WO2001043975A1

WO2001043975A1 - Direct printing device

Info

Publication number: WO2001043975A1
Application number: PCT/EP1999/010019
Authority: WO
Inventors: Joakim Larvall
Original assignee: Array Ab; Matsushita Electric Industrial Co. Ltd.
Priority date: 1999-12-16
Filing date: 1999-12-16
Publication date: 2001-06-21
Also published as: AU2661100A; JP2003516886A

Abstract

The invention relates to a direct printing apparatus in which computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle carrier (33) toward a back electrode (12) through apertures in a printhead structure (5), whereby the charged particles are deposited on an image receiving substrate (1). In order to improve the toner flow through the apertures of the printhead, and thereby reduce the scattering and deposition of toner in and around the apertures, electrodes associated with each aperture are formed with a first surface having a larger surface area than the second surface. In this way the electrode end face directed towards the aperture serves as a funnel to channel the particles into or out of the apertures. Furthermore, the electric field pattern around the edges of the electrodes is rendered more uniform thus substantially eliminating the risk of electrical breakdown due to edge effects in the electric field. This reduces the amount of toner pulled towards the control electrode and also substantially limits the scattering of toner particles.

Description

Direct Printing Device

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 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

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 from computer generated signals. Such a device includes a printhead structure through which toner particles are selectively transported in accordance with image data. The printhead structure is generally constituted by an insulating substrate layer with a control electrode array formed on its surface directed towards a charged toner particle source. The substrate layer contains apertures and the control electrodes, which are typically ring-shaped, are associated with each aperture. The control electrodes are driven to control the 'electric field in the vicinity of each aperture, such that the apertures are selectively open or closed to the charged toner particles. Deflection electrodes that are likewise associated with each aperture are provided on the underside of the substrate layer. These deflection electrodes generate asymmetric electric fields 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) and enables each individual aperture to address several dot positions.

A problem with the structure of this known image forming apparatus, and more specifically with the printhead structure is the localised disruption in the electric field around the upper edges of each aperture, particularly in the vicinity of the control electrode edge. This disruption of the electric field disturbs the flow of toner particles, and can result in scattering and also excessive deposition of toner particles on the upper edge of the aperture, leading to frequent clogging of the printhead structure. A similar problem can occur around the aperture opening on the underside of the printhead structure .

Thus there is a need for a direct electrostatic image forming arrangement that provides an improved toner flow in the vicinity of the printhead structure.

Summary of the invention

In accordance with one aspect of the present invention, this object is achieved with a printhead structure for an electrostatic printing apparatus, the structure including a substrate layer having a plurality of apertures and at least one electrically conductive layer arranged on a first surface of the substrate layer, the electrically conductive layer including at least one electrode associated with each aperture, the electrodes having first and second surfaces with the second surface arranged on a surface of the substrate layer and end faces directed towards an associated aperture, wherein the first surface of each electrode has an area that is smaller than that of the second surface.

By providing the electrodes with a larger first surface than second surface, the flow of toner particles into, through and put of the apertures is improved. This is true both for control electrodes which are arranged on a surface of the substrate facing the toner source and deflection electrodes which are arranged on the underside of the substrate layer facing the back electrode. This is due in part to the shape of the electrode end face that serves as a funnel to channel the particles into and out of the apertures. In addition, the electric field pattern around the edges of the electrodes is rendered more uniform, which reduces the amount of toner pulled towards the control electrode and also substantially limits the scattering of toner particles.

Preferably, the angle between the first surface and the end face of each electrode is greater than 90° and most preferably at least 110°. This further contributes to an improved flow of toner particles into and out of the apertures by substantially eliminating the risk of electrical breakdown due to edge effects in the electric field. Moreover, when the angle between the second surface and the end face of each electrode is at most 70°, a highly favourable electric field profile around the mouth of the apertures results.

This is still further improved when the end face of each electrode is inwardly curved from the first surface, i.e. the surface directed away from the substrate layer, to the second surface of the electrode, i.e. the surface arranged on the substrate layer.

