WO2002042081A1

WO2002042081A1 - Direct printing device with tapered aperture

Info

Publication number: WO2002042081A1
Application number: PCT/EP2000/011749
Authority: WO
Inventors: Anders Berg Palmqvist
Original assignee: Matsushita Electric Industrial Co Ltd; Array AB
Current assignee: Panasonic Holdings Corp; Array AB
Priority date: 2000-11-24
Filing date: 2000-11-24
Publication date: 2002-05-30
Anticipated expiration: 2003-05-24
Also published as: AU2001218598A1

Abstract

In an electrostatic printing apparatus electrically charged toner particles are transported from a particle carrier (33) toward a back electrode (12) through apertures in a printhead structure (5) by electric fields. An image receiving surface is arranged over the back electrode. The printhead structure has a substrate layer (50) and first (501) and second (502) insulating layers covering opposite sides of the substrate layer. Control (53) and deflection electrodes (54) are associated with the apertures, arranged on opposing sides of the substrate and covered by the first and second insulating layers, respectively. Each aperture is arranged with side walls in the second insulating layer of a length and orientation such that toner particles passing through said aperture and deflected by an electric field generated by said deflection electrodes are reflected off the side walls to follow a trajectory towards the back electrode that is substantially perpendicular to the image receiving surface. This confines the deflection process to within the aperture and renders the apparatus substantially insensitive to variations in the distance between the printhead and the image receiving surface. The aperture is tapered and widens towards the back electrode.

Description

DIRECT PRINTING DEVICE WITH TAPERED APERTURE

Direct Printing Device

5 Technical Field

The invention relates generally to direct printing apparatus. More particularly the invention is directed to a printing apparatus wherein a computer generated image is converted into a pattern of electrostatic fields, which selectively transport electrically L 0 charged particles from a particle source through a printhead structure toward a back electrode, and wherein the charged particles are deposited in image configuration on an image receiving substrate.

Background

L5

US patent No. 5 847 733 describes a direct electrostatic printing device and a method of generating text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure through which toner particles are selectively transported in

-0 accordance with image data. 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 which are controlled to selectively generate asymmetric electric fields

- 5 around the apertures, 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.

- 0 In prior art arrangements, 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. The image may be received by a belt which then transfers the image to paper; alternatively the toner particles are projected directly onto paper. However, in both cases the toner particles impact on a moving medium. Wear on the mechanism used to move the medium and also variations in the paper thickness mean that some variation in the distance between the aperture opening and the image receiving medium is inevitable. These 5 variations manifest themselves as non-uniform dot deflection over a number of lines in the image. The print quality is thus seriously degraded.

Thus there is a need for a direct electrostatic image forming arrangement that provides an improved print quality by ensuring that the deflection of toner particles obtained on L 0 an image receiving medium is substantially uniform.

Summary of the invention

The above need is met by an image forming apparatus, having a particle carrier for holding a source of charged toner particles, a back electrode for generating a

L5 background electric field for accelerating the transport of charged toner particles from the particle carrier towards the back electrode, means for transporting an image receiving surface between the particle carrier and the back electrode for intercepting the transported charged particles, and a printhead structure disposed between the particle carrier and the back electrode. The printhead structure includes a substrate

_ 0 layer having a first surface directed towards the particle carrier and a second surface directed towards the back electrode, a set of control electrodes arranged on the first substrate surface, a set of deflection electrodes arranged on the second substrate surface, and a plurality of apertures for permitting passage of charged toner particles. The control and deflection electrodes are associated with the apertures, whereby the

- 5 control electrodes alter the electric field about each aperture to selectively permit or prevent entry of the toner particles into the apertures, while the deflection electrodes alter the electric field asymmetrically to deflect toner particles leaving the apertures. Moreover, each aperture has side walls towards the second substrate surface of a length and orientation such that toner particles that pass through the aperture and are i 0 deflected by an electric field generated by the deflection electrodes are reflected off said side walls to follow a trajectory towards the back electrode that is substantially perpendicular to the image receiving surface.

By using the aperture or through hole to reflect the toner particles and subsequently cause the particles to follow a trajectory that is substantially normal to the image receiving surface the apparatus is rendered essentially insensitive to changes in the distance between the printhead and the paper or other image transfer medium. The toner particles are subjected to a deflecting electric field while still contained in the aperture. Indeed this deflection process is confined to within the aperture. Limiting the required range of the electric field also allows lower voltages to be used than conventionally employed.

In a preferred embodiment, a first insulating layer is provided on the control electrodes and first substrate surface and a second insulating layer is provided on the deflection electrodes and second substrate surface. Advantageously, the second insulating layer is thicker than said first insulating layer and/or said substrate layer at least in the vicinity of said apertures. This has the effect of extending the aperture beyond the deflection electrodes on one side of the printhead structure where the toner guidance is required. The thickness of this second insulating layer is at least 25μm, preferably at least 50μm and most preferably at least lOOμm.

