EP0816944B1 - A direct electrostatic printing (DEP) device maintaining a constant distance between printhead structure and toner delivery means - Google Patents

Description

1. Field of the Invention

This invention relates to a printing device useful in directelectrostatic printing (DEP). In DEP, electrostatic printing isperformed directly from a toner delivery means on a tonerreceiving member by means of an electronically addressableprinthead structure.

2. Background of the Invention

In DEP (Direct Electrostatic Printing) the toner or developingmaterial is deposited directly in an imagewise way on a receivingsubstrate, the latter not bearing any imagewise latentelectrostatic image. In the case that the substrate is anintermediate endless flexible belt (e.g. aluminium, polyimideetc.), the imagewise deposited toner must be transferred ontoanother final substrate. If, however, the toner is depositeddirectly on the final receiving substrate, a possibility isfulfilled to create directly the image on the final receivingsubstrate, e.g. plain paper, transparency, etc. This depositionstep is followed by a final fusing step.

This makes the method different from classical electrography,in which a latent electrostatic image on a charge retentivesurface is developed by a suitable material to make the latentimage visible. Further on, either the powder image is fuseddirectly to said charge retentive surface, which then results in adirect electrographic print, or the powder image is subsequentlytransferred to the final substrate and then fused to that medium.The latter process results in an indirect electrographic print.The final substrate may be a transparent medium, opaque polymericfilm, paper, etc.

DEP is also markedly different from electrophotography inwhich an additional step and additional member is introduced tocreate the latent electrostatic image. More specifically, aphotoconductor is used and a charging/exposure cycle is necessary.

A DEP device is disclosed in e.g. US 3,689,935. This documentdiscloses an electrostatic line printer having a multi-layeredparticle modulator or printhead structure comprising :

• a layer of insulating material, called isolation layer ;
• a shield electrode consisting of a continuous layer ofconductive material on one side of the isolation layer ;
• a plurality of control electrodes formed by a segmented layerof conductive material on the other side of the isolationlayer ; and
• at least one row of apertures.

Each control electrode is formed around one aperture and isisolated from each other control electrode.

Selected potentials are applied to each of the controlelectrodes while a fixed potential is applied to the shieldelectrode. An overall applied propulsion field between a tonerdelivery means and a receiving member support projects chargedtoner particles through a row of apertures of the printheadstructure. The intensity of the particle stream is modulatedaccording to the pattern of potentials applied to the controlelectrodes. The modulated stream of charged particles impingesupon a receiving member substrate, interposed in the modulatedparticle stream. The receiving member substrate is transported ina direction orthogonal to the printhead structure, to provide aline-by-line scan printing. The shield electrode may face thetoner delivery means and the control electrode may face thereceiving member substrate. A DC-field is applied between theprinthead structure and a single back electrode on the receivingmember support. This propulsion field is responsible for theattraction of toner to the receiving member substrate that isplaced between the printhead structure and the back electrode.The printing device as described in the original Pressman patentis very sensitive to changes in distances from the tonerapplication module towards said shield electrode, leading tochanges in image density.

The problem of keeping this distance constant has beenaddressed in several ways.

In EP-A 675 417 it is disclosed to use a magnetic brush astoner delivery means, using a two-component developer (comprisingtoner and carrier particles), and to provide "long hairs" on saidbrush so that the hairs touch the printing structure. In thatcase slight deviations in distance between the surface of thetoner delivery means and the printhead structure do no longerpresent problems, while in any case the hairs of the brush, madeup by carrier particles and toner particles are in contact withthe printhead structure. It was found that such a device couldprovide very good printing results, but yielded only adequate optical density in the print when the printing speed was not toohigh. The problem of varying image density, that can remain in adevice according to EP-A 675 417, due to a varying distancebetween the surface of the magnetic brush and the printheadstructure can further be decreased by adapting the electricalconductivity of the carrier particles used on the magnetic brushas described in European Application 96202815, filed on October10, 1996.

