EP0736822B1 - A device for direct electrostatic printing (DEP) - Google Patents

Description

Field of the invention.

This invention relates to an apparatus used in the process ofelectrostatic printing and more particularly in Direct ElectrostaticPrinting (DEP). In DEP, electrostatic printing is performed directlyfrom a toner delivery means on a receiving member substrate by meansof an electronically addressable printhead structure.

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 latent electrostaticimage. The substrate can be an intermediate endless flexible belt(e.g. aluminium, polyimide etc.). In that case the imagewisedeposited toner must be transferred onto another final substrate.Preferentially the toner is deposited directly on the finalreceiving substrate, thus offering a possibility to create directlythe image on the final receiving substrate, e.g. plain paper,transparency, etc. This deposition step is followed by a finalfusing step.

This makes the method different from classical electrography, inwhich a latent electrostatic image on a charge retentive surface isdeveloped by a suitable material to make the latent image visible.Further on, either the powder image is fused directly to said chargeretentive surface, which then results in a direct electrographicprint, or the powder image is subsequently transferred to the finalsubstrate and then fused to that medium. The latter process resultsin an indirect electrographic print. The final substrate may be atransparent medium, opaque polymeric film, paper, etc.

DEP is also markedly different from electrophotography in whichan additional step and additional member is introduced to create thelatent electrostatic image. More specifically, a photoconductor isused and a charging/exposure cycle is necessary.

A DEP device is disclosed in e.g. US-P-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 of conductive material on one side of the isolation layer ;
• a plurality of control electrodes formed by a segmented layer ofconductive material on the other side of the isolation layer ;and
• at least one row of apertures.

Each control electrode is formed around one aperture and is isolatedfrom 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 charged tonerparticles through a row of apertures of the printhead structure. Theintensity of the particle stream is modulated according to thepattern of potentials applied to the control electrodes. Themodulated stream of charged particles impinges upon a receivingmember substrate, interposed in the modulated particle stream. Thereceiving member substrate is transported in a direction orthogonalto the printhead structure, to provide a line-by-line scan printing.The shield electrode may face the toner delivery means and thecontrol electrode may face the receiving member substrate. A DCfield is applied between the printhead structure and a single backelectrode on the receiving member support. This propulsion field isresponsible for the attraction of toner to the receiving membersubstrate that is placed between the printhead structure and theback electrode.

In US-P 5,327,169 and EP-A 675 417, a DEP device having a tonercloud extracted directly from a magnetic brush, using a two-componentdevelopment system have been described. These systemshave the advantage that no special charged toner conveyer has to beincorporated in the apparatus between the toner source and theprinthead structure and that the charge of the toner particles iswell controlled. This simplifies the construction of the DEPapparatus using toner particles with well controlled charge.

A DEP device is well suited to print half-tone images. Thedensities variations present in a half-tone image can be obtained bymodulation of the voltage applied to the individual controlelectrodes. However, since the human eye is extremely sensitive tosmall density fluctuations, it is not an easy task to print at acertain grey scale density with a high degree of homogeneity.Especially a kind of "banding" i.e. stripes of slightly differentdensities can be seen in a density pattern that is intended to be totally homogeneous and even.

For that reason accurate control of the distance of said tonerapplication module and of said back electrode towards said printheadstructure is very important. It has been described in EP-A 712 056,that said problem can be tackled by stretching of said printheadstructure in said DEP device over a well-shaped bar frame.

Other descriptions in the literature are dealing with a methodfor correcting the image density, in the direction perpendicular tothe direction of the movement of the toner receiving member,according to a pattern which is superposed upon the actual imagedata and, either by time or voltage modulation, corrects for theinequality in the overall image density.

In US P 5,193,011 e.g. a pixel by pixel correction is claimed bytime-modulation of the different control electrodes. By using thiscorrection method one can change the writing-voltage and/or non-writingvoltage to each individual aperture so that after thiscorrection an homogeneous image density results. A drawback of thismethod is that part of the time modulated grey scale is consumed bythe overall density correction.

In US P 5,229,794 and EP-A 710 897, an apparatus is describedwhich comprises an apertured printhead structure in which eachindividual aperture has a distinct shield electrode and controlelectrode. By applying a different voltage to each individualshield electrode of said printhead structure, it becomes possible tocorrect said grey-scale-printing for a homogeneous image densityover the full width of said receiving member.

In EP-A 617 335 a DEP device is shown where the toner particlesare extracted from a magnetic brush whereon only magnetic tonerparticles are present, in this disclosure the sleeve of the magneticbrush rotates at a speed of 150 rpm and the substrate moves at alinear speed of 300 cm/min.

