WO2002018145A1

WO2002018145A1 - Direct printing apparatus and method

Info

Publication number: WO2002018145A1
Application number: PCT/SE2000/001680
Authority: WO
Inventors: Filip Alm
Original assignee: Array AB
Current assignee: Array AB
Priority date: 2000-09-01
Filing date: 2000-09-01
Publication date: 2002-03-07
Anticipated expiration: 2003-03-01
Also published as: AU2000275653A1

Abstract

An image forming apparatus (2000) in which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier (2033) towards an image receiving surface, wherein apertures (2052) are arranged transversely to a printing direction (PD) of a printhead structure (2005) with an average pitch (pav), and extend a first distance (Iap) from a particle-receiving plane (A) to a particle-emitting plane (B) of the printhead structure (2005). The particle-receiving plane (A) is at a second distance (Ik) from the charged toner particles (PA) on the particle carrier (2033). The image receiving surface is provided on at least one surface portion of a back electrode member (2012) and can be caused to move at least in the printing direction (PD) in relation to the printhead structure (2005) for intercepting the transported charged toner particles in an image configuration (IM).

Description

Title

Direct printing apparatus and method.

Technical field

The invention relates to a direct printing apparatus in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle carrier towards an image receiving surface through a printhead structure, whereby the charged particles are deposited in image configuration on the image receiving surface caused to move relative to the printhead structure.

More particularly, the invention relates to an image forming apparatus for direct printing. Furthermore, the invention relates to a method for direct printing utilizing the apparatus.

Background of the Invention

U.S Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image-receiving medium due to control in accordance with an image information.

The printhead structure according to US 5,036,341 is formed by a lattice consisting of intersecting wires disposed in rows and columns. Each wire is connected to an individual voltage source. Initially the wires are grounded to prevent toner from passing through the wire mesh. As a desired print location on the image receiving substrate passes below an intersection, adjacent wires in a corresponding column and row are set to a print potential to produce an electric field that draws the toner particles from the particle source. The toner particles are propelled through the square aperture formed by four crossed wires and deposited on the image receiving substrate in the desired pattern. A drawback with this construction of printhead structure is that individual wires can be sensitive to the opening and closing of adjacent apertures, resulting in imprecise image formation due to the narrow wire border between apertures.

This effect is mitigated in an arrangement described in US Patent No.5,847,733 by the present applicant. This proposes a control electrode array formed on an apertured insulating substrate. A ring electrode is associated with each aperture and is driven to control the opening and closing of the aperture to toner particles. Each aperture is further provided with deflection electrodes. These are controlled to selectively generate an asymmetric electric field around the aperture, causing toner particles to be deflected prior to their deposition on the image-receiving member. This process is referred to as dot deflection control (DDC). This enables each individual aperture to address several dot positions. The print addressability is thus increased without the need for densely spaced apertures.

U.S. Patent No. 5,606,402 discloses an image forming apparatus comprising a developing particle holder, for holding developing particles by means of either electric or magnetic forces. The disclosed apparatus further comprises a counter- electrode positioned opposite the developing particle holder, and a control grid. The control grid includes a matrix of gates, arranged between the developing particle holder and the counter-electrode, in order to regulate passage of developing particles. In order to facilitate individual control of the gates, a distance between the control grid and the developing particle holder is shorter than the pitch of the gates. The disclosed apparatus further comprises a power source section, furnishing a potential which generates a prescribed potential difference between the developing particle holder and the counter-electrode and applying a potential to the control grid. The apparatus further includes potential control means for changing an electric field existing between the developing particle holder and the counter-electrode by changing the potential applied by the power source section to the control grid according to image signals in order to form an image by controlling flight of developing particles from the developing particle holder through each gate towards the counter electrode and by adhering the developing particles to a recording medium arranged in a direction toward which the particles fly. In many cases, it has proven to be difficult to achieve a satisfactory print quality with the previously known image forming apparatuses. There are a number of reasons for such problems. One reason for this is undesired variation in the electrical field which transports and guides the charged toner particles during their passage from the toner carrier to the image receiving surface. Previously known designs where the image receiving surface is a belt or a printing paper moving across and contacting one or several back electrodes can be afflicted with such problems, since tribo-electrical recharge of the belt or printing paper often changes the resulting electrical field in an unexpected and/or undesired way.

Summary of the invention

Accordingly, a first object of the present invention is to provide an image forming apparatus which reduces the above-mentioned problems with undesired variations in the electric field to a minimum, and which apparatus ensures that an excellent print quality can be obtained in all circumstances.

This first object is achieved by means of an image forming apparatus in which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier towards an image receiving surface, which image forming apparatus includes: a background voltage source for producing a background electric field which enables a transport of charged toner particles from the particle carrier towards the image receiving surface; a printhead structure arranged in the background electric field which structure includes a plurality of apertures and control electrodes arranged in conjunction to the apertures; and control voltage sources for supplying control potentials to the control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures. The image receiving surface is arranged for movement in relation to the printhead structure for intercepting the transported charged toner particles in an image configuration. Thereby, the apertures are arranged transversely to a printing direction of the printhead structure with an average pitch and extend a first distance from a particle- receiving plane to a particle-emitting plane of the printhead structure, which is positioned so that the particle-receiving plane is located at a second distance from the charged toner particles on the particle carrier, and so that the particle-emitting plane is at a third distance from the image receiving surface.

In the image forming apparatus according to the invention, the image receiving surface is provided on a back electrode member, wherein at least one surface portion of the back electrode member can be caused to move at least in the printing direction in relation to the printhead structure for intercepting the transported charged toner particles in the image configuration. This feature ensures that an excellent print result can be achieved in all circumstances, since the undesired tribo-electrical phenomena which could be caused by a separate image receiving member moving while contacting with a back electrode member are eliminated according to the invention, as a result of the image receiving surface being provided directly on the back electrode member.

A second object of the present invention is to provide an improved method for direct printing.

This second object is achieved by means of a method according to claim 17, which method comprises to deposit toner particles in an image configuration onto an information carrier constituting a back electrode, or to transfer toner particles deposited in an image configuration on a back electrode onto the information carrier, by means of an image forming apparatus according to the invention.

Brief description of the drawings

Fig. 1 is a schematic view of an image forming apparatus where an image-receiving surface is provided on a belt arranged in an endless loop.

Fig. 2 is a schematic sectional view across a print station in an image forming apparatus, such as, for example, that shown in Fig.l.

Fig. 3 is a schematic sectional view of the print zone, illustrating the positioning of a printhead structure in relation to a particle source and an image-receiving surface. Fig. 4a is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing the toner delivery unit.

Fig. 4b is a partial view of a printhead structure of a type used in an image forming apparatus, showing the surface of the printhead structure that is facing an image receiving surface on an intermediate transfer belt.

Fig. 4c is a sectional view across a section line I-I in the printhead structure of Fig.4a and across the corresponding section line II-II of Fig.4b.

Fig. 5 shows a schematic side view of a section of an image-forming apparatus according to a preferred embodiment of the present invention.

Fig. 6 shows a schematic perspective view of a magnified portion of the image- forming apparatus in Fig. 5.

Detailed description of embodiments

In order to perform a direct electrostatic printing using the apparatus shown in Figs. 1- 4c, a background electric field is produced between a particle carrier and a back electrode to enable a transport of charged particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field between the particle carrier and the back electrode and connected to a control unit which converts the image information into a pattern of electrostatic fields which, due to control in accordance with the image information, selectively open or close passages in the electrode matrix to permit or restrict the transport of charged particles from the particle carrier. The modulated stream of charged particles allowed to pass through the opened apertures are thus exposed to the background electric field and propelled toward the back electrode. The charged particles are deposited on an image receiving surface to provide line-by line scan printing to form a visible image. A printhead structure for use in direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with an insulating layer and control electrodes are arranged between the first surface of the substrate and the insulating layer, in a configuration such that each control electrode surrounds a corresponding aperture. The apertures are preferably aligned in one or several rows extending transversally across the width of the substrate, i.e. perpendicularly to the motion direction of the image receiving surface.

According to such a method, each single aperture is utilized to address a specific dot position of the image in a transversal direction. Thus the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction.

Advantageously, a direct electrostatic printing device of the type in question includes a dot deflection control (DDC). Thereby, each single aperture is used to address several dot positions on an image receiving surface by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving surface, and thereby the location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions. For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having only 200 apertures per inch. An improved DDC method provides a simultaneous dot size and dot position control. This method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. Each aperture is surrounded by two deflection electrodes connected to respective deflection voltages Dl, D2, such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages Dl, D2 have the same amplitude. The amplitude of Dl and D2 are modulated to apply converging forces on toner particles as they are transported toward the image receiving surface, thus providing smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between Dl and D2 to deflect the toner trajectory toward predetermined dot positions.

A printhead structure for use in DDC methods generally includes a flexible substrate of electrically insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is overlaid with a first printed circuit including the control electrodes and the second surface is overlaid with a second printed circuit including the deflection electrodes. Both printed circuits are coated with insulative layers. Utilizing such a method, 60 micrometer dots can be obtained with apertures having a diameter in the order of 160 micrometer.

Fig.l shows an image forming apparatus 1000 which comprises at least one print station, preferably four print stations (Y, M, C, K), an intermediate member 1001 providing an image receiving surface, a driving roller 1010, at least one support roller 1011, and preferably several adjustable holding elements 1012. The four print stations are arranged in relation to the intermediate member 1001. The intermediate member, in Fig. 1 a transfer belt 1001, is mounted over the driving roller 1010. The at least one support roller 1011 is provided with a mechanism for maintaining the transfer belt 1001 with a constant tension, while preventing transversal movement of the transfer belt 1001. The holding elements 1012 are for accurately positioning the transfer belt 1001 with respect to each print station.

The driving roller 1010 in Fig. 1 is a cylindrical metallic sleeve having a rotational axis extending perpendicularly to the motion direction of the belt 1001 and a rotation velocity adjusted to convey the belt 1001 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing. The adjustable holding elements

1012 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station. The holding elements 1012 in Fig. 1 are cylindrical sleeves disposed perpendicularly to the belt motion in an arcuate configuration so as to slightly bend the belt 1001 at least in the vicinity of each print station in order to, in combination with the belt tension, create a stabilization force component on the belt. That stabilization force component is opposite in direction and preferably larger in magnitude than an electrostatic attraction force component acting on the belt 1001 due to interaction with the different electric potentials applied on the corresponding print station.

The transfer belt 1001 in the apparatus shown in Fig.l is an endless band of 30 to 200 microns thick composite material as a base. The base composite material can include thermoplastic poly amide resin or any other suitable material having a high thermal resistance, such as 260°C of glass transition point and 388°C of melting point, and stable mechanical properties below temperatures in the order of 250°C. The composite material of the transfer belt has a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 1001. The outer surface of the transfer belt 1001 in Fig.l is coated with a 5 to 30 microns thick coating layer made of electrically conductive polymer material having appropriate conductivity, thermal resistance, adhesion properties, release properties and surface smoothness.

The transfer belt 1001 is conveyed past the four different print stations, whereas toner particles are deposited on the outer surface of the transfer belt and superposed to form a four color toner image. Toner images can then be conveyed through a fuser unit

1013 comprising a fixing holder 1014 arranged transversally in direct contact with the inner surface of the transfer belt. The fixing holder includes a heating element 1015, suitably of a resistance type of e.g. molybdenium, maintained in contact with the inner surface of the transfer belt 1001. As an electric current is passed through the heating element 1015, the fixing holder 1014 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1001. The fusing unit 1013 further includes a pressure roller 1016 arranged transversally across the width of the transfer belt 1001 and facing the fixing holder 1014. In the apparatus shown in Fig. 1, an information carrier 1002, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 1021 and conveyed between the pressure roller 1016 and the transfer belt. The pressure roller 1016 rotates with applied pressure to the heated surface of the fixing holder 1014 whereby the melted toner particles are fused on the information carrier 1002 to form a permanent image. After passage through the fusing unit 1013, the transfer belt is brought in contact with a cleaning element 1017, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1001 for removing all non-transferred toner particles from the outer surface.

Instead of a single unit performing a combined image transfer and fusing step, separate units for transferring the image to the information carrier and for fusing/fixating the image to the information carrier can be provided.The fusing unit normally is provided with means for feeding the paper to an out-tray, from which the paper can be collected by a user.

Toner particles are retained on the surface of a particle carrier (e.g. 1033 in Figs. 2-3) by an adhesion force which essentially is related to the particle charge and to the distance between the particle and the surface of the particle carrier. The electrostatic field applied onto a control electrode to initiate toner transport through a selected aperture is selected to be sufficient to overcome the adhesion force in order to cause the release of an appropriate amount of toner particles from the particle carrier. The electrostatic field is applied during the time period required for these released particles to reach sufficient momentum to pass through the selected aperture, whereafter the transported toner particles are exposed to the attraction force from the back electrode and are intercepted by the image receiving surface.

