WO2001077754A1

WO2001077754A1 - Direct printing device and method

Info

Publication number: WO2001077754A1
Application number: PCT/EP2000/003116
Authority: WO
Inventors: Lars STRÅVIK
Original assignee: Array Ab
Priority date: 2000-04-07
Filing date: 2000-04-07
Publication date: 2001-10-18
Also published as: AU2000242943A1

Abstract

An electrostatic image forming apparatus in which computer generated image information is converted into a pattern of electrostatic fields, includes a particle source including a particle carrier which holds a layer of electrically charged particles that are transported toward a back electrode via apertures in a printhead structure or structures. The particles are received on an image receiving member to form an image. The image is formed by passing the image receiving member more than once past each printhead structure so a first part (61)of the image is formed on the first pass and further parts (62) of the image is formed on subsequent passes.

Description

Direct Printing Device and Method

Technical Field

The invention relates to a direct printing apparatus and method m which computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle source toward a back electrode through a printhead structure, whereby the charged particles are deposited m image configuration on an image receiving substrate caused to move relative to the printhead structure More specifically, the invention relates to a direct printing apparatus arranged to print m more than one pass

Background

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

A drawback of direct electrostatic printing is that the number of apertures needed to print at a particular resolution may need to be equal to the resolution This means that the density of apertures on the printhead is very high In order to meet the requirements of higher resolution printing with direct electrostatic methods, there is still a need for improving the resolution without the need to increase the density of apertures on the printhead

Summary of the invention

An ob ect of the invention is to provide an apparatus by which the print resolution may be increased without the need to increase the density of apertures

This object is achieved by an image forming apparatus in which an image information is converted nto a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, 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 back electrode member, a printhead structure arranged m said background electric field, including a plurality of apertures and control electrodes arranged m 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, an image receiving member for intercepting the transported charged particles in image configuration, and means causing the image forming member and/or the printhead structure (to move m relation to each other, wherein, the means causing the relative movement of the image receiving member and the printhead structure is so arranged that each line on the image receiving member that is transverse to the direction of said relative movement passes the printhead structure in a longitudinal direction at least twice in order to form an image, the printhead structure printing only a part of each transverse line on each pass, the moving means also include means to move the printhead structure and/or the image receiving member relative to each either between consecutive passes or during a pass so that each time that the image receiving member passes the printhead structure transversely different parts of the image receiving member are positioned to receive charged toner particles

By printing an image m more than one pass of the printhead structure the number of apertures required to print at a particular resolution is reduced The reduction in the number of apertures allows for easier manufacture of the printhead structure In a preferred embodiment the image receiving member and/or printhead structure move relatively transversely continuously during a pass, preferably at constant speed, by one dot pitch during a pass In a further preferred embodiment the trajectories of the toner particles are deflected by one or more deflection electrodes provided on each aperture

Brief description of the drawings

The invention will now be described in more detail for explanatory, and m no sense limiting, purposes, with reference to the following drawings, wherein like reference numerals designate like parts throughout and where the dimensions in the drawings are not to scale, m which

Fig 1 is a schematic view of an image forming apparatus m accordance with a preferred embodiment of the present invention,

Fig 2 is a schematic section view across a print station m an image forming apparatus, such as, for example, that shown m Fig 1,

Fig 3 is a schematic section view of the print zone, illustrating the positioning of a printhead structure in relation to a particle source and an image receiving member,

Fig 4a is a partial view of a printhead structure of a type used m 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 the intermediate transfer belt,

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

Fig 5a is a view of an image after one pass of the printhead structure of an embodiment where the complete image is formed in two passes

Fig 5b is a view of the image of Fig 5a after the second pass of the printhead structure is a view of an image after one pass of the printhead structure of an embodiment where the complete image is formed in three passes

is a view of the image of Fig 6a after the second pass of the printhead structure

is a view of the image of Fig 6a after the third pass of the printhead structure

is a view an image after one pass of the printhead structure of an embodiment where the complete image is formed m two passes and the image receiving member and/or printhead structure move relatively transversely continuously during a pass

is a view of an image after one pass of the printhead structure of an embodiment where the complete image is formed m two passes and the toner particles are deflected by a deflection electrode provided on each aperture to form two lines of print m a single pass

is a view of the image of Fig 8a after the second pass of the printhead structure

is a view of an image after one pass of the printhead structure of an embodiment where the complete image is formed in two passes and the toner particles are deflected by two deflection electrodes provided on each aperture to form three lines of print on each pass Fig 9b is a view of the image of Fig 9a after the second pass of the printhead structure