The flow behaviour of toner at each aperture is more easily controlled when each electrode substantially surrounds an associated aperture, and is still further improved when each electrode is substantially continuous around the associated aperture.

The invention further resides in an image forming apparatus, that incorporates the printhead structure described above. The image forming apparatus includes a particle carrier for holding a source of charged toner particles, a back electrode for generating a 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 member between the particle carrier and the back electrode for intercepting the transported charged particles, and the printhead structure as described above disposed between the particle carrier and the image receiving member transporting means.

In accordance with a further aspect of the invention, a method is proposed for fabricating a printhead structure for an image forming apparatus wherein charged toner particles arranged on a particle carrier are transported towards an image receiving member by a background electric field, and wherein the transport is modulated by the printhead structure . The method includes the steps of: providing a substrate layer, arranging an electrically conductive layer on the substrate layer to form a plurality of electrodes such that each electrode substantially surrounds an area of the substrate layer, providing masking means with holes having a size greater than the surrounded areas of the substrate layer but smaller than the electrodes and positioning the masking means above said electrically conductive layer with said holes substantially above the surrounded areas of the substrate layer, removing said substrate layer in the surrounded areas and at least a part of said electrically conductive layer on the upper edges of each electrode.

In this way, the advantageous electrode shaping having an first surface remote from the substrate layer with a smaller area than the second surface area arranged on the substrate layer can be achieved during fabrication of the printhead structure, and specifically in the same single step as the machining of the apertures .

Preferably, the laser energy is modified to obtain a selected target fluence at the area to be removed so as to shape electrode end face as required.

Brief description of the draw ngs

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 sectional view across a print station in an image forming apparatus, such as that shown in Fig.l,

Fig.3 is a schematic sectional 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 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 facing the intermediate transfer belt,

Fig.4c is a sectional 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, Fi gs 5a to 5c are detailed views of the printhead structure of

Fig 4c showing control electrode shapes at the edge of an aperture,

Fig 6a to 6e shows a sequence of steps for fabricating the printhead structure of Figs 4a, 4b, 4c and 5.

Detail d description

As shown m Fig.l, an image forming apparatus m accordance with a first embodiment of the present invention comprises at least one print station, preferably four print stations (Y, M, C, K) , an intermediate image receiving member 1, a driving roller 10, at least one support roller 11, and preferably several adjustable holding elements 12 The four print stations are arranged m relation to the intermediate image-receiving member 1 The image receiving member, preferably a transfer belt 1, is mounted over the driving roller 10 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 m an arcuated configuration so as to slightly bend the belt 1 at least m the vicinity of each print station m order to create a stabilisation force component on the belt m combination with the belt tension. That stabilisation force component is opposite m direction to, and preferably larger m magnitude than, an electrostatic attraction force component acting on the belt 1 due to interaction with the different electric potentials applied on the corresponding print station.

The 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 for the adjacent print stations.

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 m the order of 250°C The composite material of the transfer belt has preferably a homogeneous concentration of filler material, such as carbon or the like, which provides a uniform electrical conductivity throughout the entire surface of the transfer belt 1 The outer surface of the transfer belt 1 is preferably coated with a 5 to 30 microns thick coating layer made of electrically conductive polymer material having appropriate conductivity, thermal resistance, adhesion properties, release properties and surface smoothness

The transfer belt 1 is conveyed past the four different print stations, whereas toner particles are 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 molybdemum, maintained m contact with the inner surface of the transfer belt 1 As an electric current is passed through the heating element 15, the fixing holder 14 reaches a temperature required for melting the toner particles deposited on the outer surface of the transfer belt 1 The fusing unit 13 further includes a pressure roller 16 arranged transversally across the width of the transfer belt 1 and facing the fixing holder 14 An information carrier 2, such as a sheet of plain untreated paper or any other medium suitable for direct printing, is fed from a paper delivery unit 21 and conveyed between the pressure roller 16 and the transfer belt The pressure roller 16 rotates with applied pressure to the heated surface of the fixing holder 14 whereby the melted toner particles are fused on the information carrier 2 to form a permanent image After passage through the fusing unit 13, the transfer belt is brought m contact with a cleaning element 17, such as for example a replaceable scraper blade of fibrous material extending across the width of the transfer belt 1 for removing all untransferred toner particles from the outer surface

