FR2522839A1 - Magnetographic impression of information onto paper - uses magnetisation and selective demagnetisation of magnet material on moving band - Google Patents

Abstract

The magnetographic impression device has an intermediary registration support consisting of a continuous moving band (20) having a layer of magnetic material whose Curie point lies between zero and 300 deg. Celsius. The band, whose length is at least equal to that of the paper to be printed is held in place by two cylinders (22,23). In the first stage of operation the band makes contact with an electromagnetic head (24) which magnetises it over its entire width. In the second stage the band makes contact with a thermal head (25) which selectively demagnetises zones of the surface of the band, in spatial relation to the information to be printed, by raising their temperature above that of the Curie point of the material. In the third stage the band makes contact with a cylindrical magnetic brush (26) partially immersed in a reservoir containing a ferrite-based ink (28). The ink fixes onto the magnetised parts of the band corresponding to the information to be reproduced. During the fourth stage the band and some paper (29) from a roll (30) are put in contact and pressed between two cylinders (31,32) to transfer the ink from band to paper.

Description

 MAGNETOGRAPHIC PRINTING METHOD AND DEVICE.

 The present invention relates to the reproduction on a material support of images or texts available in the form of generally binary information. The method applies more particularly to devices for printing on paper graphic information transmitted remotely in real time or stored on information carriers such as tapes or discs.

 The invention applies even more precisely to printing devices using dot matrix capable of reproducing not only alphanumeric characters at the rate of several thousand lines per minute but also drawings possibly including shades of "gray" obtained by making vary the density of distribution of the dots printed by the printing matrix.

 There are two types of best known methods currently enabling these possibilities and printing performances to be obtained: printing by heating a heat-sensitive paper and printing by magnetic transfer of the ink.

 In the process of printing on thermal paper, a set consisting of elements in relief, each of the elements which can be heated or not according to all the possible combinations, is applied to a paper on which is fixed a large number of globules containing l ink whose envelope bursts under the effect of temperature. All elements are identical and generally only print dots placed on the same line. Several successive lines are printed to constitute, for example, a single line of alphanumeric characters; the elementary points are arranged according to the geometry of the symbols to be reproduced and sufficiently close to give the lines an impression of continuity. The major drawback of this process resides in the impossibility of constituting archives with this type of paper which degrades and makes information quickly unusable over time; in addition to the weather, other factors linked to the environment such as light or humidity are also detrimental to the conservation of this information medium. Finally, this process involves the use of special paper.

 The magnetic ink transfer printing technique also reproduces the information, by printing a mosaic of elementary dots arranged to represent symbols or drawings. The heating elements, arranged in line, of the preceding process are replaced here by as many small electromagnets controlled in a similar manner. These electromagnets magnetize in determined zones, according to the drawing to reproduce an endless band or a continuously advancing drum. The support thus magnetized, during its advancement, is first brought into contact with an ink containing ferrite particles. The ink is deposited on the magnetized parts of the tape or the drum and only on these parts.

In the next step, the ink-carrying strip or drum is brought into contact with plain paper or the ink can be transferred, for example, by simple pressure. The main drawback of this process is at the level of the recording head which must, in order to comply with CCIR standards, have less than one thousand seven hundred and twenty eight electromagnets aligned over a distance reduced to about fifteen centimeters, which is incomparably more difficult to achieve than placing under the same conditions one thousand seven hundred twenty eight contact points with thermal action as in the previous process.

 To overcome these drawbacks of the known art, the invention proposes a magnetographic printing process combining the simplicity of thermal printing and the use of plain paper. According to this method a mobile recording medium is in a first stage magnetized throughout its extent. Then thermal means are used to heat said support beyond the Curie point and demagnetize it at the places not to be inked corresponding to the blanks of the text or of the drawing. The support is then placed in the vicinity of a ferrite-based ink which is fixed on the areas of the support still magnetized and corresponding to the content of the information to be reproduced. The support is finally brought into contact with plain paper or the ink is transferred by simple pressure. The mobile support can be an endless band or a drum which makes it possible to carry out successively and repeatedly the operations which have just been indicated.

