93, 94 introduction to digital printing


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93, 94 introduction to digital printing

  1. 1. by Vince Cahill,OwnerThe ColorworksThis is the first in a series of Journalarticles describing digital printing technologies.It compares the advantages and disadvantages
  2. 2. of analog and digital methods.It goes on to describe and examine thevarieties of inkjet technologies and theirspecific advantages and disadvantages.Printing industries are increasingly adoptingdigital technologies to complementor replace analog ones. The terms digital andanalog (or analogue) designate both types ofsignals for representing data and methods ofprint reproduction. Analog signals and computersuse variations in continuous phenomenonsuch as voltage or pressure to transmitdata, while digital signals and computers relyon discontinuous patterned transmissionsof discrete amounts of electricity or light tocommunicate data.Some of the earliest electric computerswere analog.Analog printing methods reproduce imageswith like images. Screen printing, for instance,is an analog print method that employs filmpositives and negative stencils which resemblethe original image to create prints. Similarly,lithography uses analogous images on plates totransfer ink to print substrates. Flexography,
  3. 3. etching, rotogravure and other analog printmethods employ other forms of analogousimage transfer. All of these methods reproduceimages from a master image. Unlike these analogprinting technologies which use stencils orplates containing full sized images, digitalprinting approaches assemble each imageprinted from a complex of numbers and mathematicalformulas. They configure images froma matrix of dots or pixels. These use digitallycontrolled deposition of ink, toner or exposureto electromagnetic energy, such as light, toreproduce images.Market forces and the desire for digitalprinting’s advantages are driving its developmentand adoption. Print purchasers want toeliminate the risks and expenses ofmaintaining inventory. Batch printingand mass production are giving wayfor some market applications toshorter print runs and mass customization.While analog print methods bestaddress the needs of mass productionand high quality printing, digital printingcan best serve demands for variableinformation printing, personalization,customization, quick response, just-intime
  4. 4. delivery, and short print runs.Printers are employing both digitaland analog print methods to satisfytheir customers needs. They can performcomplementary functions, eachdoing what it does best.ADVANTAGES OFDIGITAL PRINTINGs Digital printing requires minimalpress setup and has multicolorregistration built-in to its system.This eliminates many of the frontendtime consuming processesand permits quick response andjust-in time print delivery.s Digital processes can vary everyprint “on-the-fly” i.e. whileproduction printing, providing variabledata, personalization,and customization.s Most digital printing technologiesare non-contact printing which permitsprinting of substrates withouttouching or disturbing them. Thiseliminates image distortion encounteredin some analog processes suchas screen printing. It also does notrequire as aggressive substrate
  5. 5. hold down methods which candistort or damage some substratessuch as fabrics.s Digital technologies can printproofing, sample and short runsmore cost effectively than analogmethods. Digital color printingprocesses offer a range of colorprocesses including 3 color process(CYM), 4 color process (CYMK),5,6,7 & 8 extended gamut coloroptions in addition to some spotcolors. These match growing marketdemand forfull color.s Most digital print processingrequires less or no color overlapor trapping.s Digital printing does not use filmmasters, stencils, screens or plates.It requires much less space forarchiving text and images thananalog printing methods require.s Generally, digital printing uses lesshazardous chemicals, produces lesswaste and results in less negative
  6. 6. environmental impact than analogtechnologies.s Digital printing is employingsophisticated color matching andcalibration technology to produceaccurate process color matching.s Digital web printers can printimages limited only by the width offabric and the length of the bolt orroll. They can print panoramas andare not restricted to repeat patterns.s Digital files are usually easier andquicker to edit and modify thananalog photographic images.s Designers, artists, photographers,architects, and draftspeople areincreasingly creating and reproducingtheir work digitally. Digitalprocessing has replaced opticaland manual methods for typesettingand page composition.Telecommunication has largelyconverted to digital processing.One can use the same digital filesfor electronic media, such asInternet, CD-ROM, Video and TV,print media and multimedia.s One can readily convert analog
