WO2000078550A1

WO2000078550A1 - Direct printing device

Info

Publication number: WO2000078550A1
Application number: PCT/EP1999/004330
Authority: WO
Inventors: Bo RYDSTRÖM; Filip Alm; Lars STRÅVIK
Original assignee: Array Ab
Priority date: 1999-06-22
Filing date: 1999-06-22
Publication date: 2000-12-28
Also published as: AU4615099A

Abstract

The invention relates to a direct printing apparatus in which computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle carrier (33) toward a back electrode (12) through a printhead structure (5), whereby the charged particles are deposited in image configuration on an image receiving substrate (1) caused to move relative to the printhead structure. In order to prevent variations in the spacing between particle carrier (33) and the back electrode (12), which can lead to a deterioration in the print quality, spacing members (60) are disposed to cooperate with said particle carrier and said back electrode to restrict the relative movement between the particle carrier and the back electrode in a direction parallel to an orthogonal projection between opposing surfaces of the particle carrier and the back electrode and in a direction transverse to said orthogonal projection.

Description

Direct Printing Device

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 source toward a back electrode through a printhead structure, whereby the charged particles are deposited in image configuration on an image receiving substrate caused to move relative to the printhead structure.

Raπkground

US patent No. 5 847 733 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 is generally constituted by 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 apertures 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 medium. 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 .

However, the actual position of a deflected dot relative to a dot formed by undeflected toner particles on the image receiving medium is affected not just by the electric field profile around the aperture, but also by the distance between the aperture, or printhead, and the image receiving medium. Accordingly variations in this distance, specifically due to unparallelity between the printhead and the image-receiving medium, will result in the relative positions of dots varying across the surface of the image. The print quality will thus be seriously degraded.

US patent No. 5,495 273 also describes a direct electrostatic printing device wherein spacers are provided on the back electrode for supporting the back electrode against the apertured control electrode. However, such an arrangement requires the control electrode to be sufficiently rigid to support the back electrode, while maintaining the required distance to the toner carrier. An alternative embodiment figures a spacer in contact with the surfaces of a back electrode and the toner carrier. However by avoiding contact with the printhead, an area of the back electrode is exposed to the toner carrier which causes the distortion of the electric field at the edges of the control electrode and results in non-uniform deflection at these edges. Furthermore, with the potential difference between the back electrode and the toner carrier of the order of 1 kV to 1.5 kV there is a danger that electrical discharge in the form of an arc may occur between these elements thus short circuiting them.

Thus there is a need for a direct electrostatic image forming arrangement that provides an improved print quality by ensuring that the deflection across the printhead is substantially uniform. 5-Surπmary of the inven on

According to the invention there is provided an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating the transport of charged toner particles from a toner carrier towards an image receiving member. A back electrode for attracting charged toner particles is connected to a voltage source. A printhead structure is disposed between the toner carrier and the back electrode and includes a plurality of apertures having associated control electrodes . Variable voltage sources are connected to the control electrodes to permit or restrict the transport of charged toner particles from the particle carrier through the apertures. An image receiving member is provided below the printhead structure for intercepting the transported toner particles in image configuration and A spacer arrangement is associated with said particle carrier for maintaining a gap between said particle carrier and said the back electrode. The spacer arrangement is disposed to co-operate with said particle carrier and said back electrode to restrict the relative movement between the particle carrier and the back electrode both in a direction parallel to an orthogonal projection between opposing surfaces of the particle carrier and the back electrode and in a direction transverse to said orthogonal projection.

The present invention thus ensures that the particle carrier and the back electrode are held parallel to one another at all times. Accordingly variations in the gap due to eccentric motion of one of these electrodes is precluded, and the print quality across the printhead structure is maintained constant

When the particle carrier and the back electrode have arcuate opposing surfaces the spacer arrangement preferably includes at least one concavely curved surface for mating with the back electrode surface. Similarly, a further concavely curved surface is provided for mating with the particle carrier. In this way, the spacer arrangement advantageously serves as a bearing for the rotary particle carrier.

