WO2007001303A1

WO2007001303A1 - Method and apparatus for reducing liquid content in electrostatic printing

Info

Publication number: WO2007001303A1
Application number: PCT/US2005/022997
Authority: WO
Inventors: Naseem Yacoub
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2005-06-27
Filing date: 2005-06-27
Publication date: 2007-01-04
Also published as: JP2008544325A; EP1904900A1

Abstract

A method of reducing liquid content of an image formed on an imaging surface (16) of an electrostatic printing apparatus (90) is disclosed. The method comprises: (a) heating a liquid developer beyond 30 degrees centigrade to a predetermined temperature being lower than 50 degrees centigrade by heater (80); (b) applying the liquid developer via outlets (106) on the imaging surface (16) so as to form the image thereon; and (c) squeezing the liquid developer via a squeegee (30). The predetermined temperature is selected so as to reduce a liquid content of the image.

Description

METHOD AND APPARATUS FOR REDUCING LIQUID CONTENT IN ELECTROSTATIC PRINTING

FIELD AND BACKGROUND OF THE INVENTION

5 The present invention relates to digital printing and, more particularly, to a method and apparatus for electrostatic printing using liquid developer.

Electrostatic printing is a very effective method of image transfer. Typically, in electrostatic printing, a latent electrostatic image is formed on an imaging surface (e.g., a photoconductor drum) by forming uniform electrostatic charge on the imaging

10 surface, and exposing it to a beam of light modulated by the image to be printed. The exposure procedure results in charged and discharged portions of the imaging surface, whereby charged portions form the print image and discharged portions form the background thereof. The latent electrostatic image is then developed, for example, by applying toner which adheres to the charged portions of the surface. The toner is

15 subsequently transferred and fused onto a transfer substrate, such as a sheet of paper.

One method to transfer the toner is by passing the transfer substrate between a roller and the imaging surface. During the toner transfer, electrostatic forces between the roller and the toner attract the toner away from the surface of the photoconductor drum onto the transfer substrate.

20 However, it is impossible in practical application to transfer all of the toner to the transfer substrate, and a residual amount remains on the imaging surface and must be removed prior to a subsequent electrostatic printing operation. From an economic standpoint, it is desirable to recycle rather than discard the residual toner after such removal.

25 Electrostatic printing may employ either dry toner or liquid toner (e.g., liquid ink). The quality of the image is related to the size of the toner particles. While it is thought that very fine particles will produce a finer image, there is a practical limitation on the size of toner particles that can be used. Dry toner particles must be of sufficient weight and size to be deposited onto the print surface without becoming

30 airborne, which is thought to lead to machinery fouling and, possibly, environmental problems. Additionally, it is difficult to recycle dry toner because the removal and collection of residual dry toner particles for the purpose of re-use is hampered, e.g., by the forces of dry friction Liquid toners have the advantage of being dispersed in a solvent, thus facilitating the use of very fine dye particles without concern for the particles becoming airborne. In addition, the recycling of liquid toner is commonly practiced in the art of electrostatic printing because the residual liquid toner can be allowed to flow downwardly under the force of gravity. Liquid toners are obtained by mixing a certain amount of toner in a carrier liquid, which is typically selected to be a highly resistant or insulating liquid (e.g., petroleum solvent), so as to facilitate efficient toner transfer. In addition, offset-preventing and release facilitating oil, such as silicone oil, is often used so as to increase the efficiency of toner transfer from the imaging surface. When using liquid toners, there is a need to remove the carrier liquid from the imaging surface after the toner has been applied thereto. This prevents the carrier liquid from being transferred from the imaging surface to the transfer substrate. Removal of the carrier liquid is necessary for various reasons, including recycling, environmental concerns and image quality (e.g., mechanical strength). A conventional electrostatic printing apparatus therefore employs a squeegee roller or another device which removes excess liquid from the imaging surface and partially dries the liquid image prior to the toner transfer process.

Carrier liquids are known to be volatile and give off bad odor, making the liquid toners unfavored for office or home use. One known solution to the problem of how to improve carrier liquid evaporation is the use of a dedicated ventilation duct through which vapors of carrier liquid are caused to evaporate during the fusing of the toner. In another method, large carrier recovery equipment is employed to suck, liquefy and collect vapors of the carrier liquid. In an additional method vapors of the carrier liquid are adsorbed and collected by a collection agent such as activated carbon or cyclodextrin.

Also of prior art of interest is U.S. patent No. 5,574,547, the contents of which are hereby incorporated by reference, which discloses a liquid electrophotographic reproduction machine having a development station which includes a porous blotter roller and a vacuum device. The roller contacts the developed image and reduces its fluid content. The roller is coupled to the vacuum device which facilitates the removal of the liquid. Additionally, the reproduction machine includes a chamber, for holding the liquid toner, and a heating element positioned therein. The heating element heats the liquid toner to an elevated temperature within a range of 50 ⁰C to 60 ⁰C, thereby reduces its viscosity.

However, the above prior art solutions for overcoming the problems of excess liquid, are far from satisfactory. Prior art techniques are power consuming, increase the overall processing time and make the electrostatic printing systems more bulky and vulnerable to mechanical failures.

There is thus a widely recognized need for, and it would be highly advantageous to have a method and apparatus for electrostatic printing, devoid of the above limitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a method of reducing liquid content of an image formed on an imaging surface of an electrostatic printing apparatus, comprising: (a) heating a liquid developer beyond 30 degrees centigrade to a predetermined temperature being lower than 50 degrees centigrade; (b) applying the liquid developer on the imaging surface so as to form the image thereon; and (c) squeezing the liquid developer; wherein the predetermined temperature is selected so as to reduce a liquid content of the image.

According to another aspect of the present invention there is provided a method of forming an image on an imaging surface of an electrostatic printing apparatus, comprising applying a heated liquid developer on the imaging surface so as to form the image thereon, and squeezing the heated liquid developer, wherein a temperature of the heated liquid developer is selected such that a liquid content of the image is below about 75 %. According to further features in embodiments of the invention described below, the liquid developer is heated by the imaging surface.

According to still further features in the described embodiments the liquid developer is heated by thermal radiation transmitted thereon.

According to still further features in the described embodiments the liquid developer is heated by a stream of hot air onto the liquid developer.

According to still further features in the described embodiments the temperature of the heated liquid developer is selected such that the liquid content of the image is reduced by at least 10 %, at least 20 %, or at least 30 %. According to still further features in the described embodiments the temperature of the heated liquid developer is selected such that the liquid content of the image of the image is below about 75 %, about 72 %, or about 70 %.

According to still further features in the described embodiments the temperature of the heated liquid developer is from about 32 degrees centigrade to about 48 degrees centigrade, or from about 34 degrees centigrade to about 46 degrees centigrade, or from about 38 degrees centigrade to about 42 degrees centigrade.

According to yet another aspect of the present invention there is provided a method of forming an image from a liquid developer on an imaging surface of an electrostatic printing apparatus, the liquid developer having solid particulates dispersed in a carrier liquid, the method comprising: (a) heating the liquid developer to a predetermined temperature; (b) applying the liquid developer on the imaging surface so as to form an image thereon; and (c) squeezing the liquid developer; wherein the predetermined temperature is selected such that, once the liquid developer is squeezed, void volumes formed between the solid particulates are significantly reduced.

According to further features in embodiments of the invention described below, the steps (a) and (b) are executed substantially contemporaneously.

According to still further features in the described embodiments the step (a) is executed while the liquid developer is on the imaging surface. According to still further features in the described embodiments the heating comprises heating the imaging surface. According to still further features in the described embodiments the heating comprises transmitting thermal radiation onto the liquid developer. According to still further features in the described embodiments the heating comprises generating a stream of hot air onto the liquid developer. According to still another aspect of the present invention there is provided an electrostatic printing apparatus, comprising: a movable imaging surface capable of carrying a latent image thereon; an exposing unit capable of emitting light on the imaging surface so as to form the latent image thereon; a developing unit, for applying a liquid developer onto the imaging surface, thereby to provide a developed image; and a heating unit, for heating the liquid developer beyond 30 degrees centigrade to a predetermined temperature being lower than 50 degrees centigrade, wherein the predetermined temperature is selected so as to reduce a liquid content of the image on the imaging surface. According to further features in embodiments of the invention described below, the heating unit comprises at least one device selected from the group consisting of an ohmic element, a blower and radiating device.

According to still further features in the described embodiments the heating unit is operatively associated with the movable imaging surface.

According to still further features in the described embodiments the heating unit comprises a heating roller engaging the liquid developer while being on the imaging surface.

According to still further features in the described embodiments the heating unit is operatively associated with the developing unit.

According to still further features in the described embodiments the developing unit comprises a container for holding the liquid developer, and further wherein the heating unit is positioned within the container.

According to still further features in the described embodiments the imaging surface is embodied on a rotating drum.

According to still further features in the described embodiments the heating unit is positioned within the rotating drum.

According to still further features in the described embodiments the apparatus further comprising a charging unit, for uniformly charging the imaging surface. According to still further features in the described embodiments the developing unit comprises at least one electrode operable to apply the liquid developer on the imaging.

According to still further features in the described embodiments the apparatus further comprising a squeegee being in contact with the imaging surface, for squeezing the liquid developer on the imaging surface.

According to still further features in the described embodiments the heating unit is operatively associated with the squeegee so as to allow the squeegee to heat the liquid developer.

According to still further features in the described embodiments the apparatus further comprising an intermediate transfer member, oppositely moving relative to the imaging surface and configured to receive the developed image from the imaging surface, and to transfer the developed image to a substrate. According to still further features in the described embodiments the developing unit is designed and constructed to apply different colors of the liquid developer on the imaging surface.

According to still further features in the described embodiments the developing unit comprises a development roller being spaced apart from the imaging surface, thereby forming a gap between the development roller and the imaging surface.

According to still further features in the described embodiments the developing unit further comprises a multicolor liquid developer sprayer, designed and constructed to spray the liquid developer onto a portion of the development roller, a portion of the imaging surface and/or a development region formed between the imaging surface and the development roller.

According to still further features in the described embodiments the heating unit is operable to heat the liquid developer while the liquid developer passes through the multicolor liquid developer sprayer. According to still further features in the described embodiments the heating unit is operatively associated with the development roller, so as to allow the development roller to heat the liquid developer.

According to still further features in the described embodiments the developing unit comprises a development roller, a main electrode and a back electrode, the main electrode and a back electrode the having a gap therebetween and configured such that the liquid developer is forced through the gap to at least partially plate the development roller.

According to still further features in the described embodiments the heating unit is configured to heat the liquid developer in a manner such that the liquid content of the image is reduced by at least 10 %, at least 20 %, or at least 30 %.

According to still further features in the described embodiments the heating unit is configured to heat the liquid developer to a predetermined temperature selected such that the liquid content of the image is below about 75 %, or below about 72 %, or below about 70 %. The present invention successfully addresses the shortcomings of the presently known configurations by providing a method and apparatus for electrostatic printing having properties far exceeding prior art. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the drawings:

Fig. 1 is a schematic illustration of a cross sectional view of an electrostatic printing apparatus, according to the teaching of the prior art.

Figs. 2a-b are schematic illustrations of spherical solid particulates dispersed in a carrier liquid, before (Figure 2a) and after (Figure 2b) application of a squeezing process.

Figs. 2c-d are schematic illustrations of non-spherical solid particulates dispersed in a carrier liquid, before (Figure 2c) and after (Figure 2d) application of a squeezing process.

Fig. 3 is a flowchart diagram of a method of reducing liquid content of an image, according to an embodiment of the present invention.

Fig. 4 is a schematic illustration of a portion of the apparatus of Figure 1, adapted to be used according to an embodiment of the present invention. Fig. 5 is a schematic illustration of an electrostatic printing apparatus, according to another embodiment of the present invention.

Fig. 6 is a schematic illustration of a developing unit, constructed in accordance with an exemplary embodiment of the present invention. Fig. 7 shows the solid content of a liquid developer on an imaging surface, as a function of the temperature, according to an embodiment of the present invention.

Fig. 8 is a shows the liquid content of a liquid developer on an imaging surface, as a function of the temperature, according to an embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

The present embodiments are of methods and an apparatus for digital printing, which can be electrostatic printing using liquid developer. Specifically, the present embodiments can be used to reduce liquid content of a liquid developer hence to increase the efficiency, in terms of energy, equipment and/or time, of the digital printing process.

For purposes of better understanding the present invention, as illustrated in Figures 2-7 of the drawings, reference is first made to the construction and operation of a conventional (i.e., prior art) electrostatic printing apparatus as illustrated in Figure 1.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Referring now to the drawings, Figure 1 schematically illustrates a cross sectional view of an electrostatic printing apparatus 1, according to the teaching of prior art. Apparatus 1 comprises a drum 10 arranged for rotation about an axle 12 in a direction generally indicated by arrow 14. Drum 10 is formed with an imaging surface

16, e.g., a photoconductive surface. Surface 16 is typically of a cylindrical shape. A charging unit 18, which can be a corotron, a scorotron, a roller charger or any other suitable charging unit known in the art, uniformly charges surface 16, for example, with positive charge.

Continued rotation of the drum 10 brings surface 16 into image receiving relationship with an exposing unit 20, which focuses a desired image onto surface 16. Unit 20 selectively discharges surface 16 in the areas struck by light, thereby forming the electrostatic latent image. Usually the desired image is discharged by the light while the background areas are left electro statically charged. Thus, the latent image normally includes image areas at a first electrical potential and background areas at another electrical potential. Unit 20 may be a modulated laser beam scanning device, an optical focusing device or any other imaging device known in the art.

Continued rotation of the drum 10 brings imaging surface 16, now bearing the electrostatic latent image, into a developing unit 22, which typically comprises electrodes 24 operative to apply a liquid developer 32 on surface 16, so as to develop the electrostatic latent image.

Liquid developer 32 can comprise charged solid particulates dispersed in a carrier liquid. The solid particulates are typically charged to the same polarity of the photoconductor. Thus, due to electrostatic repulsion forces between the developing electrode and the ink particles, ink particles adhere to areas on the photoconductor corresponding to the image regions, substantially without adhering to (developing) the background regions. In this manner a developed image is formed on surface 16.

Any liquid developer suitable for developing an electrostatic latent image can be used. One such liquid developer is known by the trade name Electrolnk®, commercially available from HP Indigo. This liquid developer is characterized by its comprising toner particulates dispersed in a carrier liquid, where the toner particulates are comprised of a core of a polymer with fibrous extensions extending from the core. When the toner particulates are dispersed in the carrier liquid in a low concentration, the particulates remain separate. When the toner develops an electrostatic image the concentration of toner particulates increases and the fibrous extensions interlock. A large number of patents and patent applications are directed toward this type of toner and charge directors which are comprised in it. These include: U.S. Pat. Nos. 4,794,651; 4,842,974; 5,047,306; 5,407,307; 5,192,638; 5,208,130; 5,225,306; 5,264,312; 5,266,435; 5,286,593; 5,300,390; 5,346,796; 5,407,771; 5,554;476; 5,655,194; 5,792,584 and 5,5923,929 the disclosures of all of which are incorporated herein by reference.

Following application of liquid developer thereto, surface 16 passes a roller 26, which is typically charged to the same polarity as the toner particles and rotates in a direction indicated by an arrow 28. Roller 26 serves for reducing the thickness of liquid developer 32. Once surface 16 passes roller 26, region of the latent image are covered, substantially exclusively, by liquid developer 32.

A typical spatial separation of roller 26 from surface 16 is about 50 microns. The electric potential of roller 26 is typically intermediate the aforementioned first and second electric potential of the latent image areas and of the background areas on surface 16. Representative examples of voltage configuration include, without limitation, roller 26: from about +300 to about +500 volts, background area: about +50 volts and latent image areas: up to about +1000 volts.

As used herein the term "about" refers to ± 10 %. Apparatus 1 may further comprises a squeegee 30, positioned downstream of roller 26, and is maintained in contacting or pressured relationship with surface 16. Squeegee 30 can be held at negative potential, e.g., from about 1000 to about 2000 volts, such that corona discharge takes place and electrical current flows from squeegee 30. Squeegee 30 repels the negatively charged particulates and causes them to more closely approach the image areas of surface 16, thus squeezing and rigidizing the liquid image thereon.

Once squeezed, the liquid image is transferred, typically via electrostatic attraction, to an intermediate transfer member 40, rotating in direction 41 which is opposite to direction 14 of drum 10. Subsequently, the image experiences a second transfer, typically aided by heat and pressure, from transfer member 40 to a substrate

42, which is supported by a roller 43.

Following the transfer of the liquid image to transfer member 40, surface 16 is engaged by a cleaning roller assembly 50, which typically comprises two oppositely rotating rollers 52 and a nozzle 54. Assembly 50 scrubs clean surface 16, e.g., using a cleaning material supplied by nozzle 54. Residual charge left on surface 16 can be removed, e.g., by flooding surface 16 with light from a lamp 58.

A major limitation in conventional electrostatic printing apparatus such as apparatus 1 is the excess of liquid (78 % or more) in the liquid image upon the first transfer. As stated in the Background section above, excess liquid (e.g., oil) is conventionally evacuated by squeezing, evaporation through a ventilation duct and/or the use of large carrier recovery equipment. The evacuation of excess liquid content in the liquid developer consumes power and causes environmental pollution due to environmentally hazardous materials present in the liquid developer.

The above limitation is primarily due to the non-spherical geometrical shape of the solid particulates, as further detailed hereinbelow with reference to Figures 2a-d.

Figure 2a-b show spherical solid particulates 62 dispersed in a carrier liquid 64, before (Figure 2a) and after (Figure 2b) application of a squeezing process, hi Figure 2a, the concentration of particulates 62 is low and the average distance between neighboring particulates is large. Upon application of the squeezing process, which can be an electrical, mechanical or magnetic process, spherical particulates 62 approach each other thus minimizing the average inter-p articulate distance. For spherical particulates, the average inter-particulate distance can be as small as 2R between the centers of neighboring particulate, where R is the average radius of the particulates. Geometrically, the ideal configuration shown in Figure 2b correspond to a liquid-solid distribution of, roughly, 40 % liquid and 60 % solid, where the liquid is confined in void volumes 68 formed in the (minimal) inter-particulate spaces. As will be appreciated by one ordinarily skilled in the art, this is the minimal liquid content (and maximal solid content) of a dispersive medium which is attainable only for spherical solid particulates.

Figure 2c-d show non-spherical solid particulates 66 dispersed in carrier liquid 64, before (Figure 2c) and after (Figure 2d) application of a squeezing process. As stated, a liquid developer (e.g., Electrolnk®) typically comprises non-spherical particulates, comprising, for example, a core polymer with fibrous extensions extending from the core. In Figure 2c, the concentration of particulates 66 is low and the average distance between neighboring particulates is large. Upon application of the squeezing process, particulates 66 approach each other to reduce the average inter- particulate distance. Unlike spherical particulates 62, however, the average inter- particulate distance is still far from the minimal possible distance, and rather large void volumes 70 (considerably larger than void volumes 68) are formed. Due to the large void volumes, the liquid-solid distribution of presently known liquid developers is about 22 % solid and about 78 % liquid, even after the squeezing process. While conceiving the present invention it was hypothesized and while reducing the present invention to practice it was realized that the liquid content of the liquid developer can be reduced by heating the liquid developer prior to, while or subsequently to the application of the liquid developer on the imaging surface. Thus, according to one aspect of the present invention there is provided a method of reducing liquid content of an image. The method of the present aspect of the invention can be executed by modifying a conventional electrostatic printing apparatus (e.g., apparatus 1), or by a specifically designed electrostatic printing apparatus as further detailed hereinunder. Thus, according to an embodiment of the present invention the liquid image is formed on an imaging surface, such as, but not limited to, imaging surface 16 of electrostatic printing apparatus 1.

Hence, the method comprises the following method steps which are illustrated in the flowchart diagram of Figure 3. It is to be understood that unless otherwise defined the method steps described hereinbelow can be executed either contemporaneously or sequentially in any combination or order of execution. Specifically, the ordering of the flowchart of Figure 3 is not to be considered as limiting. For example, two or more method steps, appearing in the description or in the flowchart of Figure 3 in a particular order, can be executed in a different order (e.g., a reverse order) or substantially contemporaneously. Referring now to Figure 3, in a first step of the method, a liquid developer is heated to a predetermined temperature, and in a second step a liquid image is formed on the imaging surface by applying the liquid developer thereon. The liquid developer can be any liquid developer known in the art which is suitable for developing an electrostatic latent image, such as, but not limited to, Electrolnk®. Additionally, the application of the liquid developer on the imaging surface can be executed in any way known in the art, e.g., using a developing unit, such as, but not limited to, developing unit 22 of electrostatic printing apparatus 1, as further detailed hereinabove.

In a third step of the method, the liquid image is squeezed on the imaging surface, for example, by application of mechanical, electrical and/or magnetic force on the liquid image. This can be done in more than one way. For example, in one embodiment, a squeegee is used to apply mechanical pressure on the liquid image; in another embodiment an electric field is generated such that a motion of the solid particulates of the liquid developer is established substantially in a direction which is perpendicular to the imaging surface; in an additional embodiment a magnetic field forces the solid particulates to approach each other, etc. A representative example includes, without limitation the use of a squeegee {e.g., squeegee 30 of apparatus 1) which can apply both mechanical and electrostatic force on the liquid image. According to an embodiment of the present invention the predetermined temperature to which the liquid developer is heated is above 30 °C and below 50 ⁰C, e.g., from about 32 °C to about 48 °C, or from about 34 ⁰C to about 46°C, or from about 36 °C to about 44°C, or from about 38 ⁰C to about 42⁰C. Additionally, the temperature is below any characteristic transition temperature the toner particles. As used herein, "characteristic transition temperature" (otherwise known as the

"glass transition temperature") refers to a temperature at which the toner particles experience a transition from one state to another, becomes softer and irreversibly coagulated. Such transitions are known in the art and include first order and a second order phase transitions, e.g., a transition from low viscosity state to high viscosity state, a transition from moderate to steep viscosity-temperature response and the like.

According to an embodiment of the present invention the temperature is selected such that the liquid content of the subsequently formed liquid image is significantly reduced. Specifically, the temperature is such that the liquid content of the liquid image is reduced by at least 10 %, or at least 20 %, or at least 30 %, e.g., a liquid content reduction of 40 % or more. For example, the predetermined temperature may be such that the resulting liquid image, after squeezing, has less than 75 % liquid content, or less than 72 % liquid content, or less than 70 % liquid content, e.g., about 68 % liquid content or less.

It is appreciated that too high temperatures of the liquid developer may result in formation of vapors in the electrostatic printing apparatus. Formation of vapors is undesired because such vapors can be toxic and may emit bad odor. Therefore, the temperature of the liquid developer can be selected so as not to substantially increase the evaporation of the liquid developer. Representative values of the predetermined temperature include, without limitation 32 ⁰C, 34 °C, 36 °C, 38 °C, 40 °C, 42 °C, 44 ⁰C and 46 °C.

As exemplified in the Example section that follows, in various experiments the Inventors of the present invention have succeeded in significantly reducing the liquid content of the image and have obtained a liquid image having a liquid-solid distribution of about 62 % liquid and 38 % solid by weight. Comparing this success to prior art teachings, in which a liquid content of about 78 % or more is inevitable, it will be appreciated the presentembodiments of the invention provide a significant improvement to the development process. Thus, suppose for example that 100 milligrams of solid particulates is to be used in a certain image. According prior art techniques, a liquid image having 100 milligrams of solid particulates also contains about 400 milligrams of liquid (in accordance with the aforementioned 22 % - 78 % ratio) which need to be evacuated by various evacuation techniques. On the other hand, if the liquid developer is heated in accordance with embodiments of the present invention, such that the liquid content is reduced to, say, 67 %, the resulting liquid image contains only half as much liquid (about 200 milligrams). This corresponds to a considerable reduction of the means (energy, equipment, time) to evacuate the liquid. The improvement if further enhanced when the present embodiments are employed in a multi-color digital imaging apparatus, whereby for each image a plurality of developing processes are employed, each developing process being dedicated to a particular color. Generally, for N colors the improvement is enhanced by a factor of N. Thus, returning to the above numerical example, the heating step facilitates a saving of about 27V milligrams of liquid for each milligram of solid particulates.

The reduction of liquid content (and consequently the increment of the solid content) of the liquid image can be explained by a shrinkage of the void volumes formed between the solid particulates (see Figure 2d). As will be appreciated by one ordinarily skilled in the art, the heating of the liquid developer decreases the viscosity of the carrier liquid, thus allowing the solid particulates to flow more freely and to approach more closely to one another. Additionally the heating increases the kinetic energy of the solid particulates in the carrier liquid hence further facilitates their approach to one another. According to an embodiment of the present invention the predetermined temperature to which the liquid developer is heated is selected so as to reduce or minimize the void volumes between the solid particulates. As stated, the heating does not exceed one or more of the characteristic transition temperatures of the liquid developer.

The heating of the liquid developer can be done in more than one way. For example, in one embodiment, the liquid developer is heated prior to the application of the liquid developer on the imaging surface, e.g., by heating a container holding the liquid developer prior to the application on the imaging surface, hi another embodiment, the liquid developer is heated substantially contemporaneously with the application on the imaging surface, e.g., using a heated roller or any other suitable developing unit in which only the applied liquid developer is heated while being applied on the imaging surface, hi an additional embodiment, the liquid developer is heated after its application, e.g., by heating the imaging surface.

Many heating methods are contemplated, including, without limitation, the use of an ohmic element, a stream of hot air and/or transmission of thermal radiation onto the liquid developer. The method of the present embodiments can be employed in any electrostatic imaging apparatus, including without limitation, apparatus 1.

Reference is now made to Figure 4 which is a schematic illustration of a portion of apparatus 1, adapted to be used according to an embodiment of the present invention. Shown in Figure 4 are drum 10, imaging surface 16 and developing unit 22 holding liquid developer 32. Other components of apparatus 1, are not shown for the sake of conciseness. Hence, according to an embodiment of the present invention apparatus 1 is supplemented by a heating unit 80, for heating the liquid developer so as to reduce the liquid content level of the liquid image as further detailed hereinabove.

Heating unit 80 can comprise any device or element which is capable of heating liquid developer 32. Thus, for example, the heating unit can be an ohmic element (e.g., an electrically resistant wire), a blower for generating a stream of hot air, a radiating device for radiating thermal energy and the like. Heating unit 80 may heat liquid developer 32 on surface 16 or in developing unit 22 (prior to the application on surface 16). Heating liquid developer 32 in developing unit 22 can be achieved by positioning heating unit 80 or a portion thereof (e.g., its heating member) within developing unit 22. Heating liquid developer 32 on surface 16 can be achieved, for example, by providing a stream of hot air on surface 16 or by engaging a heated roller with surface 16. Additionally, heating unit 80 may heat imaging surface 16 such that when liquid developer is applied on surface 16, heat is transferred from surface 16 to liquid developer 32.

Reference is now made to Figure 5 which is a schematic illustration of an electrostatic printing apparatus 90, according to another embodiment of the present invention. Apparatus 90 is used for multicolor printing. Apparatus 90 may comprise any of the elements of Apparatus 1, including, without limitation, drum 10, imaging surface 16, charging unit 18, exposing unit 20, squeegee 30 and transfer member 40, all of which can be constructed to operate as further detailed hereinabove or in any other way known in the art. Apparatus 90 further comprises a developing unit 92 for applying of liquid developer 32 on imaging surface 16. Developing unit 92 is can be designed and constructed to apply different colors (e.g., 4, 5, 6, 7 colors or more) of liquid developer 32 on imaging surface 16 in a synchronized fashion. For example, developing unit 92 can periodically apply a different color for each rotation cycle of drum 10. In the embodiment shown in Figure 5, developing unit 92 comprises a development roller 98, which is can be spaced from surface 16 thereby forming a gap between development roller 98 and surface 16. Typically, the spacing is from about 40 μm to about 150 μm. Development roller 98 can be charged to an electrical potential intermediate that of the image and its background areas. Development roller 98 is thus operative when maintained at a proper voltage to apply an electric field to aid development of the latent electrostatic image.

Development roller 98 typically rotates in the same sense as drum 10. This rotation provides for surface 16 and roller 98 to have opposite velocities in their region of propinquity. According to an embodiment of the present invention developing unit 92 further comprises a multicolor liquid developer sprayer 94, which can be mounted on an axis 96 to allow sprayer 94 to be pivoted in such a manner that a spray of liquid developer 32 can be directed either onto a portion of development roller 98, a portion of surface 16 or directly into a development region 95 between surface 16 and roller 98. Sprayer 94 can receives separate supplies of colored liquid developer from different reservoirs 99. Any number of reservoirs can be used, depending on the desired number of colors. According to an embodiment of the present invention sprayer 94 comprises a linear array of spray outlets 106, each of which communicates with a different reservoir, e.g., via a specific conduit (not shown). Spray outlets 106 can be interdigitated such that when N colors are used, every Mh outlet sprays the same color, and every group of N adjacent outlets includes outlets which spray N different colors. The flow of liquid developer 32 to each of outlet can be controlled by a controller 114. Outlets 106 are positioned at two or more levels (designated 108 and

110) to permit the minimization of separation between the outlets.

Liquid developer 32 is sprayed under pressure from each of outlets 106 into development region 95, a portion of development roller 98 and/or a portion of imaging surface 16. According to an embodiment of the present invention, the spacing of spray outlets 106 and their periodicity is selected to enable the toner for each individual given color to substantially uniformly fill region 95. This can be achieved by a substantially uniform array or the colors can be grouped in clusters each of which contains one outlet for each color. Typically these clusters have a center to center spacing of from about 40 mm to about 60 mm.

Apparatus further comprises heating unit 80, for heating liquid developer 32 so as to reduce the liquid content level of the liquid image as further detailed hereinabove. Heating unit 80 can be of any of the aforementioned types of heating units, and is can be associated with more than one element of apparatus 90. Hence, in one embodiment, heating unit 80 is positioned in or close to reservoirs 99, thereby allowing the heating to take place before the application of liquid developer 32 on imaging surface 16.

In another embodiment, heating unit 80 is positioned in or close to sprayer 94 thereby allowing heating only a portion of liquid developer 32 just before its application on imaging surface 16.

In an additional embodiment, heating unit 80 is positioned within or close to development roller 98 thereby allowing heating only a portion of liquid developer 32 while being applied on imaging surface 16.

In still another embodiment, heating unit 80 is positioned within or close to drum 10 thereby allowing heating liquid developer 32 after its application on imaging surface 16.

In yet another embodiment, heating unit 80 comprises a heating roller 82 which can be in contact with roller 98 or surface 16 so as to apply heat directly to liquid developer 32 while being rotated together with roller 98 or drum 10. When heating roller is in contact with surface 16, heating roller 82 can be positioned before squeegee 30. Alternatively squeegee 30 can be supplemented by a heating member so as to enable heating of the liquid developer by squeegee 30. Reference is now made to Figure 6, which is a schematic illustration of developing unit 92, constructed in accordance with another exemplary embodiment of the present invention. Hence, in this embodiment, developing unit 92 is encased in a housing 15. An electrode 126 is formed in two parts, a main electrode 128 and a back electrode 130. Both main 128 and back 130 electrodes are operatively associated with a development roller 98. Electrode 126 is formed with a cavity 134 into which the liquid developer is introduced via a liquid developer input portal 136.

The liquid developer is forced by pressure via a passage 138 to enter narrow spaces between electrodes 128 and 130 and roller 98. Main 128 and back 130 electrodes on the one hand, and roller 98 on the other hand, are electrified to different voltages, such that the charge solid particulates of the liquid developer are plated onto roller 98, providing a thin concentrated layer of liquid developer.

Squeegee 30 repels the negatively charged particulates to form a more concentrated layer on roller 98 as further detailed hereinabove. The layer is transferred to those portions of the latent image that are electrified to attract it, with roller 98 being electrified to aid in the transfer of the layer to image areas of the latent image and to prevent transfer to background areas of the latent image.

Developing unit 92 may further comprise a cleaning unit 142, which may have, a cleaning roller 144, a scraper 146, a sponge roller 148 and an additional squeezing roller 150. Unit 142 is used to remove the layer (or portions of a layer) that remain on roller 98. This material can be stored in the space between electrode 126 and housing

15, or it may be removed from the housing for reuse.

As stated hereinabove, heating unit 80 can be associated with more than one element of apparatus 90. With respect to the exemplary embodiment shown in Figure 6, heating unit 80 can be associated, for example, with cavity 134, squeegee 30 or roller 98, as further detailed hereinabove.

It is expected that during the life of this patent many relevant developing units will be developed and the scope of the term developing units is intended to include all such new technologies a priori.

Additional objects, advantages and novel features of the present invention will become apparent to one ordinarily skilled in the art upon examination of the following examples, which are not intended to be limiting. Additionally, each of the various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below finds experimental support in the following examples.

EXAMPLE

Reference is now made to the following example, which, together with the above descriptions, illustrates the invention in a non limiting fashion.

Two kinds of conventional electrostatic printing apparatus, ULTRASTREAM® and TURBOSTREAM®, supplied by the Assignee of the present invention were tested for the effect of heat on the liquid and solid content of the liquid developer once the liquid image is formed on the imaging surface. In both experiments, Electrolnk® was used as the liquid developer. The rotation velocity of the imaging surfaces were 120 cm/sec for the ULTRASTREAM® apparatus and 60 cm/sec for the TURBOSTREAM® apparatus. Figure 7 shows the solid content on the imaging surface of the

ULTRASTREAM® apparatus, as a function of the temperature of the Electrolnk® prior to its application on the imaging surface. Shown in Figure 7 are weight percentage values and standard deviations of the solid content of the Electrolnk® for 26 °C, 29 °C, 34 °C and 38 ⁰C. As shown in Figure 7, the solid content can be increased by about 75 % (from about 21.5 % at 26 °C to about 38 % at 38 ⁰C).

Figure 8 shows the liquid content on the imaging surface of the TURBOSTREAM® apparatus, as a function of the temperature of the Electrolnk® prior to its application on the imaging surface. Shown in Figure 8 are weight values in milligrams and standard deviations of the liquid content of the Electrolnk® for 28 ⁰C, 32 °C and 41 °C. As shown in Figure 8, the liquid content can be reduced by about 50 % (from about 110 milligrams at 28 °C to about 55 milligrams at 41 °C).