Properties such as charge amount, charge distribution, particle diameter etc. of the individual toner particles have been found to be of particularly great importance to the print performance in a direct printing method. Accordingly, the size and size distribution of the toner particles affect the printing result, since larger toner particles have a tendency to cause clogging of the apertures in the control electrode array. In addition, the toner particles allowed to pass through selected "opened" apertures are accelerated towards the image receiving under the influence of a uniform attraction field from the back electrode. In order to control the distribution of the transported particles onto a printing surface, the particles may be deflected by the application of a deflection pulse, resulting in an increase in the addressable area on the image receiving surface. Thereby, small particles having a low surface charge exhibit poor deflection properties.

Normally, toner particles are produced by the so-called melt-crushed method, which involves crushing and classifying colored resin, such as polyester resin or the like, with dispersed coloring agents and other additives using a compounding process. However, this method is not ideally suited for producing small-particle toner since it has a relatively low yield, and tends to produce a great variety of particle sizes and toner particles with a non-uniform composition. A non-uniform toner results in a poor charge uniformity and may impair the print quality.

Toner particles can also be produced in a chemical polymerization process, which is better suited for producing small toner particles of a uniform size. There are three basic processes, i.e. the suspension polymerization method, the dispersion polymerization method, and the emulsion polymerization method. The suspension and dispersion polymerization methods produce full-shaped spherical toner particles with a size between a few and up to 10 microns. The emulsion polymerization method produces polymer particles of sub-micron size or smaller, which particles are aggregated by means of different methods, e.g. heat-welding or coagulation, in order to form micron-order particles. The shape of the aggregated particles can vary from grape cluster to spherical, depending on the conditions prevailing in the aggregation process.

Toner particles can comprise a number of ingredients, e.g. a binding resin based on a cyclic polyolefin e.g. a copolymer of an alicyclic compound with double bonds, such as cyclohexene or norbornene, and an alpha-olefin, such as ethylene, propylene or butylene. Accordingly, the toner particles can be of 2-component or multi-component type. Advantageously, the toner particles have an irregular surface structure and an average diameter within the range of 3-8 microns. Depending on the application in question, electrically conductive, electrically non-conductive, or magnetic toner particles can be provided and utilized.

As shown in Fig. 2, a print station of an image forming apparatus, e.g. the one shown in Fig. 1, includes a particle delivery unit 1003 advantageously having a replaceable or refillable container 1030 for holding toner particles, the container 1030 having front and back walls (not shown), a pair of side walls and a bottom wall having an elongated opening 1031 extending from the front wall to the back wall and provided with a toner feeding element 1032 disposed to continuously supply toner particles to a sleeve 1033 through a particle charging member 1034. The particle charging member 1034 is advantageously formed of a supply brush or a roller made of or coated with a fibrous, resilient material, or a sponge or foam material. The supply brush is brought into mechanical contact with the peripheral surface of the sleeve 1033 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the sleeve. The sleeve 1033 is advantageously made of metal coated with a conductive material, and advantageously has a substantially cylindrical shape and a rotational axis extending parallel to the elongated opening 1031 of the particle container 1030. Charged toner particles are held to the surface of the sleeve 1033 by electrostatic forces essentially proportional to (Q/D)², where Q is the particle charge and D is the distance between the particle charge center and the boundary of the sleeve 1033. Alternatively, the charge unit may additionally include a charging voltage source (not shown), which supply an electric field to induce or inject charge to the toner particles. Although it is most advantageous to charge particles through contact charge exchange, the method can also be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit.

A metering element 1035 is positioned proximate to the sleeve 1033 to adjust the concentration of toner particles on the peripheral surface of the sleeve 1033, to form a relatively thin, uniform particle layer thereon. The metering element 1035 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 1035 may also be connected to a metering voltage source (not shown) which influence the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the sleeve.

