No ‘one ink’ to do it all
As much as we have become used to the adoption of digital inkjet for the production of printed textiles, it is still a relatively new process in most market segments. The many types of application often require very different qualities and have equally disparate parameters to work with.
The most important factor in any print process is the ink type used, as it forms the bases of color gamut (how much color one can reproduce), fastness properties (how well an ink performs in any circumstance such as under UV light, in wet conditions, exposed to heat, tension and rubbing) and whether it can replace or add to existing traditional processes.
Combinations and Complexity
To understand the complexity of digital inkjet based textile print production, it is important to acknowledge the many different application fields. Visual communication, soft signage and graphics use polyester fabrics; fast fashion and haute couture involves cotton and blends, while sportswear and swimwear employ polyamide, polyester and blends. Interior decoration features material suited to curtains, carpets and upholstery, as do bedlinen, towels and fashion accessories, with their inherently different surfaces.
The combination of the different fabrics and their intended use means that there is no such thing as “one” textile industry. The many niches all have their own idiosyncrasies that define the difference between success or failure, each largely based on the ink’s compatibility with the media. It is also the reason why there are so many ink suppliers with so many formulations to choose from.
Ink types have been formulated to achieve best results on a particular media; often, this is compromised by the mechanical limitations of the printer itself. This is even more relevant for digital inkjet development, where temperature, firing rate, nozzle size and other physical constraints are compensated for with changes in viscosity, surface tension and chemical composition. Logically, analogue inks have been converted into digital equivalents in order to retain their required chemical characteristics. In all cases, the search for one ink type that can apply to all substrates has been the point on the horizon for many.
The different types of ink
Pigmented ink, for the most part, is a water-based solution that has insoluble color particles that adhere to a substrate by means of a binder. Its predominant use is found with cotton and polyester-cotton blends. Since a pigment particle itself has no binding capabilities, an intermediate substance is needed to ‘glue’ the colors to the textile. There are two main approaches. One is to apply a binder to the fabric before or after printing, while the other is to include it within the ink chemistry. Combining these approaches is also possible, providing an optimized effort to give the best fixation result on these fibers. Although, recently, many suppliers have claimed to have a solution in pigment formulation for digital, the initial excitement has settled, as the required qualities, specifically color gamut and rub fastness properties, are not (yet) good enough.
Furthermore, pigment particles are relatively large and tend to clog in the print heads and nozzles. While these particles can be ground to a smaller size, this results in poorer color strength and has a knock-on effect: the greater volume of ink needed to achieve an effective result generates lower print speeds and a higher price per square meter. The recent rise in popularity is due to the availability of new print head technology that keeps the ink in motion through a recirculation system, allowing bigger pigments to be used in formulations that also hold a binder as a component. Concerns remain, however, around color vibrancy, and deep blacks are often a challenge to reproduce.
When working with traditional polyesters, disperse direct ink (or ‘high energy disperse direct ink’) is commonly used. In digital printing, this type is less common and used mostly for outdoor applications, such as flags and banners. Color fixation happens primarily through the application of dry heat, although super-heated steaming is also possible. Although high UV stability is a major benefit with this ink type, fixation must be thorough and performed at the correct, high temperature. Moreover, disperse direct inks are prone to migration. This can be prevented by running a washing cycle after printing and fixating the colorant. In traditional environments this is an accepted process step; in digital, though, it is seen as cumbersome. The particle size, in combination with non-recirculation print heads, has limited the acceptance of disperse direct ink with the digital mainstream.
Dye-sublimation ink (or ‘low energy disperse ink’), is, for most part, a heat transfer process whereby designs are printed on paper first and then applied to the textile under pressure at high temperatures. Contrary to the high energy disperse ink, this type is less prone to migration issues and is therefore more popular as a process, since the washing cycle can be omitted. But also contrary to disperse direct its light fastness is less reliable. This means that that the printed colors will deteriorate, or fade, much faster under the influence of UV light. As a result, dye-sublimation is popular for indoor and short-use visual communication and graphics production, as well as in activewear and T-shirt production.