As shown m Fig.2, a print station m an image forming apparatus m accordance with the present invention includes a particle delivery unit 3 preferably having a replaceable or refillable container 30 for holding toner particles, the container 30 having front and back walls

(not shown) , a pair of side walls and a bottom wall having an elongated opening 31 extending from the front wall to the back wall and provided with a toner feeding element 32 disposed to continuously supply toner particles to a developer sleeve 33 through a particle charging member 34 The particle-charging member 34 is preferably formed of a supply brush or a roller made of, or coated with, a fibrous, resilient material The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 33 for charging particles by contact charge exchange due to triboelectrification of the toner particles through frictional interaction between the fibrous material on the supply brush and any suitable coating material of the developer sleeve The developer sleeve 33 is preferably made of metal coated with a conductive material, and preferably has a substantially cylindrical shape and a rotation axis extending parallel to the elongated opening 31 of the particle container 30 Charged toner particles are held on the surface of the developer sleeve 33 by electrostatic forces essentially proportional to (Q/D)² , where Q is the particle charge and D is the distance between the particle charge centre and the boundary of the developer sleeve 33 Alternatively, the charge unit may additionally include a charging voltage source (not shown), which supplies an electric field to induce or inject charge to the toner particles Although it is preferred to charge particles through contact charge exchange, the method can be performed using any other suitable charge unit, such as a conventional charge injection unit, a charge induction un t or a corona charging unit, without departing from the scope of the present invention

A metering element 35 is positioned proximate to the developer sleeve 33 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 33, to form a relatively thin, uniform particle layer thereon The metering element 35 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness The metering element 35 may also be connected to a metering voltage source (not shown) which influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve.

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

As shown in Fig.4a, 4b and 4c a printhead structure 5 in an image forming apparatus in accordance with the present invention comprises a substrate 50 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve 33, a second surface facing the transfer belt 1, a transversal axis 51 extending parallel to the rotation axis of the developer sleeve 33 across the whole print area, and a plurality of apertures 52 arranged through the substrate 50 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 501 (see Fig. 5) of electrically insulating material, such as parylene, for example. 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 semiconducting material, such as silicon oxide or the like, arranged on at least a part of the second cover layer 502, facing the transfer belt 1. The printhead structure 5 is 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 control 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.

The printhead structure 5 is preferably 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.

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541, 542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541, 542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the 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 appropriate deflection angle is chosen to arctan(l/3), i.e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has an upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source Dl (not shown) , and all downstream segments 542 being connected to a second deflection voltage source D2 (not shown)

The deflection voltage sources Dl and D2 are controlled by a control unit (not shown) . Three deflection sequences (for instance. D1<D2; D1=D2, D1>D2) can be performed m 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

