EP0795802A1

EP0795802A1 - A printhead structure made from an electroless plated plastic substrate

Info

Publication number: EP0795802A1
Application number: EP97200699A
Authority: EP
Inventors: Guido Desie; André Heyvaerts; Gerhard-Dieter Wolf
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1996-03-15
Filing date: 1997-03-08
Publication date: 1997-09-17

Abstract

A printhead structure is provided wherein no adhesive layer is present between the plastic film and the metal layer(s) and wherein the metal layer is directly superimposed on a catalyst layer comprising a metal selected from the group consisting of Cu, Ag, Au, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt. Methods for manufacturing such printhead structure and a DEP (Direct Electrostatic Printing) device using such a printhead structure are also provided.

Description

1. Field of the invention.

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

2. Background of the Invention.

In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a receiving substrate, the latter not bearing any imagewise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium, polyimide etc.). In that case the imagewise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final receiving substrate, thus offering a possibility to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred 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 polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional 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 e.g. US 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle 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 of conductive material on the other side of the isolation layer ; and
at least one row of apertures.

Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode.
This kind of printing engine, however, does not produce stable results with high precision for a long writing time, since the apertures in the printhead become too easily blocked by toner particles adhering to the insulating material or shield and control electrodes.
To overcome these problems several modifications have been proposed in the literature.
In US 4,478,510 e.g. a spark discharge is used to remove toner particles adhered to the printhead, in order to set if free again. For that purpose the printing time is divided in a writing time (during which an image is written to the receiving material) and a cleaning time. During the cleaning period the voltage applied to the back electrode is enhanced so that a spark discharge occurs from printhead to back electrode. Toner particles adhered to the printhead become dislodged and are gathered upon the back electrode. Another possibility that has been described is a spark discharge between shield and control electrode providing the same effect, namely cleaning of clogged apertures in the printhead.
In US 4,755,837 an AC voltage is used for the backing electrode during the cleaning cycle. In a preferred embodiment the AC voltage on the back electrode is phase shifted by 180° if compared with the AC that is used upon the charged toner conveyor which is needed to obtain a high toner mist production, leading to high optical densities and short printing times. Further on the AC voltage can also have a certain DC-offset.
In US 4,876,561 clogging of the printhead is prevented by making the apertures large enough and/or the thickness of the isolating layer small enough.
In US 4,903,050 an AC voltage is applied to the back electrode as in UP-P 4,755,857, but a shutter and vacuum system is added in order to prevent the dislodged toner to fall onto the receiving member.
In US 5,095,322 clogging of the apertures is prevented by applying to the shield electrode a pulsed DC-voltage which is 180° out of phase if compared with the AC-voltage applied to the charged toner conveyor. In an other embodiment a DC-biased AC voltage with the same frequency as the AC voltage applied to the charged toner conveyor but 180° out of phase is used to prevent clogging of the apertures in the printhead.
In US 5,153,611 an ultrasonic vibration is applied to the printhead, yielding a better performance regarding prevention of clogging of the apertures. The same idea has also been described by Brother in US 5,202,704.
In US 5,233,392 a better performance in preventing clogging of the apertures is disclosed by using an ultrasonic vibration applied to the printhead, the improvement being changing within the writing time for each individual pixel the resonant frequency of the oscillation used by a small amount, resulting in a much better prevention of clogging.
In US 5,256,246 a printhead structure is made from a thin ceramic insulating member with control electrodes applied to said ceramic member by thin film techniques such as sputtering, vacuum deposition, ion plating, chemical vapour deposition and screen printing. It is claimed in this patent application that the absence of a sticky coating layer under the conductive layer does make the printhead structure less sensitive to clogging. A big drawback of this technique, however, is the reduced adhesive power of the conductors to the substrate.
In US 5,283,594 the level of vibration applied to the printhead is different during writing time and cleaning time. During writing time the oscillation is large enough to prevent clogging of the apertures for a great amount, during cleaning time the amplitude of the oscillating vibration is large enough to dislodge the toner particles that have partially clogged the apertures during the writing cycle. As a result the long-time performance of the DEP-apparatus is improved considerably.
In US 5,293,181 the printhead is vibrated in such a way that a mechanical vibrational propagating wave is created. The printhead also has a provision in order to prevent reflection of the mechanical vibrational propagating wave. Using these implementations a good long-time stability without clogging of the apertures is provided with a good writing characteristic.
In US 5,307,092 an antistatic coating is applied to the electrodes in the printhead so that any tribocharge that accumulates during writing can be grounded. As a result the nett tribocharge on the printhead (which is unwanted and is responsible for unpredictable results and clogging) is removed and a better long-time performance results.
In WO 90/14959 the printhead is treated with pressurized air or vacuum so that the individual toner particles do not adhere to the printhead for such a large amount if compared with a printing engine not using the air treatment. In the same document an additional improvement is described where by the magnetic toner particles are removed from the printhead by using a much stronger magnetic field during the cleaning cycle than during the writing cycle.
In DE 4,338,991 Array Printers use ionised air for blowing over the printhead so that the electrostatic interaction of the toner particles with the printhead is reduced and the toner particles are removed more easily from it than if compared with patent application WO 90/14959 where the air used is not pretreated at all.
There is thus still a need to have a system for practising DEP, that while avoiding the problems cited above, is based on a simple apparatus and that yields high quality images in a reproducible and constant way.

3. Objects and Summary of the Invention

It is an object of the invention to provide a printhead structure for use in non impact printing for printing with high density and high spatial resolution at a high printing speed.
It is a further object of the invention to provide DEP device incorporating a printhead structure, combining high spatial and density resolution with good long term stability and reliability.
It is a still further object of the invention to provide a printhead structure for use in Direct Electrostatic Printing (DEP) which makes it possible to print high resolution images and having printing apertures showing minimal or no tendency for clogging by toner particles.
It is also an object of the present invention to provide a method for manufacturing a printhead structure which makes it possible to print high resolution images and having printing apertures showing minimal or no tendency for clogging by toner particles.
Further objects and advantages of the invention will become clear from the description hereinafter.
The above objects are realized by providing a printhead structure comprising a metallic layer on a plastic substrate and an array of printing apertures through said metallic layer and said substrate, characterised in that

i) said metallic layer is directly superimposed on a catalyst layer,
ii) said catalyst layer contains a metal organic compound and,
iii) said metal in said metal organic compound is a member selected from the group consisting of Cu, Ag, Au, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt.

The objects are further realized by providing a DEP device that comprises :

a printhead structure (106), comprising a metallic layer on a plastic substrate, wherein an array of printing apertures (107) is present through which a particle flow can be electrically modulated by a control electrode (106a) formed by said metallic layer, and
a toner delivery means (101),

4. Brief Description of the Drawing

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

5. Detailed Description of the Invention

In the literature many devices have been described that operate according to the principles of DEP (Direct Electrographic Printing). All these devices are able to perform grey scale printing either by voltage modulation or by time modulation of the voltages applied to the control electrodes.
It has been described above that a problem of DEP devices is that the printing apertures that are made through said plastic substrate and conducting layers becomes too easily blocked (clogged) by passing toner particles.
This problem has been tackled by the use "cleaning means" making the DEP device much more complicated and expensive. However, the trend in any printing device, and thus also in DEP printing devices is to provide device that are as small as possible and still perform at high speed. In this view the use of additional cleaning means is not preferred.
In printhead structures, comprising a insulating material wherein printing apertures are present that are surrounded by metal control electrodes and optionally having a shield electrode on the side of the isolating material opposite to the side carrying the control electrodes, the metal for forming the electrodes is attached to said insulating material by means of an adhesive. It has been found that said adhesive layer is largely responsible for the clogging of the printing apertures.
It has also been found that a printhead structure could be produced without such an adhesive layer and that in such a printhead without adhesive layer the clogging of the printing apertures was lowered or even prevented.
The production of a printhead structure without adhesive layer between the metal forming the electrodes and the insulating plastic material, an a thin layer of catalysts in a solvent system is applied to the insulating plastic material, causing a physical entanglement of the catalyst particles with the polymeric molecules of the plastic substrate. Said interaction yielding an improved adhesion of said catalyst layer to said plastic material can be further enhanced by applying appropriate drying conditions and temperature treatments. After applying, drying and conditioning of said catalyst layer a process of electroless plating, i.e. electroless deposition of a metal, is used to convert said non-conducting catalyst surface to a conducting metallic surface. By adjusting the electroless plating conditions it is possible to produce conducting metallic layers having varying thickness. The thickness of said metallic layers can be varied between a thickness lower than 1 µm to a thickness of several µm. If even thicker conducting layers are wanted, then said conductive layer obtained by electroless plating can be further enhanced in thickness by electrodeposition processes, i.e. the deposition of a metal under influence of electric fields. At the end a plastic material is obtained having a very thin interface zone between the plastic material and the conductive material without any adhesives, e.g., low-Tg-polymers that are conventionally used for adhesive purposes. Both in the electroless plating and in the electrodeposition of a metal on the catalyst layer, any metal can be used, it is however preferred to use copper or aluminium.
Said conductive layer is then treated as known to those skilled in the art by conventional (copper) etching techniques to obtain a (copper) pattern of control electrodes. The printing apertures through both of said plastic material and conductive material can be made as known to those skilled in the art by methods such as mechanical drilling, laser burning and plasma etching. For printhead structures with a high printing addressability (high resolution) small printing apertures have to be made. Said plastic material with a thin conductive layer and a very thin non-tacky interface zone with excellent adhesive power to both of said plastic material and conductive material, is extremely well adapted for making printhead structures with these wanted properties of high resolution. The printing apertures in a printhead structure according to this invention are preferably made by a method as described in EP-A 719 648.
The catalyst used to apply the catalyst layer onto the plastic substrate can be of any type that allows good electroless plating and that gives good adhesion to the plastic substrate. Examples of catalysts useful in the production of printhead structures according to this invention, are described in several publications. In, e.g., EP-A 125 617 Ti, Zr, V and Cr are used as catalyzing metal, in, e.g., US 4,469,714 it is disclosed to use a catalyser solution comprising a UV-curable resin and a Pd catalyst, in, e.g. US 3,937,857 it is disclosed to use either Pd or Pt as catalyst, in e.g. EP-A 520 195 Pd, Pt, Ag and Au are disclosed as possible catalysts. In EP-A 256 395 it is disclosed to use metal organic compounds comprising metals selected from the subgroup 1b and group 8 of the periodic table of elements, i.e. a metal being a member of the group consisting of Cu, Ag, Au, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt. The catalysts disclosed in EP-A 256 395 are preferred for use in the manufacture of a printhead structure according to the present invention.
The catalyst solution are mostly a dispersion of the catalyst in an organic solvent or solvent mixture and comprises binders as, e.g., polyurethanes, polyacrylates, polyisocyanates, etc. Said catalyst solution can further comprise fillers as, e.g. silica, titania, etc, wetting agents, viscosity regulators, degassing promoters, etc.
Highly preferred catalysts for use in the manufacturing of a printhead structure according to the present invention are Pd-based catalysts sold by BAYER AG, Leverkusen, Germany under tradename BAYPRINT.
The layer of catalyst can be applied to the plastic substrate out of solution by any means know in the art, e.g. by screen printing, by coating, etc. Both methods, screen printing and coating, can be useful in the present invention, coating methods having the advantage that it is easier to apply the catalyst layer to plastic material in web form by coating than by screen printing. When the layer of catalyst is coated onto the plastic substrate, this coating can proceed by the known coating techniques, e.g., dip coating, rod coating, blade coating, air knife coating, gravure coating, reverse roll coating, extrusion coating, slide coating and curtain coating. An overview of these coating techniques can be found in the book "Modern Coating and Drying Technology", Edward Cohen and Edgar B. Gutoff Editors, VCH publishers, Inc, New York, NY, 1992.
Within the scope of the present invention, the layer of catalyst can be applied on only one side of a plastic material or on both sides depending on the lay-out of the printhead structure to be manufactured, e.g. a printhead structure with only control electrodes necessitates the presence of a layer of catalyst on only one side of the plastic film, a printhead structure comprising both control electrodes and a shield electrode will necessitate the presence of a layer of catalyst on both sides of the plastic film.
The layer of catalyst can be applied to any insulating substrate, although plastic substrates are preferred. From the plastic substrates, the most preferred substrates are polyimide, polyesters (e.g. polyethyleleterephthalate, polyethylenenaphthalate) and syndiotactic polystyrene.
The present invention encompasses a method for manufacturing a printhead structure, useful for use in DEP devices, but printhead structures made by the manufacturing method according to the present invention, can also be used in ink-jet devices, ionography, etc.
A first manufacturing method comprises the steps of :

i) applying a thin layer of catalyst upon a plastic substrate,
ii) converting said thin layer to a conducting metal layer by electroless plating, i.e. by electroless deposition of a metal, preferably copper or aluminum, on top of the catalyst layer,
iii) forming a plurality of control electrodes on said metal layer by patterning said metal layer on the surface of said plastic substrate and
v) forming apertures through said plastic substrate at the centre of said control electrodes.

An alternative manufacturing method, that is also encompassed in the present invention comprises the steps of :

i) applying a thin layer of catalyst upon a plastic substrate,
ii) patterning said thin catalyst layer to form a plurality of control electrodes on said thin catalyst layer,
iii) converting said control electrode patterned on said thin layer to conducting metal control electrodes by electroless plating,
iv) forming apertures through said plastic substrate at the centre of said control electrodes.

The patterning of the catalyst layer, in a method according to this invention, can proceed in several ways. A photopolymer can be applied on the layer of catalyst, the desired pattern can then be exposed on said photopolymer layer, then the photopolymer layer is exposed and the non-exposed area washed away, leaving a pattern of free catalyst layer, that can be converted to conducting metal control electrodes by electroless plating.
The catalyst layer can image-wise be ablated, e.g. by an excimer laser, and the remaining catalyst pattern can be converted to conducting metal control electrodes by electroless plating.
It is also possible to image-wise de-activate the catalyst layer, e.g. by ultraviolet light, excimer lasers, etc., the remaining active part of the catalyst layer then being used to form conducting metal control electrodes by electroless plating.
An other way to produce, in a method according to this invention, a pattern for forming control electrodes by electroless plating with a catalyst layer is to apply an inactive catalyst layer to the plastic substrate and image-wise activate the catalyst. The active part of the catalyst layer is then used to form conducting metal control electrodes by electroless plating.
In an other way, a pattern for forming control electrodes by electroless plating with a catalyst layer can be produced by covering the (active) catalyst layer by a cover layer, that can be ablated image-wise.
The catalyst layer, in manufacturing methods according to this invention, contains an metal organic compound wherein said metal is a member selected from the group consisting of Cu, Ag, Au, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt.

Description of the DEP device

A non limitative example of a DEP device comprising a printhead structure according to the present invention comprises (fig 1):

(i) a toner delivery means (101), comprising a container for developer (102), a charged toner conveyer (103) and a magnetic brush (104) , this magnetic brush forming a layer of charged toner particles upon said charged toner conveyer
(ii) a back electrode (105)
(iii) a printhead structure (106), made from a plastic insulating film, coated with a metallic film formed from a electroless deposition step on a thin layer of catalyst. The printhead structure (106) comprises a complex addressable electrode structure, hereinafter called "control electrode" (106a) around printing apertures (107), facing, in the shown embodiment, the toner-receiving member in said DEP device. Said printing apertures are arranged in an array structure for which the total number of rows can be chosen according to the field of application. In an other embodiment of the present invention a second conductive layer can be present on the other side of said plastic material, said second conductive layer being called the shield electrode layer.
(iv) conveyer means (108) to convey an image receptive member (109) for said toner between said printhead structure and said back electrode in the direction indicated by arrow A.
(v) means for fixing (110) said toner onto said image receptive member.

Although in fig. 1 an embodiment of a device for a DEP method using one electrode (106a) on printhead 106 is shown, it is possible to implement a DEP method, using toner particles according to the present invention using devices with different constructions of the printhead (106). It is, e.g. possible to implement a DEP method with a device having a printhead comprising two or even more electrode structures. The apertures in these printhead structures can have a constant diameter, or can have a broader entrance or exit diameter.
The back electrode (105) of this DEP device can also be made to cooperate with the printhead structure, said back electrode being constructed from different styli or wires that are galvanically isolated and connected to a voltage source as disclosed in e.g. US 4,568,955 and US 4,733,256. The back electrode, cooperating with the printhead structure, can also comprise one or more flexible PCB's (Printed Circuit Board).
Between said printhead structure (106) and the charged toner conveyer (103) as well as between the control electrode around the apertures (107) and the back electrode (105) behind the toner receiving member (109) different electrical fields are applied. In the specific embodiment of a device, useful for a DEP method, using a printing device with a geometry according to the present invention, shown in fig 1. voltage V1 is applied to the sleeve of the charged toner conveyer 103, voltages V3₀ up to V3_n for the control electrode (106a). The value of V3 is selected, according to the modulation of the image forming signals, between the values V3₀ and V3_n, on a timebasis or grey-level basis. Voltage V4 is applied to the back electrode behind the toner receiving member. In other embodiments of the present invention multiple voltages V4₀ to V4_n can be used. Voltage V2 is applied to the surface of the sleeve of the magnetic brush.
A DEP device according to the present invention can be operated successfully when a single magnetic brush is used in contact with a Charged Toner Conveyor (CTC) to provide a layer of charged toner on said CTC. In a DEP device according to the present invention an additional AC-source can also be connected to the sleeve of the magnetic brush.
The magnetic brush 104 preferentially used in a DEP device according to the present invention is of the type with stationary core and rotating sleeve.
In a DEP device, according to of the present invention and using a magnetic brush of the type with stationary core and rotating sleeve, any type of known carrier particles and toner particles can successfully be used. It is however preferred to use "soft" magnetic carrier particles. "Soft" magnetic carrier particles useful in a DEP device according to a preferred embodiment of the present invention are soft ferrite carrier particles. Such soft ferrite particles exhibit only a small amount of remanent behaviour, characterised in coercivity values ranging from about 3.9 kA/m up to 20 kA/m (50 up to 250 Oe). Further very useful soft magnetic carrier particles, for use in a DEP device according to a preferred embodiment of the present invention, are 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 µm. The carrier particles have preferably an average volume diameter (d_v50) between 10 and 300 µm, preferably between 20 and 100 µm. More detailed descriptions of carrier particles, as mentioned above, can be found in EP-A 675 417, that is incorporated herein by reference.
It is preferred to use in a DEP device according to the present invention, toner particles with an absolute average charge (|q|) corresponding to 1 fC ≤ |q| ≤ 20 fC, preferably to 1 fC ≤ |q| ≤ 10 fC. The absolute average charge of the toner particles is measured by an apparatus 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 femtoCoulomb (fC)) with respect to a measured toner diameter (d in 10 µm). From the absolute average charge per 10 µm (|q|/10µm) the absolute average charge |q| is calculated. Moreover it is preferred that the charge distribution, measured with the apparatus cited above, is narrow, i.e. shows a distribution wherein the coefficient of variability (v), i.e. the ratio of the standard deviation to the average value, is equal to or lower than 0.33. Preferably the toner particles used in a device according to the present invention have an average volume diameter (d_v50) between 1 and 20 µm, more preferably between 3 and 15 µm. More detailed descriptions of toner particles, as mentioned above, can be found in EP-A 675 417.
A DEP device making use of the above mentioned marking toner particles can be addressed in a way that enables it to give black and white. It can thus be operated in a "binary way", useful for black and white text and graphics and useful for classical bilevel halftoning to render continuous tone images.
A DEP device according to the present invention is especially suited for rendering an image with a plurality of grey levels. Grey level printing can be controlled by either an amplitude modulation of the voltage V3 applied on the control electrode 106a or by a time modulation of V3. By changing the duty cycle of the time modulation at a specific frequency, it is possible to print accurately fine differences in grey levels. It is also possible to control the grey level printing by a combination of an amplitude modulation and a time modulation of the voltage V3, applied on the control electrode.
The combination of a high spatial resolution and of the multiple grey level capabilities typical for DEP, opens the way for multilevel halftoning techniques, such as e.g. described in EP-A 634 862. This enables the DEP device, according to the present invention, to render high quality images.
Printing examples were made by an apparatus using a developer, comprising toner and carrier particles, as described further on.

The printhead structure (106)

A printhead structure 106 was made from a polyimide film of 75 µm thickness, KAPTON 300 HN, commercially available from Dupont, Wilmington, USA. A catalyst layer (BAYPRINT 3305, trade name, commercially available through BAYER AG, Leverkusen, Germany) was screen printed onto said polyimide layer. The amount of catalyst that was screen printed was adjusted so as to give a dry coating weight of 2 g catalyst per m². After screen printing, the material was dried and treated at a temperature of 150°C for 60 minutes.
After this treatment, the polyimide film with catalyst layer was converted to a conducting film with 1 µm thick copper through an electroless plating bath (XD-6157-T, MacDermid, USA), followed by a conventional electrodeposition step enhancing the thickness of said copper coating from about 1 µm to 5 µm. The flexprint material obtained in this way was further treated, as known to those skilled in the art, by photoresist and copper etching techniques to obtain copper control electrodes around square shaped "printing apertures" of 35 µm width staggered in 4 rows to obtain an addressability of 600 dpi (dot per inch, or 236 dots/cm). The holes through the polyimide material were made by excimer laser burning resulting in printing apertures with excellent aperture definition. Each of said control electrodes was individually addressable from a high voltage power supply. So a printhead structure was made from a polyimide plastic material with a thin copper coating with good-adhesive power, combining a high printing resolution of 600 dpi with excellent aperture and control electrode definition.

The carrier particles

A macroscopic "soft" ferrite carrier consisting of a MgZn-ferrite with average particle size 50 µm, a magnetisation at saturation of 3.6 µTm³/kg (29 emu/g) was provided with a 1 µm thick acrylic coating. The material showed virtually no remanence.

The toner particles

The toner used for the experiment had the following composition : 97 parts of a co-polyester resin of fumaric acid and bispropoxylated 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). A resistivity decreasing substance - having the following formula :
(CH₃)₃N⁺C₁₆H₃₃ Br^- was added in a quantity of 0.5 % with respect to the binder, as described in WO 94/027192.
After cooling, the solidified mass was pulverized and milled using an ALPINE Fliessbettgegenstrahlmühle type 100AFG (tradename) and further classified using an ALPINE multiplex zig-zag classifier type 100MZR (tradename). The average particle size was measured by Coulter Counter model Multisizer (tradename), was found to be 6.3 µm by number and 8.2 µm by volume. In order to improve the flowability of the toner mass, the toner particles were mixed with 0.5 % of hydrophobic colloidal silica particles (BET-value 130 m²/g).

The developer

An electrostatographic developer was prepared by mixing said mixture of toner particles and colloidal silica in a 4 % ratio (w/w) with carrier particles. 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 magnetic brush for 5 minutes, after which the toner was sampled and the tribo-electric properties were measured, according to a method as described in the above mentioned EP-A 675 417. The average charge, q, of the toner particles was -7.1 fC.

The toner delivery means (101)

The toner delivery means 101 comprised a cylindrical charged toner conveyer (103) with a sleeve made of aluminium with a TEFLON (trade name) coating an a surface roughness of 2.5 µm (Ra-value measured according to ANSI/ASME B46.1-1985) and a diameter of 20 mm. The charged toner conveyer was rotated at a speed of 50 rpm. The charged toner conveyer 103 was connected to an AC power supply with a square wave oscillating field of 600 V at a frequency of 3.0 kHz with 20 V DC-offset.
Charged toner was propelled to this conveyer from a stationary core/rotating sleeve type magnetic brush (104) comprising two mixing rods and one metering roller. One rod was used to transport the developer through the unit, the other one to mix toner with developer.
The magnetic brush 104 was constituted of the so called magnetic roller, which in this case contained inside the roller assembly a stationary magnetic core, having three magnetic poles with an open position (no magnetic poles present) to enable used developer to fall off from the magnetic roller (open position was one quarter of the perimeter and located at the position opposite to said CTC (103).
The sleeve of said magnetic brush had a diameter of 20 mm and was made of stainless steel roughened with a fine grain to assist in transport (Ra=3 µm measured according to ANSI/ASME B46.1-1985) and showed an external magnetic field strength in the zone between said magnetic brush and said CTC of 0.045 T, measured at the outer surface of the sleeve of the magnetic brush.
A scraper blade was used to force developer to leave the magnetic roller. On the other side a doctoring blade was used to meter a small amount of developer onto the surface of said magnetic brush. 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 magnetic brush 104 was connected to a DC power supply of -200 V.

The printing engine

The distance between the upper surface of the charged toner conveyor 103 and the front side of said printhead structure 106 was set to 400 µm, the distance between the back electrode 105 and the back side of the printhead structure 106 was set to 150 µm and the paper travelled at 7 mm/sec. To the individual control electrodes an (imagewise) voltage V3 between 0 V and -300 V was applied. The back electrode 105 was connected to a high voltage power supply of +600 V. To the sleeve of the CTC an AC voltage of 600 V at 3.0 kHz was applied, with 20 V DC offset.
With a DEP device using a printhead structure according to the present invention images with a high resolution could be printed without any significant density reduction as a function of printing time. Compared to prior art printhead structures (manufactured from copper-clad polyimide foil), it was even possible to print at a resolution of 600 dpi (dot per inch, or 236 dots/cm). This resolution could not be obtained when using a DEP device with a printhead structure made of conventional flexprint material (e.g. 50 µm polyimide with 20 µm adhesive and 17 µm copper).

A colour printing engine

A DEP device adapted for colour printing was made using a printhead structure manufactured by the method of the present invention. The printhead structure was made essentially in the same way as described above. A catalyst layer was applied to a polyimide film followed by electroless deposition of copper as described above. Afterwards, the thickness of the metal layer was enhanced by electrodeposition to 5 µm copper thickness. Then the control electrode were patterned and the copper was etched. The printing apertures were square shaped with a width of 100 µm and staggered in 2 rows so to obtain a printing resolution of 254 dpi (dot per inch, 100 dots per cm). The through holes were drilled by an excimer laser using the copper electrodes as mask, and further cleaned by a short isotropic plasma etching treatment. 20992 printing apertures yielded a printing width of 210 mm. A similar printhead structure was made for four different applicator modules.
As charged toner applicator module a commercially available non magnetic mono-component unit of an Apple Color Laserwriter 12/600 PS (i.e. yellow toner cartridge M3758 G/A, magenta toner cartridge M3760 G/A, cyan toner cartridge M3757 G/A, and black toner cartridge M3756 G/A) was used. The doctor blade and toner roller were connected to a high power supply delivering a 3.0 kHz square wave oscillating AC voltage of 300 Vrms and -200 V DC-component.
To the individual control electrodes a time modulated voltage of 0 to -300 V was applied, according to the image density information.
The distance between the surface of toner roller and the surface of the printhead structure bearing on that side the different control electrodes was set to 300 µm. The printhead structure was stretched over 4 roller bars in a frame as described in EP-A 712 056 and located in the printing engine in a rigid way so that correct alignment between the four different printhead structures was possible.
The image receiving paper was conducted at a printing speed of 20 mm/s over a back electrode at 500 µm distance from the back side of the printhead structure, said back electrode being connected to a high voltage power supply of 1500 V.
A multi color image was printed with this device using non magnetic mono-component toners and printhead structures made via an electroless deposition copper forming process, said image having excellent sharpness and color quality.
It is clear for those skilled in the art that many modifications can be made to this concept of slit aperture printhead without departing from the spirit of this invention, namely that an extremely inexpensive printhead structure can be constructed without difficult alignment and positioning steps.

Claims

A printhead structure, comprising a metallic layer on a plastic substrate and an array of printing apertures through said metallic layer and said substrate, characterised in that
i) said metallic layer is directly superimposed on a catalyst layer,

ii) said catalyst layer contains an metal organic compound and,

iii) said metal in said metal organic compound is a member selected from the group consisting of Cu, Ag, Au, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt.
A printhead structure according to claim 1, wherein said catalyst layer contains Pd.
A printhead structure according to claim 1 or 2, wherein said catalyst layer is non-conductive.
A printhead structure according to any of claims 1 to 3, wherein said catalyst layer is directly in contact with said plastic substrate.
A printhead structure according to any of claims 1 to 4, wherein said plastic substrate is a member selected from the group of polyimides, polyethyleneterephthalate, polyethylene naphthalate and syndiotactic polystyrene.
A method for manufacturing a printhead structure comprising the steps of :
i) applying a thin layer of catalyst upon a plastic substrate,

ii) converting said thin layer to a conducting metal layer by electroless plating,

iii) optionally, enhancing the thickness of said conducting metal layer by electrodeposition of a metal,

iv) forming a plurality of control electrodes on said metal layer by patterning said metal layer on the surface of said plastic substrate and

v) forming apertures through said plastic substrate at the centre of said control electrodes.
A method for manufacturing a comprising the steps of :
i) applying a thin layer of catalyst upon a plastic substrate,

ii) patterning said thin catalyst layer to form a plurality of control electrodes on said thin catalyst layer,

iii) converting said control electrode patterned on said thin layer to conducting metal control electrodes by electroless plating,

iv) optionally, enhancing the thickness of said metal control electrodes by electrodeposition of a metal and

v) forming apertures through said plastic substrate at the centre of said control electrodes.
A method according to claim 6 or 7, wherein said thin catalyst layer contains a metal organic compound wherein said metal is a member selected from the group consisting of Cu, Ag, Au, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt.
A method according to claim 8, wherein said catalyst layer comprises Pd.
A DEP device comprising:
a printhead structure (106), comprising a metallic layer on a plastic substrate, wherein an array of printing apertures (107) is present through which a particle flow can be electrically modulated by a control electrode (106a) formed by said metallic layer, and

a toner delivery means (101),
characterised in that said printhead structure is a printhead structure as defined in any of claims 1 to 5.