Digital printing asian dyer extended version)Document Transcript
Digital Printing: ATool For Demand ActivatedTextile 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 timemarketing concepts have put tremendous pressure on textile printers. Presentlyavailable rotary screen printing equipments are often inflexible in terms of quickcustomer response and short runs and thus not suitable for mass customization.In the present article an attempt has been made to review critically thetechnology of digital printing. The technology has achieved considerable successtill the pre-production stage and continuous efforts are being made to perfect thetechnology for production printing. It is envisaged that once the digital printingtechnology is perfected, a customer instead of purchasing the printed fabricavailable in Stores may be able to scan the design on computer, select thedesign and colour combination or create a new design and feed the informationto the digital printer and get the printed length of the fabric with a design ofher/his choice.Textile printing technology trendsRotary screen and flat bed screen printing are the major textile printingtechniques prevalent presently. The textile printing production technology trendsare shown in figure 1 and world production share in Table 1 Fig. 1 Production Technology Trends
Table 1. World production share Rotary screen 60% printing Automatic Flat Bed 18% Other methods 22% Rotary screen-printing is the most popular production technology and is likely toremain so for several years to come. Presently the share of rotary screen printingis 60% Due to decreased production in the West but strong growth in the FarEast, where flatbed production is predominantly used, has meant that the flatbedshare has increased. This reflects the cheaper labour costs that render thisrelatively labour intensive method economical to use. A further reason for theincrease in flatbed printing is the reduction in average production run length.Trends in global printing marketTextile printing is cyclical business and fashion dependent. The fashion seasonsare becoming shorter resulting in 5-6 fashion forecasts in a year. Customers aredemanding great variety of colours and unique designs. Consumers want clothesto express their individuality in homes and the clothes they wear. Due to thesereasons there has to be quick sampling and quick order turnaround. The chancesof repeat orders are becoming rare. In addition to this average run lengths arerapidly dropping. Thus the world of textile printing is rapidly changing.Globalization, quick response and ecology aiming at waste minimization andreduced environment pollution impose substantial demands on the differentcomponents of the printing process. In short: these demands have commondenominators: flexibility and versatility. In order to meet such market demandsthere must be a technology which will facilitate Mass Customization. It is a newconcept of production which specializes in short runs as little as one unit in whichthe customer dictates exactly his/her requirements. It aims of producing unlimiteddesigns of customer’s choice. If one does not mass customize one would losebusiness in today’s market.Digital printing technology supports the present industrial trends: short runs ateconomical cost, quick delivery, exclusive unique and personalized designs.Digital printing can also contribute to the ‘green image’ of textiles; the ecologicalimpact is clearly lower compared to conventional printing. Digital printing meansflexibility and a quicker response to the market demands. Digital printing isalready applied for sampling, strike-offs, short runs and mass customizedapparel. It allows the user to bypass the extremely time consuming andexpensive screen making process, providing the opportunity for quick changes tocolour or design elements prior to printing.The conventional printing requires 7-8 weeks whereas the digital printing requiresabout 2-3 weeks delivering the final printed products (Table 1). In addition to thisthe change over from one colour scheme to other and from one design toanother is also much simpler and less time consuming in case of digital printing.
Table I. Time to introduce a new product Component Conventional Digital Colour separation/Design 2 weeks editing Digital fabric samples 2-4 days Screen engraving 1week Strike off 1 week Sample yardage 3-4 weeks Total 7-8 weeks 2-3 weeks 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 androtary screen printing are known as analog printing where a master imagepresent on block or screen is reproduced onto textile in the form of print. Inanalog printing the multicolour design effect is obtained by using individualdesign screen for each colour. In Digital printing the design is in the form ofelectronic file in a computer. The computer is linked to a suitable machine e.g.inkjet printer. The design is printed onto paper or fabric in the form of image withthe help of coloured microscopic dots. The multicolour design effect is producedby 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 designscreen/block/roller for each colour.Advantages of digital printing1. Digital printing requires minimal press setup and has multicolor registrationbuilt-in to its system. This eliminates many of the time consuming preparatoryprocesses like colour separation, design screen making. It thus permits quickresponse and just-in time print delivery.2. In digital printing the colour scheme of the design or the total design can bechanged “on-the-fly” i.e. while production printing (without stopping theproduction) thus providing variable data, personalization, and customization.3. Most digital printing technologies are non-contact printing which permitsprinting of substrates without touching or disturbing them. This eliminates imagedistortion encountered in some analog processes such as screen printing. It alsodoes not require as aggressive substrate hold down methods which can distort ordamage some delicate substrates like silk or knotted fabrics.4. Digital technologies can print proofing, sample and short runs more costeffectively than analog methods. Digital color printing processes offer a range of
