Process for magnetically transferring a powder image

Abstract

 Process for transferring a powder image (2) which has been formed with magnetically attractable developing powder, which process comprises the steps of magnetically transferring the powder image (2) to an intermediate receiving member (4) comprising zones of a first material, separated from each other by a second material, in which either the first or the second material is magnetizable and the other material is non-magnetizable, and subsequently transferring the powder image (2) from the intermediate receiving member (4) to a final receiving material (9). The magnetizable material present on the intermediate receiving member (4) may be a material having no remanent magnetism, but in a preferred embodiment thin material has a remanent magnetism of at least 2 kA/m. Transfer of the powder image (2) to the intermediate receiving member (4) is effected by magnetizing the powder image (2) or the magnetizable zones of the intermediate receiving member (4) and contacting the powder image (2) with the receiving member (4).

Description

[0001] The invention relates to a process for magnetically transferring a powder image formed with the aid of a magnetically attractable powder.

[0002] According to the process of the so-called indirect electrophotographic copying system, such as this has been applied in practice, a latent electrostatic image is formed in a photoconductive material and this image is developed with a developing powder, after which the powder image obtained is transferred to a receiving material, mostly consisting of plain paper, on which it is fixed. After the transfer of the powder image the photoconductive material is cleaned and used for a next copying run.

[0003] The transfer of the powder image to the receiving material is mostly effected under influence of an electric field, which is generated between the photoconductive material and the receiving material. This electric transfer has the disadvantage, that blurred images occur, because powder particles are dispersed as a result of electric discharges which continuously take place in the transfer zone. A further objection of the electric transfer is, that with regard to the transfer efficiency and the quality of the transferred image the results obtained with it are dependent on the atmospheric conditions and the electric properties of the developing powder and the receiving material.

[0004] In order that a powder image can be transferred independently of the atmospheric conditions and the electric properties of developing powder and receiving material, it has already been proposed to use a magnetically attractable developing powder for the development of the electrostatic image and to transfer the powder image under influence of the magnetic field of a permanent magnet or electro magnet installed behind the receiving material. However, also with this magnetic transfer method blurred images occur, because during the ejection of the receiving material out of the transfer zone particles of developing powder move over the receiving material under influence of the magnetic field.
These blurred images can be prevented by simultaneously transferring the powder image magnetically, and fixing it with the aid of heat, as described in the U.S. patent specifications 3 093 039 and 3 106 479. However, an objection of these processes is, that the heat-fixation of the powder image is carried out while the image is still in contact with or in the very neighbourhood of the photoconductive material, so that it can happen that melted or softened particles of developing powder permanently deposit on the photoconductive material and consequently this material can no more be reused. An objection of the process according to U.S. patent specification 3 093 039, in which transfer and fixing of the powder image take place simultaneously under influence of a high frequency magnetic field, moreover is that a very great quantity of energy is required in order to reach the desired fixing of the image.
An objection of the process according to U.S. patent specification 3 106 479, in which a heating element is installed in the transfer zone and in the very neighbourhood of the photoconductive material, is that also the photoconductive material is heated considerably, by which its photo-electric properties quickly decrease and only a relatively limited number of copies can be made with it.
In the Dutch patent application 7209652, with reference to Fig. 8, a process for the formation of a latent magnetic image is described, which process starts from a powder image formed electrophotographically with the aid of magnetically attractable developing powder on a photoconductive material. According to this process a uniform layer of permanently magnetizable material, which layer has been magnetized according to a fine linear pattern, is brought into contact with the image-carrying surface of the photoconductive material and the magnetized layer is demagnetized in those portions which are not in contact with the powder image, with the aid of a magnetic erasing head which is installed behind the photoconductive material. During the formation of the latent magnetic image a part of the magnetically attractable developing powder is transferred to this magnetic image. However, the quantity of transferred developing powder is small, so that the process described cannot be applied without more as transfer method in an indirect electrophotographic copying system. A further objection of this process is, that for magnetizing the permanently magnetizable layer a wide magnet head must be used, which must have been manufactured with great precision, in order to obtain a magnetic field of uniform strength over its full working width.