  7. 7. images and text to digital withscanning and optical characterreading (OCR) software.s Digital files are easy to transportand communicate. One can senda digital file to any digital printeron the planet within seconds. Thispermits distribution of design tomany locations for quick responseprinting. Industries are adoptingdigitally generated and communicatedart and print copy.DISADVANTAGES OFDIGITAL PRINTINGs Most digital technologies haveslower throughput as comparedwith comparable analog printing.s Digital printing will often costmore per copy than analogprinting for longer print runs.s It often requires specially preparedand coated substrates.s Most digital printing technologiesdeposit very thin ink or tonerlayers. These limits necessitatelayering for applications requiringthicker deposits, resulting in slower
  8. 8. operation.s Digital inks and toners are limited incapacity and carry high price tags.s Most digital devices are printingtransparent chemistry whichlimits their use for white orlight substrates.s Some processes currently havedifficulty matching colorconsistently.s This is new technology whichrequires investment for trainingas well as equipment.ADVANTAGES OFANALOG PRINTINGs Analog print technologies printmany multiple copies quicklyand inexpensively.s Offset lithography and gravureproduce very high resolution andimage quality.s Analog printing usually does notrequire expensive coated substrateto print satisfactory images as mostdigital printing does.s Its inks do not require the highdegree of refinement and smallparticle pigment sizes which most
  9. 9. of digital printing’s do. Mostanalog inks cost less than mostdigital ink.s Analog screen printing providesa wide range of single pass inkdeposition thicknesses.s Screen printing can print opaqueinks which cover dark substrate surfaces.s Analog printing can print eitherspot or process colors. Printerscan maintain their own color“kitchens” from which they canmatch virtually any color.s Analog methods are existingtechnologies with existinginstalled base presses, trainedoperators and establishedmarkets and customers.DISADVANTAGES OFANALOG PRINTINGs Analog printing permits onlyvery limited variable data printing,such as letterpress numbering.s Analog methods require prepressset up and preparation.s Generally, these types of printingare not cost effective for very
  10. 10. short run printing and proofing.s Analog printing can generatesignificant waste ink, chemicalexposure and deleteriousenvironmental impact.s These printing methods use costlyfilm for screen or plate exposure.The archiving of films, plates andscreens demand considerable cataloguing,storage space and furniture.In addition, these filmsdeteriorate with age.s Images are limited to the size ofthe plate or screen image area.Larger prints require that onerepeat the pattern and that thedesign permits seamless connectionof repeated patterns.s Many analog printing operationsuse aging presses.THE WEDDING OFANALOG & DIGITALThe printing industry has begun tomarry the strengths of analog to thoseof digital printing. It has adopted digitalprocessing for operations requiringone or a small number of images, such
  11. 11. as art, page layout, screen and platemaking, while retaining analog technologyfor reproducing large numbersof copies. The Heidelberg “QuickMaster” combines the rapid imagecreation and set up of digital platemaking and controls with the fastproduction speeds and print qualityof analog offset lithography. Gerber,Luscher, Kiwo, Richmond GraphicProducts Inc & Sign-Tronics computerto screen exposure masking systems alsocombine the strengths of digital processingwith those of analog printing.Other scenarios combine digitalfor printing variable information withanalog for consistent data and graphics.Garment hang tags will combineanalog methods, such as flexographyor offset printing for logo, productidentity and trademark graphics anddigital methods, such as thermal transferto print bar codes, sizing and othervariable information. Still anotherwould screen print repetitive images onlight colored garments, but would digitallyprint personalized or customizeddrop-in names of persons or places.
