EP0082739B1 - Method and device for magnetographic copying - Google Patents

Description

The present invention relates to a magnetographic printing method which makes it possible to obtain color images on a printing medium. It also relates to a machine for implementing this method.

Magnetic printing machines are known which, in response to signals received from a control unit, make it possible to form images, such as character images for example, on a printing medium generally consisting of a strip or a sheet of paper. In these printing machines, of a type analogous to that which has been described in the French patent application published under No. 2,305,764, the printing of the images is carried out by first forming from the signals received, a magnetic image latent on the surface of a magnetic recording element generally having the shape of a rotating drum or an endless belt, this image being made up of a set of magnetized zones of very small dimensions. This latent image is then developed by depositing on this surface a powdery developer containing magnetic particles which remain only applied to the magnetized areas of the recording element to form a powder image on the surface of this element. After which, this powder image is transferred to the print medium.

For certain particular applications, it may be desirable for the image which is thus formed on the printing medium to appear in several different colors. In a known method which has been described in particular in French patent No. 1,053,634, the printing of a color image on the printing medium is carried out by first forming on the recording element an image latent magnetic corresponding to the parts of the same color of the image to be printed, by developing this latent image by means of a developer having this color, by transferring the powder image thus obtained onto the printing medium, and by repeating this operation as many times as there are colors in the image to be printed. However, such a method has the drawback of requiring a particularly long time for its implementation. In addition, despite all the care taken in framing the different powder images when they are transferred to the print medium, it is practically impossible to prevent even slight shifts between the different image parts thus which, of course, detracts from the clarity of the image ultimately formed on the print medium.

To overcome these drawbacks, a magnetographic printing process has been proposed which has been described in United States Patent No. 3,965,478 and which consists in forming on the surface of the recording element a plurality of magnetized elementary areas, all of these elementary areas constituting a latent magnetic image, each of these elementary areas being obtained by exciting a magnetic recording head by means of an electric current whose frequency is chosen as a function of the color which this elementary area must give when it is developed, the dimensions and the magnetic attraction force of this elementary area being moreover determined by the value of the frequency used. In this method, the development of the latent image formed on the recording element is carried out by means of a single developer containing particles of different colors and of different sizes, the particles of the same size being however all of a same colour. During the development of the latent image, the particles which have a determined size (and consequently, a given color) are preferentially attracted by the elementary areas whose dimensions correspond to a given force of attraction, so that, after development , each elementary area is covered with particles whose color corresponds to the frequency that was used to form this elementary area.

For the implementation of such a process, it is however necessary to use a developer, the particles of which, of different colors and of different sizes, must be carefully calibrated, the particles of the same color having in fact all to be strictly the same. size. In addition, these particles must be conditioned so as not to agglomerate between them, on pain of causing color errors during the development of the latent magnetic image. Under these conditions, the manufacture of such a developer is particularly long, delicate and relatively expensive. On the other hand, given that, depending on the color assigned to them, the elementary areas formed on the recording element do not all have the same dimensions, the images or portions of images whose hue corresponds to the areas elementary large dimensions have a definition, that is to say a degree of finesse, less good than those whose color corresponds to elementary areas of small dimensions. Finally, while the elementary areas of small dimensions are capable, during development, of attracting only the smallest particles of the developer, it is impossible to prevent that the elementary areas of large dimensions attract not only the large particles. of the developer, but also the smaller particles, which naturally causes color change.

