WO2002024461A1

WO2002024461A1 - Printhead structure and image recording device including such printhead structure

Info

Publication number: WO2002024461A1
Application number: PCT/SE2000/001837
Authority: WO
Inventors: Karin Bergman; Emma JÄRLGREN
Original assignee: Array Ab; Matsushita Electric Industrial Co., Ltd
Priority date: 2000-09-20
Filing date: 2000-09-20
Publication date: 2002-03-28
Also published as: AU2000279759A1

Abstract

Printhead structure (1) comprising a sheet-like substrate (2) of flexible and electrically insulating material, having a top surface (18) adapted to face in a direction towards a toner carrier, a bottom surface (19) adapted to face in a direction towards a recording medium, a plurality of apertures (3) arranged through the substrate (41) in at least one row and s set of deflection electrodes (7) arranged on said bottom (19) surface in conjunction with the apertures (3) wherein each deflection electrode in said set has a side facing one of said apertures (3).

Description

Printhead structure and image recording device including such printhead structure

FIELD OF INVENTION

The invention relates to a printhead structure according to the preamble of claim 1. The printhead structure is intended to be used in a direct electrostatic printing method, in which a stream of computer generated signals, defining an image information, are converted to a pattern of electrostatic fields to selectively control the deposition of charged toner particles in an image configuration directly onto an information carrier.

The invention further relates to an image recording device including such printhead structure.

DESCRIPTION OF RELATED ART

Of the various electrostatic printing techniques, the most familiar and widely utilized is xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect process since the visible image is first formed on an intermediate photoreceptor and then transferred to a paper surface.

Another method of electrostatic printing is one that has come to be known as direct electrostatic printing, DEP. This method differs from the aforementioned xerographic method in that charged toner particles are deposited directly onto an information carrier to form a visible image. In general, this method includes the use of electrostatic fields controlled by addressable electrodes for allowing passage of toner particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the toner particles to an image receiving substrate in image configuration. The novel feature of direct elecfrostatic printing is its simplicity of simultaneous field imaging and toner transport to produce a visible image on the substrate directly from computer generated signals, without the need for those signals to be intermediately converted to another fomi of energy such as light energy, as is required in electrophotographic printers, e.g. laser printers.

U.S. Patent No. 5,036,341, granted to Larson, discloses a direct printing method, which begins with a stream of electronic signals defining the image information. A uniform electric field is created between a high potential on a back electrode and a low potential on a toner carrier. That uniform field is modified by potentials on selectable wires in a two dimensional wire mesh array placed in the print zone. The wire mesh array consists of parallel control wires, each of which is com ected to an individual voltage source, across the width of the information carrier. A drawback of such a device is that, during operation of the wire mesh array, the individual wires can be sensitive to the potentials applied on adjacent wires, resulting in undesired printing due to interaction or cross-talk between neighbouring wires.

US Patent No. 5,121,144, also granted to Larson, discloses a control electrode array foπned of a thin sheet-like element comprising a plurality of addressable control electrodes and corresponding voltage sources comiected thereto. The control electrode array may be constructed of a flexible, electrically insulating material and overlaid with a printed circuit such that apertures in the material are arranged in rows and columns and are surrounded by electrodes. An electrostatic field on the back electrode attracts toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected control electrode to produce electrostatic fields which permit or restrict transport of toner particles from the particle carrier through the corresponding aperture. The modulated stream of charged particles allowed to pass through selected apertures impinges upon an infonnation carrier interposed in the particle stream to provide line-by-line scan printing to thereby form a visible image. The control electrodes are aligned in several transverse rows extending perpendicularly to the motion of the information carrier. All control electrodes are initially at a white potential Vw preventing all particle transport from the particle carrier. As image locations on the information carrier pass beneath apertures, coπesponding control electrodes are set to a black potential V_D to produce an electrostatic field drawing the toner particles from the toner carrier. Charged toner particles allowed to pass through the apertures are subsequently deposited on the information carrier in the configuration of the desired image pattern. The toner particle image is then made permanent by using heat and pressure to fuse the toner particles on the surface of the information carrier.

