Screen Printing - August/September 2017 - 25
Manufacturers of quartz flash units use parabolic reflectors (FIGURE 4) to focus the
energy evenly across the platen. Following the inverse square law (FIGURE 5), users
of blackbody panels can achieve faster flash times by decreasing the distance from the
flash to the platen.
E IS ENERGY INTENSITY AND D IS DISTANCE.
red, orange, yellow, and finally white as it reaches very high
temperatures. The shorter the wavelengths emitted (tending
toward white), the more reflective the energy becomes.
Medium-wave IR lamps are much more expensive than
the short-wave tubes used in the least expensive quartz flash
units. Short-range bulbs are much more prone to selective
color reflectance, meaning (no surprise) that white reflects
and black absorbs. The difference is that white can reflect
up to 95 percent of the energy while black can absorb up to
80 percent of it. In other words, it would take 16 times more
energy to cure the white. Obviously, the black ink or the garment would be toast long before the white ink gels.
This is a very important concept. The whiter or yellower
the light looks, the more reflective the IR will be to different colors. This means that if you see white or yellow light
from your lamps, most of the energy is being reflected at the
substrate surface by the ink.
Ideally, we would like to combine the radiance of a blackbody panel with the responsiveness of a quartz lamp. We can't
have both, so we compromise. Medium-wave and fast mediumwave IR provide good absorption across colors. There is some
color sensitivity, but it is tolerable. The frequency of mediumwave IR is between 3.4 (1100 degrees F) and 3.9 (900 degrees
F) microns. The higher the frequency, the lower the temperature and the less it reflects color. (See Figure 3.)
An item of note here: The most efficient transfer of radiant
heat, in a color-blind mode, is around 900 degrees F. This is
one of the reasons that the temperature on blackbody panels
is set so much higher than what plastisol cures at, and it's a
source of confusion for many printers.
Higher temperature settings will not help you. In fact,
the higher the temperature, the more reflective the surface
becomes, and you actually lose efficiency. Setting the unit
below 900 degrees F reduces the emitted energy, so the only
way to speed up the transfer is to lower the distance between
the panel and the surface, as we shall see.
Equipment manufacturers can theoretically pack lots of elements together to get a very high watt density, but with bulbs
costing more than $80 each, it makes sense to use as few as
possible to get the job done. (Not to mention that energy costs
would be doubled with such a design and the life of the bulbs
could potentially be shortened due to the close proximity of
the lamps to one other.) As with most things in screen printing,
the challenge comes down to balancing cost and effectiveness.
To improve the efficiency of the bulbs, manufacturers
commonly back them with parabolic reflectors that help
focus the energy evenly across the printing platen. In order to
get maximum uniformity and heat distribution, each manufacturer has designed their reflectors based on the target watt
density of their flash unit. (See Figure 4.) Each design has a
specific focal distance. If you set your lamps higher, you will
still get even heat, but the intensity will drop off quickly and
your flash times will skyrocket. If you set them too close, the
surface temperature will be very uneven and you will have
heat banding in areas where the energy is being concentrated.
How far should you set the panel or lamp head from the
print surface and what happens when you change this distance?
This is a very important question, because small changes can
make enormous differences in surface temperature.
Energy drop-off follows the inverse square law, the same
formula used in screenrooms to calculate changes in exposure as the light source is moved closer to or farther away
from the emulsion. Shops with fixed-distance LED exposure
units no longer need this calculation to burn their screens,
but for calculating surface energy, it is still very important.
The inverse square law states that energy at the surface varies inversely to the change in the distance. (See Figure 5.) The
temperature rise is measured by multiplying surface energy by
time. It sounds complicated, but it simply means that if you double the distance to the emitter, the amount of energy decreases
by a factor of 4. Conversely, if you cut the distance by half, you
quadruple the amount of energy at the surface. Put another way:
If your flash time is 2 seconds at the current distance, then by
halving the distance, your flash time will drop to half a second.
This is a great thing to know if you are using blackbody
panels and you want to shorten your flash times. Set the surface
temperature to 900 degrees F and then reduce your time according to how much you lower the panel toward the surface.
For quartz tubes with reflectors, however, lowering the
distance will create severe temperature variation. You'll likely
burn the material while having uncured ink right next to the
scorched areas. Increasing the distance with a quartz flash
will also have a compound effect on energy loss due to the
PULLING IT ALL TOGETHER
So with a better understanding of how IR heat behaves, how
do you put the pieces together? The key to high-production,