WO2000069641A1

WO2000069641A1 - Image forming apparatus and method

Info

Publication number: WO2000069641A1
Application number: PCT/SE2000/000839
Authority: WO
Inventors: Ove Larsson; Per Klockar
Original assignee: Array Ab
Priority date: 1999-05-12
Filing date: 2000-05-03
Publication date: 2000-11-23
Also published as: WO2000069639A1; AU4790400A; AU4790500A; WO2000069641A8; WO2000069640A1; AU4809299A; JP2004506533A; WO2000069638A1; AU4790600A

Abstract

The present invention relates to 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. The image forming apparatus includes 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, at least one printhead structure arranged in said background electric field, 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, an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles, a transfer unit for transferring the image on said image receiving member to a print medium, and a fusing unit for permanently fixing the image on the print medium. The invention is characterized in that said image forming apparatus comprises a buffer zone between the transfer unit and the fusing unit, and buffer means arranged in said buffer zone for temporarily accommodating said print medium being fed out from the transfer unit, before feeding it to the fusing unit. The invention also relates to a method for operating said image forming apparatus.

Description

TITLE:

Image forming apparatus and method.

TECHNICAL FIELD:

The invention relates to 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.

The image apparatus includes 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, at least one printhead structure arranged in said background electric field, 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, an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles, a transfer unit for transferring the image on said image receiving member to a print medium, and a fusing unit for permanently fixing the image on the print medium.

Furthermore, the invention relates to a method of operating an image forming apparatus of the above- mentioned type. BACKGROUND OF THE INVENTION:

U.S. Patent No. 5,036,341 discloses a direct electro^¬ static 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 member due to control in accordance with image information.

In the field of direct electrostatic printing, it is common to use an image receiving member which is caused to move in relation to the printhead structure for intercepting the transported charged particles. The image which is intercepted on the image receiving member is then transferred onto a print medium, such as a sheet of paper. The image on the paper sheet is then permanently fixed by means of a fusing unit.

A problem with the above-mentioned arrangement relates to the fact that the transfer stage can be performed relatively quickly, whereas the fusing stage is a relatively slow process. A solution to this problem is to use a fusing unit which can supply relatively high power. However, this is not always desired, since a more powerful fusing unit is also more space-consuming and costly. This is a problem, in particular due to the fact that there is a demand for office printers of small size.

SUMMARY OF THE INVENTION:

An object of the invention is to provide an improved arrangement in an image forming apparatus, in particular for solving the above-mentioned problems.

Said object is accomplished by means of an image forming apparatus of the above-mentioned type, in which said transfer unit is arranged for operating with a speed for feeding said print medium which is different than that of said fusing unit.

Said object is also accomplished by means of a method for operating said image forming apparatus, in which said image is transferred to said print medium at a different speed than the speed of said fusing unit.

BRIEF DESCRIPTION OF THE DRA INGS:

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 side view of an image forming apparatus in accordance with a first embodiment of the present invention,

Fig. 2 is a perspective view of said image forming apparatus, in particular showing a housing member forming part of the invention,

Fig. 3 is a schematic cross-sectional view across a print station in an image forming apparatus according to the invention,

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 a particle carrier,

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 an image receiving member,

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 schematic side view of an image forming apparatus in accordance with a second embodiment of the present invention,

Fig. 6 is a schematic side view of an image forming apparatus in accordance with a third embodiment of the present invention,

Fig. 7 is a schematic side view of an image forming apparatus in accordance with a fourth embodiment of the present invention,

Fig. 8 is a schematic side view of an image forming apparatus in accordance with a fifth embodiment of the present invention,

Fig. 9 is an illustration of the columns of print printed in a single pass in a two pass method,

Fig. 10 is an illustration of the columns of print shown in Fig. 9 after the second pass,

Fig. 11 is an illustration of the effect of apertures which print with a lower density in a two pass printing method,

Fig. 12 is an illustration of the printing pattern of a first embodiment, Fig. 13 is an illustration of the printing pattern of a second embodiment,

Fig. 14 is an illustration of the printing pattern of a third embodiment,

Fig. 15 is an illustration of the printing pattern of a fourth embodiment,

Fig. 16 is an illustration of the printing pattern of a fifth embodiment, and

Fig. 17 is an illustration of the printing pattern of a sixth embodiment.

PREFERRED EMBODIMENTS:

To perform a direct electrostatic printing method in accordance with the present invention, a background electric field is produced between a particle carrier and 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 an image receiving member to provide line-by line scan printing to form a visible image. A printhead structure for use m direct electrostatic printing may take on many designs, such as a lattice of intersecting wires arranged m rows and columns, or an apertured substrate of electrically insulating material overlaid with a printed circuit of control electrodes arranged m 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 member.

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 200 dpi requires a printhead structure having 200 apertures per inch in a transversal direction.

In order to clarify the apparatus according to the invention, an example of its use will now be described m connection with the accompanying drawings .

As shown m a general, slightly simplified form in Fig. 1, an image forming apparatus in accordance with the present invention comprises at least one print station, preferably four print stations la, lb, lc, Id, which are constituted by particle carriers which are adapted for printing one colour each. Preferably, the colours being used are yellow, magenta, cyan and black. Each print station la-d is preferably in the form of a generally elongated cartridge assembly which is arranged adjacent to a printhead structure 2a, 2b, 2c, 2d. Each printhead structure 2a-d, is preferably in the form of an electrode matrix provided with a plurality of selectable apertures (not shown) , which is interposed in a background electric field defined between the corresponding cartridge and a back electrode which is constituted by a printing drum 3. The drum 3 is essentially cylindrally formed and is arranged so as to rotate during operation of the image forming apparatus. To this end, the drum 3 is powered by drive means (not shown) . Furthermore, the drum 3 has a circumference which is slightly greater than the maximum vertical printed length, i.e. slightly greater than the length of the paper being used during printing.

Each of the printheads 2a-d is connected to a control unit (not shown) which converts the image information in question into a pattern of electrostatic fields so as to selectively open or close passages in the electrode matrix to permit or restrict the transport of charged particles from corresponding cartridge la-d. In this manner, charged particles are allowed to pass through the opened apertures and toward the back electrode, i.e. the drum 3. The charged particles are then deposited on the drum 3.

Due to the fact that the drum 3 is rotating during operation, the image being formed on the drum is then transferred onto an information carrier, such as a sheet of plain, untreated paper 4 or any other medium suitable for printing. The paper sheet 4 is fed from a paper delivery unit 5 and conveyed past the underside of the drum 3. For transferring the image to the paper sheet 4, it is pressed into contact with the drum 3 by means of a belt 6, which in turn is driven by means of two rollers 7, 8 around which the belt 6 extends. In this manner, the toner particles are deposited on the outer surface of the drum 3 and then superimposed to the paper sheet 4 to form a four colour image. This process is carried out in a transfer position which is defined by the contact point between the transfer belt 6 and the drum 3.

