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Thanks to digital imaging and the technology of the heat-transferred print, anyone can now own a t-shirt, coffee cup, or hat that says anything he wants it to. Screen-printing and offset lithography, the primary alternatives to heat-transferred prints, are laborious processes, messy with ink and odorous chemicals. Making a heat-transferred print, however, is relatively tidy and quick, accessible to everyone with a computer and a printer, and leaves only paper behind as waste.
There are two ways to make a heat-transferred print. Both require inks, a source of heat, and a receptive surface, or substrate, but they vary in their details. One is a topical transfer, which places the ink on top of the fabric. The other, called sublimation printing, makes the ink part of the fabric through a chemical reaction.
An iron-on applied to a t-shirt is a topical heat-transferred print at its simplest. The iron-on is a digital image printed on special transfer paper, which is coated with a clear film. The iron applied to the back of the transfer paper softens the clear film so that it, along with the image, stick to the shirt. In commercial settings, a machine, called a heat transfer press, is used instead of an iron. A heat transfer press exerts higher heat and greater, more evenly distributed pressure. The press may be either a platen, which consists of a flat bed with a hinged lid, or a roller press. The use of either creates a tighter bond between fabric and film than an iron does. Even so, the inks that compose the image link to the fibers of fabric only on the surface, and some flaking and loss of ink will occur over time.
Sublimation printing works by using particular inks that, when exposed to pressure and heat, become a part of the fabric — the inks are in the fabric rather than on it. The word sublimation comes from the Latin meaning beyond the threshold suggesting that which goes beyond the physical or palpable. Sublimation is a chemical reaction in which a substance transforms from a solid to a gas, skipping the liquid state. In a heat-transferred print produced with sublimation inks, the substrate and the transfer paper pass through a press. Under intense heat of around 400 degrees Fahrenheit (about 204 degrees Celsius), the inks sublimate and, as gas, permeate the substrate.
Not all substrates work with sublimation. Because the inks used in this way of making a heat-transferred print are largely water-insoluble, they form bonds with similarly water-insoluble materials, like polyester and other synthetics. Objects not made of synthetic materials, like ceramic coffee mugs, can, however, receive images transferred by sublimation if they are first coated with a polymer film. Things like this, however, require specially designed presses and generally can't be accomplished with existing tools at home.
@Terrificli -- I remember those well and there was a good way to minimize the damage to those decals. All you had to do was let them air dry after they ran through the washer. The heat generated by dryers is what is harmful to those decals, but washing them and letting them air dry kept them looking great for a long time.
Still, the sublimation method results in a design that will hold up a lot better. Most of the printed tee shirts you see these days were produced by that method.
Remember back in the 1970s when those heat transfer decals were very popular? You had whole stores set up so that people could pick out both the images they wanted and the shirts they wanted them on.
The problem with those things was that they looked great for a time, but several washes would fade the transfer to the point where it was almost impossible to see or the transfer would get ripped and destroy the whole thing in short order.
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