Background U. S Patent No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image receiving medium due to control in accordance with an image information.

A drawback is the printing requirements for printing color images may be different to the requirements for printing black and white images or other images in a single color.

Summary of the invention An object of the invention is to provide a apparatus by which the printing properties for one type of toner particles may be different to the printing properties for another type of toner particle.

This is achieved by an image forming apparatus in which image information is converted into a pattern of electrostatic fields for modulating a transport of charged toner particles from a particle carrier toward a back electrode member, said image forming apparatus including: a background voltage source for producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member; a plurality of printhead structures arranged in said background electric field, each including a plurality of apertures and control electrodes arranged in conjunction to the apertures; control voltage sources for supplying control potentials to said control electrodes in accordance with the image information to selectively permit or restrict the transport of charged toner particles from the particle carrier through the apertures; and an image receiving member for intercepting the transported charged particles in image configuration; the image forming member and/or the printhead structures being caused to move longitudinally in relation to each other; there being one or more printhead structures which are arranged to transport a first type of toner particles and one or more, preferably three, printhead structures which are arranged to transport a second type of toner particles; wherein the one or more printhead structures which transport the first type of toner particles are arranged to print faster than the one or more printhead structures which transport the second type of toner particles.

By printing an image faster for one type of toner particle than another type it is possible to provide in a single apparatus for different requirements regarding the printing properties of different types of toner particles.

In a preferred embodiment the apparatus is so constructed and arranged to operate that each aperture of the printhead structures which transport the first type of toner particles prints less dots to print a transverse line of print than each aperture of the printhead structures which transport the second type of toner particles. In this way the speed at which printhead structures which transport the first type of particle can be increased relative to the printhead structures which transport the second type of toner particles as they are required to transport less dots per unit time for a given printing speed.

Brief description of the drawings The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following drawings, wherein like reference numerals designate like parts throughout and where the dimensions in the drawings are not to scale, in which Fig. 1 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 side view of a first arrangement of the printhead structures in accordance with the invention.

Fig. 5A is a plan view of a first example of the arrangement of the apertures of the printhead structures of Fig. 5 taken in the direction of line V-V of Fig. 5.

Fig. 5B is a plan view of a second example of the arrangement of the apertures of the printhead structures of Fig. 5 taken in the direction of line V-V of Fig. 5.

Fig. 5C is a plan view of a third example of the arrangement of the apertures of the printhead structures of Fig. 5 taken in the direction of line V-V of Fig. 5.

Fig. 6 is a plan view of a further embodiment of the arrangement of the apertures of the printhead structures of Fig. 5 taken in the direction of line V-V of Fig. 5.

Fig. 7A shows the pattern of printing columns of print in an embodiment of the invention.

Fig. 7B shows the pattern of printing lines of print in another embodiment of the invention.

Fig. 7C shows the pattern of printing lines and columns of print in a further embodiment of the invention.

Fig. 8 is a side view of a second arrangement of the printhead structures in accordance with the invention.

Fig. 8A shows the pattern of printing columns of print in an embodiment of the invention using a printhead arrangement in accordance with Fig. 8.

Fig. 8B shows the pattern of printing columns of print in another embodiment of the invention using a printhead arrangement in accordance with Fig. 8.

Fig. 8C shows the pattern of printing columns of print in a further embodiment of the invention using a printhead arrangement in accordance with Fig. 8.

Fig. 8D shows the pattern of printing columns of print in yet a further embodiment of the invention using a printhead arrangement in accordance with Fig. 8.

Fig. 9A shows the pattern of printing columns of print in an embodiment of the invention using a printhead

arrangement in accordance with Fig. 8 printing at lower resolution.

Fig. 9B shows the pattern of printing columns of print in an embodiment of the invention using a printhead arrangement in accordance with Fig. 8 printing at lower resolution.

Fig. 10 is a side view of a third arrangement of the printhead structures in accordance with the invention.