In Fig.3, the sleeve 1033 is arranged in relation to a positioning device 1040 for accurately supporting and maintaining the printhead structure 1005 in a predetermined position with respect to the peripheral surface of the sleeve 1033. The positioning device 1040 is formed of a frame 1041 having a front portion, a back portion and two transversally extending side rulers 1042, 1043 disposed on each side of the sleeve 1033 parallel to the rotational axis thereof. The first side ruler 1042, positioned at a upstream side of the sleeve 1033 with respect to its rotation direction, is provided with fastening means 1044 to secure the printhead structure 1005 along a transversal fastening axis extending across the entire width of the printhead structure 1005. The second side ruler 1043, positioned at a downstream side of the sleeve 1033, is provided with a support element 1045, or pivot, for supporting the printhead structure 1005 in a predetermined position with respect to the peripheral surface of the sleeve 1033. The support element 1045 and the fastening axis are so positioned with respect to one another, that the printhead structure 1005 is maintained in an arcuate shape along at least a part of its longitudinal extension. That arcuate shape has a curvature radius determined by the relative positions of the support element 1045 and the fastening axis, and is dimensioned to maintain a part of the printhead structure 1005 curved around a corresponding part of the peripheral surface of the sleeve 1033. The support element 1045 is arranged in contact with the printhead structure 1005 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 1005 position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible excentricity or any other undesired variations of the sleeve 1033. That is, the support element 1045 is arranged to make the printhead structure 1005 pivotable about a fixed point to ensure that the distance between the printhead structure 1005 and the peripheral surface of the sleeve 1033 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the sleeve 1033. The front and back portions of the positioning device 1040 are provided with securing members 1046 on which the toner delivery unit 1003 is mounted in a fixed position to provide a constant distance between the rotational axis of the sleeve 1033 and a transversal axis of the printhead structure 1005. Preferably, the securing members 1046 are arranged at the front and back ends of the sleeve 1033 to accurately space the sleeve 1033 from the corresponding holding element 1012 of the transfer belt 1001 facing the actual print station. The securing members 1046 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the sleeve 1033 and a central transversal axis of the corresponding holding member 1012.

As shown in Figs. 4a, 4b, 4c, a printhead structure 1005 in an image forming apparatus, e.g. of the type illustrated in Fig. 1, can comprise a substrate 1050 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the sleeve (particle carrier), a second surface facing the transfer belt, a transversal axis 1051 extending parallel to the rotation axis of the sleeve 1033 across the whole print area, and a plurality of apertures 1052 arranged through the substrate 1050 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer

1501 of electrically insulating material, such as for example parylene or another insulating cover layer. A first printed circuit, comprising a plurality of control electrodes 1053 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 1053, is arranged between the substrate 1050 and the first cover layer 1501. The second surface of the substrate is coated with a second cover layer

1502 of electrically insulating material, such as for example parylene or another insulating cover layer. A second printed circuit, including a plurality of deflection electrodes 1054, can be arranged between the substrate 1050 and the second cover layer 1502. The printhead structure 1005 further includes a layer of antistatic material (not shown), preferably a semiconductive material, such as silicium oxide or the like, arranged on at least a part of the second cover layer 1502, facing the transfer belt 1001. The printhead structure 1005 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 1053 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 1052 during each print sequence. The control unit also can comprise deflection voltage sources (not shown) connected to the deflection electrodes 1054 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 1052. In some designs, the control unit also includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 1053 from one another, preventing electrical interaction therebetween. The substrate 1050 is advantageously a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 1050, respectively, using conventional etching techniques. The first and second cover layers 1501, 1502 are 5 to 10 microns thick parylene laminated onto the substrate 1050 using vacuum deposition techniques. The apertures 1052 are made through the printhead structure 1005 using conventional laser micromachining methods. The apertures 1052 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 1052 preferably have a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures.

In one advantageous design, the printhead structure 1005 is dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three dot locations are addressable through each aperture 1052 of the printhead structure during each print cycle. Accordingly, one aperture 1052 is provided for every third dot location in a transverse direction, that is, 200 equally spaced apertures per inch aligned parallel to the transversal axis 1051 of the printhead structure 1005. The apertures 1052 are generally aligned in one or several rows, preferably in two parallel rows each comprising 100 apertures per inch. Hence, the row aperture pitch, i.e. the distance between the central axes of two neighboring apertures in the same row is 0.01 inch or about 254 microns. The aperture rows are preferably positioned on each side of the transversal axis 1051 of the printhead structure 1005 and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations.