[0005] The object of the invention is to provide for an improved process for the magnetic transfer of a powder image which has been formed with the aid of a magnetically attractable powder, in which process the powder image is transferred under influence of a magnetic field to a first receiving support and from this receiving support is transferred direct or indirect to the final receiving support. The process is characterized in that the first receiving support comprises zones of a first material which zones are separated from each other by a second material, whilst either the first material or the second material is magnetizable and the other material is not magnetizable.
Here and also hereafter magnetizable material means a ferro- or ferrimagnetic material or a material which contains such ferro- or ferrimagnetic material in finely dispersed condition.

[0006] The process according to the invention differs from the magnetic transfer methods known up to now, in that a high transfer efficiency is achieved and in that sharp images are obtained, without necessity to fix the powder image in some way or other on the receiving support simultaneously with the magnetic transfer. In this way the objections of the processes, as described in above-mentioned U.S. patent specifications 3 093 039 and 3 106 479, are prevented in the process according to the invention.

[0007] The first receiving support applied in the process according to the invention comprises zones of a first material, which are separated from each other by a second material, whilst either the first material or the second material is magnetizable and the other material is not magnetizable. The form of the zones consisting of the first material can_be chosen arbitrarily, but for practical reasons an almost square or round form is preferred. For obtaining a good sharpness of the powder images transferred to the first receiving support, and also for achieving a high resolving power, the zones of the first material should be small, while also the inter-distance between these zones should be small. Good results are obtained when the greatest diameter of the zones built up of the first material, as well as the inter-distance between these zones lies between the diameter of the smallest developing powder particles to be transferred and 5 times the diameter of the largest powder particles to be transferred. Preferably the greatest diameter of the zones of the first material, as well as the greatest inter-distance between these zones amounts to one to two times the diameter of the largest developing powder particles. As usually developing powders with a particle size of 5 to 50 micrometres are used, the greatest diameter of the zones of the first material, as well as the greatest inter-distance between these zones will consequently amount to 5 to 250 micrometres, and preferably to about 50 to 100 micrometres.

[0008] For obtaining a good image quality, the zones of the first material should have been dispersed very uniformly over the surface of the first receiving support. Preferably about 30 to 70% of the surface of the first receiving support is covered by these zones.
The magnetizable material on the first receiving support may be any of the known, permanently or non-permanently magnetizable materials. Examples of magnetizable materials are: iron, cobalt, nickel; ferrites; alloys of Co and Ni, of Cu, Ni and Fe, of Cu, Ni and Co; chromium dioxide; y-ferrioxide and further the materials mentioned in the Dutch patent application 6806473. The magnetizable material may be present in the magnetizable regions on the first receiving support in the form of a continuous layer or as dispersion in a filmforming binding agent.
The non-magnetizable material may for instance consist of a metal such as copper or aluminium, of glass or of a plastic, in which non-magnetizable substances, such as fillers or antistatic agents, may be present.
First receiving supports with a smooth and relatively hard surface are preferred, because by their greater mechanical strength such receiving supports have a longer duration of life than receiving supports with a more or less rough and/or soft surface. First receiving supports with a smooth, metallic surface, with which consequently both the first and the second material consist of metal or a metal-alloy, are therefore preferably applied in the process according to the invention.