 The subject of the invention is therefore a method for magnetographic printing of information on a material support in a device using an intermediate recording medium made of magnetizable material comprising the steps of converting said information into electrical signals, of magnetic recording of this information by magnetizing specific areas of the surface of the intermediate recording medium in one-to-one spatial relationship with the information to be recorded, of inking the magnetized areas by attraction of a magnetizable ink, transfer of the ink onto the material support to be printed, characterized in that the recording step is carried out by magnetizing the entire surface of the intermediate recording medium followed by the selective demagnetization of this surface by raising the temperature above the point of Curie of the material constituting the support so as to leave only the magnetization of said determined zones.

 The invention also relates to a magnetographic printing device implementing this method.

The invention will be better understood and other characteristics and advantages will appear from the following description with reference to the appended figures, among which:
- Figure 1 shows some elements of a magnetic recording head according to a known method;
- Figures 2 and 3 show a thermal printing head of known type;
- Figure 4 illustrates a concrete embodiment of a device implementing the method according to the invention;
- Figures 5, 6 and 7 illustrate the magnetization head used in the device illustrated in Figure 4 ;;
- Figure 8 illustrates the thermal recording head and its control circuits used in the device described with reference to Figure 4
- Figures 9 and 10 show the cylinder used for anchoring the support in the device shown in Figure 4
- Figure 1 1 illustrates a head for perpendicular magnetization of the recording medium;
- Figures 12 and 13 illustrate the distribution of the flow in the case of a perpendicular magnetization of the recording medium;
- Figures 14 and 15 illustrate the distribution of the flow in the case of a longitudinal magnetization of the recording medium.

 Figure I shows some elements of a magnetic recording head according to a known method. This head comprises one thousand seven hundred and twenty eight magnetizing elements such as 1 arranged on line 2 of length equal to the width of an A4 page. The elements 1 are placed alternately on each side of the line 2. Each fixed element 1 can provide a magnetic field on command by means of a winding 5 and magnetize the mobile recording medium 3 which moves in the direction of the arrow 4. Each element 1 comprises a recording pole 6 and a pole 7 which ensures the closing of the magnetic circuit. The ratio of the sections between the poles 6 and 7 is such that the intensity of the magnetic field in the section of the pole 7 is lower than the recording threshold of the support while the field of the pole 6 is higher. Consequently only the pole 6, of small dimension, temporarily magnetizes the support 3 which can be a strip or a magnetic drum.

 A simple examination of FIG. 1 reveals the complexity of a process which consists in aligning with precision and over a length of less than fifteen centimeters nearly two thousand electromagnets. To the technological difficulties are added problems of possible coupling between the adjacent elements (crosstalk) and problems of electrical control linked to the time constant of these elements or to overvoltages -due to their inductive nature.

 Figures 2 and 3 show in perspective and side view of the heating elements a thermal printing head of known type limited to 13 points of contact for clarity of the figure.

 A bar 10 of approximately trapezoidal section has on its short side a succession of small relief heating elements 11 generally consisting of a conductive ink deposited by screen printing.

Each element 11 is supplied by a pair of printed copper conductors 12 accessible from the opposite face. The material constituting the bar 10 is an electrical insulator with good thermal conductivity in order to ensure the rapid cooling of the heating elements and to direct the heat flow towards the main body of the head without heating the adjacent elements (crosstalk); this material can also serve as a substrate for control electronics. The resistive element 11 through which a current flows is heated by the Joule effect.

 This description shows the simplicity of the thermal print head compared to the magnetic print head described above.

 However, this process requires the use of a special paper on the one hand and on the other hand this paper cannot be archived due to its rapid degradation over time. The object of the invention is therefore to propose a method according to which the thermal head can be used to print conventional paper.

 FIG. 4 illustrates a concrete embodiment of a device implementing the method according to the invention. The intermediate recording medium consists of an endless strip 20 moving continuously in the direction of the arrow 21. This strip 20 comprises a layer of magnetic material whose Curie point can be fixed between zero and three hundred degrees Celsius. By way of examples, chromium dioxide or alloys such as manganese-bismuth, chromium-tellurium or nickel-iron are suitable for this use.