Fig 10 is schematic side view of an embodiment for color printing m which four printhead structures each print a different color

Fig 11 is a section v ew of an embodiment of the invention having two printhead structures

Fig 12 is an illustration of the pattern of dots laid down by the printhead structures of Fig 11

Detailed description

In the apparatus m accordance with the present invention, 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 m the background electric field between the particle carrier and the back electrode and connected to a control unit which converts the image information into control signals which, due to control in accordance with the image information, selectively open or close passages m 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 the image receiving substrate to provide line-by line scan printing to form a visible image A printhead structure for use m 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 m 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, m a configuration such that each control electrode surrounds a corresponding aperture The apertures are preferably aligned m one or several rows extending transversally across the width of the substrate, I e perpendicular to the motion direction of the image receiving substrate

According to such a device, 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 m a transversal direction

According to a preferred embodiment of the present invention, a direct electrostatic printing device includes a dot deflection control (DDC) According to that embodiment, each single aperture is used to address several dot positions on an image receiving substrate by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving substrate, 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 m 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

According to a preferred embodiment, an improved DDC arrangement 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 medium, 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 msulative layers Utilizing such a method, 60 micrometer dots can be obtained with apertures having a diameter m the order of 160 micrometer

In order to clarify the apparatus according to the invention, some examples of its use will now be described in connection with accompanying drawings

As shown in Fig 1, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12 The four print stations are arranged m relation to the intermediate image receiving member 1 The image receiving member, preferably a transfer belt 1 is mounted over the driving roller 10, though a drum may be used instead of the transfer belt The at least one support roller 11 is provided with a mechanism for maintaining the transfer belt 1 with a constant tension, while preventing transversal movement of the transfer belt 1 The holding elements 12 are for accurately positioning the transfer belt 1 with respect to each print station

The driving roller 10 is preferably a cylindrical metallic sleeve having a rotation axis extending perpendicular to the motion direction of the belt 1 and a rotation velocity adjusted to convey the oelt 1 at a velocity of one addressable dot location per print cycle, to provide line by line scan printing The adjustable holding elements 12 are arranged for maintaining the surface of the belt at a predetermined gap distance from each print station The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station m order to, m combination with the belt tension, create a stabilization force component on the belt That stabilization force component is opposite m direction and preferably larger m magnitude than an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station

The transfer belt 1 is preferably an endless band of 30 to 200 microns thick composite material as a base The base composite material can suitably include thermoplastic polyamide 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 under temperatures m the order of 250°C The composite material of the transfer belt has preferably 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 1 The outer surface of the transfer belt 1 is preferably 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 1 is conveyed past the four different print stations, whereas toner particles are αeposited on the outer surface of the transfer belt and superposed to form a four color toner image Toner images are then preferably conveyed through a fuser unit 13 comprising a fixing holder 14 arranged transversally m direct contact with the inner surface of the transfer belt The fixing holder includes a heating element 15 preferably of a resistance type of e g molybdenium, maintained m contact with the inner surface of the transfer belt 1 As an electric current is passed through the heating element 15, the fixing holder 14 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1 The fusing unit 13 further includes a pressure roller 16 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 14 An information carrier 2, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 21 and conveyed between the pressure roller 16 and the transfer belt The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image After passage through the fusing unit 13, the transfer belt is brought m contact with a cleaning element 17, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1 for removing all untransferred toner particles from the outer surface

Although the above description refers to a fusing unit which includes a means for transferring the image from the transfer belt the transferring unit and the fusing unit may be provided as separate units

As shown in Fig 2, a print station in an image forming apparatus m accordance ^τιth the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a toner carrier 33 through a particle charging member 34. The particle charging member 34 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the toner carrier 33 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 toner carrier. The toner carrier 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held to the surface of the toner carrier 33 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 toner carrier 33. 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 preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction unit or a corona charging unit, without departing from the scope of the present invention. A metering element 35 is positioned proximate to the toner carrier 33 to adjust the concentration of toner particles on the peripheral surface of the toner carrier 33, to form a relatively thin, uniform particle layer thereon The metering element 35 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 35 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 toner carrier

