The idea of digital printing on textiles has been around for some time. Carpet inkjet printing machines have beenused since the early 1970s. Digital ink jet printing of continuous rolls of textile fabrics was shown at ITMA in 1995. Again at ITMA in 2003, several industrial inkjet printers were introduced to the marketplace which made digital printing on textiles the new industry standard. These new generation machines had much higher outputs, higher resolution printing heads, and more sophisticated textile material handling systems allowing a wide varieof fabrics to be printed.
One reason for the comparatively slow growth of digital printing on textiles may be related to the extreme demands of the textile applications. Although ink-jet printing onto fabric works in fundamentally the same way as any office type ink-jet prints onto paper, fabric has always been inherently more difficult to print due to its flexible nature. The level of flexibility varies from warp to weft and with each degree around the bias, so guiding the fabric under digital printer heads has proven to be very challenging. Other challenges:
- There are many types of synthetic and natural fibers, each with its own ink compatibility characteristics;
- in addition to dealing with a fabric that is stretchable and flexible, it is often a highly porous and textured surface;
- use requirements include light fastness, water fastness (sweat, too) through finishing operations and often outdoor use, heavy wear, abrasion, and cleaning;
- the fabric not only has to look good but to feel good too;
- fabric has much greater absorbency, requiring many times the ink volume compared with printing on papers.
Before any printing is carried out, the designs need to be developed in a digital format that can be read by the printers. Thus, all development has to be based on co-operation between the design software companies, the ink manufacturers and the printing machine developers.
In the face of such odds, digital textile printing is happening. And how! Digital inkjet printing has become one of the most important textile production printing technologies and is, in fact, transforming the industry. It has been influencing new workflows, business plans and creative processes. The opportunities for high-value digital printer applications are so large that many hardware and chemistry vendors are investing heavily in textile and textile-related products and systems. Between 2000 and 2005 digitally printed textile output rose by 300% to 70m square metres. This is still less than 1% of the global market for printed textiles, but Gherzi Research, in a 2008 report, suggests the growth of digital printing on fabrics to be more than 20% per year. This growth is largely driven by the display/signage sector of the market; it is only recently that interior designers, seeking unique solutions for their clients, have been turning to digital printing.
Digital processes have become so advanced that it is becoming very hard to tell digitally printed fabric apart from fabric printed the traditional way – although for my money, they’ll never replicate the artisanal hand crafted quality of hand screened or hand blocked prints, where the human touch is so delightfully evident. The lower energy, water and materials consumption means that more printers are switching to digital as it becomes competitive for shorter runs. Although there are many advantages already to digital printing, the few downsides, such as lower production speeds compared to rotary screen printing and high ink costs are both changing rapidly.
As with traditional screen printing technologies, the variables in digital technologies are as varied as in screen printing, with additional complexity of computer aided technologies requiring changes from the design stage onward. Digital textile printing output is a reflection of the design and color management software (such as Raster Image Processing or RIP) that provides the interface between the design software and the printer, the printer itself, the printing environment, the ink, the fabric, the pre-treatment, the post-treatment and last, but not least, the operator.
This print method is being heavily touted as the “greenest” option. Let’s find out why they make these claims.
In theory, inkjet technology is simple – a printhead ejects a pattern of tiny drops of ink onto a substrate without actually touching it. Dots using different colored inks are combined together to create photo-quality images. There are no screens, no cleanup of print paste, little or no wastage.
In practice, however, it’s a different story. Successful implementation of the technology is very complex. The dots that are ejected are typically sub-micron size, which is much smaller than the diameter of a human hair (70 microns); one square meter of print contains over 20 billion droplets!  They need to be positioned very precisely to achieve resolutions as fine as 1440 x 1440 dots per inch (dpi). Since the inks used must be very fluid so as to not clog the printheads, nanotechnology is a huge part of the ink development. In fact, according to Xennia, a world leader in digital printing inks, “microfluidic deposition systems are a key enabler for nanotechnology”. This precision requires multi-disciplinary skills – a combination of careful design, implementation and operation across physics, fluid mechanics, chemistry and engineering.
