CA2075948C

CA2075948C - Imaging system with intermediate transfer member

Info

Publication number: CA2075948C
Application number: CA002075948A
Authority: CA
Inventors: Ishaiau Lior; Benzion Landa; Amiran Lavon; Hanna Pinhas
Original assignee: Indigo BV
Current assignee: HP Indigo BV
Priority date: 1989-01-04
Filing date: 1990-12-13
Publication date: 2001-07-03
Anticipated expiration: 2010-12-13
Also published as: CA2075948A1; US5572274A

Abstract

Imaging apparatus for printing an image on a substrate (42) from a latent image formed on a latent image bearing surface (16) including: developing apparatus (22) for developing the latent im-age with a liquid developer to form a developed liquid toner image;
a first intermediate transfer member (4); first transfer apparatus for transferring the developed image from the latent image bearing sur-face (16) to the first intermediate transfer member (40) at a first transfer region; a second intermediate transfer member (47); second transfer means for transferring the developed image from the first image transfer member (40) to the second intermediate transfer member (47) at a second transfer region; and third transfer appara-tus for transferring the developed image from the second intermedi-ate (47) transfer member to the substrate (42).

Description

WO 9x/10793 PCTI;"~I1,991p~t1$2 _ 1 _ ~~'~~94~
1 IMAGING SYSTEM WITH INTERMEDIATE TRANSFER PdEMBER

3 The present invention relates to image transfer 4 techniques and apparatus for use in electrophotography.
BACKGROUND OF THE INVENTION
6 Various prior publications deal with the transfer of 7 single and multiple powder and liquid toner images from a 8 photoreceptor on which they are formed to an intermediate 9 transfer member for subsequent transfer to a final substrate.
U. S. Patent 3,838,919 to Takahashi describes a powder 11 toner system in which color toner images are sequentially 12 formed on an image forming member, individually transferred 13 to an intermediate transfer member and transferred at one 14 time to a recording member.
U. S. Patent 4,144,808 to Isawa et al. describes a 16 method of printing on a metal plate utilizing powder toner 17 and an intermediate transfer member where the plate is 18 heated before transfer.
19 U. S. Patent 4,518,976 to Tarumi et al. .describes a monochrome powder toner system in which a, powder image is 21 developed on a photoreceptor, and transferred 22 electrostatically to an intermediate transfer member.
23 Downstream this transfer, the intermediate transfer member 24 and the image thereon are heated before transfer to a preheated substrate.
26 U. S. Patent 4,515,460 to Knechtel, describes a powder 27 toner apparatus wherein separate toner images are 28 sequentially developed on a photoreceptor and 29 electrostatically transferred to an intermediate transfer member. After all of the individual images have been 31 transferred to the intermediate transfer member, they are 32 transferred electrostatically to the final substrate. No 33 heating of the images or substrate is disclosed.
34 U. S. Patent 4,585,319 to Okamoto et al. describes a powder developer type, single color system, utilizing a w 36 temperature controlled photoreceptor, a heated intermediate 37 transfer member and a heated transfer fixing roller which is 38 heated to a temperature slightly higher than that of the d:..~~~ t 7~~~'~ ~

WO 92/10793 ~ PCT/1~IL90/0~18Z
2~~~J4~ ~ 2 _ 1 intermediate transfer member.
2 U. S. Patent 4,690,539 to Radulski et al. describes a 3 liquid toner multi-color system in which a color image is 4 developed on a photoreceptor. and transferred to a belt type intermediate transfer member. The liquid carrier is removed 6 from the toner image'on the belt. There is no mention of 7 heating the intermediate transfer member or of the problem of 8 back transfer.
9 U. S. Patent 4,708,460 to Langdon describes a single color liquid toner system in which a developed image is 11 transferred from a photoreceptor to an intermediate transfer 12 member, heated on the transfer member and then transferred to 13 a final substrate.
14 ~ U. S. Patent 3,847,478 to Young describes a duplex printing system, wherein a developed image is transferred 16 from a photoconductor to an intermediate transfer member, a 17 second image is developed on the photoconductor and both 18 images are transferred electrostatically to opposite sides of 19 a piece of paper passed between the intermediate transfer member and the photoreceptor. ~

23 i 27 ' ~,6 'rte...... , P ~''.~'~ ~""t;~'.°' a. .,.. ia...o : b a 'e". ~ ~ 1 uew '~O 92/10793 PCT/NL90/001$2 ~a'~,a9~$

2 The present invention seeks to provide improved 3 apparatus for image transfer.
4 Tt is an object of the present invention to provide a method and apparatus for the improved transfer of an image 6 from an image bearing surface to an intermediate transfer 7 member.
8 It is a further object of the present invention to 9 provide a method and apparatus for the improved transfer of 1G an image from an image bearing surface to an intermediate 11 transfer member and subsequent transfer to a final substrate.
12 It is a further object of the present invention to 13 provide a method and apparatus far the sequential transfer of 14 a plurality of superimposed images to an intermediate transfer layer without back transfer.
16 There is thus provided in accordance with a preferred Z7 embodiment of the invention imaging apparatus far printing an 18 image on a substrate from a latent image formed on a latent 19 image bearing surface including:
developing apparatus far developing the, latent image with 21 'a liquid developer to form a developed liquid toner image;
22 a first intermediate transfer member;
23 first transfer apparatus far transferring thei developed 24 image from the latent image bearing surface to the first intermediate transfer member at a first transfer region;
26 a second intermediate transfer member;
27 second transfer apparatus far transferring the developed 28 image from the first image transfer member to the second 29 intermediate transfer member at a second transfer region; and third transfer apparatus for transferring the developed 31 image from the second intermediate transfer member to the 32 substrate.