To print an image in two or three colors on a printing medium consisting, for example, of a sheet of paper, a magnetographic printing method has also been proposed which has been described in the patent of the United States of America N ° 3.824.601 and which consists in first magnetizing the surface of a magnetic recording element, in a direction parallel to this surface, so as to form on this surface a plurality of magnetized portions constituting a latent magnetic image, these magnetized portions having different magnetization intensities, of values J1, J2 and J3 , such as J1>J2> J3. Each of these three values is associated respectively with one of three previously chosen colors, these colors being, for example, the colors yellow, magenta and cyan. When the magnetization of the surface of the magnetic recording element is finished, a first powdery developer, of yellow tint, is deposited on this surface, the particles of which are capable of being attracted magnetically by a magnetic field whose intensity is between the values J1 and J2 above. Under these conditions, these particles of first developer are only attracted by the magnetized portions of the surface whose magnetization intensity is equal to J1. Then deposited on the surface of the recording element a second powdery developer, of magenta tint, the particles of which are capable of being attracted magnetically by a magnetic field whose intensity is between the values J2 and J3 mentioned above. The magnetized portions whose magnetization intensities are equal to J1 or J2 are therefore likely to attract this second developer. However, since the magnetized portions whose magnetization intensity is equal to J1 are already covered with particles of first developer, which concentrate the magnetic fluxes of leakage from these portions and then cause a weakening of the magnetic force of these portions , only the magnetized portions whose magnetization intensity is equal to J2 are capable of attracting this second developer. Finally, a third powdery developer, cyan in color, is deposited on the surface of the recording element, the particles of which are capable of being attracted magnetically by a magnetic field whose intensity is less than J3. All the magnetized portions of this surface are therefore capable of attracting this third developer. However, given that the magnetized portions whose magnetization intensity is equal to J1 are already covered with particles of first developer and those whose magnetization intensity is equal to J2 are already coated with particles of second developer, only the magnetized portions whose magnetization intensity is equal to J3 are capable of attracting this third developer. As a result, at the end of the deposition operations, each of the magnetized portions of the recording element is covered with a single layer formed of particles of one of the three developers. The particles which constitute these three developers however have the particularity of not having all the same magnetic properties. This is because the number of magnetic grains contained in each of the particles of the first developer is less than the number of magnetic grains contained in each particle of the second developer, the latter number being itself less than the number of magnetic grains contained in each particle of third developer, so that each of these three developers is attracted only by the magnetized portions for which it is intended. To obtain such a result, it is however necessary that, not only the number of magnetic grains contained in each developer particle remains between very narrow limits (for example from 1 to 3 magnetic grains for each particle of first developer, from 4 to 5 magnetic grains for each particle of second developer and from 7 to 9 magnetic grains for each particle of third developer), but also that the mass, that is to say the size, of these particles is adjusted with great precision . Under these conditions, the manufacture of these three types of developer proves to be particularly delicate and costly. In addition, the physical characteristics of these developers can change significantly over time, so that the conditions of selective attraction of these developers by the different magnetized portions of the recording element are therefore no longer found. met, which then leads to serious alterations of the revealed image.

The present invention remedies these drawbacks and proposes a magnetographic printing method, as well as a machine using this method, which makes it possible to obtain on a printing medium, in relatively short time, high-quality color images. , while requiring only developers having the same particle size and the same magnetic characteristics.

• - à déposer sur cette surface un premier révélateur pulvérulent dont la teinte est celle d'une première desdites couleurs, ce premier révélateur subsistant sur tous les points magnétisés de cette surface,
• - à éliminer ensuite ce premier révélateur des points magnétisés dont les intensités d'aimantation sont inférieures à J1,
• - à répéter cette opération de dépôt sur cette surface avec, successivement, chacun de (p-1) autres révélateurs pulvérulents de teintes différentes, ces p révélateurs ayant cependant les mêmes caractéristiques physiques, chacune de ces opérations de dépôt étant immédiatement suivie, à l'exception de la dernière opération de dépôt, d'une opération d'élimination de révélateur, les points magnétisés concernés par cette élimination étant, au cours de chacune respective de ces (p-2) opérations d'élimination, ceux dont l'intensité d'aimantation est inférieure à, respectivement, J2, J3,..., Jp-1, de sorte que, après la dernière opération de dépôt, chaque point magnétisé ayant une intensité d'aimantation égale à J1 est recouvert de p couches superposées desdits révélateurs, chaque point magnétisé ayant une intensité d'aimantation égale à J2 est recouvert de (p-1 ) couches superposées desdits (p-1) autres révélateurs, et ainsi de suite, chaque point magnétisé dont l'intensité d'aimantation est égal à Jp étant alors uniquement recouvert d'une seule couche du p-ième révélateur,
• - et à effectuer un transfert total de toutes ces couches de révélateurs sur le support d'impression afin de former sur celui-ci une image de poudre constituée d'une pluralité de taches ponctuelles, chacune de ces taches résultant du transfert des couches de révélateurs qui ont été déposées sur chacun respectif desdits points magnétisés et ayant la couleur du révélateur qui, juste avant l'opération de transfert, se trouvait directement en contact, en ce point, avec la surface de l'élément d'enregistrement.

