelled.
the ink, it softens and the viscosity drops. At 90 degrees F, the
Ink deposited through a mesh with a 40-, 48-, or 55-micron
viscosity stabilizes and remains constant until it reaches 135
thread diameter is very easy to split. Ideally, we could use a
degrees F, where it begins to slowly thicken until it gels. This
mesh with a 27- or 31-micron thread, but with today's ink techinformation will guide you in preheating the platens.
nology, such a very thin ink film would have poor wash durabilPut the press in no-print cycle with the flash units set for a
ity and the print would fade sooner than it normally would.
1- to 2-second exposure, then run the press until the platen temThe print sequence of the overprint colors helps as well.
perature reaches ±145 degrees F. (See Figure 7.) Do this with
Starting with the color with the least area and printing the one
ink in the screens so the hot platens will warm it. Do not start
with the largest overprint area last will minimize ink transfer.
printing or color matching until the ink temperature exceeds 90
The tackiness or stickiness of the ink also comes into play.
degrees F. The goal is to minimize the difference between the
Some pigments are stickier than others. Printing small areas of
temperature of the ink and the screen and the temperature at
high-tack ink will make it more difficult to transfer the ink film.
which it gels.
When you print with the ink prewarmed and the platens
hot, the radiating heat from the platen will almost instantly
SUMMARY
increase the ink film temperature to the platen temperature.
We have covered quite a lot of ground in this article. Having a
This means that to gel the ink, you only need to raise the ink
clear understanding of the flash temperature and the differfilm by another 40 degrees F. It's an insignificant amount and
ences between flash units is the starting point. Investing in
you can do it with a very minor temperature bump versus the
fast, medium-wave IR quartz flash units is well worth it. They
heat blast common in so many shops.
are extremely controllable and will allow you to progressively
The final component is to synchronize the flash cycle to
reduce the flash duration and temperature during the run. If
the press index. Quartz flashes run at a reduced temperature
your press has surface sensing thermo probes, as in Figure
between cycles. This way, a minimum surge of power will get
2, this will happen automatically.
the lamp up to peak emitting temperature in less than half
Preheating your inks and platens and balancing your system
a second. Set your flash cycle to start as soon as the press
to control heat buildup will ensure the highest possible producbegins to index. Reduce the flash time progressively until the
tion with the least energy usage. The chart in Figure 10 can
peak of the flash occurs just before the platen lands in the
help you assess what eliminating those unnecessary seconds
next station. (See Figure 8.)
of flash delays from your print sequence can mean to your
This means the flash will be hottest before the platen is under
production. If your system is balanced, you can completely gel
it. The flash will power down as the platen is locking down
the ink without having to delay the print cycle at all.
AUGUST/SEPTEMBER 2017

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