As shown m Fig.3, the toner carrier 33 is arranged m relation with a positioning device 40 for accurately supporting and maintaining the printhead structure 5 in a predetermined position with respect to the peripheral surface of the toner carrier 33 The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the toner carrier 33 parallel with the rotation axis thereof The first side ruler 42, positioned at an upstream side of the toner carrier 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5 The second side ruler 43, positioned at a downstream side of the toner carrier 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 m a predetermined position with respect to the peripheral surface of the toner carrier 33 The support element 45 and the fastening axis are so positioned vith respect to one another, that the printhead structure 5 is maintained m an arcuated shape along at least a part of its longitudinal extension That arcuated shape has a curvature radius determined by the relative positions of the support element 45 and the fastening axis and dimensioned to maintain a part of the printhead structure 5 curved around a corresponding part of the peripheral surface of the toner carrier 33 The support element 45 is arranged m contact with the printhead structure 5 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure 5 position m both longitudinal and transversal direction about that fixed support location, m order to accommodate a possible excentπcity or any other undesired variations of the toner carrier 33 That is, the support element 45 is arranged to made the printhead structure 5 pivotable about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the toner carrier 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the toner carrier 33 The front and back portions of the positioning device 40 are provided with securing members 46 on which the toner delivery unit 3 is mounted m a fixed position to provide a constant distance between the rotation axis of the toner carrier 33 and a transversal axis of the printhead structure 5 Preferably, the securing members 46 are arranged at the front and back ends of the toner carrier 33 to accurately space the toner carrier 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station The securing members 46 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the toner carrier 33 and a central transversal axis of the corresponding holding member 12

As shown m Fig 4a, 4b, 4c, a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the toner carrier, a second surface facing the transfer belt, a transversal axis 51 extending parallel to the rotation axis of the toner carrier 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof The first surface of the substrate is coated with a first cover layer 501 of electrically insulating material, such as for example parylene A first printed circuit, comprising a plurality of control electrodes 53 disposed m conjunction with the apertures, and, m some embodiments, shield electrode structures (not shown) arranged m conjunction with the control electrodes 53, is arranged between the substrate 50 and the first cover layer 501 The second surface of the substrate is coated with a second cover layer 502 of electrically insulating material, such as for example parylene A second printed circuit, including a plurality of deflection electrodes 54, is arranged between the substrate 50 and the second cover layer 502 The printhead structure 5 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 502, facing the transfer belt 1 The printhead structure 5 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials wnich control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52 The control unit, m some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween In a preferred embodiment of the invention, the substrate 50 is 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 50, respectively, using conventional etching techniques The first and second cover layers (501, 502) are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques The apertures 52 are made through the printhead structure 5 using conventional laser micromachimng methods The apertures 52 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 52 have preferably 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 a preferred embodiment of the present invention, the printhead structure 5 is dimensioned to perform 600 dpi printing utilizing three deflection sequences m each print cycle, I e three dot locations are addressable througn each aperture 52 of the printhead structure during each print cycle Accordingly one aperture 52 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 51 of the printhead structure 5 The apertures 52 are generally aligned m one or several rows, preferably m two parallel rows each comprising 100 apertures per inch Hence, the aperture pitch, I e the distance between the central axes of two neighboring apertures of a same row is 0,01 inch or about 254 microns The aperture rows are preferably positioned on each side of the transversal axis 51 of the printhead structure 5 and transversally shifted with respect to each other such that all apertures are equally spaced m a transverse direction The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations

It should be noted that each aperture serves to provide an access for the toner particles to be directed from the toner carrier to a desired location The direction of the toner particles is dictated by the prevailing electric fields The apertures do not physically direct the particles m the sense that the shape of the borders of the aperture have no effect on the trajectory of the particles other than the effect that they may have on the electric fields The toner particles pass in general via a central area of the apertures without touching the sides of the apertures Thus the borders of the apertures provide the means for supporting the electrodes which provide the electric fields

The first printed circuit comprises the control electrodes 53 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 52, and a connector preferably extending in the longitudinal direction connecting the ring shaped structure to a corresponding control ^roltage source Although a ring shaped structure is preferred, the control electrodes 53 may take on various shape for continuously or partly surrounding the apertures 52, preferably shapes having symmetry about the central axis of the apertures In some embodiments, particularly when the apertures 52 are aligned in one single row, the control electrodes are advantageously made smaller m a transverse direction than m a longitudinal direction

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments

541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52 The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction The deflection axis 543 is dimensioned m accordance with the number of deflection sequences to be performed m each print cycle m 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), l 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 54 has a upstream segment 541 and a downstream segment

542, all upstream segments 541 being connected to a first deflection voltage source Dl and all downstream segments

542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: D1<D2; 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 52, and thus the dot position on the toner image .