5 In WO 98/24634 discloses a printhead structure, which allows increased print addressability without increasing the number of apertures and associated control electrodes and print voltage sources. For example, a transverse print addressability of 600 DPI is achieved in accordance with the present invention utilising a printhead structure having only 200 apertures per inch in a transverse direction. The increased print adressability is achieved by modulating the particle

.0 stream from a particle source through any selected aperture of the printhead structure in several consecutive print steps by a control signal and deflection signals. The control signal is supplied to a control electrode surrounding the aperture to produce an electrostatic field which due to control in accordance with the image information selectively permits or restricts the particle stream through the aperture. The deflection signals are supplied to deflection

L5 electrodes to influence the convergence and the transport trajectory of the toner particle stream. An amplitude difference between deflection signals modifies the symmetry of the electrostatic field configuration, thereby deflecting the transport trajectory of the toner particle stream toward a predetermined dot location on the information carrier. The deflection signals are dimensioned to apply converging forces on the toner particle stream in order to focus the

20 toner transport onto said predeteπnined dot location. Accordingly, several dot locations can be addressed through the same aperture during each print sequence by sequentially influencing the symmetry and convergence of the electrostatic field configuration through the aperture, thereby modifying the position and reducing the size of each printed dot.

25 The deflection electrodes influence the electrical field provided by background electrodes, in particular in and in the vicinity of the apertures. The electrical field provided by the deflection electrodes thus influences the acceleration pattern of toner particles, especially in the apertures. Using prior art printhead devices it has shown that there is a risk for clogging of toner particles in the apertures as well as a risk for air brake down between a deflection 0 electrode and a background electrode generating an electrical field transporting toner particles from the toner carrier to the print medium. It is therefore desirable to generate an electric field where the time for the electric potential from the deflection electrodes to interact on toner particles passing an aperture is sufficient for providing sufficient deflection of the particles, and wherein the risk of clogging and air brake down between the deflection electrode and the background electrode is reduced.

SUMMARY OF THE INVENTION

The object of the invention is to provide an printhead structure capable of generating an electric field where the time for the electric potential from the deflection electrodes to interact on toner particles passing an aperture is sufficient for providing sufficient deflection of the particles, and wherein the risk of clogging and air brake down between the deflection electrode and the background electrode is reduced. This object is achieved by a printhead structure according to the characterising portion of claim 1, said printhead structure comprising a sheet-like substrate of flexible and electrically insulating material, having a top surface adapted to face in a direction towards a toner carrier, a bottom surface adapted to face in a direction towards a recording medium, a plurality of apertures aπanged through the substrate in at least one row and a set of deflection electrodes arranged on said bottom surface in conjunction with the apertures wherein each deflection electrode in said set has a side facing one of said apertures. By arranging said set of deflection electrodes such that the minimum distance from said side to said one of said apertures is less than a maximum cross- section of said one of said apertures, it will be possible to generate an electrical field in the apertures which obtains adequate toner particle deflection, while reducing the risk for clogging and the risk for air break down between the deflection electrode and a background electrode. The deflection electrodes influences the background field in the apertures both by the addition of a deflection voltage, which is asymmetric and thereby deflects toner particles in an intended direction and by influencing the background electric field, such that , in the apertures, a reduction of the gradient of the potential in the direction toward a print medium is obtained. The reduction of the gradient results in that toner particles are accelerated at a lower magnitude, when the deflection electrodes are positioned closer to the aperture. This results in that the same amount of deflection can be obtained by applying a lower voltage to the deflection electrodes. Hence the risk for air brake down between the deflection electrodes and a background electrode is reduced.

Brief description of the drawinfis Figure 1 is a schematic section view across a print zone in an image recording device in which a printhead structure in accordance with the present invention is utilized to control a particle stream from a particle source to an information carrier.

Figure 2 is an enlarged partial front view of the print zone.

5 Figure 3 is a partial plane view of the top surface of a printhead structure according to a preferred embodiment of the invention

Figure 4 is a partial plane view of the bottom surface of a printhead structure according to a preferred embodiment of the invention

Figure 5 is an enlargement of the printhead structure showing four apertures and their L0 associated control electrodes and deflection electrodes in superposition.

