WO1996026469A1

WO1996026469A1 - Imaging apparatus with temperature control

Info

Publication number: WO1996026469A1
Application number: PCT/NL1996/000080
Authority: WO
Inventors: Shlomo Harush; Ilan Kander; Dvir Harmelech; Pini Perlmutter
Original assignee: Indigo N.V.
Priority date: 1995-02-21
Filing date: 1996-02-20
Publication date: 1996-08-29
Also published as: AU4848496A; IL112731A0

Abstract

A method of temperature controlled electrostatic imaging for an imaging apparatus having a photoreceptor surface and a supply of liquid toner comprising charged toner particles and carrier liquid, the method including: charging the photoreceptor surface; selectively discharging portions of the charged photoreceptor surface to form an electrostatic latent image thereon; controlling the temperature of the liquid toner; developing the electrostatic latent image with the temperature controlled liquid toner to form a developed image; and transferring the developed image to a substrate to form a printed image thereon.

Description

IMAGING APPARATUS WITH TEMPERATURE CONTROL

FIELD OF THE INVENTION The present invention relates to electrostatic imaging in general and, more particularly, to temperature control in imaging apparatus.

BACKGROUND OF THE INVENTION A substantial number of factors affect the stability and calibration of electrophotographic imaging apparatus such as printers and copiers. In general, a number of voltages are controlled to produce the required image density and other required properties. Such voltages include a voltage for charging a photoreceptor on which a latent image is formed, for example a roller voltage, a corotron voltage or a scorotron voltage. The voltage of the developer, both for liquid and powder toner development, is also generally controlled. Additionally, the intensity of light used for selective discharge of the photoreceptor in forming the latent image is also important in optimal formation of the latent image. Imaging stability is also affected by the temperatures of various elements of the imaging apparatus. Thus, imaging apparatus are sometimes provided with means for regulating temperature, for example, a fan for ventilating the imaging apparatus.