Fig. lOA is a plan view of an example of the arrangement of the apertures of the printhead structures of Fig.

10 taken in the direction of line X-X of Fig. 10.

Detailed description To perform a direct electrostatic printing method in accordance with the present invention, a background electric field is produced between a particle carrier and d a back electrode to enable a transport of charged particles therebetween. A printhead structure, such as an electrode matrix provided with a plurality of selectable apertures, is interposed in the background electric field between the particle carrier and the back electrode and connected to a control unit which converts the image information into a pattern of electrostatic fields which, due to control in accordance with the image information, selectively open or close passages in the electrode matrix to permit or restrict the transport of charged particles from the particle carrier. The modulated stream of charged particles allowed to pass through the opened apertures are thus exposed to the background electric field and propelled toward the back electrode. The charged particles are deposited on the image receiving substrate to provide line-by line scan printing to form a visible image.

A printhead structure for use in direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged in conjunction with the apertures. Generally, a printhead structure includes a flexible substrate of insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is coated with an insulating layer and control electrodes are arranged between the first surface of the substrate and the insulating layer, in a configuration such that each control electrode surrounds a corresponding aperture. The apertures are preferably aligned in one or several rows extending transversally across the width of the substrate, i. e. perpendicular to the motion direction of the image receiving substrate.

According to such a method, each single aperture is utilized to address a specific dot position of the image in a transversal direction. Thus the transversal print addressability is limited by the density of apertures through the printhead structure. For instance, a print addressability of 300 dpi requires a printhead structure having 300 apertures per inch in a transversal direction.

According to a preferred embodiment of the present invention, a direct electrostatic printing device includes a dot deflection control (DDC). According to that embodiment, each single aperture is used to address several dot positions on an image receiving substrate by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving substrate, and thereby the

location of the obtained dot. The DDC method increases the print addressability without requiring a larger number of apertures in the printhead structure. This is achieved by providing the printhead structure with deflection electrodes connected to variable deflection voltages which, during each print cycle, sequentially modify the symmetry of the electrostatic control fields to deflect the modulated stream of toner particles in predetermined deflection directions. For instance, a DDC method performing three deflection steps per print cycle, provides a print addressability of 600 dpi utilizing a printhead structure having only 200 apertures per inch.

According to a preferred embodiment, an improved DDC method provides a simultaneous dot size and dot position control. This method utilizes the deflection electrodes to influence the convergence of the modulated stream of toner particles thus controlling the dot size. Each aperture is surrounded by two deflection electrodes connected to respective deflection voltages Dl, D2, such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages D1, D2 have the same amplitude.

The amplitude of D1 and D2 are modulated to apply converging forces on toner particles as they are transported toward the image receiving medium, thus providing smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between D1 and D2 to deflect the toner trajectory toward predetermined dot positions.

A printhead structure for use in DDC methods generally includes a flexible substrate of electrically insulating material such as polyimide or the like, having a first surface facing the particle carrier, a second surface facing the back electrode and a plurality of apertures arranged through the substrate. The first surface is

overlaid with a first printed circuit including the control electrodes and the second surface is overlaid with a second printed circuit including the deflection electrodes. Both printed circuits are coated with insulative layers. Utilizing such a method, 60 micrometer dots can be obtained with apertures having a diameter in the order of 160 micrometer.

In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with accompanying drawings.

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 gap distance from each print station. The holding elements 12 are preferably cylindrical sleeves

disposed perpendicularly to the belt motion in an arcuated configuration so as to slightly bend the belt 1 at least in the vicinity of each print station in order to, in combination with the belt tension, create a stabilization force component on the belt. That stabilization force component is opposite in direction 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 transfer belt 1 is preferably an endless band of 30 to 200 microns thick 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 color 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 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 to 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 center and the boundary of the toner carrier 33. Alternatively, the charge unit may additionally include a charging voltage source (not shown), which supply 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 influence 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 excentricity or any other undesired variations of the toner carrier 33. That is, the support element 45 is arranged to made 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 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 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 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. The securing members 46 are preferably dimensioned to provide and maintain a parallel relation between the rotation axis of the toner carrier 33 and a central transversal axis of the corresponding holding member 12.