The strongest benefit of both sublimation and disperse is that it bonds with the fiber and colorants are encapsulated in the fiber, rather than being on top of the fiber or even in a coating. A notable difference between the two, besides the UV stability, is the color strength of dye-sub and, in comparison, the lesser color gamut of disperse direct: the two inksets have different colors. This also means that color management is more difficult when using more than the standard CMYK. Similarly, dye-sub inksets offer additional spot colors that allow the extension of its gamut, but this makes managing colors also more complex.
The production process involving acid ink is different than the relatively easy process of using pigment or disperse/dye-sub ink. Fixation of the colors is through normal temperature steaming. A washing cycle follows the fixation process. An acid ink is in itself not acidic, but the steaming environment allows for the chemical binding of the colorants. Acid ink has been used in traditional processes and the digital variant has largely been developed with the same characteristics in mind. An important consideration is the long fixation time of half an hour, involving expensive high volume steamers. In addition, a pre-treatment of the fabric is necessary. Acid inks are used for fabrics made from polyamide, silk and wool.
Similar to acid inks, the production process with reactive inks is based on steaming and washing. Here too, the traditional process has been transformed into a digital variant that has proven effective for many years already. For both acid and reactive inks, the quality requirements are more than sufficient in today’s digital production process. Reactive inks are used for cotton and linen, silk, wool and viscose products.
Even though not always suited for digital textile print production, for completeness, we should also mention the availability of inks that have not been specifically developed for digital production of textiles, but are sometimes used: UV-curable, solvent, latex and VAT, of which the latter is now being developed as a digital variant of the already existing traditional ink type.
Implications of variety
Following an understanding of the wide variety of combinations of media and ink, we should also consider the different types of print-heads and their specific requirements for a formulation to run correctly. On top of that comes the availability of colorants that define the color gamut of an inkset. A reactive inkset has a different color gamut than a disperse direct, for instance. And, when looking at the application needs, there are yet other layers of complexity in the print production process. One such thing is the density or volume of ink needed to fully cover the fibers. A carpet, for example, needs more ink volume as to penetrate all of the pile, a flag needs more ink to achieve print-through. A delicate silk may need much less ink and a black t-shirt often needs a first layer of opaque white ink so the colors on top will keep their vibrancy.
This emphasizes the importance of having a workflow that can incorporate and adjust to the needs of the any production variables intrinsic to textile. Ultimately, it is about the faithful translation of a digital design into a printed product. But when we understand that color profiling is hugely different for each combination of ink and fabric, it becomes apparent that a printing workflow must contain tools to cater to these differences. When we understand that ink volume will influence the print mode and print speed, a RIP software must be able to compensate for these parameters. And when we understand that similar artwork may may not appear the same automatically on another type of textile, it is imperative that the file handling is sufficient to guarantee faithful results.
This article has been published in Gamut #5 and was written by Roland Biemans, founder/owner of LMNS.
With an international background of research, development and marketing of software, hardware and supplies in the inkjet printing industry, Roland played an important role at different companies adapting and transforming technology for the development of innovative practical solutions. Among these solutions were the first refill sets and bulk ink systems for large format inkjet printers, edible ink, media winding systems, the first ever double-sided digital textile printer, software for controlling engravers, routers and cutters and an award winning contract proofing software. Most of his time was dedicated to the development and sales of software for color correction, color separation, proofing, photo editing and driving inkjet printers in the sign, banner and color reproduction industry.
Roland initiated the digital textile print competence center in Eindhoven, The Netherlands. Early 2013, he became board member of the European Specialist Printing Manufacturers Association (ESMA), and continues to support the association as Technology Partner. The Specialist Graphic Imaging Association (SGIA) awarded Roland with the outstanding service award in recognition of his effort in advancing the industry and its association.
More information about Roland can be found at his LinkedIn page at: linkedin.com/in/rolandbiemans/