WO2001068371A1

WO2001068371A1 - Printhead structure and image forming device

Info

Publication number: WO2001068371A1
Application number: PCT/SE2000/000538
Authority: WO
Inventors: Filip Alm; Akira Ishida
Original assignee: Array Ab; Matsushita Electric Industrial Co., Ltd.
Priority date: 2000-03-17
Filing date: 2000-03-17
Publication date: 2001-09-20
Also published as: AU2000244421A1

Abstract

Printhead structure (4) comprising a sheet-like substrate (41) of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis (411) adapted to be aligned with a feed direction of said recording medium and a transverse axis (410) perpendicular to said longitudinal axis, a plurality of apertures (42) arranged through the substrate (41) in at least one row parallel to said transverse axis (410), a first and a second support region (413, 414) at opposite sides of said at least one row of apertures (42) adapted for supporting the printhead structure (4) in a position where a portion (420) of the substrate surrounding said at least one row is maintained in an arcuate shape at a distance from an outer surface of a particle carrier when arranged in conjunction with said particle carrier, control electrodes (43) arranged in conjunction with the apertures (42) and a conductor matrix (44) including a first set of conductors (45) arranged on said substrate providing a connection to each control electrode.

Description

TITLE

Printhead structure and image forming device.

FIELD OF INVENTION The invention is within the field of electrographical printing devices. More specifically, the invention relates to a printhead structure for an image forming apparatus and an image forming apparatus including such a printhead. The printhead structure includes a sheet-like substrate having a plurality of apertures arranged through the substrate. In operation, the printhead structure is brought into co-operation with a particle source to modulate a stream of toner particles from the particle source through the apertured sheet-like substrate.

In particular the invention relates to a printhead structure comprising a sheet-like substrate of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis adapted to be aligned with a feed direction of said recording medium and a transverse axis perpendicular to said longitudinal axis, a plurality of apertures arranged through the substrate in at least one row parallel to said transverse axis, control electrodes arranged in conjunction with the apertures and a conductor matrix including a first set of conductors arranged on said substrate providing a connection to each control electrode.

DESCRIPTION OF RELATED ART

US Patent No.5, 036, 341 granted to Larson 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. The Larson patent discloses a method in which an electrode matrix, arranged between a back electrode and a rotating particle carrier, generates a pattern of electrostatic fields which, due to control in accordance with an image information, modulate a transport of toner particles toward the back electrode. An electrostatic field on the back electrode attracts the toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected individual control electrodes to create

SUBS i i i^"u i OΠL I^" RULE 26) electrostatic fields which either permit or restrict the transport of toner particles from the particle carrier through the electrode matrix. In effect, these electrostatic fields "open" or "close" selected apertures in the electrode matrix to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged toner particles allowed to pass through the opened apertures impinges upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.

An electrode matrix for use in direct electrostatic printing devices may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or a screen-shaped, apertured printed circuit. Generally, the matrix is formed of a thin, flexible substrate of electrically insulating material, such as polyimide, provided with a plurality of apertures and overlaid with a printed circuit of control electrodes arranged in connection to the apertures, such that each aperture is surrounded by an individually addressable control electrode.

An essential requirement of the aforementioned method is to maintain a constant, uniform gap distance between the control electrodes and the toner particle layer on the particle carrier. The gap distance can vary from machine to machine because it is determined by a combination of independent factors such as manufacturing variations in the size and placement of the particle carrier and the electrode matrix, as well as the thickness of the toner layer on the particle carrier. Because the gap distance is only in the order of 10-30 microns, even the slightest mechanical imperfections can result in a drastic degradation of the print quality.

When arranging a sheet-like substrate in conjunction with a particle carrier in an image processing device and the substrate is bent in order to obtain a curvature of the sheet-like structure for positioning the apertures at a correct distance from the peripheral outer surface of the particle carrier, it has shown that the distance varies in dependence to properties of the printhead structure, and in particular to the bending stiffness in a critical region to around the apertures.

SUMMARY OF THE INVENTION

The object of the inventions is thus to provide a printhead structure that allows the distance, Lk, from the peripheral outer surface of a particle carrier to the apertures in the substrate to remain even over one or several rows of apertures, when arranged in conjunction with a particle carrier in an image forming apparatus. It has shown that the possibility to flex the flexible substrate for achieving a low variation in the distance Lk is dependent on the bending stiffness of the substrate.