After the image has been formed on the paper sheet 4 by said charged particles, the paper sheet 4 is fed to a fusing unit 9, in which the image is permanently fixed onto the paper sheet 4. In particular, the fusing unit 9 comprises a fixing holder (not shown) which includes a heating element preferably of a resistance type of e.g. molybdenium. As an electric current is passed through the heating element, the fixing holder reaches a temperature required for melting the toner particles deposited on the paper sheet 4. The fusing unit 9 further includes a pressure roller (not shown) arranged transversally across the width of the paper 4. Also, the fusing unit 9 is provided with means for feeding the paper 4 to an out tray (not shown) , from which the paper 4 can be collected by a user.

After passage through the fusing unit 9, the paper sheet 4 can also be brought in contact with a cleaning element (not shown) , such as for example a replaceable scraper blade of fibrous material extending across the width of the paper sheet 4, for removing untransferred toner particles from the paper sheet 4.

The print stations la-d and the printhead structures 2a-d are mounted in a generally cylindrically shaped housing element 10, which is shown in a schematical form m Fig. 2 by means of broken lines. Said housing element 10 is shown in greater detail in Fig. 2, which is a perspective view of said housing 10 and a print station la, the latter being shown in a position before it is mounted in the housing 10.

As indicated in Fig. 2, the housing 10 is designed with a generally cylindrical shape, so as to enclose the drum, which is not shown in Fig. 2 but which is arranged to be accommodated inside the housing 10. Furthermore, the housing 10 is provided with means for supporting the four print stations, said means comprising four elongated guide elements 11a, lib, lie, lid, which extend in the longitudinal direction of the housing 10 and consequently also in the longitudinal direction of the drum. The guide elements lla-d are intended for insertion of the four corresponding print stations, of which one print station la is shown in Fig. 2. The housing is also provided with means for supporting the four printhead circuits, of which one printhead structure 2a is shown in Fig. 2. Said means for supporting the printhead structure 2a comprises an elongated support element 12 provided with a slot 13 facing one of the side portions of the printhead structure 2a. The other side portion of the printhead structure 2a is mounted into two holding elements 14a, 14b, each of which are provided with a through hole 15a, 15b for accommodating a pin-shaped element 16 on the printhead structure 2a. In this manner, one end of the printhead structure is hinged in said holes 15a, 15b, whereas the other end is resting on the surface being defined in the slot 13 in the support element 12. Similar means for supporting the remaining three printhead structures are also provided on the housing, as shown in Fig. 2. Consequently, the print stations and the printhead structures are supported by the housing 10 in an accurate manner and are maintained in predetermined positions with respect to the drum. In particular, the printhead structures 2a-d are held in a predetermined position in relation to the peripheral surface of the drum and in relation to the print stations la-d.

The print station la has a generally elongated shape and is adapted for inserting into the corresponding guide element 11a. To this end, the guide element 11a is formed with a generally C-shaped cross-section forming a longitudinally extending cavity or slot, whereas the print station la comprises an insertion rod 17 intended to cooperate with said guide element 11a, so as to lock the insertion rod 17 from being removed from the housing

10 when inserted. The insertion rod 17 comprises a tip

17a which simplifies the alignment of the insertion rod

17 before mounting.

Furthermore, the print station la comprises a particle delivery unit 18, which now will be described in detail in Fig. 3.

As indicated in Fig. 3, a print station la forming part of an image forming apparatus in accordance with the present invention includes a particle delivery unit 18 preferably having a replaceable or refillable container 19 for holding toner particles, the container 19 having front and back walls (not shown) , a pair of side walls and a bottom wall having an elongated opening 20 extending from the front wall to the back wall and provided with a toner feeding element 21 disposed to continuously supply toner particles to a developer sleeve 22 through a particle charging member 23. The particle charging member 18 is preferably formed of a supply brush or a roller made of or coated with a fibrous, resilient material, and is arranged to be rotated as indicated by means of an arrow in Fig. 3. The supply brush is brought into mechanical contact with the peripheral surface of the developer sleeve 22 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 22 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 20 of the particle container 18. The developer sleeve 22 is arranged to be rotated as indicated by means of an arrow in Fig. 3.

Charged toner particles are held to the surface of the developer sleeve 22 by electrostatic forces essentially proportional to (Q/D) ², where Q is the particle charge and D is the distance between the particle charge center and the boundary of the developer sleeve 22. 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 invention can be carried out 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 24 is positioned proximate to the developer sleeve 22 to adjust the concentration of toner particles on the peripheral surface of the developer sleeve 22, to form a relatively thin, uniform particle layer thereon. The metering element 24 may be formed of a flexible or rigid, insulating or metallic blade, roller or any other member suitable for providing a uniform particle layer thickness. The metering element 24 may also be connected to a metering voltage source (not shown) which influences the triboelectrification of the particle layer to ensure a uniform particle charge density on the surface of the developer sleeve.

As indicated in Fig. 3, a printhead structure 2a is arranged adjacent to said developer sleeve 22 during operation of the image forming apparatus according to the invention. The printhead structure 2a is supported in a manner described above and shown in Fig. 2, and will be described in greater detail below with reference to Figs. 4a-c. The particle delivery unit 18 is provided with a spacer element 42 for defining a predetermined distance between the developer sleeve 22 and the printhead structure 2a. Furthermore, a section of the drum 3 is also indicated in Fig. 3. It is to be understood that the embodiment according to Fig. 1 includes four print stations la-d and four printhead structures 2a-d of the same type as shown in Fig. 3, wherein said print stations la-d are intended for toner particles of different colours .

It can be noted that the printhead structure 2a is maintained in an arcuated shape along at least a part of its longitudinal extension. That arcuated shape has a curvature radius so as to maintain a part of the printhead structure 2a curved around a corresponding part of the peripheral surface of the developer sleeve 22. In this manner, the printhead structure 2a is arranged so that the distance between the printhead structure 2a and the peripheral surface of the developer sleeve 22 remains constant along the whole transverse direction at every moment of the print process, regardless of undesired mechanical imperfections of the developer sleeve 22.

All four print stations la-d can be used simultaneously for printing on the drum 3. This means that all four colours (CMYK) are printed on the drum 3 simultaneously, and with a print density which corresponds to the resolution defined by the dimensions of the apertures in the printhead structures 2a-d. Normally, the printhead structures 2a-d are provided with 200 equally spaced apertures per inch, said apertures being aligned parallel with the longitudinal axis of the drum 3. For example, said apertures can be arranged in two rows each comprising 100 apertures per inch. Referring back to Fig. 2, the line of apertures formed on the printhead structure 2a is indicated by means of reference numeral 25.

With reference to Fig. 2, it can be noted that the developer sleeve 22 is arranged in the print station la. The print station la is mounted in the housing 10 by first inserting the insertion rod 17 into the matching slot formed by the corresponding guide element 11a. After that, the print station la is pushed in a direction as indicated by means of an arrow in Fig. 2, i.e. along the longitudinal direction of the housing 10, until the insertion rod 17 is fully inserted in the guide element 11a and the print station la is positioned so as to extend generally along the entire length of the housing 10. After that, the delivery unit 18 is pivoted about the axis of rotation which is defined by the insertion rod 17, in a direction towards the housing 10. The delivery unit 18 is pivoted until it reaches a position in which it is positioned adjacent to the printhead structure 2a. In this position, the print station la is preferably releasably locked by means of locking means (not shown) preventing the print station la from being removed from its correct position. Said locking means is preferably arranged so that a user easily can remove it from its (fully inserted) operating position. This operation will occur for example when an empty delivery unit 18 is to be replaced with a new one.