SUMMARY OF THE INVENTION The present inventors have found that the temperatures of certain elements of the imaging apparatus are particularly effective in stabilizing the properties of the image produced by the apparatus . These temperatures include the temperature of the photoreceptor surface , the temperature of liquid toner compri s ing charged toner particles dispersed in carrier liquid , the carrier liquid tempera ure, the temperature of an intermediate transfer member , i f used , and the temperatures of certain environments in the imaging apparatus . Direct control of at least some of these temperatures has been found to improve image quality and image stability . It is , therefore , an obj ect of the present invention to provide a temperature- controlled imaging apparatus, wherein at least some of the above mentioned temperatures are controlled. Speci f ical ly , it has been found by the present inventors that in liquid toner imaging apparatus the toner temperature converges to a certain maximum temperature , higher than room temperature, but only after a substantial period of operation. A curve showing toner temperature as a function of time in conventional imaging apparatus is shown in Fig . 5A. According to the present invention, the liquid toner is , first , preheated to a temperature substantially equal to or slightly higher than the normally reached maximum temperature and , then , maintained at that temperature by periodic reheating. A heating element in the l iquid toner supply i s preferably used for both the preheating and the periodic reheating of the toner. It has been also found that control of the temperature of the carrier liquid and the temperature of the intermediate trans fer member , if used , improves image stability. Thus, in a preferred embodiment of the invention, the temperature of the intermediate transfer is maintained at an optimal temperature by periodic, controlled, heating and the carrier liquid is maintained at an optimal temperature by control led cool ing . A halogen lamp i s preferably used for periodic heating of the intermediate transfer member . Cooling of the carrier liquid supply is preferably performed by circulating carrier liquid through a chiller. To further improve image stability, the environmental temperature within the housing of the imaging apparatus is also control led , preferably , us ing a control led air- conditioning system . The carrier liquid chiller i s preferably associated with the air-conditioning system. It has been shown by the present inventors , that control of the toner temperature , the carrier liquid temperature, the intermediate transfer member temperature and the imaging environment temperature, as described above, results in a substantially constant, optimal , temperature of the photoconductive imaging surface . Thus , a preferred embodiment of the present invention provides a comprehensive temperature control mechanism which considerably improves image stability. In accordance with a preferred embodiment of the present invention there is thus provided , in an imaging apparatus having a photoreceptor surface and a supply of liquid toner comprising charged toner particles and carrier liquid, a method of temperature-controlled electrostatic imaging including : charging the photoreceptor surface; selectively di scharging portions of the charged photoreceptor surface to form an electrostatic latent image thereon; controlling the temperature of the liquid toner; developing the electrostatic latent image with the temperature controlled liquid toner to form a developed image; and transferring the developed image to a substrate to form a printed image thereon. In a preferred embodiment of the invention, controlling the temperature of the liquid toner includes heating the liquid toner to a predetermined toner temperature. Additionally, in a preferred embodiment , controlling the temperature of the liquid toner includes maintaining the temperature of the liquid toner substantially at the predetermined toner temperature . Pre ferably , in thi s preferred embodiment of the invention , maintaining the temperature of the liquid toner includes sensing the temperature of the liquid toner and periodically heating the liquid toner in response to the sensed toner temperature . The imaging apparatus preferably includes a heating element associated with the supply of liquid toner and periodical ly heating the liquid toner preferably includes periodically activating the heating element . In a further preferred embodiment of the present invention , the imaging apparatus further includes an intermediate transfer member ( ITM ) and transferring the developed image to the substrate includes , first , transferring the developed image to a surface of the ITM to form an intermediate image thereon and, then, transferring the intermediate image from the surface of the ITM to the substrate . According to this preferred embodiment , the method further includes controlling the temperature of the ITM. Preferably, in this preferred embodiment of the invention, controlling the temperature of the ITM includes heating the ITM to a predetermined ITM temperature and maintaining the temperature of the ITM substantially at the predetermined ITM temperature. In a preferred embodiment of the invention, maintaining the temperature of the ITM includes sensing the temperature at the surface of the ITM and periodically heating the ITM in response to the sensed ITM surface temperature. Preferably, the imaging apparatus further includes a heating lamp associated with the ITM and periodically heating the ITM includes periodically activating the heating lamp. In an additional preferred embodiment of the present invention, the imaging method further includes controlling the temperature of at least a portion of the imaging environment. Preferably, controlling the temperature of at least a portion of the imaging environment includes cooling at least the portion of the imaging environment. Further, in accordance with this preferred embodiment of the invention, controlling the temperature of at least a portion of the imaging environment includes sensing the temperature of at least the portion of the imaging environment and cooling at least the portion of the imaging environment in accordance with the sensed environmental temperature. In a further preferred embodiment of the present invention, the imaging apparatus further includes a supply of carrier liquid and the imaging method further includes controlling the temperature of the carrier liquid. Preferably, in this preferred embodiment, controlling the temperature of the carrier liquid includes cooling the carrier liquid to a predetermined carrier liquid temperature. Additionally or alternatively, in this preferred embodiment, controlling the temperature of the carrier liquid further includes maintaining the temperature of the carrier liquid toner substantially at the predetermined carrier liquid temperature. Preferably, according to this embodiment, maintaining the temperature of the carrier liquid includes sensing the temperature of the carrier liquid and cooling the carrier liquid in response to the sensed carrier liquid temperature. In a preferred embodiment, the imaging apparatus includes a chiller and cooling the carrier liquid includes circulating at least a portion of the carrier liquid through the chiller.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention wi l l be understood and appreciated more fully from the f ol lowing detai led description, taken in conj unction with the drawings in which: Fig . 1 is a simpli f ied sectional i llustration of temperature-controlled electrostatic imaging apparatus constructed and operative in accordance with a preferred embodiment of the present invention; Fig . 2 is a simplified enlarged sectional illustration of the temperature-controlled imaging apparatus of Fig . 1 ; Fig . 3 is a schematic, block diagram, illustration of a preferred temperature control system of the imaging apparatus of Fig . 1 ; Fig . 4A is a schematic illustration of a curve showing the surface temperature of an intermediate transfer member ( ITM ) as a functions of time in the temperature-controlled imaging apparatus of Fig . 1 ; Fig. 4B is a schematic illustration of a curve showing the temperature in the vicinity of an ITM heating lamp when the ITM is heated as in Fig . 4A; Fig. 5A is a schematic illustration of a curve showing toner temperature as a function of time in conventional imaging apparatus ; Fig. 5B is a schematic illustration of a curve showing toner temperature as a function of time in the temperature - controlled imaging apparatus of Fig. 1 ; Fig . 6 is a schematic illustration of a curve showing carrier liquid temperature as a function of time in the temperature-controlled imaging apparatus of Fig . 1 ; and Fig. 7 is schematic illustration of an air conditioning system used for controlling environmental temperature in the imaging apparatus of Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Reference is now made to Figs . 1 and 2 which illustrate a multicolor electrostatic imaging system constructed and operative in accordance with a preferred embodiment of the present invention . As seen in Figs . 1 and 2 there i s provided an imaging sheet , preferably an organic photoreceptor 12 , typically mounted on a rotating drum 10. Drum 10 is rotated about its axis by a motor or the like ( not shown ) , in the direction of arrow 18 , past charging apparatus 14 , preferably a corotron, scorotron or roller charger or other suitable charging apparatus as are known in the art and which is adapted to charge the surface of sheet photoreceptor 12. The image to be reproduced is focused by an imager 16 upon the charged surface 12 at least partially discharging the photoconductor in the areas struck by light, thereby forming the electrostatic latent image. Thus , the latent image normally includes image areas at a f irst electrical potential and background areas at another electrical potential . Photoreceptor sheet 12 may use any suitable arrangement of layers of materials as is known in the art , however, in the preferred embodiment of the photoreceptor sheet , certain of the layers are removed from the ends of the sheet to facilitate its mounting on drum 10. This preferred photoreceptor sheet and preferred methods of mounting it on drum 10 are described in a co- pending application of Belinkov et al . , IMAGING APPARATUS AND PHOTORECEPTOR THEREFOR , f i led September 7 , 1994 , assigned serial number 08/301 , 775 , the disclosure of which is incorporated herein by reference . Alternat ively , photoreceptor 12 may be deposited on the drum 10 and may form a continuous surface. Furthermore, photoreceptor 12 may be a non-organic type photoconductor based, for example, on a compound of Selenium. In a preferred embodiment of the present invention, imaging apparatus 16 is a modulated laser beam scanning apparatus, or other laser imaging apparatus such as is known in the art . 1 Also associated with drum 10 and photoreceptor sheet