In order to prevent distortion of the electric field at the edge of the printhead structure and exclude the danger of discharge between the particle carrier and back electrode, the spacer arrangement usefully includes at least an electrically insulating portion that is adapted to be coupled to the back electrode and to extend essentially parallel with a surface of said back electrode along the gap at least up to an area within the gap located directly between the back electrode and the printhead structure. In this manner the back electrode is electrostatically shielded from the particle carrier throughout its length by at least one of the printhead structure and the spacer arrangement. This electrically insulating portion may further be integrally formed with the spacer arrangement as a whole .

The spacer arrangement is preferably detachably mounted in a unit that also carries a toner particle container. This permits the spacer arrangement to be replaced at intervals, for example when an empty toner container is replaced by a full one, so that the use of a hard, abrasion-resistant material for the spacer arrangement is not critical.

Brief description of the drawings

The invention will now be described in more detail for explanatory, and in 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, in which

Fig.l is a schematic view of an image forming apparatus -. τ-£s -P , present invention,

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

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 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 the intermediate transfer belt,

Fig.4c is a section 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 is partial view of the print zone shown in Fig. 3 viewed from the side illustrating a spacer according to the present invention that defines the relative positions of the particle source and the image receiving member.

Detailed description

As shown in Fig.l, an image forming apparatus in accordance with a first embodiment of the present invention comprises at least one print station, preieraciy r^u..-. iim b Liuub , π, >--, Λ , cui 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 in relation to the intermediate image -receiving member 1. The image receiving member, preferably a transfer belt 1, is mounted over the driving roller 10. 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 belt 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 in order to create a stabilisation force component on the belt in combination with the belt tension. That stabilisation force component is opposite in direction to, and preferably larger in 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 holding elements 12 are provided with an electrically conducting surface which is connected to a voltage source for generating a background electric field. serve as back electrodes and are connected to a high voltage source of opposite polarity to the The transfer belt 1 is preferably an endless band of 30 to 200 microns thick having 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 in 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 deposited on the outer surface of the transfer belt and superposed to form a four colour toner image. Toner images are then preferably conveyed through a fuser unit 13 comprising a fixing holder 14 arranged transversally in 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 in 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 bel . 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 in 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.

As shown in Fig.2, a print station in an image forming apparatus in accordance with 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 developer sleeve 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 developer sleeve 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 developer sleeve. The developer sleeve 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 developer sleeve 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 developer sleeve 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown) , which supplies 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 developer sleeve 33 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 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 influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve.

As shown in Fig.3, the developer sleeve 33 is arranged in 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 developer sleeve 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 developer sleeve 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at an upstream side of the developer sleeve 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 developer sleeve 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the developer sleeve 33. The support element 45 and the fastening axis are so positioned with respect to one another, that the printhead structure 5 is maintained in 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 developer sleeve 33. The support element 45 is arranged in 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 in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible eccentricity or any other undesired variations of the developer sleeve 33. That is, the support element 45 is arranged to make 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 developer sleeve 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 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 in a fixed position to provide a constant distance between the rotation axis of the developer sleeve 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 developer sleeve 33 to accurately space the developer sleeve 33 from the corresponding holding ' element 12 of the transfer belt 1 facing the actual print station. As shown in 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 developer sleeve 33, a second surface facing the transfer belt 1, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 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 in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in 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 semiconducting material, such as silicon 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 which 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, in 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 micromachining 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 in each print cycle, i.e. three dot locations are addressable through 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 in one or several rows, preferably in two parallel rows each comprising 100 apertures per inch. Hence, the aperture pitch, i.e. the distance between the central axes of two neighbouring 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 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 the control electrodes 53 each 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 voltage source. Although a ring shaped structure is preferred, the control electrodes 53 may take on various shapes 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 in a transverse direction than in 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 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 54 has an 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. In accordance with the invention, the deflection voltage sources Dl and D2 are controlled by a control unit 65 (see Fig. 9), which will be discussed in more detail below. 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 .