In US-A 4 491 855 also a DEP device is shown where the tonerparticles are extracted from a magnetic brush whereon only magnetictoner particles are present, in this disclosure the sleeve of themagnetic brush rotates at a speed of 150 rpm and the substrate issaid to move at a linear speed of 1500 cm/min.

Although the solutions that have been proposed do improve thehomogeneity of even density patterns printed by DEP, when viewed inthe direction perpendicular to the direction of movement of thetoner receiving member, there is still a need for a DEP system thatmakes it possible to print even density patterns with high homogeneity, when viewed in the direction of movement of the tonerreceiving member and that does not require complicated electronic ofmechanical measures to ensure high homogeneity (no banding visible)in even density patterns.

Objects of the invention

It is an object of the invention to provide an improved DirectElectrostatic Printing (DEP) device, printing at high densityresolution with high image homogeneity and no "banding".

It is a further object of the invention to provide a DEP devicecombining high spatial and density resolution with good long termstability and reliability.

It is still a further object of the invention to provide a DEPdevice, wherein said toner application module gives a constant andreliable flux of charged toner particles towards said printheadstructure as a function of printing time.

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

The above objects are realized by providing a DEP(DirectElectrostatic Printing) device comprising

• a back electrode (105),
• a printhead structure (106), comprising an array of aperturesthrough which a particle flow can be electrically modulated,
• means for moving a receiving substrate (109) at a speedV_sub ≥ 10 cm/min between said back electrode (105) and saidprinthead structure (106),
• a toner delivery means (101), at the front side of saidprinthead structure, with a magnetic brush assembly (103)comprising a core and a sleeve, wherein said sleeve of saidmagnetic brush assembly is coupled to means for rotating it at aspeed V_rot higher than 100 rpm (rotations per minute) and
• developer in said toner delivery means containing at leasttoner particles and magnetically attractable carrier particles;

   characterised in that said means for rotating said sleeve ofsaid magnetic brush assembly and said means for moving saidsubstrate are equipped for being operated so that
V_rot / V_sub ≥ 2 rotations/cm.

In a preferred embodiment said receiving substrate (109)moves at a speed V_sub ≥ 28 cm/min and said sleeve of said magneticbrush assembly is rotated at a speed V_rot so that V_rot / V_sub ≥ 5rotations/cm.

Brief Description of the Drawings

Fig. 1 is a schematic illustration of a possible embodiment of a DEPdevice according to the present invention.

Detailed Description of the Invention

In the literature many devices have been described that operateaccording to the principles of DEP (Direct Electrographic Printing).All these devices are able to perform grey scale printing either byvoltage modulation or by time modulation of the voltages applied tothe control electrodes. Most of the disclosed DEP devices operatewith a Charged Toner Conveyer (CTC) to bring toner particles in thevicinity of a printhead structure. The toner particles aremagnetic, as disclosed in, e.g., EP-A 617 335, or are non-magnetic.In that latter case the toner particles are applied to the CTC by aconventional magnetic brush from a multi-component developercomprising toner particles and magnetic carrier particles. Examplesof such devices are a.o. found in e.g., US 5,327,169 and US5,214,451.
The DEP-devices, as described in EP-A 675 417, operating with amulticomponent developer comprising magnetic carrier particles andtoner particles and wherein the toner particles are directly broughtto the printhead structure by a magnetic brush, can give raise to akind of "banding", especially in the direction of movement of thetoner receiving member, in printed patches of even density. Thiswas especially so when fast printing was to be achieved. In thecontext of the present invention, fast printing means that the tonerreceiving substrate travels past the printhead structure at a speedV_sub ≥ 10 cm/min. This "banding" is due to density fluctuations,during the time that the printing of the even density patch proceed.We have found that reproducible density modulation as a function ofprinting time is possible without the introduction of time-propagatingelectrical signals applied to either the controlelectrodes (106a), the shield electrode (106b) or the tonerapplication module (103). The most important parameter found toimprove the "banding" (i.e. diminish said banding) was the rotationspeed of the sleeve of the magnetic brush. This proved to be trueboth for a magnetic brush of the stationary core/rotating sleevetype and for a magnetic brush of the rotating core/rotating sleevetype. It was found that when the rotation speed (V_rot) of said sleeve was higher than 100 rotations per minute (rpm) the bandingphenomenon was clearly diminished. It was found that the bandingphenomenon was even more diminished when V_rot (in rpm) was tuned tospeed (V_sub) in cm/min of the receiving substrate moving between theback electrode and the printhead structure so thatVrot/Vsub ≥ 2 rotations/cm.