Figure 6 is a section view of the printhead structure across the section line I-I of Figure 5.

Figure 7 illustrates a printing method in accordance with the present invention, in which a transverse line, formed of nine dots is printed through three adjacent apertures.

Figures 8a and 8b illustrate examples of control functions during a print sequence including L5 three consecutive steps, whereas three dots are printed through a single aperture.

Figure 9a illustrates a section view of an aperture in a printhead structure and the associated convergence field.

Figure 9b illustrates a section view of an aperture in a printhead structure and the associated convergence and deflection field.

20 Figure 10 is an enlargement of an alternative embodiment of the printhead structure showing six apertures and their associated print electrodes and deflection elecfrodes in superposition, wherein four deflection electrodes are provided for each aperture.

Figure 11 illustrates the control functions during print sequence for the embodiment of figure 10 wherein alternate print sequences are performed in reverse order.

25 Figure 12 illustrates the dot location addressed during two consecutive print sequences by the embodiment of figure 10 when controlled by the control functions illustrated in Figure 11. Figure 13 show in detail a side view of a printhead structure, including one aperture and deflection and control electrodes aπanged in conjunction with the aperture.

Figure 14 show in detail a top view of a printhead structure, including one aperture and deflection and control electrodes arranged in conjunction with the aperture.

Figure 15 show equipotential lines for a printhead structure where the deflection electrodes are positioned at the same distance from an aperture as the control electrode.

Figure 16 show equipotential lines for a printhead structure where the deflection electrodes are positioned further from an aperture than the control electrode.

Detailed description of the preferred embodiments

A print zone in an image recording device, as schematically illustrated in Figures 1 and 2, consists of an electric field generated between a particle source 10 and a back electrode 13 to transport charged toner particles 17 therebetween; a printhead structure 1 positioned in the electric field to modulate the transport of charged toner particles 17 and an information carrier 11 onto which the transported particles 17 are deposited in an image configuration.

Image recording devices include generally several print zones each of which corresponds to a specific colour of the toner particles 17. The information carrier 11 is then fed in a single path consecutively through the different print zones whereas dots of different colours are superposed on the information carrier 11 to form coloured image configurations.

According to a preferred embodiment of the invention, a printhead structure 1 is preferably positioned between a particle source 10, such as a rotating cylindrical sleeve or any other device suitable for toner delivery, and an information carrier 11, such as a sheet of plain, untreated paper or any other medium suitable for direct printing, caused to move through the print zone at a predetermined, constant feed velocity Vp (arrow 12)-

As it is more apparent from Figure 3 and Figure 4, the printhead structure 1 includes:

an electrically insulating substrate layer 2 preferably formed of a non-rigid, flexible material, such as polyimide or the like, having a top surface (18, Figure 3) facing the particle source 10, a bottom surface (19, Figure 4) facing the information carrier 11 and a plurality of apertures 3 arranged through the substrate layer 2 to enable toner transport from the particle source 10 toward the information carrier 11;

a first printed circuit arranged on the top surface 18 of the substrate layer 2, comprising a set of control electrodes 4 each of which is disposed in relation to a corresponding aperture 3 in the substrate layer 2;

a second printed circuit arranged on the bottom surface 19 of the substrate layer 2, comprising at least one set of deflection electrodes 7; and

The first printed circuit arranged on the top surface 18 of the substrate layer 2, including the control electrodes, are connected to variable voltage sources 6 through a conducting part 5 for supplying control signals Nprint in accordance with the image information to the control elecfrodes 4.

Furthermore, the second printed circuit arranged on the bottom surface 19 of the substrate layer 2, including the deflection electrodes, are connected to at least one deflection voltage source 8, 9 for supplying deflection signals Dl, D2 in predetermined sequences to each set of deflection electrodes 7.

Although a printhead structure can take on various design without departing from the scope of the present invention, a preferred embodiment will be described hereinafter with reference to Figures 3,4,5, and 6.

