sublimation technology

8/14/2019 Sublimation Technology

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5.6.1 Overview of ThermographyTechnologies

As explained in figure 5.1-4, the NIP technology ofthermography can be in the main diided into thermaltransfer and thermal sublimation.

In both processes, the in! is applied to a donor "sheet

or #eb$ and then transferred to the substrate by the ap-plication of heat "or, depending on the system, first toan intermediate carrier #hich subse%uently transfers itto the substrate$.

&igure 5.'-1 adds to the illustration in figure 5.1-4 bysub-diiding thermography into direct thermographyand transfer thermography. (ransfer thermography isfurther sub-diided into thermal transfer and thermalsublimation.

Thermography

Direct Thermography Tansfer Thermography |

[Thermal Transfer ; fhermal Sublimation

Mass/Ink Transfer Dye Diffusion Thermal Transfer(D2T2! Sublimation or "blation

(constant ink film thickness/color #ensity in pi$el

(%ariable ink film thickness/color #ensity in pi$el

&

'onstant ariable)i$el Si*e )i$el Si*e

1 appro$imately'onstant )i$el Si*e

+ig, -,.& 0%er%ie1 of thermographic processes in thermography

)irect (hermography. In direct thermography the sub-strate is treated #ith a special coating, #hich changes itscolor #hen sub*ected to heat. (his !ind of special paperis often used for applications in fax machines and forlabeling and coding "e.g., bar codes$. +achines usingthermal printing systems "thermal printers$ are label

printers or receipt printers. )irect thermal printing #illnot be dealt #ith in detail here those processes are de-scribed that are largely independent of the substrate and#here the in! is supplied ia the system.

(ransfer (hermography. In thermal transfer, in con-trast to direct thermography, the in! is stored on adonor and is transferred to the substrate by the appli-cation of heat.

Put simply, part of the in! layer is released from thedonor and transferred to the substrate "a large %uantityof in! is transferred$. (he in! on the donor may be #axor a special polymer "resin$. &or this reason thermal

transfer is sometimes also called "thermal mass transfer?In thermal sublimation, on the other hand, the in! is

transferred from the donor to the substrate by diffu-sion. (he heat melts the in! and initiates a diffusionprocess onto the paper. (his re%uires a special coatingon the substrate to ta!e on the diffused colorants. (hephysically and chemically precise term for thermal sub-limation is "dye diffusion thermal transfer',' #hich isshortened to )(.

In! )onor. &igure 5.'- sho#s the structure of the in!donors. It sho#s the importance of selecting a suitablecombination of coating on the printing material and

in! layer on the donor material, particularly in thermalsublimation ")($.

hereas in thermal transfer the donor material is al-#ays in contact #ith the substrate during transfer, theremay be a small gap bet#een the receiing layer and thein! layer in thermal sublimation. (his can be achieedby spacers, for instance, #hich are integrated into either


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732 5 Printing Technologies without a Printing Plate !"P Technologies#

Ink carrier(#onor eat supply

"

)rotecti%elayer

)olyestercarriermaterial

)rimer

Ink layer(1a$! resin

Impression cylin#er

eat supply

Inkcarrier(#onor

Substrate(imagecarrier

3'oating$)aper

$ig. 5.6%2 Structure of ink #onor material,a Thermal transfer;b Thermal sublimation4 coating of paper 1ith #iffusion layer (inkrecei%ing layer

the receiing layer or the in! donor "e. g., spherical par-ticles, forming a special surface structure$.

Printing Unit. A simplified illustration of the funda-

mental principle of a thermography-based printingunit is gien in figure 5.'-/. In multicolor printing, forexample, the colors blac!, yello#, magenta, and cyanare applied to a donor. A thermal printing head is incontact #ith the donor material. 0y controlling theheating elements in this head in accordance #ith theimage "systems #ith a resolution of ' dpi are aail-able$ the in! is transferred from the donor to the pa-per. As mentioned preiously, the donor is in directcontact #ith the paper "or other substrate$.