[0009] The first receiving support for application in the process according to the invention can be manufactured in various ways. A very suitable manufacturing method is the photomechanical method. According to this method a layer, which consists of magnetizable or non-magnetizable material, and which has been applied on a non-magnetizable support, is provided with a layer of lacquer, which layer can be crosslinked by light, and this layer of lacquer is exposed under a suitable screen pattern, for instance a crossline-screen or autotype- screen, as frequently applied in the graphic arts, after which the unexposed parts of the layer of lacquer are removed. The uncovered parts of the underlayer are then removed by treatment with a suitable solvent or an etching liquid and subsequently a layer of non-magnetizable respectively magnetizable material is applied on these places. Finally also the exposed parts of the layer of lacquer are removed and the surface of the receiving support thus obtained is preferably made smooth by a suitable treatment, for instance by polishing.
Instead of a non-magnetizable support, which has been provided with a non-magnetizable layer, it is also possible to use a self-supporting non-magnetizable material, for instance a copper or aluminium plate, belt or cylinder, or a glass plate or cylinder. The light-sensitive layer of lacquer which is applied over the magnetizable or non-magnetizable layer, may for instance be a layer of a photopolymer, such as described in the U.S. patent specifications 2 732 301, 3 357 831 and 3 506 440, the British patent specifications 1 065 665 and 1 128 850, French patent specification 1 528 490 and the Dutch patent applications 6702407 and 6703214.
The application of the non-magnetizable respectively the magnetizable material on the places where the underlayer has been removed, may take place according to a commonly known method. When metal material is taken, it can for instance be applied via the galvanical way, via the catalytical-chemical way or by vapour-coating. A non-metallic material, for instance plastic or plastic in which magnetizable material has been finely dispersed, can be applied by applying a solution or dispersion of the plastic, in which the magnetizable material, if necessary, has been finely dispersed, and by drying the layer at raised temperature and by hardening it, if necessary.

[0010] In a way different from the photomechanical way, a suitable first receiving support for application in the process according to the invention can be obtained by pressing a relief in a surface of a plastic film, or a plastic layer, which has been applied on a suitable non-magnetizable support, if so desired after the surface of the plastic film or the plastic layer has been softened with a suitable swelling agent, and by subsequently filling up the deepened parts of the relief with a magnetizable material, for instance a fine dispersion of magnetizable pigment in a filmforming binding agent. It is also possible to manufacture the first receiving support by coating a non-magnetizable support with a solution of a filmforming binding agent, in which solution granulated magnetizable particles with a particle size between 5 and 250 micrometres and possibly non-magnetizable pigment particles have been dispersed, in such a way that a layer is formed which contains separate magnetizable particles which have been separated from each other by non-magnetizable material (binding agent and possibly non-magnetizable pigment).

[0011] Under application of a first receiving support, in which either the first or the second material is permanently magnetizable, the magnetic transfer of the magnetically attractable powder image takes place by magnetizing the magnetizable material on the first receiving support and by then bringing the receiving support, which has been magnetized locally in this way, into contact with the powder image. The magnetizing of the magnetizable material on
the first receiving support can take place in a simple way by conveying the receiving support through a homogeneous magnetic field of sufficient strength. In order that in the way described above an almost complete transfer of the powder image to the first receiving support is obtained, the magnetizable zones should have a remanence of at least 2 kA/m. In order to guarantee a good transfer efficiency, a magnetic auxiliary field can still be generated in the transfer zone, for instance by installing in the transfer zone a magnet behind the first receiving support or by installing two unlike magnet poles opposite to each other behind the first receiving support and the support which carries the powder image to be transferred.

[0012] First receiving supports of which the magnetizable material is not permanently magnetizable, can be used for the transfer of powder images which have been formed with the aid of a permanently magnetizable developing powder. The transfer of such powder images to the first receiving support is executed by magnetizing the powder images and by subsequently bringing them into contact with the first receiving support, or by bringing the first receiving support into contact with the powder image and by simultaneously generating a magnetic field in the contact zone which magnetic field is strong enough for magnetizing the developing powder. When the first receiving support is separated from the other support, the magnetic powder is kept adhering to the magnetizable material of the first receiving support by influence.