More specifically, the invention relates to a magnetographic printing method making it possible to obtain, on a printing medium, an image in p colors previously chosen, p being an integer at least equal to two, this method consisting in magnetizing the surface of a magnetic recording element, in a direction perpendicular to this surface, so as to form a set of magnetized points constituting a magnetic latent image, these magnetized points all having the same size and having the same magnetic polarity, but different magnetization intensities, of values J1, J2, J3, ..., Jp such as J1>J2>J3>...> Jp, each of these values being associated with one of the p colors of said image , the magnetized points intended for the formation of the images or parts of image which, on the support, must appear in the same color all having the same intensity of magnetization, the said method being characterized in that it further consists of:

• - depositing on this surface a first powder developer, the color of which is that of a first of said colors, this first tor remaining on all the magnetized points of this surface,
• - then to eliminate this first developer of the magnetized points whose magnetization intensities are less than J1,
• - repeating this deposition operation on this surface with, successively, each of (p-1) other powdery developers of different shades, these p developers having however the same physical characteristics, each of these deposition operations being immediately followed, at except for the last deposition operation, a developer elimination operation, the magnetized points concerned by this elimination being, during each respective of these (p-2) elimination operations, those whose intensity of magnetization is less than, respectively, J2, J3, ..., Jp-1, so that, after the last deposition operation, each magnetized point having a magnetization intensity equal to J1 is covered with p superimposed layers of said developers, each magnetized point having a magnetization intensity equal to J2 is covered with (p-1) superimposed layers of said (p-1) other developers, and so on, each magnetized point whose magnetization intensity is equal to Jp then being only covered with a single layer of the p-th developer,
• - And to carry out a total transfer of all these layers of developers on the printing medium in order to form on the latter a powder image consisting of a plurality of punctual spots, each of these spots resulting from the transfer of the layers of developers which have been deposited on each respective one of said magnetized points and having the color of the developer which, just before the transfer operation, was in direct contact, at this point, with the surface of the recording element.

• - des moyens de calibrage de courant interposés entre la source d'impulsions et chacune des têtes pour permettre à chacune des impulsions de courant envoyées par cette source d'être ajustée sélectivement à l'une de p valeurs d'amplitude prédéterminées et former ainsi sur la surface d'enregistrement une image magnétique latente dont les points magnétisés présentent la même polarité magnétique, mais ont des intensités d'aimantations différentes de valeurs J1, J2,..., Jp, telles que J1>J2>...>Jp, chacune de ces valeurs étant associée à l'une respective des p susdites couleurs, les points magnétisés destinés à la formation des parties d'image qui, sur le support d'impression, doivent apparaître en une même couleur ayant tous la même intensité d'aimantation,
• - (p-1) autres dispositifs applicateurs répartis le long du trajet suivi par la surface d'enregistrement au cours de son déplacement, chacun de ces dispositifs applicateurs permettant de déposer sur chaque point magnétisé de cette surface une couche de chacun respectif de (p-1) autres révélateurs pulvérulents, chacun de ces (p-1) révélateurs ayant pour teinte l'une respective desdites couleurs autres que la première couleur, tous ces p révélateurs ayant cependant les mêmes caractéristiques physiques,
• - (p-1) dispositifs de retouche placés chacun en aval, par rapport au sens de déplacement de la surface, de chacun respectif desdits dispositifs applicateurs, à l'exception, du dernier, le premier de ces dispositifs de retouche étant établi pour retirer le premier révélateur des points magnétisés dont l'intensité d'aimantation est inférieure à J1, le deuxième dispositif de retouche étant établi pour retirer le deuxième révélateur des points magnétisés dont l'intensité d'aimantation est inférieure à J2, et ainsi de suite,
• - et un dispositif de transfert disposé en aval du dernier dispositif applicateur pour transférer en totalité sur le support d'impression les différentes couches de révélateurs qui recouvrent les points magnétisés de la surface lorsque ces points sont passés devant ledit dernier dispositif applicateur.