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, a second surface facing the transfer belt, 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 antistatic material (not shown), preferably a semiconductive material, such as silicium oxide or the like, arranged on at least a part of the second cover layer 502, facing the transfer belt 1. The printhead structure 5 is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 53 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 52 during each print sequence.

The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 54 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 52.

The control unit, in some embodiments, even includes a shield voltage source (not shown) connected to the shield electrodes to supply a shield potential which electrostatically screens adjacent control electrodes 53 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 50 is a flexible sheet of polyimide having a thickness on the order of about 50 microns. The first and second printed circuits are copper circuits of approximately 8-9 microns thickness etched onto the first and second surface of the substrate 50, respectively, using conventional etching techniques. The first and second cover layers (501,502) are 5 to 10 microns thick parylene laminated onto the substrate 50 using vacuum deposition techniques. The apertures 52 are made through the printhead structure 5 using conventional laser

micromachining methods. The apertures 52 have preferably a circular or elongated shape centered about a central axis, with a diameter in a range of 80 to 120 microns, alternatively a transversal minor diameter of about 80 microns and a longitudinal major diameter of about 120 microns. Although the apertures 52 have preferably a constant shape along their central axis, for example cylindrical apertures, it may be advantageous in some embodiments to provide apertures whose shape varies continuously or stepwise along the central axis, for example conical apertures.

In a preferred embodiment of the present invention, the printhead structure 5 is dimensioned to perform 600 dpi printing utilizing three deflection sequences in each print cycle, i. e. three dot locations are addressable through each aperture 52 of the printhead structure during each print cycle. Accordingly, one aperture 52 is provided for every third dot location in a transverse direction, that is, 200 equally spaced apertures per inch aligned parallel to the transversal axis 51 of the printhead structure 5. The apertures 52 are generally aligned in one or several rows, preferably in two parallel rows each comprising 100 apertures per inch. Hence, the aperture pitch, i. e. the distance between the central axes of two neighbouring apertures of a same row is 0,01 inch or about 254 microns. The aperture rows are preferably positioned on each side of the transversal axis 51 of the printhead structure 5 and transversally shifted with respect to each other such that all apertures are equally spaced in a transverse direction. The distance between the aperture rows is preferably chosen to correspond to a whole number of dot locations.

It should be noted that each aperture serves to provide an access for the toner particles to be directed from the toner carrier to a desired location. The direction of the

toner particles is dictated by the prevailing electric fields. The apertures do not physically direct the particles in the sense that the shape of the borders of the apertures have no effect on the trajectory of the particles other than the effect that they may have on the electric fields. The toner particles pass in general via a central area of the apertures without touching the sides of the apertures. Thus the borders of the apertures provide the means for supporting the electrodes which provide the electric fields.

The first printed circuit comprises the control electrodes 53 each of which 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 shape for continuously or partly surrounding the apertures 52, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 52 are aligned in one single row, the control electrodes are advantageously made smaller in a transverse direction than in a longitudinal direction.

The second printed circuit comprises the plurality of deflection electrodes 54, each of which is divided into two semicircular or crescent shaped deflection segments 541,542 spaced around a predetermined portion of the circumference of a corresponding aperture 52. The deflection segments 541,542 are arranged symmetrically about the central axis of the aperture 52 on each side of a deflection axis 543 extending through the center of the aperture 52 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 543 is dimensioned in accordance with the number of deflection

sequences to be performed in each print cycle in order to neutralize the effects of the belt motion during the print cycle, to obtain transversally aligned dot positions on the transfer belt. For instance, when using three deflection sequences, an appropriate deflection angle is chosen to arctan (l/3), i. e. about 18,4°. Accordingly, the first dot is deflected slightly upstream with respect to the belt motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the belt motion, thereby obtaining a transversal alignment of the printed dots on the transfer belt. Accordingly, each deflection electrode 54 has a upstream segment 541 and a downstream segment 542, all upstream segments 541 being connected to a first deflection voltage source D1, and all downstream segments 542 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: D1<D2 ; D1=D2 ; D1>D2) can be performed in each print cycle, whereby the difference between D1 and D2 determines the deflection trajectory of the toner stream through each aperture 52, and thus the dot position on the toner image.