[0013] The image transferred to the first receiving support is subsequently transferred in a known way direct or indirect to the final receiving support, which will mostly consist of plain paper. The direct transfer of the powder image to the final receiving support can for instance be effected in the way as described in the Dutch patent application 7209652 already mentioned, in which the final receiving support is pressed against the powder image and the image transferred as a result of the pressure-execution is subsequently fixed in a suitable way, for instance by heating, on the final receiving support. The indirect transfer of the powder image to the final receiving support can for instance be executed in the way as described in British patent specification 1 245 426, in which the powder image is transferred under influence of pressure on a resilient medium and subsequently is transferred under influence of pressure and heat from the resilient medium to the final receiving support and is fixed at the same time.
The process according to the invention is especially attractive for application in so-called indirect electrophotographic copying systems in which for the development of the electrostatic image a magnetically attractable developing powder, electrically conductive or not-conductive, is used. The reason of this is, that in comparison with known processes the process according to the invention has the great advantage, that a good transfer of the powder image is realized under conditions which are very favourable for the duration of life of the photoconductive medium which is usually very vulnerable. Besides the fact, that in the process according to the invention no heat has to be supplied to the powder image to be transferred, so that thermal charge of the photoconductive medium is prevented, only a slight contact-pressure between photoconductive medium and first receiving support is necessary, so that also the mechanical charge of the photoconductive surface is limited to a minimum. In last-mentioned respect the process according to the invention differs from the process as described in British patent specification 1 245 426 in a favourable way.

[0014] When applying the process according to the invention in the so-called indirect electrophotographic copying systems, the transfer efficiency can even be increased by exposing the photoconductive medium, before or during the transfer, in order to eliminate the electrostatic force of attraction acting on the developing powder particles.
The powder images to be transferred according to the process of the invention can be formed with the known magnetically attractable, electrically conductive or not-conductive developing powders. Suitable developing powders are for instance described in the German patent application 1 937 651, the Dutch patent application 7203523 and the U.S. patent specification 3 093 039.

[0015] The invention will now further be explained in below examples. Example 1.

[0016] A photoconductive belt, manufactured as described in the example of British patent specification 1 408 252 was provided in a known way, by successively electrostatically charging and imagewise exposing, with a latent charge image, and this charge image was developed according to the known magnetic brush method with a magnetically attractable, one-component developing powder, which had a particle size between 10 and 30 micrometres, and a specific resistance of 8 x 10⁸ ohm.cm.
The developing powder was prepared according to the method as described in example 3 of the Dutch patent application 7508056.
The powder image thus formed on the photoconductive belt was transferred according to the process of the invention to a receiving paper, by conveying the photoconductive belt through a transfer device having the installation as schematically represented in the Figure.

[0017] In the transfer device the photoconductive belt 1, which carries the powder image 2 to be transferred, is conveyed over a supporting roller 3 and with slight contact-pressure is brought into contact with an image receiving roller 4, of which the sleeve 5 comprises permanently magnetizable zones and non-magnetizable zones. The supporting roller 3 and the sleeve 5 of the image receiving roller 4 are driven in the direction indicated by the arrows. Within the rotating sleeve 5 a stationary bar magnet 6 extending in axial direction is installed in such a way that its magnet field is only effective in the nip between the roller 3 and the sleeve 5. The magnet field generated in the nip by the magnet has a strength of about 24 kA/m. During the first revolution of the sleeve 5 the magnet 6 effects a permanent magnetization of the magnetizable zones on the sleeve and further serves as auxiliary magnet for the transfer of the powder image to the magnetized zones. For improving the transfer efficiency a lamp 7 is installed just before the nip between the roller 3 and the sleeve 5, which lamp exposes away the charge image still present on the photoconductive belt. The powder image transferred to the sleeve 5 is transferred in the nip between the sleeve 5 and the elastic pressure roller 8 under influence of pressure to a sheet of receiving paper 9 supplied from a stock pile. Finally the powder image is fixed on the receiving paper by heat.