The invention also relates to a magnetographic printing machine for implementing the method indicated above, this machine comprising a recording element provided with a magnetic recording surface, a plurality of magnetic heads controlled by pulses. electric and established to, in response to these pulses, magnetize said recording surface, in a direction perpendicular to this surface, to form thereon a set of magnetized dots constituting a latent magnetic image, drive means for causing a relative displacement between the recording element and the magnetic heads, a pulse source established to selectively send electrical pulses to said heads, and an applicator device making it possible to deposit a powdery developer on said recording surface, this developer does not remaining applied only on the magnetized points of this surface to form a powder image, said machine being characterized in that, said developer comprising particles whose color is of a first of p colors previously chosen, it further comprises:

• - current calibration means interposed between the pulse source and each of the heads to allow each of the current pulses sent by this source to be selectively adjusted to one of p predetermined amplitude values and thus form on the recording surface a latent magnetic image whose magnetized points have the same magnetic polarity, but have different magnetization intensities of values J1, J2, ..., Jp, such as J1>J2>...> Jp , each of these values being associated with a respective one of the p above colors, the magnetized points intended for the formation of the image parts which, on the printing medium, must appear in the same color all having the same intensity of magnetization,
• - (p-1) other applicator devices distributed along the path followed by the recording surface during its movement, each of these applicator devices making it possible to deposit on each magnetized point of this surface a layer of each respective of (p -1) other powdery developers, each of these (p-1) developers having a respective one of said colors other than the first color, all these p developers having however the same physical characteristics,
• - (p-1) touch-up devices each placed downstream, relative to the direction of movement of the surface, of each respective one of said applicator devices, with the exception of the last, the first of these touch-up devices being set up to remove the first developer of the magnetized points whose magnetization intensity is less than J1, the second retouching device being set up to remove the second developer of the magnetized points whose magnetization intensity is less than J2, and so on,
• - And a transfer device arranged downstream of the last applicator device for transferring entirely onto the printing medium the various layers of developers which cover the magnetized dots on the surface when these dots have passed in front of said last applicator device.

• Les figures 1A à 1G: illustrent les différentes phases du procédé d'impression magnétographique selon l'invention,
• La figure 2: représente un mode de réalisation d'une machine imprimante qui met en oeuvre le procédé d'impression de l'invention;
• La figure 3: est une vue montrant le principe de magnétisation transversal de l'élément d'enregistrement faisant partie de la machine représentée sur la figure 2,
• La figure 4: représente un schéma des circuits électriques utilisés pour commander les différentes têtes magnétiques d'enregistrement de la machine représentée sur la figure 2,
• La figure 5: est une vue montrant la disposition des points magnétisés qui ont été formés sur l'élément d'enregistrement pour constituer l'image magnétique latente d'un caractère, et
• La figure 6: montre des courbes illustrant la façon dont varie la force d'attraction magnétique exercée par chaque point magnétisé formé sur l'élément d'enregistrement faisant partie de la machine représentée sur la figure 2.

The invention will be better understood and other objects, details and advantages thereof will appear better in the following description, given without limitation, and with reference to the appended drawings in which:

• FIGS. 1A to 1G: illustrate the different phases of the magnetographic printing process according to the invention,
• Figure 2: shows an embodiment a printing machine which implements the printing method of the invention;
• FIG. 3: is a view showing the principle of transverse magnetization of the recording element forming part of the machine shown in FIG. 2,
• FIG. 4: represents a diagram of the electrical circuits used to control the different magnetic recording heads of the machine shown in FIG. 2,
• FIG. 5: is a view showing the arrangement of the magnetized dots which have been formed on the recording element to constitute the latent magnetic image of a character, and
• FIG. 6: shows curves illustrating how the magnetic attraction force exerted by each magnetized point formed on the recording element forming part of the machine shown in FIG. 2 varies.

FIG. 1A shows, in enlarged section, a recording element 10, of known type, which can be used for implementing the method of the invention. In the example described, it will be considered that this magnetic recording element is of a type analogous to that which has been described and represented in French patent No. 2,402,921 and that it comprises a support 11 formed from a material having high magnetic permeability, such as iron or mild steel, this support being coated with a layer of magnetic material with high coercivity such as, for example, the nickel-cobalt magnetic alloy. In the embodiment of the printing method according to the invention, this recording element 10 is magnetized transversely by means of a cu of several recording heads 13 of the type which has been shown in the figure 3. Referring to FIG. 3, it can be seen that this recording head 13 comprises a magnetic core 14 on which is wound a winding E connected to an electrical excitation circuit which will be described a little later. This magnetic core 14 has substantially the shape of a U and it is profiled so as to present at its ends a recording pole 15 and a flux closing pole 16. These two buds are arranged, as shown in FIG. 3 , near the surface of the magnetic layer 12, so that the magnetic core 14, the support 11 and the two regions 100 and 101 which are included between this core and this support and which are located respectively below the poles 15 and 16 form a closed magnetic circuit. It should be noted here that, although in the case illustrated in Figure 3, the poles 15 and 16 are located near the surface of the magnetic layer 12, we could adopt another arrangement in which these two poles would be placed in contact of this surface.