There are two general designs of ink jet printers: continuous inkjet (CIJ) and drop-on-demand (DOD). As the names imply, these designs differ in the frequency of generation of droplets.
In continuous ink jet printers, droplets are generated continually with an electric charge imparted to them. As shown schematically in Figure 1, the charged droplets are ejected from a nozzle. Depending upon the nature of the imposed electric field, the charged droplets are either directed to the media for printing, or they are diverted to a recirculation system. Thus, while the droplets are generated continuously, they are directed to the media only when/where a dot is desired. Historically, continuous ink jet printing has enjoyed an advantage over other inkjet technologies in its ability to use inks based on volatile solvents, allowing for rapid drying and aiding adhesion on many substrates. The disadvantages of the technology include relatively low print resolution, very high maintenance requirements and a perception that CIJ is a dirty and environmentally unfriendly technology due to the use of large volumes of volatile solvent-based fluids. Additionally, the requirement that the printed fluid be electrically chargeable limits the applicability of the technique.
FIGURE 1.Continuous ink jet (schematic). Charged droplets leaving the nozzle are directed either toward a substrate or toward an ink recirculation system, depending upon the imposed electric field.
In DOD ink jet printers, droplets are generated only when they are needed. There are two subcategories in DOD jet printers:
- The droplets can be generated by heating the ink to boil off a droplet, called thermal ink jet. Thermal inkjet technology (TIJ) is most used in consumer desktop printers but is also making some inroads into industrial inkjet applications. In this technology, drops are formed by rapidly heating a resistive element in a small chamber containing the ink. The temperature of the resistive element rises to 350-400ºC, causing a thin film of ink above the heater to vaporise into a rapidly expanding bubble, causing a pressure pulse that forces a drop of ink through the nozzle. Ejection of the drop leaves a void in the chamber, which is then filled by replacement fluid in preparation for creation of the next drop. The advantages of thermal inkjet technology include the potential for very small drop sizes and high nozzle density. High nozzle density leads to compact devices, lower printhead costs and the potential for high native print resolution. The disadvantages of the technology are primarily related to limitations of the fluids which can be used. Not only does the fluid have to contain a material that can be vaporised (usually meaning an aqueous or part-aqueous solution) but must withstand the effects of ultra high temperatures. With a poorly designed fluid, these high temperatures can cause a hard coating to form on the resistive element (kogation) which then reduces its efficiency and ultimately the life of the printhead. Also, the high temperature can damage the functionality of the fluid due to the high temperatures reached (as is the case with certain biological fluids and polymers).
- Alternatively, the droplets can be ejected mechanically through the application of an electric stimulation of a piezoelectric crystal (usually lead zirconium titanate) to elicit a deformation. This distortion is used to create a pressure pulse in the ink chamber, which causes a drop to be ejected from the nozzle. This method is shown in Figure 2. Piezo drop-on-demand inkjet technology is currently used for most existing and emerging industrial inkjet applications. In this technology, a piezoelectric crystal (usually lead zirconium titanate) undergoes distortion when an electric field is applied. This distortion is used to create a pressure pulse in the ink chamber, which causes a drop to be ejected from the nozzle. There are many variations of piezo inkjet architectures including tube, edge, face, moving wall and piston, which use different configurations of the piezo crystal and the nozzle. The advantages of piezo inkjet technology include the ability to jet a very wide variety of fluids in a highly controllable manner and the good reliability and long life of the printheads. The main disadvantage is the relatively high cost for the printheads, which limits the applicability of this technology in low cost applications.
FIGURE 2.Piezoelectric drop on demand ink jet (schematic). In a DOD ink jet printer, upon application of a mechanical pulse, the ink chamber is deformed. This results in the ejection of a droplet toward the substrate.
As with screen printing, there are steps other than printing which are often overlooked: the first step in digital printing is the pretreatment of the fabric. Because many chemicals and/or auxiliaries cannot be incorporated into the printing ink, they must be applied to the fabric during the pretreatment. The entire process has to be designed to control bleeding, but also to achieve the hand, color, and fastness required in the finished textile. For basic fabric pretreatment, the elements of this solution can include:
- Antimigrants – To prevent migration of ink and prevent “bleeding.”