2 12, in the preferred embodiment of the invention, are a

3 multicolor liquid developer spray assembly 20, a developing

4 assembly 22, color specific cleaning blade assemblies 34, a

5 background cleaning station 24, an electrified squeegee 26,

6 a background discharge device 28, an intermediate transfer

7 member 30, cleaning apparatus 32, and, optionally, a

8 neutralizing lamp assembly 36.

9 Developing assembly 22 preferably includes a

10 development roller 38. Development roller 38 is preferably

11 spaced from photoreceptor 12 thereby forming a gap

12 therebetween of typically 40 to 150 micrometers and is

13 charged to an electrical potential intermediate that of the

14 image and background areas of the image. Development roller

15 38 is thus operative, when maintained at a suitable voltage,

16 to apply an electric field to aid development of the latent

17 electrostatic image.

18 Development roller 38 typically rotates in the same

19 sense as drum 10 as indicated by arrow 40. This rotation

20 provides for the surface of sheet 12 and development roller

21 38 to have opposite velocities at the gap between them.

22 Multicolor liquid developer spray assembly 20, whose

23 operation and structure is described in detail in U.S.

24 Patent 5,117,263, the disclosure of which is incorporated

25 herein by reference, may be mounted on axis 42 to allow

26 assembly 20 to be pivoted in such a manner that a spray of

27 liquid toner containing electrically charged pigmented toner

28 particles can be directed either onto a portion of the

29 development roller 38, a portion of the photoreceptor 12

30 or directly into a development region 44 between

31 photoreceptor 12 and development roller 38. Alternatively,

32 assembly 20 may be fixed. Preferably, the spray is directed

33 onto a portion of the development roller 38.

