Digital textile printing is seemingly everywhere, and many sources tout it as a high growth area. New product announcements are also coming thick and fast. But what are the basics of digital textile printing?
Excerpts taken from an article by Richard Romano
 Published: March 7, 2016 in whattheythink.com

If you have been to any major trade show in the industry you know that textile printing is all the rage, and more and more traditional wide-format printing equipment manufacturers are introducing units that will print on fabrics. Soft signage has also been an oft-bruited printing application for the past couple of years and continues to grow. Here is a  general primer on textile printing.

What Can We Print?

First of all, what do we mean by “textile printing”? In general, we can identify three distinct categories:
Sign and display—This category would include banners and posters, trade show graphics, point-of-sale/point-of-purchase (POP/POS) displays, flags, outdoor graphics, and so on. This is also where we would classify “soft signage” in all its myriad forms.
Garment and apparel—This category includes anything that can be worn such as T-shirts, hoodies, caps/hats, uniforms and spiritwear (that is, sports  or other team uniforms), swimwear, towels, bags (OK, bags aren’t worn, but can carry things you wear), and so on.
Décor—Think not just wall coverings, but also upholstery for chairs and sofas, as well as drapes and curtains, bed sheets, coverings for tables and other furniture, even carpeting.
As you can tell, these three categories include a bewildering variety of products (and, yes, there is some overlap of categories), not all of which can be produced on the same equipment. As in general commercial printing, and indeed in wide-format printing, different printing technologies are better suited to certain applications than others.

How Can We Print?

For decades—if not centuries—textile printing was done using analog printing of various kinds. One of the most conspicuous perhaps is screen printing, which emerged in the 1960s as perfect for T-shirts (such as concert tees) and other kinds of garments. At the high end, industrial textile presses are used for continuous sheets of fabric—that is, printed fabrics that come off press in rolls and are then cut and sewn into finished products.
As digital printing has grown and evolved, it has proven capable of handling many of these same textile substrates. When we talk about digital fabric printing today, we are largely—although not exclusively—talking about dye-sublimation. But this is not to exclude other ink technologies. Such as:

Solvent and ecosolvent—The same printers that can be used for traditional wide-format output can be used to print on fabrics, although it may not be the most desirable way of doing it. Solvent printing on fabric requires specially coated textiles in order for the ink to adhere properly. This may not make it suitable for items that will be worn, for example. Still, solvent inks are abrasion-resistant, fade-resistant, waterproof, and long-lasting and durable. Solvent inks are also inexpensive when compared to other inks.
Ultraviolet (UV)—You can also use a UV flatbed to print on fabrics. UV inks can be expensive compared to other inks, and since UV printing essentially lays down a thin polymer film, this can also result in stiff printed fabrics, again not entirely suitable if something is going to be worn, or at least worn comfortably. Still, UV printing is durable and lightfast, as you would expect, UV-cured materials are UV light-resistant.

Latex—Latex printers use water-based inks and can print on a wide variety of coated and uncoated substrates, including textiles. Indeed, latex printer vendors tout soft signage as an ideal application for this ink technology. Latex-printed materials are lightfast and solvent-resistant (so they can be dry-cleaned if necessary), but can produce muted, less-than-vibrant colors, and on some types of porous materials nay not be scratch-resistant.
If you have any of these types of wide-format printers, they can be versatile enough to handle textile printing, if that is an area that a shop wishes to expand into.
Before we look at dye-sublimation, it is worth mentioning that there is a class of textile printers called direct-to-garment printers (these are distinct from the dye-sublimation direct-to-fabric printers we’ll look at shortly). These use a special type of inkjet ink to print directly on pre-made garments such as T-shirts, hoodies, tote bags, hats, and the like. These devices are usually used for short-run printing of these garments, not high-volume industrial production. Unlike dye-sublimation, these work best on cotton substrates.

How Do We Print Using Dye-Sublimation?

Transfer Printing—In this form of dye-sub, currently the most common form of digital dye-sublimation printing, the printer images on a paper that has a coating expressly designed to hold and then later on release (under heat and pressure) the printed image. After printing, the paper is brought into contact with the fabric in (or on) a heat press. The ink on the paper is then “gassed” directly onto the fibers of the substrate. This is the sublimation process where the solid colorant is converted into a gas to penetrate into the fabric. For chemical reasons, polyester fabrics are best-suited to transfer dye-sub printing. (That said, there are different kinds of dye-sub inks that can print on different kinds of natural and synthetic fibers.)