&igure 5.'-/ sho#s ho# multicolor printing is carriedout by the thermographic transfer of color separationsonto the substrate. (he different in!s are positioned on

the in! donor ribbon one after the other. In multipassprinciple the color separations are collected on the paper.&igures 5.'-4 and 5.'-5 gie a simplified illustration

of the principles of thermal transfer and thermal subli-mation.

$ig. 5.6%3)rinciple of ink transfer onto the substrate in thermal transfer (multipass system

Thermal Transfer. As sho#n in figure 5.'-4, thermaltransfer is based on the in! melting onto the carrierfilm #hen heated. (he li%uefied in! is transferred to thesubstrate under lo# pressure. In the simple binaryprocess, the optical density of the print is set in adanceby producing the donor #ith a specified thic!ness oflayer, pigment concentration, and hue.

(his is the original ersion of thermal transfer a bi-nary in! transfer process - illustrated in figure 5.' -4 - thatconsists of controlling the heating element of the print-ing head and transferring the entire in! layer per pixel

onto the paper. hen the heating element is s#itchedoff there is no in! transfer. (he use of micromechanicaland microelectronic techni%ues in the mechanical de-sign of the thermal head facilitates finely controlledheating of the image area. (his ma!es it possible to trans-fer different %uantities of in!. )ue to the composition ofthe in! layer, ho#eer, the in! concentration of thetransfer remains constant, although the dot si2e mayary. (his means that smaller or larger amounts of in!can be transferred by defined melting. (his ariant ofthermal transfer is also called 3)( (variable dot thermaltransfer - see also the oerie# in fig. 5.'-1$.

Thermal Sublimation. &igure 5.'-5 sho#s the principleof thermal sublimation. In thermal sublimation, thein! eaporates locally through the application of heat,#hich triggers sublimation. In physical terms, subli-mation is the apori2ation of a solid #ithout the inter-mediate formation of a li%uid. (his is not necessarilythe case #ith thermal sublimation - the better, or gen-erally more accurate, term for the flo#ing process is de-fined by diffusion effects (dye diffusion thermal trans-

fer, D2T2. )epending on the thermal energy supplied

)aper

Substrate

,)rotecti%e layer

5i6'arrier material

57 Ink layer2f8(#yes

9:5 Diffusion laye r


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ariable pixel si2e as explained aboe, here thediameter of the dot remains roughly the samealthough the color density changes.

Ink Donor Transfer and Configuration. As illustratedin figures 5.'-4-b and 5.'-5, a complete section of the

donor material is used for each color in the format,#hich is designed specifically for the e%uipment.After the in! has been transferred, the remainingsurface of the donor sheet can no longer be used forprinting, #hich means a relatiely poor utili2ationof donor sheets is achieed.

(he in! donors "fig. 5.'-$, in the form of sheet or#eb material, typically hae a thic!ness of around 1 6im


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73& 5 Printing Technologies without a Printing Plate !"P Technologies#

the in! layer itself has a thic!ness of around / 7m. Inaddition to this there is a protectie layer of around #m. (he protectie layer has the tas! of ensuring goodheat transfer from the imaging system and simultane-ously facilitating secure handling of the thin material.&urther information on the structure of the material isgien in figure 5.'-.

As explained aboe, the $eb material is configured insuch a #ay that the indiidual colors are arrangedbehind one another on a #eb. As sho#n in figure 5.'-/the colors are transferred onto the printing substratesuccessiely in a printing unit. (hermal transfer formulticolor printing is also done #ith only the three col-ors cyan, magenta, and yello# blac! is produced sub-se%uently by oerprinting.

(he in! donors are also aailable in the form of sheet

material and are fed into the printing process using aspecial deice "figs. 5.'-8 and 5.'-1$.

(he substrate must be fed into the printing unit se-eral times "multipass system$. As detailed later, there arealso multicolor printing options that use the unit de-sign principle in order to raise productiity "fig. 5.'-9$.

:omparison of (hermography Processes. In figure5.'-'sections of printed imagesproduced by thermaltransfer and thermal sublimation are compared. It be-comes apparent that in thermal transfer only t#o grayalues can be produced #hile in thermal sublimation"fig. 5.'-'b$ arious gray alues are possible per dot

of the same si2e. "(he possibility of transferring ar-ious dot si2es in thermal transfer is not sho#n see fig.5-'-8b.$

A surey of the current state of thermography is gi-en by #ay of example in ;5.'-1< and ;5.'-


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5.6 Thermography 735

e%uipment designed in unit design is also aailable, assho#n in figure 5.'-9.

In the digital printing systems illustrated in figure5.'-9 four separate thermal transfer printing units (sin-

glepass system, unit design transfer each color separa-tion onto the substrate "sheet material$. (he %uality ofthe multicolor image is determined by the %uality ofthe in!s, the register precision of the oerprinting, andthe resolution. Page-#ide imaging systems "around/ mm$ are aailable #ith resolutions of up to ' dpi.(he system sho#n in figure 5.'-9 is e%uipped for print-ing #ith / dpi and a printing speed of A4 pagesper minute. In figure 5-'=9b an early ersion of theprinting system is sho#n to clarify the principle.

In figure 5.'-> another thermal transfer printing sys-tem is sho#n. It is designed for printing larger formats.

(he printing is carried out using four printing stations#ith a resolution of 4 dpi on $eb material #ith animage #idth of up to 8 mm.

&igure 5.'-8 sho#s a digitalproofing system for theA/? format that produces a four-color image by ther-mal transfer "fig. 5.'-8:$ #ith a ariable pixeldot si2e"fig. 5.'-8b$. (he color separations are successiely

$ig. 5.6%(Thermal transfer printing system for largeformat multicolor printing; unit #esign! resolution BAA #pi! printing spee# & m/min! image1i#th EAAmm (MS) :.! Matan

$ig. 5.6%7Digital multicolor printing system in unit #esign base# on the 3I) technology of thermal transfer; resolution :AA #pi! printing spee#2A "B pages per minute!": format,a Mo#el '>Ma$ :2BA (",F, Dick/Datametrics;b Mo#el G"?@" (Datametrics


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736 5 Printing Technologies without a Printing Plate !"P Technologies#

transferred from a color ribbon onto an intermediatecarrier at a resolution of / dpi. In a subse%uentprocess the image is then transferred from the inter-mediate carrier to the paper through the application ofpressure and heat. (he system sho#n in figure 5.'-8 hasa resolution of / dpi. )ots of arying si2e can be pro-duced by special screening, #hich is carried out in stepsin the paper transport direction that are smaller thanthe pixel distance related to the resolution "called 3@,variable resolution screeningby &u*i, fig. %.&-gb.

A higher %uality of image reproduction is possible than#ith a t#o-dimensional resolution of / dpi. (hetechnical implementation is also based on specialdonor material ;5.'-/


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&igure 5.'-1 sho#s a system for digital proofing #iththe highest resolution. (his system, as already sho#nin figure /.-9', images #ith a multibeam thermal laser

system "around >/ nm, beams$ using thermal trans-fer color sheets on an intermediate carrier "fig. 5.'-ioa$.

(his intermediate carrier "transfer base material$ isfixed onto a drum in the form of a sheet. (he system cantransfer the indiidual color separations onto the inter-mediate carrier #ith a resolution of up to / dpi. (heprocess is based on thermal transfer, #here the dots aretransferred in a binary fashion. (he halftone dot is builtup from seeral indiidual pixels "similar to the illustra-tion in fig. 5.'-'a, only #ith much higher resolution$. Inthe system sho#n in figure 5.'-1 the heat is supplied#ithout contact by laser light "thermal, about >/ nm$.

ith this high resolution the screen structure of multi-color printing can be reproduced, as it is in the offsetprocess for producing the print *ob #ith high run length."(he system sho#n in fig. 5.'-ioc is a multi-function sys-tem and can also be used for imaging printing plates #iththe same data file.$ (o transfer the indiidual color sep-arations to the image carrier, the operator inserts thecorresponding donor sheets into the system one by one.(he donor sheets are fixed to the drum by a separate ac-uum system. (he color image on the intermediate car-rier is transferred onto paper in a separate machine"laminator$ by the application of pressure and heat.

In the system sho#n in figure 5.'-1 the image is firsttransferred to an intermediate carrier and then ontopaper. (his substrate is specially conditioned to ensure

$ig. 5.6%1Digital thermal transfer proofing system formulticolor halftone proofing; resolution upto :2AA #pi! format4 "&C (J "B pages! 2Aminutes per"& fourcolor print, a Gayertransfer by thermal transfer; b alftonestructure 1ith four color separations;c )roofing system (Tren#Setter Spectrum4multifunctional system for platemaking an#proofing! ei#elberg/'reo/lmation

9/>


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5 Printing Technologies without a Printing Plate !"P Technologies#

stability and %uality. It is not possible to directly use theconentional production paper as it is used in a #iderange in offset printing. (here are in! donor structuresthat do allo# transfer onto this paper "as for example inthe PolaProof process by Polaroid, see fig. /.-99$. o#-eer, a special laminate is later imposed onto the print-ed image to stabili2e the transferred in!.

&oils "colorless$ #ith arious surface structures areaailable to create or imitate differentgloss effects.

5.6.3 Thermal 'u*limation Printing'ystems

(he principle of thermal sublimation #as gien in fig-ure 5.'-5.

In thermal sublimation a large number of different

gray values can be produced through controlled diffu-sion per dot of the in!ing materials "see also fig.5.'-'b$. As in thermal transfer the imaging can be car-

ried out using arrays of indiidually controllable heatelements, but also through heat transfer #ith thermallaser light sources.

Bince both thermal transfer and thermal sublimationare based on in! transfer using thermal energy, thesys-tems can, in principle, be used multifunctionally for bothprocesses #hen using suitable in!s. In the e%uipmentexample in figure 5.'-' it is possible to #or! #ith a sys-tem follo#ing either the thermal transfer or the thermalsublimation process. 3arious donor materials are com-bined #ith the same imaging system "thermal printhead$ depending on the process. In! donors coated #ith#ax are used for thermal transfer. In thermal sublima-tion high %uality, multi-layer treated in! donors are em-ployed, enabling production of different gray alues#ith the same dot si2e in the diffusion process.

&igure 5.'-11 sho#s one of the first large-format ther-mal sublimation proofing systems. In this highly auto-mated system the color separations are transferred to an

$ig. 5.6%11Digital color proofing system base# onthermal sublimation, a +unction principleof the "ppro%al system; resolution &JAA#pi! ":Cformat; b )roofing system 1ithlaminator ("ppro%al! o#ak;c )roofing system for "2C format; resolution 2BAA #pi; color #ensity for each colorapplication can be set at 22 le%els per

color separation for %arious #ensities! &-minutes per "2 )roof ("ppro%al N)B!o#ak )olychrome raphics

)rinting #rum (co%ere# 1ith the interme#iate sheet

Spent #onors Imaging hea#/ Image interme# iate proof

-~- ' ' " O O PPPP @ecei%ing hopper+ilm han#ling system(#onors an#interme#iate carrier

5.6 Thermography

'utter

'ompactcompressor(for suction


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73)

intermediate carrierby thermal laser e#posure ia donor

sheets cut from #ebs. (he material for the intermediatecarrier is also housed in the storage system for the donorreels of the process colors cyan, magenta, yello#, andblac!. Cne position is designated for a special color. (heintermediate carrier sheet is fixed onto a drum byacuum. (he color separations are transferred onto theintermediate carrier one by one in precise register iathe four donor sheets. (hese are held on the carrierdrum by means of a separate suction system. (he print-ed image is transferred from the intermediate carrieronto normal paper by a laminator "in fig. 5.'- 11b, sho#nin the bac!ground next to the proofing system$.

0y controlling the energy supplied to the dots bylaser, arious %uantities of in! can be diffused to pro-duce dots of the same si2e #ith ariable color density

"optical density, chroma$. (o be able to use this systemto produceproofs for the offset process, an imaging modeis of adantage that creates pixels #ith identical colordensities "basically the same principle as #ith conen-tional offset printing processes$ and the halftone dotsmade up of seeral pixels of the same density. (he sys-

tem can be operated as a Dthermal transfer e%uipmentDfor this purpose. (he system sho#n in figure 5.'-na hasa resolution of 1> dpi, #ith #hich a good replicationof the dot structure is possible.

&urther deelopments hae ta!en place on the prin-ciple of the system sho#n in figure 5.'-11. Bystems #ithan addressabilityresolution of 4 dpi are aailablefor the A? format. ith the system sho#n in figure5.'-nc a four-color proof in A format #ith a resolu-tion of 4 dpi is produced in about 15 minutes.

In principle, the system can also be operated as athermal transfer or as a thermal sublimation system#ith the adantage of thermal sublimation to generateseeral gray alues per pixel to reproduce continuous-tone color images. (he appropriate in! donor sheets,intermediate carriers, and receier paper sheets hae to

be used. (he laser imaging system must also be con-trolled at the appropriate energy leels "5' leels> bitsis possible$.

&igure 5.'-1 sho#s the layer structure of the donorsheet for thermal sublimation and the transfer to theintermediate carrier and later to theproduction paper

$ig. 5.6%12

Thermal sublimation 1ith interme#iate

carrier,

a Gayer structure of ink #onors! interme#i

ate carrier! transfer process onto pro

#uction paper;

b )rocess steps for in#irect thermal subli

mation (printing %ia interme#iate carrier

onto substrate; b&4 onto interme#iate

carrier! b24 laminating onto pro#uction

paper ("ppro%al! Mo#ak


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$ig 5.6%1&Digital proofing system

base# on thermalsublimation (D2T2 process; resolution :AA #pi!2-. gray %alues! ":Cformat! appro$, - minutesper ": proof (D') E-AA)roofer! o#ak )olychromeraphics

$ig. 5.6%15Digital thermal sublimation printer for

plastic car#s (:AA #pi! appro$, t1ocar#s per minute for printing 1ith fourcolors an# a##itional coating film; canalso be a#Huste# for operation as thermal transfer printer (T0)! +U0lectronic Systems

pact systems and toin! used in in! *etsystems. o#eer,the higher cost of in! is to some extent compensatedby the fact that certain components, such as a de-eloping unit for supplying toner in electrophotogra-phy, is no longer re%uired and that the thermal printingheads are of a simpler design than is the case #ith in!*et systems. Printing systems based on thermal transferhae a relatiely simple structure.

@eferences in -,.;5.'-1< 0IB Btrategic )ecisions "Fd.$G (hermal Printing in

the 188s. Cerie# and Cutloo!, Nor#ell "+A$ 188.

;5.'-< 0IB Btrategic )ecisions"Fd.$G (hermal Printing 1885 -Ne# Products, Applications,and Cpportunities. Nor#ell

"+A$ 1885.;5.'-/< Na!amura, . et al.G igh Huality alftone

(hermal Imaging (echnology by D(hin-layer (hermal(ransferD "/($ (echnology sed for &I@B( P@CC&J.NIP 1/G International :onference on )igital Printing(echnologies "Proceedings$, (he Bociety for ImagingBcience and (echnology "IBK($, Bpringfield "3A$ 1889,pp. 9'8-99-

;5.'-4< )e0oer, :G Laser (hermal +edia - (he Ne#Mraphic Arts Paradigm. ournal of Imaging Bcience and(echnology, ol. 4,188>, pp. '/-'8.

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