Digital Textile Printing — Too Many Choices

The use of synthetic fabrics printed by dye sublimation inkjet technology has exploded as fabric has both replaced paper and plastics in many applications, and has provided opportunities for new applications. New dedicated fabric printers as well as modifications to standard printers enabling them to produce these products are proliferating. A lot of confusion results from this surge of activity. This is a new world of printing for many wide-format print providers. Some have dabbled in this technology in the past, but now it is going “mainstream” with new and exciting approaches to producing fabric prints. Here we will try to help you understand some of the choices and try to clarify some of the confusion.

TRANSFER VS. DIRECT PRINT

There are two ways to produce printed fabric by dye sublimation technology: 1) print onto transfer paper designed for the ink and transfer it to the fabric on a heat press, or; 2) print directly onto the fabric using an inkjet printer with dye sublimation ink followed by a heat treatment to fix the print. There are pros and cons to each approach; and to complicate matters, there are at least three different types of inks and three classes of dyes in those inks. Oh what fun!

Let’s start with the pros and cons of the transfer process. There is clearly more labor involved with a two-step transfer process vs. a one-step process on a direct printer with an integrated heating/fixing system. If you print onto paper and have a printing problem like a blocked nozzle, or some other quality problem, you only lose the ink and paper and some time. If you print directly on the fabric you lose the ink and expensive fabric. This can be a significant issue depending on the fabric being used. When transferring from paper to the fabric you can have defects as well like non-uniform transfer (non-uniform heating) or slippage of the fabric against the paper distorting the print.

One of the advantages of printing on paper first (in addition to being able to check for print defects before risking the fabric), is the ability to separate the printing process from the variations in substrate. The ability to control dot shape and size on the paper is the same each time. Color curves on the paper will be much more reproducible than on the variety of fabrics being printed.

The type of fabrics available varies widely, depending on thickness, weight, fiber content and weave. Each fabric being used needs its own color calibration to achieve the desired color match. Printing directly results in deeper penetration of the dye into the fabric prior to fixing. This may be good or it may be bad depending on the end use. With transfer, the dye stays closer to the surface of the fabric. Penetration of the dye is dictated by the dye used, the fabric and transfer conditions. This can be complex as can direct printing.

THE DYES MORE CHOICES
Dye selection is often made by the printer manufacturer. Low-energy dyes are the least expensive, the least difficult to transfer since they require less heat to sublime, and they are less stable once fixed, especially in light fastness. High energy dyes are more expensive, require more energy to transfer but are also more stable.

The most stable are disperse dyes which do not sublime and must be printed directly onto the fabric. Their fixing may require higher temperature with moisture to effectively diffuse the dye into the fiber. These dyes are similar to the dyes used commercially in the dying of synthetic fibers. They are the most stable dyes by far.

In addition to the dyes some ink providers add UV absorbers to the ink to try to improve the light stability of the dyes. UV absorbers may yellow with time, so one should be cautious and choose the ink systems which best suits the application for which the printed fabric is intended. Remember, none of these dye inks are as stable to light as pigment inks.

MORE CHOICES
Disperse dyes are printed direct and steam-treated offline for the best results. The moisture helps the dye particles dissolve and diffuse into the fiber. The ink vehicle, the liquid part, is generally mostly water for disperse dyes. They are ground and suspended in the ink vehicle like pigments. Sublimation dyes may be dispersed as particles in water based inks or dissolved in solvent or oil based inks. The water-based inks can be transferred or printed directly. Today this is probably the most common type of ink used in desktop as well as wide-format textile printers.

Dedicated textile printers using “sticky belt” media transport systems hold the fabric straight and tight while printing, rather than simply stretching the fabric tightly between two rollers. These “sticky belts” are similar to the techniques used in traditional screen printing. Several companies now make them available and they are being used very successfully with water based inks.

For grand format inkjet textile printing solvent-based inks have become more and more popular. This enables the user to retrofit an existing solvent-based vinyl printer which uses pigmented inks to one which uses similar compatible dye sublimation solvent inks for printing directly onto textiles. Of course dedicated solvent based fabric printers are now available from several companies. Printers are also available that have both options on the same printer, allowing the user to quickly switch back and forth between vinyl printing and fabric printing.

Oil-based inks are less commonly used, but offer very reliable jetting since the ink does not evaporate. The inks are used for transfer printing and the oil remains in the transfer paper and only the dye sublimes onto the fabric in the post printing heat treatment.

THE HEAT TREATMENT A CRITICAL STEP IN THE PROCESS
Whichever dye or ink is selected, and whether transfer or direct processes are used, getting the heat treatment correct is critical to the proper fixing of the dye into the fiber of the fabric. You can always tell when the treatment is not right when the dye rubs off or washes out of the fabric.

The mechanism of fixing is the diffusion of dye molecules into the spaces within the fibers as they open up under heat (heat and moisture with disperse dyes). Dwell time and temperature are critical to this process. In transfer printing you need pressure as well, and must heat the paper uniformly and with sufficient heat and dwell time to sublime the dye so that it will transfer properly. Normally, the ink or printer manufacturer will recommend appropriate transfer or fixing conditions. Trying to print too fast or lowering the temperature may result in incomplete fixing.

WHAT ABOUT SAFETY?
Clearly if you are exposing a solvent-printed fabric to high temperature you must take care not to accidentally ignite the solvent fumes. But there are safety concerns even with water-based inks as there are glycol co-solvents being evaporated in the transfer and fixing of the fabric.

The fumes and solvents may be very significant in the fixing process. This is especially true when printing at high volumes as with a grand-format printer. It is advisable in all cases to use some method to capture fumes and ink aerosols with a carbon bed scrubber or other air filtering system to remove these chemicals from the vicinity of the printer.

In addition, all inks when jetted form some aerosols which should not be inhaled since they contain both solvents (or co-solvents in water-based inks) and dyes which can be harmful to humans. Also, with high-throughput printers the amount of organic material which may condense in and on the printer may cause damage to the printer itself.

CONCLUSION
Clearly we are embarking on a new wave of fabric printing in the print-for-pay marketplace. As with most new technologies, there is much to learn before selecting the equipment and materials required to provide the right solution for your particular applications. Study the options and learn as much as you can about the technology before choosing. Then enjoy all the opportunities that these new capabilities can provide.