color processes including 3 color process (CYM), 4 color process (CYMK), 5,6,7& 8 extended gamut color options in addition to some spot colors. These matchgrowing market demand for full color.5. Digital printing does not use film masters, stencils, screens or plates. Itrequires much less space for archiving text and images than analog printingmethods require.6. Generally, digital printing uses less hazardous chemicals, produces less wasteand results in less negative environmental impact than analog technologies.7. Digital printing is employing sophisticated color matching and calibrationtechnology to produce accurate color matching.8. Digital files are usually easier and quicker to edit and modify than analogphotographic images. 9. One can readily convert analog images and text to digital with scanning andoptical character reading (OCR) software.10. Digital files are easy to transport and communicate. One can send a digitalfile to any digital printer on the planet within seconds. This permits distribution ofdesign to many locations for quick response printing. Industries are adoptingdigitally generated and communicated art and print copy.disadvantages of digital printing1.Most digital technologies have slower throughput as compared with mostpopular rotary or automatic flat bed screen analg printing.2. Digital printing will often cost more than analog printing for longer print runs.3. It often requires specially prepared and coated substrates.4 Most digital printing technologies deposit very thin ink or toner layers. Theselimits applications requiring thicker deposits, resulting in slower operation.5.Digital inks and toners are limited in colours and carry high price tags.6. Digital printing inks are transparent. Therefore printing on black or other darkshade dyed fabric is not possible.7 This is new technology which requires investment for equipment as well astraining.Advantages of analog printing1.Analog print technologies print many multiple copies quickly and inexpensively.2. Analog printing usually does not require expensive coated substrate to printsatisfactory images as mostdigital printing does.3. Its inks do not require the high degree of refinement in terms of particle size andparticle size distribution. Most analog inks cost less than digital inks.4. Analog screen printing provides a wide range of single pass ink deposition thicknesses.5. Screen printing can print opaque inks which cover dark substrate surfaces.
6.Analog printing can print spot colors. Printers can maintain their own color“kitchens” from which they can match virtually any color.7. Analog methods are existing technologies with existing installed base presses, trainedoperators and established markets and customers.Disadvantages of analog printing1. Analog printing permits only very limited variable data printing, such as letterpressnumbering.2. Analog methods require prepress set up and screen preparation bebefore printing.3. Generally, these types of printing are not cost effective for very short run printing andproofing.4. Analog printing can generate significant waste ink, chemical exposure and deleteriousenvironmental impact. 5. These printing methods use costly film for screen exposure. The archiving of films,plates and screens demand considerable cataloguing, storage space and furniture. Inaddition, these films deteriorate with age.6. Images are limited to the size of the screen image area. Larger prints require that onerepeat the pattern and that the design permits seamless connection of repeated patterns.Digital and analog printing comparisonA comparison between conventional and digital, printing is outlined in table 1.Table 1; comparison between conventional and DP printingConventional printing Digital printing Analog print uses master image in the Digital printing bypasses the use ofform of screens, blocks, design rollers screens. Elimination of screens providesetc. advantage in sampling and small runs at economical cost. The master design in the form of photograph or any design can be scanned and printed onto textiles.Speed 30-80 m/min. 1-5 min/minPretreatment not necessary. Whereas post Pre and post treatment necessarytreatment necessary .dpi,125 but 225 max. . dpi >200, upto 1440Screen costs, engraving, washing, Storage No screens storage No colour kitchen Aqueous pastes made up on site in large Special inks in small canisters. (Usually lbatches litre capacity)
Contact with fabric . Non-contact with fabric .. Half-tones no problem .Usually designs mis registration a set up Instant registration Strike-off (sample) may be different from Strike-offs on bulk machinebulkHalf-tones shades are difficult Half-tones no problemPaper printing versus textile printing Inkjet textile printing is quite differentcompared to printing on paper. • Compared to paper printers the amounts of ink required to be fired from textile ink jet printer are several orders of magnitude higher due to the much greater absorbency of textile compared to paper. This difference has important implications in speed, drop size, drop frequency, and nozzle life for ink jet printing of fabrics, • Issues of image quality are also vastly different in the two types of printing; the colour gamut required for textile printing is greater than the gamut traditionally