[0018] The sleeve 5 comprising permanently magnetizable and non-magnetizable zones was manufactured as follows:

A copper sleeve was provided with a positively working, light-sensitive layer of lacquer (photoresist PK 13 of Kalle A.G., Wiesbaden, West-Germany) and the layer of lacquer was exposed under a 54 points screen, after which the exposed parts of the layer of lacquer were removed. The uncovered parts of the copper sleeve were subsequently etched with a usual etching liquid on basis of ferrichloride and hydrochloric acid to a depth of about 3 micrometres. The etched parts of the sleeve were subsequently filled up via the galvanical way with a permanently magnetizable Co-Ni alloy. Finally the unexposed parts of the layer of lacquer were removed and the surface of the sleeve was made smooth by polishing. The sleeve thus obtained carried at its surface fine, pointlike zones of copper, which were separated from each other by permanently magnetizable zones of Co-Ni.

[0019] The transfer method described above resulted into sharp copies of very good quality, with which a resolving power of more than 5 pairs of lines per mm was reached. The transfer efficiency with the transfer of the powder image to the first receiving support was equal to the efficiency which was achieved with the usual electrical transfer methods. Equally good results were obtained, when instead of a sleeve with permanently magnitizable Co-Ni zones a similar sleeve was used of which the magnetizable zones consisted of a fine dispersion of permanently magnetizable chromium dioxide particles in epoxy resin in the volume proportion 1:1.

Example 2.

[0020] The process of example 1 was repeated, but now a permanently magnetizable, one-component developing powder was used for the development of the electrostatic image, which powder consisted of thermoplastic particles, which contained 40% by weight of epoxy resin and 60% by weight of permanently magnetizable y-ferrioxide and which carried a layer of electrically conductive carbon at their surface. The specific resistance of the developing powder amounted to 3 x 10⁸ ohm.cm while the particle size lied between 10 and 30 micrometres.
As first receiving support a similar sleeve as in example 1 was used, which sleeve now however carried non-permanently magnetizable zones of nickel instead of permanently magnetizable zones of Co-Ni.
Also now copies of very good quality were obtained. The transfer efficiency in the first transfer step was again almost equal to that of the usual electric transfer methods.

Claims

1. Process for the transfer of a powder image formed with the aid of magnetically attractable developing powder, in which under influence of a magnetic field this powder image (2) is transferred to a first receiving support (4) and is subsequently transferred direct or indirect from the first receiving support (4) to the final receiving support (9), characterized in that the first receiving support (4) comprises zones of a first material, which are separated from each other by a second material, in which either the first or the second material is magnetizable and the other material is not-magnetizable.

2. Process according to claim 2, characterized in that the zones of the first material have an almost square or round shape.

3. Process according to one or more of the preceding claims, characterized in that the greatest diameter of the zones of the first material as well as the greatest inter-distance between these zones lies between the diameter of the smallest powder particle to be transferred and maximally 5 times the diameter of the largest powder particle to be transferred.

4. Process according to claim 3, characterized in that the greatest diameter of the zones of the first material as well as the greatest inter-distance between these zones amounts to one to two times the diameter of the largest powder particle to be transferred.

5. Process according to one or more of the preceding claims, characterized in that the zones of the first material together cover 30-70% of the surface of the first receiving support.

6. Process according to one or more of the preceding claims, characterized in that the first receiving support (4) has a smooth, metallic surface.

7. Process according to one or more of the preceding claims, characterized in that the magnetizable material consists of a material which has a remanent magnetism of at least 2 kA/m.

8. Electrophotographic process in which in a photoconductive material a latent electrostatic image is formed, this latent image is developed with a magnetically attractable developing powder, and the powder image (2) is transferred under influence of a magnetic field to a receiving support (4), characterized in that the transfer of the powder image (2) takes place according to the process of one or more of the preceding claims.

Drawing