Digital printing asian dyer (final) asian dyer, nov dec.2006 43-48
Digital Printing: ATool For Demand Activated Textile Printing R. B. CHAVAN Department of Textile Technology Indian Institute of Technology Hauz-Khas, New Delhi 110016 e-mail email@example.comAbstractWide spread acceptance of demand activated manufacturing and just in time marketingconcepts have put tremendous pressure on textile printers. Presently available rotaryscreen printing equipments are often inflexible in terms of quick customer response andshort runs and thus not suitable for mass customization. In the present article an attempthas been made to review critically the technology of digital printing. The technology hasachieved considerable success till the pre-production stage and continuous efforts arebeing made to perfect the technology for production printing. It is envisaged that once thedigital printing technology is perfected, a customer instead of purchasing the printedfabric available in Stores may be able to scan the design on computer, select the designand colour combination or create a new design and feed the information to the digitalprinter and get the printed length of the fabric with a design of her/his choice. What is digital printingThe art and science of decorating fabric with colourful designs is known as textileprinting. Conventional textile printing techniques such as block printing, flat and rotaryscreen printing are known as analog printing where a master image present on block orscreen is reproduced onto textile in the form of print. In analog printing the multicolourdesign effect is obtained by using individual design screen for each colour. In Digitalprinting the design is in the form of electronic file in a computer. The computer is linkedto a suitable machine e.g. inkjet printer. The design is printed onto paper or fabric in theform of image with the help of coloured microscopic dots. The multicolour design effectis produced by superimposing dots of usually four colours viz. cyan, magenta, yellow andblack (CMYK). This means that the master design in the form of electronic data isconverted into analog image without the help of individual design screen/block/roller foreach colour.
Textile printing technology trendsRotary screen and flat bed screen printing are the major textile printing techniquesprevalent presently. The textile printing production technology trends are shown in figure1 Rotary screen-printing is the most popular production technology and is likely to remainso for several years to come. Presently the share of rotary screen printing is 60% Due todecreased production in the West but strong growth in the Far East, where flatbedproduction is predominantly used, has meant that the flatbed share has increased. Thisreflects the cheaper labour costs that render this relatively labour intensive methodeconomical to use. A further reason for the increase in flatbed printing is the reduction inaverage production run length.Trends in global printing marketTextile printing is cyclical business and fashion dependent. The fashion seasons arebecoming shorter resulting in 5-6 fashion forecasts in a year. Customers are demandinggreat variety of colours and unique designs. Consumers want clothes to express theirindividuality in homes and the clothes they wear. Due to these reasons there has to bequick sampling and quick order turnaround. The chances of repeat orders are becomingrare. In addition to this average run lengths are rapidly dropping. Thus the world oftextile printing is rapidly changing. Globalization, quick response and ecology aiming atwaste minimization and reduced environment pollution impose substantial demands onthe different components of the printing process. In short: these demands have commondenominators: flexibility and versatility. In order to meet such market demands theremust be a technology which will facilitate Mass Customization. It is a new concept ofproduction which specializes in short runs as little as one unit in which the customerdictates exactly his/her requirements. It aims of producing unlimited designs ofcustomer’s choice. If one does not mass customize one would lose business in today’smarket.
Digital printing technology supports the present industrial trends: short runs ateconomical cost, quick delivery, exclusive unique and personalized designs. Digitalprinting can also contribute to the ‘green image’ of textiles; the ecological impact isclearly lower compared to conventional printing. Digital printing means flexibility and aquicker response to the market demands. Digital printing is already applied for sampling,strike-offs, short runs and mass customized apparel. It allows the user to bypass theextremely time consuming and expensive screen making process, providing theopportunity for quick changes to colour or design elements prior to printing.The conventional printing requires 7-8 weeks whereas the digital printing requires about2-3 weeks delivering the final printed products (Table 1). In addition to this the changeover from one colour scheme to other and from one design to another is also muchsimpler and less time consuming in case of digital printing. Table I. Time to introduce a new product Component Conventional Digital Colour separation/Design editing 2 weeks 2 weeks Digital fabric samples 2-4 days Screen engraving 1week Strike off 1 week Sample yardage 3-4 weeks Total 7-8 weeks 2-3 weeksDigital printing technologiesThere are various technologies available for digital printing. Among these the mostpopular is ink jet printing technology. The principle of operation involves directingminute droplets of ink, from a nozzle, onto the printing substrate. Although there aredifferent ways of producing the droplets, a common feature is the computer control ofdroplet position on the substrate by their response to high frequency digital electronicsignals. The droplet formation involves the application of a controlled pressure on theliquid ink in its reservoir as it streams into the printing nozzles the ink stream is brokeninto droplets.There are two main technologies applied to ink jet printers, continuous ink jet and dropon demand. Their application can be further subdivided as indicated in Figure 2