obtained in printing on paper. • The number of picks and ends in fabrics will impose a structure on the image that will limit the resolution that can be achieved without regard to the printers capability. It has been estimated that with spot colour a resolution of 200 dpi will give very good image quality in printed fabrics, but process colour may require in excess of 360 dpi. • Demands for fastness properties are quite different for textiles in contrast to paper printing, textile printing demands in most cases nigh fastness levels, particularly wash, abrasion, light, rubbing fastness and crock resistance; • Digital printing on textile deals with different types of fibres (natural, synthetic), yarn type and fabrics ( weight, thickness, stretchable, flexible, often highly porous and textured surface). These differences in paper and fabric as substrates for ink-jet printing and the different approaches taken to achieve image quality have important implications in the design of print engines for textile printing.Digital printing technologiesThere are various technologies available for digital printing. Among these themost popular is ink jet printing technology. The principle of operation involvesdirecting minute droplets of ink, from a nozzle, onto the printing substrate.Although there are different ways of producing the droplets, a common feature isthe computer control of droplet position on the substrate by their response to high
frequency digital electronic signals. The droplet formation involves the applicationof a controlled pressure on the liquid ink in its reservoir as it streams into theprinting nozzles the ink stream is broken into droplets.There are two main technologies applied to ink jet printers, continuous ink jet anddrop on demand. Their application can be further subdivided as indicated inFigure 2 Fig. 2 Ink jet technologiesEssentially, all jet-print systems put colour on the substrate in the same way;however, the method of drop generation and their route to the fabric can vary. Asimple classification is that of "drop on demand (DOD)" and continuous inkjet(Cl])". In both of these technologies, high numbers of nozzles are used for eachcolour: nozzles are between 10 and 100 microns in diameter, thus ensuring aresolution of up to 720 drops per inch (DPI). Between a thousand and a millionink droplets are processed per second. Both systems generally operate withconstant basic process colours (4 or even more up to 12), which are mixed onthe substrate. The depth of the shade is controlled by the number of dots appliedand pale shades are created by more base fabric being visible at any given pointin the design. Using the same primary colours all the time means that no colourkitchen is needed. Both of these types of engines have advantages anddisadvantages for textile printing and both have served as the basis for thedevelopment of printers for textile applications.Continuous jetIn continuous ink-jet systems droplets of inks are selectively printed ontotextile substrate. In this technology, a continuous stream of electricallyconductive ink is produced by forcing it through a narrow nozzle at a pressure ofabout 3x105 Pa. The resulting high velocity breaks the ink stream into droplets(approx. 100,000 per second). Directional control over the droplets is obtained byselectively inducing an electrostatic charge on them as they leave the nozzle.The charged droplets then pass through a set of like-charged plates which repeland deflect the droplets either to the required position on the substrate and theuncharged droplets to the reservoir for recycling or vice versa.The drop volumes can be significantly larger than most DOD ink- jet devices sothat the volume of ink delivered per unit time is higher than with DOD printers.
This can be a significant factor in printing on textile substrates that require highink volumes. Due to a continuous flow clogging of the nozzles is minimisedcompared to DOD systemThe main advantages are: accuracy: the pump is running constantly and is not a start-stop operationInk formulation: as the inks are not heated, the formulation can be less criticaland hence the inks less expensive.The disadvantages inherent to CIJ systems are: • Complexity: The current heads are all manufactured manually • Each nozzle has its own pump. • Relatively high price • Need for electrically conductive inkBinary continuous inkjetThe simplest of these is the binary CIJ. In this approach inks drops areselectively charged. The uncharged drops then strike the substrate to form theimage. Drops carrying a charge are deflected to a catcher or gutter by deflectionplates for recycle. The operating principle is shown in Fig.3 Fig. 3 Binary continuous inkjet Main players of this system are Stork, Iris and Silver Reed. The disadvantages are • most complex print head technology • poorly adapted to process colours. • technology is expensive to manufacture and maintain. • lower resolution compared to drop on demand printers,the need for low viscosity electrolytic inks, and the need for refiltration resulting insome waste ink.Multideflection CIJ-systems