 The strip 20 whose width is at least equal to that of the paper to be printed is held in place by the cylinders 22 and 23. In a first step the strip 20 meets a simplified electromagnetic head 24 which magnetizes it to its full extent. In the next step, the strip 20 comes into contact with a thermal head 23 which demagnetizes it in places devoid of information to be printed by bringing it to a temperature higher than that of the Curie point of the material. In the third step the strip 20 comes into contact with a rotating cylindrical magnetic brush 26 partially immersed in a reservoir 27 containing an ink 28 based on ferrite. In contact with this brush, the ink is fixed on the still magnetized parts of the strip which correspond to the information to be reproduced. The description and operation of the magnetization head 24, the thermal head 23 and the magnetic brush 26 are presented later.

 During the fourth step, the strip 20 and the paper 29 extracted from a roll 30 are brought into contact and pressed between two cylinders 31 and 32 this operation ensures the transfer of the ink between the strip 20 and the paper 29. After printing the paper undergoes a drying operation by thermal radiation by means of a parabolic reflector 33 comprising a heat source 34. The printed paper is finally conveyed towards the outlet of the device by means of the cylinders 35 and 36. After its passage between the cylinders 31 and 32 the strip 20 is rid of ink residues which could remain by means of a cylindrical brush 37 partially immersed in a cleaning product placed in a reservoir 38. The cylinder 31 can be used to ensure the entrainment of the whole device.

 Table I presented in the appendix at the end of this description summarizes the typical characteristics and performances obtained with such a device.

 Figures 3, 6 and 7 illustrate the magnetization head used in the device illustrated in Figure 4.

 Figure 3 is an enlarged view of the magnetization head 24. The head 24 is made of magnetic material, for example ferrite; it has a groove 30 in which is disposed a control winding not shown in this figure.

 Two magnetization poles 51 and 32 of small cross section are in contact with the mobile support 20 which advances continuously in the direction of arrow 21.

Figure 6 is a cross section of the head 24 and Figure 7 a longitudinal section in the plane of symmetry showing how the control winding 33 is arranged. The magnetic field generated by rewinding 33 is closed by the poles 31 and 52 through the magnetic material of the movable support 20; this magnetization leaves in the support 20 a remanent magnetic field distributed as indicated in FIG. 3. Lines of alternating north and south magnetic polarities create over the entire extent of the support a flux gradient sufficient to fix the ink molecules to ferrite base. Such a type of magnetization is known by the term longitudinal magnetization.

 The distance d between two successive lines of magnetization is not only related to the distance which separates the poles 51 and 52 from the head 24. In fact, as shown in FIG. 7, the winding 53 is powered by a generator sinusoidal current 54 whose frequency is around a thousand hertz. It follows that the distance d is a function of this frequency, the speed of travel of the support 20 and the distance between the poles 51 and 52.

 To obtain good print quality, the distance d must be approximately half that between two successive printed dots. In other words, the support 20 is magnetized with a resolution frequency about twice that of the printed information.

 FIG. 8 illustrates the thermal recording head and its control circuits used in the device described with reference to FIG. 4.

 A head 25 comparable to the head described in FIGS. 2 and 3 of length at least equal to the width of a sheet of paper of A4 format comprises one thousand seven hundred and twenty eight heating elements such as 11 in accordance with CCIR standards. These one thousand seven hundred twenty eight elements 11 are electrically divided into forty eight sections each comprising thirty six elements according to the technique known as "de-multiplexing". The time allocated by the standards to print a line of points is such that it is not necessary to print all the points of this line simultaneously. If this were the case, the equivalent of one thousand seven hundred and twenty eight switches should be assigned to the one thousand seven hundred and twenty eight heating elements.

In the present case, the forty-eight sections are printed successively, starting, for example, from left to right; each section of rank n is made active for a time determined by the conduction of the corresponding transistor S.