Turning now to Figs. 5a, 5b and 5c, there are illustrated three embodiments of a printhead structure m accordance with the present invention These figures show a detailed sectional view of the printhead structure 5 with opposing sides of one aperture 52 and the opposing end faces of a control electrode 53. In all three embodiments the control electrode 53 has a lower surface 532 lying on the substrate 50 and an upper surface 531 covered by the first cover layer 501. The control electrode 53 also has an end face 533 directed towards the aperture 52. The deflection electrodes 54 are also illustrated The first and second cover layers 501 and 502 may also extend over the sides of the aperture 52 In this figure, the deflection electrodes are positioned directly below the control electrodes, however, it will be understood that the deflection electrodes may also be spaced slightly from the aperture edges This may have a beneficial effect on the flow of the toner particles exiting the aperture The end faces of the deflection electrodes 54 are substantially straight and parallel, such that the edges of deflection electrodes 54 facing the aperture 52 are essentially right-angled. In contrast, the end faces of the control electrodes 53 are sloped such that the lower edge of the control electrode 53 formed between the lower surface 532 and the end face 533 extends further towards the aperture than the upper edge formed between the upper surface 531 and the end face 533. As a result the area of the upper surface 531 of the control electrode 53 is larger than the area of the lower surface 532. This conical shape improves the toner flow at the aperture 52 and aids in substantially reducing toner scattering and deposition of toner particles on the aperture walls, which leads to clogging of the printhead structure. Moreover, as a direct result of the recessed or offset upper edge of the control electrode, the electric field pattern at the mouth of the aperture 52 is more homogenous. In particular, the effects of the local electric field density around the upper edge of the control electrode 53 is significantly reduced at the mouth of the aperture .

Turning specifically to Fig. 5a, the end faces 533 of the control electrodes are inwardly curved or concave from the upper surface 531 to the lower surface 532. As a result of this shaping, the lower edge formed between the lower surface 532 and the end face 533 forms an acute angle, while the upper edge formed between the upper electrode surface 531 and the end face 533 is at an angle that is at least 90°. Preferably, the lower edge of the control electrode 53 encloses an angle of 70° or less. Consequently, the end face 533 of the control electrode surrounding the aperture 52 forms a substantially conical funnel at the mouth of the aperture 52.

In the embodiment illustrated in Fig. 5b, the end face 533 of the control electrode 53 is inwardly curved or concave over part of its length so that the upper edge of the control electrode is substantially square, but preferably greater tl an 90°. However, in contrast to the embodiment of Fig. 5a, the lower portion of the end face 533 curves outwardly before terminating at an angle of at least 90° to the lower surface 532 of the control electrode With this embodiment also, the advantageous funnelling effect is obtained to substantially improve toner flow into the aperture 52 Moreover, the electric field pattern at the lower edge of the control electrode 53 is rendered more homogenous by the blunted or rounded lower edge of the electrode 53, so further reducing scattering of toner particles due to variations m electric field density through the aperture.

A preferred third embodiment of the invention is illustrated in Fig. 5c In this embodiment, the upper edge of the control electrode 53 formed between the upper surface 531 and the end face 533 is blunted or rounded, such that the end face 533 falls off from the upper surface 531 at an angle that is substantially greater than 90°. The remaining, lower, portion of the end face 533 is substantially straight, and the lower edge of the control electrode encloses an acute angle that is preferably no greater than 75° In addition to providing the improved electric field pattern and funnelling effect of the previous embodiments shown in Figs. 5a and 5b, the roundmg-off of the control electrode 53 additionally substantially reduces the risk of electrical breakdown at the upper control electrode edge

As a result of the funnel-shaped control electrode common to the embodiments of Figs. 5a to 5c, the flow of charged toner particles into and through the aperture is greatly improved and the risk of clogging due to excessive deposition of toner on and around the aperture is substantially reduced.

The manufacture of the three embodiments of the printhead structure shown m Figs. 5a to 5c will now be described with reference to Figs. 6a to 6e.