Since the apertures 52 and their surrounding areas will under some circumstances need to be cleaned from residual toner particles which agglomerate there, an image forming apparatus in accordance with the present invention preferably further includes a cleaning unit 6 which is used to prevent toner contamination. Due to undesired variations in the charge and mass distribution of the toner material, some of the toner particles released from the toner carrier 33 do not reach sufficient momentum during a print sequence to be deposited onto the transfer belt 1 and contribute to image formation. Some toner particles having a charge polarity opposite to the intended, so called wrong signed toner, may be repelled back to the printhead structure 5 after passage through the apertures under influence of the background field, and

adhere on the printhead structure 5 in the area surrounding the apertures 52. Some particles may be deviated during transport and agglomerate on the apertures walls, obstructing the aperture 52. Residual toner particles have to be removed periodically during an appropriate cleaning cycle, for example after a predetermined number of image formation cycles or due to control in accordance with a sensor measuring the amount of residual toner.

With respect to the description which follows reference is made to an image or printable area. In the present context an image is formed by the toner particles over an area of the transfer belt 1. The image also includes those printable areas that could receive toner particles but do not receive the particles because the information content of the image does not require this. Typically, an image covers approximately the area of an A4 sheet of paper, though possibly reduced by a small area around the margins that is not printed. The image may, for example, comprise a plurality of pictures or printed areas which would be printed on the same sheet of paper. Although reference is made to A4 paper this reference is not limiting as the image could be the size of A3 or A5 paper or other standard paper sizes or any other chosen size.

A column of printing is a longitudinal line of the transfer belt which is subject to printing of dots by an aperture or apertures even if not all the parts of the line receive dots due to the information content of the image being formed requiring some parts of the column to be left without dots. A transverse line of printing is a transverse line of the transfer belt which is subject to printing of dots from a plurality of apertures, even if not all the parts of the line receive dots due to the information content of the image being formed requiring some parts to be left without dots. The closest distance

between two adjacent columns or lines of print is defined as the pitch or the distance between two addressable pixel locations.

Where reference is made to the number of dots required to print a column or a line of print, the number of dots is that which is required to print a complete line with each possible addressable location receiving a dot. In use, to form a desired image some of the possible addressable locations do not receive a dot so that an image having the desired information content is formed.

The transverse direction is the direction which, in the case that the image receiving member is a drum, is perpendicular to a radial vector of the cylinder towards the printhead structure at the surface of the drum and parallel to the axis of rotation of the drum along the surface of the drum. In the case of a transfer belt it is the direction in the plane of the belt perpendicular to the direction of movement of the belt, the said movement being the movement required to allow the belt to move around the rollers 10,11. Thus, the transverse direction will normally be parallel to the axes of these rollers 10, 11. The longitudinal direction is the direction perpendicular to the transverse direction and in the plane of the surface of the image receiving member, i. e. transfer belt or drum. In the case of the drum the longitudinal direction is the direction perpendicular to the transverse direction and along the circumference so the drum. In the case of a transfer belt the longitudinal direction is the direction at any point on its surface in the direction perpendicular to the axis of rotation of the rollers and in the plane of the surface of the drum.

In the following, reference is made to printhead structures which are arranged to black toner particles.