For devices working at quite high printing speeds, the use ofa charged toner conveyer (a CTC), whereon the toner particles canbe deposited by a magnetic brush or any other means known in theart, presents advantages. But the problem of uneven density(white banding) in a direction perpendicular to the printingdirection has to be solved.

In EP-A 740 224 a device is described in which the frequencyof said density banding (in a direction perpendicular to theprinting direction) due to the variation of the distance from thetoner application module towards said printhead structure isdiminished. To achieve this better evenness in printing, it isdisclosed to give the toner bearing surfaces of the toner deliverymeans rather high rotational speeds. Since the surfaces that bearthe toner particles rotate very fast and the distance between saidtoner bearing surfaces and the printhead structure is low, theparticles are exposed to quite large shearing force. This highshearing force can give raise to agglomeration and/or deformationof the toner particles (especially when in the toner particlespolymeric toner resins with low (< 60 °C) Tg are used. Thus theprinting apertures can be clogged by agglomerated or deformedtoner particles, leading to images with missing dots and bad imagequality.

In US 5,552,814 it is disclosed to use a device wherein theCTC and the printhead structure are in close contact. Such adevice does indeed decrease the banding in the directionperpendicular to the printing direction, but, as with the fastmoving CTC's in EP-A 740 224 referred to above, the particles areexposed to quite large shearing force. This high shearing forcecan give raise to agglomeration and/or deformation of the tonerparticles and thus to some clogging of printing apertures. Todiminish that problem it has been proposed in US 5,497,175 toprovide a layer with very low coefficient of friction on the faceof the printhead structure contacting the CTC or, in US 5,539,438,to provide a layer with low coefficient of friction on the surface of the CTC. These layers may influence the charge or thechargeability of the toner particles and can thus, in someinstances, negatively influence the printing quality.

In US 5,448,272 an other approach to diminish the shearingforces on the toner particles in a DEP device wherein the CTCcontacts the printhead structure has been disclosed. On the faceof the printhead structure contacting the CTC a kind of guidingmembers are provided in the spacing between the printingapertures, and only these guiding members are in contact with theCTC. The guiding members are wedge shaped, with the point of thewedge against the toner feeding direction. In operation theguiding members, that keep the distance between the printheadstructure and the CTC constant, "plough" through the layer oftoner particles on the CTC and guide the particles to the printingapertures. A drawback of this device is the difficulty ofmanufacturing such a printhead structure with the desired accuracyfor high resolution printers (50 dpi (dot per inch) or 20 dots/cm)or higher. A high resolution printer necessitates a printheadstructure with small apertures and small spacing between theprinting apertures, necessitating a very accurate positioning ofthe guiding members.

In WO-A-95/24 675 a guiding or spacer means is placed before the printing apertures over the full length.

In JP-A 08/300715 a printhead structure with "guiding means"is disclosed, wherein the guiding means are placed before theprinting apertures and form an angle with the direction ofmovement of the toner delivery means. Again at least one guidingmeans per printing aperture is provided. Thus also in this devicea very accurate positioning of the guiding members is required,which complicates the manufacture of the printhead structure.

It is thus still desired to have DEP printing devices whereinthe distance between the toner delivery means and the printheadstructure is kept constant, wherein the toner particles are notsubjected to excessive shear and that is relatively simple tomanufacture.

3. Objects and Summary of the Invention

It is an object of the invention to provide a directelectrostatic printer that makes it possible to print at highspeed, with good evenness of printing.

It is an other object of the invention to provide a directelectrostatic printer that makes it possible to print at highspeed and wherein the clogging of the printing apertures by toner particles is minimized.

Further objects and advantages of the invention will becomeclear from the description hereinafter.