- Acids/Alkalis – To support reactions of acid and reactive inks, respectively.
- Urea/Glycols – To increase moisture content of the fabric, giving high, even fixation of the inks.
- “Effects” Chemicals – Vary widely in purpose. Although there are too many effects to mention here, they can include chemicals to improve the brightness of the prints, water and stain repellants, UV absorbers to improve the fabric’s resistance to sunlight, fabric softeners/stiffeners, even antimicrobials to provide resistance to mildew and germs.
Many patented and proprietary formulations for pre treatment exist, ranging from simple formulations of soda ash, alginate and urea to more sophisticated combinations of cationic agents, softeners, polymers and inorganic particulates such as fumed silica. Many of these have been aimed at fashion fabrics such as cotton, silk, nylon and wool. The processing of the fabric during pretreatment is also an important factor in producing a superior finished printed fabric. Fabrics must be crease-free and even in width. Some producers provide fabrics that are backed with removable paper to allow companies with graphic printers that have been retrofitted with textile inks to print fabrics. This paper, and the adhesive that holds it to the fabric, must be properly applied so that the paper can be removed easily from the fabric.
Inks used in digital printing are thinner than those used for traditional printing, so the fabric also needs to be prepared by soaking it in a thickening agent. This agent reacts to moisture by swelling. As soon as a drop of dye touches the pre treated fabric, the thickener will swell up, keeping the dye in its place. Without this agent, the dye would run and bleed on the fabric.
Inkjet inks must be formulated with precise viscosities, consistent surface tension, specific electrical conductivity and temperature response characteristics, and long shelf life without settling or mould-growth. The inks, made up of pigments or dyestuffs of high purity, must be milled to very fine particle size and distributed evenly in solution. In addition, further properties such as adequate wash-, light- and rub-fastness are necessary.
Inkjet inks contain dyes or pigments but like screen printing inks they contain other things too:
- Liquid carriers (water or other solvents)
- Rheology modifiers
- Functional materials
- Adhesion promoters
- Other additives
- Colorants (dyes or pigments)
The inks used in digital printing today have comparable color performance and fastness as compared to traditional screen printing inks. They fall into four general categories:
- Water based – can contain glycol plus pigments or dyes. These inks are designed to run specifically in printers with thermal and piezo-electric print-heads. Dyes used include:
Reactive dyes, particularly suited to cotton, viscose and other cellulosic materia
Acid dyes, used for wool, silk and nylon.
Disperse dyes are used for synthetics like polyester and nylon.
- Pigments (as well as disperse dyes) present a more difficult set of problems for ink makers. Both exist in water as a dispersion of small particles. These inks must be prepared with a high degree of expertise so that the particles will not settle or agglomerate (flocculate) and clog the printheads. The particle size must have an average of 0.5 micrometer and the particle size distribution must be very narrow with more than 99% of the particles smaller than 1 micrometer in order to avoid clogging of the nozzles. The major outstanding problem with the use of pigments in inkjet systems is how best to formulate and apply the resins which are required to bond the pigment particles to the fabric surface. Several different approaches, from coating pigment particles with advanced surfactants, to spraying resin through a separate jet head to screen printing binder over an inkjet printed color have been suggested.
- Solvent based – Solvent-based inks are relatively inexpensive and have the advantage of being able to produce good vivid colors. However, their main ingredients are volatile organic compounds (VOCs) which produce harmful emissions. These inks need to be employed in machines which have ducting to extract the solvents to atmosphere. It is possible to remove the VOC’s using activated carbon filters without ducting to outside the building however you still have to dispose of the solvent laden graphite. Fabrics produced using solvent-based inks have a strong odor. The higher the level of the solvent, the greater the keying, or bonding, with the material’s surface to give a durable finish. Types of solvent range from eco-solvent, low and mild solvent through to hard or full solvent. The term eco-solvent does not necessarily mean less environmentally damaging than conventional solvent, as discussed in the post entitled “Textile Printing and the Environment”.