A printhead structure according to the invention comprises a sheet-like substrate of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis adapted to be aligned with a feed direction of said recording medium and a transverse axis perpendicular to said longitudinal axis, a plurality of apertures arranged through the substrate in at least one row parallel to said transverse axis, control electrodes arranged in conjunction with the apertures and a conductor matrix including a first set of conductors arranged on said substrate providing a connection to each control electrode.

When flexing the substrate around the transverse axis for arranging the membrane in conjunction with a particle carrier, the shape of a flexure of the substrate at a cross section through the membrane along the longitudinal axis at an aperture chosen by random is dependent on the bending stiffness of the substrate and the contact forces which flexes the substrate. Furthermore, when flexing the substrate around the transverse direction, an edge effect due to the finite length in the transverse direction flexes the substrate around the longitudinal axis. The flexure around the longitudinal axis is dependent on the contact forces and the bending stiffness of the substrate.

Still further, local variations in the bending stiffness of the substrate give rise to variation of the shape of the flexure at different locations of the substrate.

In a first embodiment of the invention it is provided a printhead structure according to the characterising portion of claim 1, thereby providing a printhead device allowing improved properties regarding the correctness of position of the apertures when arranged in conjunction with a particle carrier in an image forming device. By providing a thin substrate the bending stiffness could be reduced and the desired shape of the flexure could be obtained with low contact forces and the flexure around the longitudinal axis could be reduced giving an even distribution of the distance Lk at different positions along the transverse axis.

SUBSTITUTE P^πir:rτ ""IF 26) In a second embodiment of the invention it is provided a printhead structure according to the characterising portion of claim 18, thereby providing a printhead device allowing improved properties regarding the correctness of position of the apertures when arranged in conjunction with a particle carrier in an image forming device. By providing an even bending stiffness at a portion of the substrate surrounding said at least one row the desired shape of the flexure could be obtained at different positions along the transverse axis.

In a third embodiment of the invention it is provided a printhead structure according to the characterising portion of claim 19, thereby providing a printhead device allowing improved properties regarding the correctness of position of the apertures when arranged in conjunction with a particle carrier in an image forming device. By providing a low bending stiffness at a portion of the substrate surrounding said at least one row, the desired shape of the flexure could be obtained with low contact forces and the flexure around the longitudinal axis could be reduced, giving an even distribution of the distance Lk at different positions along the transverse axis.

In a fourth embodiment of the invention it is provided a printhead structure according to the characterising portion of claim 20, thereby providing a printhead device allowing improved properties regarding the correctness of position of the apertures when arranged in conjunction with a particle carrier in an image forming device. By providing a low and even bending stiffness at a portion of the substrate surrounding said at least one row, the desired shape of the flexure could be obtained with low contact forces and the flexure around the longitudinal axis could be reduced, giving an even distribution of the distance Lk at different positions along the transverse axis and due to the even bending stiffness the desired shape of the flexure could be obtained at different positions along the transverse axis.

In a fifth embodiment of the invention it is provided a printhead structure according to the characterising portion of claim 21, thereby providing a printhead device allowing improved properties regarding the correctness of position of the apertures when arranged in conjunction with a particle carrier in an image forming device. By providing bending stiffness per transverse length unit, EI/L, of less than 5*10^"3 Nm at a portion of the substrate surrounding said at least one row of apertures, where El is the bending stiffness and L is the length in the transverse direction of the printhead structure within said portion at a portion of the substrate surrounding said at least one row, the desired shape of the flexure could be obtained with low

SUBS^'I iTUTE SH__tT κULΞ 26 contact forces and the flexure around the longitudinal axis could be reduced, giving an even distribution of the distance Lk at different positions along the transverse axis.

The application also includes dependent claims relating to preferred embodiments of the invention.

The invention further relates to an image recording apparatus wherein a printhead structure according to the invention is incorporated.

In a preferred embodiment of the invention the substrate is arranged with at least one additional flexure thereby stabilising the substrate against flexing around the longitudinal axis.