  12. 12. Whether one chooses digital oranalog print methods or a combinationof them for applications, one needs tomeasure and examine a number ofparameters to effectively compare andchoose among the technologies. Theparameters include: image qualityand resolution, production print andprocessing speeds, indoor and outdoorapplication durability, environmentalimpact and cost, color and imageconsistency and reproducibility,and costs per print.IMAGE QUALITY& RESOLUTIONBoth analog and digital imagequality and print resolution involvea complex of phenomena. One needsto understand the relationship of theseto compare different printing technologieseffectively. Resolution is usuallyindicated with terms such as lines perinch (lpi), lines per centimeter (lpc),dots per inch (dpi), or dots per centimeter(dpc). Inkjet images are composedfrom a grid of pixels, i.e. pictureelements or dots. The number of these
  13. 13. dots per linear inch in the x directionand the number of dots per linear inchin the y direction indicate the dpi resolutionof a print. Software can usuallyinstruct a printer to construct its linesfrom one or more rows of contiguouspixels. If one used 4 pixels per line ona 360 dpi matrix, one would be printing90 lpi output. In actuality theseterms, dpi and lpi, describe only oneaspect of what is termed apparent resolution,i.e. the resolution we perceive.Another factor is grayscale or graylevels.This involves the number ofdifferent sizes or grays a pixel dot in amatrix can have. In inkjet printing, thisinvolves the number of drops addressedper pixel or dot, or the numberof possible droplet sizes per pixel dot.Matrix cluster patterns and RasterImage Processor (RIP) algorithms canalso affect apparent resolution, butthese are largely software rather thanhardware factors. Digital printingaddresses its ink or toner to a grid ormatrix. Digital software image processingcan offer a large number of matrixpatterns including random patterning.
  14. 14. It also permits a single master or printimage to contain multiple matrices.For instance lettering might be generatedin a pattern which favors lineacuity, while a photographic image isgenerated in a pattern which permitsthe desired degree of contrast andpixel gradation.Focusing on the relationshipbetween image output quality andprint hardware factors, RodneyShaw of Hewlett-Packard suggests“digital image-quality descriptorswhich allow for absolute performancecomparisons between diverse imagingtechnologies, both analog and digital.”One of these descriptors is a digitalnoise scale (DNS). This model forcomparison adapts electronic communication’ssignal-to-noise ratioanalysis. It uses a 0 to 10 noise scale,delineated in Table 1, which encompassesthe gamut of conventionalphotographic image noise or whatis commonly called graininess.
  15. 15. Adapted from: Rodney Shaw, “Image QualityConsiderations for Printing Digital Photographs ”in The Proceedings of IS&T’sNIP 13: 1997 International Conference onDigital Printing Technologies102468DNS2400dpi1200dpi600
  16. 16. dpi300 dpi150 dpi 75 dpiVery CoarseCoarseMediumVery FineMicrofine24Improved Image Quality8 16 32 64Higher resolution comes at the price of tone and imagecontour contrasts. The frequency of one’s dots (dpi) is inverselyproportional to the square root of the tone level. Increasingdpi to obtain finer detail and reduce matrix visibility will alsoreduce the picture’s tone contrasts and increase the chances offalse contouring and perceived image distortion. Print applicationviewing distance is the key factor in striking this balance.At a far distance, one needs higher tone levels but not highdot frequency for the perception of continuous tone, whileone requires high frequencies and apparent resolution closeup but not high tone levels. Printers have to find the balancebetween high resolution and high tone quality to suit eachapplication. Billboards viewed from a distance of a 100 feetcould have a resolution at of 20 dpi with a binary grayscalebut appear to be a continuous tone images, while images
  17. 17. viewed at a distance of 2 feet would require a 1000 dpi resolutionwith binary grayscale. Binary grayscale means that eitherthere is one or zero dots of one invariable size at pixel cells inan image matrix. The higher thenumber of possible grays (or value variations) per pixel, thehigher the apparent resolution. Also the higher the number ofdots or lines per inch, the greater the resolution.Table 1:For digital printing, such as inkjet, thenoise level is a function of a print’s dpiand the number of its gray levels. Thehigher the DNS number, the lesser theapparent resolution, the lower thegreater. Figure 1 plots the relationship ofdpi and gray-levels against the digitalnoise scale.