The apertures 3 are preferably aligned in parallel rows 8 and columns, the parallel rows 8 extending transversally across the width of the print zone, preferably at a right angle to the feed motion 12 of the information carrier 11, and the columns being aligned at an appropriate angle to the feed motion 12 of the information carrier 11 to ensure complete coverage of the information carrier by providing an addressable area at every point across a line in a direction transverse to the feed motion 12 of the information carrier 11.

As it is more apparent from Figure 5 and Figure 6, the apertures 3 have preferably a circular section with a central axis 31 extending perpendicularly to the substrate layer 2. Each control electrodes 4 comprises a preferably ring-shaped part surrounding the periphery of its corresponding aperture 3, with a symmetry axis coinciding with the central axis 31 of the aperture 3 and an inner diameter which is equal to or sensibly larger than the aperture diameter.

Each aperture 3 is related to a first and a second deflection electrode 71, 72 spaced around a first and a second segment of the circumference of the aperture 3, respectively. The deflection electrodes 71, 72 are preferably semicircular or crescent-shaped and disposed symmetrically on each side of a deflection axis 32 extending diametrically across the circular aperture 3 at a predetermined deflection angle δ to the feed motion 12 of the information carrier, such that the deflection electrodes 71, 72 substantially border on a first and a second half of the circumference of their corresponding aperture 3, respectively.

All first and second deflection electrodes 71, 72 are connected to a first and a second deflection voltage sources 8, 9, respectively. The deflection voltage sources 8, 9 supply deflection signals Dl, D2 to the first set and the second set of deflection electrodes 71, 72, respectively, such each aperture is exposed to a superposition of Dl and D2.

Each pair of deflection electrodes 71, 72 is disposed symmetrically about the central axis 31 of its corresponding aperture 3 such that the electric field configuration remains substantially symmetric about the central axis 31 of the aperture 3 when Dl and D2 have the same amplitude.

As illustrated in Figure 5 and 6 at least one guard layer 15 is preferably arranged on the top surface 18 of the substrate layer 2 as a part of said first printed circuit. The guard layer 15 extends between the control electrodes 4 and is set on a guard potential which electrically shields the control electrodes 4 from each other thereby preventing interaction between adjacent control fields. As apparent from Figure 6, the printhead structure is preferably embedded within a thin protective layer 16 of electrically insulating material such as parylene or the like, arranged on both printed circuits to at least partially cover on both surfaces of the substrate layer and the inner wall of each aperture. The protective layer significantly reduces the interaction between the fields generated within an aperture by the corresponding control electrode and deflection electrodes.

The second circuit further includes a layer of semiconductive material (not shown) such as Silicium oxide or the like, arranged by sputtering or by any other suitable method on the protective layer to remove eventual charge accumulation due to undesired toner agglomeration in the vicinity of the apertures. The present invention is preferably used within an image recording device using a printing method performed as follows:

A substantially uniform electric field is produced between a background potential NBE ^on the back electrode 13 and a potential (preferably 0 N) on the particle source 10 to apply attractive electric forces on charged toner particles located on the particle source 10.

As image locations on the information carrier 11 pass beneath a row 8 of apertures 3, print sequences are performed to influence said attractive electric forces in order to modulate the stream of toner particles 17 in accordance with the image information.

Each print sequence includes several steps during each of which the particle stream through any selected aperture is controlled by the corresponding control electrode and deflection electrodes.

During each step, a control signal Nprint is supplied to each control elecfrode 4 to produce an electrostatic field about the coπesponding aperture.

The control signal Nprint has an amplitude chosen to be above or below a predetermined threshold value to respectively permit or restrict the transport of toner particles from the particle source through the actual aperture. The amplitude may have any level between a white potential N preventing all toner transport, and a black potential Nb coπesponding to full density dot. The control signal Nprint has a pulse width chosen as a function of the amount of toner particles intended to pass through the aperture. The pulse width may have any value between 0 and tb.

Every control signal pulse Nprint is followed by a period t_w during which new toner particles are supplied to the particle source.

During each step, a deflection signal Dl is supplied to a first set of deflection elecfrodes 71 and a deflection signal D2 is supplied to a second set of deflection electrodes 72, resulting in that an electric potential difference is produced between both sets of deflection electrodes.