Direct-to Fabric—The next step for dye-sublimation is to take the transfer paper out of the process, for fairly obvious reasons: it’s an expense (about 10 cents a square foot for a decent quality transfer paper) and once transfer is complete it’s waste that needs to be disposed of. So a system that prints directly on the fabric is highly desirable. The problem, however, is that in order to do so, the fabric needs to be pretreated to accept the ink. Historically, pretreated fabrics were expensive and had inconsistent quality, but as dye-sub has taken off, substrate manufacturers have improved the consistency and quality of pretreatments. In direct-to-fabric printing, the ink penetrates further into the fabric than with heat transfer. The result can be less vibrant colors and softer text and images, as well as more show-through on the other side of the fabric. This is why the number one application for direct-to-fabric dye-sub at present is flags, where a high level of show-through is desired. (At present, duplexing with dye-sublimation is impractical, and for two-sided printing, you often need to print the second side on a separate piece of fabric and then sew or otherwise attach it to the first side.) That said, a lot of development is going into direct-to-fabric—both on the equipment/ink and the substrate side—and many of its limitations are starting to fall away. This is one area to keep close tabs on.

How Do We Finish Printed Textiles?

Finishing for textile printing differs in many respects from other kinds of printing, even wide-format printing, but there are some common and some uncommon finishing processes.
Cutting—Cutting is the most basic finishing process in any kind of printing, and in the case of textile printing can involve simply cutting multiple impressions on a roll into individual panels (not unlike paper), to cutting the fabric according to a pattern in order to manufacture a garment.
Calendering—You may think that if you are using a direct-to-fabric system, you can avoid the heat press, but that’s not true. You still need a heat press or calender (despite Autocorrect’s insistence, “calender” is the correct spelling even though you can print textile-based calendars) to fix the ink onto the fabric to make it washproof and otherwise durable and long-lasting.
Sewing—If you’re manufacturing garments, the cut fabric needs to be sewn to form the garment itself. Even if you’re just printing soft signage, you still need some kind of sewing capabilities to hem the edges of the sign, to add pockets for support posts, or to otherwise handle the mounting and display hardware. Traditional sewing machines are often used for these processes, and there is still a fair amount of hand sewing done. Seamstresses are still very much in demand in the soft signage industry. How a banner or sign is going to be displayed or hung will largely define the finishing processes and skills required. Does it need to be stretched over a frame? If it’s an outdoor graphic, are things like post holes or pockets sewn strongly enough to withstand a strong wind? Do the hems of a sign get easily frayed? Will your customer’s nerves get even more frayed?

A Closer Look at Digital Dye-Sublimation Printing

There are a number of different printing technologies used to print fabrics and textiles. One of the most common—and the one that is getting the most attention these days—is dye sublimation. What is dye sublimation printing, and what do you need to take advantage of the technology?

Above , when we look at Textile Printing , we see that dye-sublimation printing has become the go-to technology for digitally printed fabrics. However, it’s not suitable for every fabric, nor is it suitable only for textile printing. Let’s have a closer look at 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 (see below), 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.

Sublime Chemistry

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 paper. The 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 transfer 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.

Beyond Textiles

Although the emerging trend is away from using transfer-based dye-sublimation and toward direct-to-fabric printing, the use of transfer paper does offer one distinct advantage: the final substrate doesn’t have to be fabric. Indeed, the same transfer paper that is used on fabrics can also be used on other materials or objects, like ceramic tiles, commemorative plates, coffee mugs, or other tchotchkes, mementos, and ad specialties. As long as the surface—wood, glass, metal, ceramic, plastic, etc.—is pretreated with a polyester coating, dye sublimation can be used for many of the same applications as UV flatbed printing, or even high-end ceramic printing. The surface or object may also require a post coating and if you want to print mugs or glasses, you will need a “mug press” (a heat press designed specifically for cylindrical objects like mugs), This is not going to be a high-volume production line, but can give a shop a diverse array of products to offer customers.

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