The mounting of the print station la can be further aided by means of spring elements (not shown in the drawings) which can be used to force the print station la so as to assume its correct operation position. Such spring elements can also be used so as to force the particle delivery unit 18 in a manner so that its spacer element 42 (cf. Fig. 3) contacts the printhead structure

2a, thereby defining the correct position of the printhead structure in relation to the developer sleeve.

Furthermore, the housing 10 is preferably also provided with support means (not shown) for supporting the print station la in a predetermined, correct position along the axial direction of the drum.

During printing with a printhead structure, the resolution of the printed image (i.e. the number of printed dots per inch) generally depends on the number of apertures per inch. If a higher resolution than 200 dots per inch is desired, some form of multiplexing method is required for using one single aperture in a printhead structure for producing several dots on the image receving member. According to a first embodiment of the present invention, such a multiplexing method is accomplished by rotating the drum 3 three revolutions, between which revolutions the drum 3 is diplaced sideways

(i.e. in its longitudinal direction) a certain distance.

Said distance then corresponds to a pitch which equals one single dot. This means that a printhead structure having 200 apertures per inch will produce undeflected centre dots in three consecutive complete revolutions of the drum 3. As a result, the printhead structure having 200 apertures per inch can be used for producing an image on the drum 3 which presents a resolution of 600 dots per inch. Consequently, this "multi pass" method, which will be described in detail below with reference to Figs. 9-17 increases the print addressability of the printhead structure without requiring an increased number of apertures in the printhead structure.

According to a second embodiment of the invention, a multiplexing method in the form of so-called dot deflection control (DDC) is utilized. According to this method, each single aperture of the printhead structure is used to address several dot positions on an image receiving member by controlling not only the transport of toner particles through the aperture, but also their transport trajectory toward the image receiving member, and thereby the location of the obtained dot. The DDC method, which is known per se, 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 an embodiment involving DDC, 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 DI, D2, such that the electrostatic control field generated by the control electrode remains substantially symmetrical as long as both deflection voltages DI, D2 have the same amplitude. The amplitude of DI and D2 are modulated to apply converging forces on toner particles as they are transported toward the image receiving member, thus providing smaller dots. The dot position is simultaneously controlled by modulating the amplitude difference between DI 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 the following, a printhead structure provided with deflection electrodes for said DDC method will be described. However, in the case of the above-mentioned first embodiment involving said multi pass method, no such deflection electrodes are necessary.

As shown in Figs. 4a, 4b, 4c, a printhead structure 2a in an image forming apparatus in accordance with the present invention (and being intended for dot deflection control) comprises a substrate 28 of flexible, electrically insulating material such as polyimide or the like, having a predetermined thickness, a first surface facing the developer sleeve (cf. Fig. 2), a second surface facing the drum, a transversal axis 29 extending parallel to the rotation axis of the developer sleeve of the print station across the whole print area, and a plurality of apertures 30 arranged through the substrate 28 from the first to the second surface thereof. The first surface of the substrate is coated with a first cover layer 31 of electrically insulating material, such as for example parylene. A first printed circuit, comprising a plurality of control electrodes 32 disposed in conjunction with the apertures, and, in some embodiments, shield electrode structures (not shown) arranged in conjunction with the control electrodes 32, is arranged between the substrate 28 and the first cover layer 31. The second surface of the substrate is coated with a second cover layer 33 of electrically insulating material, such as for example parylene. A second printed circuit, including a plurality of deflection electrodes 34, is arranged between the substrate 28 and the second cover layer 33. The printhead structure 2a 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 33, facing the drum 3.

The printhead structure 2a is brought in cooperation with a control unit (not shown) comprising variable control voltage sources connected to the control electrodes 32 to supply control potentials which control the amount of toner particles to be transported through the corresponding aperture 30 during each print sequence. The control unit further comprises deflection voltage sources (not shown) connected to the deflection electrodes 34 to supply deflection voltage pulses which controls the convergence and the trajectory path of the toner particles allowed to pass through the corresponding apertures 30. 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 32 from one another, preventing electrical interaction therebetween. In a preferred embodiment of the invention, the substrate 28 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 28, respectively, using conventional etching techniques. The first and second cover layers 31, 33 are 5 to 10 microns thick parylene laminated onto the substrate 28 using vacuum deposition techniques. The apertures 30 are made through the printhead structure la using conventional laser micromachining methods. The apertures 30 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 30 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.

The first printed circuit comprises the control electrodes 32 each of which having a ring shaped structure surrounding the periphery of a corresponding aperture 30, 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 32 may take on various shape for continuously or partly surrounding the apertures 30, preferably shapes having symmetry about the central axis of the apertures. In some embodiments, particularly when the apertures 30 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 34, each of which is divided into two semicircular or crescent shaped deflection segments

35, 36 spaced around a predetermined portion of the circumference of a corresponding aperture 30. The deflection segments 35, 36 are arranged symmetrically about the central axis of the aperture 30 on each side of a deflection axis 37 extending through the center of the aperture 30 at a predetermined deflection angle d to the longitudinal direction. The deflection axis 37 is dimensioned in accordance with the number of deflection sequences to be performed in each print cycle. 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 drum motion, the second dot is undeflected and the third dot is deflected slightly downstream with respect to the drum motion, thereby obtaining a transversal alignment of the printed dots on the drum. Accordingly, each deflection electrode 34 has an upstream segment 35 and a downstream segment

36, all upstream segments 35 being connected to a first deflection voltage source DI, and all downstream segments 36 being connected to a second deflection voltage source D2. Three deflection sequences (for instance: DKD2; D1=D2; D1>D2) can be performed in each print cycle, whereby the difference between DI and D2 determines the deflection trajectory of the toner stream through each aperture 30, and thus the dot position on the toner image .

In the embodiment shown in Figs. 4a-c, the printhead structure 2a is suitable for performing 600 dpi printing utilizing three deflection sequences in each print cycle, i.e. three dot locations are addressable through each aperture 30 of the printhead structure during each print cycle. Accordingly, one aperture 30 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 29 of the printhead structure 2a. The apertures 30 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 29 of the printhead structure 2a 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.

As explained above, the invention can be implemented with a multiplexing method involving said multi pass method. In such cases, the deflection electrodes described with reference to Figs. 4a-c are not necessary.

Referring back to Fig. 1, it can be noted that the transfer stage and the fuser stage in the image forming apparatus are arranged in sequence. This means that after the image information in question has been transferred onto the paper sheet 4, the paper sheet 4 is forwarded to the fusing unit 9. Moreover, due to the demand for a small physical size of a printer using the image forming apparatus according to the invention, the fusing unit 9 is preferably of relatively small size. Since the fusing unit 9 is relatively small, it will also generate a relatively low power for use during fusing of the image onto the paper sheet 4. The operation involving transferring image information from each of the print stations la-d onto the paper sheet 4 can be made with a speed of appoximately 36 pages per minute (ppm) , whereas the relatively low-power fusing unit 9 presents a fusing speed of approximately 12 ppm. However, the invention can be implemented using other particular speeds for the transfer and fusing stages.