That potential difference may have any value within a range -D to D, where -D corresponds to maximal deflection in a first direction and D corresponds to maximal deflection in the opposite direction. Every level of the potential difference corresponds to a specific transport trajectory of the toner particles. The deflection signals Dl, D2 apply repelling forces on toner particles causing the particle stream to converge toward a predetermined transport trajectory. Due to the symmetrical disposition of the deflection elecfrodes 71, 72 about the central axis 31 of their corresponding aperture 3, the field configuration remains substantially symmetrical as long as Dl = D2.

During each step, the deflection signals Dl and D2 produce a deflection field which applies converging forces on the particle stream. Those converging forces focus the stream upon a predetermined dot location. The dot location coincides with the central axis 31 of the aperture 3 only when Dl = D2. Deflected dots are obtained by producing an inequality Dl ≠ D2, thereby modifying the symmetry of the field configuration.

For instance, as illustrated in Figure 7, nine dots are printed in a continuous transverse line using apertures A, B, C. A print sequence comprises three consecutive steps tl, t2, t3. During a first step tl, the symmetry of the electrostatic field is modified to deflect the particle stream from its initial trajectory in a first direction, while the convergence of the electrostatic field is increased in that direction rl to focus the particle stream upon a first dot location. During a second step t2, the symmetry of the elecfrostatic field remains unaltered while its convergence is increased toward a central axis 31 of the aperture 3 to focus the particle stream upon a second, central dot location. During a third step t3, the symmetry of the electrostatic field is modified to deflect the particle stream from its initial trajectory in a direction r2 opposite to rl, while the convergence of the electrostatic field is increased about r2 to focus the particle stream upon a third dot location.

Accordingly, three focused dots can be printed through each single aperture during each print sequence. For instance, by modulating the deflections signal to obtain appropriate convergence and symmetry variations of the field configuration during the consecutive steps, the dot size and the dot deflection can be adjusted to meet the requirement of a 600 DPI print resolution utilizing 200 apertures per inch.

As shown in Figure 7, a first print sequence is performed as the dot locations pass beneath the first row 8a of apertures, whereas dots are printed through apertures A and C, and a second print sequence is performed similarly as the dot locations reach the second row 8b of apertures, whereas dots are printed through aperture B.

Figure 8a is a diagram showing the control signal Nprint and the deflection signals Dl, D2 as a function of time during a print sequence T wherein three transverse dots are printed. Figure 8b is a diagram showing another example of a control function with the confrol signal Nprint and the deflection signals Dl, D2 as a function of time during a print sequence T wherein three transverse dots are printed.

During a first step tl, the deflection signals Dl and D2 are dimensioned to deflect the dots in a first predetermined direction rl obliquely against the feed motion 12 of the information carrier 11.

During a second step t2, the deflection signals Dl and D2 have the same level, whereby the dots remains undeflected.

During a third step t3, the relation between Dl and D2 is reversed to obtain deflection in a direction r2 opposite to r 1.

Each step is characterized by a predetermined relation between both deflection signals Dl and D2. In the example shown in Figure 8, the deflection voltage sources are activated such that Dl > D2 during tl, Dl = D2 during t2, and Dl < D2 during t3.

Figure 9a shows a printhead structure, in which the toner particle stream is controlled by a control electrode 4 and deflection electrodes 71, 72 set on the same potential (Dl = D2). The field configuration preserves its symmetry and a convergence field is generated by the deflection electrodes 71, 72 to focus the toner particle stream toward a central axis 31 of the aperture 3, resulting in a focused, undeflected dot.

Figure 9b shows a printhead structure, in which the toner particle stream is controlled by a control electrode 4 and deflection elecfrodes 71, 72 set on different potentials (Dl ≠ D2). In that case, the toner particle stream is exposed to both a convergence field and a deflection field. The deflection field determines the transport trajectory 35 of the toner particle stream and the convergence field focus the stream toward the so determined transport trajectory 35.

