Heat transfer print: what is it?

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Heat transfer printing allows for easy customization of items like t-shirts and coffee cups. Iron-on transfers can be made using either topical transfer or sublimation printing, with the latter creating a chemical bond between ink and fabric. Sublimation printing works best on synthetic materials, but can also be used on non-synthetic items with a polymer coating. Commercial settings use a heat transfer press for better results.

Thanks to digital imaging and heat transfer printing technology, anyone can now own a t-shirt, coffee cup or hat that says anything they want. Screen printing and offset lithography, the main alternatives to heat transfer prints, are laborious processes, messy with ink and smelly chemicals. Making an iron-on transfer, however, is relatively neat and quick, accessible to anyone with a computer and printer, and leaves nothing but paper as waste.

There are two ways to make an iron-on transfer. Both require inks, a heat source and a receptive surface, or substrate, but vary in the details. One is a topical transfer, which places 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 transfer print in its simplest form. Iron-on is a digital image printed on special transfer paper, coated with a transparent film. The iron applied to the back of the transfer paper softens the transparent film so that, together with the image, it adheres to the shirt. In commercial settings, a machine, called a heat transfer press, is used instead of an iron. A heat transfer press exerts more heat and pressure that is greater and more evenly distributed. The press can be a platen, which consists of a flat bed with a hinged lid, or a roller press. Using both creates a tighter bond between fabric and film than an iron. Even so, the inks that make up the image bond with the fibers of the fabric only on the surface, and ink flaking and leakage will occur over time.

Sublimation printing works by using special inks which, when exposed to pressure and heat, become 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 is beyond the physical or palpable. Sublimation is a chemical reaction in which a substance changes from a solid to a gas, skipping the liquid state. In a thermal transfer print produced with sublimation inks, the substrate and transfer paper go through a press. Under intense heat of about 400 degrees Fahrenheit (about 204 degrees Celsius), the inks sublimate and, like gases, permeate the substrate.

Not all substrates work with sublimation. Because the inks used in this way to make a heat transfer print are largely insoluble in water, they form bonds with materials that are also insoluble in water, such as polyester and other synthetics. Non-synthetic items, such as ceramic coffee mugs, can still receive sublimation-transfer images if they are first coated with a polymer film. Things like this, however, require specially designed presses and generally cannot be made with existing tools at home.

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