As explained mitally, the situation involving a highspeed transfer stage and a low-speed fuser stage leads to a problem, since the paper sheets 4 would then be fed out of the transfer stage at a much higher speed than the fuser stage can absorb. The invention can still be implemented with said transfer unit operating with a speed for feeding said print medium which is different than that of said fusing unit, m particular since the invention comprises buffer means for temporarily accommodating a paper sheet which is fed out from the transfer stage. The paper sheet being accommodated in the buffer means will then be fed into the fusing unit 9 as soon as a previous paper has been fed out of the fusing

According to the first embodiment of the invention shown in Fig. 1, said buffer means comprises a buffer zone 38 which is designed as a zone which extends between the transfer stage and the fuser stage through which the paper sheet 4 is being passed. The buffer zone 38 has a predetermined length 1-., which is adapted to the speed of the fuser stage, the speed of the transfer stage and the length of the paper sheet being used, so that a paper sheet which is fed out of the transfer stage is not colliding with a previous paper sheet, i.e. a paper sheet being fed into the fusing unit 9. Consequently, when a first paper sheet is fed through the fusing unit 9, the image forming apparatus (including the transfer stage) can be activated to generate a subsequent paper sheet at a point in time which is suitable for feeding the subsequent paper into the fusing unit 9 closely after the first paper sheet. Normally, the length l_x of the buffer zone 38 is approximately the same as the length of the paper sheets being used.

According to a second embodiment of the invention shown in Fig. 5, the paper sheet 4 which is fed out from the transfer stage is guided around a set of cylindrical rollers, for example two rollers 39a, 39b which are arranged in a buffer zone 38^' and which extend essentially transverse to the direction of travel of the paper sheet 4. In this manner, the physical length 1₂ of the buffer zone 38' can be made smaller than that of the buffer zone 38 as shown in Fig. 1.

In order to further improve the buffer zone 38^', the set of rollers 39 can be movably arranged. More precisely, the rollers 39a, 39b can be moved in a direction which defines an angle with respect to the direction of travel of the paper sheet 4 from the transfer to the fusing step. Preferably, the rollers 39a, 39b are then arranged to be displaced in a direction which is perpendicular to the direction of travel of the paper sheet 4, for example as shown in Fig. 6. In this manner, the rollers 39a, 39b can then be displaced a certain distance away from each other, which results in a longer available distance of travel for the paper sheet 4. In other words, a longer sheet of paper can be used while maintaining the same length 1₂ of the buffer zone 38^' as shown in Fig. 5.

In order to improve the buffer zone arrangement according to Fig. 6, the two rollers 39a, 39b can be arranged to be displaced automatically depending on the type of paper currently being used. In this manner, a buffer zone 38^' having a predetermined length 1₂ can be automatically adapted for different paper types, i.e. paper types of various lengths. To this end, the image forming apparatus according to the invention is provided with means (normally a computer control unit) for determining the length of the paper sheets being currently used and for setting the rollers 39a, 39b in suitable positions depending on said length.

Fig. 7 discloses an image forming apparatus according to a further embodiment of the invention. In this case, the paper sheet 4 which is fed from the transfer stage is bent slightly before being forwarded to the fusing unit 9. To this end, a displaceable stop element 40 is arranged in the buffer zone 38''. The stop element 40 is set in a position so as to stop a paper sheet from the transfer stage while it is fed towards the fuser stage. As a consequence, the paper will bend slightly. This means that the necessary length 1₃ of the buffer zone will decrease (as compared with the length li as shown in Fig. 5) depending on the degree of bending of the paper sheet. When the paper sheet 4 can be fed into the fusing unit 9, the stop element 40 will be moved away by means of a suitable drive means (not shown) , thereby allowing the paper sheet 4 to move.

Fig. 8 discloses an image forming apparatus according to yet another embodiment of the invention. In this case, the fuser unit 9' is arranged so as to be displaced in the direction of travel of the paper sheet 4. More precisely, the fuser unit 9 is arranged on a transport unit 41 comprising drive means (not shown) adapted to move the fuser unit 9 generally between two different end positions, i.e. a first position relatively close to the transfer belt 6 and a second position relatively far from the transfer belt 6. The movement of the fusing unit 9 is indicated by means of an arrow m Fig. 8. The first position of the fusing unit 9' is shown with a broken line, whereas the second position is shown with a full line. According to this embodiment, the fusing unit 9' is positioned m said first position when a first paper sheet 4 having a transferred image is to be fed out of the transfer stage. As the paper sheet 4 is fed into the fusing unit 9' and being processed m the fusing unit 9', the fusing unit 9' is gradually moved away from the transfer belt 6 towards the second position. During the final part of this motion, a second paper sheet is being printed and treated m the transfer stage. When the first paper sheet has been discharged from the fusing unit 9', said fusing unit 9' is once again displaced towards the first position m order to receive said second paper sheet .

With reference to the above-mentioned embodiments, it can be noted that the invention is arranged for operating with a speed for feeding the print medium which is different than that of said fusing unit. In particular, the transfer unit is arranged for a higher speed for transferring the image to the print medium and feeding the print medium as compared with the speed of said fusing unit.

In the following, the above-mentioned "multi pass" method, which be described in detail below with reference to Figs.

9-17. By means of said method, the resolution achieved by a printhead structure 2 for a given number of apertures 30 may be increased without necessarily the use of deflection electrodes 34. In order to achieve a printing resolution greater than the number of apertures in the printhead structure 2 the printing takes place in two or more passes of the image receiving member, i.e. the drum 3 according to the above-mentioned embodiment. By a pass is meant a movement of the image receiving member which passes a section of the drum 3 to be printed with a movement relative to a given printhead structure 2 and allows the printhead structure 2 to deposit a plurality of longitudinal columns of printing. A column of printing is a longitudinal line of the image receiving member 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 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 image receiving member 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 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. After the first pass the next passes may be in the same or opposite longitudinal directions to that of the first pass.

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 that the image receiving member is 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 two rollers (not shown) . Thus, the transverse direction will normally be parallel to the axes of these rollers. 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.

With respect to the description which follows reference is made to image or printable area. In the present context an image is formed by the toner particles over an area of the drum 3. The image also includes those printable areas that could receive toner particles but do not receive the particles because the 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 paper sizes or any other chosen size.

In order to better understand the invention an example will first be described with respect to performing just two passes with each pass taking place in the same direction. In this case the number of apertures 30 per unit length is half that needed to achieve the desired resolution with a single pass. In a first pass a first half of the image is formed on the drum 3. This first half of the image comprises alternate longitudinal columns of print of the intended final image, i.e. alternate columns are printed and alternate columns are not printed. The drum 3 and printhead structure are then moved relative to each other in the direction transverse to the direction of movement of the drum, but in the plane of the drum. This relative movement may be carried out by any suitable means known to the person skilled in the art. Then, in a second pass, the remaining columns of print are printed to form a complete image. The second pass can carried out with the drum traveling in the same longitudinal direction as the first pass or the opposite longitudinal direction. This effect is illustrated in Figs. 9 and 10. Fig. 9 represents a section of the drum after the first pass. The areas that are printed in the first pass are shown as hatched areas 61. Fig. 10 represents the same section of the drum after the second pass. The areas that are printed in the first pass are shown as hatched areas 61, whilst the areas that are printed in the second pass are shown as differently hatched areas 62.