FIG. 3 also shows that the width d of the recording pole 15 is very small compared to that D of the flux closing pole 16. Under these conditions, if an electric current of intensity I is sent to the winding E , this current creates, inside the magnetic core 14, a magnetic flux whose line of average force is represented by the line in dashed lines 17. In the portion of the magnetic layer 12 which is in the region 100 of the pole recording 15, the magnetic field is perpendicular to the surface of this layer 12, so that, in this portion, the magnetization of the magnetic layer 12 is effected transversely. In this portion, the magnetic field created by the head 13 is greater than the saturation field of the magnetic layer 12 and therefore causes, in this portion, the appearance of a practically punctual magnetized area, usually called magnetized point, this magnetized area remaining even when the winding E then ceases to be traversed by a current. On the contrary, in the portion of the magnetic layer 12 which is located in the region 101 of the flux closure pole 16, because the width of this pole is much greater than that of the recording pole 15, the value of the magnetic field created by the head 13 is much lower than that of the saturation field of the magnetic layer 12, so that this flux closure pole 16 can neither cause the formation of a magnetized area in the layer 12, nor a modification of the magnetized zones already formed in this layer. Under these conditions, the magnetic layer 12 can be magnetized so that the magnetized zones thus formed constitute a slow-moving magnetic image having a determined configuration, for example, the configuration of a character. By way of example, FIG. 5 shows a set of magnetized zones A arranged in a rectangular matrix comprising seven lines and five columns and distributed, inside this matrix, so as to constitute the image of the character "H". It should be noted here that the spacing pitch P of the rows and columns of this matrix is at least equal to the dimension L of a magnetized area. Under these conditions, it has been found that, even in the case where this pitch P was substantially equal to this dimension L, the magnetizations presented by two neighboring magnetized zones had practically no influence on each other. It will now be recalled that, in known methods, the latent magnetic image which has thus been formed on the surface of the recording element is then developed by depositing on the surface of the magnetic layer 12 a powdery developer comprising finely particles. divided, each formed of an organic thermoplastic resin in which a pigment and magnetic particles have been incorporated. After which, the surface of the magnetic layer 12 is subjected to a retouching operation which makes it possible to remove the developer particles which are in excess on this surface, so that, at the end of this operation, only the magnetized areas of this layer remains covered with a developer film, thus forming on the surface of layer 12 a powder image, the configuration corresponds to that of the magnetized zones. This powder image is then transferred to a printing medium generally consisting of a strip of paper. In the present invention, on the contrary, so that the image which is formed on the printing medium appears in p previously selected colors, p being an integer at least equal to 2, the following method is used, the different phases of which will now be described with reference to FIGS. 1A to 1G.