This approach differs from the binary CIJ in that the ink drops are given avariable charge that gives different deflections as the drops pass through thedeflection plates. This allows multiple positioning of ink drops (up to 30) on thesubstrate to be printed from a single jet. The system is shown in Fig.Fig. 4 Multideflection CIJThis is also a complex technology, but has proven more reliable and lessexpensive than binary continuous to manufacture and maintain. The heads canbe used with a wide range of inks, produce larger drop sizes and cover morepixels with fewer print heads. In this version of continuous inkjet technology, it isthe undeflected drops which are recycled while the electrostatically deflecteddrops hit the fabric.These heads are generally considered well suited to textile printing at higherspeeds and are attracting an increasing amount of interest and developmenteffort. However, they are only suited to relatively low viscosity inks.This technology has been employed by Imaje in printers for industrial markingand served as the basis for T-shirt printers developed by Embleme. Linx, Jemtex,Willet. Imaje company developed a demonstration T-shirt printing deviceemploying the continuous inkjet heads and UV curable water-based pigmentedinks. This first direct digital garment printer could Print on cotton, linen, rayon,silk, wool, polyester, polyamides, Lycra, and sponge. The printed imagesmarginally altered the fabric hand. In addition to the UV inks textile inks, Toxot,the research and development arm of Imaje, developed a water-based pigment-loaded thermally cured inkjet color ink which exhibits greater color density, washfastness, and adhesion than its water soluble UV curable predecessor.The advantages of multilevel continuous inkjets are their speed, their ability tocover a larger band width print area with one pass, reliable operation and longprint head life over thermal or piezo drop on demand printers,The disadvantages are that they initially cost more than drop on demandprinters, they currently operate at resolutions lower than most drop on demandprinters, they are limited by their requirement for inks with extremely low viscosity
between 3 to 6 cp and electrical conductivity that usually involves the addition ofsoluble salts.In general, the initial cost of CIJ heads currently prohibit their use for low volumeapplications. However, they are cost effective and reliable means to digitallyprint larger volumes simple designs. Air jet deflection In this system the ink droplets are deflected by an air stream. for example, the MillikenMillitron. It is most suitable for relatively low definition (20-30 dpi) and high inkvolume applications such as for carpet printing. The system is shown in Fig.5Fig.5 Airjet deflectionThis type of inkjet prints carpets better than analog technologies due to itsability to vary dye delivery pressure so as to penetrate color into different carpetpile types and thickness. Due to their larger orifice and drop size, carpet printersdeposit a larger volume of ink with somewhat higher viscosity in the 100 to 400cps range, while other inkjets use ink in the 1+ to 30 cps range. This enables theprinting of durable intense colors which do not wick excessively with lessexpensive dye chemistry.These printers have proven successful because they operate at profitable productionspeeds (about 20 running meters per minute), with print resolutions acceptable to themarket. Their disadvantages are their low resolution and that they generate waste ink.This technology is not adaptable to the needs of the mainstream textile printer.Drop-on-demand technologyDOD-systems DOD ink-jet engines deliver a drop of ink only when required forprinting: it produces "drops on demand", i.e. only when and where needed in thedesign. These systems are environmentally friendly, since the entire colour goesdirectly onto the fabric: "no paste, no waste". Moreover, a special advantage iscost effectiveness of short runs, which allows customized and personalizedtextiles to be produced at an acceptable price.Two 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 is only intermittently applied when a droplet is needed.Thermal ink jet or bubble jet technologyThe bubble jet printer (Canon) relies on a thermal pulse to generate the ink drop.This technology was the first of the drop on demand. The technique boils thewater content of the ink and the resulting steam pressure forces a droplet of inkout of the nozzle. In these printers, the computer signal heats a resistor to a hightemperature (> 360 °C) which creates a vapour bubble in a volatile componentin the ink, the vapour bubble expands and exits the nozzle followed by acontraction of the bubble causing a drop of ink to be ejected on the textilesubstrate. The vapour bubble must then cool and collapse allowing the inkchamber to refill from a reservoir. Cycle time is limited to approximately 10.000drops per second and the volume per drop of ink is typically 150 to 200 picoliters.Thus, a single thermal ink jet can deliver approximately O.I ml of ink per minute.The operating principle is shown in Fig .Figure 6 Thermal inkjet printer: (a) Bubble jet chamber; (b) bubbleformationIn thermal inkjet printers following two variations are availableSide shooter thermal inkjet
Fig. 7 Side shooter thermal inkjetRear shooter thermal inkjetFig.8 Rear shooter thermal inkjetThermo inkjet printers are capable of printing cellulosic fibers with fiber reactivedyes, synthetics with disperse dyes, and nylon and protein fibers with acid dyes.These printed fabrics require conventional post processing including steamingand washing. Canon extended the life of its print heads from 8 to 14 and then to130 hours of continuous operation, which it guarantees. It achieved this with theaddition of a head cleaning mechanism and the reformulation and refining ofinks. The Canon Bubble Jet Textile Printer delivers 360 dpi resolution, 8 colorcapacity, over 1,000 nozzles in 16 print heads and prints up to 1.65 meter widthsat the rate of 1 linear meter per minute. The high force associated with dropletejection from Bubble jet heads provides the advantage of fabric penetration andthe disadvantage of increased ink splatter. Some coarser, pile fabrics benefitfrom its advantage while hiding the splatter, while tightly woven fine fiber fabricsreveal splatter.