The density of a dot, i.e. the quantity of toner particle used to form the dot, may vary according to the position of the aperture on the printhead structure due to insufficient toner particles being available. This is known as the starvation effect. The variation in dot density may take place between apertures within the same row and/or between apertures in different rows.

In the example there is just one row of apertures 30 and the row is moved transversely by one dot pitch between the two passes. In this case pairs of adjacent rows will be printed by the same aperture. This is illustrated in Fig. 11 in which the first positions of the apertures is shown by the reference numeral 70 and the positions of the same apertures during a second pass are shown by the reference numeral 71. Each aperture then prints a double column of print by printing two adjacent columns. If, by way of example, every fourth aperture suffered from starvation effect then every fourth aperture would produce columns which have less density than the columns produced by the remaining apertures. If the row of apertures is moved transversely by one dot pitch then the adjacent column will also be printed in less density. The result is then a column of a width that is double the width which would be due to printing by a single aperture. Such a double width column is more visible to a viewer. Columns of lesser density produced each by a single aperture in two passes are lighter shaded and indicated by reference numeral 72 and columns of greater density each produced by a single aperture in two passes are heavier shaded and are indicated by reference numeral 73 in Fig. 11.

A first embodiment of the multi pass method is illustrated in Fig. 12. In order to reduce the effect of the problem of starvation mentioned above, the row of apertures is moved transversely by more than one dot pitch between passes. The row is moved transversely by an amount equal to 2N+3 number of times the transverse pitch length L, where NX is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. The pitch length L is the distance between adjacent dots. For 600 dpi (dots per inch) the pitch length is approximately 42 microns. The movement for one row of apertures printing in two passes at 600 dpi is approx. 127 microns or a higher integer multiple as specified in the preceding formula. This is illustrated in Fig. 12. In Fig. 12 the row of apertures has moved from the first position indicated by reference numeral 80 in which the first pass took place to the position indicated by reference numeral 81 in which the second pass took place. The apertures that are lighter shaded represent apertures that produce dots having lower density. The columns that are lighter shaded represent columns of print that have a lower density. Columns of lesser density produced each by a single aperture in two passes are indicated by reference numeral 82 and columns of greater density each produced by a single aperture in two passes are indicated by reference numeral 83 in Fig. 12. As can been seen from Fig. 16 the columns of less density are of narrower width than those in Fig. 15 and spaced apart from each other. These narrower columns will be less visible to a viewer. As is evident from the figure at the areas at the lateral sides of the image not every column of print may be printable by an aperture. The number of apertures in a row is chosen such that not all the apertures are needed to print the intended image. The printing from apertures at the end of the rows which are outside the area to be printed is then suppressed.

A second embodiment of the multi pass method is illustrated in Fig. 13. In this example the printing is carried out in three passes. In Fig. 13 the row of apertures has moved from the first position indicated by reference numeral 91 in which the first pass took place to the position indicated by reference numeral 92 in which the second pass took place and then to the position indicated by reference numeral 93. The number of apertures per unit of length transverse is one third that needed to achieve the same resolution as with a single pass. In a first pass a first one third of the image is formed on the drum. This first third comprising one third of the columns of print indicated by reference numeral 94 of the intended final image. The drum and printhead structure are then moved relative to each other in the direction transverse to the direction of movement of the drum, but m the plane of the drum, preferably by moving the drum transversely. Then, m a second pass, a second set of columns of the image indicated by reference numeral 95 are printed. The printhead structure is moved transversely by an amount equal to 3N+2 number of times the transverse pitch length, where N is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. The second pass can occur with the drum traveling m the same longitudinal direction as the second pass or in the opposite longitudinal direction. In a third and final pass, the remainder of the columns of the image indicated by reference numeral 96 are printed. Between the second and third pass the printhead structure is moved transversely by an amount equal to 3N+2 number of times the transverse pitch length L, where N is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. The third pass can occur with the drum traveling in the same direction as the first pass or m the opposite direction. This embodiment is illustrated in Fig. 13 which shows the drum at the end of the first pass. The apertures that are lighter shaded represent apertures that produce dots having lower density. The columns that are lighter shaded represent columns of print that have a lower density. As can be seen these columns of lower density are not adjacent each other. In accordance with this embodiment between each pass the row is moved transversely by an amount equal to 3N+2 number of times the transverse pitch length L. In an alternative the row could be moved transversely by an amount equal to 3N+4 number of times the transverse pitch length between each pass where N is an integer including 0. As in the first embodiment the number and transverse extent of the apertures in a row is chosen such that not all the apertures are needed to print the intended image. The printing from apertures at the end of the rows which are outside the area to be printed is then suppressed. The apparatus may be arranged such that the non-used apertures are at both ends of the row or rows of apertures and during a pass an aperture or apertures at both ends are simultaneously not used.

In general, for printhead structures having a single row of apertures the movement could at least be PN+P+1 or PN+P-1 times the pitch length where P is number of passes needed to complete an image, and N is an integer including 0. However for certain numbers of passes there may be more allowable movement possibilities. So for 5 passes the movements may be 5*N + X times the pitch length where X may be 2, 3, 4 or 6 and N is an integer including 0. In this case the values of X = 4 and 6 correspond to the general formula, whereas the values of X = 2 and X = 3 are extra values. Furthermore for 7 passes the movements may be 7*N + X times the pitch length where X may be 2, 3, 4, 5, 6 or 8 and N is an integer including 0. In this case the values of X = 6 and 8 correspond to the general formula, whereas the values of X = 2, 3, 4 or 5 are extra values. Extra values in particular occur where the number of passes is a prime number. In this case the number of pitch lengths may be neither 1 nor an integer multiple of 7.

The printhead structure may comprise one or more transverse rows of apertures. The number of apertures in each transverse row may be equal or unequal . The pitch between each aperture in a row may be equal or unequal . The pitch between apertures in a row may be the same in each row, or different rows may contain apertures with different pitches. The apertures in one row are in staggered relationship with the apertures of another row. In a printhead structure containing two rows of apertures the apertures in one row may be arranged to be centered between the apertures of the other row. Alternatively the apertures of one row may arranged to be off centre relative to the apertures of the other row, whilst avoiding being in longitudinal alignment. There may alternatively three or more rows of apertures per printhead. The number of rows of apertures may be the same on each printhead or different.

This is illustrated in a third, preferred, embodiment in Fig. 14. In this embodiment the printhead structure includes two rows of apertures 101, 102. The apertures in one row 101 are transversely displaced relative the apertures in the other row 102. The apertures as shown are spaced apart from each other transversely by the same distance, though this is not essential. Each row of apertures includes one sixth of the number of apertures per unit length required to print the complete image so that the two rows of apertures together include one third of the number of apertures per unit length required to print the complete image. The image is printed in three passes of the printhead structure. The positions of the rows of apertures for the first, second and third passes are indicated by 103, 104 and 105 respectively. Between each pass the printhead structure and image receiving member are moved relative to each other by a distance equal to four times the pitch length L. In Fig. 14 the columns of print printed by the second row of apertures is indicated by shading. Columns printed during the first, second and third passes are indicated by 106, 107 and 108 respectively. As can be seen in the figure the movement by four times the pitch length results in adjacent columns of print being printed by apertures which belong to different rows.