- Oil based – requires the use of a printer which is compatible; otherwise similar to water and solvent based inks. Oil-based inks are less commonly used, but offer very reliable jetting since the ink does not evaporate.
- UV curable – generally made of synthetic resins which have colored pigments mixed in. Curing is a chemical reaction that includes polymerization and absorption by the fabric. UV inks consist of oligomers, pigments, various additives and photoinitiators (which transfer the liquid oligomers and monomers into solid polymers).
- Phase change – ink begins as a solid and is heated to convert it to a liquid state. While it is in a liquid state, the ink drops are propelled onto the substrate from the impulses of a piezoelectric crystal. Once the ink droplets reach the substrate, another phase change occurs as the ink is cooled and returns to a solid form instantly.
Once you have digitally printed the fabric, you must perform some process to fix the ink. What process this is depends on the type of ink you used. Each dye type needs a specific finishing agent.
Finally, the fabric needs to be washed to remove the excess dye and thickening agents. Fabrics are washed in a number of wash cycles at different temperatures to make the print washfast.
So at the end of this process, you can see that there is no real difference in the amount – or kinds – of chemicals used, except perhaps those lost through wastage. So what exactly are the green claims based on?
The traditional printing industry produces large amounts of waste – both dyes/pastes and water, and it has high energy useage. There are also large space reqirements to operate presses, which produce a lot of noise. In a project sponsored by the European Union’s LIFE Program, an Italian printing company, Stamperia di Lipomo, transferred from conventional printing to digital. They found that these new digital presses lowered water, energy and materials consumption significantly. The following reductions were achieved:
- Production space required by 60%
- Noise by 60%
- Thermal energy usage by 80%
- Wastewater by 60%
- Electricity consumption by 30%
- By-production of waste dyes = eliminated entirely
Digital printing has other advantages, which include:
- Minimal set up costs – short runs and samples are economical – so traditional mill minimums can be avoided. Costs per print are the same for 1 or 1000000.
- There is no down time for set up – the printer is always printing – so there is also increased productivity.
- Faster turnaround time – and very fast design changes. Turnaround time for samples can be reduced from 6 to 8 weeks to a few days.
- Print on demand, dramatically reducing time to market.
- Just-in-time customization or personalization
- Theoretically no limit on number of colors.
- Decreases industrial waste and print loss.
The disadvantages most often cited, that of high cost of inks and shorter printing speeds, are quickly being overcome by the manufacturers.
One concern I have is that of the use of nanotechnology, which seems to be an inextricable part of the equation. Already nanotechnology is enabling manufacturers to offer functional finishes in post processing, such as stain and water repellants, fire retardants, and UV blocking . It is also being used in smart clothing: To harness the energy of the sun, flexible thin film modules are sewn onto clothes. However, since they show clearly when sewn, digital textile printing makes these modules inconspicuous.
The traditional industry still looks at digital textile printing parameters from the context of what it “can’t do,” compared to conventional printing (much of which is already history). For a much smaller group of designers, textile artists, fine artists, costumers, wide-format printers and the like, this technology is much more about what it “can do” to provide to provide products and services the market has never before seen. For these people, textile printing offers parameters not available with conventional printing: unlimited repeat size, tonal graphics, engineered designs that cross several seam lines, quicker samples, customization and short-run production. And the use of the technology is beginning to catch the imagination of more and more textile designers, as they realize that their old reaction to computer generated graphics (dismissive to say the least) is truly outdated. Claire Lui, Print magazine associate editor, points out that in ultra-custom milieus, design and printing become more like art than common manufacturing.
The traditional textile industry needs to understand that, in the same way the Internet is not going to replace the television as a form of entertainment or information, this new digital technology isn’t about replacing existing processes , but rather about leveraging the expanded parameters to offer new niche products and services. And we must remember too that digital printing is not the panacea it’s touted to be for the environment, though it seems to have less of a pollution footprint than traditional screen or rotary printing.