Canon, Hewlett-Packard and Lexmark have continually advanced thermal inkjettechnology. They have developed printheads withnmore nozzles capable ofhigher droplet generation and print resolution. For its BJC 7000 series printers,Canon developed a 480 nozzle printhead. Hewlett-Packard refined its 800 seriesprintheads with a new more reliable 192 nozzle configuration. On older models,each nozzle can generate 32 pL (picoliter) ink droplets at 6,000 drops per second(6 kHz). On newer models, each can generate 10 pL droplets at 12,000 dropsper second (12 kHz). Lexmark developed a printhead capable of 1,200 x 1,200resolution. All of these printheads have been incorporated into printer systemswhich deliver photographic quality images.Advantages and disadvantages of thermal inkjet printersThe major advantage of the thermal ink-jet technology is the low cost of nozzlefabrication. It is made using the mass production technique based on theintegrated circuits. Thus, thermal ink jets offer low-cost print heads but sufferfrom reliability and slow speed. Thermal printers are well suited for low-volumeprinting. The system restricts the use of binder containing pigment inks. Themajor problem with the thermal ink jet is the high nozzle and resistor failure rateresulting from rapid thermal cycling. As the heater to boil the water has to work ina semi-explosive way, the temperature can rise up to 360 °C, which can causethe nozzle to burn out. The high temperatures cause often decomposition of inkcomponents, which leads to poor heat transfer and/or nozzle clogging. Therefore,only thermal stable inks can be used. These defects are unpredictable. Poorquality production results are possible.Compared to piezo-systems the dropletsize is larger resulting in a lower resolution. Main players are Canon, Encad,Color Wings, HP, and Direct Imaging Systems.Piezo drop on demand technologyThis is one of the simplest ways of generating drops on demand. It makes use ofthe piezoelectric effect in which small electronic impulses delivered to suitablecrystalline materials (transducer) causes them to expand. This transducer,incorporated in the ink chamber, enables pressure pulses to be created in theink. Droplets are generated intermittently according to the electronic signalsreceived. The piezo on removal of the potential returns to its normal dimensionsand the ink chamber is filled from an ink reservoir by capillary action. crystals ofLead-Zirconium-Titanate (PZT) or ceramics are used as transducer.The cycle time of the piezo-based printers is limited by the ink replenishment rateand can be somewhat higher (14,000 cycles per second) than the thermal ink-jetbut drop volumes are usually somewhat smaller (150 picoliters). The small dropsize allows the piezo-based printers to produce very high- resolution prints (1440dpi is commercially available). Piezo-print engines are now in use in a number ofprinters for textile substratesTypes of piezoelectric print headsSeveral variations exist of the basic mechanism by which crystals of Lead-Zirconium-Titanate (PZT) (transducer) turn electrical signals into mechanical
pressure 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.Piezo electric shear modeShear mode print heads use an electric field perpendicular to the polarizationof the piezoelecric PZT driver. Electric charge causes a shearing action in thedistortion of the PZT piezoplates against the ink causing ink drops to eject fromthe nozzleFig. 9 Piezo shear mode Shear mode piezoelectric printheads include those from Spectra, Xaar,Brother,and Olympus.Spectra heads have the advantage of high reliability, proven performance, robustcapability, wide ink choice, and can process inks in the 20 to 25 cP (centipoise)range, which is relatively high for piezoelectric inkjet. Spectra manufactures anumber of head versions made of materials varying from sintered graphite tostainless steel. Spectra Inc. has advanced shear mode with the use of CNCmachined sintered polycrystalline graphitic carbon as the structural print headbase, the placement of a filter between the piezo pumpingchamber and the nozzle, and the edge shooting placement of the piezoelectrictransducer. The shear mode action makes it possible to achieve tightly packedassembly of many jets in a printhead with just one piece of piezoelectric plate.printheads developed with shear mode technology can deliver inks at higherspeeds with superior jet uniformity for jetting various inks on a wide variety ofsubstrates. Xaar heads typically have the advantage of relatively low cost. butsuffer from limited reliabilityPiezoelectric bend mode
Fig. 10 Piezo bend modeTektronix, Sharp, Epson, and On-Target Technologies print heads employpiezoelectric bend mode, in which its electrically excited piezoceramic plateexpands placing pressure on the ink forcing some through the nozzle thusforming ink droplets. Printers which use this form of piezo inkjet are Tektronix’sPhaser 300 and 350 and the Epson Color Stylus 400, 600, 800, and 5000 inkjetprinters.The advantages of these heads are that they form very accurate circulardroplets, and they are mass producedand low cost. Their disadvantage lies in the requirement of extremely lowviscosity inks of about 1.6 to 3 cp.Piezoelectric push modeFig.11 Piezo Push mode
Push mode is similar to bend mode in that the electrical field between itselectrodes is parallel with the piezoplate polarization, and that the piezoceramicpushes against a transducer foot which places pressure on the ink to ejectdroplets. Dataproducts, Trident and older Epson printers use this technology. It isa relatively high energy process which ejects droplets which are more elongated.These companies have considerable experience with these printers and havesteadily improved them.They are relatively robust and can use inks in the 10 to 20 cP range.Piezoelectric squeeze modeFig. 12 Piezo Squeez modeSqueeze mode refers to technology which S.L. Zoltan of Clevite Corp. developed(1970 announced, 1974 US patent) and Seimens used in its PT-80 printer(1977). In this mode, electrical charge deforms a piezoceramic tube so that itsqueezes the ink within the tube forcing it out through the nozzle end. Seimenssuccessfully marketed this technology for office printing.Coupled piezo electric crystal jetThe Calcomp Topaz CrystalJet printheads are a hybrid or “coupled” piezotechnology which combine shear mode with bend mode technology to squeezeand push ink through its nozzles. Three PZT side walls and roof collapse in onthe ink channel to eject droplets.