33 In a preferred embodiment of the invention the imaging 34 apparatus includes heating apparatus for heating the developed liquid toner image to a first temperature higher 36 than room temperature at the first transfer region and to a 37 second temperature higher than the first 'temperature at the 38 second transfer region.
~ ~.g ,~ s..~
~~~~~J i 1 r ~ ~ r ve W ..:r 'NO 92/i0793 P9CT/NL9~lO(91$2 1 In a preferred'embodiment of the invention the imaging 2 apparatus includes ~ 'in'terinediate transfer member heating 3 apparatus for heating the first intermediate transfer member 4 to a first temperature and for heating the second intermediate transfer member tW a second temperature higher 6 than the first temperature.v 7 In a further preferred embodiment of the invention liquid 8 toner image transfer from the image bearing surface is 9 enhanced at temperatures above a first given temperature;
liquid 'toner image transfer to the final substrate is 11 enhanced at temperatures above a second given temperature, 12 higher than the first given temperature; and 13 undesirable image transfer from the intermediate transfer 14 member to the image bearing surface is increased at temperatures above a third given temperature, higher than the 16 first given temperature and lower than the second given 17 temperature, and 18 the first temperature is above the first given 19 temperature and below the third given temperature, and wherein the second temperature is above the second given 21 temperature.
22 In a preferred embodiment of the invention the imaging 23 apparatus also includes first voltage apparatus for 24 maintaining the first intermediate transfer member at a first voltage. Preferably at least a portion of the latent image 26 bearing surface is at a second voltage and the first voltage 27 is different from the second voltage. Preferably the 28 apparatus also includes second voltage apparatus for 29 maintaining the second intermediate transfer member at a third voltage.
31 In a preferred embodiment of the invention the developing 32 apparatus~is operative for developing a latent image to form .
33 a second developed liquid toner image thereon after transfer 34 of the developed liquid toner image therefrom to the intermediate transfer member and the first transfer apparatus 36 is operative to transfer the second liquid toner image to the 37 first intermediate transfer member, without substantial back-38 transfer of the first image to the image bearing member, to S°U~STI'D'UTE So'-IEET

°

~V~D 92/107<)3 PC~/NL9U/Of)182 _ 5 _ ~~'~~9~~
1 form a composite image. Preferably the second transfer 2 apparatus is operative to transfer. the composite image to the 3 second intermediate transfer member.
4 In a preferred embodiment of the invention the developing apparatus is operative to develop a different latent image to 6 form a different liquid toner image on the image bearing 7 surface: and the first.transfer apparatus is operative to , 8 transfer the different liquid toner image to the first 9 intermediate transfer member subsequent.to transfer of the developed liquid toner image therefrom to the second image 11 transfer member. Preferably the third transfer apparatus the 12 third transfer apparatus includes apparatus for supplying a 13 substrate to the second transfer region; and apparatus for 14 urging the first and second transfer members against each other whereby the developed liquid toner image is transferred 16 to one side of the substrate and the different liquid toner 17 image is transferred to the other side of the substrate.
18 There is further provided in a preferred embodiment of 19 the invention imaging apparatus for printing an image from a latent image formed on a latent image bearing surface 21 includinga 22 developing apparatus for developing the latent image 23 with a liquid developer to form a developed liquid toner 24 image: .
a heated intermediate transfer member for receiving the 26 developed image from the latent image bearing surface at a 27 first transfer region, fox subsequent transfer to a final 28 substrate at a second transfer region: and 29 cooling apparatus for cooling a portion of the intermediate transfer member prior to transfer of a portion 31 of the developed image to the cooled portion of the 32 intermediate transfer member.
33 In a preferred embodiment ~f the invention liquid toner 34 image transfer from the image bearing surface is enhanced at temperatures above a first given temperature; liquid toner 36 image transfer to the final substrate is enhanced at 37 temperatures above a second given temperature, higher than 38 the first given temperature; and undesirable image transfer S~3~S'T1'1~'~'~ ~~-l~r ~

WO 92/10793 ~~ 'r.~~ ~~ P~'T/NL90/fl0182 V
_ 6 _ 1 from the intermediate transfer member to the image bearing 2 surface is increased at temperatures above a third given 3 temperature, higher than the first given temperature and 4 lower than the second given temperature, and wherein the intermediate transfer member is heated to a temperature above 6 the second temperature at the second transfer region and the 7 cooling apparatus is operative to cool the intermediate 8 transfer member to a temperature above the first temperature 9 and below the third temperature at the first transfer region.
2n a preferred embodiment of the invention the developing 11 apparatus is operative for developing a latent image to form 12 a second developed liquid toner image thereon after transfer 23 of the developed liquid taper image therefrom to the 14 intermediate transfer member; gnd the intermediate transfer member and the cooling apparatus are operative to transfer 16 the second developed liquid toner image to the intermediate I7 transfer member, without substantial back-transfer of the -18 first liquid toner image to the image bearing member, to form 19 a composite image. .
