EP0675417A1 - Procédé et dispositif d'impression électrostatique directe (DEP) - Google Patents

Procédé et dispositif d'impression électrostatique directe (DEP) Download PDF

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Publication number
EP0675417A1
EP0675417A1 EP95200603A EP95200603A EP0675417A1 EP 0675417 A1 EP0675417 A1 EP 0675417A1 EP 95200603 A EP95200603 A EP 95200603A EP 95200603 A EP95200603 A EP 95200603A EP 0675417 A1 EP0675417 A1 EP 0675417A1
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EP
European Patent Office
Prior art keywords
toner
magnetic brush
brush assembly
particles
magnetic
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EP95200603A
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German (de)
English (en)
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EP0675417B1 (fr
Inventor
Serge C/O Agfa-Gevaert Naamloze Tavernier
Guido c/o Agfa-Gevaert naamloze Desié
Jacques C/O Agfa-Gevaert Naamloze Leonard
Luc C/O Agfa-Gevaert Naamloze Van Aken
Leo C/O Agfa-Gevaert Naamloze Alaerts
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Agfa Gevaert NV
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Agfa Gevaert NV
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00—Apparatus for electrographic processes using a charge pattern
    • G03G15/22—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20
    • G03G15/34—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20 in which the powder image is formed directly on the recording material, e.g. by using a liquid toner
    • G03G15/344—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20 in which the powder image is formed directly on the recording material, e.g. by using a liquid toner by selectively transferring the powder to the recording medium, e.g. by using a LED array
    • G03G15/346—Apparatus for electrographic processes using a charge pattern involving the combination of more than one step according to groups G03G13/02 - G03G13/20 in which the powder image is formed directly on the recording material, e.g. by using a liquid toner by selectively transferring the powder to the recording medium, e.g. by using a LED array by modulating the powder through holes or a slit

Definitions

  • This invention relates to the process of electrostatic printing and more particularly to Direct Electrostatic Printing (DEP).
  • DEP electrostatic printing is performed directly on a substrate by means of electronically addressable printheads.
  • the toner or developing material is deposited directly in an imagewise way on a substrate, the latter not bearing any imagewise latent electrostatic image.
  • the substrate can be an intermediate, in case it is preferred to transfer said formed image on another substrate (e.g. aluminum, etc..), but it is preferentially the final receptor, thus offering a possibility to create directly the image on the final receptor, e.g. plain paper, transparency, etc.... after a final fusing step.
  • the final substrate can be different materials, such as a transparent medium, opaque polymeric films, paper, etc....
  • DEP is also markedly different from electrophotography in which an additionnal step and additionnal member is introduced to create the latent electrostatic image, more specifically, a photoconductor is used and a charging/exposure cycle is necessary.
  • a DEP device is disclosed in US-P 3,689,935.
  • This document discloses an electrostatic line printer comprising a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conductive material on one side of the layer of the insulating material and a segmented layer of conductive material on the other side of the layer of the insulating material.
  • the printhead comprises also at least one row of apertures.
  • Each segment of the segmented layer of conductive material is formed around a portion of an aperture and is insulatively isolated from each other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer.
  • An overall applied field projects charged particles through a row of apertures of the particle modulator (printhead) and the intensity of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer.
  • the modulated stream of charged particles impinges upon a print-receiving medium interposed in the modulated particle stream and translated in a direction relative to the particle modulator (printhead) to provide a line-by-line scan printing.
  • the segmented electrode is called the control electrode and the continuous electrode is called the shield electrode.
  • the shield electrode faces, e.g., the toner supply and the control electrode faces the image recording member.
  • a DC field is applied between the printhead and a backing electrode so as to attract the toner to the imaging receiving member that is placed between the printhead and the backing electrode.
  • GS 2,108,432 different measures are disclosed to overcome some of the mentioned problems.
  • Means are disclosed for realising a stable and uniform supply of toner particles to the printhead structure and for avoiding clogging of the apertures in the printhead structure by toner particles. Therefore a conveying member is provided on which a layer of toner particles is deposited and an AC voltage is applied between the toner conveying member and the continuous layer of conductive material on the printhead structure. Due to this AC voltage the toner particles "jump" between the toner conveying member and the surface of the printhead facing said toner conveying member, forming a "toner-cloud".
  • the AC-voltage is adjusted such as to allow the toner particles to reach the printhead structure, thus enabling the overall DC voltage laid between the printhead structure and the substrate bearing member to extract said toner particles after modulation from said powder cloud.
  • the overall DC voltage propels the toner particles, after said modulation, onto the image receiving member interposed between the printhead and a backing electrode. It is believed that the 'touching' toner particles will assist in delaying the contamination of the printhead structure and clogging of the apertures.
  • a special design of the apertures in the printhead structure and a special selection of the material from which the printhead structure is made is claimed to assist in delaying the clogging.
  • a last measure which is proposed is to 'clean' the printhead structure by periodical electrical bursts.
  • DE-OS 3,411,948 an apparatus is disclosed wherein the toning particles are presented to the printhead structure in layer form on a conveying member.
  • Said conveying member has a special design and AC/DC fields are used to realise jumping transport along said printhead structure.
  • the quality of the "toner-cloud” is addressd to make the process easier.
  • This document teaches means to operate a DEP-system with larger distance between said conveying member and the printhead structure than before and enabling the use of lower electrical fields, in such a way that no disruptive discharge between the printhead structure and the backing electrode is possible.
  • the toner is moreover continuously agitated such as to prevent clumping of the separate toner particles.
  • EP-A 266 960 a toner delivery system is disclosed in which a monolayer of toner is deposited on the surface of the toner conveying means using a multi-component developer (carrier/toner) and a conventional magnetic brush.
  • a multi-component developer results in a favorable charge distribution in the toner and hence in a reduction of the contamination rate of the printhead.