According to the aforementioned method, a print resolution of 600 DPI is easily obtained by performing three-steps sequences on a 200 DPI printhead structure. A 200 DPI printhead structure comprises preferably two parallel rows comprising 100 aperture per inch, which implies that the distance between the central axis of two adjacent apertures of a row is 0.01 inch. Dots in a range 60 to 80 microns are obtained using apertures having generally a diameter in the order of 120 to 150 microns. In that case, the deflection length, i.e. the displacement of a deflected dot with respect to the central axis of the coπesponding aperture, is preferably 1/600 inch or about 42 microns.

The deflection angle δ is chosen to compensate the motion of the information carrier during a step, in order to provide transversally aligned dots. Thus, the deflection angle is dependent on the number of steps performed during a print sequence. The deflection angle is defined by the relation tan δ = 1/N, where N is the number of steps performed during a print sequence. For three-steps sequences, as described above, the deflection angle is thus preferably chosen to be about 18,4° while the deflection angle is about 45° when only two steps are performed. However, the present invention is neither limited to a specific number of steps nor a particular design of the deflection elecfrodes, the aforementioned embodiments being given only as illustrative examples.

The present invention is not either limited to two different sets of deflection electrodes. In some applications, it may be convenient to utilize more than two deflection electrodes around the apertures. For instance, it has been observed that the deflection field can be made more uniform by reversing every second print sequence, to alternate both deflection directions rl, r2. Instead of providing three transversally aligned dots in identical series (rl, center, r2) as described above, the series can be reversed to obtain rl, center, r2 - r2, center, rl. Hereby, the deflection field has not to be shifted between two opposite directions, resulting in constant, uniform step transitions. Such an embodiment is illustrated in Figure 10. A printhead structure is provided with four deflection elecfrodes 73, 74, 75, 76, spaced around each aperture 3 such that each deflection electrode borders on a segment of the periphery of the aperture 3. All similarly located deflection electrodes are connected to a corresponding deflection signal (Dl, D2, D3, D4). The deflection field is produced between two symmetrically disposed pairs of deflection electrodes. Figure 11 shows a control function with Dl, D2, D3, D4 as a function of time during two consecutive print sequences. For instance, every second print sequence is performed with three steps in the following order:

Dl = D2 > D3 = D4 during tl

Dl = D2 = D3 = D4 during t2, and

Dl = D2 < D3 = D4 during t3

and the remaining print sequences are performed in a reversed order: Dl = D4 > D2 = D3 during tl

Dl = D2 = D3 = D4 during t2, and

Dl = D4 < D2 = D3 during t3.

Accordingly, the dot locations addressed during two consecutive print sequences are alternated as illustrated in Figure 12, in a series [rl, center, r2, r3, center, r4], where r2 = -rl; r4 = - r3; rl and r3 are reversed with respect to the direction 12 of the motion of the information carrier 11.

From the foregoing it will be recognized that numerous variations and modifications may be effected without departing from the scope of the invention as defined in the appended claims.

Control electrodes are in some literature referred to as print electrodes.

Fig 13 and 14 show the position of the deflection electrode 7 in a printhead structure 1 in more detail. The printhead structure 1 comprises a sheet-like subsfrate 2 of flexible and electrically insulating material, having a top surface 18 adapted to face in a direction towards a toner carrier and a bottom surface 19 adapted to face in a direction μtowards a recording medium. The substrate could be made of any suitable material having proper electric and mechanical properties, such as for instance XXXX. The substrate has a thickness T of less than 200 μm, preferably less than 100 μm, still preferably less than 50μm. A plurality of apertures 3 are arranged through the substrate 2. The apertures are preferably arranged in at least one row, in the feed direction of a print medium, stretching across the substrate. In conjunction with the apertures 3, a set of deflection electrodes 7 is aπanged on said bottom surface 19. The deflection electrodes are, in a prefeπed embodiment covered by an insulation layer 16. The top surface of the substrate 2 and the control electrodes are preferably covered with one guard layer 15. The deflection electrodes 7 are preferably produced by etching, but could be arranged on the substrate 2 in any other way known to the skilled in the art.