The objects of the invention are realized by providing a DEPdevice comprising a printhead structure having a first and asecond major face, and a toner delivery means having an externalsurface wherein

i) said first major face of said printhead structure faces saidexternal surface of said toner delivery means, and

ii) said external surface moves in a direction A with respect tosaid first major surface, and is kept at a constant distance D ofsaid first major face by at least two spacing means, forming anangle between 45 ° and 0° with said direction A, characterised inthat said spacing means are placed at a distance d from each othersuch that 1 cm ≤ d ≤ 50 cm.

In a preferred embodiment said spacing means are placed at adistance d from each other such that 5 cm ≤ d ≤ 35 cm.

In a further preferred embodiment said constant distance D issuch that 25 µm ≤ D ≤ 500 µm.

4. Brief Description of the drawings.

Fig. 1 shows a schematic view, viewed from the outer surface ofthe toner delivery means, of a printhead structure according tothis invention.

Fig. 2 shows a schematic sectional view of a printhead structureaccording to this invention, mounted in contact with a tonerdelivery means.

Fig. 3 shows a schematic view, viewed from the outer surface ofthe toner delivery means, of a further embodiment of a printheadstructure according to this invention.

5. Detailed Description of the Invention

Hereinafter the wording "toner delivery means" is used asthat part of the device that brings toner particles in thevicinity of the printhead structure.

It was found that it was possible in a DEP printing device tokeep the distance D between the printhead structure and the tonerdelivery means constant by far less spacing means than disclosedin e.g. US 5,448,272. It was found that it was possible to keepsaid distance D constant using between the printhead structure and the toner delivery means spacing means that were at least from 1cm up to 50 cm apart (the distance between the spacing means ismeasured between the faces of the spacing means facing eachother). In a printer having a resolution of 20 dots per cm (50dpi or dots per inch) this means that, with the spacing means 1 cmapart, at most every 20 printing apertures a spacing means ispresent, which largely facilitates the construction of theprinting device. It was found that a very satisfying compromisebetween ease of manufacture and evenness of the printing could beachieved when the spacing means were between 5 and 35 cm, bothlimits included, apart.

It was further found that the spacing means, in a deviceaccording to this invention, were advantageously constructed suchthat the distance D between the printhead structure and the tonerdelivery means was between 25 and 500 µm, more preferably between50 and 300 µm. The advantage of the larger distance D (in e.g. US5,448,272 it is said that a distance D of one toner diameter isenough) is not only a substantial prevention of toner smearing byshearing forces exerted on the toner and of the clogging of theprinting apertures, but also that the device can be operated withmore than 1 layer of toner particles on the surface of the tonerdelivery means. Having more than 1 layer of toner particles onthe toner delivery means, brings more toner particles per unit oftime available for printing and thus provides high printingdensity at high printing speed.
It was also found that, in a device according to this invention,it was advantageous for high printing speed that on the tonerdelivery means an AC-field was superimposed. This AC-field isthen applied to the surface (sleeve) of the toner delivery means.This AC-field creates, in the gap determined by the distance D, akind of cloud of free vibrating toner particles. This cloud offree vibrating toner particles brings even more toner particlesper unit of time available for printing and thus provides in highprinting density at high printing speed. The AC-field, in adevice according to this invention, has preferably a frequencybetween 1 and 5 kHz and can have a sinusoidal, square ortriangular shape, it can be a symmetrical as well as anasymmetrical signal. In a device according to this invention thisAC-field can be applied on the sleeve of the toner delivery means,when the DC-voltage on the sleeve of the toner delivery means is 0(i.e. the sleeve is grounded) as well as when the DC-voltage onthe sleeve has either a positive or a negative value. The root mean square voltage (V^rms) of the AC-field superimposed on the DC-fieldbetween printhead structure and toner delivery means,preferably relates to the voltage of the DC-field. More preferablythe root means square voltage of the AC-field (V^rms)is preferablyat least 10 times higher than the voltage of the DC-field on whichit is superimposed. When the voltage of the DC-field on thesleeve is zero (i.e. when the sleeve is grounded) and an AC-fieldis applied, then |AC/DC| = ∞ , which is the upper limit for theratio |AC/DC|.