Since the apertures 52 and their surrounding areas will under some circumstances need to be cleaned from residual toner particles which agglomerate there, an image forming apparatus in accordance with the present invention preferably further includes a cleaning unit 6 which is used to prevent toner contamination. Due to undesired variations in the charge and mass distribution of the toner material, some of the toner particles released from the toner carrier 33 do not reach sufficient momentum during a print sequence to be deposited onto the transfer belt 1 and contribute to image formation. Some toner particles having a charge polarity opposite to the intended, so called wrong signed toner, may be repelled back to the printhead structure 5 after passage through the apertures under influence of the background field, and adhere on the printhead structure 5 m the area surrounding the apertures 52. Some particles may be deviated during transport and agglomerate on the apertures walls, obstructing the aperture 52. Residual toner particles have to be removed periodically during an appropriate cleaning cycle, for example after a predetermined number of image formation cycles or due to control in accordance with a sensor measuring the amount of residual toner.

In accordance with the invention the resolution achieved by the printhead structure 5 for a given number of apertures 52 may be increased with or without the use of deflection electrodes 54. In order to achieve a printing resolution greater than the number of apertures m the printhead structure 5 the printing takes place in two or more passes of the drum or transfer belt 1 By pass is meant a movement of the drum or transfer belt which passes a section of the drum or transfer belt 1 to be printed with a movement relative to the printhead structure 5 and allows the printhead structure 5 to deposit a plurality of columns of printing A column of printing is a column of the drum or transfer belt which is subject to printing of dots even if not all the parts of the column receive dots due to the content of the image being formed requiring some parts to be left without dots Each column has a width of one dot After the first pass the next passes may be m the same or opposite directions to that of the first pass.

The transverse direction is the direction which m the case that the image receiving member is a drum is perpendicular to a radial vector of cylinder towards the printhead structure at the surface of the drum and parallel to the axis of rotation of the drum along the surface of the drum In the case of a transfer belt it is the direction in the plane of the belt perpendicular to the direction of movement of the belt, the said movement being the movement required to allow the belt to move around the rollers 10, 11 Thus, the transverse direction will normally be parallel to the axes of these rollers 10, 11 The longitudinal direction is the direction perpendicular to the transverse direction and m the plane of the surface of the image receiving member, i.e transfer belt or drum In the case of the drum the longitudinal direction is the direction perpendicular to the transverse direction and along the circumference so the drum In the case of a transfer belt the longitudinal direction is the direction at any point on its surface m the direction perpendicular to the axis of rotation of the rollers and in the plane of the surface of the drum

With respect to the description which follows reference is made to an image In the present context an image is what is formed by the toner particles over an area of the drum or transfer belt 5 The image also includes those areas that could receive toner particles but do not receive the particles because the content of the image does not require this Thus, the term image covers those areas which are printable, irrespective of whether the whole area actually is printed Typically, an image covers approximately the area of an A4 sheet of paper, though possibly reduced by a small area around the margins that are not printed The image may for example comprise a plurality of pictures or printed areas which would be printed on the same sheet of paper Although reference is made to A4 paper this reference is not limiting as the image could be the size of A3 or A5 paper or other paper sizes or any other chosen size

In order to better understand the invention it will first be described with respect to performing just two passes with each pass taking place in the same direction In this case the number of apertures 52 is half that needed to achieve the desired resolution with a single pass The direction of movement of the drum or transfer belt during printing around the holding elements 12 is defined as the longitudinal direction The direction perpendicular to this longitudinal direction and parallel to the axis of the drum is defined as the transverse direction In the case of a belt it is the direction perpendicular to the longitudinal direction in the plane of the belt under the printhead In a first pass a first half of the image is formed on the drum or transfer belt 1 This first half of the image comprises alternate longitudinal columns of print of the intended final image, 1 e alternate columns are printed and alternate columns are free The drum or transfer belt 1 and printhead structure 5 are then moved relative to each other in the direction transverse to the direction of movement of the drum or transfer belt, but in the plane of the belt, preferably by moving the drum or transfer belt transversely In the case of a drum this transverse movement therefore comprises a movement of the drum about its axis This relative movement may be carried out by any suitable means known to the person skilled m the art Then, m a second pass, the other longitudinal columns of print are printed to form a complete image The second pass can be carried out with the belt traveling m the same longitudinal direction as the first pass or the opposite longitudinal direction This embodiment is illustrated m Figs 5a and 5b Fig 5a represents a section of the drum or transfer belt 1 after the first pass The areas that are printed m the first pass are shown as shaded areas 61 Fig 5b represents the same section of the drum or transfer belt 1 after the second pass The areas that are printed in the first pass are shown as shaded areas 61, whilst the areas that are printed in the second pass are shown as differently shaded areas 62

