A Closer Look at Digital Dye-Sublimation Printing
Some of us may remember when dye-sublimation printers started appearing on the market in the 1980s. They became somewhat popular in the 1990s for color proofing, and small dye-sub photoprinters were sold (or at least marketed) as accessories for the first generations of digital cameras. However, these were not really dye-sublimation printers, as the dye didn’t really sublimate , although the understanding at the time was that they did. In retrospect, these were more correctly called dye-diffusion printers. In today’s printers used for textile printing—among other things—the dyes do indeed sublimate.
 
A key perhaps to understanding dye sublimation is understanding what sublimation is. As you probably recall from high school chemistry or physics, sublimation is the physical process whereby a substance transitions from the solid phase directly to the gas phase without first passing through the liquid phase. The emblematic example of sublimation in action is dry ice, which is frozen carbon dioxide which immediately become a gas at room temperature. Sublimation takes place because of heat, because of pressure, or both.
 
(By the way, sublimation in this context is distinct from the psychological concept of sublimation, which Freud defined as “the process of deflecting sexual instincts into acts of higher social valuation.” As far as I know, this type of sublimation has yet to be applied to a printing technology, although I bet it would liven up press conferences.)
 
The opposite of sublimation is called deposition, the process by which a gas transitions directly into a solid without first becoming a liquid. Examples of this are snowflakes and frost, which is the result of water vapor (a gas) becoming a solid without first condensing into liquid water.
 
How does this work in the context of dye-sublimation printing? As you well know, an ink comprises two basic elements: a colorant, which is a pigment or a dye, and a vehicle, which is a liquid that is used to transport the colorant to the substrate. It’s common to think of dyes as liquids and pigments as solids, but actually both are solids, and the real difference between them is solubility (among many other things). Generally speaking, dyes and dyestuffs are soluble in water and other solvents, while pigments are not.
 
So, in traditional (if we can use that term at this stage) dye-sublimation printing, the ink consists of solid dye particles in a liquid suspension, usually water-based. The dye-sublimation printer transfers the ink to a transfer medium, usually sublimation paper. The sublimation transfer paper has a special coating that will not only accept this ink, but also facilitate its release in the next step of the process. It should also be noted that the image is printed in reverse on the dye sublimation paper.
 
The next step, variously called fixation or outgassing, involves a heat press, which can either be rotary or flatbed. Usually the term heat press refers to flatbed devices used for the fabric equivalent of sheets, while the term calender is used to refer to a fixation unit used with rolls.
 
In the heat press, the printed transfer paper is brought into contact with the fabric that is to be imaged. If you are planning to print on a stretchy material, you may get better results by using a tacky transfer paper that will adhere slightly to the fabric so that it won’t shift during fixing and cause blurring, ghosting, or other imaging imperfections.
 
Here’s the sublime part. Under the heat (around 375° to 410°F, depending on the fabric) and pressure of the heat press or calendering unit, the solid dye on the printed transfer paper is converted to a gas and penetrates (gasses or outgasses) into the fabric. The dye then resolidifies on the fibers of the fabric which makes the colors quite durable, wash-resistant, and colorfast.
 
This process will yield best results on 100-percent (or at the very least predominantly) polyester materials. Why not natural fibers like cottons? The same heat that sublimates the dye also slightly melts the polyester, which is basically a plastic. (Cotton and other natural fibers will not melt when exposed to the heat of a heat press, but will instead burn, which is a bad thing.) By “melting” I don’t mean that the fabric turns into a gooey mess,  but it’s just enough to open up tiny gaps in the polyester fibers, and the gaseous dye then penetrates into these gaps. When the heat is removed, both the dye and the fibers resolidify, fused as one. This explains why dye-sub printing is so durable and colorfast.
 
By the way, if you use a calender rather than a flatbed heat press, you may need to use a protection paper to protect the fabric and the rollers of the calender.
sublimation paper