BRIEF DESCRIPTION OF DRAWINGS

The invention will in the following be further described in non-limiting way under reference to the accompanying drawings in which:

Fig.l is a schematic view of an image forming apparatus ,

Fig. 2a is a schematic section view across a print station of the image forming apparatus of Fig.l,

Fig.2b is an enlargement of a part of Fig.2a, showing a print zone in a print station,

Fig.3a is a schematic plane view of a printhead structure in an image forming apparatus as that shown in Fig.l,

Fig. 3b is an enlargement of a part of Fig.3a,

Fig.4 is a schematic perspective view of a printhead structure and its position in print station,

Fig.5 is a schematic section view of the printhead structure of Fig.4, across the longitudinal axis thereof,

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SUBS Tϋϊc SU ΞT (RULE 25) Fig. 6 is a schematic view of conventional positioning of a printhead structure in an image forming device according to prior art,

Fig.7 is a schematic view of positioning of a printhead structure in an image forming device,

Fig.8 is an embodiment of the positioning of a printhead structure in conjunction to a toner carrier,

Fig.9 is another embodiment of the positioning of a printhead structure in conjunction to a toner carrier,

Fig.10 is another embodiment of the positioning of a printhead structure in conjunction to a toner carrier, in which spacer means are provided to maintain a constant gap distance therebetween,

Fig. 11a is an embodiment of a printhead structure according to the invention,

Fig. l ib is an enlargement of a portion of fig 11a,

Fig. 12 is a side-view of a printhead structure.

Fig 13 shows a printhead structure arranged in conjunction with a particle carrier, where the printhead structure includes an additional flexure and

Fig 14 is an alternative embodiment of the arrangement of the printhead structure.

DETAILED DESCRIPTION OF EMBODIMENTS 1. General description of the apparatus referring to Fig. l The present invention relates to an image recording apparatus such as that schematically illustrated in Fig.l, in which an image receiving medium, such as an intermediate transfer belt

1 , is conveyed in a longitudinal direction (arrow D) successively past four print stations (Y,C,M,K), each corresponding to a specific toner color (generally yellow, cyan, magenta and black), to intercept a modulated stream of toner particles from each print station whereby the so obtained four image configurations are directly superposed onto the transfer belt 1 , forming a visible four color toner image. That toner image is subsequently brought into contact with an information carrier (P) in a transfer unit (TU), whereas the toner image is transferred to the information carrier and thereafter made permanent in a fusing unit (not shown).

2. General description of a print station referring to Fig.2a, 2b

A print station includes:

(a) a particle delivery unit 2 for conveying charged pigment particles to a position adjacent to the image recording medium 1, said particle delivery unit 2 including a particle carrier 21 ; (b) a background electrode 3 positioned in conjunction with the particle carrier 21;

(c) a background voltage source V_BE for producing a background electric field between said background electrode 3 and said particle carrier 21, which background electric field enables a transport of charged pigment particles from the particle carrier 21 toward the background electrode 3; (d) a printhead structure 4 positioned in said background electric field between the toner carrier 21 and the background electrode 3, said printhead structure 4 including: a substrate 41 of electrically insulating material; a plurality of apertures 42 arranged through the substrate 41 ; and control electrodes 43 arranged in conjunction with the apertures 42;

(e) variable voltage sources 5 connected to said control electrodes 43 to produce a pattern of electrostatic control fields influencing said background electric field in accordance with an image information for selectively permitting or restricting said transport of charged pigment particles through the apertures 42; and

(f) a positioning device for positioning the printhead structure 4 in relation to the particle carrier 21, including a fastening element 61 and a supporting element 62.

3. General description of the different parts of the print station (a) The particle delivery unit 2 (as shown in Fig.2a) includes a casing 20 , a partition 22 which separates the casing 20 into a toner container 23 for storing toner particles T, and a buffer chamber 24 for regulation of the amount of toner to be supplied. Provided in the toner container 23 is a rotating stirring member 231 for supplying toner from the toner container 23 to the buffer chamber 24. The buffer chamber 24 includes a rotating, substantially cylindrical toner carrier 21 having a transversal rotation axis, a predetermined radius R and a peripheral outer surface on which a toner layer is formed by means of a supply element 21 1 for delivering toner to the toner carrier, and a regulation element 212 which ensures a uniform toner layer thickness. (b) The background electrode 3 (as shown in Fig.2a) is a rotating, substantially cylindrincal roller supporting the image transfer belt 1. The background electrode roller 3 has a transversal rotation axis.

(c) The background voltage source V_BE (as shown in Fig.2a) produces an electric potential difference between the particle carrier 21 and the back electrode roller 3 for generating a uniform background electric field therebetween, which exposes the toner layer for an attraction force toward the background electrode roller 3 thereby enabling a toner transport through the selected apertures 42. The electric potential difference can be in the order of + 1500 V.