In another embodiment the printing is carried out m three passes In this case the number of apertures 52 is one third that needed to achieve the same resolution as with a single pass In a first pass a first one third of the image is formed on the drum or transfer belt 12 This first third comprising one third of the longitudinal columns of print of the intended final image The drum or transfer belt and printhead structure 5 are then moved relative to each other m the direction transverse to the direction of movement of the drum or transfer belt, but in the plane of the belt preferably by moving the belt ZJ

transversely Then, in a second pass, a second set of longitudinal columns of the image are printed The second pass can occur with the belt traveling m the same longitudinal direction as the second pass or m the opposite longitudinal direction In a third and final pass, the remainder of the longitudinal columns of the image are printed The third pass can occur with the belt traveling m the same direction as the first pass or the opposite direction This embodiment is illustrated m Figs. 6a to 6c Fig 6a represents the drum or transfer belt at the end of the first pass The areas that are printed m the first pass are shown as shaded areas 63 Fig 6b represents the drum or transfer belt 1 at the end of the second pass The areas that are printed in the first pass are shown as shaded areas 63, whilst the areas that are printed in the second pass are shown as shaded areas 64 Fig 6c represents the drum or transfer belt 1 at the end of the second pass The areas that are printed m the first and second passes are shown as shaded areas 63 and 64, whilst the areas that are printed in the second pass are shown as shaded areas 65

In a further embodiment (not shown) the number of passes is four and the number of apertures 52 is one fourth that needed to achieve the same resolution as with a single pass It is also possible to extend the process to five or more passes with a corresponding reduction m the number of apertures

The number of apertures provided for printing a transverse line may be greater than the number of dots to be printable per line In particular, the apertures can be arranged such that they are able to print over a width that is greater than the width of the image to be printed In this way it is possible to move the belt and the printhead structure relative to each other a distance greater than the minimum distance needed to form an image In this case the apertures which are outside the image area do not contribute to forming all the columns of the image In this way there are always sufficient apertures that can contribute to forming the image that the image can be formed The apertures that are outside the image area for a transverse line do not contribute to forming the image on that transverse line. Thus, there are always sufficient apertures that can contribute to forming the image, even with a large relative transverse movement between passes

Although in the above embodiments the number of apertures is reduced for more passes it is alternatively possible to provide the number or apertures necessary to achieve the desired resolution but to reduce the number of apertures actually used for printing in a pass Thus m the case of the embodiment with two passes the number of apertures used for printing could be half the number provided and required for printing with a single pass

In order to allow the transverse movement between passes the image should not m the above mentioned embodiments extend all the way along the drum or transfer belt 1 Between the end of the image and the start of the image the belt should contain an image free area The drum or transfer belt may then be moved transversely while the image free area is passing the printhead structure 5

in the above embodiments the drum or transfer belt 1 is moved relative to the printhead structure 5 m two or more passes with a relative movement between the drum or transfer belt 1 and the printhead structure 5 between each pass In another embodiment however the drum or transfer belt 1 and the printhead structure 5 are moved relative to each other transversely m a continuous manner, preferably at constant speed, at the same time as they are moved relative to each other longitudinally The printing column of each aperture then describes a column on the drum or transfer belt 1 which is at an angle to the direction of movement of the drum or transfer belt 1 This is shown m Fig 7 In Fig 7 the direction of movement is indicated by the arrow 66 and the column of print of a single aperture is indicated by 67 If it is arranged that the ends of the image are arranged end-to- end on drum or transfer belt 1 without an image free area, then the second pass can take place m the same direction as the first pass and without the need for a separate transverse movement between the passes It is not however essential that the image is arranged end-to- end on the drum or transfer belt, m which case the transverse relative movement continues over the image- free area This process produces an image that is at an angle to the longitudinal direction of the drum or transfer belt, since each of the print columns is at such an angle due to the continuous relative movement The continuous transverse movement continues until the image is completed