The first printed circuit comprises control electrodes 1053 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 1052, and a connector preferably extending in the longitudinal direction, connecting the ring shaped structure to a corresponding control voltage source. Although a ring-shaped structure is preferred, the control electrodes 1053 may take on various shapes for continuously or partly surrounding the apertures 1052, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 1052 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises a plurality of deflection electrodes 1054, each of which is divided into two semicircular or crescent shaped deflection segments 1541, 1542 spaced around a predetermined portion of the circumference of a corresponding aperture 1052. The deflection segments 1541, 1542 are arranged symmetrically about the central axis of the aperture 1052 on each side of a deflection axis 1543 extending through the center of the aperture 1052 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 1543 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan(l/3), i.e. about 18.4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 1054 has a upstream segment 1541 and a downstream segment 1542, all upstream segments 1541 being connected to a first deflection voltage source Dl, and all downstream segments 1542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: DKD2; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between Dl and D2 determines the deflection trajectory of the toner stream through each aperture 1052, thus the dot position on the toner image.

Also a so-called multi-pass technique can be utilized. When utilizing such a technique, the image forming apparatus is provided with moving means causing the image receiving surface and the printhead structure to move in relation to each other, i.e. the image receiving surface or the printhead structure, or both, are movable. Thereby, the relative movement is so arranged that each line on the image-receiving surface that is transverse to the direction of the relative movement passes the printhead structure at least twice in order to form an image. Accordingly, the printhead structure prints only a part of each transverse line on each pass. When utilizing multi-pass technique, the moving means further includes means to move the printhead structure and the image receiving surface relative to each other (i.e. the printhead structure or the image recieving surface, or both, are moved) between consecutive passes or during a pass, so that each time that the image receiving surface passes the printhead structure, different parts of the image receiving surface are positioned to receive charged toner particles. The multi-pass technique increases the print addressability without requiring a larger number of apertures in the printhead structure which, if desired, can eliminate the need for special deflection electrodes or the like.

In many cases, the multi-pass technique results in an improved resolution and print quality in comparison to when an image is printed in one single printing pass.

Multi-interlacing (MIC) is a further developed technique where an image forming apparatus utilizing multi-pass techniques is so constructed and arranged that adjacent columns of print are not printed by the same aperture in different passes. When utilizing MIC-technique, columns of print from different passes (partial images) are "interlaced" with each order in order to form the completed image. It has been found that MIC-techniques can improve print resolution and print quality even further in comparison to conventional multi-pass techniques.

Also dot deflection techniques (DDC) can be utilized in an image forming apparatus of the types in question. According to the DDC-method, each single aperture is used to address several dot positions on an image receiving surface by controlling not only the transport of toner particles through the aperture, but also their transport trajectory towards the image receiving surface, and thereby the location of the obtained dot. The DDC-methpd increases the print addressability without requiring a larger number of apertures in the printhead structure.

By means of utilizing a combination of multipass-, MIC- and/or DDC-techniques, the print addressability/number of apertures, and the print resolution can be optimized.

In the following, an image forming apparatus according to a preferred embodiment of the invention will be described in greater detail with particular reference to the attached Figs. 5 and 6.

In the image forming apparatus 2000 according to the invention, an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier 2033 towards an image receiving surface. The apparatus includes a background voltage source (not shown in the drawings) for producing a background electric field which enables a transport of charged toner particles from the particle carrier 2033 towards the image receiving surface, and a printhead structure 2005 arranged in the background electric field. The printhead structure includes a plurality of apertures 2052 and control electrodes 2053 arranged in conjunction to the apertures. Furthermore, the apparatus includes control voltage sources (not shown in the drawings) for supplying control potentials to the control electrodes 2053 in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier 2033 through the apertures 2052. The apertures 2052 are arranged transversely (but not necessarily perpendicularly) to a printing direction PD of the printhead structure 2005 with an average pitch p_av. The apertures 2052 extend a first distance l_ap from a particle- receiving plane A to a particle-emitting plane B of the printhead structure 2005 which is positioned so that the particle-receiving plane A is located at a second distance Ik from the charged toner particles PA on the particle carrier 2033, and so that the particle-emitting plane B is at a third distance 1, from the image receiving surface. According to the invention, and in the preferred embodiment, the image receiving surface is provided on a back electrode member 2012, wherein at least one surface portion of the of the back electrode member 2012 can be caused to move at least in the printing direction PD in relation to the printhead structure 2005 for intercepting the transported charged toner particles in the image configuration IM.

Accordingly, in the apparatus according to the invention, the toner particles are deposited in the desired image configuration directly on the back electrode member 2012. Thereby, the above-discussed problems with undesired variations in the electrical field caused by tribo-electrical phenomena is eliminated.

