Screen Printing - August/September 2017 - 23
FIGURE 1 Look familiar? Flashing too long, at too high a temperature, can spell disaster with the heat-sensitive fabrics in high demand today. Telltale
smoke during the flashing process (left) produces scorch marks (right) that mean a rejected shirt, in addition to lost production time and wasted energy.
(Photographs by Jerome Vieh, with thanks to JW Contract Screen, Covington, Kentucky, for allowing us to shoot in their facility.)
Learn how to increase your production while
reducing cost, heat, and wasted energy.
MARK A. COUDRAY, ASDPT
lash curing has been an essential part of textile and
T-shirt production for over 40 years. Yet, for all its benefits,
flash curing has become a Band-Aid many printers use to correct poor printing practices. It can be a great problem solver
for novice printers: Flashing between colors seems to fix
registration problems, ink build-up on the screens, and other
The problem is, flash curing means heat, and heat is wasted
energy. Besides being expensive, heat also affects mesh tension,
registration, production speed, and the performance of subsequent colors in the print sequence. Overuse of flash curing leads
to heat retention on the platen and the surface of the garment.
The heat can't dissipate into the environment, so it gets transferred to subsequent screens, where it can heat or gel the ink
and lead to image defects. The substrate also absorbs the excess
heat, which can be a disaster with sensitive fabrics. We always
want to use the minimum amount of heat to get the job done.
Unfortunately, new ink systems and the mandate by some
of the major brands to remove PVC from the formulations
have dramatically increased the need for individual flashing
between colors. High solids acrylic (HSA) water-based inks,
for example, require flashing and a cooldown station after
every color. This triples the number of heads required on a
press. A conventional 16-color press now requires 48 heads
plus load and unload stations. The energy usage and high capital cost of such very large oval presses with multiple flashes
is considerable, even with high-volume production orders.
And it's not just the inks: The technical nature of the fabrics we're printing more often today also makes flash curing
a critical control point in production. Materials like moisturewicking polyester, performance apparel, stretch fabrics
with Lycra, low-energy sublimated jerseys, and lightweight
tri-blend yarns demand precise control of heat to achieve
the right results. These substrates often have very narrow
processing windows that result in high shrinkage, sublimation dye migration, or scorching if too much heat is applied or
allowed to build progressively over time. (See Figure 1.)
And remember: Heat is the enemy. The hotter the printroom, the more difficult controlling the surface temperature of
the garment will be. Cold rooms present different challenges,
with wide temperature swings and rapid cooldown of the print
surface. Often, you'll see wide variations in image quality and
color consistency when printing resumes after a press delay. If
the job involves critical color matching, this can be a disaster.
With a better understanding of how flash curing works,
you can apply it more efficiently and effectively in your plant.
It may surprise you to hear that it's possible to achieve flash
times of half a second or less without risking the problems
associated with heat buildup. With a correct flash setup, you
can control surface temperature, dwell time, and the curing
needs of different garment and ink colors, balancing heat
throughout the process while minimizing the time needed to
gel the ink. You can flash at the fastest speed of the press with
little, if any, residual heat passed onto subsequent screens.
You can also save a lot of cash. In many shops, energy
usage can be 10 times more than what is actually needed.
THE INTEGRATED HEAT SYSTEM
Flash curing systems first appeared around 1978. The two most
common heating elements in the early units were cal-rods and
blackbody panels, both of which produce radiant heat. These
first-generation elements are inexpensive to manufacture and
use, but not particularly responsive or accurate.
Panel flashes are still in common use today, mostly with
manual printers and entry-level automatic presses. They are
operated either in a continuous cycle, with the power always
on, or they have proportional timers that surge the unit with
power on a 15- to 30-second cycle. Either way, they are either
power on or power off. (While it's possible to reduce the