 Each of the thirty six heating elements of each of the forty eight sections can be, depending on the information to be printed, connected to the source by means of thirty six transistors referenced el, e2 e36. The electrical states (conductive or not) of these thirty six transistors are updated with each change of section according to the information to be printed. A diode in series with each heating element prevents the sections at rest from being supplied by the section being printed; moreover, if the control members are no longer, as in FIG. 8, the equivalent of switches but of two-state logic circuits, all the sections are supplied simultaneously in the absence of such diodes. sequential printing of the same line eighty four control members (48 + 36) are sufficient instead of one thousand seven hundred twenty eight and their integration can be done directly on the material constituting the print head. The electrical signals suitable for printing by dots can come from a transmission network to which are connected fax machines operating according to this printing technique. Compatible electrical signals recorded on conventional information media (magnetic tapes, discs, etc.) can also serve as sources. Generally and in all cases, an interface member (transcoder, character generator, etc.) can be inserted in the transmission chain to ensure compatibility.

 Figures 9 and 10 show the cylinder used for anchoring the support in the device shown in Figure 4. A cylinder 26 of length at least equal to the width of the writing support comprises according to generatrices magnetized tracks 60 alternately of north and south polarities. The cylinder is, in its lower part, in contact with ink 28 placed in a container 27. The ink molecules containing ferrite particles are arranged, largely in the air, along the lines of force from poles 60 creating in space very complex structures whose figure 10 presents a simplified aspect. These very fragile structures are easily and partially carried away by the magnetized inscription support and permanently renewed by the rotation of the cylinder 26.

 The device which has just been described is only one of the possible combinations of means implementing the method according to the invention.

 The initial magnetization of the recording medium 20 can be carried out by lines of force parallel to the plane of the medium in all possible directions, two particular directions being the longitudinal magnetization used in the example described and the so-called perpendicular "transverse" magnetization in the direction of scrolling of the support. It is also possible to magnetize the support by a flux oriented perpendicular to this support; FIG. 11 illustrates a magnetization head 24 ′ designed to apply a flux perpendicular to the support 20. FIG. 12 gives an indication of the distribution of the flux; at the point of contact 72 of the head 24 'with the ferromagnetic layer 20' of the support 20, the surface is small and the induction high while the flux closes through the support in areas 73 of large area therefore of low induction . After removal of the inducing field, a remanent field exists at the magnetized point 72 where an elementary magnetic dipole is located located in the thickness of the ferromagnetic material and perpendicular to the plane of the latter. FIG. 13 represents the succession of dipoles 71 obtained by the combined action of an alternating inductive field and the continuous advancement of the support 20. In contrast, FIGS. 14 and 13 illustrate the orientation of the magnetic dipoles 71 ′ obtained in the case of the device described according to FIG. 5. The elements with the same references as in the preceding descriptions fulfill the same functions and are not described a second time.

 The signals exciting the head 24 may be different in shape and frequency from those used. Magnetization can be carried out by a head with more than two poles. An appropriately shaped head can be driven in a rotational or translational movement and possibly only have magnets. The carrier cylinder 22 can be designed to directly magnetize the support 20.

 Punctual demagnetization of the recording medium by direct contact with heating elements is, in the prior art, a preferred option; other means aiming at the same goal such as for example a scanning by beam of radiant energy modulated in time or a fixed beam covering a line and modulated in space as well as any technique within the reach of the skilled person remain in the part of the invention.

 The overall magnetization step and the point demagnetization step can be carried out at two successive points, located on the surface of a single cylinder generally called a drum.

 In the device described using FIG. 4, the support is anchored using a magnetic cylinder; another solution is to pass the recording medium through a liquid containing suspended ink.

 Just as the transfer of ink between the recording medium and the paper is carried out by simple pressure between two cylinders, it can also be carried out by electrostatic transfer; in this case the paper is loaded with respect to the support using a high voltage e "ink is subjected on the part of the paper to an electrostatic attraction force greater than that of the magnetic field which retains it which causes its transfer.

 It is possible to transfer the information to a support other than paper, supports such as those used in overhead projectors and known as "transparencies" represent another example.

 The method according to the invention makes it possible to produce devices combining the simplicity of thermal printers and the printing of information on a good quality material support which is stable over time and does not require any particular prior treatment.

TABLE I
Typical characteristics and performances obtained with an example of a device implementing the method.