Fig.13. Coupled PiezoThey consist of 256 nozzles per printhead with one printhead per color. Thisproduces 180 dpi with one pass, 360 dpi with two passes and 720 dpi with 4passes. These are relatively fast and robust printheads which can use a range ofink types. They have the advantages of relatively fast processing speeds and theability to produce variable droplet sizes with higher viscosity inks than thermalinkjet.Advantages of piezo D-O-D systemsAdvantages inherently linked to piezo-systems are: • Greater print head life than the thermal-based systems (100 times). A number of companies (e.g. Konica, Mimaki, Epson) are producing and developing wide format piezo printers for printing wide width fabrics. • Ink formulation: as the inks are not heated, the formulation can be less critical and hence the inks are less expensive. • most piezo designs are able to jet higher viscosity inks of wide range of formulations based on solvent, water, UV curable, pigment binders and even thermoplastic (phase-change) or heat-sensitive inks. • Despite their higher cost, piezo heads are generally more reliable and better suited to higher volume industrial printing applications than thermal heads. • Certain piezo heads can achieve high resolution using very small droplet sizes, down even to 2 or 3 picolitres. Other types produce much larger droplet sizes, more suitable for achieving good colour saturation on thicker textiles.
Major players of inkjet printersThe major players manufacturing different types of printers are summarized intableTable Main ink-jet head types and leading manufacturers (excluding carpetprinting)Thermal (bubble) DOD Piezo DODCanon AprionHewlettPackard BrotherLexmark(formerly-IBM) EpsonXerox (recent) Hitachi-Koki (formerly Dataproducts) Konica SII Spectra Tektronix Trident Xaar XeroxBinary continuous Multi-deflection continuousDomino JemtexScitex LinxStork Imaje/ToxotToxot Marconi (formerly VideoJet) WillettEssential 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 ofmicroscopic ink droplets and direct them to the substrate.ComputerComputer system for data processingSoftwareIncluding printer drivers, raster image processing (RIP) and colour managementsystems to convert computer-based designs into the electronic signals whichcontrol the scanning inkjet head and machine. These systems can also ensurefaithful and reproducible results with different batches of fabric, and provide atotal interface with the other components of a digital design, sampling andproduction environment.
Textile substratesOften in the digital world called “media”. Woven or knitted fabrics are suitable forinkjet printing using appropriate inkjet inks.Fabric pretreatmentThe bleached fabric without any further pretreatment is adequate forconventional printing. The print chemicals and auxiliaries such as thickener, acid/alkali, urea, anti-foaming agents etc are incorporated in the print paste. The printpaste is quite viscous to get sharp prints without spreading. In case of inkjetprinting it is not possible to incorporate print chemicals and auxiliaries in the inkbecause of the danger of nozzle clogging. Therefore, the print chemical areincorporated on the fabric in the form of pre-treatment before inkjet printing.Unstable fabrics such as knits and lighter weight woven fabrics frequently needto be precoated by a stiffening binder, or temporary lamination to a supportpaper. One of the fabric precoat machines is shown in Fig. 14Fig. 14 Fabric pre-coat machine for digital printingFabrics are pre-treated with various surface active agents, gums and fineparticulate coatings in order to maximize the absorbency and reactivity of thetextile substrate towards the inks, while minimizing their spreading to preventloss of definition and colour intensity. Many patented and proprietary formulationsexist, ranging from simple formulations of soda ash, alginate and urea to moresophisticated combinations of cationic agents, softeners, polymers and inorganicparticulates such as fumed silica. Many of these have been aimed at fashionfabrics such as cotton, silk, nylon and wool. 3P InkJet Textiles (Germany) ismarketing pretreated fabrics ready for inkjet printing.A partial alternative to the pre-treatment of fabrics is the development of inkjethead technologies capable of using much higher viscosity inks. However, theongoing need for some special preparation and pre-treatment of fabrics isprobably unavoidable. Eventually, this may become as widely accepted if itresults in significantly improved colour yield and reduced consumption ofexpensive inkjet inks. Novel preparative methods such as plasma treatment mayalso have much to offer in terms of primary adhesion and dyestuff fastness.