Each deflection electrode 7 in said set has a side 20 facing one of said apertures 3. The deflection elecfrodes 7 are preferably arranged in pairs 7a, 7b arranged, preferably on opposite sides of an aperture for creating an electric field providing a deflection of a toner particle trajectory passing through the aperture. In a preferred embodiment the substrate holds two deflection electrodes per aperture whereby a possibility of deflection in two mutually opposite directions are possible by applying different potentials to the deflection electrodes. It is also possible to arrange more than two deflection electrodes per aperture, for example four, whereby deflection along more than one deflection direction is possible. According to the invention the deflection elecfrodes are aπanged such that a minimum distance d from the side 20a, 20b of each deflection electrode 7a, 7b to the closest aperture 3 is less than a maximum cross-section D of a corresponding apertures 3. The maximum cross- section of an aperture is normally in the range of 120 - 150 μm. The thickness of a deflection electrode, in a direction vertical to the surface of the substrate, is preferably less than 20μm.

In a preferred embodiment of the invention the minimum distance d from said side 20 to said one of said apertures 3 is less than the thickness T of the sheet-like substrate 2. In a still more prefeπed embodiment of the invention the minimum distance d from the side of the deflection electrode 3 to the closest aperture is less than 150μ. In an embodiment of the invention the minimum distance d from the side of the deflection elecfrode 3 to the closest aperture is less than lOOμ and in a further embodiment of the invention the minimum distance d from the side of the deflection electrode 3 to the closest aperture is less than 50μ.

In a preferred embodiment where the deflection electrodes 7 are arranged in pairs, the deflection electrodes 7a, 7b are arranged with a maximum distance M from a point on one of the sides on a portion P of one of the deflection electrodes 7a in said pair to a diametrically opposite point on a coπesponding portion of said side of the other deflection electrode 7b in said pair is less than 3 times the largest aperture cross-section D. This restriction regarding the distance from one deflection electrode to a corresponding deflection electrode in a pair holds in a portion P which encompasses a centre angle α of said aperture 3 of at least 20°. By this aπangement, the distance from each deflection electrode in the deflection direction is kept below a maximum distance, whereby deflection could be arranged without applying to high potentials to the electrodes. In a preferred embodiment, the centre angle encompasses 45°. In order to facilitate for keeping low variation in the maximum distance from the electrode at a specified angle, measured from the centre C of an aperture , from the transverse direction of substrate, the deflection electrodes are preferably formed such that the portions (P) of the deflection electrodes 7a, 7b are arcuate with concave sides 20a,20b facing one of said apertures 3. Furthermore, in a preferred embodiment, a set of control electrodes 4 are arranged on said top surface in conjunction with the apertures 3, each control electrode in said set having a side 21 facing one of said apertures 3. The control electrodes 4 are aπanged such that a minimum distance d_c from the side 21 of said control electrode 4 to said one of said apertures 3 is less than the minimum dd distance from the side of said at least one deflection electrode 7 to said one of said apertures 3.

In a preferred embodiment, the confrol electrodes 4 and the deflection electrodes 7 are arranged such that the minimum distance _d from the side 20 of said deflection elecfrode 7 to said one of said apertures 3 exceeds the minimum distance d_cfrom the side 21 of said control electrode 4 to said one of said apertures 3 with more than 5%.

In an other embodiment of the invention the side 21 of said control electrodes 4 includes a lower edge 22 facing said top surface rSTThe sid 2O!TFuιeJleΗection electtodeT7^"mclude_r an upper edge 23 facing the bottom surface 19 of the substrate 2.The edges of the control electrodes 4 and the deflection elecfrodes 7 are arranged such that a tangent T through said upper edge and said lower edge forms an angle β of between 10° and 65° with a surface normal N of the printhead structure at any cross section of a portion of the printhead structure encompassing a deflection direction 24 with an angle of at least ± 10°.

In a preferred embodiment of the invention the control elecfrodes are essentially ring shaped.