  18. 18. PRODUCTION PRINT& PROCESSING SPEEDSWhen comparing systems, oneneeds to look not only at the throughputspeed of a printer, but also at thetotal processing time to create imagesincluding pre- and post-press time.The rate at which printing equipmentproduces product or adds value to it,in part, determines how much onecan spend for its purchase or use.OTHER POINTSOF COMPARISON Indoor & OutdoorApplication Durability Environmental Impact & Cost  Color & Image Quality,Consistency & Repeatability Costs Per Print Capital Equipment& Training Costs  Turnaround TimeDIGITAL PRINTINGTECHNOLOGIESDigital printing encompassesmany technologies. These includevarious forms of inkjet, thermography,electrophotography and electrostatic
  19. 19. printing, ionography, magnetography,and digital photographicimaging and developing. None ofthese require a physical master butinstead rely on digital data tocreate images.HYBRID DIGITAL-ANALOGPRINTING TECHNOLOGIESBoth analog and digital printingmethods have advantages which theother lacks. Numerous opportunitiesexist for combining the strengths ofeach to garner the best of bothworlds. The digital take over of prepressanalogue operations illustratesthis example. Pre-press requires thegeneration of a single master whichis best generated digitally. Once created,analogue printing can reproducelarge numbers of it costeffectively. Digital can print variableinformation in a print job, whileconventional prints the unchangingelements. Other marriages arealso possible to use the best of both.CONCLUSIONSInkjet technology has developed toa point where it is driving a growing
  20. 20. multi-billion dollar industry worldwide.The development of chemistryfor this printing technology is expandingits applications beyond paperprinting to textile and industrial printingand fluid deposition. Other digitaltechnologies are also providing theadvantages of digital printing. We willexamine the role of inkjet chemistryand other digital printing technologiesin future articles.Market demands for reducedinventory risk, quick response,variable information processing, personalizationand customization aredriving the adoption of digital imagingand inkjet printing technologies. Theyhave grown to dominate niches andsegments of the Printing Industry. Itsexpansion will continue for newapplications and to replace analogprinting’s market share.IT Strategies projects that the greatestgrowth for inkjet printer sales willoccur for the in-house Corporate marketfollowed by the Professional printfor pay market. Current inkjet configurationscover productivity ranges from
  21. 21. 1 to 200 square meters per hour. Asproduction speeds and print qualityincreases and capital and consumablecost decrease, more print providerswill adopt one or more forms of digitaltechnology with inkjet leading the way.Trends indicate that wide-format electrostaticprinting will experience somewhatslower growth. Novel digitaltechnologies have to develop sometrack record before one can predicttheir role and market penetration.Analog printing technologies will continueto hold quality and productionspeed advantages for the next decade,but digital improvements will continueto erode analog market share.REFERENCESBoar, Hanley, & Williams, The Numbers:Worldwide Printer & Supplies Market Report,IT Strategies, Hanover, MA, 1998Cahill, Hanley, O’Sullivan, & Williams,Digital Color Printing of Textiles, ITStrategies, Hanover, MA, 1996J. P. Stefanini, F. Noll, & A. Dunand,Inkjet Printing For The Textile Industry,Toxot, Valence, France 1994Ross N. Mills, Inkjet Printing-Past, Present
  22. 22. and Future, IS&T’s Tenth InternationalCongress on Advancesin Non-Impact Printing Technologies, 1994Hue P. Le, Progress and Trends in InkjetPrinting Technology, The Journal of ImagingScience and Technology, January/February1998, Volume 42, Number 1, p. 49-61Jerome L. Johnson, Principles of NonImpact Printing, Palatino Press, Irvine, CA,1986, 1992Binery continuous inkjetThe advantages of binary inkjets are their speed, proven track record, highreliability,a large range of available chemistry for many applications. The disadvantagesare initial
  23. 23. systems costs and lower resolution of some current offerings than drop ondemand printers,the need for low viscosity electrolytic inks, and the need for refiltrationresulting insome waste ink. Hertz binary continuous inkjet, such as those from Stork andIris, offerhigh resolution but with slow image production speeds, drum-limited format,and highcapital and consumable costs.Binary Continuous InkjetFIGURESMultilevel continuous ink jetThe now defunct technology application development company, Embleme,developed
  24. 24. a demonstration T-shirt printing device employing the Imaje continuous inkjetheads andUV curable water-based pigmented inks. This first direct digital garmentprinter couldimage on cotton, linen, rayon, silk, wool, polyester, polyamides, Lycra, andsponge. Theprinted images altered the character of the fabric’s hand so slightly as to beindistinguishable.In addition to the UV inks textile inks, Toxot, the research and developmentarm ofImaje, developed a water-based pigment-loaded thermally cured inkjet colorink whichexhibits greater color density, wash fastness, and adhesion than its watersoluble UV curablepredecessor. The Embleme team established that the Imaje heads and UV inkcouldprint textiles. Continuous inkjet heads can cost about $5,000 per head,piezoelectric headsrange from about $30 to $3,000, and thermal inkjet heads about $20.Continuous headsoffer very high reliability and resistance to failure commensurate with theircost.Continuous inkjet heads are hand assembled while the production of thermalinkjets and
  25. 25. the Epson multi-layer Stylus systems are automated. A few tens of thousands ofcontinuousinkjet print heads satisfy worldwide demand each year. While companies, suchas Hewlett Packard, Canon and Lexmark produce drop on demand inkjets bythe millions.Large scale production of many drop on demand print heads keep their costsvery low. Their reliability is considerably less than continuousprinters. This makes CIJ effective for high volume, long run printing.Embleme had produced commercially acceptable T-shirt prints with a set offour CYMK scanning Imaje/Toxot inkjets operating at120 dpi. Theoretically, a configuration involving dedicated arrays ofImaje/Toxot continuous inkjets could print bolt textile at a rate of600 running meters per hour at 180 dpi. For 1.5 meter wide bolts of fabric,this would yield productions rates of 900 square meters, orabout 9,000 square feet, per hour. (Toxot is currently constructing a 120 dpi 6color multilevel CIJ array capable of printing two meterwidths of floor vinyl at production rates of 2,400 square meters per hour.)Though still falling far short of rotary screen printing’scruising production rates of 60 to 90 meters per minute or 3600 to 5400meters per hour, it is considerably faster than drop on demandinkjet printing capability. In addition, rotary screen printers usually printelastomeric and other dimensionally unstable fabrics at slowerspeeds of about 20 meters per minute. For print jobs which could accept itsresolution, continuous inkjet could compete with rotary
  26. 26. screen printing for short to medium production runs, elastomeric fabrics, andfor on-demand printing.The advantages of multilevel continuous inkjets are their speed, their ability tocover a larger band width print area with one pass,reliable operation and long print head life over thermal or piezo drop ondemand printers, proven track record, and available chemistry.The disadvantages of multilevel continuous inkjets are that they initially costmore than drop on demand printers, they currentlyoperate at resolutions lower than most drop on demand printers, they arelimited by their requirement for inks with extremely lowviscosity between 3 to 6 cp and electrical conductivity that usually involves theaddition of soluble salts.In general, the initial cost of CIJ heads currently prohibit their use for lowvolume applications. Binary CIJ and multilevel CIJ systemscurrently offer the most cost effective and reliable means to digitally printlarger volumes.Air jet deflection Milliken Millitron
  27. 27. These printers have proven successful because they operate at profitableproductionspeeds (about 20 running meters per minute, with print resolutions acceptableto the market.They permit customization, personalization and on the fly design and colorchanges. Thistype of inkjet also prints carpets better than analog technologies due to itsability to vary dyedelivery pressure so as to penetrate color into different carpet pile types andthickness. Dueto their larger orifice and drop size, carpet printers deposit a larger volume ofink with somewhathigher viscosity than other inkjets. The rug dying inkjets use ink with viscosityin the
  28. 28. 100 to 400 cps 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 dyechemistry. Since most rug yarns are coarser than apparel yarns, carpet inkjetoutput withcoarse resolution in the 20 to 30 dpi range do not encounter customerresistance, becausethe coarseness of carpet inkjet printing matches that of the carpet yarn.The advantages of air deflected continuous inkjets are that they can printcontinuously at production rates on 3-dimensionalsurfaces reliably. Their disadvantages are their low resolution and that theygenerate waste ink.Side shooter thermal inkjetAir Jet Deflection -Bar
  29. 29. In 1977, Canon received its first patent for the side-shooter thermal bubble jet.Soonafter Hewlett-Packard received patents for its roof-shooter thermal inkjettechnology.These companies cross licensed their patents to each other. IBM licensed rightsto produceand develop the HP technology. When IBM sold its printer division, these rightstransferredto the new company, Lexmark. Hewlett-Packard produced the first commercialthermalinkjet printer in 1984. Other thermal inkjet head manufacturers includeOlivetti andXerox. Many other companies have fabricated printers using print heads fromthese headmanufacturers.Rear shooter thermal inkjet
  30. 30. The wide-format thermal inkjet manufacturers include Encad, Hewlett-Packard,Calcomp,Xerox, Colorspan (formerly LaserMaster), Mutoh, Mechatron, and others. Theycan directprint flexible substrates, such as paper, vinyl and coated canvas for use asbanners. Theflat-bed versions can print rigid surfaces. The Mechatron and other XY plotterbasedprinters can also mask printing plates and screens for imaging.Canon extended the limits of its Bubblejet technology with its development andrefinementof the Canon Bubble Jet Textile Printer. It introduced this printer at the end ofits joint developmentventure with Japanese textile printer Kanebo in Vienna, Austria, June 1996.This
  31. 31. device prints cellulosic fibers with fiber reactive dyes, synthetics with dispersedyes, and nylonand protein fibers with acid dyes. These printed fabrics require conventionalpost processingincluding steaming and washing. Canon extended the life of its print headsfrom 8 to 14 andthen to 130 hours of continuous operation, which it guarantees. It achieved thiswith the additionof a head cleaning mechanism and the reformulation and refining of its inkstoo. TheCanon Bubble Jet Textile Printer delivers 360 dpi resolution, 8 color capacity,over 1,000 nozzlesin 16 print heads and prints up to 1.65 meter widths at the rate of 1 linearmeter per minute. The high force associated with dropletejection from Bubblejet heads provides the advantage of fabric penetrationand the disadvantage of increased ink splatter. Somecoarser, deeper pile fabrics benefit from its advantage while hiding thesplatter, while tightly woven fine fiber fabrics reveal splatter.Canon, Hewlett-Packard and Lexmark have continually advanced thermal inkjettechnology. They have developed printheads withmore nozzles capable of higher droplet generation and print resolution. For itsBJC 7000, Canon developed a 480 nozzle printhead.Hewlett-Packard refined its 800 series printheads with a new more reliable192 nozzle configuration. On older models, each nozzle can
  32. 32. generate 32 pL (picoliter) ink droplets at 6,000 drops per second (6 kHz). Onnewer models, each can generate 10 pL droplets at 12,000drops per second (12 kHz). Lexmark developed a printhead capable of 1,200 x1,200 resolution. All of these printheads have been incorporatedinto printer systems which deliver photographic quality images.The advantages of thermal inkjets are:  Their low equipment cost Very large installed base  Available inks and ink development  Available media The development resources and guarantees of giants such as Hewlett Packard, Canon and LexmarkTheir disadvantages include:  Limited head life due to kogation and resistor failure  Limitations of their low viscosity inks.Piezo electricPiezoelectric printheads can be grouped into one of the following types basedon theway electrical current deforms the piezoceramic activator plate: shear mode,bend mode,
  33. 33. push-piston mode, and squeeze tube. In addition, one device is a hybridcombiningtwo of these modes in the same printhead on separate piezoceramic plates.Piezoelectricdriver plates are almost universally Lead-Zirconium-Titanate (PZT).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 thedistortionof the PZT piezoplates against the ink causing ink to eject from the nozzleopening in drops. Shear mode piezoelectric printheads include those fromSpectra, and those based on Xaar’s patents, such as Xaar, Nu-Kote MIT,Brother,and Olympus.LaserMaster, now ColorSpan, employs Spectra sheer mode heads for its hot
  34. 34. melt DisplayMaker Express. Polaroid uses them for its DryJet Color ProofingSystem. Luscher and Kiwo employ them for their computer to screen printers.3DSystems manufactures prototype building devices which print dimensionalprototypemodels using Spectra shear mode printheads. These print systems meltthermoplasticresins which Spectra heads shoot layer upon layer to form models.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 toachieve tightly packed assembly of many jets in a printhead with just onepiece of piezoelectric plate. Although these heads have the disadvantageof high sticker price, printheads developed with shear mode technology candeliver lower cost per jet at higher speeds withsuperior jet uniformity for jetting various inks on a wide variety of substrates.
  35. 35. The Xaar type of this technology is also known as shared-wall shear mode. Itselectrodes are exposed in the ink channel in theirnative unpassivated mode. Water-based inks corrode these electrodes unlessthey are passivated, that is coated to prevent corrosion.Without passivation, these heads must use non-corroding solvent-based inks.Xaar, Brother and others have successfully passivated theprinthead electrodes to permit the printing of water-based inks. The Xaarpiezoelectric print technology is termed shared wall becauseeach of the piezoelectric activated membrane walls is shared with itsneighboring ink channel. This means that only every other chambercan fire simultaneously. In actuality, one can only fire every third chamber dueto the possibility of accidental droplet generationfrom the chambers immediately adjacent. By adjusting the angular orientationof the printheads to the direction of substrate movement,one can achieve a workable pattern of ink deposition to compensate for thishead firing limitation. The Nu-kote/MIT version ofthe Xaar print heads can print either 200 or 360 dpi depending on the angle oforientation.A number of OEMs employ Xaar shear mode printhead technology. Using theMIT version are Raster Graphics for its PiezoPrint 5000and 6000 printers, and Daniel Instruments Ltd. Of Winterthur, Switzerland forits Polijet Digital Inkjet Print System (DIPS). These systemsuse three rows of precision positioned printheads to enable continuous lineprinting. Xerox employs the Olympus version of Xaar technology
  36. 36. in a two row array producing up to 720 dpi resolution. Mechatron is also usingan array of Xaar piezo printheads for use withits XY flatbed plotter system’s tool head. Xaar heads typically have theadvantage of relatively low cost. They have improved in performance,but still suffer from limited reliability and the restrictions of in channelelectrodes and shared-wall activation.Piezoelectric Shear Mode Coupled piezo electric crystal jetThe Calcomp Topaz CrystalJet printheads are a hybrid or “coupled” piezotechnologywhich combine shear mode with bend/normal technology to squeeze and pushinkthrough its nozzles. Three PZT side walls and roof collapse in on the inkchannel toeject droplets. They use separated rather than shared walls and can generate anumber
  37. 37. of different drop sizes per pixel for 12 gray levels to be expanded to 16. Theyconsist of256 nozzles per printhead with one printhead per color. This produces 180 dpiwithone pass, 360 dpi with two passes and 720 dpi with 4 passes. These arerelatively fastand robust printheads which can use a range of ink types. They have theadvantages ofrelatively fast processing speeds and the ability to produce variable dropletsizes andgray levels with higher viscosity inks than thermal inkjet. They own thedisadvantageof a short track record.Piezoelectric bend mode
  38. 38. Tektronix, Sharp, Epson, and On-Target Technologies print heads also employpiezoelectric bend mode, in which its electrically excited piezoceramic plateexpandsplacing pressure on the ink forcing some through the nozzle thus forming inkdroplets.The electrical field between its electrodes is in parallel with the piezoplatepolarization.The piezo transducer plates are bonded with the printhead diaphragm which isoppositethe nozzle plate. Printers which use this form of piezo inkjet are Tektronix’sPhaser 300 and 350 color phase change inkjets and the Epson Color Stylus400, 600, 800,and 5000 inkjet printers. The Tektronix printheads and print system producevery good saturated color which makes excellent overhead transparencies.Rather
  39. 39. than printing directly to a print surface, they print to a transfer roll which inturn printsthe surface. These systems are more complex and expensive than the Epsonsystems.In 1997, Epson changed its multi-layer piezo design to bend mode technologyfor useon its Color Stylus 400, 600, and 800, 3000 and 5000 inkjet printers. Epsonhas combinedits multi-layer vertically laminated piezoelectric disk technology with itsMicroWeavealgorithm to eliminate banding and ultra-fine ink for uniform ink droplets andimproved print quality.Epson provides piezoelectric print heads to Original Equipment Manufacturers(OEM)Mimaki Engineering, Raster Graphics, and Roland Digital Group for themanufacture ofwide format color printers. These include the Mimaki’s JV-1300 and JV-1800,RasterGraphics PiezoPrint 1000 and Roland’s CammJet series of printers.Those printers which are capable of 1440 dpi, such as the 400, 600, 800,5000, useEpson’s MicroPiezo refinement of its multi-layer technology. The advantages ofthese
  40. 40. Epson heads are that they form very accurate circular droplets, and they aremass producedand low cost. Their disadvantage lies in their delicate multi-layer piezo louverswhich can only tolerate extremely low viscosity inks of about 1.6 to 3 cP.Piezoelectric Bend Mode Piezoelectric push modePush mode is similar to bend mode in that the electrical field between its electrodesis parallel with the piezoplate polarization, and that the piezoceramic pushes against atransducer foot which places pressure on the ink to eject droplets. Dataproducts, Tridentand older Epson printers use this technology. It is a relatively high energy process whichejects droplets which are more elongated and may form satellite droplets. Thesecompanieshave considerable experience with these printers and have steadily improved them.The Idanit 162Ad and other large high production digital print devices employ theDataproducts heads. Trident has been serving the major marking and coding companies
  41. 41. and their industry for a number of years. Its high energy droplet ejection level can beanadvantage or disadvantage, depending upon its application and the need forpenetration.They are relatively robust and can use inks in the 10 to 20 cP range. Satellitedroplets, however, usually compromise image quality.Multi-layer piezoelectric technology refers to Epson’s technology which first employedpush mode for its Stylus Color (1994) and Stylus II (1995) printer printheads. These pushmode heads contain 64 nozzles per printhead.Piezoelectric squeeze 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
  42. 42. this mode, electrical charge deforms a piezoceramic tube so that it squeezesthe inkwithin the tube forcing it out through the nozzle end. Seimens successfullymarketedthis technology for office printing.SummaryHot melt, also known as phase change, technology currently uses piezoelectricprint heads. Teletype Corporation probably initiatedhot melt technology in the late 1960s using continuous inkjet technology.Exxon claimed invention of this technology using piezoelectricinkjet. Dataproducts developed the first commercial application of thistechnology for monochrome printing. In 1987, Dataproductsacquired Exxon’s rights. Howtek introduced the first color printer using hot meltpiezo. Tektronix of Wilsonville, Oregon owns thephase change trademark and manufactures a number of phase change printers.It uses the bender mode piezoelectric driver. Spectradeveloped its own version of hot melt and licensed Dataproducts rights as well.Brother has also been active in hot melt piezoelectricprinting.The time it takes to refill the ink firing capillary or chamber is the primaryfactor limiting the speed of a piezoelectric printhead andnot the response time of the piezoelectric transducer. Transducers in continuousinkjets operate at hundreds of thousands of cycles per
  43. 43. second. Drop on demand printers are constantly stopping and starting the flowof ink, whereas CIJ print heads receive acontinuous flow of ink and do not have to overcome inertia constantly as dodrop-on-demand print heads. Continuous inkjet also canguide ejected droplets with electrical force, while drop on demandpiezoelectric inkjet relies on the force and direction of expulsionand gravity as its only droplet guiding forces.The advantages of the more robust Xaar, Tektronix, Trident, Calcomp,Dataproducts, and Spectra piezoelectric printheads are theirability to print higher viscosity inks, a wide variety of inks with differentsolvents, heat sensitive inks and colors, and hot melt thermoplasticand paraffin. Piezoelectric printheads generally have high reliability and willlast for months to years of operation. Piezoelectricinkjets generally deposit consistent ink droplets which result in sharp images.The disadvantages of piezoelectric inkjets have been speed and cost. Newconfigurations of these heads are producing throughputwhich is much greater than that of comparable thermal inkjet printers asevidenced by the Calcomp CrystalJet, Raster GraphicsPiezoPrint 5000 and the Xerox Xpress 36 & 54, which are 2 to 3 times fasterthan Encad or HP thermal inkjet wide format printers.Summary of Piezoelectric