Factors enabling industrial digital textile printing

Introduction

Textile production has been transformed in the last 10 years by the introduction of industrial digital printing based on inkjet technology. While there were early forays into digital printing in the 1970s with the Millken Millitron digital carpet printer, the first commercial inkjet textile printers were introduced in the 1990s with early examples like the Stork TruColor Jet Printer (1991) and the Ichinose Image Proofer (1999). As the name of the Ichinose machine indicates, these early machines were aimed firmly at producing samples and proofs of designs, intended for later production on conventional rotary screen printers. For many years there seemed no prospect of digital technology being usable for production of printed textiles in industrial quantities.

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Early in the new century, the introduction of a number of solutions pointed the way to the future as for the first time there were digital machines usable for production, albeit on a small scale and at a relatively high cost. Systems like the DuPont/Artistri 2020, Reggiani DReAM and Monna Lisa enabled high quality proofing of new designs, but also production of high end, low volume products like silk scarves for the high fashion market. The existence of such high value, low volume markets provided an outlet for digital technology, giving an introduction to textile mills of the potential advantages (and pitfalls) of the technology, while enabling inkjet system and component manufacturers to justify further investment in next-generation systems.

This investment has led to a vast improvement in the capability of printing systems and inks, leading to introductions of solutions since around 2010 that are able to rival conventional printing systems in quality, capability and increasingly in throughput and printing cost. In the next sections we will review the developments that have enabled industrial digital textile printing, as well as those that will continue to enable performance and cost improvements over the next few years.

Printheads

The component at the heart of the printing system is the inkjet printheads, and development of improved printheads is a highly important factor enabling industrial printing. The major factors in printhead performance are maximum jetting frequency, number of nozzles, drop volume, jetting straightness and uniformity, operating window and cost. For many years piezo drop-on-demand printheads have given the best compromise in speed, quality, robustness and range of ink types that can be used, and are used in almost all textile applications. Other possible technologies include continuous inkjet, which has been used in the past and maintains some interest, and thermal drop-on-demand, which may yet show some promise in textiles, having been developed by Hewlett Packard and Memjet for industrial applications.

The rise of digital textile machines to industrial applicability has almost entirely been dependent on one printhead up to now – the Kyocera KJ4. The combination of high speed, aqueous compatibility, large nozzle count and greyscale capability with a suitable range of drop sizes for textile printing meant that successful printers could be built around it. These range from scanning machines with one printhead per colour, up to single pass machines with several hundred printheads in total. However, due to the construction of the printhead (and perhaps also due to its dominant market position) the head remains a high cost per nozzle component.

Increasingly alternative printheads are becoming available to system manufacturers, often based on silicon (Si) MEMS (micro-electro-mechanical systems) construction that has become a popular approach to building industrial piezo printheads. The advent of single pass printer architectures has generated a need for printheads with higher nozzle counts, tighter packing densities and smaller drop sizes. This need for miniaturisation fits well with precise feature size control inherent in the photolithographic and micromachining techniques used in MEMS processes. Silicon and silicon oxide provide excellent chemical compatibility with most ink families used in inkjet textile printing. Careful selection of upstream construction materials and bonding epoxies help to push the envelope for applications requiring compatibility with complex crosslinking inks, functional materials and aggressive maintenance fluids. Finally, silicon MEMS manufacturing holds the promise of enjoying the economies of scale so important in the semiconductor industry. As the total number of units shipped grows and printhead manufacturers learn how to take advantage of this, the high fixed cost of operating a MEMS fab can be spread across a larger number of units, lowering the per unit cost of manufacture (and potentially therefore the per nozzle cost of printheads when purchased by system manufacturers). It remains to be seen how rapidly, and with what effect, the adoption of Si-MEMS printheads will progress in the textile market, but it remains a very promising technology.

Ink supply

While often treated as a secondary item, the ink supply system that ensures the ink is delivered to the printheads is vital in ensuring reliability in an industrial production context. While simple in principle, the ink supply is often a source of problems that can be extremely difficult to track down. The ink supply has to maintain the correct ink temperature, pressure and flow rate under varying external conditions, while also preventing particles and other contaminants from reaching the printhead and avoiding chemical interaction and other reliability problems. Importantly, it also needs to be easy to use and refill under production conditions.