Figures 15 and 16 illustrate the influence of the position of deflection electrodes on the electrical field, in particular in the apertures. The figures show equipotential lines when a background field is generate by a field electrode and specific potentials are applied to the deflection electrodes and the control electrodes. The control electrodes are kept at a potential of 300 N and the deflection elecfrodes are kept at 100 N and 120 N respective. In fig 14 the control electrodes and the deflection electrodes are positioned with a 50 μm centrum distance. In fig 15 the position of the deflection electrodes is changed to a centrum distance of 150 μm. When comparing fig 16 with fig 16 it can be seen that the gradient of the potential is greater in the aperture in the example shown in fig 16, where the deflection electrodes are arranged at greater distance, than in the example shown in fig 15. This means that the time available for deflection of toner particles are greater in the example shown in fig 15 since the acceleration in the z- direction, i.e, towards a recording medium, is greater in the example in fig 16. As a result, the amount of deviation is greater in fig 15 than in fig 16. The x-axis is graded in the quantity μm and the y-axis in Nolt.

Claims

Printhead structure (1) comprising a sheet-like substrate (2) of flexible and electrically insulating material, having a top surface (18) adapted to face in a direction towards a toner carrier, a bottom surface (19) adapted to face in a direction towards a recording medium, a plurality of apertures (3) arranged through the substrate (2) and a set of deflection elecfrodes (7) aπanged on said bottom surface (19) in conjunction with the apertures (3) wherein each deflection electrode (7) in said set has a side (20) facing one of said apertures (3) characterised in that the minimum distance (d) from said side (20) to said one of said apertures (3) is less than a maximum cross-section (D) of said one of said apertures (3).

Printhead structure according to claim 1, characterized in that the minimum distance (d) from said side (20) to said one of said apertures (3) is less than the thickness (T) of the sheet-like substrate (2).

Printhead structure according to claim 1 or 2, characterized in that the minimum distance (d) from said side to said one of said apertures (3) is less than 150μ.

Printhead structure according to claim 3, characterized in that the minimum distance (d) from said side to said one of said apertures (3) is less than lOOμ.

Printhead structure according to claim 3, characterized in that the minimum distance (d) from said side to said one of said apertures (3) is less than 50μ.

Printhead structure according to any of claims 1-5, wherein the set of deflection elecfrodes (7) includes a pair of deflection electrodes (7a, 7b) for each aperture (3), each deflection electrode (7a,7b) in said pair having a side (20a,20b) facing one of said apertures (3) arranged at diametrically opposite sides of said at least one aperture (3), characterized in that the maximum distance (M) from a point on one of the sides on a portion (P) of one of said deflection electrodes (7a) in said pair to a diametrically opposite point on a coπesponding portion of said side of the other deflection electrode (7b) in said pair is less than 3 times the largest aperture cross-section (D), wherein said portion (P) encompasses a centre angle (α) of said aperture (3) of at least 20°.

Printhead structure according to claim 6, characterized in that said portion encompasses a centre angle (α) of said aperture (3) of at least 45°.

Printhead structure according to claim 6 or 7, characterized in that said portions (P) are arcuate with concave sides (20a,20b) facing one of said apertures (3).

Printhead structure according to any of claims 1 -8, wherein a set of control electrodes (43) are arranged on said top surface in conjunction with the apertures (3), each control electrode in said set having a side facing one of said apertures (3) characterized in that the minimum distance from the side of said control electrode to said one of said apertures (3) is less than the minimum distance from the side of said at least one deflection electrode (7) to said one of said apertures (3).

Printhead structure according to claim 9, characterized in that the minimum distance from the side of said deflection electrode (7) to said one of said apertures (3) exceeds the minimum distance from the side of said control electrode to said one of said apertures (3) with more than 5%.

Printhead structure according to claims 9 or 10, wherein said side of each confrol electrode has a lower edge facing said top surface (18), said side of each deflection elecfrode (7) has an upper edge facing said bottom surface (19), c h a racterized in that a tangent through said upper edge and said lower edge forms an angle of between 10° and 65° with a surface normal of the printhead structure at any cross section of a portion of the printhead structure encompassing a deflection direction with an angle of at least +10°.

Printhead structure according to claims 9 or 10, characterized in that each control electrode includes an arcuate portion having a concave side facing one of said apertures (3), wherein said minimum distance occurs at the arcuate portion.

Printhead structure according to claim 11, characterized in that the control electrodes are essentially ring shaped.

Printhead structure according to any of the preceding claims, characterized in that the apertures are arranged in at least one row extending transversally to a print direction.

Image recording device including a printhead structure according to any of the preceding claims.