The rows of apertures may not receive the same quantity of toner particles when printing. Since one row is always upstream or downstream of another row relative to the movement of the toner carrier the row which is upstream will have more toner available than the row which is downstream. The effect of this is that the downstream row or rows may produce dots of a lower density than other rows. If adjacent columns of print are printed by apertures in the same row then the effect of the lower density will be more visible as double width columns of low density will be produced. In accordance with the preceding embodiment no two adjacent columns of print are produced by the same row of apertures. This ensures that the columns of lower density are always spaced from each other and hence are less visible. Although, described with a relative movement between passes of four times the column width the movement could also be eight times the column width. Similarly to the first and second embodiments the number and transverse extent of the apertures in the rows is chosen such that not all the apertures in each row are needed to print the intended image. The printing from apertures at the end of the rows which are outside the area to be printed is then suppressed. The apparatus may be arranged such that the non-used apertures are at both ends of the row or rows of apertures and during a pass an aperture or apertures at both ends are simultaneously not used.

In a still further embodiment DDC control of the apertures may be used. When DDC control is applied, each aperture is able to print more than one column of print in a single pass. The DDC control is preferably arranged to print columns from a single aperture which are not adjacent to each other, though in a less preferred embodiment they could print adjacent columns in a single pass. In an example (see Fig. 15) the DDC control is arranged to print two non-adjacent columns of print per pass from each aperture, in this embodiment the columns are separated from each other by a distance of twice the pitch length. In Fig. 15 the row of apertures has moved from the first position indicated by reference numeral 110 in which the first pass took place to the position indicated by reference numeral 113 in which the second pass took place. The columns printed by a single aperture 111 are indicated by shaded lines 112 in the drawing. The position of the aperture 111 producing the columns is indicated by shading. The aperture in this embodiment produces columns of print that are separated by a single column. The drum and printhead structure are then moved relative to each other by 5 pitch lengths L in the direction transverse to the direction of movement of the drum, but in the plane of the drum. Then, in a second pass, a second set of columns of the image are printed. The position 113 of the apertures in the second pass are indicated by the second row of apertures. The columns printed by the aperture 111 in the second pass are indicated by shaded lines 114 in the drawing. The printhead structure is moved transversely by an amount equal to N*2 + 5 number of times the transverse pitch length L, where N is an integer including 0. N will have a maximum value dependent upon the number of apertures and the width of the image to be printed such that the transverse movement leaves enough apertures available to print the image. In this embodiment N is equal to 0 so that the relative movement between passes is equal to 5. As can be seen the relative movement is just sufficient to ensure that the columns printed by a single aperture are not adjacent each other. The relative movement could however be greater than 5, e.g. 7, 9 etc.

In a still further embodiment using DDC control (see Fig. 16) each aperture prints two columns of print which are separated from each other by four times the pitch length. In Fig. 16 the row of apertures has moved from the first position indicated by reference numeral 120 in which the first pass took place to the position indicated by reference numeral 123 m which the second pass took place. The columns printed by a single aperture 121 are indicated by shaded lines 122 m the drawing. The position of the aperture 121 producing the columns is indicated by shading. The position 123 of the apertures m the second pass are indicated by the second row of apertures. The columns printed by the aperture 121 m the second pass are indicated by shaded lines 124 m the drawing.

In another embodiment the relative movement is less if the distance between the columns printed m a pass is at least six. In this case the relative movement may be only three pitch lengths. This is possible because the individual columns printed by a single aperture are sufficiently far apart to allow an intermingling of columns printed from different passes by the same aperture. This is illustrated in Fig. 17. In Fig. 17 the row of apertures has moved from the first position indicated by reference numeral 130 in which the first pass took place to the position indicated by reference numeral 133 m which the second pass took place. The columns 132 printed from the aperture 131 on the first pass are printed m hatched shading and the columns 134 printed on the second pass are printed in differently hatched shading. As is visible in the drawings, columns from one pass intermingle columns from the other pass.

In a further example each aperture prints two columns per line and pass, the distance between the two columns is three times the pitch length and the image is printed in three passes. In this case the relative transverse movement between passes may be 5, 7 or more times the pitch length, according to the formula N*3 + 5 or N*3 + 7, where N is an integer including 0.

In a yet another example each aperture prints three columns per line and pass, the distance between the three columns is two times the pitch length and the image is printed in two passes. In this case the relative transverse movement between passes may be 7, 9 or more times the pitch length according to the formula N*2 + 7, where N is an integer including 0.

In a further example DDC control is used to print adjacent columns of print. Each aperture prints two adjacent columns so the spacing is one pitch length. The image is printed in two passes. In this case the relative transverse movement between passes may be 6, 10 or more times the pitch length according to the formula N*4 + 6, where N is an integer including 0.

In a yet a further example DDC control is used to print adjacent columns of print. Each aperture prints two adjacent columns so the spacing is one pitch length. The image is printed in three passes. In this case the relative transverse movement between passes may be 4, 8 or more times the pitch length according to the formulae N*6 + 4 or N*6 + 8, where N is an integer including 0.

In a yet a still further example DDC control is used to print adjacent columns of print. Each aperture prints three adjacent columns so the spacing is one pitch length. The image is printed in two passes. In this case the relative transverse movement between passes may be 9, 15 or more times the pitch length according to the formulae N*6 + 9, where N is an integer including 0.

It is also possible to use DDC control in combination with multiple rows of apertures.

The amount of transverse movement of the printhead structure relative to the image receiving member is normally greater than the transverse distance between the apertures in the printhead structure. This means that for any one aperture its transverse position during a subsequent pass is beyond the position of the aperture which was transversely adjacent to said one aperture in the previous pass. Alternatively, any one aperture is beyond the position of the aperture which was transversely adjacent to said one aperture in the previous pass plus one, i.e. two passes previously. This means that the an aperture passes beyond the position of its neighbour either at the next pass or over next pass.

The spacing of the transverse spacing of the apertures in the printhead structure may assume any suitable value. Preferably the value is between 1 and 9 times the pitch length more preferably it is between 2 and 6 times the pitch length or less. Even more preferably it is between 3 and 5 times the pitch length.

The multi pass method which now has been described will now be further explained with reference to Fig. 1.

The image receiving member in this embodiment is a drum 3. The drum 3 rotates about an axis. Around the periphery of the drum 3 are arranged four print stations la, lb, lc and Id. The print stations respectively contain differently coloured toner particles to allow colour printing. One of the print stations may contain black toner particles to allow black and white printing. There is also provided a transfer station 6 for transferring the image to another medium such as a paper sheet 4. Transfer may be effected by electrostatic attraction or by pressure transfer. A cleaning structure can be provided for cleaning the printhead structures of toner particles as required. Preferably, the cleaning station then comprises a vacuum source, for example in the form of an audio loudspeaker. The vacuum source acts through one or more transversely aligned rows of apertures in the drum so that a suction force may be effected on a printhead structure. Such cleaning of the printhead structures preferably is performed after each pass. Alternatively, the cleaning is performed after an image has been formed. In a further alternative the cleaning is performed after two or more images have been formed.