The ratio of AC/DC is taken as an absolute value, (|AC/DC|) sothat the ratio is independent of the sign of the DC-field.

The spacing means in a device according to this invention canhave different shapes, it can be a row of dots, a row of bars, abar, they can be rectangular, cylindrical, triangular, etc as longas they perform the effect of keeping the distance between theprinthead structure and the toner delivery means constant.The spacing means can be made of any material, although spacingmeans made of insulating material, e.g. polymeric material,ceramic material, are largely preferred. The insulating materialcan preferably be a flexible polymeric material as e.g. apolyester, a polyimide, a polyamide, a polyurethane, apolycarbonate, etc.

The face of the spacing means contacting the outer surface ofthe toner delivery means can be provided with a friction reducinglayer for aiding the smooth gliding of the face of the spacingmeans over the surface of the toner delivery means. Such a layercan comprise a solid lubricant dispersed in a binder, e.g.disulfide of molybdenum dispersed in a binder, as disclosed in US5,497,175, the layer can be made with a perfluorpolymer (e.g.TEFLON (tradename), the friction reducing layer can comprisematting agents protruding above the layer, which diminish thesurface of the spacing means contacting the surface of the tonerdelivery means. Such spacing particles can advantageouslycomprise a lubricant (e.g. a wax) as described in EP-A 241 600 orcomprise fluor-containing compounds.

The spacing means used in a device according to this inventioncan be permanently attached to the face of the printhead structurefacing the external surface of the toner delivery means or can beplaced between said face of the printhead structure and thesurface of the toner delivery means. In the latter case thespacing particles are kept in place by pressing the toner deliverymeans against the spacing means that are pressed against the printhead structure.

In a device according to this invention, the printheadstructure is preferably kept in contact with the surface of thetoner delivery means via the spacing means, permanently mounted onthe printhead structure or simply interposed between the printheadstructure and the outer surface of the toner delivery means, byresilient means. When the printhead structure comprises aresilient substrate, e.g. a polymeric sheet material, beingbetween 50 and 400 µm thick, wherein printing apertures arepresent the contact of the printhead structure with the tonerdelivery means, via the spacing means, can be assured by the ownresilience of the printhead structure. Suitable material to beused as polymeric sheet material, with a certain resilience, in aprinthead structure useful in this invention are e.g. polyester,polyimide, polyamide, polyurethane, polycarbonate, etc.

Even if the printhead structure has a certain resilience ofits own, it is preferred, in a device according to this invention,to use resilient means to keep printhead structure and tonerdelivery means in contact.

The resilient means keeping the printhead structure and thetoner delivery means in contact via the spacing means can besprings, coil springs, rubber bands, etc.

Fig. 1 shows a schematic view, viewed from the outer surfaceof the toner delivery means, of a printhead structure according tothis invention. The printhead structure (100) comprises an arrayof printing apertures (101) located between two spacing means(102). The spacing means are positioned over a distance (d)apart. When such a printhead structure is mounted in a DEP devicewith the spacing means (102) in contact with the toner deliverymeans, several forces, in fig. 1 represented by the arrows, areexerted on the printhead structure. In the direction parallelwith the array of printing apertures (herein after termed thelength of the printhead structure), forces f_ℓ1, f_ℓ2, f_ℓ3 and f_ℓ4are exerted on the printhead structure. The sum of these forceswill be referred to as F_ℓ. In the direction perpendicular to thearray of printing apertures (herein after termed the width of theprinthead structure), forces f_w1, f_w2, f_w3, f_w4, f_w5 and f_w6 areexerted on the printhead structure. The sum of these forces willbe referred to as F_w. The forces f_w2 and f_w5 are exerted betweenthe spacing means, and the sum of them will be referred to as F_wb.Forces f_w1, f_w3, f_w4 and f_w6 are exerted outside the spacingmeans, and the sum of them will be referred to as F_wo. The values of F_ℓ and F_w are related to the distance whereover they act onthe printhead structure and are thus expressed in N/m. In fig.1the forces f_ℓ1 and f_ℓ2 have points of application spaced by adistance of n meter, thus F_ℓ = (f_ℓ1 + f_ℓ2 + f_ℓ3 + f_ℓ4)/n . In fig.1 forces f_w1 and f_w3 have points of application spaced by adistance of m meter, thus F_w = (f_w1 + f_w2 + f_w3 + f_w4 + f_w5 +f_w6)/m . In fig. 1 F_w is made up of six forces, two acting betweenthe spacing means and four acting outside the spacing means, thusF_wb = 2/6 times F_w and F_wo = 4/6 times F_w.

