Screen Printing - August/September 2017 - 26
efficient flashing is knowing how to direct and sequence the
transfer of heat.
Whether you are using plastisol, water-based, or silicone
ink, the goal of flashing is to temporarily set the ink so it does
not transfer to the subsequent screens. Here, physics again
come into play. Ink transfer happens because the printed ink
film splits between the substrate surface and the backside of
the following screens. The adhesive force of the ink binding
to these surfaces is greater than the cohesive force holding
the ink film together, so it splits in the middle, with some ink
transferring from the substrate to the back of the next screen.
Our goal is to gel the ink layer to the point where the
cohesive force is greater than the adhesive force, hence no
transfer. We can do several things to help accomplish this.
First, use highly tensioned mesh with an off-contact distance
of 1.5 millimeters (0.06 inches). Mesh tension of 25 newtons per
centimeter is sufficient to minimize surface contact between the
screen and the wet printed ink film. If you can limit the point of
surface contact to the squeegee blade, there will not be enough
force to lift the ink off the garment. While some transfer is inevitable, the vast majority of ink will remain on the garment.
Second, the critical ink film to flash is the underbase,
which has different requirements than the surface or overprint layers. The primary objective with the underbase is to
cause enough ink to penetrate into the garment to provide
an anchor with the knit of the T-shirt. This ensures excellent
washability for many cycles. But at the same time, we want
the thinnest ink deposit possible.
Proper mesh selection is essential in meeting those
requirements. My favorite is a 225/40 "S" thread mesh, which
delivers excellent ink release with minimum squeegee
pressure. It gives you a very thin, opaque layer of ink with
minimum penetration into the garment, which will be very
important once we apply heat to it. Our goal with the flash
is to gel the ink just to the point where it won't transfer. We
definitely do not want to cure it completely because this
creates a whole new set of problems. You know you have
FIGURE 6 Plastisol ink is thermally sensitive. The colder it
is, the higher its viscosity and the more unstable it will be
the right amount of gel when you can lightly touch the ink
surface after flashing with no ink transferring to your hand.
To achieve this with the lowest amount of heat and shortest
time, you need to preload heat into the system - a fancy way of
saying to preheat your platens and warm up the ink. The key to
maximum efficiency is to consider how much heat is actually
needed to gel the ink. In shops that don't understand this, flashes
are too long, garments get saturated with heat, energy is wasted,
and presses must be slowed down to deal with the consequences.
Plastisol is still the most common ink used for T-shirt printing. It cures by fusion of a resin with a liquid plasticizer. Previously, polyvinyl chloride (PVC) was the primary resin; modern
inks have moved away from PVC, but the principle is still the
same. Think of the resin as grains of sugar dropped into a cup
of tea. The solid sugar sinks to the bottom, and as you stir the
tea, the crystals dissolve. The same thing happens with plastisol.
Preheating the platens (FIGURE 7) will allow for quicker flash times. Cycle the press with inks in the screens to preheat the
ink as well, lessening the work the flash unit must do. With quartz units and thermo probes tied to the control panel, the
flashes cycle on and off as necessary (FIGURE 8) to maintain the ideal temperature, making it impossible to overflash.