(d) The printhead structure 4 (as shown in Fig. 3a and 3b) includes a sheet-like substrate 41 of electrically insulating, flexible polymere material, such as polyimide or the like, having a substantially rectangular shape with a transversal axis 410 along the Y-axis and a longitudinal axis 41 1 parallel to the X-axis. The substrate 41 has a predetermined thickness, a first surface facing the particle carrier 21, a second surface facing the background electrode roller 3, a front part 413 located upstream of the transversal axis 410 with respect to the rotation of the particle carrier 21, and a rear part 414 located downstream of the transversal axis 410 with respect to the rotation of the particle carrier 21. The printhead structure 4 has a plurality of apertures 42 arranged through the substrate 41 and aligned in at least one transversal row, for example extending along the transversal axis 410. The first surface of the substrate is overlaid with a first printed circuit including a plurality of control electrode 43 each of which surrounds a corresponding aperture 42 in the substrate. The first printhead circuit is coated with a first cover layer (not shown) of electrically insulating material.

According to a preferred embodiment of the present invention, the front and rear parts 413, 414 of the substrate 41 are at least partially laminated with a thin sheet of rigid material. A central portion 415 of the substrate 41 , located between the front part 413 and the rear part

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SUBSTITUTE SHEET RULE 25 414 remains unlaminated and thus flexible. The apertures 43 are arranged on the flexible central portion 415 of the subtrate 41. Accordingly, the substrate 41 has a variable flexibility along its longitudinal axis 411, such that the central portion 415 can be given an arcuate shape following the curvature of the particle carrier 21. (e) The variable voltage sources (not shown) are generally conventional IC-drivers supplying a stream of control voltage pulses, defining an image information, to the control electrodes for electrostatically opening or closing corresponding apertures in the substrate, so as to permit or restrict the influence of the background electric field on the toner layer through the opened apertures, and thus modulate a selective toner transport through the printhead structure toward the background electrode. The control voltage pulses have a magnitude and a pulse- width dimensioned to control the amount of toner particles allowed to pass through the corresponding aperture during a print sequence.

(f) The positioning device (as shown in Fig.2a) includes a fastening element 61 for securing the front part of the substrate to the particle delivery unit along a transversal fastening line, and a supporting element 62 for supporting the rear part of the substrate at a predetermined supporting point 620 on the longitudinal axis of the substrate.

4. The position of the printhead structure in relation to the particle carrier The position of the printhead structure 4 relative to the particle carrier according to a preferred embodiment of the present invention is schematically illustrated in Fig.4 and Fig.5.

According to most prior art (Fig.6), the printhead structure is conventionally positioned on a X-Y plane and arranged between two transversally extending fastening elements which maintain the substrate in a stretched state at a predetermined gap distance from the peripheral outer surface of the particle carrier. According to the present invention, a fastening element 61 extends transversally across the width of the substrate 41 at a predetermined distance Zf from the X-Y plane and at a predetermined distance Xf from the Y-Z plane. The securing element 62 is preferably disposed to support the substrate 41 at a predetermined position on its longitudinal axis 411. The supporting element 62 is arranged at a predetermined distance Zs from the X-Y plane and at a predetermined distance Xs from the Y-Z plane. Front and rear parts 413, 414 of the substrate 41 are made rigid, whereby only the central part 415 of the substrate 41 is allowed to be bent . Accordingly, the rigid parts of the substrate 41 will extend at angles a and b from the X-Y plane, which in turn results in a curvature of the central flexible part 415 of the substrate 41. That curvature is determined by the distances Xf, Zf, Xs, Zs which are adapted to the radius R of the particle carrier 21. Since the substrate 41 is supported at a single point, the whole printhead structure is allowed to pivot in both longitudinal and tranversal direction in order to accomodate the mechanical variations of the particle carrier.

5. Embodiments of the positioning of a printhead structure in conjunction with a particle carrier.

Shown in Fig.7 and 8 is an embodiment of the positioning of a printhead structure in conjunction with a particle carrier, in which the fastening and supporting elements 61, 62 are disposed so as to provide a contact area 63 between the peripheral outer surface of the particle carrier 21 and the first surface of the substrate 41. The contact area 63 is located between the apertures 42 and the supporting element 62, downstream of the apertures 42 with respect to the rotation of the particle carrier 21. The curvature radius of the substrate 41 is made slightly larger than the radius R of the particle carrier 21, which results in a uniform gap distance Lk between the apertures 42 and the peripheral outer surface of the particle carrier 21. Since the substrate 41 is held in contact with the particle carrier 21, the substrate 41 follows the shape of the particle carrier 21, resulting in that the gap distance Lk remains constant and independent of undesired mechanical variations. The supporting element 62 maintain the substrate 41 in contact with the particle carrier 21 along a transversal contact line onto the peripheral outer surface thereof, and allows the substrate 41 to pivot in order to compensate both radial and transversal variations of the shape of the particle carrier 21. Accordingly, the pivotability of the substrate 41 ensures a uniform gap distance Lk even when the particle carrier 211 is not perfectly centered, nor perfectly parallel to the X-Y plane.

Shown in Fig.9 is an embodiment of the positioning of a printhead structure in conjunction with a particle carrier, in which the curvature of the substrate 41 is dimensioned to provide a contact area 63 on both sides of the apertures 42. The curvature radius of the substrate between the contact areas is slightly smaller than the radius of the particle carrier 21, resulting in that a uniform gap distance Lk is formed between the peripheral outer surface of the particle carrier and the apertures.

Shown in Fig.10 is an alternate embodiment of the positioning of a printhead structure in conjunction with a particle carrier including spacer layers 71, 72 arranged between the substrate 41 and the particle carrier 21. In such an embodiment the gap distance Lk is mainly determined by a predetermined thickness of a spacer layer 71 arranged upstream of the apertures 42 with respect to the rotation of the particle carrier. Preferably, the positioning device includes an upstream spacer layer 71 interposed between the front part 413 of the substrate 41 and the peripheral outer surface of the particle carrier 21. Spacer layers 71, 72 may also be arranged on both upstream and downstream positions as examplified in Fig.10. The spacer layer 71 is preferably a removable sheet of flexible material, such as polyimide, having a predetermined thickness corresponding to an appropriate value of the gap distance Lk.

In one embodiment of the invention, as shown in figures 13 and 14, the substrate 41 is arranged with at least one additional flexure 52 along said transverse axis outside a portion 417 of the substrate surrounding said at least one row is maintained in an arcuate shape. Said portion of the substrate is stretching at least 3 mm from the region surrounding the apertures 416. The additional flexure could have any direction including the same or an opposite direction to the flexure of said portion of the substrate. An additional flexure is defined by the existence of a discontinuity of the change of the radius of curvature at one point of the substrate outside, but in the vicinity of said portion. In fig 12 it is shown a substrate 41 having an additional flexure 52 having a generally opposite direction as the flexure inside the portion 420. In fig 13 it is shown a substrate 41 having an additional flexure 52 having generally the same direction as the flexure inside the portion 420

6. Embodiments of the printhead structure according to the invention

The printhead structure 4 (as shown in Fig. 11a and 1 lb) includes a sheet-like substrate 41 of electrically insulating, flexible polymere material, such as polyimide or the like, having a substantially rectangular shape with a transversal axis 410 along the Y-axis and a longitudinal axis 41 1 parallel to the X-axis. In a preferred embodiment, the substrate 41 is made of liquid crystal polymer, for example the liquid crystal polymer sold under the tradename BIAC CC.

In one embodiment of the invention the sheet-like substrate 41 is thinner than 75 μm. By making the substrate sufficiently thin it is possible to arrange the substrate so that it follows the curvature of a particle carrier while maintaining low contact forces and thereby reducing the tendency for the substrate to bend around the longitudinal axis which is an effect of the strain within the substrate caused by the bending of the substrate and the fact that the substrate has a finite length in the transverse direction, i.e. edge effects of the substrate.

In another embodiment the edge effects are reduced by providing a substrate having a bending stiffness per transverse length unit, EI/L of less than 5*10^"3 Nm, where El is the bending stiffness and L is the length in the transverse direction of the printhead structure within said portion, in a region 416 surrounding the apertures 42. The region surrounding 416 the apertures is defined as the area between two transverse lines 418, 419 that are in as close contact as possible with a row of apertures without touching the apertures or control electrodes 43 or deflection electrodes 46, 47 surrounding the apertures 42 . The lines are situated at opposite sides of the at least one row of apertures in the longitudinal direction.

By providing a substrate that is thinner than or by providing a substrate having a bending stiffness of less than 5*10^"3 Nm, it is provided a printhead structure that has a low bending stiffness allowing a deviation in position of any of said apertures from a straight line not to exceed 5 μm when said structure is arranged in conjunction with a particle carrier, which is claimed as a third embodiment of the invention.

The substrate furthermore includes a first surface adapted to face a particle carrier 21, a second surface adapted to face a background electrode roller 3, a front part 413 located upstream of the transversal axis 410 with respect to the rotation of a particle carrier 21 , when arranged in conjunction with such a particle carrier and a rear part 414 located downstream of the transversal axis 410 with respect to the rotation of a particle carrier 21, when arranged in conjunction with such a particle carrier.

The printhead structure 4 has a plurality of apertures 42 arranged through the substrate 41 and aligned in at least one transversal row, for example extending along the transversal axis 410. The first surface of the substrate is overlaid with a first printed circuit including a plurality of control electrode 43 each of which surrounds a corresponding aperture 42 in the substrate. The printhead structure furthermore includes a conductor matrix 44 including a first set of conductors 45 providing a connection to each control electrode 43. Furthermore the printhead could optionally include one or several sets of deflection electrodes 46, 47, that is one or several deflection electrodes for each aperture. The deflection electrodes 46, 47 deflects charged particles when passing through the apertures, thereby making is possible to use each aperture for producing an image dot at a multitude of positions. The deflection electrodes are connected to a connecting bar 48 by a second set of conductors 49 in the conductor matrix. The connecting bar is preferably thinner than 10 μm, and is preferable positioned at least 2 mm from the region 416 surrounding the apertures. In a preferred embodiment the conductors in the conductor matrix 44 are thinner than 10 μm. The connecting bar is arranged along the transverse axis.

In a preferred embodiment of the invention, the conductors 45, 49 in the conductor matrix 44 in a portion of the sheet-like substrate outside a region 416 surrounding the at least one row of apertures and in close range to said apertures are arranged along the longitudinal axis 41 1. The close range is defined as at least 3 mm from the edge of the region 416.

In a preferred embodiment all conductors in the conductor matrix have uniform width at any chosen cross-section along the transverse axis of the sheet like substrate. The width could vary along the longitudinal axis, but should vary in the same fashion for all conductors, thereby providing isotropic conditions of the bending stiffness along the transverse axis.

By arranging the conductors 45, 49 of the conductor matrix along the longitudinal axis, the connecting bar along the transverse axis, by making the conductors and connecting bar this and by arranging the connecting bar at a distance from the region 416 surrounding the apertures, the printhead structure given more isotropic properties in the region 416 surrounding the apertures and in the close range to said apertures. This reduces the variation of the bending stiffness of the printhead structure in the region surrounding the apertures and in a close range to the apertures, thereby providing a printhead structure has a even bending stiffness allowing two curvature radius at different cross-sections along said longitudinal axis through any of said apertures positioned in the same row, not to vary more than 1% when said structure is arranged in conjunction with a particle carrier.

In one embodiment of the invention, as shown in fig 12, the printhead circuit is coated with a first cover layer 50 of electrically insulating material. In a preferred embodiment of the invention the cover layer has thickness of less than 26 μm, which is equivalent to less than 1/1000 inch. In another preferred embodiment the insulation material is made of layer is made of Aluminium nitride or Silicon nitride. The layer of Aluminium nitride or Silicon nitride is made thinner than 5 μm.

In one embodiment of the invention the electrically insulation cover layer is coated with a semi-conducting layer 51 having a thickness of less than 5 μm. The semi-conducting layer is arranged for removing static electricity from the surface of the printhead structure, thereby preventing particles from adhering to the surface of the printhead structure. The semi- conducting layer has preferably a lower module of elasticity than the sheet-like substrate.

In one embodiment of the invention the conductors of the conductor matrix are electoplated with polyimid. By electroplating the conductors, the smallest possible amount of material is added to only the conductors, providing insulation. Thereby the insulation adds as little as possible to the bending stiffness.

According to a preferred embodiment of the present invention, the front and rear parts 413, 414 of the substrate 41 are at least partially laminated with a thin sheet of rigid material. A central portion 415 of the substrate 41 , located between the front part 413 and the rear part 414 remains unlaminated and thus flexible. The apertures 43 are arranged on the flexible central portion 415 of the subtrate 41. Accordingly, the substrate 41 has a variable flexibility along its longitudinal axis 41 1, such that the central portion 415 can be given an arcuate shape following the curvature of the particle carrier 2.

Claims

1 Printhead structure (4) comprising a sheet-like substrate (41) of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis (411) adapted to be aligned with a feed direction of said recording medium and a transverse axis (410) perpendicular to said longitudinal axis, a plurality of apertures (42) arranged through the substrate(41) in at least one row parallel to said transverse axis (410), a first and a second support region (413, 414) at opposite sides of said at least one row of apertures (42) adapted for supporting the printhead structure (4) in a position where a portion (420) of the substrate surrounding said at least one row is maintained in an arcuate shape at a distance from an outer surface of a particle carrier when arranged in conjunction with said particle carrier, control electrodes (43) arranged in conjunction with the apertures (42) and a conductor matrix (44) including a first set of conductors (45) arranged on said substrate providing a connection to each control electrode characterised in that the sheet-like substrate is thinner than 50 μm.