The fabrication process starts with the provision of a substrate layer 50, which is preferably a flexible sheet of polyimide having a thickness of the order of about 50 microns. A layer of conductive material is then deposited on the upper and lower surfaces of the substrate 50 and etched into the required configuration to form the first and second printed circuits with control electrodes 53 and deflection electrodes 54, resulting m the structure shown in Fig. 6b. The conductive material is preferably copper deposited to a thickness of approximately 8-10 microns. The structure is then positioned below a mask 60 which has holes arranged at predefined intervals corresponding approximately to the required positions of the apertures 52 as shown m Fig 6c The mask 60 and the substrate layer 50 are arranged such that the holes m the mask are preferably centred over the areas of the substrate layer 50 surrounded by the control electrodes 53, however the positioning does not have to be precise. This mask 60 is preferably made of metal, for example stainless steel The substrate layer 50 and the mask 60 are preferably arranged in a suitable j lg to prevent relative movement during the machining process. The holes in the mask 60 are substantially circular in shape and of a slightly larger diameter then the required aperture diameters, such that they also cover part of the control electrodes 53. However, the mask holes do not extend outwardly beyond the area covered by the control electrodes 53, so that only the areas of the substrate layer 50 that are contained within the control electrodes 53 are exposed by the mask 60 The apertures 52 are then formed m the substrate layer 50 by laser micromachimng, specifically by directing the laser beam through the holes in the mask 60 as indicated by the dotted lines in Fig. 6c. Preferably the laser is an excimer laser As the laser energy is targeted on the substrate layer 50 enclosed by the control electrodes 53 the polyimide will be progressively ablated resulting in an aperture 52. Since the laser beam is targeted on an area that is larger than the exposed substrate portion 50, the mask 60 is not used to shape the apertures 52. Instead, the control electrodes 53 themselves serve as integrated precision masks. The size of the apertures 52 obtained with the laser machining step is thus defined by the internal diameter of the control electrodes 53, since the control electrode material protects the underlying substrate layer 50 from vaporisation by the laser. The apertures 52 preferably have a circular shape with a diameter in a range of 80 to 120 microns. The finished apertures 52 are shown in Fig. 6d. At the same time, the upper surface 531 of the control electrode 53 immediately adjacent the aperture 52 is also ablated by the laser.

The amount of control electrode 53 that is ablated depends on the energy density or fluence generated by the laser at the target. If the target fluence is too low, the shape of the aperture 52 will be distorted. If the apertures 52 are not circular as a result of this distortion, the flow of toner particles into and out of the apertures 52 will be non-uniform, resulting in a degradation of print quality. If the target fluence is too high, the control electrode will be ablated fully in the exposed zone defined by the mask 60. In accordance with the present invention, the laser energy is adjusted to remove only part of the thickness of the control electrode 53 to obtain an electrode 53 with a larger surface area remote from the substrate layer 50 than that arranged on the substrate layer. In accordance with a preferred embodiment of the invention a laser target fluence of between 1 J/cm² and 2 J/cm² is utilised to form apertures with the desired circular periphery and the ablation of the control electrode material from the upper surface 531 and end face 533, and specifically from the upper edge of the control electrode 53 to form a blunt or rounded upper edge as in the preferred control electrode shape illustrated in Fig. 5c. The control electrode shape illustrated in the embodiment of Fig. 5b is obtained by using a higher laser target fluence, specifically between 2 J/cm² and 4 J/cm². Finally, the control electrode shape of the embodiment shown m Fig. 5a is obtained by using a target fluence of between about 4 J/cm² and less than 7 J/cm².

The structure is then preferably cleaned to remove any deposits of waste material created during the laser machining process using a procedure causing low mechanical impact, such as plasma cleaning. The fabrication process is then completed by the deposition of the first and second cover layers 501, 502 as shown m Fig. 6e. The first and second insulating cover layers 501, 502 are 5 to 10 microns thick parylene or polyimide laminated onto the substrate 50 using vacuum deposition techniques .

Although the apertures 52 below the control electrodes 53 preferably have substantially parallel walls along their central axis it may be advantageous m some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures .

While m the fabrication process described above, the control electrodes 53 serve as masks for the apertures 52, it will be appreciated that the mask 60 may be dimensioned to provide the desired aperture size. A laser target fluence may then be utilised to ablate both the substrate layer 50 and the control electrodes 53 in the exposed areas below the mask. In order to ablate both the substrate layer 50 and the control electrode film 53, a target fluence of at least about 7 J/cm² would be used. The final shape of the control electrode 53 would have a form similar to that shown in Fig. 5a.