In the preferred embodiment of the invention, the surface portion or portions of the back electrode member 2012 is/are provided on a shell of a rotatable cylinder or on a surface layer on the shell. Thereby, the cylinder shell can be made of e.g. a metal or metal alloy or another suitable electrically conductive material. When present, the surface layer can be e.g. a protective coating and/or a release coating of a suitable material.

It is also conceivable with embodiments where the surface portion(s) is/are provided on a back electrode member of another shape than cylindrical. In one embodiment, the surface portion or portions of the back electrode member is/are provided on a belt (1001 in Fig. 1) arranged in an endless loop, wherein the belt has a resistivity at 20 °C which is lower than 2 ^• 10^"6 Ω-m. Accordingly, in this embodiment, the belt in itself constitutes the back electrode.

In another embodiment (not illustrated in the drawings), the surface portion or portions of the back electrode member is/are provided on an information carrier with a resistivity which is lower than 10ⁿ Ω-m. Preferably, the resistivity of the information carrier is lower than 10¹⁰ Ω-m, and most preferably lower than 10⁹ Ω-m. In these embodiments, the information carrier is an electrically conductive paper, a conductive polymer film, or another electrically conductive information carrier which is able to function as a back electrode. In another advantageous embodiment of the image forming apparatus according to the invention, the surface portion or portions of the back electrode member 2012 is/are provided on an information carrier through which the electrical resistance is lower than 10⁷ Ω-m², preferably lower than 10⁶ Ω-m², and most preferably lower than 10⁵ Ω-m².

In still another embodiment, the surface portion(s) of the back electrode member 2012 is/are arranged for being able to move in the printing direction PD in relation the printhead structure 2005, in a way enabling each line printed on the back electrode 2012 to pass the printhead structure 2005 at least twice in order to form the image configuration IM, so that the printhead structure 2005 prints only a part of each line during each pass in order to form longitudinal columns of said image configuration IM. In this embodiment, the back electrode 2012 particularly advantageously has a plurality of said surface portions located in different positions transversely to the printing direction PD, which surface portions can be moved relative to the printhead structure 2005, either between consecutive passes or during a pass, in a way enabling said different surface portions to receive charged toner particles in order to form said image configuration IM.

The second distance Ik is preferably smaller than the average pitch p_av, since this improves the print quality.

In case the apertures are arranged in a plurality of rows with an average pitch within each of the rows which is larger than 250 μm, the second distance I_k preferably is smaller than 130 μm.

In case the apertures 2052 are arranged in a single row with an average pitch which is smaller than 130 μm, also the second distance I_k preferably is smaller than 130 μm.

Most advantageously, the second distance I is smaller than 30 μm, since this enables an excellent print quality to be achieved. Preferably, the total l_ap + Ik + li of the first, second and third distances is smaller than 500 μm, whereas the total Ik + li of the second and third distances preferably is smaller than 300 μm.

Furthermore, a ratio lk/1; between the second Ik and third distance li advantageously is between 1:1 and 1:25, and preferably the ratio yi, is between 1:1 and 1:20.

The average pitch p_av and the first distance l_ap preferably are determined by the geometrical design of the printhead structure 2005. The second distance I preferably is determined by a thickness l_s of a spacer element 2004. The layer thickness of the charged toner particles PA on the particle carrier 2033 is denoted l_t. Thereby, the third distance li is determined by the positioning of the particle carrier 2033 in relation to the back electrode member 2012, and by the first l_ap and second I distances and the layer thickness l_t. Accordingly, the distance X between the surface of the particle carrier and the surface of the back electrode member can be calculated by means of the equation X = l_t + l_s + l_ap + li.

The method according to the invention comprises to deposit toner particles in an image configuration onto an information carrier constituting a back electrode, or to transfer toner particles deposited in an image configuration on a back electrode onto the information carrier, by means of an image forming apparatus of the above- described type according to the invention. Thereby, the information carrier can be a printing paper, or another suitable substrate.

The present invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims.

