Screen Printing - August/September 2017 - 24
FIGURE 2 Presses with quartz flash units and thermo probes that interface with the control panel allow the surface temperature of the platens
to be continuously maintained during a run.
amount of current through the proportional timers, it's not
commonly done with this heat source.)
Blackbody panels produce uniform IR heat when energized.
The amount of energy (or radiance) they emit is expressed in
watts of energy per unit of area (W/sq in. or J/sq cm), sometimes called the watt density of the emitter. The higher the watt
density, the shorter the time needed to cure. Think of high watt
density as the difference between a match and a blowtorch.
The theory of the energy distribution of a blackbody surface
was developed by German theoretical physicist Max Planck
in 1900. Planck's law describes the spectral energy density of
electromagnetic radiation (wavelength of light) a blackbody
emits at a given temperature. In order for the temperature of a
surface to be raised, it must absorb this radiant energy.
Planck defined a blackbody as a surface that completely
absorbs all incident radiation. It also emits radiation at the
maximum possible monochromatic intensity in all directions and at all wavelengths. This means that all colors have
the same intensity of radiance. In other words, the emitted
energy is color-blind - all colors absorb equally.
Planck further established that the radiance at each IR
wavelength is the maximum amount possible for a given temperature. If we used an IR spectrometer to measure this emitted energy and plotted the result, it would follow the Planck
distribution. Planck hypothesized that energy can be absorbed
or emitted only in fixed units of photons of energy. This
absorption does not change and is known as Planck's constant.
What does this mean for us? The key takeaway is that IR
energy emitted by a blackbody will have no visible color, and
so all colors will absorb the energy equally. White will cure at
the same rate as black. This is particularly attractive when you
have heat-sensitive materials like tri-blends and you want all
ink colors to gel at the same rate without overheating the yarn.
Unfortunately, blackbody panels are not particularly
responsive and their cycle times are pretty broad. Temperature is usually controlled by retracting the unit away from
the printing platen. The panel cycles in for the flash and pulls
back after some designated time interval. This is fine for
plastisols printed on cotton, but definitely not satisfactory for
more sophisticated inks and fabrics.
The solution came with the introduction of quartz tube flash
units, which use incandescent tungsten filaments. Think of them
as incandescent light bulbs on steroids. They are highly accurate, instantly responsive, precise, and controllable - but they
are also much more expensive than panel flash units. They can
be controlled with percentage timers, thermostats that measure
bulb temperature, or surface thermo probes that shut the lamps
off when the desired surface temperature has been reached.
Quartz flash units are very popular because they can be
cycled down to a standby temperature and almost instantly energized to the target IR frequency. Because they are highly controllable, we can use Planck's law to precisely calculate how much
energy we need to get the job done. Better yet, they allow the
use of thermo probes to monitor surface temperature continuously and adjust lamp intensity based on the target temperature.
As platen temperatures rise during the run, the thermo probes
reduce the flash cycle time to only raise the surface temperature
to the target level. (See Figure 2.) This is a major advantage
when dealing with temperature-sensitive materials like rayon
(tri-blends), acrylics, and sublimation-prone polyesters.
Quartz units operate in what's known as the near- to
medium-IR region. In flash curing, IR frequencies do the
work and are invisible to us. The light you see emitted from
the bulbs is waste energy and is a function of the operating
temperature of the bulbs. As the temperature of the filament
increases, it begins to emit color - first a deep red, then
FIGURE 3 The ideal flash temperature to generate the medium-wave IR that works
across the range of colors is between 900 and 1100 degrees F. Narrower wavelengths
below 3.4 microns generate more heat that will be more reflective.