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims

WHAT IS CLAIMED IS:

1. A method of reducing liquid content of an image formed on an imaging surface of an electrostatic printing apparatus, comprising:

(a) heating a liquid developer beyond 30 degrees centigrade to a predetermined temperature being lower than 50 degrees centigrade;

(b) applying said liquid developer on the imaging surface so as to form the image thereon; and

(c) squeezing the liquid developer; wherein said predetermined temperature is selected so as to reduce a liquid content of the image.

2. The method of claim 1, wherein said steps (a) and (b) are executed substantially contemporaneously.

3. The method of claim 1, wherein said step (a) is executed while the liquid developer is on the imaging surface.

4. A method of forming an image on an imaging surface of an electrostatic printing apparatus, comprising applying a liquid developer on the imaging surface so as to form the image thereon, heating said liquid developer to a predetermined temperature, and squeezing said liquid developer while being at said predetermined temperature, in a manner such that a liquid content of the image is below about 75 %.

5. The method of claim 1 or 4, wherein said liquid developer is heated by the imaging surface.

6. The method of claim 1 or 4, wherein said liquid developer is heated by thermal radiation transmitted thereon.

7. The method of claim 1 or 4, wherein said liquid developer is heated by a stream of hot air onto said liquid developer.

8. An electrostatic printing apparatus, comprising: an imaging surface capable of carrying a latent image thereon; an exposing unit capable of emitting light on said imaging surface so as to form said latent image thereon; a developing unit, for applying a liquid developer onto said imaging surface, thereby to provide a developed image; and a heating unit, for heating said liquid developer beyond 30 degrees centigrade to a predetermined temperature being lower than 50 degrees centigrade.

9. The apparatus of claim 8, wherein said heating unit comprises at least one device selected from the group consisting of an ohmic element, a blower and radiating device.

10. The apparatus of claim 8, wherein said heating unit is operatively associated with said movable imaging surface.

11. The apparatus of claim 10, wherein said heating unit comprises a heating roller engaging said liquid developer while being on said imaging surface.

12. The apparatus of claim 8, wherein said heating unit is operatively associated with said developing unit.

13. The apparatus of claim 8, wherein said developing unit comprises a container for holding said liquid developer, and further wherein said heating unit is positioned within said container.

14. The apparatus of claim 8, wherein said imaging surface is embodied on a rotating drum.

15. The apparatus of claim 14, wherein said heating unit is positioned within said rotating drum.

16. The apparatus of claim 8, further comprising a charging unit, for uniformly charging said imaging surface.

17. The apparatus of claim 8, wherein said developing unit comprises at least one electrode operable to apply said liquid developer on said imaging.

18. The apparatus of claim 8, further comprising a squeegee being in contact with said imaging surface, for squeezing said liquid developer on said imaging surface, thereby to reduce a liquid content of said image on said imaging surface.

19. The apparatus of claim 18, wherein said heating unit is operatively associated with said squeegee so as to allow said squeegee to heat said liquid developer.

20. The apparatus of claim 8, further comprising an intermediate transfer member, oppositely moving relative to said imaging surface and configured to receive said developed image from said imaging surface, and to transfer said developed image to a substrate.

21. The apparatus of claim 8, wherein said developing unit is designed and constructed to apply different colors of said liquid developer on said imaging surface.

22. The apparatus of claim 21, wherein said developing unit comprises a development roller being spaced apart from said imaging surface, thereby forming a gap between said development roller and said imaging surface.

23. The apparatus of claim 21, wherein said developing unit further comprises a multicolor liquid developer sprayer, designed and constructed to spray said liquid developer onto a portion of said development roller, a portion of said imaging surface and/or a development region formed between said imaging surface and said development roller.

24. The apparatus of claim 23, wherein said heating unit is operable to heat said liquid developer while said liquid developer passes through said multicolor liquid developer sprayer.

25. The apparatus of claim 23, wherein said heating unit is operatively associated with said development roller, so as to allow said development roller to heat said liquid developer.

26. The apparatus of claim 8, wherein said developing unit comprises a development roller, a main electrode and a back electrode, said main electrode and a back electrode said having a gap therebetween and configured such that said liquid developer is forced through said gap to at least partially plate said development roller.

27. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is selected such that said liquid content of said image is reduced by at least 10 %.

28. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is selected such that said liquid content of said image is reduced by at least 20 %.

29. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is selected such that said liquid content of said image is reduced by at least 30 %.

30. The method or apparatus of claim 1 or 18, wherein said predetermined temperature is selected such that said liquid content of said image of said image is below about 75 %.

31. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is selected such that said liquid content of said image of said image is below about 72 %.

32. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is selected such that said liquid content of said image of said image is below about 70 %.

33. The method or apparatus of claim 4 or 18, wherein said predetermined temperature is from about 30 degrees centigrade to about 50 degrees centigrade.

34. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is from about 32 degrees centigrade to about 48 degrees centigrade.

35. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is from about 34 degrees centigrade to about 46 degrees centigrade.

36. The method or apparatus of claim 1, 4 or 18, wherein said predetermined temperature is from about 38 degrees centigrade to about 42 degrees centigrade.