Since the demand for digitally created fabrics and carpets is increasing, more sign makers need to learn how to provide their customers with high quality fiber-based imaged products.
Achieving good results is not easy for everyone. It is a much more complex system than ink or toner on paper or plastic.
With both direct and transfer printing the variables controlling color and image quality require some art as well as science to produce customer pleasing products. Here I hope to answer some of your questions about dye sublimation printing and to add some clarity to your understanding of this printing and dyeing process.
A LITTLE HISTORY
Synthetic fibers like polyester and nylon were developed as an alternative to natural fibers like cotton, silk and wool. They are the fabrics of choice, particularly polyester, for most signage applications.
Unlike natural fibers, polyester will not dye with conventional dyes because it has few functional groups or “hooks” on the fiber’s surface for traditional dyes to bond to.
Reactive dyes, for example, react with “hooks” on the surface of cotton to give bright colors with good fastness properties. When exposed to reactive dyes polyester will remain nearly white. The polyester will not react with the dye and it can easily be washed away. For fabrics rich in polyester this is a serious problem and could have limited the acceptance of this fiber in many applications.
As a result the chemists at DuPont and other large chemical companies who pioneered the development of polyester and nylon fibers had to develop a class of dyes and the dyeing processes that would allow the successful coloring of these synthetic fibers.
For some applications they manufactured the fibers with pigments embedded in the fibers themselves. Clearly for printed applications this would not meet the need. So they developed dyes which would “dissolve” or “disperse” in the polyester fibers under heat, moisture and pressure. These dyes are called disperse dyes.
Unlike traditional direct dyes, reactive dyes and acid dyes used on natural fibers do not have the reactive groups needed to bond with natural fibers so they do not dye natural fibers effectively. So these disperse dyes are only used to dye synthetic fibers.
Dye sublimation dyes are a subset of disperse dyes. They are smaller molecule versions of these dyes and share similar chemistry. They differ in that with relatively low levels of heat they are transformed from a solid to a gas (i.e., sublimation) directly without going through the liquid phase. This enables the dye molecules to “dissolve” on a molecular level in the polyester, penetrating well into the fiber to give good rub and wash fastness.
More traditional disperse dyes may be sublimed but require such a high temperature that exposing the polymer surface to this high temperature will damage the fiber. So to avoid this, these dyes are “dissolved” in the polyester by using heat and pressure in the presence of moisture. The heat and moisture allows the nearly insoluble dye to slowly dissolve in the water. The heat and moisture swells the fibers allowing the individual molecules of dye to diffuse into the fiber.
One might ask why bother with the traditional disperse dyes. There are several reasons. The traditional dyes are more permanent than sublimation dyes and for bulk dyeing the fabric can be dyed in a pressure vessel in a dye bath using superheated steam very cost effectively.
One side benefit from using sublimation dyes is they provide a much “greener” dyeing process. Since no water is used there is no waste water containing chemicals and residual dye as is common with traditional disperse dyes, reactive dyes, direct dyes and acid dyes. When heated, the dye simply becomes a gas and diffuses into the polyester fiber.
DIRECT AND TRANSFER PRINTING PROCESSES
There are several different ways to print fabric or carpet with these dyes. Traditionally, dyes used in transfer printing are printed using rotogravure printing technology using an engraved cylinder to pick up the ink and deposit it on transfer paper.
The transfer paper is then placed against the fabric to be printed, and then heat and pressure are applied to sublime the dye so that it can diffuse into the fabric, giving a “dissolved” dye in polyester print.
The inks and paper are designed to control dot gain and allow for a complete transfer to the receptive fabric or carpet. This is in use today for printing onto polyester fabric for flags, apparel, carpets, etc.
Engraving rolls is an expensive and time consuming process and limits printing to a repeat equal to the diameter of the roll. In addition short run printing using this technology is too costly and time consuming to be viable for many short run sign and banner making applications. With digital printing these limitations are overcome in cost, run length and the repeat limitations presented by the diameter of the roll.