Claims

1. An image forming apparatus in which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier towards an image receiving surface, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said image receiving surface; a printhead structure arranged in said background electric field, including a plurality of apertures and control electrodes arranged in conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; said image receiving surface being arranged for movement in relation to the printhead structure for intercepting the transported charged toner particles in an image configuration;

wherein said apertures (2052) are arranged transversely to a printing direction (PD) of said printhead structure (2005) with an average pitch (p_av), and said apertures (2052) extend a first distance (l_ap) from a particle-receiving plane (A) to a particle-emitting plane (B) of said printhead structure (2005) which is positioned so that said particle- receiving plane (A) is located at a second distance (Ik) from said charged toner particles (PA) on said particle carrier (2033), and so that said particle-emitting plane (B) is at a third distance (li) from said image recieving surface, characterized in that the image receiving surface is provided on a back electrode member (2012), wherein at least one surface portion of the back electrode member (2012) can be caused to move at least in the printing direction (PD) in relation to the printhead structure (2005) for intercepting the transported charged toner particles in the image configuration (IM).

2. An image forming apparatus according to claim 1, characterized in that the surface portion or portions of the back electrode member (2012) is/are provided on a shell of a rotatable cylinder or on a surface layer on said shell.

3. An image forming apparatus according to claim 1 , characterized in that the surface portion or portions of the back electrode member is/are provided on a belt (1001) arranged in an endless loop, wherein the belt has a resistivity at 20 °C which is lower than 2 ^• 10^"6 Ω-m.

4. An image forming apparatus according to claim 1, characterized in that the surface portion or portions of the back electrode member is/are provided on an information carrier with a resistivity which is lower than 10^u Ω-m, preferably lower than 10¹⁰Ω-m, and most preferably lower than 10⁹Ω-m.

5. An image forming apparatus according to claim 1, characterized in that the surface portion or portions of the back electrode member is/are provided on an information carrier through which the electrical resistance is lower than 10⁷ Ω-m², preferably lower than 10⁶ Ω-m², and most preferably lower than 10^s Ω-m².

6. An image forming apparatus according to any one of claims 1 - 5, characterized in that the surface portion(s) of the back electrode member (2012) is/are arranged for being able to move in the printing direction (PD) in relation the printhead structure (2005), in a way enabling each line printed on the back electrode (2012) to pass the printhead structure (2005) at least twice in order to form the image configuration (IM), so that said printhead structure (2005) prints only a part of each line during each pass in order to form longitudinal columns of said image configuration (IM).

7. An image forming apparatus according to claim 6, characterized in that the back electrode (2012) has a plurality of said surface portions located in different positions transversely to the printing direction (PD), which surface portions can be moved relative to the printhead structure (2005), either between consecutive passes or during a pass, in a way enabling said different surface portions to receive charged toner particles in order to form said image configuration (IM).

8. An image forming apparatus according to any one of claims 1 - 5, characterized in that the second distance (Ik) is smaller than the average pitch (p_av).

9. An image forming apparatus according to any one of claims 1 - 5, characterized in that the apertures are arranged in a plurality of rows with an average pitch within each of said rows which is larger than 250 μm, wherein the second distance (I_k) is smaller than 130 μm.

10. An image forming apparatus according to any one of claims 1 - 5, characterized in that the apertures (2052) are arranged in a single row with an average pitch which is smaller than 130 μm, wherein also the second distance (I_k) is smaller than 130 μm.

11. An image forming apparatus according to any one of claims 1 - 5, characterized in that the second distance (I_k) is smaller than 30 μm.

12. An image forming apparatus according to any one of claims 1 - 5, characterized in that the total (l_ap + Ik + li) of the first, second and third distance is smaller than 500 μm.

13. An image forming apparatus according to any one of claims 1 - 5, characterized in that the total (I + 1,) of the second and third distance is smaller than 300 μm.

14. An image forming apparatus according to any one of claims 1 - 5, characterized in that a ratio (yij) between the second (l and third distance (1;) is between 1:1 and 1:25.

15. An image forming apparatus according to any one of claims 1 - 5, characterized in that the ratio (lk/li) is between 1:1 and 1:20.

16. An image forming apparatus according to any one of claims 1 - 5, characterized in that the average pitch (p_av) and the first distance (l_ap) are determined by the geometrical design of the printhead structure (2005), that the second distance (I_k) is determined by a thickness (l_s) of a spacer element (2004), wherein the third distance (li) is determined by the positioning of the particle carrier (2033) in relation to the back electrode member (2012), by the first (l_ap) and second (I_k) distances, and by a layer thickness (l_t) of the charged toner particles (PA) on the particle carrier (2033).

17. A method for direct printing, characterized in that the method comprises to deposit toner particles in an image configuration onto an information carrier constituting a back electrode, or to transfer toner particles deposited in an image configuration on a back electrode onto the information carrier, by means of an image forming apparatus according to any one of claims 1-16.