Such printhead structures are hereinafter referred to as

black printhead structures. The expression black also includes shades which are near to black, in particular grey or dark grey. The image forming apparatus further comprises printhead structures for transporting colored toner particles. Such printhead structures are hereinafter referred to as color printhead structures. By colored is meant colors which are not comprised within the definition of black as given above. Multicolor printing requires the presence of two, three, four or more compatible colors so chosen that combinations of the amounts of these colors allow a larger range of colors than the individual colors to be printed. The color printhead structures therefore each print once on each transverse line of print so as to ensure that the line of print contains all the required colors.

A first embodiment of the invention is illustrated in Fig.

5 which shows a side view of the arrangement of the printhead structures and transfer belt. There is one black printhead structure 5 and three color printhead structures 5a, 5b, 5c. The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow.

The black printhead structure 5 is arranged to be able to print faster than the color printhead structures 5a, 5b, 5c. In a first alternative the black printhead structure is provided with more apertures printing each longitudinal column of print than the color printhead structures. This may be achieved by providing more apertures on a transverse row of apertures in the printhead structure. In a specific example shown in plan view in Fig. 5A which shows schematically the arrangement of the apertures on the printhead structures, the black printhead structure 5 contains twice as many apertures per row as the color printhead structures 5a, 5b, 5c. In a further example shown in plan view in Fig. 5B which shows schematically

the arrangement of the apertures on the printhead structures, the black printhead structure may contain three times as many apertures per row as the color printhead structures. In further examples there may be four times as many per row as the color printhead structures.

In examples where the number of apertures in the black printhead structure is greater than the number of apertures in the color printhead structures the extra apertures may be provided in the black printhead structure by reducing the spacing between the apertures. The spacing between the apertures in the black printhead structure will, in such cases, be less than the spacing between the apertures in the color printhead structures.

Alternatively, there may be more rows of apertures in the black printhead structure than in the color printhead structures. An example of this is illustrated in Fig. 5C which shows schematically the arrangement of the apertures on the printhead structures. The black printhead structure 5 contains the same number of apertures per row as the color printhead structures 5a, 5b, 5c, but contains twice as many rows as the color printhead structures.

The color printhead structures, and optionally the black printhead structures, may print with dot deflection control so that each aperture prints more than one dot on a transverse line of print. The apertures may each print two, three, four, five or more dots per transverse line.

The color printhead structures may have more dot deflection control, i. e. print more dots per aperture per transverse line, and have less apertures than the black printhead structures In a specific example each aperture in the color printhead structures may print three dots per aperture for each transverse line of print and each aperture in the black printhead structure prints one dot for each transverse line of print. There are three times

as many apertures in the black printhead structure as in the color printhead structures and the apertures may have one third of the transverse spacing from each other as the apertures in the color printhead structures. The arrangement of the apertures in the printhead structures is thus the same as in Fig. 5B. In a further example illustrated in Fig. 6 which shows schematically the arrangement of the apertures on the printhead structures, each aperture in the color printhead structures may print three dots per aperture for each transverse line of print.

Each aperture in the black printhead structures may print two dots per aperture for each transverse line of print.

There are fifty percent more apertures in the black printhead structure as in the color printhead structures.

The apertures in the black printhead structure have a transverse spacing which is two-thirds of the transverse spacing of the apertures in the color printhead structures. The amount of dot deflection control, numbers of apertures in the black and in the color printhead structures, and the transverse spacing of the apertures may be varied in many possible combinations, so long as the black printhead structure is able to print faster than the color printhead structures.

In another embodiment of the invention the black printhead structures are arranged to print at lower transverse and/or longitudinal resolution, than the color printhead structures. ~In an example of this embodiment, the black printhead structures may print with lower transverse dot density than the color printhead structures. In an example with one black printhead structure the black printhead structure may print every other longitudinal column of print compared to the color printhead structures so that the transverse resolution is reduced by a factor of two.