34 Color specific cleaning blade assemblies 34 are

35 operatively associated with developer roller 38 for separate

36 removal of residual amounts of each colored toner remaining

37 thereon after development. Each of blade assemblies 34 is

38 selectably brought into operative association with developer roller 38 only when toner of a color corresponding thereto is supplied to development region 44 by spray assembly 20. The construction and operation of cleaning blade assemblies is described in PCT Publication WO 90/14619 and in US patent 5,289,238, the disclosures of which are incorporated herein by reference. Each cleaning blade assembly 34 includes a toner directing member 52 which serves to direct the toner removed by the cleaning blade assemblies 34 from the developer roller 38 to separate collection containers 54, 56, 58, and 60 for each color to prevent contamination of the various developers by mixing of the colors. The different color toners collected by collection containers 54, 56, 58 and 60 are recycled to corresponding toner reservoirs 55, 57, 59 and 61. A final toner directing member 62 always engages the developer roller 38 and the toner collected thereat is supplied into collection container 64 and thereafter to a carrier-liquid reservoir 65 via a separator 66 which is operative to separate relatively clean carrier liquid from the various colored toner particles. The separator 66 may be typically of the type described in U.S. Patent 4,985,732, the disclosure of which is incorporated herein by reference. In accordance with a preferred embodiment of the present invention, reservoirs 55, 57, 59 and 61 are associated with respective heating elements 255, 257, 259 and 261, the activation of which are controlled by a toner temperature control unit 204. Reservoirs 55, 57, 59 and 61 are preferably also associated with respective toner temperature sensors 355, 357, 359 and 361, which generate electric outputs responsive to the toner temperatures of reservoirs 55, 57, 59 and 61, respectively. In a preferred embodiment of the invention, as described in detail below, heating elements 255, 257, 259 and 261 are selectively activated by control unit 204 based on the outputs of sensors 355, 357, 359 and 361, to heat the liquid toners in reservoirs 55, 57, 59 and 61, respectively. In a preferred embodiment of the invention, as described in U.S. Patent 5,255,058, the disclosure of which is incorporated herein by reference, where the imaging speed is very high, a background cleaning station 24 typically including a reverse roller 46 and a fluid spray apparatus 48 is provided. Reverse roller 46 which rotates in a direction indicated by arrow 50 is electrically biased to a potential intermediate that of the image and background areas of photoconductive drum 10, but different from that of the development roller. Reverse roller 46 is preferably spaced apart from photoreceptor sheet 12 thereby forming a gap therebetween which is typically 40 to 150 micrometers. Fluid spray apparatus 48 receives liquid toner from carrier-liquid reservoir 65 via conduit 88 and operates to provide a supply of preferably non-pigmented carrier liquid to the gap between sheet 12 and reverse roller 46. The liquid supplied by fluid spray apparatus 48 replaces the liquid removed from drum 10 by development assembly 22 thus allowing the reverse roller 46 to remove charged pigmented toner particles by electrophoresis from the background areas of the latent image. Excess fluid is removed from reverse roller 46 by a liquid directing member 70 which continuously engages reverse roller 46 to collect excess liquid containing toner particles of various colors which is in turn supplied to reservoir 65 via collection container 64 and separator 66. In accordance with a preferred embodiment of the present invention, reservoir 65 is associated with a chiller 265 and a carrier liquid pump 267. When pump 267 is operated, carrier liquid from reservoir 265 is circulated via chiller 265 and cooled thereby. Reservoir 65 is preferably also associated with a carrier liquid temperature sensor 216, which generates an electric output responsive to the temperature of the carrier liquid in reservoir 65. In a preferred embodiment of the invention, as described in detail below, pump 267 is periodically operated by control unit 206, based on the output of sensor 216, to cool the carrier liquid in reservoir 65. The apparatus embodied in reference numerals 46, 48, 50 1 and 70 is generally not required for low speed systems, but

2 is preferably included in high speed systems.

3 Preferably, an electrically biased squeegee roller 26

4 is urged against the surface of sheet 12 and is operative to

5 remove liquid carrier from the background regions and to

6 compact the image and remove liquid carrier therefrom in the

7 image regions. Squeegee roller 26 is preferably formed of

8 resilient slightly conductive polymeric material as is well

9 known in the art, and is preferably charged to a potential

10 of several hundred to a few thousand volts with the same

11 polarity as the polarity of the charge on the toner

12 particles.

13 Discharge device 28 is operative to flood sheet 12 with

14 light which discharges the voltage remaining on sheet 12,

15 mainly to reduce electrical breakdown and improve transfer

16 of the image to intermediate transfer member 30. Operation

17 of such a device in a write black system is described in

18 U.S. Patent 5,280,326, the disclosure of which is

19 incorporated herein by reference.

20 Figs. 1 and 2 further show that multicolor toner spray

21 assembly 20 receives separate supplies of colored toner

22 typically from four different reservoirs 55, 57, 59 and 61.

23 Figure 1 shows four different colored toner reservoirs 55,

24 57, 59 and 61 typically containing the colors Yellow,

25 Magenta, Cyan and, optionally, Black respectively. Pumps 90,

26 92, 94 and 96 may be provided along respective supply

27 conduits 98, 101, 103 and 105 for providing a desired amount

28 of pressure to feed the colored toner to multicolor spray

29 assembly 20. Alternatively, multicolor toner spray assembly

30 20, which is preferably a three level spray assembly,

31 receives supplies of colored toner from up to six different

32 reservoirs (not shown) which allows for custom colored tones

33 in addition to the standard process colors.

34 A preferred type of toner for use with the present

35 invention is that described in Example 1 of U.S. Patent

36 4,794,651, the disclosure of which is incorporated herein by

37 reference or variants thereof as are well known in the art.

38 For colored liquid developers, carbon black is replaced by 1 color pigments as is well known in the art. Other toners may

2 alternatively be employed, including liquid toners and, as

3 indicated above, including powder toners. Preferred liquid

4 toners are also described in the various patents and patent

5 applications referred to herein and/or incorporated herein

6 by reference.

7 Intermediate transfer member 30 may be any suitable

8 intermediate transfer member having a multilayered transfer

9 portion such as those described below or in U.S. Patents 0 5,089,856 or 5,047,808 or in U.S. Patent application S.N. 1 08/371,117, filed January 11, 1995, the disclosures of which 2 are incorporated herein by reference. Member 30 is

13 maintained at a suitable voltage and temperature for

14 electrostatic transfer of the image thereto from the image 5 bearing surface. Intermediate transfer member 30 is

16 preferably associated with a pressure roller 71 for transfer

17 of the image onto a final substrate 72, such as paper,

18 preferably by heat and pressure.

19 In accordance with a preferred embodiment of the

20 present invention, intermediate transfer member (ITM) 30 is

21 heated by a heater 230 which preferably includes a sealed

22 halogen lamp 232. Activation of lamp 232 is preferably

23 controlled by an ITM temperature controller 202. The surface

24 of ITM 30 is preferably associated with an ITM temperature

25 sensor 212, which generates an electric output responsive to

26 the surface temperature of ITM 30. In a preferred embodiment

27 of the invention, as described in detail below, lamp 232 is

28 activated by control unit 202, based on the output of sensor

29 212, to heat the surface of ITM 30 to an optimal working

30 temperature.

31 Cleaning apparatus 32 is operative to scrub clean the

32 surface of photoreceptor 12 and preferably includes a

33 cleaning roller 74, a sprayer 76 for spraying a non polar

34 cleaning liquid, preferably chilled carrier liquid from

35 reservoir 65, and a wiper blade 78 to complete the cleaning

36 of the photoconductive surface. The sprayed carrier liquid

37 assists in the scrubbing process and cools the photoreceptor

38 surface. Cleaning roller 74 which may be formed of any synthetic resin known in the art for this purpose is driven in the same sense as drum 10 as indicated by arrow 80, such that the surface of the roller scrubs the surface of the photoreceptor. Any residual charge left on the surface of photoreceptor sheet 12 may be removed by flooding the photoconductive surface with light from optional neutralizing lamp assembly 36, which may not be required in practice. In accordance with a preferred embodiment of the invention, after developing each image in a given color, the single color image is transferred to intermediate transfer member 30. Subsequent images in different colors are sequentially transferred in alignment with the previous image onto intermediate transfer member 30. When all of the desired images have been transferred thereto, the complete multi-color image is transferred from transfer member 30 to substrate 72. Impression roller 71 only produces operative engagement between intermediate transfer member 30 and substrate 72 when transfer of the composite image to substrate 72 takes place. Alternatively, each single color image is separately transferred to the substrate via the intermediate transfer member. In this case, the substrate is fed through the machine once for each color or is held on a platen and contacted with intermediate transfer member 30 during image transfer. Alternatively, the intermediate transfer member is omitted and the developed single color images are transferred sequentially directly from drum 10 to substrate 72. It should be understood that the invention is not limited to the specific type of image forming system used and the present invention is also useful with any suitable imaging system. The specific details given above for the image forming system are included as part of a best mode of carrying out the invention, however, many aspects of the invention are applicable to a wide range of systems as known in the art for electrophotographic and offset ink printing and copying. Reference is now made also to Fig. 3 which 1 schematically illustrates a temperature control system in

2 accordance with a preferred embodiment of the present

3 invention . The temperature control system pref erably

4 includes at least one processor 200 which controls the

5 operation of ITM temperature controller 202 , toner

6 temperature controller 204 , carrier-liquid temperature

7 controller 206 and an air-condition controller 208 , using

8 appropriate control signals , as described below . Processor

9 200 may be implemented on a computer or in the form of

10 dedicated hardware which may include a microprocessor.

11 In response to ITM control signals from processor 200,

12 ITM temperature controller 202 periodically activates lamp

13 232 of heater 230. In response to toner control signals from

14 processor 200, toner temperature controller 204 selectively

15 activates one or more of heating elements 255, 257 , 259 and

16 261 . In response to carrier liquid control signals from

17 processor 200, carrier-liquid controller 206 activates pump

18 267 to circulate carrier liquid through chiller 265 . In

19 response to environment control signals , air-condition

20 controller 208 operates an air-conditioning system 250 which

21 regulates the air temperature in the housing of the imaging

22 apparatus . Control of all the temperatures mentioned above

23 will now be described in detail with reference to Figs . 4A -

24 7C.

25 Fig. 4A schematically illustrates the temperature of

26 ITM 30 as a function of time. Fig. 4B schematically

27 illustrates the temperature in the vicinity of lamp 232 as

28 a function of time. It is appreciated that lamp 232, which

29 is preferably a halogen lamp, will generally not tolerate

30 temperatures above 350°C in the vicinity thereof. Thus, lamp

31232 is operated periodically according to a predetermined

32 duty cycle even when continuous heating is required, for

33 example during initiation of the imaging apparatus when ITM

34 30 is still cold. Fig. 4B shows the periodic temperature

35 variation in the vicinity of lamp 232. The right-hand side

36 of the curve in Fig. 4B corresponds to a typical 4-color-

37 print state of the imaging apparatus while the left-hand

38 side of the curve corresponds to a typical ready-to-print state. During operation of the imaging apparatus , processor 200 receives the output of ITM temperature sensor 212 and generates the above mentioned ITM temperature control signals in response thereto . In a preferred embodiment of the invention , lamp 232 is activated by ITM temperature controller 202 only when the difference between the sensed I TM surf ace temperature and a des ired I TM surf ace temperature is above a preselected threshold . Fig . 4A illustrates a preferred embodiment of the present invention in which the ITM core temperature is maintained at approximately 139 . 5±1 . 5 ^β C . The resultant temperature at the surface of ITM 30, in the 4 -color-print state , is approximately 97 ^β C with a time fluctuation of approximately ±2°C and a spatial variation of approximately 7 °C between the middle and the edges of the ITM surface. In thi s embodiment , lamp 232 i s powered by a voltage of approximately 225 Volts and with a duty cycle o f approximately 36 percent . Fig . 5A schematically illustrates the temperature of the toner supply of a conventional imaging apparatus, where toner temperature control is not used , as a function of time . Fig . 5B illustrates toner temperature as a function of time in one of reservoirs 55, 57 , 59 or 61. Fig. 5B shows a preferred embodiment of the present invention in which the toners in reservoirs 55 , 57 , 59 and 61 are preheated to a temperature substantially equal to or slightly higher than the maximum toner temperature of conventional imaging apparatus as shown in Fig. 5A. Peaks 375 and 380 in Figs . 5A and 5B , respectively, represent an abrupt change in toner emperature which is generally experienced during priming of the imaging apparatus . In a preferred embodiment of the present invention, the toners are maintained at desired temperatures by periodically activating heating elements 355, 357 , 359 and 361. Direct cooling of the toners is generally not required in this embodiment of the present invention, since the toner is cooled indirectly during imaging . However toner cooling may be incorporated, in addition to toner heating, to further improve the control of toner temperature. During operation of the imaging apparatus, processor 200 receives the electric outputs of toner temperature sensors 355, 357, 359 and 361 and generates the above mentioned toner temperature control signals in response thereto. In a preferred embodiment of the invention, a given heating element 255, 257, 259 or 261 is activated by toner temperature controller 204 only when the difference between the sensed toner temperature of the respective reservoir 55, 57, 59 or 61 and a desired toner temperature is above a preselected threshold. Fig. 6 schematically illustrates the temperature of the carrier-liquid in carrier liquid reservoir 65 as a function of time, when the carrier-liquid temperature control of the present invention is employed. As shown in Fig. 6, periodic cooling of the carrier liquid, which generally tends to heat-up by transfer of heat from the photoreceptor, prevents the carrier liquid from being heated beyond a predetermined maximum temperature. Thus, in a preferred embodiment of the invention, carrier-liquid temperature is maintained within a predetermined narrow range, for example 21+2°C as shown in Fig. 6. During operation of the imaging apparatus, processor 200 receives the electric output of carrier- l iquid temperature sensor 216 and generates the above mentioned toner temperature control signals in response thereto. In a preferred embodiment of the invention, pump 267 ( Fig . 