Fig. 2 Ink jet technologiesContinuous jetIn this technology, a continuous stream of ink is produced by forcing it through a narrownozzle at a pressure of about 3x105 Pa. The resulting high velocity breaks the ink streaminto droplets. Directional control over the droplets is obtained by inducing an electrostaticcharge on them as they leave the nozzle. The charged droplets then pass through a set oflike-charged plates which repel and deflect the droplets to the gutter (collection reservoir)for recycling (Continuous binary inkjet) or to the required position on the substrate.(Continuous multideflection inkjet). Figure 3 and 4 shows the principle of operation of asingle jet or binary continuous ink jet system and multideflection inkjet systemsFig. 3 Continuous binary inkjet
Fig 4 Continuous multideflection inkjetAir jet deflection In this system the ink droplets are deflected by an air stream. The system has been incommercial use for a number of years for example, the Milliken Millitron. It is mostsuitable for relatively low definition (20-30 dpi) and high ink volume applications suchas for carpet printing. The system is shown in Fig.5Fig.5 Airjet deflectionThe continuous inkjet is the oldest technology. Currently the main direction of researchhas moved to ‘Drop on demand’ digital printing technologiesDrop-on-demand technologyTwo main types of print head technologies are available in this category1. Thermal (or bubble-jet)2 Piezo drop on demand.
In these technologies the pressure applied to the ink reservoir is not continuous, but isonly intermittently applied when a droplet is needed.Thermal ink jet or bubble jet technologyThe bubble jet printer (Canon) uses a small heating element to create pressure droplets ondemand within the ink reservoir. The small quantity of ink present in each nozzle isheated by a resistive heating element actuated by the digital data stream. The ink boilscreating a bubble which forces an equivalent volume of ink droplet through the nozzleand onto the substrate (Figure 6).Thermal ink jets offer low-cost print heads but suffer from reliability and slow speed.Figure 6 Thermal inkjet printer: (a) Bubble jet chamber; (b) bubble formationIn thermal inkjet printers following two variations are availableSide shooter thermal inkjet and Rear shooter thermal inkjet (Fig. 7,8)Fig. 7 Side shooter thermal inkjet Fig.8 Rear shooter thermal inkjetThermal printers are well suited for low-volume printing. The system restricts the use ofbinder containing pigment inks. The major problem with the thermal ink jet is the highnozzle and resistor failure rate resulting from rapid thermal cycling. The hightemperatures cause often decomposition of ink components, which leads to nozzleclogging. Therefore, only thermally stable inks can be used. Poor quality production
results are possible. Finally, compared to piezo-systems the droplet size is larger resultingin a lower resolution. Main players are Canon, Encad, Color Wings, HP, and DirectImaging SystemsPiezo drop on demand technologyThis is one of the simplest ways of generating drops on demand. It makes use of thepiezoelectric effect in which small electronic impulses delivered to suitable crystallinematerials (transducer) causes them to expand. This transducer, incorporated in the inkchamber, enables pressure pulses to be created in the ink. Droplets are generatedintermittently according to the electronic signals received. (Figure 9)Figure 9 Piezo Drop on demand technologySeveral variations exist of the basic mechanism by which crystals of Lead-Zirconium-Titanate (PZT) (transducer). turn electrical signals into mechanicalpressure pulses to produce ink droplets. These are shear mode, bend mode,push mode, squeeze mode and hybrid or “coupled” mode.A number of patented designs and manufacturers exploit these variousmechanisms to differing effect.Fig. 10 Piezo shear mode
Fig. 13 Piezo Squeez modeFig.14. Coupled PiezoThese printers have the advantage of much greater print head life than the thermal-basedsystems (100 times). Piezo inkjet printers are now in use in a number of printers fortextile substrates. A number of companies (e.g. Konica, Mimaki, Epson) are producingand developing wide format piezo printers for printing wide width fabrics.Advantages inherently linked to piezo-systems are:• Ink formulation: as the inks are not heated, the formulation can be less critical andhence the inks are less expensive