Fabric Feeding systemThis system feeds and presents the fabric to the traversing inkjet heads, andensures perfect registration and alignment throughout, even for delicate andunstable fabrics such as knits or fine silks. If required, this machine may also pre-heat, dry or set the printed fabric, before finally rolling-up the output smoothly andwith even tension.Fig. 15 Fabric feeding Fabric exitMachine vendors are increasingly focusing their attention on improved fabric feedand take-up mechanisms, as well as devices such as adhesive printing belts(fitted with washers to prevent a build-up of printed-through ink). Establishedscreen printing manufacturers such as Ichinose and Zimmer are at the forefrontof such developments. Ichinose uses a conveyor belt to transport and align thetextile substrate. The system is schematically shown in figure 16.Figure 16 Ichinose-unit: conveyor belt and dryerAt the entry end the cloth is fixed to the conveyor belt with the help of suitableadhesive. The conveyor belt carrying the fabric gently moves ahead for inkjetprinting operation. The print head nozzles are set up right above the carrier belt,and the cloth printed with the inks sprayed from the head nozzles. This can
prevent the inks from bleeding onto the cloth. After printing operation the cloth atthe exit end is released from the conveyor belt. The conveyor belt can becleaned whenever necessaryInkjet inks Ink formulations for DP-applications are probably the most difficult problem thatmust be solved for further penetration of this printing technology in the textileindustry. The inks comprising of pigments or dyestuffs of high purity must bemilled to very fine particle size and particle size distribution. Inkjet inks must beformulated with precise viscosities, consistent surface tension, specific electricalconductivity and temperature response characteristics, and long shelf life withoutsettling or mould-growth. Other important parameter is colour build up onsubstrate. The colorants must have very high strength and high chroma toachieve a broad colour gamut with a minimum number and amount of depositedcolorant. In addition, further properties such as adequate wash-, light- and rub-fastness are necessary.To achieve reasonable throughput the frequency of drop firing and the number ofnozzles must be high. High frequency, larger numbers of nozzles and very smallnozzles place a very difficult burden on the ink designer. Viscosities of ink-jetfluids must be quite low compared to rotary screen-printing pastes. Theviscosities of fluids for several typical types of ink-jet heads are shown in tableTable 3 Typical operating parameters for ink-jet enginesPrint head Viscosity cps Drop volume Pico litre plContinuous 1-10 400Thermal 1-3 200Piezo 5-30 100High surface tension of the ink formulation and the need to use very high puritycomponents (to prevent nozzle clogging in most ink jet heads) are furtherlimitations. The average particle size of disperse ink must be approx. 0,5 micrometer or lower in order to avoid clogging of the nozzles. Electrostatic deflectionsystems also require that the ink is electrically conducting which is difficult toachieve in organic solvent based systems.Reactive and acid dyesFrom the outset, suppliers of textile inkjet inks were quick to offer products basedon reactive and acid dyestuffs. Reactive dyes are particularly suited to cotton,viscose and other cellulosic materials, whereas acid dyes are used for wool, silkand nylon. Both are fully water soluble and relatively easy to formulate for a wide
range of inkjet heads, especially the widely installed thermal drop on demand jettypes.Pigment colours and disperse dyesDisperse dyes (for polyester and nylon) and pigments present a more difficult setof problems 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 particleswill not settle or agglomerate (flocculate). The particle size must have an averageof 0.5 micrometer and the particle size distribution must be very narrow withmore than 99% of the particles smaller than 1 micrometer in order to avoidclogging of the nozzlesSince pigment printing accounts for over 50% of all conventional textile printing, itis an attractive target for inkjet developers. Several of the major jet ink producershave recently launched new pigment systems. Although still prone to someproblems of handle and rub fastness, they offer excellent wash and light fastnessand have the great advantage of universal application to almost all fibres andsubstrates. In addition, the after treatments are limited to a dry fixation process.The fixation unit could potentially be mounted between a printinting and singleply cutting unit Research is going on to develop UV-curable pigment inks in-stead of thermal curable inks. UV formulations bring distinct advantages to inkjetprinting on certain substrates. The major problem with use of pigments in inkjetsystem is how best to formulate and apply the resins binder which are required tobond the pigment particles to the fabric surface. Several different approaches,from spraying resin through a separate jet head to screen printing binder over aninkjet printed colour have been suggested. In the long run, improved resintechnology seems likely to prevail, allowing trouble- free formulation and printingfrom a single inkjet head for each colour.While reactive and acid dyes will always retain some place within the overallmarket (for example, for the brightness of shade, excellent handle and fastnessoffered by reactive dyes on cellulosics), it seems increasingly likely that disperseand pigment inks will represent the way forward for inkjet printing of textiles. Printhead and machine design and materials handling arrangements will need toreflect this trend.Commercial water based inksInk specialists such as DuPont, Ciba Specialty Chemicals, Dystar (BASF), CHT,Dohmen, Lyson, Brookline, ECS and Kimberly are developing digital versions ofconventional dyes such as, reactive, acid, disperse and pigment These inksallow printers to produce prints on their specific type of fabric using often thesame type of post printing processes, as in the analog printing process. Theimportant water based ink systems and their suitability for different fibres areshown in Table 3 and 4Table 4 Commercial water-based inks