The deflection experienced by charged toner particles due to the application of an asymmetric electrostatic field across an aperture in the printhead 5 depends on a number of factors including the mass and size of the toner particles and the deflecting electric field strength. However, since deflected toner will describe a path that deviates from the normal by a substantially constant angle, the final position of a deflected toner dot depends strongly on the length of the path. Thus, the distance between the printhead 5 and the image receiving surface, which in this case is the transfer belt 1 is critical for obtaining a uniform degree of deflection across all the apertures of the printhead 5. While the securing members 46 are preferably dimensioned to provide and maintain an essentially parallel relation between the rotation axis of the developer sleeve 33 and a central transversal axis of the corresponding holding member 12, eccentricities in the rotation of either of these members can result in sufficient relative motion to cause nonuniformity of deflection transverse to the movement of the transfer belt. This non-parallelity is mitigated by the arrangement shown in Fig. 5.

Fig. 5 shows a perspective view of one end of a developer sleeve 33 carrying the charged toner particles and an opposing holding member 12 that serves as a back electrode in a print station. A portion of a printhead 5 is shown in section. According to the invention, the developer sleeve 33 is provided with two spacer members 60 at either end, one of which is shown in Fig. 5. Each spacer member has an arcuate surface with a curvature corresponding to that of the developer sleeve. The concave mating surface of the spacer member 60 is smooth to allow the developer sleeve 33 to rotate freely within it. The smooth finish of the mating surface may be obtained by polishing or by providing a suitable coating, such as Teflon (polytetrafluoroethylene) , for example. The developer sleeve 33 fits snugly in the upper mating surface of the spacer member 60 and rotates within it; the spacer thus serves as a bearing for the developer sleeve. The spacer member 60 further extends downwards essentially normally towards the opposing surface of the holding member 12.

At its lower end, the spacer member 60 is provided with a second arcuate mating surface with a radius of curvature corresponding to that of the holding member. The arcuate mating surface in contact with the holding member 12 is of electrically insulating material. The remaining portion of the spacer member 60 is preferably formed integrally with the lower mating surface and is therefore similarly made of electrically insulating material. However, while this implementation is preferred to minimise the manufacturing tolerances, it will be understood that other materials, including electrically conductive materials, may be utilised for the remaining parts of the spacer member 60. In a similar manner to the upper mating surface, the arcuate surface of the spacer member 69 that mates with the holding member 12 is smooth to allow the holding member 12 to rotate freely. Alternatively, free rotation of the holding member 12 may be enabled by providing rotary elements, such as rollers, ball bearings and the like on the lower mating surface of the spacer member 60.

The arcuate mating surfaces of the spacer member 60 assure the parallelity between the developer sleeve 33 and the holding member 12 by restricting the relative movement of these rollers both in a vertical plane as viewed in Fig. 5, i.e. in a plane containing the axes of both rollers, and in a transverse plane. It will be understood that since the opposing surfaces of both the developer sleeve 33 and the holding member 12 are curved, any relative movement transverse to the vertical plane will also result in a change in the dimensions of the gap between the developer sleeve 33 and the holding member 12. The described spacer member restricts the movement of the developer sleeve 33 and the holding member 12 in all directions except for the axial direction. The liklihood of non-parallelity occurring is thus effectively prevented.

The lower electrically insulating surface of the spacer member 60 in contact with the holding member 12 extends inwardly parallel to the axis of the holding member 12 to an area directly below the printhead 5. In this manner, the developer sleeve 33 is shielded throughout its length from the high voltage back electrode 12 either by the printhead 5 or the electrically insulating portion of the spacer member 60. Preferably there is a slight overlap between the spacer member 60 and the printhead structure 5 to minimise the possible distortion of the electric field at the edges of the printhead structure 5 and co safely prevent the short-circuiting of the holding member 12 and developer sleeve 33 by discharge across the gap.

The spacer member 60 is mounted in the particle delivery unit 3 (Fig. 2) by non-shown connecting arms. Since the spacer member 60 is constantly in contact with the rotating surfaces of the developer sleeve 33 and the holding member 12, the mating and/or bearing surfaces are subjected to a considerable amount of wear and risk abrasion that may alter the spacing distance with time. For this reason it is preferred that the spacer member 30 be easily removable from the particle delivery unit, so that it may be periodically replaced, possibly at similar intervals to the toner particle container 30.

Variations in the gap between developer sleeve 33 and holding member 12 can be still further reduced by keeping the holding member 12 stationary. This will prevent variation by runout during rotation of this member 12.

The spacer member arrangement is not limited to the embodiments described above but may also be utilised in a print station that projects toner directly onto a final image receiving surface, such as paper. However, it will be understood that variations in paper thickness will influence the flight distance of the toner particles, and therefore the deflection. Accordingly, it is preferable that a transfer belt with a controllable uniform thickness be utilised to avoid variations in print quality. Alternatively, the functions of the transfer belt 1 and holding member 12 may be assured by a single entity in the form of a drum, which transfers a received image to the final medium in a similar manner to the belt 1.

Claims

What is claimed is:

l.An image forming apparatus in which image information is converted into 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 in said background electric field between said back electrode member and said particle carrier, including a plurality of apertures and control electrodes associated with 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 caused to move in relation to the printhead structure for intercepting the transported charged particles in image configuration; and spacer means associated with said particle carrier for maintaining a gap between said particle carrier and said the back electrode; characterized in that, the spacer means (60) are disposed to co-operate with said particle carrier (33) and said back electrode (12) to restrict the relative movement between the particle carrier and the back electrode in a direction parallel to an orthogonal projection between opposing surfaces of the particle carrier and the back electrode and in a direction transverse to said orthogonal projection.

2.An image forming apparatus as defined in claim 1, characterised in that opposing surfaces of said particle carrier (33) and said back electrode (12) are at least partially arcuated, and said spacer means comprise at least one concave surface for mating with the arcuated back electrode surface .

3. An image forming apparatus as defined in claim 2, characterized in that said spacer means (60) include at least one concave surface for mating with the arcuated surface of the particle carrier (33) .

4. An image forming apparatus as defined in any previous claim, characterized in that the spacer means (60) include at least an electrically insulating portion that is adapted to be coupled to the back electrode and to extend essentially parallel with a surface of said back electrode along said gap at least up to an area within said gap located directly between said back electrode (12) and said printhead structure (5) such that said back electrode is electrostatically shielded from said particle carrier by said printhead structure (5) and said spacer means (60) .

5. An image forming apparatus as defined in any previous claim, characterized in that the electrically insulating portion of said spacer means (60) is integrally joined to said spacer means .

6. An image forming apparatus as defined in any previous claim, characterized in that the spacer means (60) serve as a bearing, whereby the particle carrier (33) is rotatably mounted.

7. An image forming apparatus as defined in claim 6, characterized in that the spacer means (60) are mounted in a unit (3) that further contains means (30) for supplying toner particles to said particle carrier.

8. An image forming apparatus as defined in claim 7, characterized in that the spacer means (60) are detachably mounted in said unit (3) .

9. An image forming apparatus as defined in any previous claim, characterized in that the back electrode is stationary.

10. An image forming apparatus as claimed in any preceding claim, characterized in that the image- receiving member is an image transfer member (1) adapted to transfer an image to a final image-receiving medium.

11. An image forming apparatus as defined in any previous claim, characterized in that the spacer means include at least two spacer elements (60) extending between the particle carrier (33) and back electrode (12) , and the printhead structure (5) is coupled to said particle carrier (33) , wherein said printhead structure (5) is furthermore disposed between said spacer elements (60) .