• Well suited for high-volume printing (industrial)• Reliability is built-in through design• Uses a wide range of ink formulations (solvent, water, UV curable)• Produces high resolution by using small drop size• Capable of using binder containing pigment inksEssential elements of inkjet printingThe essential elements of a textile inkjet system are:Inkjet printerInkjet printer with one or more inkjet print heads, generates the streams of microscopicink droplets and direct them to the substrate.ComputerComputer system for data processingSoftwareIncluding printer drivers, raster image processing (RIP) and colour management systemsto convert computer-based designs into the electronic signals which control the scanninginkjet head and machine. These systems can also ensure faithful and reproducible resultswith different batches of fabric, and provide a total interface with the other componentsof a digital design, sampling and production environment.Textile substratesOften in the digital world called “media”. Woven or knitted fabrics are suitable for inkjetprinting using appropriate inkjet inks.Fabric pretreatmentUnstable fabrics such as knits and lighter weight woven fabrics frequently need to besupported by a stiffening binder, or temporary lamination to a support paper. One of thefabric precoat machines is shown in Fig. 15Fig. 15 Fabric pre-coat machine for digital printing
Fabrics are pre-treated with various surface active agents, gums and fine particulatecoatings in order to maximize the absorbency and reactivity of the textile substratetowards the inks, while minimizing their spreading to prevent loss of definition andcolour intensity. Many patented and proprietary formulations exist, ranging from simpleformulations of soda ash, alginate and urea to more sophisticated combinations ofcationic 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. 3PInkJet Textiles (Germany) is marketing pretreated fabrics ready for inkjet printing.A partial alternative to the pre-treatment of fabrics is the development of inkjet headtechnologies capable of using much higher viscosity inks. However, the ongoing need forsome special preparation and pre-treatment of fabrics is probably unavoidable.Eventually, this may become as widely accepted if it results in significantly improvedcolour yield and reduced consumption of expensive inkjet inks. Novel preparativemethods such as plasma treatment may also have much to offer in terms of primaryadhesion and dyestuff fastness.Fabric Feeding systemThis system feeds and presents the fabric to the traversing inkjet heads, and ensuresperfect registration and alignment throughout, even for delicate and unstable fabrics suchas knits or fine silks. If required, this machine may also pre-heat, dry or set the printedfabric, before finally rolling-up the output smoothly and with even tension.Fig. 16 Fabric feeding Fabric exitMachine vendors are increasingly focusing their attention on improved fabric feed andtake-up mechanisms, as well as devices such as adhesive printing belts (fitted withwashers to prevent a build-up of printed-through ink). Established screen printingmanufacturers such as Ichinose and Zimmer are at the forefront of such developments.Ichinose uses a conveyor belt to transport and align the textile substrate. The system isschematically shown in figure 17.
Figure 17 Ichinose unit: conveyor belt and dryerAt the entry end the cloth is fixed to the conveyor belt with the help of suitable adhesive.The conveyor belt carrying the fabric gently moves ahead for inkjet printing operation.The print head nozzles are set up right above the carrier belt, and the cloth printed withthe inks sprayed from the head nozzles. This can prevent the inks from bleeding onto thecloth. After printing operation the cloth at the exit end is released from the conveyor belt.The conveyor belt can be cleaned whenever necessaryInkjet inksInk formulations for DP-applications are probably the most difficult problem that must besolved for further penetration of this printing technology in the textile industry. The inkscomprising of pigments or dyestuffs of high purity must be milled to very fine particlesize and particle size distribution. Inkjet inks must be formulated with precise viscosities,consistent surface tension, specific electrical conductivity and temperature responsecharacteristics, and long shelf life without settling or mould-growth. Other importantparameter is colour build up on substrate. The colorants must have very high strength andhigh chroma to achieve a broad colour gamut with a minimum number and amount ofdeposited colorant. In addition, further properties such as adequate wash-, light- and rub-fastness are necessary.Reactive and acid dyesFrom the outset, suppliers of textile inkjet inks were quick to offer products based onreactive and acid dyestuffs. Reactive dyes are particularly suited to cotton, viscose andother cellulosic materials, whereas acid dyes are used for wool, silk and nylon. Both arefully water soluble and relatively easy to formulate for a wide range of inkjet heads,especially the widely installed thermal drop on demand jet types.Pigment colours and disperse dyesDisperse dyes (for polyester and nylon) and pigments present a more difficult set ofproblems for ink maker. Both exist in water as dispersion of small particles. These inks
must be prepared with high degree of expertise so that the particles will not settle oragglomerate (flocculate). The particle size must have an average of 0.5 micrometer andthe particle size distribution must be very narrow with more than 99% of the particlessmaller than 1 micrometer in order to avoid clogging of the nozzlesSince pigment printing accounts for over 50% of all conventional textile printing, it is anattractive target for inkjet developers. Several of the major jet ink producers have recentlylaunched new pigment systems. Although still prone to some problems of handle and rubfastness, they offer excellent wash and light fastness and have the great advantage ofuniversal application to almost all fibres and substrates. The major outstanding problemwith their use in inkjet systems is how best to formulate and apply the resins which arerequired to