  • a set-up is necessary to recover the non-used toning material from said conveying belt and either dispose of it or reuse it by adding it back to the magnetic brush applicator used to deposit the active material on said conveying member.
  • DEP Direct Electrostatic Printing
  • the above objects are realized by providing a method for direct electrostatic printing (DEP) on an intermediate substrate or on a final substrate, using a device that comprises a back electrode (5), a printhead structure (6) comprising a control electrode in combination with apertures (7), a toner delivery means (1) presenting a cloud (4) of toner particles in the vicinity of said apertures (7), characterised in that
  • the reference surface of said magnetic brush assembly is placed at a distance (l) from the surface of the printhead structure facing said magnetic brush assembly, wherein l fulfils the condition : 2/3 L ⁇ l ⁇ 1000 + L wherein all dimensions are expressed in ⁇ m and L is defined as the maximum thickness of the developer layer on said magnetic brush assembly in the absence of said oscillating field and measured according to TEST A.
  • reference surface of said magnetic brush assembly is to be understood as the outer surface of the magnetic brush assembly when no developer is present on said outer surface.
  • said toner particles used in the method of the present invention have a average charge (q) expressed in fC such that 1 fC ⁇ q ⁇ 20 fC, more preferably such that 1 fC ⁇ q ⁇ 10 fC.
  • said toner particles used in the method of the present invention have a charge distribution, measured according to TEST B, with a coefficient of variability, ⁇ , lower than 0.5, preferably lower than 0.33.
  • Fig. 1 is a schematic illustration of an possible embodiment of a DEP device according to the present invention.
  • Fig. 2 represents a schematic cross-sectional drawing of an apparatus used in the determination of the above defined average charge and coefficient of variability of the charge distribution of charged toner particles.
  • the thickness (L) of the developer layer of the magnetic brush i.e. the length of the "hairs” of the magnetic brush
  • the electrical fields needed to perform the jetting process are within usefull ranges and that also obstruction of the apertures within the printhead structure can be markedly reduced. It has been found that both latter effects are improved when an oscilatory field interacts with the magnetic brush on said magnetic brush assembly.
  • the thickness, L, of the developer layer on the magnetic brush is measured according to TEST A.
  • the thickness of the developer layer on the magnetic brush is determined as follows :
  • the developer is introduced within the developing unit.
  • the unit is operated for 5 minutes in order to establish a steady state situation.
  • the developing unit is mounted on a reference rack, containing a reference plane parallel to the axis of the magnetic brush assembly, and provided with a micrometer screw allowing to adjust the distance between the surface of the sleeve of the magnetic brush assembly and the reference plane and thus enabling to read the distance between both bodies with an accuracy of 10 ⁇ m.
  • the development unit is operated with the normal settings, the development unit is stopped and the sleeve is moved to the reference plane over such a distance that the tops of the hairs of the brush touch the reference plane. This touching is observed visually using a illumination through the slit.
  • the distance is recorded, and the measurement is repeated 5 times.
  • the average of these 5 measurements is denoted 'L', the thickness of the developer layer on the magnetic brush and is expressed in ⁇ m.
  • the reference surface of said magnetic brush assembly is placed at a distance (l) from the surface of the printhead structure facing said magnetic brush assembly, wherein l fulfils the condition: 2/3 L ⁇ l ⁇ 1000 + L wherein all dimensions are expressed in ⁇ m and L is defined as the maximum thickness of the developer layer on said magnetic brush assembly in the absence of said oscillating field and measured according to TEST A.
  • the reference surface of said magnetic brush assembly is at a distance (l) from the surface of the printhead structure facing said magnetic brush assembly, wherein l fulfils the condition L ⁇ l ⁇ 500 + L wherein all dimensions are expressed in ⁇ m and L is defined as the maximum thickness of the developer layer on said magnetic brush assembly in the absence of said oscillating field and measured according to TEST A.
  • a device for implementing a DEP method according to the present invention comprises (fig 1):
  • a DEP method according to the present invention using two electrodes (6a and 6b) on printhead 6 is shown, it is possible to implement a DEP method according to the present invention using devices with different constructions of the printhead (6). It is, e.g. possible to implement the DEP method according to the present invention with a device having a printhead comprising only one electrode structure aswell as with a device having a printhead comprising more than two electrode structures.
  • the DEP method according to the present invention can also be implemented by using a DEP device comprising a electrode mesh array as printhead structure.
  • said toner delivery means is a layer of multi-component developer on a magnetic brush assembly, and the toner cloud is directly extracted from said magnetic brush assembly and not via a conveyer belt or the like.
  • the back electrode, the printhead structure, the conveying means for the image receptive member and the fixing means in a DEP device according to the present invention can be constructed in any suitable way, as disclosed in, e.g., US-P 3,689,935, GB 2,108,432, DE-OS 3,411,948, EP-A 266 960, US-P 4,743,926, US-P 4,912,489, US-P 5,038,322, US-P 5,202,704 etc.
  • a special semiconductive layer is put on the surface of the back electrode to reduce uncontrolled electrical discharges.
  • a semiconductive layer can be, e.g. a layer of semiconductive rubbery material.
  • the fixing means (10) are not incorporated, but the back electrode (5) is heated and thus acts also as fixing means for toner particles.
  • the magnetic brush assembly to be used in a DEP device according to the present invention can be either of the type with stationary core and rotating sleeve or of the type with rotating core and rotating or stationary sleeve.
  • a clear description of typical development units of both type, the stationary core/rotating sleeve type and the rotating core/rotating or stationary sleeve type can be found in the Hitachi Metals publication, Hitachi components for electrophotographic printing systems, p.5 - p.11, published by Hitachi Metals, International Ltd., 2400 Westchester Avenue, Purchase, New York, 10577, USA.
  • the rotating core type developing unit is commonly used for monocomponent developers.
  • a small toner cloud is formed in the vicinity of the apertures of the printhead due to a DC-field between the sleeve of said magnetic brush assembly and the printhead structure, without the presence of an oscillating field.
  • a sufficiently dense toner cloud is only formed when as an oscillatory field an AC-field (i.e. an oscillating field of electrical nature) is combined with said DC-field between the sleeve of said magnetic brush assembly and the printhead structure.
  • the first effect is the typical movement for a stationary core magnetic brush assembly, i.e. the developer is transported over the magnetic brush because all magnetic brush hairs are tumbling from one stationary magnetic pole to an other since the base of each hair is forced to follow the slightly roughened surface of the roller.
  • the other effect is the movement due to the fact that the top of each hair is tumbling towards the magnetic pole which is moving into its direction. The hairs will tumble from each magnetic pole to the next as said poles pass underneath the developer.
  • a sufficiently dense toner cloud is formed in the vicinity of the apertures of the printhead due to a DC-field between the sleeve of said magnetic brush assembly and the printhead structure, without an additional AC-field combined with said DC-field between the sleeve of said magnetic brush assembly and the printhead structure.
  • the rotating magnetic core of the magnetic brush assembly already provides an oscillating field of magnetic nature, originating from changing magnetic fields induced by moving different magnetic poles beneath the surface of said magnetic brush assembly.
  • the formation of a toner cloud can advantageously be enhanced by combining also an AC-field with said DC-field between the sleeve of said magnetic brush assembly and the printhead structure.
  • the carrier particles are preferably "soft" magnetic particles (Carrier A).
  • a first interesting embodiment of "soft" magnetic carrier particles to be used in a DEP process according to the present invention is offered within the range of the so called soft ferrite carrier particles.
  • Such soft ferrite particles exhibit only a small amount of remanent behaviour, characterised in coercivity values ranging from about 50 up to 250 Oe.
  • An other embodiment of "soft” magnetic carrier particles can be derived from metal based particles. Both types of “soft” particles constitute macroscopic large particles of uniform composition over the whole particle. These particles may be used as such as well as in an resin coated version (Carrier A1 and A2).
  • composite magnetic particles with a pronounced soft magnetic character.
  • These composite particles comprise essentially the same type of magnetically active materials as the macroscopic large particles, with the exception that the magnetically active material is used in a very fine, quasi microscopic, form, so called pigment form and are bonded together over some binding matrix to form the particles.
  • the advantages of using said particles is twofold first the magnetic properties of carrier particles comprising microscopic magnetic pigments inbedded in a binder resin can easily be adjusted by changing the formulation of the composite and second carrier particles comprising microscopic magnetic pigments imbedded in a binder resin have a lower specific gravity, giving rise to lower wear of the particles due to lower mutual mechanical impact, thus extending the lifetime of the developer (Carrier A3).
  • soft magnetic pigments a variety of materials can be used, which comprise magnetic metal pigments such as fine powder, Fe powder, other metals and/or alloys, as well as magnetic oxide pigments both pure iron-based, such as magnetite, mixed iron oxide, etc. and mixed oxide magnetic pigments, commonly referred to as ferrites of the soft type.
  • Ferrites can be represented by the general formula: MeO.Fe2O3 wherein Me denotes at least one divalent metal such as Mn2+, Ni2+ Co2+, Mg2+, Ca2+, Zn2+, and Cd2+, furtheron doped with monovalent or trivalent ions.
  • FeO.FeO3 magnetite
  • the pigments referred to as soft are characterized by a coercivity of at most 250 Oe, as found by applying the procedure described below.
  • the coercivity of a magnetic material is the minimum external magnetic force necessary to reduce the remanence Br to zero while it is held stationary in the external field, and after the material has been magnetically saturated.
  • a variety of apparatus and methods for the measurement of the coercivity of the carrier particles used according to our invention can be employed.
  • a Princeton Applied Research Model 155 Vibrating Sample Magnetometer available from Princeton Applied Research Co., Princeton, N.J., is used to measure the coercivity of powder particle samples.
  • the powder was mixed with a nonmagnetic polymer powder (90 percent magnetic powder : 10 percent polymer by weight).
  • the mixture was placed in a capillary tube, heated above the melting point of the polymer, and then allowed to cool to room temperature.
  • the filled capillary tube was then placed in the sample holder of the magnetometer and a magnetic hysteresis loop of induced magnetism (in emu/gm) versus external field (in Oersted units) was plotted. During this measurement, the sample was exposed to an external field of 0 to 8000 Oersted.
  • a maximum, or saturated magnetic moment, Bsat When a powdered material is magnetically saturated and immobilized in an applied magnetic field H of progressively increasing strength, a maximum, or saturated magnetic moment, Bsat, will be induced in the material. If the applied field H is further increased, the moment induced in the material will not increase any further. When the applied field, on the other hand, is progressively decreased till zero, reversed in polarity and thereafter increased again, the induced moment B of the powder will ultimately become zero and thus be on the threshold of polarity reversal in induced moment.
  • the value of the applied field H necessary to bring about the decrease of the remanence, Br, to zero is called the coercivity Hc of the material.
  • the described soft magnetic pigments to be used in carrier particles for the DEP method of the present invention using a stationary core/rotating sleeve magnetic brush assembly exhibit a coercivity of less than 250 Oersted when magnetically saturated, preferably a coercivity of at most 200 Oersted and most preferably a coercivity of at most 100 Oersted.
  • composite carrier particles comprising a resin binder and a mixture of two magnetites having a different particle size as described in EP-B 289 663.
  • the particle size of both magnetites will vary between 0.05 and 3 ⁇ m.
  • the carrier particles are preferably "hard” magnetic particles (Carrier B).
  • the homoparticles are preferably hard ferrite macroparticles.
  • hard magnetic macroparticles are understood particles with a coercivity of at least 250 Oe, most preferably 1000 Oe, when magnetically saturated, the magnetisation being at least preferably 20 emu/g of carrier material.
  • Useful hard magnetic materials include hard ferrites and gamma ferric oxide.
  • the hard ferrite are represented by a similar composition as cited above, whereby specific ions such as Ba, Pb, or Sr are used as disclosed in US Patent No. 3,716,630.
  • composite particles it is preferred to use composite particles as they give a lower specific gravity and are more flexible in design.
  • the hard magnetic particles are present in a fine form, called pigment, but are essentially of the same chemical composition (Carrier B1).
  • the hard magnetic pigments then show a coercivity of at least 250 Oe, preferably at least 1000 Oe, and more preferably at least 3000 Oe.
  • a coercivity of at least 250 Oe preferably at least 1000 Oe, and more preferably at least 3000 Oe.
  • magnetic materials having coercivity levels of 3000 and 6000 Oersted have been found useful, there appears to be no theoretical reason why higher coercivity levels would not be useful.
  • hard magnetic pigments include hard ferrites and gamma ferric oxide.
  • the hard ferrites are represented by a similar composition as cited above, whereby specific ions such as Ba2+, Pb2+, or Sr2+ are used as disclosed in US Patent No. 3,716,630.
  • composite carrier comprising a binder resin and a mixture of both "soft” and “hard” magnetic particles can be used as the "hard” magnetic carrier to be used in a DEP method according to the present invention.
  • said carrier particles comprise a mixture of magnetic pigment particles wherein a portion (A) of said pigment particles has a coercive force of more than 250 Oe and an other portion (B) of said magnetic pigment particles has a coercive force of less than 250 Oe, the weight ratio of said portions (A) and (B) being in the range of 0.1 to 10.
  • said carrier particles of Carrier A and Carrier B preferably have, independently of the type of magnetic brush used in a DEP device according to the present invention, an induced magnetic moment B between 10 and 100 emu/gm, more specifically between 20 and 75 emu/g based on the weight of the carrier, when present in a field of 1000 Oersted, after full magnetisation.
  • the typical particle size of the carrier particles (Carrier A as well as Carrier B) to be used in accordance with the present invention can be choosen over a broad range. It is however useful to define the particle size small enough in order to increase the specific surface area of the carrier and hence its capability to offer a larger interacting surface to the toner particles. On the other hand some care should be taken not to go for too fine particles, as they might become too weakly bond to the magnetic field of the magnetic brush assembly. In such a case they may become airborn from the moving brush by centrifugal forces or may be stripped too easily in electrical fields or be lost from the brush by mechanical impact of the magnetic hairs with interacting components of the marking engine e.g. the printhead structure.
  • the diameter refers to the typical volume average particle diameter of the carrier beads, as it may be determinated by sieving techniques.
  • the carrier beads can be used as such, i.e. uncoated, or they may be coated with inorganic as well as organic or mixed coatings. Typical coating thickness is in the range of 0.5 to 2.5 ⁇ m.
  • the coating may be used to induce different properties such as for example triboelectrical charging, friction reduction, wear resistance, etc.
  • the toner particles used in a DEP process according to the present invention can essentially be of any nature as well with respect to their composition, shape, size, and preparation method and the sign of their tribo-electrically acquired charge.
  • a PEP process according to the present invention it is possible to use black toners and coloured toners.
  • the toner composition can comprise charge controlling additives, flow regulating agents etc. Examples of useful toner compositions can be found in, e.g., EP 058 013, US-P 4,652,509, US-P 4,647,522, US-P 5,102,763.
  • the toner for use in combination with carrier particles in a DEP process according to the present invention can be selected from a wide variety of materials, including both natural and synthetic resins and charge controlling agents as disclosed e.g. in US-P 4,076,857 and 4,546,060.
  • the shape of the toner particles can be irregular, as is the case in ground toners, or spheroidal. Spheroidization may proceed by spray-drying or the heat-dispersion process disclosed in US-P 4,345,015.
  • the toner particles according to the present invention have preferably an average volume diameter (d v,50 )between 3 and 20 ⁇ m, more preferably between 5 and 10 ⁇ m when measured with a COULTER COUNTER (registered trade mark) Model TA II particle size analyzer operating according to the principles of electrolyt displacement in narrow aperture and marketed by COULTER ELECTRONICS Corp. Northwell Drive, Luton, Bedfordshire, LC 33, UK.
  • COULTER COUNTER registered trade mark
  • Model TA II particle size analyzer operating according to the principles of electrolyt displacement in narrow aperture and marketed by COULTER ELECTRONICS Corp. Northwell Drive, Luton, Bedfordshire, LC 33, UK.
  • toner particles for use in DEP The main problem encountered with toner particles for use in DEP is the presence of wrong sign toners. To avoid the occurence of wrong sign toners, it is possible to give the toner particles in triboelectric contact with the carrier particles a very high charge (either positive or negative). This can be achieved by matching the toner resins with the composition and species present in the carrier (e.g. the coating of the carrier particles) with respect to their respective triboelectric properties.
  • the toner particles will acquire, upon triboelectric contact with the carrier particles, a charge (q), expressed in fC (femtoCoulomb) and that can be either negative or positive, such that 1 fC ⁇ q ⁇ 20 fC, more preferably such that 1 fC ⁇ q ⁇ 10 fC.
  • q a charge expressed in fC (femtoCoulomb) and that can be either negative or positive, such that 1 fC ⁇ q ⁇ 20 fC, more preferably such that 1 fC ⁇ q ⁇ 10 fC.
  • the toner particles useful according to the present invention contain:
  • Said coefficient of variation ( ⁇ ) is the standard deviation (s) divided by the median value (x).
  • the spread of charge values of individual toner particles containing said ingredients (1) and (2) is called standard deviation (s) which for obtaining statistically realistic results is determined at a particle population number of at least 10,000.
  • Said standard deviation divided by said median yields according to the present invention an absolute number equal to or smaller than 0.5, when the median q value is expressed in fC and stems from a curve of a percentage distribution of frequency of occurence of a same charge (in y-ordinate) versus number of observed toner particles (in x-abscissa), said median being the value of the x-coordinate at which the area under the curve is bisected in equal area parts.
  • the coefficient of variation ( ⁇ ) is preferred since it is more useful and significant to measure the spread in relative terms than using the standard deviation (s) alone; it is independent of the units in which the variate is measured, provided that the scales begin at zero [ref. Christopher Chatfield "Statistics for technology” A course in applied statistics - Third ed. (1986) Chapman and Hall Ltd, London, p. 33.].
  • triboelectric properties of toner particles are measured by means of a charge spectrograph apparatus operating as schematically shown in Fig. 2.
  • the apparatus involved is sold by Dr. R. Epping PES-Laboratorium D-8056 Neufahrn, Germany under the name q-meter".
  • the q-meter is used to measure the distribution of the toner particle charge (q in fC) with respect to a measured toner diameter (d in 10 ⁇ m).
  • the measurement result is expressed as percentage particle frequency (in ordinate) of same q/d ratio on q/d ratio expressed as fC/10 ⁇ m (in abscissa).
  • the measurement is based on the different electrostatic deflection according to their q/d ratio of triboelectrically charged toner particles making part of a bunch of toner particles carried by a laminar air flow in a long narrow tube 1 at a mean speed v m while passing through an electrical field E maintained perpendicular to the axis of said tube 21 by a registration electrode plate 22 and plate electrode 23 of opposite charge sign with respect to the registration electrode.
  • Said electrodes are forming a condensor with plate distance y (5 cm).
  • a bunch of triboelectrically charged toner particles is injected by air-pulse into said tube 21 from a little pot 24 containing an air injection inlet 25 and a certain amount of triboelectrically charged toner to be tested.
  • the developer is composed of magnetic carrier particles mixed with toner particles.
  • the carrier particles are retained in the pot 24 by means of a magnetic field stemming from an electromagnet situated at the bottom of the pot, whereas the toner particles are taken away in a laminar air flow.
  • q E 3 ⁇ ⁇ v m d y/x
  • E is the electric field in kV/y
  • d is in 10 ⁇ m units
  • ⁇ 3.14..
  • ⁇ is the air viscosity
  • x and y are in mm.
  • the average charge of the toner particles is calculated by wherein d v,50 is expressed in ⁇ m.
  • Toner compositions showing a narrow charge distribution are disclosed in WO 94/029770, WO 94/027192 and WO 94/027191, these application are incorporated herein by reference.
  • said substance (2) capable of lowering the volume resistivity of said binder by a factor of at least 3.3 when present in said binder in a concentration of 5 % by weight relative to the weight of said binder is within the following classes of compounds :
  • onium compounds in the present invention is understood “compounds containing an organic cation” for the term is intended to cover not only compounds named with the use of the suffix "onium” but also “olium”, “inium”, “ylium”, “enium”, etc. (see Chemical Abstracts - Vol.56 (1962) January-June, Nomenclature, pages 59N to 60N).
  • the measuring procedure for selecting the resistivity decreasing substance proceeds by a test R described hereinafter.
  • the resin or resin mixture to be tested is melt-blended with the resistivity decreasing substance being added in an amount of 5 % by weight with respect to the resin mass.
  • the melt-blending proceeds at 110 °C for 30 minutes using a laboratory melt-kneader Type W50H (sold by Brabender OGHGoodstra E 51-55 D4100 Duisburg 1).
  • the product After melt-mixing the product is solidified and milled using a laboratory mill Type A10 (sold by Janke and Kunkel - Germany). The product is sieved over 63 ⁇ m mesh. The fraction passing through is collected and compressed with a pressure of 10 ton full load for 1 minute to form a circular tablet having a diameter of 13 mm and height of 1.15 mm.
  • the conductivity is measured after conditioning at 20 °C and 50 % relative humidity for 24 h.
  • Toner particles and carrier particles, as described above are finally combined to give a high quality electrostatic developer.
  • This combination is made by mixing said toner and carrier particles in a ratio (w/w) of 1.5/100 to 15/100, preferably in a ratio (w/w) of 3/100 to 10/100.
  • flow improving additives are preferably extremely finely divided inorganic or organic materials the primary (i.e. non-clustered) particle size of which is less than 50 nm.
  • fumed inorganics of the metal oxide class e.g. selected from the group consisting of silica (SiO2), alumina (Al2O3), zirconium oxide and titanium dioxide or mixed oxides thereof which have a hydrophilic or hydrophobized surface.
  • the fumed metal oxide particles have a smooth, substantially spherical surface and are preferably coated with a hydrophobic layer, e.g. formed by alkylation or by treatment with organic fluorine compounds. Their specific surface area is preferably in the range of 40 to 400 m2/g.
  • the proportions for fumed metal oxides such as silica (SiO2) and alumina (Al2O3) are admixed externally with the finished toner particles in the range of 0.1 to 10 % by weight with respect to the weight of the toner particles.
  • Fumed silica particles are commercially available under the tradenames AEROSIL and CAB-O-Sil being trade names of Degussa, Franfurt/M Germany and Cabot Corp. Oxides Division, Boston, Mass., U.S.A. respectively.
  • AEROSIL R972 (tradename) is used which is a fumed hydrophobic silica having a specific surface area of 110 m2/g. The specific surface area can be measured by a method described by Nelsen and Eggertsen in "Determination of Surface Area Adsorption measurements by continuous Flow Method", Analytical Chemistry, Vol. 30, No. 9 (1958) p. 1387-1390.
  • a metal soap e.g. zinc stearate, as described in the United Kingdom Patent Specification No. 1,379,252, wherein also reference is made to the use of fluor containing polymer particles of sub-micron size as flow improving agents, may be present in the developer composition to be used in a DEP process according to the present invention.
  • a DEP method according to the present invention can be addressed in a way that enables it to give not only black and white, i.e. being operated in a "binary way” but also to give an image with a plurality of gray levels.
  • Gray level printing can be controled by either an amplitude modulation of the voltage V3 applied on the control electrode or by a time modulation of the voltage V3 applied on the control electrode. By changing the duty cycle of the time modulation at a specific frequency, it is possible to print accurately fine differences in gray levels. It is also possible to control the gray level printing by a combination of an amplitude modulation and a time modulation of the voltage V3, applied on the control electrode.
  • the improved stability of the DEP process, according to the present invention makes it also possible to operate it in a reproducable way at higher resolution by the fact that obstruction of the jetting process even in smaller apertures is strongly reduced.
  • a DEP device wherein the toner cloud is directly extracted from a magnetic brush assembly, in one apparatus together with a classical electrographic device, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible.
  • the PEP device according to the present invention and the classical electrographic device are two different printing devices used to print both images with various gray levels and alphanumeric symbols and/or lines on one sheet of substrate.
  • the DEP device according to the present invention can be used to print fine tuned gray levels (e.g. pictures, photographs, medical images etc. that contain fine gray levels) and the classical electrographic device can be used to print alphanumeric symbols, line work etc. that do not need the fine tuning of gray levels.
  • Such an apparatus combining a DEP device, according to the invention with a classical electrographic device the strenghts of both printing methods are combined.
  • a printhead structure was made from a polyimide film of 100 ⁇ m thickness, double sided coated with a 15 ⁇ m thick copperfilm.
  • the printhead structure had one continuous electrode surface opposed to the toner delivering means, and a complex addressable electrode structure facing the receptor surface. No third electrode was used in this particular example.
  • the addressable electrode structure was made by conventonial techniques used in the micro-electronics industry, and using fotoresist material, film exposure, and subsequent etching techniques. No surface coatings were used in this particular example.
  • the appertures were 150 ⁇ m in diameter, being surrounded by a circular electrode structure in the form of a ring with a width of 225 ⁇ m measured radialy from the edge of the 150 ⁇ m apertures.
  • the apertures were arranged in such a way as to obtain a pitch of 100 ⁇ m, giving an overall addressability of the image of 250 dpi.
  • the electrodes could be changed in their potential individualy, whereas other elements were connected to one electrical potential for their whole corresponding structure.
  • the toner delivery means is a stationary core/rotating sleeve type magnetic brush (Magbrush A) as described below.
  • the development assembly comprised two mixing rods used to transport the developer through the unit and to mix toner with developer and one metering roller.
  • the magnetic brush assembly was constituted of the so called magnetic roller, which in this case contained inside the roller assembly a stationary magnetic core, showing 9 magnetic poles of 500 Gauss magnetic field intensity and with an open position to enable used developer to fall of from the magnetic roller.
  • the magnetic roller contained also a sleeve, fitting around said stationary magnetic core, and giving to the magnetic brush assembly an overall diameter of 20 mm.
  • the sleeve was made of stainless steel roughened with a fine grain to assist in transport ( ⁇ 50 ⁇ m).
  • a scraper blade was used to force developer to leave the magnetic roller.
  • a doctoring blade is used to meter a small amount of developer onto the surface of said magnetic brush assembly.
  • the sleeve was rotating at 100 rpm, the internal elements rotating at such a speed as to conform to a good internal transport within the development unit.
  • the kneaded mass was pulverised in an impact mill and powder particles sizing between 36 and 100 ⁇ m were separated by sieves of suited mesh.
  • the magnetic properties viz. magnetisation at 1000 Oe in emu/g and coercitive force (Hc) in Oe of the carrier were measured to be 53 emu/g and 97 Oe.
  • Toner T1 97 parts of a copolyester resin of fumaric acid and propoxylated bisphenol A, having an acid value of 18 and volume resistivity of 5.1 x 1016 ohm.cm was melt-blended for 30 minutes at 110 °C in a laboratory kneader with 3 parts of Cu-phthalocyanine pigment (Colour Index PB 15:3).
  • resistivity decreasing substance 0.5 % with respect to the binder of an onium salt K having the furtheron defined structural formula was added.
  • the solidified mass was pulverized and milled using an ALPINE Fliessbettarnastrahlmühle type 100AFG (tradename) and further classified using an ALPINE multiplex zig-zag classifier type 100MZR (tradename).
  • the resulting particle size distribution of the separated toner measured by Coulter Counter model Multisizer (tradename) was found to be 6.3 ⁇ m average by number and 8.2 ⁇ m average by volume.
  • the average particle size by volume is represented hereinafter by d v,50 .
  • the toner particles were mixed with 0.5 % of hydrophobic colloidal silica particles (BET-value 130 m2/g).
  • An electrostatographic developer was prepared by mixing said mixture of toner particles and colloidal silica in a x % ratio (w/w) with carrier particles as defined above (in table 1 the various values of x are shown).
  • the triboelectric charging of the toner-carrier mixture was performed by mixing said mixture in a standard tumbling set-up for 10 min.
  • the developer mixture was run in the development unit (magnetic brush assembly Magbrush A) for 5 minutes after which the toner was sampled and the triboelectric properties measured according to TEST B described above.
  • Direct electrostatic prints were made using a developer comprising carrier A1 and 4 % w/w with respect to the carrier of toner T1, brought into magnetic brush assembly Magbrush A.
  • the brush appeared to be very thin and smooth.
  • the thickness of the brush measured according to TEST A, L, was 350 ⁇ m.
  • front side of the printhead structure means the side of the printhead structure facing the sleeve of the magnetic brush assembly (i.e. the reference surface of the magnetic brush assembly).
  • the distance between the rear side of the printhead structure and the paper running over the back electrode was 150 ⁇ m and the paper travelled at 2 cm/sec.
  • the electric potentials on the different electrodes were (referring to figure 1)
  • Direct electrostatic prints were made in the same way as described in example 1, but the distance, l, between the front side of the printhead structure and the sleeve of the magnetic brush assembly, was set at 275 ⁇ m.
  • Direct electrostatic prints were made in the same way as described in example 1, but the distance, l, between the front side of the printhead structure and the sleeve of the magnetic brush assembly, was set at 225 ⁇ m. In these conditions independentely of the potential settings at the various electrodes, injection of the carrier particles in the printhead structure and onto the prints was observed, resulting in carrier loss and deterioration of the image.
  • Direct electrostatic prints were made in the same way as described in example 1, but the distance, l, between the front side of the printhead structure and the sleeve of the magnetic brush assembly, was set at 1500 ⁇ m. Only when the AC-field that was combined with V1 at the sleeve of the magnetic brush was increased to 9,000 volts peak to peak toner deposition was observed. This high voltage impeded stable functioning of the power supply.Also the amount of deposited toner became unstable in the time.
  • toner T1 at a concentration of 4 % w/w, in combination with carrier A2, has a very small charge, a wide charge distribution, however without wrong sign particles.
  • the thickness of the developer layer on the sleeve of the magnetic brush assembly, L, was 500 ⁇ m and the distance, l, between the reference surface of the magnetic brush assembly and the front side of the printhead structure was set at 750 ⁇ m.
  • toner T1 at a concentration of 7 % w/w, in combination with carrier A3, has a charge that is intermediate between the charge of the toner particles used in example 1 and the charge of the toner particles used in example 11.
  • the thickness of the developer layer on the sleeve of the magnetic brush assembly, L, was 500 ⁇ m and the distance, l, between the reference surface of the magnetic brush assembly and the front side of the printhead structure was set at 500 ⁇ m.
  • the toner delivery means was a rotating core/rotating sleeve type magnetic brush (MagBrush B)
  • the obtained particles were magnetically characterized after melting to a solid mass after magnetisation. A coercivity of 275 Oe and magnetic induction at 1000 Oe of 60 emu/g were measured. The carrier was magnetised up to saturation.
  • Toner T1 was used in this example.
  • An electrostatographic developer was prepared by mixing a mixture of toner particles (toner T1) and colloidal silica in a 4 % w/w ratio with carrier particles (carrier B1) as defined above.
  • the triboelectric charging of the toner-carrier mixture was performed by mixing said mixture in a standard tumbling set-up for 10 min.
  • the developer mixture was run in the development unit (magnetic brush assembly Magbrush B) for 5 minutes after which the toner was sampled and the triboelectric properties measured according to TEST B described above.
  • Toner T1 in combination with carrier B gave following tribolelectric properties (table 2) :
  • Direct electrostatic prints were made using a developer comprising carrier B1 and 4 % w/w of toner T1, brought into magnetic brush assembly Magbrush B.
  • the brush appeared to be of intermediate thickness but very smooth.
  • the thickness of the brush measured according to TEST A, L, was 500 ⁇ m.
  • front side of the printhead structure means the side of the printhead structure facing the sleeve of the magnetic brush assembly.
  • the distance between the rear side of the printhead structure and the paper running over the back electrode was 150 ⁇ m and the paper travelled at 2 cm/sec.
  • the electric potentials on the different electrodes were (referring to figure 1)
  • the sleeve of the magnetic brush rotated clockwise at 80 rpm and the core rotated counterclockwise at 1500 rpm.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Printers Or Recording Devices Using Electromagnetic And Radiation Means (AREA)
EP19950200603 1994-03-29 1995-03-14 Procédé et dispositif d'impression électrostatique directe (DEP) Expired - Lifetime EP0675417B1 (fr)

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Cited By (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0725317A1 (fr) 1995-01-30 1996-08-07 Agfa-Gevaert N.V. Procédé de fabrication d'une suspension de polymère pour le procédé de préparation des particles de toner
EP0809158A2 (fr) * 1996-05-21 1997-11-26 Agfa-Gevaert N.V. Dispositif d'impression électrostatique directe utilisant une composition de toner ayant une bonne conductivité
EP0811894A1 (fr) * 1996-06-06 1997-12-10 Agfa-Gevaert N.V. Méthode d'impression d'information sur des substrats munis d'éléments de sécurité
EP0823676A1 (fr) * 1996-08-08 1998-02-11 Agfa-Gevaert N.V. Méthode d'impression électrostatique directe sur un substrat isolant
EP0827046A1 (fr) * 1996-08-14 1998-03-04 Agfa-Gevaert N.V. Dispositif d'impression électrostatique directe comprenant des moyens pour l'alimentation de toner avec une brosse magnétique contenant un rouleau rotatif magnétique
EP0836124A1 (fr) * 1996-10-10 1998-04-15 Agfa-Gevaert N.V. Méthode d'impression électrostatique directe avec extraction de particules de toner d'un développateur à deux composants comprenant un porteur conducteur
EP0851316A1 (fr) 1996-12-23 1998-07-01 Agfa-Gevaert N.V. Imprimante et méthode d'impression
US5880760A (en) * 1996-06-06 1999-03-09 Agfa-Gevaert Method and device for printing information on substrates having security features
US6003975A (en) * 1995-07-14 1999-12-21 Agfa-Gevaert N.V. DEP printhead structure and printing device having an improved printing electrode structure
US6049680A (en) * 1998-05-08 2000-04-11 Agfa Gevaert N.V. Apparatus for conditioning moisture content temperature of media
US6050677A (en) * 1996-08-08 2000-04-18 Agfa-Gevaert Method for direct electrostatic printing (DEP) a substrate comprising a conductive layer
US6070966A (en) * 1996-10-10 2000-06-06 Agfa-Gevaert Method for direct electrostatic printing in which toner particles are extracted directly from a magnetic brush carrying a two-component developer with conductive carrier
US6074112A (en) * 1996-12-19 2000-06-13 Agfa-Gevaert Printer for large format printing
US6102523A (en) * 1996-12-19 2000-08-15 Agfa-Gevaert Printer for large format printing using a direct electrostatic printing (DEP) engine
US6151047A (en) * 1996-08-14 2000-11-21 Agfa-Gevaert Direct electrostatic printing apparatus having a magnetic brush with a core rotating at high speed
US6227655B1 (en) 1997-04-09 2001-05-08 Agfa-Gevaert DEP (direct electrostatic printing) device maintaining a constant distance between printhead structure and toner delivery means
US6551754B2 (en) 2000-04-11 2003-04-22 Xeikon, N.V. Method for coating carrier particles
US6690837B1 (en) 1998-11-03 2004-02-10 Agfa-Gevaert Screening method for overlapping sub-images

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JPS5881177A (ja) * 1981-11-06 1983-05-16 Canon Inc 画像形成装置
JPS60263962A (ja) * 1984-06-13 1985-12-27 Konishiroku Photo Ind Co Ltd 画像記録装置
EP0266960A2 (fr) * 1986-11-03 1988-05-11 Xerox Corporation Appareil d'impression électrostatique direct et ensemble de nettoyage de la tête d'impression pour cet appareil
US5132708A (en) * 1990-07-02 1992-07-21 Xerox Corporation DEP apparatus for selectively creating monochrome highlight color or process color images
US5233392A (en) * 1991-08-19 1993-08-03 Brother Kogyo Kabushiki Kaisha Image recording apparatus having a particle control electrode

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JPS5881177A (ja) * 1981-11-06 1983-05-16 Canon Inc 画像形成装置
JPS60263962A (ja) * 1984-06-13 1985-12-27 Konishiroku Photo Ind Co Ltd 画像記録装置
EP0266960A2 (fr) * 1986-11-03 1988-05-11 Xerox Corporation Appareil d'impression électrostatique direct et ensemble de nettoyage de la tête d'impression pour cet appareil
US5132708A (en) * 1990-07-02 1992-07-21 Xerox Corporation DEP apparatus for selectively creating monochrome highlight color or process color images
US5233392A (en) * 1991-08-19 1993-08-03 Brother Kogyo Kabushiki Kaisha Image recording apparatus having a particle control electrode

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PATENT ABSTRACTS OF JAPAN vol. 7, no. 177 (M - 233)<1322> 5 August 1983 (1983-08-05) *

Cited By (20)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0725317A1 (fr) 1995-01-30 1996-08-07 Agfa-Gevaert N.V. Procédé de fabrication d'une suspension de polymère pour le procédé de préparation des particles de toner
US6003975A (en) * 1995-07-14 1999-12-21 Agfa-Gevaert N.V. DEP printhead structure and printing device having an improved printing electrode structure
EP0809158A2 (fr) * 1996-05-21 1997-11-26 Agfa-Gevaert N.V. Dispositif d'impression électrostatique directe utilisant une composition de toner ayant une bonne conductivité
EP0809158A3 (fr) * 1996-05-21 1997-12-10 Agfa-Gevaert N.V. Dispositif d'impression électrostatique directe utilisant une composition de toner ayant une bonne conductivité
US5880760A (en) * 1996-06-06 1999-03-09 Agfa-Gevaert Method and device for printing information on substrates having security features
EP0811894A1 (fr) * 1996-06-06 1997-12-10 Agfa-Gevaert N.V. Méthode d'impression d'information sur des substrats munis d'éléments de sécurité
EP0823676A1 (fr) * 1996-08-08 1998-02-11 Agfa-Gevaert N.V. Méthode d'impression électrostatique directe sur un substrat isolant
US6050677A (en) * 1996-08-08 2000-04-18 Agfa-Gevaert Method for direct electrostatic printing (DEP) a substrate comprising a conductive layer
US6151047A (en) * 1996-08-14 2000-11-21 Agfa-Gevaert Direct electrostatic printing apparatus having a magnetic brush with a core rotating at high speed
EP0827046A1 (fr) * 1996-08-14 1998-03-04 Agfa-Gevaert N.V. Dispositif d'impression électrostatique directe comprenant des moyens pour l'alimentation de toner avec une brosse magnétique contenant un rouleau rotatif magnétique
EP0836124A1 (fr) * 1996-10-10 1998-04-15 Agfa-Gevaert N.V. Méthode d'impression électrostatique directe avec extraction de particules de toner d'un développateur à deux composants comprenant un porteur conducteur
US6070966A (en) * 1996-10-10 2000-06-06 Agfa-Gevaert Method for direct electrostatic printing in which toner particles are extracted directly from a magnetic brush carrying a two-component developer with conductive carrier
US6074112A (en) * 1996-12-19 2000-06-13 Agfa-Gevaert Printer for large format printing
US6102523A (en) * 1996-12-19 2000-08-15 Agfa-Gevaert Printer for large format printing using a direct electrostatic printing (DEP) engine
EP0851316A1 (fr) 1996-12-23 1998-07-01 Agfa-Gevaert N.V. Imprimante et méthode d'impression
US6227655B1 (en) 1997-04-09 2001-05-08 Agfa-Gevaert DEP (direct electrostatic printing) device maintaining a constant distance between printhead structure and toner delivery means
US6049680A (en) * 1998-05-08 2000-04-11 Agfa Gevaert N.V. Apparatus for conditioning moisture content temperature of media
US6690837B1 (en) 1998-11-03 2004-02-10 Agfa-Gevaert Screening method for overlapping sub-images
US7231096B2 (en) 1998-11-03 2007-06-12 Agfa-Gevaert N.V. Screening method for overlapping sub-images
US6551754B2 (en) 2000-04-11 2003-04-22 Xeikon, N.V. Method for coating carrier particles

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