There is further provided in a preferred embodiment of 21 the invention imaging apparatus for printing an image on a 22 substrate from a latent image formed on a latent image 23 bearing surface includinga i 24 developing apparatus for developing the latent image with a liquid developer to form a developed liquid toner image;
26 an intermediate transfer member heated to a first 27 temperature;
28 first transfer apparatus for transferring the developed 29 image from the latent image bearing surface to the intermediate transfer member at a first transfer region;
31 second transfer apparatus for transferring the developed 32 image from the intermediate transfer member to the substrate 33 the second transfer apparatus including 34 apparatus for heating the substrate to a second 3S temperature higher than the first temperature.
36 Tn a preferred embodiment of the invention the apparatus 37 for heating comprises a heating backing roller operative to 38 apply heat and pressure to the image during the second ~E.°''' °'~ "'" ~~-;~ ~z -iT1 4.:':,J 1 ~ a ~ 6 n.s l~.~a f °~m' 'iW0 92/10793 FLi'/~1L90/Ut31$Z
- 7 _ 20'~~~~:~
1 transfer.
2 In a preferred embodiment of the invention the developing 3 apparatus is operative for developing a latent image to form 4 a second developed liquid toner image thereon after transfer ' of the developed liquid toner image therefrom to the 6 intermediate transfer member; and .
7 the first transfer apparatus is operative to transfer the 8 second developed liquid toner image to the intermediate 9 transfer member, without substantial back-transfer of the ' developed liquid toner image to the image bearing member, to 11 form a composite image.
12 BRIEF DESCRIPTION OF TFiE DRAWINGS
13 The present invention will be understood and appreciated Z4 more fully from the following detailed description, taken in conjunction with the drawings in which:
16 Fig. 1 is a simplified sectional illustration of 17 electrophotographic apparatus constructed and operative in 18 accordance with a preferred embodiment of the present 19 invention;
Fig. 2 is a simplified sectionalv illustration of 21 electrophotographic apparatus constructed and operative in 22 accordance with another preferred embodiment of the present 23 invention;
i 24 Fig. 3 is a simplified sectional illustration of ' electrophotographic apparatus constructed and operative in 26 accordance wzth yet another preferred embodiment of the 27 present invention;
28 Fig. 4 is a simplified sectional illustration of a 29 electrophotographic apparatus constructed and operative in accordance with yet another preferred embodiment of the 31 present invention;
32 Fig: 5 is a simplified sectional illustration of 33 electrophotographic apparatus constructed and operative in 34 accordance with yet another preferred embodiment of the present invention;
36 Fig. 6 is a simplified sectional illustration of 37 electrophotographic apparatus constructed and operative in 38 accordance with yet another preferred embodiment of the e°~'.r~~~rL~~ ~ i~ v» sa:.~

WO ~2/107~3 ~~~~~ PCT/~(L9(~/~3(ll$2 _ g _ 1 present invention; and 2 Fig. 7 is a graphical illustration of the temperature 3 variation along a low thermal mass intermediate transfer 4 member in an arrangement such as that illustrated in Fig. 6.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
6 Reference is now made to Fig. 1 which illustrates 7 electraphotographic imaging apparatus constructed and 8 operative in accordance with a preferred embodiment of the 9 present invention. This and other embodiments of the invention are described in the context of liquid developer 11 systems with negatively charged toner particles and 12 positively charged photoreceptors. Such systems operate in a 13 "write-white" mode, for which areas which are not to be toned 14 are exposed to light. The. invention may be useful for other combinations of toner charge, photoreceptor charge as well as 16 for other writing systems, such a~ °'write-black" systems.
17 The apparatus of the invention is described using a 18 liquid developer system. In accordance with a preferred l9,embodiment of the invention the liquid developer of Example 1 of U. S. Patent 4,794,651 can be used, bu°t other suitable 21 developers may be used in the practice of the invention.
22 Especially useful are liquid developers comprising toner .
23 particles which solvate the carrier liquid of the develoger 24 at elevated temperatures, above room temperature.
As in conventional electrophotographic systems, the 26 apparatus of Fig. 1 comprises a drum 10 arranged for rotation 27 about an axle 12 in a direction generally indicated by arrow 28 14. Drum 10 is formed with a cylindrical photoreceptor 29 surface 16.
A corona discharge device 18 is operative to generally 31 uniformly charge photoreceptor surface 16 with a .positive 32 charge. ~ Continued rotation of drum 10 brings charged , 33 photoreceptor surface 16 into image receiving relationship 34 with an exposure unit including a lens 20. Lens 20, focuses a , desired image, which may be laser generated, onto charged 36 photoreceptor ,surface 16, selectively discharging the 37 photoreceptor surface, thus producing ~an electrostatic 38 latent image thereon.
ur ':.. i.:. ...~ i 1: a :ad 'e ~ a...'Z ie Continued rotation of drum 10 brings charged photoreceptor surface 16 bearing the electrostatic latent image into operative association with a development unit 22, operative to apply a liquid developer to develop the electrostatic latent image. For multicolor copying or printing, the development unit 22 can, for example, comprise a plurality of developers, one for each color, which are selectively engaged with the photoreceptor, as described, for example, in U.S. Patent 4,690,539, or a single development station where the liquid toner is changed between colors, or any other suitable development system. In general this development process takes place at a relatively low temperature, namely approximately the temperature of the environment of the system.
In accordance with a preferred embodiment of the invention, following application of toner thereto, photoreceptor surface 16 passes a typically positively charged rotating roller 26, preferably rotating in a direction indicated by an arrow 28.
Roller 26 functions as a metering roller and reduces the thickness of liquid on photoreceptor surface 16. Typically the spatial separation of roller 26 from photoreceptor surface 16 is about 50 microns.
Preferably the voltage on roller 26 is intermediate the voltages of the latent image areas and of the background areas on the photoreceptor surface. Typical voltages are: roller 26:
+200V, background area: +50V and latent image areas: up to about +looov.
Liquid which passes roller 26 should be relatively free of pigmented particles except in the region of the latent image.
Downstream of roller 26 there is preferably provided a rigidizing roller 30. Rigidizing roller 30 is preferably formed of a resilient polymeric material, for example a slightly conductive resilient polymeric material as described in either or both of U.S. Patents 3,959,574 and 3,863,603. Roller 30 is preferably resiliently urged against photoconductive surface 16.

-1~-In a preferred embodiment of the invention , an electrically biased squeegee roller is used as roller 30. Roller 30 is negatively charged to a potential of at least several hundred and up to 2000 volts with the same sign as the charge on the pigmented toner particles, so that it repels similarly charged pigmented particles and causes them to more closely approach the image areas of the photoreceptor surface 16, thus compressing and rigidizing the image.
Downstream of rigidizing roller 30 there is provided an intermediate transfer member 40, which rotates in a direction opposite to that of photoreceptor surface 16, as shown by arrow 41, providing zero relative motion between their respective surfaces at the point of propinquity. Intermediate transfer member 40 is operative for receiving the toner image from photoreceptor surface 16 and for transferring the toner image to a receiving substrate 42, such as paper. Disposed internally of intermediate transfer member 40 there may be provided a heater 46, to heat intermediate transfer member 40.
Various types of intermediate transfer members are known and are described, for example in U.S. Patent 4,684,238, PCT
Publication WO 90/04216 and U.S. Patent 4,974,027.
Following the transfer of the toner image to intermediate transfer member 40, photoreceptor surface 16 engages a cleaning station 49. This station may be any conventional cleaning station, comprising a cleaning roller 50 which may comprise a suitable resilient material such as foam polyethylene or neoprene.
Cleaning roller 50 may be wetted by clean lubricating cleaning liquid, which preferably comprises liquid developer from which all or nearly all of the toner particles have been removed. Cleaning roller 50 is driven so that its surface moves opposite to surface 16 at their nip, to provide scrubbing action for removal of residual particles and carrier liquid from photoreceptor surface 16. A scraper 56 completes the removal of any WO 92/10793 PC~1'/1'AIL9b/bb382 - 11 '~~~7~48 1 residual toner which may not have been removed by cleaning 2 statian 49.
3 A lamp 58 completes the cycle by removing any residual 4 charge, characteristic of the previous image, from semiconductor surface 16.
6 Transfer of the image to intermediate transfer member 40 7 is preferably aided by providing electrification of 8 intermediate transfer member 40 to a voltage opposite that of 9 the charged particles, thereby causing transfer by i0 electrophoresis. It has been found by the inventors, that, at 11 least for the preferred developer, raising the temperature of 12 the developed toner image to a temperature higher than the 13 development temperature and room temperature aids this first 14 transfer, even when the transfer is by electrophoresis.
Subsequent final transfer o~ the image from intermediate 16 transfer member 40 to substrate 42 is preferably aided by 17 heat and pressure. A higher temperature.than that used for 18 first transfer is preferably utilized for this subsequent 19 finml transfer, in accordance with the present invention.
In the prior art a liquid tonero image was first 21 transferred to an intermediate transfer member. The toner 22 image was heated during the interval'between first and second 23 transfer so as to aid in final transfer.
i 24 In the present invention the preferred first transfer step, i.e., the transfer of the liquid toner image to the 26 intermediate transfer member includes the heating of the 27 image either before or during first transfer. The preferred 28 final transfer step, i.e., the transfer of the liquid toner 29 image to the final substrate, includes the Further heating of the image before and/or during second transfer. This further 31 heating can be achieved by heating the image on intermediate 32 transfer. member 40, far example by heat transfer from 33 intermediate transfer member 40 during the interval between 34 first and final transfer and/or by external heating of the image. Preferably the image is heated to a temperature at 36 which it solvates liquid to form a single phase, without 37 evaporating substantial amounts of °liquid carrier.
38 Alternatively or additionally the further heating can be ~d1's."~s~~L,~e,s ~,.:~_.

'~/~ ~Z/i0793 f~:.f/~~JL90100182 ~~~ ~c~ f~$ - 12 -1 achieved by conduction heating of the image from the final 2 substrate during final transfer.
3 These preferred first and second transfer steps improve 4 the quality of the image on the final substrate both for single color and for~multi-color images.
6 For multicolor systems it is useful to sequentially 7 transfer the separate colors to intermediate transfer member 8 40 in alignment with and generally superimposed and in ' 9 registration with each other and then to transfer them together to paper or other substrate 42. It has then been 11 found that for this configuration, there is a tendency ~or 12 the heated images previously transferred to the intermediate 13 transfer member at a lower temperature, to transfer back, in 14 whole or in part, to photoreceptor surface 16, when the previously transferred image returns to the point of first 16 transfer.
17 The embodiments of the invention described herein 18 provide improved first and final transfer and for multicolor 19 systems can solve the back transfer problem.
In general, some of the embodiments o~ the invention are 21 characterized in that photoreceptor 16 is at a first, 22 relatively low temperature; intermediate transfer member 40 23 is at a second, somewhat higher temperature, to provide for 24 improved first transfer;. and final substrate 42 is at a third, even higher temperature to provide for good transfer 26 from intermediate transfer member 40 to substrate 42.
27 Alternatively or additionally, some of the embodiments 28 can be characterized in that, when a toner image is 29 transferred from photoreceptor surface 16 to intermediate transfer member 40, and there to final substrate 42, the toner 31 image is hotter during transfer to the intermediate transfer 32 member than it was an the photoreceptor surface and the image 33 is hotter when it is transferred to the final substrate, than 34 during the earlier transfer.
Alternatively or additionally, some of the embodiments 36 can be characterized in that, when multiple toner images are 37 transferred sequentially from photoreceptor surface 16 to 38 intermediate transfer member 40, and then to final substrate ~~~:~k~~~l~

1A/t~ 92l 10793 f C j'l ~l L90/00182 _ 13 _ ~Q ~~~~$
1 42 as a group, the composite, multicolor toner image is 2 hotter when it is transferred to the final substrate than 3 during any contact of earlier transferred images with the 4 photoreceptor.
One embodiment of the izwention can be characterized in 6 that the image is transferred from a photoreceptor surface, 7 at a first relatively low temperature to a first intermediate 8 transfer member at a second intermediate temperature. The 9 image is then transferred to a second intermediate transfer member. Final transfer takes place from the second 11 intermediate transfer member to the final substrate at a 12 third, higher temperature. Preferably, the image temperature ,.
13 during first transfer is higher than that of that portion of 14 the photoreceptor surface not in contact with the intermediate transfer member.
16 Returning now to Fig. 1, intermediate transfer member 40 17 is heated to a temperature sufficient to enhance the 18 electrophoretic transfer of toner particles from 19 photoreceptor surface 16 to intermediate transfer: member 40.
The image is heated during transfer to 3nte~medxate transfer 21 member 40, and the heating continues while the image is on 22 intermediate transfer member 40 until the image is at the 23 temperature of intermediate transfer member 40. Rotation of 24 intermediate transfer member 40 brings the heated intermediate transfer member 40 into image transfer 26 relationship with a final substrate 42, which is pressed 27 against the intermediate transfer member by a heated backing 28 roller 43. Heated backing roller 43 heats the paper and 29 thereby heats the image in contact therewith by conduction from the paper, to a sufficient degree to ensure that 31 complete or nearly complete final transfer of the image to 32 the substrate, by heat and pressure, takes place.
33 While the invention has been described in a 34 monochromatic version, where it gives improved transfer from the photoreceptor to the intermediate transfer member and 36 from the intermediate transfer member to the final substrate, 37 the invention is particularly useful in a'multicolor system, 38 wherein images of different colors are sequentially formed on SUBSTITUTE aHEEi' 1~V0 92/1(1793 ~~ ~'~ ~~ ~cri~r~.~oioomz 1 photoreceptor surface 16, and transferred one by one in 2 mutual alignment to image transfer member 40 prior to a ' 3 single transfer of all of the images, which form a multicolor 4 image, to final substrate 42.
Final substrate 42 is brought into transfer engagement 6 with intermediate transfer member 40 only when all of the _ 7 colors have been transferred to intermediate transfer member 8 40, for final transfer of the multicolor image to substrate 9 42.
ZO As noted above, it is appreciated that during first 11 transfer of subsequent images from photoreceptor surface 16 12 to image transfer member 40, earlier transferred images 13 return to the region of fizst transfer. Any back transfer of 14 previously transferred images to photoreceptor surface 16 will result in undesirable artifacts in the final printed 16 image.
17 Generally if the intermediate transfer member is heated 18 to a temperature which is useful for good final transfer, 19 then there is a tendency for the image to back transfer to the photoreceptor.
21 The arrangement of Fig. 1, with proper choice of 22 temperatures far intermediate transfer member 40 at first 23 transfer, and for final substrate 42 arid the image at i second 24 transfer in accardance with the present. invention, substantially eliminates the problem o~ back transfer to 26 photoreceptor surface 16, by keeping the image temperature, 27 when the image on the intermediate transfer member returns to 28 the photoreceptor, low enough so that it is not tacky enough 29 to stack to the photoreceptor.
Fig. 2 shows a second embodiment of the invention in 31 which all of the parts and operation are generally the same 32 as those of the apparatus of Fig. 1, except that heated 33 backing roller 43 is replaced by an unheated backing roller 34 44, and final substrate 42 is preheated by a heating lamp 45.
A combination of the embodiments of Figs. 1 and 2 is also 36 useful, whereby paper 42 is pre-heated by lamp 45, and heated 37 roller 43 is used. °
38 A third embodiment of the apparatus of the invention is ~.,~~J~'~~~~~ ~W.s a WO 92/1793 PL.'t/ML90/001$2 15 ~-1 shown in Fig. 3. In this case intermed ~t~ ~~r~~sfer member 40 2 is heated to a first, moderate, temperature which is high 3 enough to enhance first transfer, but not so high as to cause 4 substantial back transfer of previously transferred images from intermediate transfer member 40 to photoreceptor surface 6 16. The images are transferred to a second intermediate 7 transfer member 47 which is heated by an internal heater 48 8 to a higher temperature, sufficient to assure good final 9 transfer to final substrate 42.
In a preferred embodiment of the invention, intermediate 11 transfer member 40 is maintained at a first voltage 12 (different from the voltage of the photoreceptor surface 16) 13 to enhance transfer of the image thereto from photoreceptor 14 surface 16, and second intermediate transfer member 47 is electrified to a second voltage, different from the first 16 voltage, to enhance transfer of the image thereto from 17 intermediate transfer member 40.
18 Transfer to second intermediate transfer member 41 can 19 occur sequentially for each of the images, or preferably the images are collected on first intermediate vtransfer member 40 21 and then the multicolor image is transferred as a whole to 22 second intermediate transfer member 47 for final transfer to 23 the final substrate 42.
I
24 A dupleac embodiment of the invention, for printing two sides of a substrate at the same time is shown in Fig. 4. The 26 separate color images which make up the multi-colored image 27 to be printed on a first side of substrate 42 are first 28 transferred sequentially to intermediate transfer member 40 29 and then are transferred, pa~eferably as a group, to second intermediate transfer member 47. Second image transfer member 31 47 is preferably heated to a higher temperature than 32 intermediate transfer member 40. The images to be printed on 33 the other side of the page are subsequently transferred 34 sequentially to intermediate transfer member 40, which is meanwhile kept out of transfer engagement with second 36 intermediate transfer member 47.
37 Final substrate 42 is then passed between intermediate 38 transfer member 40 and second intermediate transfer member ~mr V V L ~4.' 7 wa WO 92/10793 ~ F'C.'.TlNL90/00182 r _ 16 -1 47, while pressing the two intermediate transfer members 2 together to effect transfer of the images to bath sides of 3 the paper by heat and pressure. It is understood that 4 preferably second intermediate transfer member 47 heats substrate 42 and the image to a suitable temperature to 6 assure good transfer of the image on intermediate transfer 7 member 40 to substrate 42. Alternatively or additionally, the 8 paper may be heated before transfer as described above in 9 connection with Fig. 2.
ZO In same preferred embodiments of the invention 11 intermediate transfer member 40 acts to heat the image to a 12 first temperature during first transfer from photoreceptor 16 13 to intermediate transfer member 40, and to heat the image to 14 a second higher temperature before second and final transfer from intermediate transfer member 40 to final substrate 42.
16 Exemplary embodiments include the apparatus shown in 17 Fig. 5. This apparatus is generally the same as the apparatus 18 of Fig. 1, except that a cooling station 60 is operatively 19 associated with intermediate transfer member 40~just before it returns. to make contact with photoreceptor surface 16.
21 Intermediate transfer member 40 is cooled at cooling station 22 60 to locally reduce the temperature of intermediate transfer 23 member 40 before and during contact with the image on the 24 photoreceptor. This local cooling allows the liquid toner ,image to be hotter at the point of final transfer from 26 intermediate transfer member 40 to final substrate 42 than it 27 is at first transfer from photoreceptor surface 16 to 28 intermediate transfer member 40.
29 Cooling station 60 may comprise, for example, apparatus for providing a stream of cool air to the surface of the 31 photoreceptor or a cooled roller in contact with the 32 phatoreceptor surface. Either or both cooling systems cool 33 intermediate transfer member 40 to a temperature, higher than 34 room temperature, but lower -than the final transfer temperature.
36 In a multicolor system, if a roller cooler is used it is 37 coated with a non-stick coating to avoid transfer of the 38 image from intermediate transfer member 40 to the roller of ~u~s°re~uT~ sr~~~-r cooling station 60.
Another exemplary embodiment of this type is illustrated in Fig. 6, which is essentially the same as Fig. 8 of WO
90/04216 previously referenced. Here an intermediate transfer member 140 is of low heat capacity, and is heated only after first transfer is completed. As shown in Fig. 7, which is the same as Fig. 9 of the above referenced application, the temperature at the first transfer is above room temperature in order to improve first transfer, and the temperature at second transfer is even higher to assure complete or nearly complete second transfer. For a multi-color system the temperatures and heat capacities are selected so that the first transfer takes place at a temperature low enough to avoid back transfer.
In the above embodiments, intermediate transfer members 40 and 47 have been described as having heaters placed internal to the core to heat each of them to its required temperature. Other methods of heating intermediate transfer members known in the art can also be used in the practice of the invention.
Examples Colored liquid developer is prepared in the following manner:
Preparation of Black Liquid Developer 10 parts by weight of Elvax 5720 (E. I. Du Pont) and 5 parts by weight of Isopar L* are mixed at low speed in a jacketed double planetary mixer connected to an oil heating unit for one hour, the heating unit being set at 130 degrees C.
A mixture of 2.5 parts by weight of Mogul ~ carbon black (Cabot) and 5 parts by weight of Isopar L~ are then added to the mix in the double planetary mixer and the resultant mixture is further mixed for one hour at high speed. 20 parts by weight of Isopar L preheated to 110 degrees C are added to the mixer and mixing is continued at high speed for one hour.
The heating unit is then disconnected and mixing is continued until the temperature of the mixture drops~to 40 degrees C.
The resulting mixture is transferred to an S-1 attritor * Trade raarks device equipped With 3/16 inch carbon steel media, diluted with Isopar L*to a 16% solids ratio and ground without cooling until the temperature rises to about 60 degrees C.
Cooling, which reduces the temperature to about 30 degrees is then commenced and grinding is continued for a total of 24 hours. The mixture is removed from the device and diluted with Isopar L to 1.5% by weight solids concentration. The particles in the resultant toner concentrate have an average diameter of 2.5 microns.
Charge director as known in the art, is added to give the final liquid developer. In a preferred embodiment of the invention the charge director of Example 1 of PCT publication WO 90/14617 the disclosure of which is incorporated herein by reference, is added to give the final liquid developer.
Preparation of Colored Developer 10 parts by weight of Elvax 572t~ (E. I. Du Pont) and 5 parts by weight of Isopar L are mixed at low speed in a jacketed double planetary mixer connected to an oil heating unit for one hour, the heating unit being set at 130 degrees C.
Pre-heated Isopar L is then added to reduce the solids concentration to preferably 35% and mixing is continued at high speed for one hour. The heating unit is then disconnected and mixing is continued until the temperature of the mixture drops to 40 degrees C.
The mixture is then transferred to an S-1 attritor device equipped With 3/16 inch carbon steel media and pigment is added to the material in the attritor. The mixture is diluted with Isopar L to about a 12-16% solids ratio, depending on the viscosity of the material and is ground without cooling until the temperature rises to about 60 degrees C. Cooling, which reduces the temperature to about 30 degrees, is then commenced and grinding is continued for a total of 24 hours. The mixture is removed from the device and diluted with Isopar L* to 1.5% by weight solids concentration. The particles in the resultant toner concentrate had an average diameter of 2.5~microns.
Charge director as known in the art, is added to give * Trade marks the final liquid developer. In a preferred embodiment of the invention the charge director of Example 1 the above referenced PCT publication WO 90/14617 is added to give the final liquid developer.
Appropriate colored pigments known in the art of liquid ~3eveloper manufacture, for example the list given in U. S.
Patent 4,794,561 can be used. Other suitable pigments are Sico Fast Yellow D1350 (BASF), Lithol Rubin D4576 (BASF), Lyonol Hlue FG7351*(TOYO) and Lyonol Yellow 761310 (TOYO). in amounts and combinations depending on the color and intensity required. Optionally, Aluminum Stearate can be added in small amounts. For pigments which are discolored by steel, other grinding media such as zirconia may be used.
These developers are used to form the individual color liquid toner images on photoreceptor surface 16 which comprise a relatively high concentration of toner particles in carrier liquid.
Photoreceptor surface 16 is preferably formed of selenium. Intermediate transfer member 40 is preferably formed of a cylindrical aluminum core cowed with a 1 mm thick layer of very soft polyurethane having a hardness of 20-25 Shore A. This layer is covered by an offset printing blanket, preferably a KYNIO AIRTACK offset blanket, which is much harder than the polyurethane. J~ thin conducting layer of conducting acrylic covers this layer and is covered in turn by a 0.1 mm layer of polyurethane of shore A Hardness 20.
This layer is overcoated by a thin layer of Syl-Otf*type 291 or 294 silicone release coating.
Liquid developer prepared in accordance with the method described above is used in the equipment of Fig. 1.
Preferably the temperature of the intermediate transfer ~ayer should be less than about 50 degrees C. For temperatures greater than about 50 degrees, there is a tendency for the previously transferred colors to back transfer to photoreceptor surface 16. Heating intermediate transfer member 40 improves image transfer to intermediate transfer member 40. Intermediate transfer member 40 is preferably heated to a temperature somewhat below that at .which back * Trade marks WO 92/10793 IyCT/i''iL9U/f703$Z
_ 20 _ 1 transfer begins to occur.
2 It is believed that the improvement in first transfer 3 when the intermediate.transfer member is heated may be a 4 consequence of partial solvation of carrier liquid by the pigmented toner particles in the image.
6 One characteristic of the liquid developers preferred in 7 the practice of this invention is that the pigmented toner 8 particles contained therein solvate the carrier liquid at 9 elevated temperatures. It is believed that there is a partial solvation of the carrier liquid in the toner particles during 11 first transfer to heated intermediate transfer member 40 12 which may cause the particles to partially coalesce and form 13 a film during first transfer. Coalesced toner is believed to 14 transfer better than uncoalesced toner part~.cles.
Furthermore, when the taper material solvates some of 16 the carrier liquid, the taper particles separate from the 17 unsolvated carrier liquid. It is ;believed that this separated 18 carrier liquid forms a film between the toner image and the 19 photoreceptor which seduces the adhesion of the image to the photoreceptor, aiding complete transfer of~the image to the 21 intermediate transfer member.
22 It is to be understood that the heating of the image 23 before and/or during final transfer insures the complete or 24 nearly complete transfer of the image from the intermediate transfer member to the final substrate. Where this image 26 heating comes solely by conduction from the paper, it has 27 been found experimentally that the paper should be at a 28 temperature o~ at least about 70 degrees C. Higher 29 temperatures such as 80 or 90 degrees can also be used, but substantially lower temperatures do not tackify the image 31 enough to assure complete transfer from intermediate transfer 32 member 40 to paper 42.
33 The precise 'temperatures used for particular 34 configurations and combinations are a function of the material properties of the toner particles and the carrier 36 liquid as well as of the quality of the release layer on the 37 intermediate transfer member. Hack transfer occurs due to the 38 tackiness of the image, but is also influenced by the ~°'AIi','".~"s~'r~~.... ~m,~~.°~.'~' ~~.it Lv...: % p y, r .r a... :.~.

PCT'/ i~a L90/Of) i 82 1 relative adhesion of the image to the release layer on the 2 intermediate transfer member and to the photoreceptor. It 3 would be possible to increase the temperature of the 4 intermediate transfer member if the release properties of the surface of the intermediate transfer member were poorer. This 6 however would also result in poorer transfer to the final 7 substrate.
8 In particular representative; operating examples the 9 following temperatures are used. In a first example, which is used for the transfer of single color images, the il intermediate transfer member is heated to a surface 12 temperature of 100 degrees C and the paper is not heated.
13 Calculations shave that the image is at a temperature o~ 52 to 14 63 degrees C during first transfer. During the interval between first and second transfer the image temperature rises 16 to the intermediate transfer member's temperature of 100 17 degrees C, and the image is cooled during second, final 18 transfer to paper to a temperature of 73 to 78 degrees C.
19 In a second, representative, operating example for sequential transfer of multiple images to the intermediate 21 transfer member, the intermediate transfer member is heated 22 to 50 degrees C and backing roller 43 is heated. to 120 23 degrees C. The image temperature on first transfer is 24 approximately 43 degrees C and on second transfer it is 75 to 78 degrees C.
26 The temperatures shaven in figure 7 are also 27 representative of values suitable for single image transfer.
28 For multi-image transfer to intermediate transfer member 140, 29 the first transfer temperature must be low enough to assure that no back transfer takes place.
31 It will be understood that certain features and sub-32 combinations of the invention are useful, and may be employed 33 without other features and sub-combinations. It is noted that 34 various changes may be made in details within the scope of the claims without departing from the spirit of the 36 invention. It is therefor to be understood that the 37 invention is not to be limited to the specific details shown 3$ and described.
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Claims

WE CLAIM

1. A method of transferring a first liquid toner image from a first surface having a first temperature to a second surface having a second liquid toner image thereon to form multiple images on the second surface characterized in that it includes the steps of:

heating the second surface to a second temperature at transfer, which second temperature is higher than the first temperature; and transferring the first and second images to a final substrate from the second surface.

2. A method according to claim 1 wherein the second temperature is lower than the temperature at which the second liquid toner image would transfer to the first surface from the second surface.

3. A method according to claim 1 or claim 2 wherein the step of transferring is by electrophoresis.

4. A method according to any of the preceding claims and including the steps of:

forming the first liquid toner image on an image bearing surface using a liquid developer, comprising carrier liquid and charged toner particles; and transferring the first liquid toner image to the second surface .

5. A method according to claim 4 wherein the step of forming the first liquid toner image includes the step of developing a latent electrostatic image on the image bearing surface.

6. A method according to any of the preceding claims and including the steps of:

forming the first liquid toner image on the first surface using a liquid developer comprising carrier liquid and charged toner particles;

transferring the first liquid toner image to the second surface at least partially superimposed on the second liquid toner image to form multiple images thereon.

7. A method according to claim 6 wherein the step of forming the second liquid toner image includes the step of developing a latent electrostatic image on the first surface.

8. A method according to any of the preceding claims wherein during the step of transferring to the final substrate the multiple images are at a higher temperature than during the step of transferring the second liquid toner image to the second surface.

9. A method according to any of the preceding claims wherein the step of transferring the first and second images to the final substrate comprises:
transferring the first and second images to a second intermediate transfer member; and transferring the first and second images to the final substrate.

10. A method according to claim 9 and including the steps of forming at least one additional image on the first surface after transfer of the first and second images therefrom;
transferring the at least one additional image to an intermediate transfer member after the transfer of the first and second images from the intermediate transfer member to the second intermediate transfer member, but prior to their transfer to the final substrate; and passing the final substrate through a nip formed between the intermediate transfer member and the second intermediate transfer member such that the first and second images are transferred to one side of the final substrate and the at least one additional image is transferred to the other side of the final substrate.

11. Imaging apparatus for printing multicolor image comprising:
means (20, 22) for forming a liquid toner image of a first color, means (20, 22) for forming a second liquid toner image of a second color;
an intermediate transfer member (40);
first transfer means for sequentially transferring the first and second liquid toner images to the intermediate transfer member in mutual alignment to form a multicolor image thereon; and second transfer means for transferring the multicolor image to a final substrate from the intermediate transfer member, characterized in that it includes:
first heating means (46) for heating the formed images before completion of their respective transfers to the intermediate transfer member.

12. Apparatus according to claim 11 wherein the means for forming (20, 22) the first and second liquid toner images comprises means (22) for developing an electrostatic latent image on a photoreceptor.

13. Apparatus according to claim 11 or claim 12 wherein the second transfer means includes a second intermediate transfer member (47) adapted to receive images from the intermediate transfer member (40) prior to transfer of the images to the final substrate (42).

14. Apparatus according to claim 13 wherein the intermediate transfer member and the second intermediate transfer member are configured to form a nip therebetween and including means for passing the final substrate between the intermediate transfer member and the second intermediate transfer members carrying first and second multicolor images respectively, such that said first and second images are transferred to opposite sides of the final substrate.

15. Apparatus according to any of claims 11-14 and comprising:
second heating means (45, 43, 46, 48, 114) for further heating of the developed first transferred image such that it is at a higher temperature during its transfer to the final substrate than during its transfer to the intermediate transfer member.

16. A method of printing images on two sides of a substrate comprising:
(a) forming a first image on a first surface;
(b) transferring the first image to a second surface from the first surface;
(c) forming a second image on the first surface;
(d) transferring the second image to a third surface from the first surface; and (e) simultaneously transferring the first and second images from the second and third surfaces to the two sides of a substrate.

17. A method according to claim 16 wherein at least one of the first and second images comprises a multicolor image formed of a plurality of monochrome images.