The forces, acting on the printhead structure, are in figure 1shown as being exactly parallel or exactly perpendicular to thedirection of the printing apertures. However the resilient meansexerting said forces, can, in a DEP device according to thisinvention, be placed in a position deviating from the exactparallel or exact perpendicular position.

Although with any resilient positioning of a printheadstructure in a DEP device according to the present invention couldachieve good printing density at acceptable speed combined withgood printing evenness and could largely avoid the smearing oftoner and the clogging of printing apertures, it was found thatthe printing quality with a device according to the presentinvention could further be ameliorated when the forces exerted onthe printhead structure by the resilient means were adapted toeach other. The printing quality was found to be ameliorated whenF_w/F_ℓ ≤ 0.5, , and substantially ameliorated when F_w/F_ℓ ≤ 0.25.When further F_w was adapted to the distance d between the spacingmeans, such that F_w/d < 5000, the printing quality was furtherenhanced. Further improvement of the printing quality could beobserved when F_wb/F_wo ≤ 1.00.

In a device according to this invention, the printheadstructure is mounted in contact, via the spacer means, with thetoner delivery means in such a way that the part of the printheadstructure being in contact, via the spacer means, with the tonerdelivery means has a kind of a bulge extending between 0.2 and 20mm above the line connecting the surface of supporting means forthe printhead structure, that faces the printhead structure,preferably between 1 and 5 mm (both limits included). A schematicview of a printhead structure according to thus invention combinedwith a toner delivery means is given in fig 2. In this figure,the numerical 104 denotes means for supporting theprintheadstructure 100 and 103 denotes the toner delivery means, thedriving means for said toner delivery means and the supporting means to support the toner delivery means in the DEP device arenot shown. Arrows f_w3 and f_w4 denote forces exerted on theprinthead structure in the length of this printhead structure andare thus part of F_w. Theprinthead structure 100 is pressedagainst the toner delivery means 103 by the forces f_w3 and f_w4over spacing means 102. A portion of spacing means 102 shown by adotted line, to show theprinting apertures 101 in the printheadstructure. The distance between theline 105 connecting thesurfaces of the supportingmeans 104 and the printing apertures (bin fig. 2) is the kind of bulge describe immediately above. Theheight of the bulge (b) is preferably such that 0.2 ≤ b ≤ 20 mm,more preferably such that 1 ≤ b ≤ 5 mm.

A printhead structure according to this invention can be madein various forms : the printhead structure can carry one array ofprinting apertures (throughout this document, an "array ofprinting aperture" is used to indicate at least one row ofprinting apertures) between spacing means. It can carry aplurality of arrays of printing apertures, each of said arraysbeing located between two spacing means and the arrays beingstaggered. In fig. 3 such a printhead structure is schematicallyshown : Two arrays of printing apertures (101a, 101b) are locatedadjacent to each other and each is located between two spacingmeans (102a, 102b and 102c) spacing means 102b being common to thetwo arrays of apertures. A third array of printing apertures(101c) between two spacing means (102d and 102e) is located in astaggered way with respect to the first two arrays of printingapertures. The spacing means are located at a given distance ofeach other (d, d' and d''). In figure 3 d, d' and d'' are shownas being equal, but this is not necessarily so. Any printheadcarrying a plurality of arrays of printing apertures, said arrayshaving equal or different numbers of printing apertures, each ofsaid arrays being located between two spacing means and the arraysbeing staggered, is within the scope of the invention. E.g. aprinthead structure with more than three arrays of printingapertures, that are equal or different in length, that comprisethe same or a different number of rows of printing apertures andwherein the spacing means are equally or differently spaced is aprinthead structure according to this invention.