Supplier Trade name TypeDyestar Helizarine Pigment Bafixan Disperse (Transfer) Reactive Reactive (MCT)Ciba Terasil (D) Disperse Terasil (T) Disperse 9Transfer) Irgaphor TBI HC Pigment + binder Cibacron Reactive MCT Lanaset AcidDohmenn Dorasyn AcidDupont Artistry 1000 Pigment (CMYK) Artistry 500 Pigment + BinderColorspa Colorspan ReactivenTable 5 Digital printing inks for different substratesFibre Colorant Aftertreatment After wash (Fixation)Cotton, Viscose Reactive Steam Yes Silk, wool, Reactive/Acid Steam YesPolyamidePolyester Disperse HT steam YesAll fibres Pigment Polymerization No Thermal, UV curingA water based ink formulation contains different components as pointed out intable 5Table 6 Ink formulationComponent Content %Water < 80Solvent (Ethylene 30 MaxGlycol*Additives (Wetting 10andAntifoaming agentsDye Upto 10* Avoids drying out of the nozzlesSpot colours versus process coloursSpot colours
The inks used in Conventional printing systems are known as spot colours. Thismeans the required shade is prepared by mixing appropriate colours beforeprinting. Although it is a skilled job, it allows matching the desired shade asclosely as possible. Since colours are pre-mixed they do not suffer fromvariations due to order of printing. This gives an extremely large colour gamut,less variation in colour in solid areas, and a cleaner brighter shades.Process coloursThe inks used in inkjet printing are known as process colours. The desired shadeis produced on fabric itself during printing operation by blending the primaries-cyan, magenta, yellow and black (CMYK) drop by drop sequentially over a smallarea rather than being premixed in an ink kitchen prior to printing Each primarymust be transparent so that light passing one colour ink will not absorb or scatterthe light from another colour ink beneath it.With screen-printing the inks may be dried between colours, with ink-jet allcolours are printed simultaneously, wet on wet. The colour gamut obtainable withspot colours is larger than with process colours. The major limitations of thisapproach lie in the inability of any given set of CMYK process colours to generatea full colour gamut. You may be able to theoretically produce 16.7 million colours;however, only 1.5 million might be useful for most textile printing out of this 1million colours may be outside the colour space possible from this system. Inorder to improve the colour gamut and to obtain extremely fine images specialcolour systems are developed. Hexachrome® (Pantone Inc.) is a 6-color processconsisting of the four basic colours plus orange and green inks. This approachresults in more brilliant continuous-tone images and in almost twice the numberof colours that can be obtained using CMYKFigure 17 Colour gamut with 4 and 6 inksoutside: Hexachrome gamutInside: CMYK gamutWith the introduction of 6, 7, 8, and even 12-color digital printers into the market,these systems come closer to achieving the results obtained using analogprinting. . However, this increasing number of colours in the design of systemsfor ink-jet printing of textiles is problematical. Each additional colour headincreases the problems of data handling rate and nozzle failure. It also
significantly reduces the fraction of the printer that is not actively printing at anygiven moment, thus significantly reducing machine efficiency.Fabric post processingPost-treatments are associated with the printing operation; examples are baking,steaming and/or washing. Fig. 18 Steam fixation and washing unitsThese processes are similar to those for conventional textile prints, except thatthe process is undertaken with a much smaller batch size, typically a few tens ofmetres or even individual sample lengths.One barrier to new entrants is the need to finish fabrics after printing, in order todevelop and fix the colours to acceptable industry standards of wash, rub andlight fastness, handle and appearance. Often there is a need for final applicationof flame retardant, soil, stain and crease-resistant finishes. Such processes callfor the use of specialized capital equipment such as steamers, washers, driers,bakers and stenters. Apart from the cost and space requirements (water, energy,effluent etc.), many new potential users of inkjet technology have neither theknow-how nor inclination to embark upon conventional textile processing in thisway. Some suppliers of inkjet equipment or suppliers of pre-treated fabrics nowoffer small desk top steamers capable of handling short sample lengths of printedfabric (typically up to 30 metres). However, these are far from ideal for evensmall-scale production and still leave many aspects of fabric finish andperformance uncontrolled, for example, shrinkage and final width.Slow adoption of digital textile printingFollowing issues are responsible for slow commercial adoption of digital textileprinting.• 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 where labour intensive processesprevail.• Main stream textile printers are geared to low cost mass production businessmodel and long 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 tocontinue with modest growth.2. Bulk production for batches less than 1000 metres. This is the vision of manyand interest is at what point digital technologies can “compete” successfully withscreen printing.3. Mass-customization: The creation of new niche markets for small-mediumbatches of printed textiles for specific customers. It may be possible that garmentmakers decide to buy a digital printer and attach it to a laser cutting table. Afterprinting, the fabric could be cut single ply using a computerized system and thenconverted to made-ups.Major inkjet manufacturers are working to resolve the issue of production speedand it is hoped that inkjet printers will be available with a speed to compete withrotary screen printing. The ITMA 2003 exhibition in Birmingham, UK, was asignificant milestone for digital printing, with 27 companies offering textile digitalprinting equipment. Many of the machines shown were said to print at over 50m2 per hour, and the Reggiani printer was said to print at 150m2/hour. Howeverthis far less than rotary screen printing (3600 meters/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 tocarry out the printing production. In Bangkok a printing unit has 25 StorkSapphire machines run much like a traditional weaving department.ConclusionsDigital printing provides an opportunity to meet the present day market trends ofmass customization. It has established as an acceptable technology for sampleproduction. Among other technology problems speed of printing is the mainhurdle in commercialization of technology. Attempts are being made to achievecommercially acceptable printing speeds. Till then the practice of combination ofdigital printing for sampling and rotary screen printing for production will continue.What now seems certain is that there is sufficient industrial investment andcommitment by manufacturers to ensure that commercial ink jet textile printingwill become a reality.