2 Printhead structure according to claim 1 characterised in that the printhead structure further comprises at least one set of deflection electrodes (46, 47) arranged in conjunction with the apertures (42), the conductor matrix (44) further includes a second set (49) of conductors providing connection to each deflection electrode (46, 47) and that each conductor in the second set of conductors are connected to a connecting bar (48).

3 Printhead structure according to claim 2 characterised in that said connecting bar (48) is positioned at least 1 mm in the longitudinal direction from the at least one row of apertures (42) and that said at least one connecting bar is arranged in parallel to said at least one row.

4 Printhead structure according to claims 2 or 3 characterised in that said connecting bar (48) is thinner than 8 μm. Printhead structure according to any of the preceding claims characterised in that the conductors in the conductor matrix (44) are thinner than 8 μm.

Printhead structure according to any of the preceding claims characterised in that the conductors in the conductor matrix in a portion of the sheet-like substrate outside a region 416 surrounding the at least one row of apertures (42) and in close range to said apertures are arranged along the longitudinal axis (411).

Printhead structure according to claim 6 wherein said portion of the sheet-like substrate (41) where the conductors in the conductor matrix (44) are arranged along the longitudinal axis stretches from the edge of said at least one row of apertures (42) to a position of at least 3 mm from said edge in the longitudinal direction (410).

Printhead structure according to any of the preceding claims characterised in that the sheet-like substrate (41 ) is made of a liquid crystal polymer.

Printhead structure according to any of the preceding claims characterised in that the sheet-like substrate (41) is covered by an insulation layer (50) having a thickness of less than 26 μm.

Printhead structure according to claim 9 characterised in that the insulation layer (50) has a lower module of elasticity than the sheet-like substrate.

Printhead structure according to claim 9 or 10 characterised in that the insulation layer (50) is made of one of the following materials: Aluminium oxide, Benzo cyklo butene, Strontium titanat or Silicon nitride.

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SUBSTITUTE GH ____ ^-'6) Printhead structure according to claim 11 characterised in that the insulation layer (50) is thinner than 5 μm.

Printhead structure according to any of the preceding claims characterised in that conductors in the conductor matrix (44) are covered with electroplated polyimid.

Printhead structure according to any of the preceding claims characterised in that the sheet-like substrate (41) is covered by a semi- conducting layer (51).

Printhead structure according to claim 14characterised in that the semi-conducting layer (51) is thinner than 5 μm.

Printhead structure according to claim 14 or 15 characterised in that the semi-conducting layer (51) has a lower module of elasticity than the sheetlike substrate.

Printhead structure according to any of the preceding claims characterised in that the printhead structure furthermore comprises an area with higher bending stiffness (421) positioned at an end region in the longitudinal direction of said printhead structure.

Printhead structure (4) comprising a sheet-like substrate (41) of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis (411) adapted to be aligned with a feed direction of said recording medium and a transverse axis (410) perpendicular to said longitudinal axis, a plurality of apertures (42) arranged through the substrate(41) in at least one row parallel to said transverse axis

(410), a first and a second support region (413, 414) at opposite sides of said at least one row of apertures (42) adapted for supporting the printhead structure (4) in a position where a portion (420) of the substrate surrounding said at least one row is maintained in an arcuate shape at a distance from an outer surface of a particle carrier when arranged in conjunction with said particle carrier, control electrodes (43) arranged in conjunction with the apertures (42) and a conductor matrix (44) including a first set of conductors (45) arranged on said substrate providing a connection to each control electrode c h a r a c t e r i s e d i n that the printhead structure has a even bending stiffness allowing two curvature radius at different cross-sections along said longitudinal axis through any of said apertures positioned in the same row, not to vary more than 1% when said structure is arranged in conjunction with a particle carrier.

Printhead structure (4) comprising a sheet-like substrate (41) of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis (41 1) adapted to be aligned with a feed direction of said recording medium and a transverse axis (410) perpendicular to said longitudinal axis, a plurality of apertures (42) arranged through the substrate(41) in at least one row parallel to said transverse axis (410), a first and a second support region (413, 414) at opposite sides of said at least one row of apertures (42) adapted for supporting the printhead structure (4) in a position where a portion (420) of the substrate surrounding said at least one row is maintained in an arcuate shape at a distance from an outer surface of a particle carrier when arranged in conjunction with said particle carrier, control electrodes (43) arranged in conjunction with the apertures (42) and a conductor matrix (44) including a first set of conductors (45) arranged on said substrate providing a connection to each control electrode c h a r a c t e r i s e d i n that the printhead structure has a low bending stiffness allowing a deviation in position of any of said apertures from a straight line not to exceed 50 μm when said structure is arranged in conjunction with a particle carrier.