A printhead structure useful in the present invention can alsobe a mesh shaped type of printhead structure as disclosed in e.g.US 5,036,341. Preferably a printhead structure for use in adevice according to this invention comprises a sheet of isolating material, preferably a polymeric material, wherein printingapertures are present. The toner flow form the toner deliverymeans to the receiving substrate in a DC-field between said tonerdelivery means and said substrate is controlled by controlelectrodes on the printhead structure. Various forms of controlelectrodes are possible, there can be a control electrode aroundevery printing aperture, a single control electrode can control arow of printing apertures, as disclosed in e.g. US 5,121,144, thecontrol electrodes around each printing aperture can be segmentedas disclosed in e.g. WO 94/26527. A printhead structure used inthis invention can comprise a shield electrode on the face of theprinthead structure opposite to the face carrying the controlelectrodes. Such a shield electrode can have various forms, itcan be a continuous electrode, or it can have a form adapted tothe printing apertures, as disclosed in European Application96201622, filed on June 11, 1996.

The DC-field between the toner delivery means and the tonerreceiving substrate can be provided by applying a potentialdifference between said toner delivery means and a back electrodepositioned behind said substrate, or it can be provided byapplying a potential difference between said toner delivery meansand said substrate itself as disclosed in European Application96202228, filed on August 8, 1996.

The toner delivery means in a DEP device according to thisinvention, is a means comprising an endless outer surface forcarrying charged toner particles. It may be cylindrical or canhave the form of an endless belt. The charged toner particles canoriginate from a magnetic mono-component developer. In that caseit is beneficial that the toner delivery means comprise a magneticfield for attracting the charged toner particles to its surface.In other embodiment of this invention, the toner delivery means isan applicator using non-magnetic mono-component developer, i.e.the surface of the toner delivery means carries non-magneticcharged toner particles.
In a further embodiment of the invention, the charged tonerparticles on the surface of the toner delivery means originatefrom a multi- (two-)component developer comprising magneticcarrier particles and non-magnetic charged toner particles, thenthe toner particles are deposited from a magnetic brush (whereinboth carrier and charged toner particles are present) on the tonerdelivery means, whereon only charged toner particles are present. DEP devices using a magnetic brush carrying magnetic carrierparticles and non-magnetic charged toner particles for bringingcharged toner particles on to the toner delivery means, aredescribed in e.g. EP-A 740 224.

In still an other embodiment of the invention the tonerdelivery means is a magnetic brush comprising magnetic carrierparticles and non-magnetic toner particles and the toner particlesare directly extracted from the "hairs" of the magnetic brush. Inthis embodiment the spacing means contact the sleeve of themagnetic brush and keep the distance between the magnetic brushand the printhead structure constant.

Whatever the way to bring toner particles (optionally incombination with carrier particles as "hairs" of a magnetic brush)to the surface of the toner delivery means it is preferred thatthe width of the toner layer (or of the layer of hairs of themagnetic brush) applied to the toner delivery means is smallerthan or equal to the distance between the spacing means at the endof the array of printing apertures. By doing so, the spacingmeans do not squeeze toner particles between the surface of thetoner delivery means and the surface of the spacing meanscontacting the toner delivery means, this means that no physicalstrain is imposed on the toner particles and that thus tonersmearing, clogging of printing apertures by deformed (half molten)toner particles are largely prevented.