The printhead structure provided with each print station is preferably of the type illustrated in Figs. 4a-c, i.e. two parallel rows of apertures with constant pitch between the apertures in a row. The apertures of one row are staggered in relationship to the apertures of the other row. The apertures of one row may be centered in the spaces between the apertures of the other row, though they could be arranged eccentrically.

During a pass the each transverse line of the image to be formed on the drum passes the printhead structures in turn. The transverse line then passes the transfer station 6. While the drum is rotating it is moved along its axis. The printhead structures and drum are thus moved continuously relatively to each other in the transverse direction parallel to the axis of the drum. Each rotation of the drum causes a pass of the printhead structures. After two or more passes or rotations of the drum during which printing is effected the transfer station starts to transfer the image to paper as soon as the leading edge of the image reaches the fuser unit. This transfer may start before the other parts of the image have passed all the printhead structures. The cleaning structure is preferably permanently so that cleaning of each printhead structure may be effected on each pass.

The image preferably occupies a major portion of the circumference of the drum, in particular more than 50%, preferably more than 75%. Where the image occupied a sufficient portion of the circumference of the drum the start of a further pass for the leading edge of an image may start to be printed before the previous pass has been completed by all printhead structures.

The relative transverse movement between or during passes may take on the following values. In a first example for three or four passes and two rows of apertures per printhead structure a step distance of (P + RxPxN + 1) or of (P + RxPxN - 1) times the pitch length give suitable values for the transverse movement, where P = number of passes, R = number of rows, N is an integer including 0. In a second example for five passes and two rows of apertures per printhead structure a step distance of (P + RxPxN + X) or of (P + RxPxN - X) times the pitch length give suitable values for the transverse movement, where X can take the values: +3, +1, -1, -3. In a third example for two passes and three rows of apertures per printhead structure a step distance of RxPxN - 2 is possible. In a fourth example for three passes and three rows of apertures per printhead structure a step distance of RxPxN + X, where X has the values -7 or -5 are possible.

The above examples are particularly useful where the starvation effect leads to a variation in dot density between different rows of apertures on the printhead structure. However the starvation effect may occur over several adjacent apertures which are spaced from each other in the transverse direction. In this case it may be appropriate to have a larger transverse movement. For example it may be two or more times the extent of the starvation effect. The printhead structure or another part of the printer may include an instrument for measuring the optical density of the image. The instrument may detect the transverse extent of the starvation effect. The output of the instrument may be used to cause a transverse movement sufficient that that the apertures affected by the starvation effect do not print columns adjacent to columns which were formed by the starved apertures in a preceding pass .

After a number of passes the direction of movement of the drum relative to the printhead structures will be reversed. To effect this a pass without any printing is performed during which the direction of movement is changed. Preferably the change in direction takes place after one image has been completed and before another image is commenced. A pass without printing may also be made where it is desired to change the speed and/or pattern of the transverse motion of the drum.

The drum can be formed of an electrically conducting material. The material may optionally be covered on its surface facing outwardly towards the toner carrier with a thin layer of an electrically insulating material, preferably less than 100 microns thick. The electrically conducting material is preferably a metal though any material is possible so long as it conducts electricity. The metal is preferably aluminium. The thin layer of insulating material is sufficiently thin that the electric field lines pass through sufficiently to allow a mirror charge to be formed which mirrors the charge on the toner on the surface of the transfer belt or drum. This mirror charge increases the force holding the charged toner to the transfer belt or drum. The insulating materials may be any suitable material, in particular aluminium oxide. The aluminium oxide may be combined with any conducting material for the drum, but is particularly advantageous when used with a drum with an aluminium surface. The above form of drum is particularly useful when the transfer of the image is to be effected by pressure as the stronger material of the drum allows a higher pressure to be used.

This form of drum is particularly useful with a multi pass printer as hereinbefore described, but may be used with other types of printer, particularly those with high surface speeds of the drum or belt.

In any of the above embodiments of the invention the pitch

(distance between centers of dots) may be varied. The distance between dots on the transverse lines (horizontal pitch) may be varied and/or the distance between dots in a longitudinal column (vertical pitch) may be varied. The horizontal pitch may be varied by varying the amount of relative transverse movement between passes. The vertical pitch can be varied by varying the amount of longitudinal movement between the printing of lines.

The invention is not limited to the embodiments described above but may be varied within the scope of the appended patent claims. For example, the invention is not limited to use of an image receiving member in the form of a drum. For example, the image receiving member can be a drum of the kind mentioned above, or may alternatively be an endless belt.

Furthermore, said image receving member can be formed for example by a sheet-like or belt-like element which is bent so that its end portions meet. In the position where these end portions meet, the above-mentioned opening or openings can be formed by means of for example a joining element which joins said end portions while providing a slit or gap between the end portions.

The invention can be used for colour printing, as described above. It can also be used for black and white printing. In the latter case, a printing apparatus using only one printing station and only one cartridge with black toner particles being mounted on the housing can then be used.

The invention can be used with a multiplexing method, such as the above-mentioned multi pass method or DDC method, or may alternatively be operated without any multiplexing method.

The invention is not limited to arrangements having a cylindrical drum and a cylindrical housing, but can be used in other image forming arrangements.

Claims

CLAIMS :

1. 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; at least one printhead structure arranged in said background electric field, 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; an image receiving member caused to move in relation to the printhead structure for intercepting the transported charged particles; a transfer unit for transferring the image on said image receiving member to a print medium; and a fusing unit for permanently fixing the image on the print medium; characterized in that said transfer unit is arranged for operating with a speed for feeding said print medium which is different than that of said fusing unit .

An image forming apparatus according to claim 1, characterized in that said transfer unit is arranged for a higher speed for feeding said print medium than said fusing unit.

3. An image forming apparatus according to claim 2, characterized in that it comprises a buffer zone between the transfer unit and the fusing unit, and buffer means arranged in said buffer zone for temporarily accommodating said print medium being fed out from the transfer unit, before feeding it to the fusing unit.

4. An image forming apparatus according to claim 3, characterized in that said buffer zone is of a predetermined length which is adapted to the speed of transfer to said print medium, the speed of fusing said image and to the length of said print medium.

5. An image forming apparatus according to claim 3 or 4, characterized in that the buffer zone comprises a set of rollers around which said print medium is guided.

6. An image forming apparatus according to claim 5, characterized in that said set of rollers are displaceable in dependence of the length of said print medium.

7. An image forming apparatus according to any one of claims 3-6, characterized in that the buffer zone is provided with means for sligthly bending said print medium before feeding it to the fusing unit.

8. An image forming apparatus according to claim 1, characterized in that said fusing unit is movably arranged between a first position relatively close to the transfer unit and a second position relatively far from the transfer unit, said fusing unit being arranged for moving towards said first position when said print medium is fed out of said transfer unit.

9. An image forming apparatus according to any one of the preceding claims, characterized in that the relative movement of the image receiving member and the printhead structure is so arranged that each line on the image receiving member that is transverse to the direction of said relative movement passes the printhead structure in a longitudinal direction at least twice in order to form an image, the printhead structure printing only a part of each transverse line on each pass to form longitudinal columns of print, the printhead structure and/or the image receiving member being moved relative to each other either between consecutive passes or during a pass so that each time that the image receiving member passes the printhead structure transversely different parts of the image receiving member are positioned to receive charged toner particles, the image forming apparatus being so constructed and arranged to operate that adjacent columns of print are not printed by the same aperture in different passes.