The invention has been described with specific reference to the shape of the control electrodes. However, the it will be understood that the improved electric field pattern around the ap _rture openings may also be obtained by applying this advantageous shaping to the deflection electrodes 54 on the underside of the substrate layer 50, or indeed on any other electrode structure associated with the apertures, for example shield electrodes, to substantially improve the toner flow both into and out of the printhead apertures 52.

Claims

What is claimed is:

5 1. A printhead structure for an electrostatic printing apparatus, said structure (5) including a substrate layer (50) having a plurality of apertures (52) and at least one electrically conductive layer arranged on a surface of said substrate layer (50) , said electrically 0 conductive layer including at least one electrode (53, 54) associated with each aperture (52), the electrodes (53, 54) having first and second surfaces (531, 532) with the second surfaces (532) arranged on said surface of said substrate layer (50) and end faces 5 (533) directed towards an associated aperture (52) , characterised in that, the first surface (531) of each electrode (53, 54) has a smaller area than that of the second surface (532) .

0 2. A printhead structure as claimed m claim 1, characterised in that the angle between the first surface (531) and the end face (533) of each electrode (53, 54) is at least 90° .

5 3. A structure as claimed m claim 1 or 2, characterised in that the angle between the first surface (531) and the end face (533) of each electrode (53, 54) is at least 110° .

0 4. A structure as claimed m any previous claim, characterised m that the edge formed between the first surface (531) and the end face (533) of each electrode (53, 54) is rounded

_^ 5. A structure as claimed m any one of claims 1 to 3 , characterised in that the angle between the second surface (532) and at least part of the end face (533) of each electrode (53, 54) is at most 70°

6. A structure as claimed in any previous claim, characterised in that the end face (533) of each electrode (53, 54) is inwardly curved at least in an area adjacent the first surface (531) of the electrode.

7. A structure as claimed in any one of claims 1 to 6 , characterised in that the end face (533) of each electrode (53, 54) is substantially straight at least toward the second surface (532) of the electrode.

8. A structure as claimed in any previous claim, characterised in that each electrode (53, 54) substantially surrounds an associated aperture.

9. A structure as claimed m claim 8, characterised in that each electrode (53) is substantially continuous around the associated aperture (52) .

10. A structure as claimed in any previous claim, characterised in that an electrically conductive layer is arranged on both surfaces of said substrate layer (50), wherein each electrically conductive layer includes at least one electrode.

11. 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 member (1) between said particle carrier (33) and said back electrode (12) for intercepting the transported charged particles, and a printhead structure (5) as claimed in any one of claims 1 to 7 disposed between said particle carrier (33) and said image receiving member transporting means.

12. An apparatus as claimed in claim 8, characterised by voltage sources connected with said electrodes (53, 54) for applying potentials to said electrodes to control the transport of charged toner particles through said apertures (52 ) .

13. A method of fabricating a printhead structure for an image forming apparatus wherein charged toner particles arranged on a particle carrier (33) are transported towards an image receiving member (1) by a background electric field, and wherein said transport is modulated by said printhead structure (5), the method including, providing a substrate layer (50) , arranging an electrically conductive layer on said substrate layer to form a plurality of control electrodes (53) such that each control electrode (53) substantially surrounds an area of said substrate layer (50) , characterised by providing masking means (60) with holes having a size greater than the surrounded areas of said substrate layer (50) but smaller than said control electrodes (53) and positioning said masking means (60) above said electrically conductive layer with said holes substantially above the surrounded areas of said substrate layer (50), removing said substrate layer in said surrounded areas and at least a part of said electrically conductive layer on the first edges of each control electrode (53) .

14. A method as claimed in claim 13, characterised by removing said substrate layer (50) and at least part of said electrically conductive layer (53) with a laser.

15. A method as claimed in claim 14, characterised by utilising a laser fluence of less than 7 J/cm².

16. A method as claimed in claim 14 or 15, characterised by using a laser fluence of between 1 J/cm² and 2 J/cm²

17. A method as clained in any one of claims 13 to 16, characterised in that said masking layer (60) is a metal.