Resolutions in points per cm:
Horizontally: 80
Vertically: 40
Printing time for
A4 page size: 500 ms

Claims (11)

 1. A method of magnetographic printing of information on a material support in a device using an intermediate recording medium made of magnetizable material comprising the steps of converting said information into electrical signals, of magnetic recording of this information by magnetization specific areas of the surface of the intermediate recording medium in one-to-one spatial relationship with the information to be recorded, inking of the magnetized areas by attraction of a magnetizable ink, transfer of the ink onto the material support to be printed, characterized in that the recording step is carried out by magnetizing the entire surface of the intermediate recording medium followed by the selective demagnetization of this surface by raising the temperature above the Curie point of the material constituting the medium so as to leave only the magnetization of said determined zones.  2. Method according to claim 1 characterized in that, the intermediate support being plane and moving in a direction parallel to this plane, the magnetization of this support is effected by applying a magnetic field whose lines of force are parallel to the plane of the support and to the direction of movement, said magnetic field having a determined and constant polarity and intensity along any direction orthogonal to the direction of movement and variable along the direction of movement so as to record in this direction a succession of close magnetic states of alternating polarities  3. Method according to claim 1 characterized in that, the intermediate support being plane and moving in a direction parallel to this plane, the magnetization of this support is effected by applying a magnetic field whose lines of force are perpendicular to the plane of the support, said magnetic field having a determined and constant polarity and intensity in any direction orthogonal to the direction of movement and variables according to the direction of movement so as to record in this direction a succession of close magnetic states of alternating polarities.  4. Method according to claim 1 characterized in that the selective demagnetization of the recording medium is effected by heating a succession of points arranged on a line of direction orthogonal to that of the displacement, the displacement resulting in selective demagnetization line by line extended to the entire surface of the support.  3. A device for magnetographic printing of information on a material support implementing the method according to any one of claims 1 to 4 comprising means for converting this information into electrical signals, recording means (24,25 ) of this information on an intermediate magnetizable support (20), inking means (26) of the intermediate support, means of transfer (31, 32) of information from the intermediate support to the material support (29) charac- terized in that the recording means comprise a magnetic head (24) for magnetizing the entire surface of the intermediate recording medium (20) and a thermal head (2S) for selectively demagnetizing, in one-to-one spatial relationship with the information to be recorded, the surface of the intermediate support (20) by exceeding the Curie point of the material constituting the support.  6. Device according to claim 5 characterized in that, the intermediate support being plane in displacement in a direction parallel to this plane, the magnetic head used to magnetize the intermediate support (20) is an electromagnet (24) elongated and unique covering the support in a direction orthogonal to its movement and comprising two magnetic poles in contact with said support (20).  7. Device according to claim 6 characterized in that it further comprises a sinusoidal current generator with audio frequency (S4) permanently supplying the winding (33) of the electromagnet (24).  8. Device according to claim 3 characterized in that the intermediate support (20) comprises a layer of magnetic material in chromium dioxide or consisting of a chromium-tellurium, nickel-iron or manganese-bismuth alloy.  9. Device according to any one of claims 5; 8 characterized in that the intermediate support is an endless belt (20) with continuous advancement carried by two cylinders (22,23).  10. Device according to claim 5 characterized in that, the intermediate support being plane and moving in a direction parallel to the plane, the thermal head (25) serving to demagnetize the intermediate support (20) comprises an alignment of heating elements ( 11) offset from each other by a constant pitch and extending over the entire support (20) in a direction orthogonal to said movement, said heating elements (11) being in contact with said support (20).  11. Device according to claim 10 characterized in that it comprises a selective control circuit for each of the heating elements in one-to-one relation with the content of the information to be printed.  12. Device according to claim 11 characterized in that the control circuit comprises a multiplexing circuit acting sequentially on independent sets of heating elements (11).  13. Device according to claims 7 and 10 characterized in that said sinusoSdal generator delivers a signal of determined frequency so that the distance which separates two successive magnetic states of opposite polarities on the intermediate support (20) is half the pitch which separates two heating elements (11) of the thermal head (25).  14. Device according to claim 5 characterized in that the material support (29) is a conventional sheet of paper or a transparent film used for projection;  15. Device according to claim 5 characterized in that the means for converting information into electrical signals comprise fax machines connected to a transmission network.