10. An image forming apparatus according to claim 9, wherein the image forming apparatus is so constructed and arranged to operate that adjacent columns of print are not printed by the same aperture in any pass.

11. An image forming apparatus according to claim 9 or claim 10, wherein the printhead structure includes apertures arranged in two or more longitudinally separated transverse rows, the apertures in one row not being in longitudinal alignment with the apertures in another row, and the apparatus is arranged to print adjacent columns by apertures from different rows of apertures.

12. An image forming apparatus according to claim 11, wherein the apertures in a row are transversely equidistantly spaced apart from each other.

13. An image forming apparatus according to claim 11 or claim 12, wherein the apertures in one row are transversely displaced relative to the apertures in another such that the apertures in one row are not in longitudinal alignment with the apertures in another row.

14. An image forming apparatus according to any preceding claim, wherein the apparatus is arranged to have a relative transverse movement between the image forming member and the printhead structure between consecutive passes which is greater than the transverse distance between the apertures in the printhead structure.

15. An image forming apparatus according to any preceding claim, wherein the apparatus is arranged to have a relative transverse movement between the image forming member and the printhead structure which is of the same amount between each of the passes in the formation of an image.

16. An image forming apparatus according to any of claims 11 to 13, wherein the transverse relative movement between passes is equal to PN+P+1 or PN+P-1 times the pitch length where P is the number of passes used to form an image and N is an integer including 0.

17. 7Λn image forming apparatus according to any of claims 11 to 16, wherein the printhead structure includes apertures arranged in two transverse rows and the transverse spacing of the apertures is such that that either three or four passes are required in order to print an image .

18. An image forming apparatus according to claim 17 wherein, the step distance between passes is given by the formulae P + RxPxN + 1, or P + RxPxN - 1 times the pitch length, where P = number of passes, R = number of rows of apertures and N is an integer including 0.

19. An image forming apparatus according to any of claims 11 to 16, wherein the printhead structure includes apertures arranged in two transverse rows and the transverse spacing of the apertures is such that five passes are required in order to print an image and the step distance between passes is given by the formulae P + RxPxN

+ X, or P + RxPxN - X times the pitch length, where P = number of passes, R = number of rows, N is an integer including 0, and X can take on the values +3, +1, -1, -3.

20. An image forming apparatus according to any of claims 11 to 16, wherein the printhead structure includes apertures arranged in three transverse rows and the transverse spacing of the apertures is such that three passes are required in order to print an image and the step distance between passes is given by the formulae RxPxN + X, or RxPxN - X times the pitch length, where P = number of passes, R = number of rows, N is an integer including 0, and X can take on the values -7 or -5.

21. An image forming apparatus according to claim 17, wherein the distance between adjacent apertures on a row is six times the pitch length required for the resolution.

22. An apparatus according to any preceding claim wherein, the control electrodes of one or more apertures are arranged to enable the aperture to print at two or more locations separated transversely in a single pass so as to produce two or more columns of print per aperture per pass.

23. An apparatus according to any preceding claim, wherein the number and transverse extent of the apertures in the row or rows of the printhead structure is more than is necessary to print the transverse width of the image and the apparatus is arranged not to use apertures which are only able to print transversely outside the image area to be printed during a pass.

24. An apparatus according to claim 23, wherein the apparatus is arranged such that the non-used apertures are at both transverse ends of the row or rows of apertures and an aperture or apertures at both ends are simultaneously not used during a pass.

25. An apparatus according to any preceding claim, wherein the image receiving member is either formed from electrically conducting material or has a layer of electrically conducting material, for example aluminum, surrounding the circumference of the image receiving member.

26. An apparatus according to claim 25, wherein the surface of the conducting material that is facing the at least one printhead structure is at least partly covered by an electrically insulating material, for example aluminum oxide, such that a mirror electrostatic charge is formed on the conducting material, the mirror charge corresponding to the charge on any toner particles on the surface of the layer of electrically insulating material.

27. 7Λn apparatus according to any preceding claim, wherein the image receiving member includes perforations in at least part of its surface which receives toner particles, there being a vacuum source provided on the side of the image receiving member which is opposite to the at least one printhead structure, the vacuum source and the image receiving member being arranged such that the vacuum source can remove toner particles from the printhead structure via the perforations.

28. An apparatus according to any preceding claim, wherein the apparatus is constructed and arranged to be capable of varying the distance between adjacent columns of print and/or adjacent transverse lines of print.

29. An apparatus according to any preceding claim, wherein the image receiving member is in the form of a generally cylindrical drum which rotates about the axis of the cylinder such that lines on the surface of the drum which are parallel to the said axis pass the at least one printhead structure when the drum rotates.

30. An apparatus according to claim 29, wherein the drum is arranged for movement parallel to its own axis to effect transverse movement relative to the at least one printhead structure and at least some of the apertures in the at least one printhead structure are arranged in at least one row which is arranged parallel to the axis of the drum.

31. An apparatus according to claim 30, wherein the transverse movement of the drum relative to the at least one printhead structure is continuous while the image to be formed passes the at least one printhead structure.

32. An apparatus according to claim 31, wherein a partial revolution, or one revolution, or more than one revolution of the drum is effected without printing during a printing operation, preferably while changing the direction of the transverse movement relative to the at least one printhead structure to the opposite direction.

33. An apparatus according to claim 32, wherein said partial revolution, or one revolution, or more than one revolution of the drum without printing is effected between the printing of successive images.

34. An apparatus according to any of claims 29 to 33, wherein there are four or more printhead structures arranged around the circumference of the drum and at least one of the printhead structures is arranged to transport black toner particles.

35. An apparatus according to claim 34, further comprising a transfer station for transferring the image formed on the drum to another medium, e.g. paper.

36. An apparatus according to claim 35, wherein the transfer station is arranged such that the transfer of the image is started before the whole of the image has passed all of the printhead structures for the last time before the image is transferred by the transfer station.

37. An apparatus according to any of claims 34 to 36, wherein the apparatus is constructed and arranged such that one or more of the printhead structures starts the printing of a subsequent image before one or more of the other printhead structures have finished printing the preceding image.

38. Method for operating 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 method including: producing a background electric field which enables a transport of charged toner particles from said particle carrier towards said back electrode member, wherein at least one printhead structure is arranged in said background electric field, said printhead structure including a plurality of apertures and control electrodes arranged in conjunction to the apertures; 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; intercepting the transported charged particles by means of an image receiving member caused to move in relation to the printhead structure; transferring the image on said image receiving member to a print medium; and feeding said print medium to a fusing unit for permanently fixing the image on the print medium; characterized in that said method comprises: transferring said image to said print medium at a different speed than the speed of said fusing unit.

39. Method according to claim 38, characterized in that it comprises transferring said image to said print medium at a higher speed than that of said fusing unit.

40. Method according to claim 39, characterized in that it comprises temporarily accommodating said print medium in a buffer zone after transferring the image on the image receiving member to the print medium and before feeding said print medium to said fusing unit.