In order to obtain sufficient deflection between dots printed through the same aperture and at the same time accurately control the amount of toner passed through the aperture at any one time, the aperture is wider in the second insulating layer than in the first insulating layer. The side walls of the aperture further slope outwardly substantially continuously at least in the second insulating layer. The aperture opening in the second insulating layer may be substantially circular. Preferably however the aperture opening in the second substrate layer is substantially elongate in the direction of an axis of deflection defined by the deflection electrodes.

The apparatus also includes control means for controlling voltages applied to the control and deflection electrodes for generating electric fields about said apertures. Preferably the control means are arranged to control the electrodes associated with each aperture to deflect a first stream of toner particles in a first direction and to deflect a second stream of particles in a second direction opposite the first direction to obtain two separate dots on said image receiving member. In this way two dots are printed via each aperture. Alternatively, three dots may be printed from each aperture. To this end the control means may be arranged to deflect a first stream of toner particles in a first direction, to pass a second stream of toner particles through the aperture undeflected and to deflect a third stream of toner particles in a second direction opposite the first direction to obtain three separate dots on said image receiving member.

It will be appreciated that more than three dots may also be printed via each aperture by controlling the degree of deflection applied to each toner stream.

The invention further resides in a printhead structure as defined in the claims.

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 the figures

Fig.l is a schematic view of an image forming apparatus in accordance with a preferred embodiment of the 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 a detail of the sectional view of the printhead structure shown in Fig. 4c according to a first embodiment of the present invention,

Fig. 6 is a detail of the sectional view of the printhead structure shown in Fig. 4c according to a second embodiment of the present invention, and

Fig. 7 shows an enlarged view of the printhead structure shown in Fig. 6.

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, preferably four print stations (Y, M, C, K), an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12. The four print stations are arranged 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 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. These elements 12 thus serve as back electrodes.

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 belt. The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image. After passage through the fusing unit 13, the transfer belt is brought 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 toner sleeve or carrier 33 through a particle charging member 34. The particle- charging member 34 is preferably formed of a supply brush or a roller made of, or coated with, a fibrous, resilient material. The supply brush is brought into mechanical contact with the peripheral surface of the toner carrier 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the toner carrier. The toner carrier 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30. Charged toner particles are held on the surface of the toner carrier 33 by electrostatic forces essentially proportional to (Q/D)² , where Q is the particle charge and D is the distance between the particle charge centre and the boundary of the toner carrier 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 toner carrier 33 to adjust the concentration of toner particles on the peripheral surface of the toner carrier 33, to form a relatively thin, uniform particle layer thereon. The metering element 35 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 35 may also be connected to a metering voltage source (not shown) which influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the toner carrier.

As shown in Fig.3, the toner carrier 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 toner carrier 33.

The positioning device 40 is formed of a frame 41 having a front portion, a back portion and two transversally extending side rulers 42, 43 disposed on each side of the toner carrier 33 parallel with the rotation axis thereof. The first side ruler 42, positioned at an upstream side of the toner carrier 33 with respect to its rotation direction, is provided with fastening means 44 to secure the printhead structure 5 along a transversal fastening axis extending across the entire width of the printhead structure 5. The second side ruler 43, positioned at a downstream side of the toner carrier 33, is provided with a support element 45, or pivot, for supporting the printhead structure 5 in a predetermined position with respect to the peripheral surface of the toner carrier 33. The support element 45 and the fastening axis are so positioned 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 toner carrier 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 toner carrier 33. That is, the support element 45 is arranged to make the printhead structure 5 pivotal about a fixed point to ensure that the distance between the printhead structure 5 and the peripheral surface of the toner carrier 33 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the toner carrier 33. The front and back portions oi 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 toner carrier 33 and a transversal axis of the printhead structure 5. Preferably, the securing members 46 are arranged at the front and back ends of the toner carrier 33 to accurately space the toner carrier 33 from the corresponding holding element 12 of the transfer belt 1 facing the actual print station.

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 toner carrier 33, a second surface facing the transfer belt 1, a transversal axis 51 extending parallel to the rotation axis of the toner carrier 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 501 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 53 disposed 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 anti-static material (not shown), preferably a semi-conducting 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 coupled to 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. In some embodiments, the control unit may even include 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.

In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness of the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thick deposited and etched on the first and second surface of the substrate 50, respectively, using conventional 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.

In a preferred embodiment of the present invention, the printhead structure 5 is dimensioned to perform 600 dpi printing utilising 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.

5 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

L 0 the centre 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 neutralise 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

.5 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

!0 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 5 controlled by a control unit (not shown). 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.

0 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 toner carrier 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.

Referring now to Fig. 5, an enlarged view of the printhead structure 5 about a single aperture 52 cut through the line II-II of Fig. 4b is shown. From Fig. 5 it can be seen that the second insulating layer 502 covering the deflection electrodes is thicker than the first insulating layer 501. This second layer 502 is preferably also deeper than the substrate layer 50. It has a thickness of between 25μm and lOOμm, and is preferably around 50μm thick. The portion of the apertures 52 extending through the first insulating layer 501 and the substrate 50 is preferably of a circular or elongated shape centred 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 maj or diameter of about 120 microns .

The side walls of the aperture 52 in the second insulating layer 502 actually serve as a guide for the deflected toner particles. The diameter of the aperture in the direction or axis of deflection, that is transverse to the axis 543 shown in Fig. 4b, determines the deflection position on the image receiving medium 1. The aperture 52 may be formed with an elongate opening in the second insulating layer 502 that is longer parallel to this deflection axis than transverse to the deflection axis. The thickness of the second insulating layer 502 and the slope of the aperture side walls are selected to obtain the desired deflection distance.

As is apparent from Fig. 5 the aperture 52 slopes outwardly from the first insulating layer 501 to the second insulating layer 502. The smaller aperture opening in the first insulating layer 501 is thus directed to the toner carrier 33, while the larger opening is directed towards the transfer belt 2 or other image receiving medium. A stream of charged toner particles released from the toner carrier 33 on application of a control voltage to the control electrodes 53 is illustrated by a dashed arrow 60. Three possible toner paths 61, 62, 63 through the aperture are also illustrated. These correspond to toner trajectories subjected to different deflecting electric fields generated by the deflection electrodes 54. Toner particles follow a first path 61 when a higher repelling voltage is applied to the deflection electrode 54 located on the right-hand side of the aperture in Fig. 5 than to the left-hand illustrated deflection electrode 54. Toner particles are deflected within the aperture 52 away from the repelling electrode 54 until they impact with the side wall of the aperture 52. They are then reflected downwardly substantially normally towards the image receiving member 2. Toner particles follow the centre path to impact as a dot on the image receiving member substantially normally when the deflection electrodes are fed with no voltages or with a symmetrical voltage. Finally the third path 63 is followed when a deflection voltage is applied to the deflection electrodes 54 that is substantially equal and opposite to that applied for the first path 61.

The aperture side walls limit and determine the deflection distance, but also ensure that the deflection applied to a stream of toner particles is essentially unaffected by a change in distance between the printhead structure 5 and the image receiving member 2.

It will be appreciated that a single aperture may be used to print at least three separate dots in each printing pass using the three trajectory paths 61, 62, 63 shown in Fig. 5. Alternatively, each aperture may be used to print at most two dots. In this latter case only the two deflection paths 61 and 63 are used to maintain uniform spacing between dots.

In a further embodiment of the invention, several degrees of deflection can be obtained through each aperture. This is illustrated in Figs. 6 and 7. In this embodiment, the aperture 52 has side walls that are sloped outwardly from the first insulating layer 501 through to the second insulating layer 502, however the angle of slope is not the same at different points through the aperture 52. With reference to the enlarged view of Fig. 7, a first portion of the aperture within the second insulating layer 502 close to the substrate layer 50 is sloped at an angle of θi to the normal, while a second portion of the aperture wall extending between this first portion and the opening in the second insulating layer 502 is sloped at an angle of θ₂ to the normal. This shaping of the aperture walls can be obtained using conventional laser micromachining techniques.

5

By applying different intensities of deflection voltages to the deflection electrodes 54 the toner stream can be made to impact on different portions of the aperture wall, so resulting in a lesser or greater deflection of the printed toner dot. This is illustrated in Fig. 6, where a smaller deflection is obtained by applying higher deflection voltages

LO to the deflection electrodes 54. The toner particles illustrated by the arrow 611 and

631 thus impact the aperture walls higher up the aperture where the aperture diameter is smaller, so resulting in a smaller deflection. A lower deflection voltage causes the toner particles to impact with the aperture walls at a lower location, where the slope angle is smaller as shown by the paths 61 and 63. This results in a greater deflection.

L5

The slope of the aperture walls illustrated in Figs. 6 and 7 is not continuous, however, it may not be necessary to provide different angles of slope at different locations along the aperture. The angle is selected such that toner particles impacting on the aperture wall will be reflected substantially vertically downwards towards the image

-0 receiving member 2 and impact with this member substantially normally. This naturally depends on the angle of incidence of the toner particles on the walls. The deflection caused by the deflection electrodes 54 is not straight, but the degree of curvature of the toner path prior to hitting the aperture wall will depend in part on the position and shape of the deflection electrodes 54. It may therefore be possible to

-5 position the deflection electrodes 54 such that toner particles will be impact substantially normally on an image receiving member 2 without providing different aperture slope angles.

50

Claims

What is claimed is:

5 1. A printhead structure for an electrostatic printing apparatus, said structure (5) including a substrate layer (50) with opposing first and second surfaces and having a first set of electrodes (53) disposed on said first surface, a set of deflection electrodes (54) arranged on said second surface of said substrate layer and a plurality of apertures (52) for permitting the passage of charged 0 toner particles, wherein said first set of electrodes are arranged to control an electric field around said apertures to permit or prevent entry of said toner particles and said deflection electrodes are arranged generate an asymmetric electric field around said apertures to deflect toner particles leaving said aperture, characterised in that 5 each aperture (52) has side walls towards the second substrate surface that are arranged to reflect charged toner particles that are deflected by said deflection electrodes at an angle that is substantially normal to said first substrate surface.

2. A printhead structure as claimed in claim 1, characterised in that the aperture

10 is wider towards said second substrate surface than towards said first substrate surface.

3. A printhead structure as claimed in claim 1 or 2, characterised in that the side walls of said aperture slope outwardly substantially continuously at least 5 towards said second substrate surface.

4. A printhead structure as claimed in any previous claim , characterised by a first insulating layer arranged to encapsulate said first set of electrodes on said first substrate surface and a second insulating layer arranged to encapsulate i 0 said second set of electrodes on said second substrate surface, wherein said second insulating layer is thicker than said first insulating layer and/or said substrate layer at least in the vicinity of said apertures (52).

5. A printhead structure as claimed in claim 4, characterised in that said deflection electrode define an axis of deflection about each aperture and said second insulating layer is substantially elongate in the direction of said axis at least around the opening of each aperture (52).

6. An image forming apparatus, including a particle carrier (33) for holding a source of charged toner particles, a back electrode (12) for generating a background electric field for accelerating the transport of charged toner particles from said particle carrier towards said back electrode, means for transporting an image receiving surface (1 ) between said particle carrier (33) and said back electrode (12) for intercepting the transported charged particles, and a printhead structure (5) disposed between said particle carrier (33) and said back electrode (12), wherein said printhead structure includes a substrate layer (50) with opposing first and second surfaces and having a first set of electrodes

(53) disposed on said first surface, a set of deflection electrodes (54) arranged on said second surface of said substrate layer and a plurality of apertures (52) for permitting the passage of charged toner particles, wherein said first set of electrodes are arranged to control an electric field around said apertures to permit or prevent entry of said toner particles and said deflection electrodes are arranged generate an asymmetric electric field around said apertures to deflect toner particles leaving said aperture, characterised in that each aperture (52) has side walls towards the second substrate surface of a length and orientation such that toner particles passing through said aperture (52) and deflected by an electric field generated by said deflection electrodes are reflected off said side walls to follow a trajectory towards said back electrode (12) that is substantially perpendicular to the image receiving surface

(1).

7. An apparatus as claimed in claim 6, characterised by a first insulating layer arranged to encapsulate said first set of electrodes on said first substrate surface and a second insulating layer arranged to encapsulate said second set of electrodes on said second substrate surface, wherein said second insulating layer is thicker than said first insulating layer and/or said substrate layer at least in the vicinity of said apertures (52).

8. An apparatus as claimed in claim 7, characterised in that said second insulating layer has a thickness of at least 25μm.

5

9. An apparatus as claimed in claim 7 or 8, characterised in that said second insulating layer has a thickness of at least 50μm.

10. An apparatus as claimed in any one of claims 7 to 9, characterised in that said .0 second insulating layer has a thickness of at least 1 OOμm.

11. An apparatus as claimed in any one of claims 6 to 10, characterised in that the aperture is wider towards said second substrate surface than towards said first substrate surface.

.5

12. An apparatus as claimed in any on of claims 7 to 11, characterised in that the side walls of said aperture slope outwardly substantially continuously at least in said second insulating layer.

! 0 13. An apparatus as claimed in any one of claims 7 to 12, characterised in that said deflection electrodes define an axis of deflection about each aperture and said second insulating layer is substantially elongate in the direction of said axis at least around the opening of each aperture (52).

¹5

14. An apparatus as claimed in any one of claims 6 to 13, characterised by control means for controlling voltages applied to said control and deflection electrodes for generating electric fields about said apertures, wherein for each aperture said control means are arranged to deflect a first stream of toner particles in a first direction and to deflect a second stream of particles in a second direction

0 opposite from said first direction to obtain two separate dots on said image receiving member.

15. An apparatus as claimed in any one of claims 6 to 13, characterised by control means for controlling voltages applied to said control and deflection electrodes for generating electric fields about said apertures, wherein for each aperture said control means are arranged to deflect a first stream of toner particles in a first direction and to pass a second stream of toner particles through said aperture undeflected and to deflect a third stream of toner particles in a second direction opposite from said first direction to obtain three separate dots on said image receiving member.