In a DEP device according to the present invention, V_sub ≥ 10cm/min and the ratio between V_rot and V_sub fulfils preferably theequation V_rot/V_sub ≥ 5 rotations/cm. In a preferredembodiment of the present invention, V_sub ≥ 28 cm/min and the ratiobetween V_rot and V_sub fulfils preferably the equation V_rot/V_sub ≥ 5rotations/cm. In the most preferred embodiment of the presentinvention, V_sub ≥ 28 cm/min and V_rot/V_sub ≥ 10 rotations/cm. The dimensions ofV_rot/V_sub are a number of rotation over cm.

The printhead structure used in a preferred embodiment of thepresent invention is made in such a way that reproducible printingis possible without clogging and with accurate control of printingdensity. Such a printhead structure has been described in EP-A 719648 and is preferentially stretched over a 2-bar or 4-bar frame asdescribed in EP-A 712 056.

Description of the DEP device

A non limitative example of a device for implementing a DEPmethod using toner particles according to the present inventioncomprises (fig 1):

(i) a toner delivery means (101), comprising a container fordeveloper (102) and a magnetic brush assembly (103), this magneticbrush assembly forming a toner cloud (104),

(ii) a back electrode (105),

(iii) a printhead structure (106), made from a plasticinsulating film, coated on both sides with a metallic film,

(iv) conveyer means (108) to convey a receiving substrate (109)for said toner between said printhead structure and said backelectrode in the direction indicated by arrow A,

(v) means for fixing (110) said toner onto said image receptivemember.

In fig 1., the printhead structure (106) comprises one continuous electrode surface, hereinafter called "shield electrode"(106b) facing in the shown embodiment the toner delivering means anda complex addressable electrode structure, hereinafter called"control electrode" (106a) around printing apertures (107), facing,in the shown embodiment, the receiving substrate (109) in said DEPdevice. Said printing apertures are arranged in an array structurefor which the total number of rows can be chosen according to thefield of application. In a preferred embodiment as described lateron e.g. an array of printing apertures consisting of 2 individualrows of apertures can be used. The location and/or form of theshield electrode (106b) and the control electrode (106a) can, inother embodiments of a device for a DEP method using toner particlesaccording to the present invention, be different from the locationshown in fig. 1.

Although in fig. 1 an embodiment of a device for a DEP methodusing two electrodes (106a and 106b) onprinthead 106 is shown, itis possible to implement a DEP method, using toner particlesaccording to the present invention using devices with differentconstructions of the printhead (106). It is, e.g. possible toimplement a DEP method with a device having a printhead comprisingonly one electrode structure as well as with a device having aprinthead comprising more than two electrode structures. It is alsopossible to implement a DEP device according to the presentinvention using a mesh isolated wires as printhead structure, asdisclosed in e.g., US 5,036,341. The apertures in these printheadstructures can have a constant diameter, or can have a broaderentrance or exit diameter.The back electrode (105) of this DEP device can also be made tocooperate with the printhead structure, said back electrode beingconstructed from different styli or wires that are galvanicallyisolated and connected to a voltage source as disclosed in e.g. US-P4,568,955 and US-P 4,733,256. The back electrode, cooperating withthe printhead structure, can also comprise one or more flexiblePCB's (Printed Circuit Board).

Between said printhead structure (106) and the magnetic brushassembly (103) as well as between the control electrode around theapertures (107) and the back electrode (105) behind the tonerreceiving member (109) as well as on the single electrode surface orbetween the plural electrode surfaces of said printhead structure(106) different electrical fields are applied. In the specificembodiment of a device, useful for a DEP method, wherein the sleeve of the magnetic brush rotates at least at 100 rpm, according to thepresent invention, shown in fig 1. voltage V1 is applied to thesleeve of themagnetic brush assembly 103, voltage V2 to theshieldelectrode 106b, voltages V3₀ up to V3_n for the control electrode(106a). The value of V3 is selected, according to the modulation ofthe image forming signals, between the values V3₀ and V3_n, on atimebasis or grey-level basis. Voltage V4 is applied to the backelectrode behind the toner receiving member. In other embodimentsof the present invention multiple voltages V2₀ to V2_n and/or V4₀ toV4_n can be used.

Themagnetic brush assembly 103 used in a DEP device accordingto the present invention can be either of the type with stationarycore and rotating sleeve or of the type with rotating core androtating or stationary sleeve.

The use of a magnetic brush of the rotating sleeve/stationarycore is preferred in a DEP device according to the presentinvention. Especially preferred is a magnetic brush with rotatingsleeve and stationary core, said magnetic brush having a curvaturein the development zone fulfilling the equation I :R >C24.25B + 0.25   wherein
   the curvature R of said magnetic brush in the development zoneis expressed as the radius (in mm) of a circle that best fits tosaid curvature of said magnetic brush in the development zone, B isthe distance between the surface of said sleeve of said magneticbrush to the surface of said printhead structure, facing saidmagnetic brush and C is the extension (in mm) of the array ofprinting apertures (107) in the direction of the movement of saidreceiving substrate (109) measured from the middle of the aperturesin the first row to the middle of the apertures in the last row. Amagnetic brush fulfilling the equation above has been described inEP-A 731 394.

In a DEP device, according to the present invention, any type ofknown carrier particles and toner particles can successfully beused. It is however preferred to use "soft" magnetic carrierparticles. "Soft" magnetic carrier particles useful in a DEP device according to the present invention are soft ferrite carrierparticles. Such soft ferrite particles exhibit only a small amountof remanent behaviour, characterised in coercivity values rangingfrom about 50 up to 250 Oe. Further very useful soft magneticcarrier particles, for use in a DEP device according to the presentinvention, are composite carrier particles, comprising a resinbinder and a mixture of two magnetites having a different particlesize as described in EP-B 289 663. The particle size of bothmagnetites will vary between 0.05 and 3 µm. The carrier particleshave preferably an average volume diameter (d_v50) between 10 and 300µm, preferably between 20 and 100 µm. More detailed descriptions ofcarrier particles, as mentioned above, can be found in EP-A 675 417,titled "A method and device for direct electrostatic printing(DEP)".

It is preferred to use in a DEP device according to the presentinvention, toner particles with an absolute average charge (|q|)corresponding to 1 fC ≤ |q| ≤ 20 fC, preferably to1 fC ≤ |q| ≤ 10 fC. Moreover it is preferred that the chargedistribution is narrow, i.e. shows a distribution wherein thecoefficient of variability (ν), i.e. the ratio of the standarddeviation to the average value, is equal to or lower than 0.33.Preferably the toner particles used in a device according to thepresent invention have an average volume diameter (d_v50) between 1and 20 µm, more preferably between 3 and 15 µm. More detaileddescriptions of toner particles, as mentioned above, can be found inEP-A 675 417, titled "A method and device for direct electrostaticprinting (DEP)".

A DEP device making use of the above mentioned marking tonerparticles can be addressed in a way that enables it to give blackand white. It can thus be operated in a "binary way", useful forblack and white text and graphics and useful for classical bilevelhalftoning to render continuous tone images. A DEP device accordingto the present invention is especially suited for rendering an imagewith a plurality of grey levels. Grey level printing can becontrolled by either an amplitude modulation of the voltage V3applied on thecontrol electrode 106a or by a time modulation of V3.By changing the duty cycle of the time modulation at a specificfrequency, it is possible to print accurately fine differences ingrey levels. It is also possible to control the grey level printingby a combination of an amplitude modulation and a time modulation of the voltage V3, applied on the control electrode. The combinationof a high spatial resolution and of the multiple grey levelcapabilities typical for DEP, opens the way for multilevelhalftoning techniques, such as e.g. described in the EP-A 634 862with title "Screening method for a rendering device havingrestricted density resolution". This enables the DEP device,according to the present invention, to render high quality images.

EXAMPLES

Aprinthead structure 106 was made from a polyimide film of 50µm thickness, double sided coated with a 17 µm thick copper film.Theprinthead structure 106 had two rows of printing apertures(107), said apertures having a square shape of 200 by 200 micron. Atthe back side of said printhead structure each aperture had a squarecopper electrode of 50 micron around each aperture, said 2 rows ofapertures isolated from each other by a 100 micron broad isolationzone. This printhead structure had a resolution of 127 dpi (50 dotsper cm) and was fabricated using the technique of plasma etching.Each of said control electrodes was individually addressable from ahigh voltage power supply. On the front side of the printheadstructure, facing the toner delivery means, a common shieldelectrode was present.

The toner delivery means 101 was a stationary core/rotatingsleeve type magnetic brush (103) comprising two mixing rods and onemetering roller. One rod was used to transport the developer throughthe unit, the other one to mix toner with developer.

Themagnetic brush assembly 103 was constituted of the so calledmagnetic roller, which in this case contained inside the rollerassembly a stationary magnetic core, showing nine magnetic poleswith an open position to enable used developer to fall off from themagnetic roller. The magnetic roller contained also a sleeve,fitting around said stationary magnetic core, and giving to themagnetic brush assembly an overall diameter of 20 mm. The sleeve wasmade of stainless steel roughened with a fine grain to assist intransport (Ra=3 µm) and showed an external magnetic field strengthin the developing nip of 0.450 T.
A scraper blade was used to force developer to leave the magneticroller. And on the other side a doctoring blade was used to meter a small amount of developer onto the surface of said magnetic brushassembly. The sleeve was rotating at a speed as tabulated in table1, the internal elements rotating at such a speed as to conform to agood internal transport within the development unit. Themagneticbrush assembly 103 was connected to an AC power supply with a squarewave oscillating field of 600 V at a frequency of 3.0 kHz with 0 VDC-offset.

A macroscopic "soft" ferrite carrier consisting of a MgZn-ferritewith average particle size 50 µm, a magnetisation atsaturation of 29 emu/g was provided with a 1 µm thick acryliccoating. The material showed virtually no remanence.

The toner used for the experiment had the followingcomposition : 97 parts of a co-polyester resin of fumaric acid andpropoxylated bisphenol A, having an acid value of 18 and volumeresistivity of 5.1 x 10¹⁶ ohm.cm was melt-blended for 30 minutes at110° C in a laboratory kneader with 3 parts of Cu-phthalocyaninepigment (Colour Index PB 15:3). A resistivity decreasing substance -having the following structural formula : (CH₃)₃N⁺C₁₆H₃₃Br^- wasadded in a quantity of 0.5 % with respect to the binder. It wasfound that - by mixing with 5 % of said ammonium salt - the volumeresistivity of the applied binder resin was lowered to 5x10¹⁴ Ω.cm.This proves a high resistivity decreasing capacity (reduction factor: 100).

After cooling, the solidified mass was pulverized and milledusing an ALPINE Fliessbettgegenstrahlmühle type 100AFG (tradename)and further classified using an ALPINE multiplex zig-zag classifiertype 100MZR (tradename). The resulting particle size distribution ofthe separated toner, measured by Coulter Counter model Multisizer(tradename), was found to be 6.3 µm average by number and 8.2 µmaverage by volume. In order to improve the flowability of the tonermass, the toner particles were mixed with 0.5 % of hydrophobiccolloidal silica particles (BET-value 130 m²/g).

An electrostatographic developer was prepared by mixing saidmixture of toner particles and colloidal silica in a 4 % ratio (w/w)with carrier particles. The. tribo-electric charging of the toner-carriermixture was performed by mixing said mixture in a standardtumbling set-up for 10 min. The developer mixture was run in thedevelopment unit (magnetic brush assembly) for 5 minutes, after which the toner was sampled and the tribo-electric properties weremeasured, according to a method as described in the above mentionedEP-A 675 417, giving q = -7.1 fC, q as defined in said application.

The distance B between the front side of theprintheadstructure 106 and the sleeve of the magnetic brush assembly 3, wasset at 400 µm. The distance between theback electrode 105 and theback side of the printhead structure 106 (i.e.control electrodes106a) was set to 150 µm. The receiving substrate (109) was paperand moved at various speeds (V_sub in cm/min) as indicated in table1. Theshield electrodes 106b, 106c were grounded : V2 = 0 V. To theindividual control electrodes an (imagewise) voltage V3 between 0 Vand -300 V was applied. Thebackelectrode 105 was connected to ahigh voltage power supply of +400 V. To the sleeve of the magneticbrush an AC voltage of 600 V at 3.0 kHz was applied, without DCoffset.

Several prints of even densities were made with this kind ofDEP device. In the different printing experiments the rotationspeed of the magnetic brush (V_rot) and the speed of movement of thetoner receiving substrate (V_sub) were changed. The differentcombinations of V_rot and V_sub are listed in table 1.
In the same table 1 the homogeneity of the even density patches isgiven under heading SIG and have been measured according to test Aas described hereafter.

TEST A: MEASUREMENT OF PRINT QUALITYMEASURING THE STANDARD DEVIATION OF THE DENSITY

The printing was done on paper and the density patches were measuredin reflection mode.
The homogeneity of a patch of even densities was expressed withrespect to the visibility of density differences, i.e. to the way ahuman observer would perceive these differences. Therefore, themeasured values of density variations (in fact a well known σ_D) wererecalculated to density variations as perceived by a human observer.In practice, a sample of even density patches printed on paper wasscanned in the direction of the movement of the receiving substratewith a slit of 2 mm by 27 µm and a spatial resolution of 10 µm. Thesampling distance was 1 cm and 1024 data points were sampled. Thesampling proceeded in reflection mode and the reflectances wheremeasured.
Said obtained scan of the reflectances was converted to a"perceived" image by means of a perception model. This conversioncomprises the following steps :

(i) applying visual filtering, describing the spatial frequencycharacteristics of the "early" eye, i.e. only taking in account thereceiving characteristics of the eye. The filter used has beendescribed in detail by J. Sullivan et al. in IEEE Transactions onSystems, Man and Cybernetics, vol. 21, n° 1 p. 33 to 38, 1991.Contrary to the filter described in said reference, the filter wasnot levelled off to a value of one for frequencies lower than thefrequency of maximum sensitivity of said early eye. This means thatin measurement A a band-pass filter was used, instead of a low-passfilter in the reference cited above. The viewing distance was 40cm.

(ii) transforming the reflectances (R), that have beentransformed in step (i) by the filtering, to visual densities(D_vis), by following formula's :Dvis = 2.55 x (1 - R1/3)

when the reflectance (R) is higher than orequal to 0.01, and
D_vis = 2.00 when the reflectance (R) is lower than 0.01, while theeye can differentiate reflectances below 0.01.
In the thus obtained "perceived" image the standard deviation of thedensity fluctuation (SIG) was calculated.
The results of this analysis are given in table 1. A value for theparameter SIG smaller than 0.045 means acceptable image quality, interms of homogeneity of even density patterns, a value smaller than0.030 means excellent quality, a value of 0.025 to 0.020 is typicalfor offset high-quality.

Experiment n°	Vsub cm/min	Vrot rpm	Vsub/Vrot	SIG
1	56	150	2.6	0.041
2	56	300	5.3	0.040
3	28	300	10.7	0.039
4	14	300	21.4	0.028
5	14	105	7.5	0.041
6	28	1000	35.7	0.022
7	28	50	1.8	0.060
8	56	100	1.8	0.048
9	56	60	1.1	0.049

Having described in detail preferred embodiments of the currentinvention, it will now be apparent to those skilled in the art thatnumerous modifications can be made therein without departing fromthe scope of the invention as defined in the following claims.Better homogeneous printing of even density patches with a DEPdevice utilizing charged toner conveyer (CTC) means different from amagnetic brush (e.g. a belt conveying the charged toner to theprinthead structure) can also be improved by giving the CTC aminimum speed in the neighbourhood of the printhead structure and byadapting said minimum speed to the travelling speed of the tonerreceiving substrate.

Claims (5)

1. A DEP (Direct Electrostatic Printing) device comprising

   a back electrode (105),

   a printhead structure (106), comprising an array of aperturesthrough which a particle flow can be electrically modulated,

   means for moving a receiving substrate (109) at a speed V_sub≥ 10 cm/min between said back electrode (105) and said printheadstructure (106),

   a toner delivery means (101), at the front side of saidprinthead structure, with a magnetic brush assembly (103)comprising a core and a sleeve, wherein said sleeve of saidmagnetic brush assembly is coupled to means for rotating it at aspeed V_rot higher than 100 rpm (rotations per minute) and

   developer in said toner delivery means containing at leasttoner particles and magnetically attractable carrier particles;characterised in that said means for rotating said sleeve ofsaid magnetic brush assembly and said means for moving saidsubstrate are equipped for being operated so that
   V_rot / V_sub ≥ 2 rotations/cm.
2. A DEP device according to claim 1, wherein said means forrotating said sleeve of said magnetic brush assembly and said meansfor moving said substrate are equipped for being operated so thatV_rot / V_sub ≥ 5 rotations/cm.
3. A DEP device according to claim 2, wherein said means for movingsaid receiving substrate (109) is equipped for moving it at a speedV_sub ≥ 28 cm/min.
4. A DEP device according to claim 1, wherein said means for movingsaid receiving substrate (109) is equipped for moving it at a speedV_sub ≥ 28 cm/min and said means for rotating said sleeve of saidmagnetic brush assembly is equipped to be operated so as to have aVrot so that V_rot / V_sub ≥ 10 rotation/cm.
5. A DEP device according to claim 1, wherein said magnetic brush isof the stationary core/rotating sleeve type.

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