bond the pigment particles to the fabric surface. Several different approaches,from spraying resin through a separate jet head to screen printing binder over an inkjetprinted colour have been suggested. In the long run, improved resin binder technologyseems likely to prevail, allowing trouble- free formulation and printing from a singleinkjet head for each colour.While reactive and acid dyes will always retain some place within the overall market (forexample, for the brightness of shade, excellent handle and fastness offered by reactivedyes on cellulosics), it seems increasingly likely that disperse and pigment inks willrepresent the way forward for inkjet printing of textiles. Print head and machine designand materials handling arrangements will need to reflect this trend.Spot colours versus process coloursThe inks used in Conventional printing systems are known as spot colours. This meansthe required shade is prepared by mixing appropriate colours before printing. Although itis a skilled job, it allows matching the desired shade as closely as possible. The inks usedin inkjet printing are known as process colours. The desired shade is produced on fabricitself during printing operation. This is achieved by super imposing the dots of limitednumber of colours essentially, cyan, magenta, yellow and black (CMYK). In some casesadditional colours may be used to increase the colour gamut.Fabric post processingPost-treatments are associated with the printing operation; examples are baking, steamingand/or washing. Fig 18 Steam fixation and washing unitsThese processes are similar to those for conventional textile prints, except that theprocess is undertaken with a much smaller batch size, typically a few tens of metres oreven individual sample lengths.
One barrier to new entrants is the need to finish fabrics after printing, in order to developand fix the colours to acceptable industry standards of wash, rub and light fastness,handle and appearance. Often there is a need for final application of flame retardant, soil,stain and crease-resistant finishes. Such processes call for the use of specialized capitalequipment such as steamers, washers, driers, bakers and stenters. Apart from the cost andspace requirements (water, energy, effluent etc.), many new potential users of inkjettechnology have neither the know-how nor inclination to embark upon conventionaltextile processing in this way. Some suppliers of inkjet equipment or suppliers of pre-treated fabrics now offer small desk top steamers capable of handling short samplelengths of printed fabric (typically up to 30 metres). However, these are far from ideal foreven small-scale production and still leave many aspects of fabric finish and performanceuncontrolled, for example, shrinkage and final width.Slow adoption of digital textile printingFollowing issues are responsible for slow commercial adoption of digital textile printing.• Existing machines do not fit the mainstream market needs.• The existing speeds adequate for sample printing but not for bulk production• Availability of printing inks at reasonable cost• Colour matching problems in flat colours• Reproducibility of results from one printer to another printer.• Migration of manufacturing capacity to Asia• Main stream textile printers are geared to low cost mass production business model andlong response time• Niche market has to be build up from scratch• Educating the consumers about the potentialities of digital printingA vision of the futureApplications for digital technologies may be analysed in three categories1. Sampling: This is the traditional application area and this may be expected to continuewith modest growth.2. Bulk production for batches less than 1000 metres. This is the vision of many andinterest is at what point digital technologies can “compete” successfully with screenprinting.3. Mass-customization: The creation of new niche markets for small-medium batches ofprinted textiles for specific customers. It may be possible that garment makers decide tobuy a digital printer and attach it to a laser cutting table. After printing, the fabric couldbe cut single ply using a computerized system and then converted to made-ups.Major inkjet manufacturers are working to resolve the issue of production speed and it ishoped that inkjet printers will be available with a speed to compete with rotary screenprinting. The ITMA 2003 exhibition in Birmingham, UK, was a significant milestone fordigital printing, with 27 companies offering textile digital printing equipment. Many ofthe machines shown were said to print at over 50 m2 per hour, and the Reggiani printerwas said to print at 150m2/hour.The other possibility is .that inkjet printing technology may be used as weavingtechnology where printers may have large number of inkjet printers like looms to carry
out the printing production. In Bangkok a printing unit has 25 Stork Sapphire machinesrun much like a traditional weaving department.ConclusionsDigital printing provides an opportunity to meet the present day market trends of masscustomization. It has established as an acceptable technology for sample production.Among other technology problems speed of printing is the main hurdle incommercialization of technology. Attempts are being made to achieve commerciallyacceptable printing speeds. Till then the practice of combination of digital printing forsampling and rotary screen printing for production will continue. What now seemscertain is that there is sufficient industrial investment and commitment by manufacturersto ensure that commercial ink jet textile printing will become a reality.