This is illustrated in Fig. 7A which illustrates the printing of print columns by the black printhead structure 5. The printhead structure prints every other print column

indicated by reference numeral 70 in the drawing. The columns inbetween indicated by 71 are not printed. The direction of movement of the belt is indicated by an arrow. It is also possible to reduce the resolution by a factor of three or even more. In an alternative to the previous example, the black printhead structure could print at a lower longitudinal resolution. This is illustrated in Fig. 7B which illustrates the printing of print lines by the black printhead structure 5. The printhead structure prints every other transverse print line indicated by reference numeral 72 in the drawing. The lines inbetween indicated by 73 are not printed. The direction of movement of the belt is indicated by an arrow. So that the black printhead structure could for example print every second transverse line print compared to the color printhead structures. This would reduce the longitudinal resolution by a factor of two. In a further alternative both the longitudinal and the transverse resolutions could be reduced. This is illustrated in Fig.

7C which illustrates the printing of print columns and lines by the black printhead structure 5. The printhead structure does not print every other print column and every other print line. The areas printed are indicated by reference numeral 74 in the drawing. The areas inbetween the printed areas are indicated by 75 and are not printed.

The direction of movement of the belt is indicated by an arrow. In this case, every alternate longitudinal line and every alternate transverse line of print is subject to printing as compared to the color printhead structures.

This means that only half of each printed column is printed. Correspondingly only half of each printed line is printed. The total number of dots required for printing is thus reduced by a factor of four. The reduction in the number of dots required for printing allows the black printhead structure to print faster. The reduction in the resolution may be achieved by lower dot deflection control.

In yet another embodiment of the invention there are provided two or more black printhead structures. Each black printhead structure only prints a portion of the printable area. In this way the black printhead structures can print faster than the color printhead structures. An example of this embodiment of the invention illustrated in Fig. 8 which shows a side view of the arrangement of the printhead structures and transfer belt. There are two black printhead structure 5,5'and three color printhead structures 5a, 5b, 5c. The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow.

In a first example the black printhead structures 5,5' may be arranged such that each black printhead structure prints different transverse lines of print. If, for example, there are two black printhead structures then each printhead structure may each print alternate transverse lines of print. This is illustrated in Fig. 8A wherein lines printed by a first printhead structure 5 are indicated by 80 and lines printed by the second printhead structure 5'are indicated by 81. The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow. If, for example, there are three black printhead structures (not shown), each printhead structure may each print every third transverse line of print. This is illustrated in Fig. 8B wherein lines printed by a first printhead structure 5 are indicated by 82, lines printed by the second printhead structure 5'are indicated by 83 and lines printed by a third printhead structure (not shown) are indicated by 84.

The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow. There may be four or more black printhead structures which each print a corresponding number of transverse lines of print.

Alternatively, the black printhead structures are each arranged to print less longitudinal columns of print per aperture than the color printhead structures by using less dot deflection control and/or having less spacing between the apertures in a row compared to the color printhead structures. The longitudinal columns of print that are not printed by one black printhead structure are printed by another black printhead structure. In this manner the black printhead structures print faster than the color printhead structures, since each aperture is required to produce less dots per transverse line. In one example two black printhead structures are provided each printing one dot per aperture and transverse line of print to print alternate longitudinal columns of print. This is illustrated in Fig. 8C wherein columns printed by a first printhead structure 5 are indicated by 85 and columns printed by the second printhead structure 5'are indicated by 86. The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow. In a further example three black printhead structures are provided each printing one dot per aperture for each transverse line of print. This is illustrated in Fig. 8D wherein columns printed by a first printhead structure 5 are indicated by 87, columns printed by a second printhead structure 5'are indicated by 88 and columns printed by a third printhead structure (not shown) are indicated by 89. Two, three or more black printhead structures may thus be provided each printing one or more dots per aperture per transverse line of print.

It is also possible in an example which is not shown to combine the two preceding examples so that there are two or more black printhead structures which each print alternate transverse lines of print and alternate longitudinal columns of print.