Transfer printing offers a very flexible and easier-to-control process for short and long runs but it involves more process steps than direct printing. With transfer printing the substrate is fixed. It is dimensionally stable.
The ink, printer and paper are designed and color calibrated to provide the resolution and amount of ink which will eventually be transferred under controlled conditions to the surface of the fabric or carpet to give the design and colors the customer requires. Each step in the process is carefully controlled to allow the reproduction of the correct colors on the final fabric while maintaining good image quality.
Direct printing is simpler but the ink behavior on each fabric or carpet is different. Instead of a standardized well-designed paper surface, the ink must provide the needed penetration and resolution on a wide variety of surfaces both in texture, depth and surface chemistry.
The degree of penetration of the inks into the fabric during the printing process, the order of printing of the colors and finally the temperature at which each dye color diffuses into the fiber can cause artifacts such as banding to degrade image quality and make color calibration difficult. The substrate is rarely as dimensionally stable as the transfer paper causing issues in maintaining proper transport and alignment of the fabric during printing.
Achieving good results is a function of controlling the variables in the process. Direct printing makes this more difficult.
DIGITAL PROCESSES–TONER AND INKJET
Both toner and inkjet technologies have been applied to dye sublimation printing. Basically the sublimation dyes are pigments which are incorporated into toners or dispersed as water-based pigmented inks. Each has its advantages and disadvantages depending upon the needs of the sign maker.
Liquid toner transfer dye sublimation printing is a very fast process. Medium run carpets or banners can be prepared at low cost and with short delivery times using modified Scotchprint 2000 electrostatic printers with electrostatic paper and commercial heat transfer presses.
Dry toner can also be used in special xerographic printers (without fusers turned on) on transfer paper designed for toners. This approach is limited to small format applications.
For inkjet there are many options for both transfer printing and direct printing. For transfer printing there are both water based and solvent based (i.e., “oil” based) dye sublimation inks available. Water based is the most frequently used.
Here dye sublimation dyes are dispersed in water based inks as pigment dispersions designed to print on transfer paper.
Mimaki JV4 printers are commonly used for this purpose and both nylon and polyester dye sublimation dyes are available for them.
On the desktop, converted Epson printers are being used with water based sublimation dye inks. Dedicated printers are required since the sublimation dye inks and traditional dye or pigmented inks designed for paper or plastic printing are incompatible and will clog print heads if intermixed.
Generally these print heads are not designed for these inks and are not easily cleaned to enable changeover from one type of ink to another. Over time most of these sublimation inks have a tendency to build up in the ink system and cause continued degradation of image quality until the printer fails to function properly. You should expect to replace a sublimation dye converted desktop printer in about one year.
Solvent or oil based inks have been developed for printing onto transfer paper. Mimaki solvent ink printers can print these “oil” based (solvent which does not evaporate) inks. These inks have a big advantage over water based inks in that the dye is dissolved in the solvent and is not present as a less stable pigment dispersion. The stability and reliability of these inks appears to be much better than the water based inks. The “oil” remains with the paper and the dye transfers to the polyester very effectively.
Recently several printer and ink sets have been introduced that print directly on fabric and in some cases provide the heat to sublime the dye on the printer.
The Texpress is such a printer. It even provides a cutter which tracks the side to side movement of the fabric during printing.
The fabric used is critical in the way it accepts the ink as is the transport mechanism which must track the fabric evenly during the printing process. The inks are designed to print on fabric and not on paper and are generally not interchangeable with sublimation transfer inks. Where it meets the needs of the end use, these direct printing devices save steps in the process, labor and floor space for the sign maker.
Here we have tried to give an overview of this complex set of choices available to the sign maker who wants to print fabric or carpet for his customers. As you make your choices ask many questions of the systems providers. Be sure the solution you choose meets the needs of your client’s applications.