Inks are complex chemical fluids with a wide range of possible constituents, including particulates and binder resins in the case of pigmented inks. This makes it very difficult to find materials for the parts of the ink system in contact with the ink that will not interact with that ink chemically. Piping in scanning systems has to be carefully designed to avoid pressure fluctuations that lead to banding in the printed result. It is only continued learning and development of ink supply systems and components that has allowed inkjet printing systems to gain sufficient reliability to be a realistic option for production textile printing.

Automated maintenance

As the nozzle/printhead count in systems increases and the requirement for uptime in production limits the time available for nozzle maintenance, the need for fast, automated maintenance configurations becomes more pressing. In fact in many large systems, manual nozzle maintenance is simply impossible as many of them are inaccessible. Nozzles become compromised due to satellite ink and misting collecting on the printhead faceplate, debris being trapped in non-printing nozzles, vibration leading to ink seepage, dust and fibres from the substrate and other contaminants from the printing environment, air bubbles either being drawn into the nozzle or in suspension in the ink, and ink drying in the nozzle. All of these can cause jetting to be compromised or stopped altogether.

Many systems in production today in textile mills rely on manual nozzle maintenance, and the development of reliable and fast automated maintenance is a significant factor in the continued adoption of inkjet into production textile printing.

Substrate handling and motion systems

Motion systems are required to move the substrate or printheads, or both, in order to scan the entire textile and produce the printed result. An industrial motion system for digital printing, no matter what the configuration, needs to have smoothness and consistency of motion, accuracy of positioning, handling of substrates to ensure dimensional stability during printing, and the suppression of vibrations that can lead to visible print artefacts. There are a number of specific problems faced by motion system designers, as systematic errors in dot placement are highly visible to the human eye and generally undesirable.. A combination of sound mechanical design and (in some cases) compensation for issues using software is required for optimum print quality.

While promoting dimensional stability of textile substrates using ‘sticky rollers’ has been known for many years, the requirement and challenges are more testing for digital printing. The rotary screens in conventional printing act to hold the textile in place, while with inkjet the non-contact nature of the printing provides an additional challenge, which becomes ever more difficult as printing speeds increase. Again, significant development has been required to give good textile handling performance for production, and new and more difficult problems needed to be solved for single pass systems.

Single pass printing

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Single pass printing systems, where the printheads remain stationary in a complete line across the textile roll and the substrate moves beneath them in a continuous manner, allow for greatly increased throughput from a single printing system. Single pass printing systems have productivities that rival rotary screen systems for the first time, with the trade-off of greatly increased cost over scanning systems. A potential issue with single pass printing is the fact that there is no opportunity to use interlacing of multiple print swathes, as is commonly used in scanning printers, to help in masking print defects. Another factor to consider is that with single pass printing there is no opportunity to perform nozzle spitting during a print run – a process that is commonly used in scanning printers to ensure all nozzles continue working correctly. These factors mean that single pass print production is at a higher risk of rejection due to print defects, with the high printing speeds also meaning these print defects can extend over large areas before being recognised.

For these reasons, single pass printing has not been adopted widely so far in production textile printing, with many textile mills choosing to add productivity to their factories by ordering additional scanning machines rather than going down the single pass route. However, some of the largest textile mills have been using single pass systems successfully for several years, and the introduction of new entrants into this market suggests that single pass systems may show larger market penetration from now on.

Software and electronics

Another important area is the printing software that manages the printing system and controls the supply of data to the printheads. Development of powerful and easy to use software is a significant factor in adoption of industrial printers, especially in a production environment like a textile mill. A good user interface allows easy access to the most important controls, while enabling more detailed changes to be made by qualified users.

Meanwhile the image pipeline is responsible for converting an input image file into the data that determines when each nozzle fires as the textile is being printed. This involves a number of steps including colour management (to ensure the printed colours are as expected), screening (to reproduce continuous tonal variations in the best possible way using a matrix of dots), and splitting (deciding which data to send to each printhead depending on the printer configuration). Single pass systems again make huge demands on electronics and software to handle the large data throughputs required and development in this area has been crucial to allow single pass systems to be successful.