Printhead structure (4) comprising a sheet-like substrate (41) of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis (411) adapted to be aligned with a feed direction of said recording medium and a transverse axis (410) perpendicular to said longitudinal axis, a plurality of apertures (42) arranged

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SUBSTITUTE SHEET RULE 25 through the substrate(41) in at least one row parallel to said transverse axis (410), a first and a second support region (413, 414) at opposite sides of said at least one row of apertures (42) adapted for supporting the printhead structure (4) in a position where a portion (420) of the substrate surrounding said at least one row is maintained in an arcuate shape at a distance from an outer surface of a particle carrier when arranged in conjunction with said particle carrier, control electrodes (43) arranged in conjunction with the apertures (42) and a conductor matrix (44) including a first set of conductors (45) arranged on said substrate providing a connection to each control electrode c h a r a c t e r i s e d i n that the printhead structure has a even bending stiffness allowing two curvature radius at different cross-sections through any of said apertures, positioned in the same row, along said longitudinal axis not to vary more than 1 % and has a low bending stiffness allowing a deviation in position of any of said apertures from a straight line not to exceed 50 μm when said structure is arranged in conjunction with a particle carrier.

Printhead structure (4) comprising a sheet-like substrate (41) of flexible and electrically insulating material, having a first surface adapted to face in a direction towards a toner carrier, a second surface adapted to face in a direction towards a recording medium, a longitudinal axis (41 1) adapted to be aligned with a feed direction of said recording medium and a transverse axis (410) perpendicular to said longitudinal axis, a plurality of apertures (42) arranged through the substrate(41) in at least one row parallel to said transverse axis (410), a first and a second support region (413, 414) at opposite sides of said at least one row of apertures (42) adapted for supporting the printhead structure (4) in a position where a portion (420) of the substrate surrounding said at least one row is maintained in an arcuate shape at a distance from an outer surface of a particle carrier when arranged in conjunction with said particle carrier, control electrodes (43) arranged in conjunction with the apertures (42) and a conductor matrix (44) including a first set of conductors (45) arranged on said substrate providing a connection to each control electrode c h a r a c t e r i s e d i n that the printhead structure has a bending stiffness per transverse length unit, EI/L, at a portion of the substrate surrounding said at least one row of apertures, of less than 5* 10^"3 Nm, where El is the bending stiffness and L is the length in the

19 ^" ' transverse direction of the printhead structure within said portion.

An image recording apparatus for forming an image on an image recording medium, said apparatus including:

(1) a particle delivery unit for conveying charged pigment particles to a position adjacent to the image recording medium, said particle unit including a particle carrier;

(2) a background electrode positioned in conjunction with the particle carrier;

(3) a background voltage source for producing a background electric field between said background electrode and said particle carrier, which background electric field enables a transport of charged pigment particles from the particle carrier toward the background electrode;

c h a r a c t e r i s e d i n that

the image recording apparatus further includes: (4) a printhead structure according to any of the preceding claims positioned in said background electric field between the toner carrier and the background electrode.

An image recording apparatus according to claim 22 c h a r a c t e r i s e d i n that the image recording apparatus further includes:

(5) variable voltage sources connected to said control electrodes to produce a pattern of electrostatic control fields influencing said background electric field in accordance with an image information for selectively permitting or restricting said transport of charged pigment particles through the apertures;

(6) a positioning device for positioning the printhead structure in relation to the particle carrier, said positioning device comprising a fastening element and a supporting element wherein:

20 Hr τ ^,πL^c 26) a first part which is fastened in a fixed position by means of said fastening element, and a second part which is supported by said supporting element in such a position, that a portion of the substrate surrounding said at least one row is maintained in an arcuate shape around a portion of an outer surface of the particle carrier.

An image recording apparatus according to claim 23 characterised in that the substrate is arranged with at least one additional flexure along said transverse axis outside said portion, said additional flexure stabilising the substrate against bending around the longitudinal axis.

An image recording apparatus according to claim 24 characterised in that the flexure outside said portion having a generally opposite direction as the flexure inside said portion creating said arcuate shape.