1 ) is activated by carrier-liquid temperature controller 206 only when the di f ference between the sensed carrier- liquid temperature and a desired toner temperature is above a preselected threshold . Activation of pump 267 circulates carrier liquid through chiller 265 thereby to cool the carrier liquid as described above . In a preferred embodiment, chiller 265 is associated with air-conditioning system 250 as described below with reference to Fig. 7A. Fig . 7 schematically illustrates a preferred embodiment of air conditioning system 250. Air-conditioning system 250 includes a compressor 300 which compresses cooling gas into a condenser 310. An air blower 320 helps to remove removing heat from the cooling gas in condenser 310. A capillary tube 335 controls the release of cooling gas from condenser 310 towards an evaporator 315. In a preferred embodiment of the invention, the released cooling gas flows through chiller 265, cooling the carrier liquid therein, before reaching evaporator 315. As known in the art, the expanded cooling gas in evaporator 315 is operative for cooling the air in the vicinity of the evaporator. The cooled air is circulated through the interior of the imaging apparatus by virtue of an air blower 325. The cooling gas then p asses through an accumulator 305 and recompressed by compressor 300 for another cooling cycle. In accordance with a preferred embodiment of the invention, air-conditioning system 250 is further provided with a warm gas bypass valve (WGBV) 330 which, when opened, directs some of the compressed cooling gas back to evaporator 315 without cooling at condenser 310. It should be appreciated that the cooling efficiency of system 250 when WGBV 330 is opened is considerably lower than the cooling efficiency when WGBV 330 is closed. In a preferred embodiment of the invention, optimal environmental temperatures in the interior of the imaging apparatus are maintained by periodically switching WGBV from a closed position to an open position. During operation of the imaging apparatus, processor 200 receives the electric output of the at least one environmental temperature sensor 218 and generates the above mentioned environmental temperature control signals in response thereto. In a preferred embodiment of the invention, WGBV 330 is periodically opened and closed by air-condition controller 208 in accordance with the at least one sensed environmental temperature. Thus, a substantially stable imaging environment is maintained. Environmental temperature sensor 218 may be located at any suitable location of the imaging apparatus, for example sensor 218 may be located in the development environment where developer roller 38 engages photoreceptor surface 12. It should be appreciated that when the ITM temperature, the toner temperature, the carrier liquid temperature and the environmental temperature are all controlled, the temperature of photoreceptor surface 12 will also be substantially controlled. In other words, the system will reach a steady state in which the temperature of the photoreceptor surface is substantially equal to a predetermined linear combination of all the temperatures mentioned above. Therefore, in a steady state, the following equation holds: ^T0PC ^{■ aT}ITM ^{+ bτ}CL ⁺ 9^TToner ^{+ dτ}Air' wherein: ₀pc ^is the steady state temperature of the surface of the photoreceptor; T_ITM is the steady state temperature at the surface of the ITM; T_CL is the steady state temperature of the carrier liquid; T_Toner is the steady state temperature of the liquid toner; T_Air is the steady state temperature of the development environment; and a, b, g and d are constants dependent on the specific imaging apparatus. Since the initial temperature of the photoreceptor, e.g. room temperature, is generally lower than the desired steady state temperature, T_0PC, a preferred embodiment of the invention preferably includes a pre-imaging, cold-start, mode in which the photoreceptor temperature is raised to approximately T_0PC before imaging begins. According to this preferred embodiment of the invention, the cold-start mode is carried out during a pre-imaging period of time in which the imaging apparatus is operated with ITM 30, developer roller 38 and reverse roller 46 all engaging photoreceptor surface 12, but without forming a latent image on surface 12 and without transferring an image to final substrate 72. It should be appreciated that during the cold-start period, the temperature of surface 12 is affected by the ITM temperature, by the carrier liquid temperature, by the imaging environment temperature and by the toner temperature, if toner is applied, all of which temperatures are controlled as described above . Once photoreceptor surface 12 reaches a temperature of approximately T_QPC, the imaging apparatus is ready for use and imaging may be commenced. While the present invention has been described in the context of a four color imaging system, the invention is equally applicable to a system using a greater number of colors, such as a six color system, or a lesser number of colors such as a monochrome or three color system. It will be appreciated by persons skilled in the art that the present invention is not limited by the description and example provided hereinabove. Rather, the scope of this invention is defined only by the claims which follow:

Claims

1. In an imaging apparatus having a photoreceptor surface and a supply of liquid toner comprising charged toner particles and carrier liquid, a method of temperature- controlled electrostatic imaging comprising: charging the photoreceptor surface; selectively discharging portions of the charged photoreceptor surface to form an electrostatic latent image thereon; controlling the temperature of the liquid toner; developing the electrostatic latent image with the temperature controlled liquid toner to form a developed image; and transferring the developed image to a substrate to form a printed image thereon.

2. A method according to claim 1 wherein controlling the temperature of the liquid toner comprises heating the liquid toner to a predetermined toner temperature.

3. A method according to claim 2 wherein controlling the temperature of the liquid toner further comprises maintaining the temperature of the liquid toner substantially at the predetermined toner temperature.

4. A method according to claim 3 wherein maintaining the temperature of the liquid toner comprises: sensing the temperature of the liquid toner; and periodically heating the liquid toner in response to the sensed toner temperature.

5» A method according to claim 4 wherein the imaging apparatus further comprises a heating element associated with the supply of liquid toner, and wherein periodically heating the liquid toner comprises periodically activating the heating element. 1

2 6. A method according to any of claims 1 - 5, wherein the

3 imaging apparatus further comprises an intermediate transfer

4 member (ITM) and wherein transferring the developed image to

5 the substrate comprises, first, transferring the developed

6 image to a surface of the ITM to form an intermediate image

7 thereon and, then, transferring the intermediate image from

8 the surface of the ITM to the substrate and wherein the

9 method further comprises controlling the temperature of the 10 ITM.

11

12 7. A method according to claim 6 wherein controlling the

13 temperature of the ITM comprises:

14 heating the ITM to a predetermined ITM temperature; and

15 maintaining the temperature of the ITM substantially at

16 the predetermined ITM temperature. 17

18 8. A method according to claim 7 wherein maintaining the

19 temperature of the ITM comprises:

20 sensing the temperature at the surface of the ITM; and

21 periodically heating the ITM in response to the sensed

22 ITM surface temperature. 23

24 9. A method according to claim 8 wherein the imaging

25 apparatus further comprises a heating lamp associated with

26 the ITM and wherein periodically heating the ITM comprises

27 periodically activating the heating lamp. 28

29 10. A method according to any of the preceding claims and

30 further comprising controlling the temperature of at least a

31 portion of the imaging environment. 32

33 11. A method according to claim 10 wherein controlling the

34 temperature of at least a portion of the imaging environment

35 comprises cooling at least said portion of the imaging

36 environment. 37

38 12. A method according to claim 10 or claim 11 wherein controlling the temperature of at least a portion of the imaging environment comprises: sensing the temperature of at least said portion of the imaging environment; and cooling at least said portion of the imaging environment in accordance with the sensed environmental temperature.

13. A method according to any of the preceding claims wherein the imaging apparatus further comprises a supply of carrier liquid and wherein the method further comprises controlling the temperature of the carrier liquid.

14. A method according to claim 13 wherein controlling the temperature of the carrier liquid comprises cooling the carrier liquid to a predetermined carrier liquid temperature.

15. A method according to claim 14 wherein controlling the temperature of the carrier liquid further comprises maintaining the temperature of the carrier liquid toner substantially at the predetermined carrier liquid temperature.

16. A method according to claim 15 wherein maintaining the temperature of the carrier liquid comprises: sensing the temperature of the carrier liquid; and cooling the carrier liquid in response to the sensed carrier liquid temperature.

17. A method according to claim 16 wherein the imaging apparatus comprises a chiller and wherein cooling the carrier liquid comprises circulating at least a portion of the carrier liquid through the chiller.