Textile-specific software is needed to handle textile design images, including flat and continuous tone designs and separation files. Also required are the acceptance of a wide variety of image file formats from CAD design and screen separation programs, handling of very large image files, support for spot and process colours with expanded process colour sets, colourway variations of designs, real time image repeating, and many more functions. Screen simulation features bridge the gap between digital and screen-printed fabrics, especially if both techniques are still to be used. Several software vendors have incorporated features useful in simulating and matching to screen printed production fabric, such as simulating screen resolution/mesh size, colour mixing and overprinting, colour trapping, and incorporating gradation curves for tonal separations. 

Another area where software has a part to play is in the automated recognition and compensation of print defects. Advanced vision and analysis systems have the potential to be able to recognise a wide variety of print defects (much of this work still relies on the human eye in many printing applications, including textiles). Automation of defect recognition promises to be faster, more objective and more reliable, while also enabling rapid response to the defect by triggering maintenance action, reprinting a job and even using nozzle compensation schemes where a neighbouring or backup nozzle is pressed into service to replace or hide a missing nozzle.

Inks

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Inks is another area which has shown rapid advancement over the last few years, with further development required and expected to deliver all of the possibilities required by textile printers. Reactive dye inks have shown advancements in colour performance and reliability, while sublimation inks have transformed the way textiles are decorated for sportswear and soft signage applications. An area requiring further development in order to truly tap the potential of digital printing, particularly in home furnishings, is the availability of pigmented inks with good colour, feel and fastness performance at a reasonable cost per metre. This is currently a significant gap in the market, but ink companies are developing technology in this area, and it is expected that further advances in performance, and reduction in cost, will occur over the next few years.

The future

A number of technological factors are not yet significant in production textile printing, but may well become so in the next few years, with the potential to make further inroads for digital technology into textile (and garment) production.

Digital finishing

Digital finishing – using inkjet or other technology to deposit functional materials as part of the production process is an area of niche interest currently, but is expected to become more important in the future. Companies and institutions have demonstrated a variety of materials with functions like conductivity, water repellence, UV and or IR blocking, etc. There is the potential to create elaborate ‘smart’ fabrics and garments using the ability of digital deposition to deposit the functions only where they are needed, but this requires a rethink of the normal textile supply chain and is at an early stage of implementation currently.

Digital dyeing

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 Another area that has been expected to show growth is in digital dyeing – using inkjet technology to colour textiles a single colour. This is counterintuitive, as inkjet is typical used for (and is arguably best at) creating fine patterns, but the potential for this technology is enormous. Dyeing is one of the most environmentally damaging processes in textiles, using a huge amount of water per kilo of fabric. If inkjet technology can be used instead, the water and power savings are highly significant. Current implementations of inkjet technology may not be ideal for this application, as they are aimed at creation of patterns. Decorating solid areas is a weakness, as it readily exposes print defects that are not noticeable in printed patterns. Also the total coverage capability for creating rich solid colours is limited. One technology that has started to see adoption is digital thread dyeing, where threads are printed on as they pass into an embroidery or other machine, allowing colours to be controlled in a novel way.

New deposition technologies

Both digital finishing and digital dyeing may in fact benefit from new deposition technologies being developed. These are digitally addressable like inkjet but designed to deposit much larger quantities of material, giving greater coverage and the capability to deposit materials that inkjet struggles with (high viscosities, corrosive materials, large particulates, etc.). Companies like The Technology Partnership, Alchemie Technology and Archipelago Technologies have all shown digital deposition technologies aimed at deposition applications where resolution of fine detail is less important, all based on different electro-mechanical configurations that enable ejection of materials under partial or complete digital control. In the future we may see these technologies being adopted and enabling further forays into the world of textile production, with possibilities that are endlessly exciting.

Tim Phillips, IMI Europe & Catenary Solutions

This article was originally published in Digital Textile magazine, where Tim Phillips is Consulting Editor.