EXAMPLESThe printhead structure.

A printhead structure was made from a polyimide film of 50 µmthickness, double sided coated with a 5 µm thick copper film. Onthe back side of the printhead structure, facing the receivingsubstrate, a rectangular control electrode was arranged aroundeach rectangular aperture. Each of said control electrodes wasconnected over 2 MΩ resistors to a HV 507 (trade name) highvoltage switching IC, commercially available through Supertex,USA, that was powered from a high voltage power supply. On thefront side of the printhead structure, facing the toner deliverymeans, a common shield electrode was present. The printheadstructure had two rows of apertures. The apertures had anaperture size of 360 µm x 120 µm The size of the copper controlelectrodes was 520 µm x 280 µm. The rows of apertures were staggered to obtain an overall resolution of 85 dpi. Theprinthead structure comprised a shield electrode, having a slit of1.6 mm wide, the printing apertures being located in said slit.This is a shield electrode and a printhead structure according tothe disclosures in European Application 96201622 filed on June 11,1996. The printhead structure comprised two strips ofpolyurethane as spacing means. The spacing means were 300 µm thickand 10 mm wide and were placed at a distance (d) of 0.165 m.

The toner delivery means

The toner delivery means was a commercially available tonercartridge comprising non magnetic mono component developer, theCOLOR LASER TONER CARTRIDGE MAGENTA (M3760GIA), for the COLORLASER WRITER (Tradenames of Apple Computer, USA).

The printing engine

The printhead structure and the charged toner conveyer weremounted in a frame and the printhead structure was stretched insaid frame by spring coils and pressed against said charged tonerconveyer, via the spacing means. The distance between the chargedtoner conveyer and the printhead structure was 300 µm, i.e. thethickness of the spacing means. The printhead structure wasmounted in contact, via the spacer means, with the toner deliverymeans in such a way that the part of the printhead structure beingin contact with the toner delivery means has a kind of a bulgeextending 1 mm above the line connecting the surface of supportingmeans for the printhead structure.
A back electrode was present behind the paper whereon the printingproceeded, the distance between the back electrode and the backside of the printhead structure (i.e. control electrodes) was setto 750 µm and the paper travelled at 2 cm/sec. The shieldelectrode was grounded : V2 = 0 V. To the individual controlelectrodes an (imagewise) voltage V3 between 0 V and - 280 V wasapplied. The back electrode was connected to a high voltage powersupply of + 1000 V. To the sleeve of the toner delivery means anAC voltage of 250 V(V^rms, root means square voltage) at 3.0 kHzwas applied, with -100 V DC offset. Thus |AC/DC| = 2.5.

PRINTING EXAMPLES

The printing quality of the printing examples below wasvisually judged on two criteria and given a quality figure from 5,very good to 1, bad. The two criteria were evenness of density inthe printing direction (EPD) and evenness of density in adirection perpendicular to the printing direction (EPPD). A totalprinting quality (TQ) was given as (EPD + EPPD)/2.

Also the density that was obtained was judged on a relativescale (D_max).

For all examples and for the comparative example, the resultsare tabulated in table 1.

EXAMPLE 1 (E1)

A printhead structure as described above was used, with F_ℓ =2220 N/m, acting over four application points being 50 mm apartand F_w = 1500 N/m, F_wb = 500 N/m, F_wo = 1000 N/m, acting over sixapplication points being 200 mm apart. |AC/DC| = 2.5. Thedistance, d, between the spacing means was 0.165 m.

EXAMPLE 2 (E2)

A printhead structure as described above was used, with F_ℓ =2220 N/m, acting over four application points being 50 mm apartand F_w = 1000 N/m, F_wb = 500 N/m, F_wo = 500 N/m, acting over eightapplication points being 200 mm apart. |AC/DC| = 2.5. Thedistance, d, between the spacing means was 0.165 m.