In a yet a further alternative to the present embodiment there are two or more black printhead structures which print at a lower transverse and/or longitudinal resolution print than the color printhead structures. In an example with two black printhead structures each printhead structure would print every fourth longitudinal column and/or transverse line of print so that every second longitudinal column and/or transverse line of print is printed print compared to the color printhead structures.

This is illustrated in Figs. 9A and 9B wherein columns printed by a first printhead structure 5 are indicated by 90 and columns printed by the second printhead structure 5'are indicated by 91 and lines printed by a first printhead structure 5 are indicated by 92 and lines printed by the second printhead structure 5'are indicated by 93. The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow. The speed of printing is increased by a factor of upto four compared to a single printhead structure. The longitudinal and/or transverse resolution would, for example, be reduced from 600 dpi to 300 dpi (dpi is dots per 2.54 cm (inch)).

In a specific example (not shown) utilizing one of the above mentioned principles, there are three color printhead structures and two black printhead structures.

The color printhead structures each print three dots per aperture for each transverse line of print. The black printhead structures each print one dot per aperture and alternate transverse lines of print. The spacing of the apertures in the transverse direction of the black printhead structures is one third of the spacing of the apertures of the color printhead structures. The black printhead structures can print at six times the speed of the color printhead structures.

In yet another embodiment there are provided three black printhead structures and three color printhead structures printing different colors. An example of this embodiment of the invention illustrated in Fig. 10 which shows a side view of the arrangement of the printhead structures and transfer belt. There are three black printhead structure 5,5', 5"and three color printhead structures 5a, 5b, 5c. The direction of movement of the transfer belt relative to the printhead structures is indicated by an arrow. The arrangement of the apertures in the printhead structures is shown in Fig. 10A. Each color printhead structure prints three dots per aperture for each transverse line of print using dot deflection control to print all the dots on a transverse line. Each black printhead structure prints one dot per aperture per transverse line of print. Each aperture prints every third dot on a transverse line. The apertures in the black printhead structures may, as illustrated, be arranged such that the apertures of the respective printhead structures are not in longitudinal alignment with each other. The black printhead structures can print three times faster than the color printhead structures.

In the forgoing embodiments of the invention the color printhead structures have been illustrated as upstream of the black printhead structures relative to the movement of the transfer belt 1. However, alternatively the black printhead structures could be arranged upstream of the color printhead structures. In a further arrangement the black printhead structures could be arranged to be interspaced between the color printhead structures.

In a further embodiment of the invention the black printhead structures print in single pass and the color printhead structures print in more than one pass. By a pass is meant a movement of the transfer belt which passes a section of the transfer belt 1 to be printed with a

longitudinal movement relative to the printhead structure 5 and allows the printhead structure 5 to deposit a plurality of longitudinal columns of printing. After or during a pass the printhead structure or structures and/or the image transfer member are moved transversely relative to each other to allow printing of the unprinted areas of the image in one or more further passes. After the first pass the next passes may be in the same or opposite longitudinal directions to that of the first pass.

The image receiving member may be an image transfer member which receives the image and transfers the image to an information carrier, such as paper. The image transfer member may be in the form of a belt or drum.

Alternatively, the image receiving member may be an information carrier, such as paper. Where in the preceding embodiments reference has been made to a transfer belt it to be understood that this may be replaced by a drum or other suitable image receiving means.

By'faster printing'is meant that the image receiving member moves faster longitudinally relative to the printhead structures during printing so that each image is printed in less time.

When the black printhead structures are printing then optionally the toner carrier 33 of the color printhead structures may cease to turn. Correspondingly, when the color printhead structures are printing then the toner carriers 33 of the black printhead structures may cease to turn.

The above description has described faster printing with respect to black printhead structures printing faster than color printhead structures. However, where, in the foregoing, reference is made to black then this can also be applied to color and vice-versa. In this way it is

possible to arrange particular colors to be printed faster or for all colors to be printed faster than black, if required.

The invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims.