WO2001036207A1

WO2001036207A1 - Direct printing device and method

Info

Publication number: WO2001036207A1
Application number: PCT/SE1999/002125
Authority: WO
Inventors: Daniel Nilsson
Original assignee: Array Ab
Priority date: 1999-11-19
Filing date: 1999-11-19
Publication date: 2001-05-25
Also published as: AU1701600A

Abstract

The present invention relates to an image forming device in which charged toner particles are transported from a particle source to an image receiving member, said image forming device including: a particle delivery unit including said particle source; a printhead structure provided with apertures arranged between said particle source and said image receiving member; and a spacer element arranged between said printhead structure and said particle source. According to the invention, said spacer element has a first portion which is secured to a demountable component of said image forming device, and a second, free portion which is held in contact with the printhead structure to maintain the particle source at a predetermined distance form said apertures. By means of the invention, an improved image forming device is provided, which allows easy access to said spacer element, for example for cleaning thereof.

Description

TITLE

Direct printing device and method

TECHNICAL FIELD The invention relates to a direct printing apparatus in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle source toward a back electrode member 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.

More specifically, the invention relates to a device and method for accurately positioning the printhead structure in relation to the particle source.

BACKGROUND ART

US Patent 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 the image receiving substrate due to control in accordance with an image information. The printhead structure is formed by a lattice consisting in intersecting wires disposed in rows and columns. Each wire is connected to an individual voltage source. Initially the wires are grounded to prevent toner from passing through the wire mesh. As a desired print location on the image receiving substrate passes below an intersection, adjacent wires in a corresponding column and row are set to a print potential to produce an electric field that draws the toner particles from the particle source. The toner particles are propelled through the square aperture formed by four crossed wires and deposited on the image receiving substrate in the desired pattern. A drawback with this construction of printhead structure is that individual wires can be sensitive to the opening and closing of adjacent apertures, resulting in imprecise image formation due to the narrow wire border between apertures. This effect is mitigated in an arrangement described in US Patent No. 5,847,733 by the present applicant. This proposes a control electrode array formed on an apertured insulating substrate. A ring electrode is associated with each aperture and is driven to control the opening and closing of the aperture to toner particles. Each aperture is further provided with deflection electrodes. These are controlled to selectively generate an asymmetric electric field around the aperture, causing toner particles to be deflected prior to their deposition on the image receiving member. This process is referred to as dot deflection control (DDC). This enables each individual aperture to address several dot positions. The print addressability is thus increased without the need for densely spaced apertures.

An essential requirement of such a method is that all control electrodes of the printhead structure are positioned at a same distance from the particle source to ensure a good print uniformity. However, that gap distance can vary from one machine to another due to a combination of independent factors, such as manufacturing variations in the size and placement of the particle source and the printhead structure, as well as the thickness of the toner particle layer on the particle source. Because the gap distance is only in the order of 10-30 microns, even the slightest mechanical imperfections may result in a drastic degradation of the print uniformity. For instance, the particle source can be a rotating, cylindrical developer sleeve having a rotation axis which is not perfectly centered, or a peripheral surface which is neither perfectly round nor perfectly smooth. Further, the toner particle themselves may vary in their diameter and degree of sphericity, and the toner particle layer may vary in thickness along the surface of the developer sleeve.

To accommodate all of these dimensional variations, spacer constructions have been proposed in U.S. Patent No. 5,666,147 by the present applicant. This discloses a direct electrostatic printing device having improved means for maintaining a constant minimal gap between the printhead structure and a developer sleeve, while providing a uniform toner particle layer on the surface of the sleeve. To meet that requirement, a spacer or scraper blade is mounted on the first surface of the substrate to engage the sleeve on it, and the portion of the substrate supporting the spacer can move slightly radially towards and away from the sleeve to accommodate imperfections in the sleeve surface and variations in the toner layer thickness.

A drawback of such a spacer construction is the surface of the spacer, brought in frictional contact with the toner layer, may be worn out or deteriorated due to abrasion forces between the spacer and the toner particles. The frictional forces may increase in the contact area between the spacer and the toner layer, heating and melting small toner particles which adhere onto the spacer surface. This defect is usually referred to as filming. A thin film of melted toner material deposited on the spacer surface will influence not only the thickness but also the conductivity of the spacer and, in some circumstances, degrade the print uniformity. It has been observed that filming occurs after 250-300 pages continuous printing.

Therefore, there is still a need for providing improved spacer arrangements in which the filming effect is eliminated.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved image forming device, by means of which the above-mentioned drawbacks, in particular regarding said filming effect, can be eliminated. This object is accomplished by means of an image forming device according to the appended claim 1.

The present invention relates to an image forming device in which charged toner particles are transported from a particle source to an image receiving member. The image forming device includes a particle delivery unit including said particle source, a printhead structure provided with apertures arranged between said particle source and said image receiving member and a spacer element arranged between said printhead structure and said particle source. The invention is characterized in that said spacer element has a first portion which is secured to a demountable component of said image forming device, and a second, free portion which is held in contact with the printhead structure to maintain the particle source at a predetermined distance from said apertures. By means of the invention, an improved image forming device is accomplished, by means of which an advantage as regards allows easy access to said spacer element is obtained, for example for the purpose of cleaning the spacer element.

A further object of the invention is to provide an improved method for accurately positioning a printhead structure of said image forming apparatus in relation to a particle source. This object is achieved by means of a method according to appended claim 23. Said method includes the following steps: providing a positioning device including a transversal ruler extending in a first direction (X-axis), at least one fastening element, and a least one support element; fastening the printhead structure to said transversal ruler by means of said at least one fastening element; securing the printhead structure on said at least one support element; moving said at least one support element in a second direction (Y-axis) toward said transversal ruler to bend at least a part of the printhead structure in the Y-Z-plane about an curvature axis parallel to the X-axis; providing a spacer element having a free portion made of bendable material; and securing the particle source to said positioning device so as to guide said free portion of the spacer element between the particle source and the printhead structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of an image forming apparatus according to the invention. Fig. 2 is a schematic section view across a print station of the image forming apparatus, wherein a particle delivery unit is also shown in a condition in which it is not mounted on the image forming apparatus. Fig. 3 is a schematic perspective view of a positioning device in accordance with the invention. Fig. 4 is a section view of across the section line l-l of Figure 3. Fig. 5 illustrate the adjustment of the curvature of the printhead structure in the positioning device of Fig. 3 and 4. Fig. 6 illustrates the position of the particle delivery unit on the positioning device of Fig. 3 and 4. Fig. 7 is a partial view of a printhead structure showing the part of the substrate facing the particle delivery unit. Fig. 8 is a partial view of a printhead structure showing the part of the substrate facing the belt unit. Fig. 9 is a partial section view across the section lines 11-11 or Ill-Ill in Fig. 7 or 8. Fig. 10 is an enlargement of a part of Fig. 9 showing an aperture in the printhead structure. Fig. 11 is a partial exploded view of the printhead structure illustrating its different layers. Fig. 12 is a schematic view of a printhead structure according to the invention, in which a developer sleeve is shown in a condition in which it is not mounted to the printhead structure. Fig. 13 is a schematic view of an assembled printhead structure according to the invention. Fig. 14 is a schematic view of a printhead structure according to a second embodiment of the invention, in which a developer sleeve is shown in a condition in which it is not mounted to the printhead structure. Fig. 15 is a schematic view of a printhead structure according to said second embodiment, in which a developer sleeve is shown in a print position.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT As shown in Fig. 1 , an image forming apparatus in accordance with a preferred embodiment of the invention comprises four print stations (Y,M,C,K) arranged in cooperation with a belt unit. However, it will be understood that an image forming apparatus in accordance with the invention comprises at least one print station, and an image receiving member. Allthough the illustrated embodiment is a four color printer, the present invention is neither restricted to a particular number of print stations nor a particular arrangement of the image receiving member. For instance toner particles could be projected directly on paper or any other material suitable for direct printing, without the need for an intermediate transfer belt. Alternatively, a solid drum could be provided for receiving the image and subsequently transferring this to paper or other final medium.

1. Belt Unit The image forming apparatus illustrated in Fig. 1 comprises a belt unit including a driving roller 10, at least one support roller 11 and several adjustable holding elements 12. The 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 (arrow 15) of the transfer belt 1 and a rotation velocity adjusted to convey the transfer belt 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 1 at a predetermined gap distance from each print station. The holding elements 12 are preferably cylindrical sleeves disposed perpendicularly to the belt motion 15 in an arcuated configuration so as to slightly bend the belt 1 at least in a vicinity of each print station in order to create a stabilization force component on the belt in combination with the belt tension. That stabilization force component is opposite in direction to, and preferably larger in magnitude than an electrostatic attraction force component acting on the belt 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 thickness having composite material as a base. The base composite material can suitably include thermoplastic polyimide 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 electric conductivity throughout the entire surface of the transfer belt. The outer surface of the transfer belt 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 print stations, whereas toner particles are deposited on the outer surface of the transfer belt and superposed to form a four color toner image. Toner images are then conveyed through a fuser unit 16 comprising a fixing holder 161 arranged transversally in direct contact with the inner surface of the transfer belt 1. The fixing holder 161 includes a heating element 162 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 162, the fixing holder 161 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1. The fusing unit 16 further includes a pressure roller 163 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 161. An information carrier 170, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 17 and conveyed between the pressure roller 163 and the transfer belt 1. The pressure roller 163 rotates with applied pressure to the heated surface of the fixing holder 161 whereby the melted toner particles are fused on the information carrier 170 to form a permanent image. After passage through the fuser unit 16, the transfer belt 1 is brought in contact with a cleaning element 18, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt for removing all untransferred toner particles from the outer surface.

2. Particle delivery unit

As shown in Fig. 2, a print station in the image forming apparatus illustrated in Fig. 1 , includes a particle delivery unit 2 preferably having a replaceable or refillable container 20 for holding toner particles T, the container having front and back walls (not shown), a pair of side walls and a bottom wall having an elongated opening extending from the front wall to the back wall and provided with a toner feeding element 22 disposed to continuously supply toner particles to a developer sleeve 21 through a particle charging member 23.

A particle charging member 23 is preferably formed of a supply brush or roller made of or coated with a fibrous, resilient material. The supply brush 23 is brought into mechanical contact with the peripheral surface of the developer sleeve 21 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 21. The developer sleeve 21 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 of the particle container 20. Charged toner particles are held on the surface of the developer sleeve 21 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. 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 invention.

A metering element 24 is positioned proximate to the developer sleeve 21 to adjust the concentration of toner particles on the peripheral surface of the sleeve 21 , to form a relatively thin, uniform particle layer thereon. The metering element 24 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 24 may also be connected to a metering voltage source (not shown) to produce an electric potential which influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the sleeve surface.

A spacer element 5 is arranged proximate to the developer sleeve 21 to space the peripheral surface of the sleeve 21 from the printhead structure when the particle delivery unit 2 is secured on the positioning device 3. The spacer element 5 has a portion that is fastened to one of the side walls of the container 20, and a free portion made of flexible material, which is guided to a predetermined fixed position between the sleeve 21 and the printhead structure 4 as the particle delivery unit 2 is secured on the positioning device 3.

3. Positioning device The developer sleeve 21 is arranged in relation to a positioning device 3 for accurately supporting and maintaining the printhead structure 4 in a predetermined position with respect to the peripheral surface of the developer sleeve 21. As shown in Fig. 3 and Fig. 4, the positioning device 3 is formed of a frame 30 having a front portion 301 , a back portion 302 and two transversally extending side rulers 303, 304 disposed on each side of the developer sleeve 21 parallel with its rotation axis. The first ruler 303, positioned at an upstream side of the developer sleeve 21 with respect to its rotation direction, is provided with fastening means 31 to secure the printhead structure 4 along a transversal fastening axis 32 extending across the entire width of the printhead structure 4. The second side ruler 304, positioned at a downstream side of the developer sleeve 21, is provided with a support element 33, or pivot, for supporting the printhead structure 4 in a predetermined position with respect to the peripheral surface of the developer sleeve 21. The support element 33 and the fastening axis 32 are so positioned with respect to one another, that the printhead structure 4 is maintained in an arcuated shape along at least a part of its longitudinal extension. As illustrated in Fig. 5, that arcuated shape has a curvature radius determined by the position of the support element 33 which can be adjusted by moving the support element in a longitudinal direction. The bending curvature of the printhead structure 4 is dimensioned to maintain a part of the printhead structure 4 curved around a corresponding part of the peripheral surface of the sleeve 21. The support element 33 is arranged in contact with the printhead structure 4 at a fixed support location on its longitudinal axis so as to allow a slight variation of the printhead structure position in both longitudinal and transversal direction about that fixed support location, in order to accommodate a possible excentricity or any other undesired variations of the developer sleeve 21. That is, the support element 33 is arranged to made the printhead structure 4 pivotable about a fixed point to ensure that the distance between the printhead structure 4 and the peripheral surface of the developer sleeve 21 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 21. Preferably, parts of the printhead structure 4 are reinforced by stiffening element 41 , and only a central portion of the printhead structure is bendable. The front and back portions 301 , 302 of the positioning device 3 are provided with securing members 34 on which the particle delivery unit 2 can be removably secured in a predetermined print position, which is illustrated in Fig. 6. As the particle delivery unit 2 is secured in its print position, the free portion of the spacer element 5 is pressed against the curvated part of the printhead structure 4.

4. Printhead structure

As shown in Fig. 7-11, a printhead structure 4 in an image forming apparatus in accordance with the invention comprises a substrate 40 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve 21 (Fig. 7), a second surface facing the transfer belt 1 (Fig. 8), a transversal axis 46 extending parallel to the rotation axis of the developer sleeve 21 across the whole print area, and a plurality of apertures 42 arranged through the substrate 40 from the first to the second surface thereof. The first surface of the substrate 40 is coated with a first printed circuit, comprising a plurality of control electrodes 43 disposed in conjunction with the apertures 42, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 43. The second surface of the substrate is coated with a second printed circuit, including a plurality of deflection electrodes 44. The printhead structure 4 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, facing the transfer belt 1. The printhead structure 4 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 43 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 42 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 44 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 42. In some embodiments, 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 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 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, respectively, using conventional etching techniques. The printed circuits are insulated with cover layers 45 (shown in Fig. 10). Cover layers are preferably parylene films of 5 to 10 microns thickness, laminated onto the substrate 50 using vacuum deposition techniques. The apertures 42 are made through the printhead structure 4 using conventional laser micromachining methods. The apertures 42 have preferably a circular or elongated shape centered about a central axis 420 (shown in Fig. 11). The aperture diameter is in a range of 80 to 120 microns. Alternatively, elongated apertures have a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 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 invention, the printhead structure 4 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 42 of the printhead structure 4 during each print cycle. Accordingly, one aperture 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 of the printhead structure. The apertures 42 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 46 of the printhead structure 4 and transversally shifted with respect to each other such that all apertures 42 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.

As can be seen in Fig. 11 , the first printed circuit comprises control electrodes 43 each of which having a ring-shaped structure 431 surrounding the periphery of a corresponding aperture 42 and a connector 432 preferably extending in the longitudinal direction, connecting the ring-shaped structure 431 to a corresponding control voltage source. Although a ring-shaped structure is preferred, the control electrodes 43 may take on various shape for continuously or partly surrounding the apertures 42, preferably shapes having symmetry about the central axis 420 of the apertures 42. In some embodiments, particularly when the apertures 42 are aligned in one single row, the control electrodes 43 are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises a plurality of deflection electrodes 44, each of which is divided into two semicircular or crescent-shaped deflection segments 441 , 442 spaced around a predetermined portion of the circumference of a corresponding aperture 42. The deflection segments 441 , 442 are arranged symmetrically about the central axis 420 of the aperture 42 on each side of a deflection axis 443 extending through the center of the aperture at a predetermined deflection angle d to the longitudinal direction. The deflection axis 443 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(1/3), i.e. about 18,4°.

5. Spacer arrangement

As shown in Fig. 12 and Fig. 13, a spacer element 5 is arranged between the printhead structure 4 and the developer sleeve 21 to maintain a constant gap distance G between the apertures 42 and the toner layer on the peripheral surface 210 of the sleeve 21. The spacer element 5 is located on the upstream side of the aperture rows with respect to the rotation direction of the sleeve 21. The spacer element 5 is a sheet of flexible material having a fixed portion secured to a part of the particle delivery unit (not shown), and a free portion positioned proximate to the sleeve 21. The spacer element 5 is suitably secured to the particle delivery unit since said unit is easily demountable from the image forming apparatus (cf. Fig. 2). In this manner, the spacer element is relatively easy to clean by first removing the particle delivery unit, which provides access to the spacer element.

The free portion of the spacer element 5 has a main part 51 and a edge part 52. The printhead structure 4 is provided with a threshold 45, against which the main part 51 of the spacer element 5 is guided as the sleeve 21 is brought in its print position. The edge part 52, which has initially a substantially plane extension (Fig. 12), is caused to follow the curvature of the sleeve 21 (Fig. 13) as the main part 51 is fastened against the threshold 45. The threshold position on the printhead structure 4 and the length L_s of the edge part 52 are chosen to obtain a spacer position where the end of the edge part 52 is spaced from the upstream row of apertures by a predetermined distance b. The edge part 52 has a thickness dimensioned to a predetermined gap G. The edge part 52 has a curvature corresponding to that of the sleeve surface 210, so as to obtain a contact surface against the toner layer.

It should be noted that the invention is not limited to an embodiment in which the spacer element is secured on the particle delivery unit. As an alternative to the particle delivery unit, the spacer element can be attached to other demountable components of the image forming device. Accordingly, in an alternate embodiment, illustrated in Fig. 14 and Fig. 15 the spacer element 5 has its fixed part fastened to the positioning device 3, instead of to the particle delivery unit. Regardless of which embodiment is chosen, an advantage of the invention is to provide a spacer element that can be easily removed, for example for cleaning, and replaced at its initial position. This is enabled by the use of a threshold 45 or any other securing element used for guiding the edge part 52 to its position.

The invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims. For example, the invention can be used in image forming devices lacking the above-mentioned deflection electrodes.

Claims

1 An image forming device in which charged toner particles are transported from a particle source to an image receiving member, said image forming device including a particle delivery unit including said particle source, a printhead structure provided with apertures arranged between said particle source and said image receiving member, and a spacer element arranged between said printhead structure and said particle source, characterized in that said spacer element has a first portion which is secured to a demountable component of said image forming device, and a second, free portion which is held in contact with the printhead structure to maintain the particle source at a predetermined distance from said apertures

2 An image forming device according to claim 1, characterized in that said demountable component is constituted by said particle delivery unit

3 An image forming device according to claim

in that said demountable component is constituted by a positioning device for positioning the printhead structure in relation to the particle source

4 An image forming device as defined in any one of the preceding claims, characterized in that the spacer element is a sheet of bendable material

5 An image forming device as defined in any one of the preceding claims, characterized in that the particle source is a rotating element having a rotation axis extending in a first direction and an outer surface carrying particles

6 An image forming device as defined in claim 5, characterized in that the spacer element is arranged on a upstream side of the particle source with respect to the rotation direction of said particle source

7. An image forming device as defined in any of claims 5 or 6, characterized in that said rotating element has a substantially cylindrical shape.

8. An image forming device as defined in any of claims 5 to 7, characterized in that said free portion of the spacer element has a contact surface which is held in contact with said outer surface of the particle source, tangentially to said outer surface of the particle source, the particles being thereby carried between said contact surface and said outer surface of the particle source.

9. An image forming device as defined in claim 8, characterized in that said contact surface of the spacer element extends in said first direction.

10. An image forming device as defined in claim 8, characterized in that said free portion of the spacer element is bent to follow a part of said outer surface of the particle source.

11. An image forming device as defined in claim 10, characterized in that said free portion of the spacer element extends tangentially in relation to said outer surface of the particle source, so as to obtain a contact axis, in said first direction, between said free portion of the spacer element and said outer surface of the particle source, said free portion of the spacer element being bent about said contact axis, with a curvature corresponding to the curvature of said outer surface of the particle source in the vicinity of said contact axis, thereby providing a contact surface between the particle source and the spacer element.

12. An image forming device as defined in claim 11, in which particles form a layer on said outer surface of the particle source, said layer being conveyed through said contact surface in frictional contact with the spacer element.

13. An image forming device as defined in any of claims 5 to 12, characterized in that said free portion of the spacer element has a free edge extending in said first direction.

14. An image forming device as defined in any of claims 5 to 13, characterized in that said apertures are aligned in at least one row extending in said first direction.

15. An image forming device as defined in any of claims 13 or 14, characterized in that the printhead structure includes at least one securing element against which said free edge is maintained at a predetermined distance from said apertures.

16. An image forming device as defined in any of claims 13 or 14, characterized in that the printhead structure includes at least one securing element, and said free portion of the spacer element includes a main portion and a edge portion, said main portion being secured against said securing element, whereby said edge portion is maintained in a predetermined position.

17. An image forming device as defined in any of claims 13 to 16, characterized in that said free edge is maintained at a predetermined distance from said apertures, said predetermined distance being smaller than 1 mm.

18. An image forming device as defined in any of claims 14 to 17, characterized in that said at least one securing element is a threshold formed on the printhead structure and extending in said first direction.

19. An image forming device as defined in any of claims 14 to 17, characterized in that said at least one securing element is a track formed in the printhead structure and extending in said first direction.

20. An image forming device as defined in any of claims 14 to 17, characterized in that said at least one securing element is made of spikes or other protuberances formed in the surface of the printhead structure.

21. An image forming device as defined in any one of claims 1- 16, including a positioning device for positioning the printhead structure in relation to the particle source, characterized in that the printhead structure has at least one bendable part which is maintained in an arcuated position about a curvature axis extending in said first direction, and a curvature corresponding to the curvature of said outer surface of the particle source in the vicinity of said curvature axis

22 An image forming device as defined in claim 17, c h a r a c t e r i z e d i n that said positioning device includes a fastening element for fastening the printhead structure along a fastening axis extending in said first direction, and at least one support element for supporting the printhead structure at at least one support point, said fastening axis and said support point being so positioned with respect to each other, that said at least one bendable part of the printhead structure is maintained in an arcuated position with a curvature axis extending in said first direction, and a curvature corresponding to the curvature of said outer surface of the particle source in the vicinity of said curvature axis

23 A method for positioning a printhead structure in relation to a particle source, said method including the following steps providing a positioning device including a transversal ruler extending in a first direction (X-axis), at least one fastening element, and a least one support element, fastening the printhead structure to said transversal ruler by means of said at least one fastening element, securing the printhead structure on said at least one support element, moving said at least one support element in a second direction (Y-axis) toward said transversal ruler to bend at least a part of the printhead structure in the Y-Z- plane about an curvature axis parallel to the X-axis, providing a spacer element having a free portion made of bendable material, and securing the particle source to said positioning device so as to guide said free portion of the spacer element between the particle source and the printhead structure