EXAMPLE 3 (E3)

A printhead structure as described above was used, with F_ℓ =2220 N/m, acting over four application points being 50 mm apartand F_w = 90 N/m, F_wb = 0 N/m, F_wo = 90 N/m, acting over fourapplication points being 200 mm apart. |AC/DC| = 2.5. Thedistance, d, between the spacing means was 0.165 m.

EXAMPLE 4 (E4)

Example 3 was repeated, except for the DC and AC-field. A DC-fieldof - 30 V was applied to the sleeve of the toner deliverymeans and an AC voltage of 1000 V (V^rms). |AC/DC| = 30.3.

COMPARATIVE EXAMPLE (CE)

A printhead structure, as described above, was mounted on afour frame bar, as described in EP-A 712 056, with four points ofapplication for F_ℓ, acting over 50 mm and giving a force perlinear m of 2000 N/m and with six points of application for F_w,acting over 200 mm, and giving a force per linear m of 750 N/m.On the outer surface of the toner delivery means a DC-field of-100V was applied and an AC-field of 300 V^rms superimposed to saidDC-field. |AC/DC| = 3.0.

The toner delivery means was arranged at 300 µm of theprinthead structure and NO spacing means were present. I.e. theprinthead structure and the toner delivery means made NO contactat all.

Nr	Fw/Fℓ	Fw/d	|AC/DC|	EPD	EPPD	TQ	Dmax
E1	0.676	9090	2.5	5	2.5	3.75	1
E2	0.450	6060	2.5	5	4	4.5	1
E3	0.041	550	2.5	5	5	5	1
E4	0.041	550	30.3	5	5	5	2
CE	0.375	n.a.	3.0	1	5	3	1

The headings of the table have the meaning as explained in thetext, n.a. means not applicable.

It is clear that by using a printhead structure according tothis invention a higher printing quality can be achieved than witha DEP device wherein no spacing means are present.

Claims (10)

1. A DEP device comprising a printhead structure (100) having afirst and a second major face, and a toner delivery means (103) havingan external surface wherein

   i) said first major face of said printhead structure faces saidexternal surface of said toner delivery means, and

   ii) said external surface moves in a direction A with respect tosaid first major surface, and is kept at a constant distance D ofsaid first major face by at least two spacing means (102), forming anangle between 45 ° and 0° with said direction A, characterised inthat said spacing means are placed at a distance d from each othersuch that 1 cm ≤ d ≤ 50 cm.
2. A DEP device according to claim 1, wherein said spacing meansare placed at a distance d from each other such that 5 cm ≤ d ≤ 35cm.
3. A DEP device according to claim 1 or 2, wherein said constantdistance D is such that 25 µm ≤ D ≤ 500 µm.
4. A DEP device according to claim 1 or 2, wherein said constantdistance D is such that 50 µm ≤ D ≤ 300 µm.
5. A DEP device according to any one of the preceding claims,wherein said printhead structure comprises a polymeric substratewherein printing apertures are present.
6. A DEP device according to claim 5, wherein said printheadstructure has a plurality of arrays of printing apertures (101a, 101b, 101c),each of said arrays are located between two spacing means (102a - 102e) and saidarrays are staggered.
7. A DEP device according to any of the preceding claims, whereinsaid printhead structure and said toner delivery means are kept incontact, via said spacing means, by resilient means.
8. A DEP device according to claim 7, wherein said resilient meansexert a force on said printhead structure in a direction parallelto said printing apertures (F_ℓ) and a force in a directionperpendicular to said printing apertures (F_w) and wherein F_w/F_ℓ ≤0.5.
9. A DEP device according to claim 8, wherein F_w/d < 5000.
10. A DEP device according to any of the preceding claims, whereinan AC-voltage is superimposed on a DC-voltage applied to saidouter surface of said toner delivery means and wherein10 ≤ |AC/DC| ≤ ∞.

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