The continuous relative movement can be arranged such that a transverse relative movement of one dot pitch is achieved during a single pass of the printhead However, it is also possible to arrange that the relative transverse movement is more than one dot pitch per pass or less than one dot pitch per pass Where the relative transverse movement is less than one dot pitch it will be necessary that there is a further movement between the first and second passes to bring the complete relative movement up to one dot pitch This ensures that the printing on the second pass takes place m the areas that were not printed on the first pass If transverse movement during a pass is an integer number of dot pitches then a relative movement between passes is not required If transverse movement during a pass is not an integer number of dot pitches then a relative movement between passes is required to bring the total relative movement up to an integer number of dot pitches The relative movement may take place between passes

The transverse movement between passes may take place each time m the same direction until an image is completed The drum or transfer belt 1 and/or printhead structure 5 may then be returned to their respective starting positions ready to print the next image Alternatively, m the cases of non- continuous relative movement, the next image may be printed by a transverse movement opposite to the direction of the first transverse movement so as to bring the drum or transfer belt 1 and/or printhead structure 5 back to their starting positions during the formation of the next image In this way the total number of transverse movements is reduced

It is also possible to form several images m a single set of passes where there is space on the drum or transfer belt 1 for more than one image For example, instead of making two passes to form a first image, followed by two passes for the second image, etc there is a first pass of both images followed by a second pass of both images

The pitch of the printing may be varied dependent upon the number of passes Preferably the pitch is inversely proportional to the number of passes For example m a printer which normally prints at 600 dpi and s printing in three passes then the pitch is increased to 400 dpi /hen printing is m two passes and reduced to 800 dpi vhen the printing is m four passes A further embodiment of the invention includes two or more printhead structures 5 The printhead structures 5 may be spaced from each other along the longitudinal direction of travel of the drum or transfer belt 1 Each printhead structure 5 is provided with apertures such that the total number of apertures of the printhead structures 5 corresponds to that necessary to print an image m the number of passes that is intended For example, if there are two printhead structures 5 and it is intended to print in two passes then each printhead structure 5 could have one fourth of the number of apertures necessary to print from a single printhead structure 5 m a single pass at the desired resolution This is illustrated m Figs 8a and 8b Fig 8a shows the situation after the first pass The columns of print indicated by 68a come from the first printhead structure 5 and the columns of print indicated by 68b come from the second printhead structure 5 Fig 8b shows the situation after the second pass The columns of print indicated by 69a come from the first printhead structure 5 and the columns of print indicated by 69b come from the second printhead structure 5 The use of two or more printhead structures 5 allows the printhead structures 5 to be manufactured with a lower density of apertures If three passes are used instead of two passes then the number of apertures may be lowered correspondingly

If three printhead structures 5 are provided then three columns of print may be printed per pass with, for example, three unpπnted columns of print The three unpnnted columns are then printed in the second pass It is also possible to leave six or another multiple of three unpnnted and to print with three or more passes In a preferred embodiment of the invention the above described embodiments of the invention are provided with the dot deflection control (DDC) as already described above In this case one or more, preferably all, of the apertures 52 are each provided with one, two, or more dot deflection electrodes 54 or deflection segments 541, 542 so as to allow the dots to be deflected m one or two dimensions m the plane of the drum or transfer belt 1. As an example, the apertures 52 are each provided with two deflection segments 541, 542 to enable three dots to be transferred, one without deflection and the other two m opposite directions This example is illustrated m Figs 9a and 9b In this case the formation of an image may take place m two passes with sets of three adjacent columns of print 68a being formed, the groups of three columns alternating with a space being formed on a first pass, as illustrated in Fig 9a The second three columns of print 68b are formed on a second pass m the spaces left between the first three columns of print, as illustrated in Fig 9b The process may be extended to more than two passes with appropriate extra space being left between the columns of the first pass This may also be applied to apertures provided with a different number of deflection segments Where there is only one deflection segment which forms two adjacent columns it will be necessary to leave two columns free before the next two adjacent columns are printed if the printing is to take place m two passes If the printing is to take place m three passes then the free space on the first pass will be double the space occupied by two adjacent columns, I e the equivalent of four columns The second pass prints half of this free space and the third pass completes the remaining space

A plurality of printhead structures is also suitable for color printing Such an arrangement is shown m schematic form m Fig 10 In this example four printhead structures 5a', 5b¹, 5c', 5d', are shown The four printhead structures are provided in longitudinal order relative to the drum or transfer belt 1 with the arrow adjacent the belt indicating the direction of travel of the belt The first printhead structures 5a', 5b¹, 5c¹, may be used, for example, for color printing with the colors yellow, magenta and cyan respectively The remaining printhead structure 5d' may then be used for black printing Alternatively, the black printhead structure could be arranged before the color printhead structures Although described here with four printhead structures it is also possible for there to be only three printhead structures 5a¹, 5b', 5c', providing color printing or even ust two In a further alternative there may be more than four printhead structures, which may be further color and/or black printhead structures

Printing then may take place in exactly the same manner as the embodiments described with respect to a single printhead structure These means that each of the printhead structures 5a', 5b', 5c¹, 5d', prints in the same way as already described This allows color and/or black and white image formation from two or more passes of the printhead structures It should be noted however that where each of the printhead structures prints a different color then each printhead structure prints on the same columns of print during a single pass so as to create the color image Alternatively, the different colors may be printed on different passes The dots of one pass will be printed on the dots of the previous passes as required, so as to form the final colors of each dot

in a further multiple printhead structure embodiment two or more printhead structures 5' ' of the same color, preferably black, are provided This is schematically illustrated m the case of two printhead structures 5 ' ' in Fig 11 The two printhead structures 5' ' are provided m longitudinal order relative to the drum or transfer belt 1 with the arrow adjacent tne belt indicating the direction of travel of the belt

With two printhead structures 5 ' ' each structure prints alternative dots in the longitudinal direction along a longitudinal column of print This means that in a single pass each print head structure 5' ' has printed only half the total transverse lines If the apertures of the two printhead structures 5 ' ' are aligned with each other m the longitudinal direction then the two printhead structures 5' ' have together printed the whole of each longitudinal column of print that is printed m a single pass The remaining transverse lines of print are printed m one or more further passes m the same manner as already described with respect to a single printhead structure 5 An alternative embodiment is illustrated in simplified schematic form m Fig 12 The position of dots printed is indicated by 70 though the extent of each dot is only indicated schematically The dot size is such that when all the dots are printed the whole area is covered by the dots In this embodiment the apertures of the two printhead structures may not be aligned but arranged such that their apertures 54 alternate The apertures of the two printhead structures do not then print along the same longitudinal column but each prints alternate dots 70 on its own column of print With this arrangement each printhead structure is printing on a different, e g alternate, longitudinal column of print during a single pass For a second pass the two printhead structures are moved relati^ve to each other, with or without an overall movement relative to the drum or transfer belt 1 An example of such movement is indicated by the arrows adjacent the printhead structures 5' ' m Fig 12 Each printhead structure will then print along a different longitudinal column for the second pass compared to that printed on the first pass The spaces between the dots printed on the first pass can then be filled m by the dots printed on the second pass to complete the image

In the above example the printing takes place with each printhead structure printing on different transverse lines during a single pass However, it can be arranged that the two printhead structures print on the same transverse line during a pass, so that during a pass alternate transverse lines are printed The remaining transverse lines are printed by the two printhead structures during a second pass This process can also be extended to printing every third or more transverse line and completing the image m three or more passes

Where the image is transferred from the drum or transfer belt 1 to an information carrier 2 then this transfer may begin as soon as the final pass has commenced so that those parts of the image that have been completed are transferred to the information carrier at the same time as the remaining parts of the image are still making their final pass of the printhead structure In this way the time required for printing an image is reduced

The drum or transfer belt 1 may be in any convenient form which is capable of moving relative to the printhead structure 5 Alternatively, it may be replaced any other suitable means capable of receiving and transferring an image It is also possible for the image to be formed directly on the information carrier 2 without the intermediary of the drum or belt 1 After an image has been formed the drum or transfer belt 12 may be cleaned of toner particles to allow the formation of a new image The cleaning does not take place between each pass of the printhead structure, as this would remove part of the image

The printhead structures 5 may be each provided with a cleaning device to clean the printhead structure of excess toner particles. Alternatively, the cleaning device for the printhead structures may be provided in association with or integrated into the drum or transfer belt Such a cleaning device could take the form of a vacuum source acting through perforations m the drum or transfer belt 1 The printhead structures are cleaned as the perforations of the drum or transfer belt passes opposite the printhead structures 5

In any of the above embodiments of the invention the pitch (distance between centers of dots) may be varied. The distance between dots on the transverse lines

(horizontal pitch) may be varied and/or the distance between dots in a longitudinal column (vertical pitch) may be varied The horizontal pitch may be varied by varying the amount of relative transverse movement between passes The vertical pitch can be varied by varying the amount of longitudinal movement between the printing of lines

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

Claims

What is claimed is:

1 An image forming apparatus m which an image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier (33) toward a back electrode member, 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 back electrode member, a printhead structure (5, 5a^' - 5d, 5'') arranged m said background electric field, including a plurality of apertures (52) and control electrodes (53) arranged m 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 (52), an image receiving memoer (1) for intercepting the transported charged particles m image configuration, and means causing the image forming member (1) and/or the printhead structure 5, 5a^' - 5d, 5^'') to move m relation to each other, characterized in that, the means causing the relative movement of the image receiving member (1) and the printhead structure (5, 5a - 5d, 5^'') is so arranged that each line on the image receiving member (1) that is transverse to the direction of said relative movement passes the printhead structure (5, 5a^' - 5d, 5'^') in a longitudinal direction at least twice in order to form an image the printhead structure (5, 5a - 5d, 5 ) printing only a part of each transverse line on each pass, the moving means also including means to move the printhead structure (5, 5a^' - 5d, 5^'') and/or the image receiving member (1) relative to each either between consecutive passes or during a pass, so that each time that the image receiving member passes the printhead structure (5, 5a' - 5d, 5'') transversely different parts of the image receiving member (1) are positioned to receive charged toner particles

An image forming apparatus as defined m claim 1, wherein the number of passes is selected from the group comprising two, three, four, and more than four

An image forming apparatus as defined in either of claims 1 or 2, wherein the moving means are so arranged that after an image is formed the image receiving member and/or printhead structure move back to their positions occupied at the start the formation of the image before starting to make the next image

An image forming apparatus as defined m either of claims 1 or 2, wherein the moving means are so arranged that after an image is formed the image receiving member and/or printhead structure form the next image by moving relatively transversely m a direction opposed to the direction of relative movement adopted for making the preceding image

An image forming apparatus as defined m any one of claims 1 - 4, wherein the moving means are so arranged that the image receiving member and/or printhead structure move relatively transversely continuously during a pass

An image forming apparatus as defined in claim 5, wherein the moving means are so arranged that the image receiving member and/or printhead structure move relatively transversely, preferably at constant speed, by one dot pitch during a pass

An image forming apparatus as defined m claim 5 or claim 6, wherein the continuous relative movement of the image receiving member and/or printhead structure continues until an image is completed

An image forming apparatus as defined in any preceding claim, wherein the printhead structure includes apertures extending a width greater than the width of the printable image and that during printing of a transverse line one or more apertures at one or both ends of the line are not used to form the image

An image forming apparatus as defined m any preceding claim, wherein the apparatus is arranged to be able to vary the pitch m the transverse direction dependent upon the number of passes, preferably to vary the pitch inversely dependent upon the number of passes

An image forming apparatus as defined m any one of the preceding claims, wherein there are two or more printhead structures (5''), each printhead structure forming a different portion of the image

An image forming apparatus as defined in claim 10, wherein each printhead structure prints a portion of the longitudinal columns of print on a first pass and prints the remaining columns of print on the subsequent passes

12 An image forming apparatus as defined m claim 10, wherein each printhead structure prints only a portion of each longitudinal column of print on each pass

13. An image forming apparatus as defined in any preceding claim, wherein there are two or more printhead structures (5a' -5b') and the printhead structures print in at least two different colors to form at least partially a color image

14 An image forming apparatus as defined m any preceding claim, wherein the image receiving member (1) is an image transfer member which receives the image and transfers the image to an information carrier, such as paper.

15 An image forming apparatus as defined claim 14, wherein the image transfer member (1) is in the form of a belt or drum

16 An image forming apparatus as in any of claims 1 to 13, wherein the image receiving member (1) is an information carrier, such as paper

17 An image forming apparatus as defined m any preceding claim, wherein each aperture (52) is provided with one or more deflection electrodes (54) which deflect the trajectories of the toner particles

18 An image forming apparatus as defined n claim 17, wherein the trajectories are deflected such as to transport the toner particles from each aperture to more than one transverse position m each pass

An image forming apparatus as defined m any preceding claim, wherein means are provided to clean the image receiving member of toner particles before the start of the formation of an image

An image forming apparatus as defined in claim 19, wherein the cleaning means are arranged not to clean the image forming member of toner particles between one or more passes during the formation of an image

An image forming apparatus as defined m any preceding claim, wherein there is a cleaning device provided for each printhead structure to clean the printhead structure of toner particles

An image forming apparatus as defined m any preceding claim, wherein there is a single cleaning device provided to clean all the printhead structures of toner particles