In the phase illustrated in FIG. 1A, the recording element 10 is magnetized so as to form on its surface magnetized zones which all have the same dimensions and which have magnetizations all oriented in the same direction. In FIG. 1A, only three of these zones, referenced respectively A1, A2, and A3 have been shown for reasons of simplification, but it is understood that the number of these magnetized zones can be absolutely arbitrary. FIG. 1A also shows the north (N) and south (S) magnetic polarities as well as the respective magnetizations J ₁ , J ₂ and J ₃ of the areas A1, A2 and A3, each of these magnetizations being represented by a arrow whose length is proportional to the value of this magnetization. We then see, in FIG. 1A, that the magnetizations J ₁ , J ₂ and J ₃ of the respective zones A1, A2 and A3 are indeed all oriented in the same direction, but have different values, the magnetization J ₂ having in indeed, in this figure, a value greater than that of the magnetization J ₃ , but less than that of the magnetization J _i . In general, the magnetized zones which are intended to form, on the printing medium, images or parts of an image which must appear at the same determined value have magnetizations before all of the same value. However, the magnetization of these zones has a value different from the magnetization of the zones intended to form, on the printing medium, images or parts of image which must appear in another determined value. There are, therefore, as many magnetization values as there are different colors in the image or images to be formed on the printing medium, that is to say in the most case general, p values of magnetizations J "J ₂ , ... J _p . Thus, for example, the zones A1, A2 and A3 which have been represented in FIG. 1A and which all have magnetizations of the same sense, but of different values, are intended to form on the printing paper three punctual spots all having different colors. We will consider, for example, that the magnetized zone A1 is intended to form a punctual spot of red color, that the magnetized zone A2 is intended to form a spot spot of black color and that the magnetized zone A3 is intended to form a point spot of yellow color. On the other hand, it will be considered, in the example illustrated by FIG. 1A, that the values of the magnetizations J, and J ₂ have been previously chosen and are such that we have J ₁ = 1.6 .J ₃ and that J ₂ = 1,25.J ₃ . To form these three magnetized zones A1, A2 and A3, three identical recording heads can be used, of the type which has been represented in FIG. 3, and excite these three heads with currents flowing in the same direction in each respective one. windings of these heads, but having respective intensities 1 ₁ , 1 ₂ and 1 ₃ such that these currents cause the formation, in the recording element, of magnetized zones of respective magnetizations J ₁ , J ₂ and J ₃ .

The recording element 10 having been magnetized in the manner which has just been indicated, a first powdery developer is then deposited on the surface of this element, the hue of which is one of p colors previously chosen. In the example described, it will be considered that this first powdery developer is red in color. This first developer, which is brought into contact with the entire surface of the recording element 10, is however only attracted by the magnetized zones of this element, so that, if we manage to leave this developer only on these magnetized areas, for example by depositing the recording element so that, when the developer is applied to this element, each developer particle is subjected to the action of an oriented force of gravity opposite to the magnetic attraction force exerted by the magnetized areas, each of these magnetized areas is then coated, as shown in FIG. 1B, with a layer 18 of first developer, the thickness of this layer being all the more important as the magnetization of the magnetized zone on which it is deposited has a higher value. It should be pointed out that the force with which each of the developer particles which have been deposited on the same magnetized zone of the recording element 10 is attracted depends not only on the value of the magnetization of this zone and on the distance h which separates each particle from this zone, but also the physical characteristics of this developer, such as the particle size state and the percentage of magnetic particles of this developer. Under these conditions, the magnetic force F _m exerted by each of the magnetized zones on each of the developer particles which have been deposited on this zone varies, as a function of the distance h which separates this particle from this zone, according to a law of variation. illustrated by the curves which have been represented on the diagram of FIG. 6, the curve in dashed lines 60 representing the variations, as a function of h, of the magnetic force exerted by a magnetized zone of strong magnetization, such as A1, the curve in solid lines 61 representing the variations as a function of h, of the magnetic force exerted by a magnetized zone of medium magnetization such as A2, and the curve in dashed lines 62 representing the variations, as a function of h, of the magnetic force exerted by a magnetized area of weak magnetization, such as A3. FIG. 6 shows only the three curves corresponding to the three values J ₁ , J ₂ and J ₃ of the magnetizations of the zones which are intended to form, on the printing medium, an image comprising, in the example described, three different colors. However, it will be readily understood that, in the most general case, there are as many curves as there are colors in the image to be printed, that is to say p curves if this image comprises p colors, each of these p curves corresponding to a respective one of the p magnetization values J ₁ , J ₂ , J ₃ , ... J _p of the magnetized zones. In FIG. 6, the value F _G of the force of gravity mentioned above has also been shown, this force being exerted on each particle of developer during the application of this developer on the surface of the element. registration. We then see that there exists, for each magnetized zone, a particular value of the distance h for which this force of gravity F _G is equal to the magnetic attraction force exerted by this magnetized zone, this particular value thus determining the thickness of the layer of this developer remaining in this area. Thus, in the example described, this particular value for each of the magnetized zones A1, A2 and A3 is equal to e _o , e ' _o and e " _o , respectively, so that the thickness of the layer of developer which remains on zone A1 is equal to e _o , that of the layer which remains on zone A2 is equal to e ' _o and that of the layer which remains on zone A3 is equal to e " _o , these thicknesses having substantially, in the case illustrated by FIG. 6, the following values: e _o = 100 μm, e ' _o = 65 pm, e " _o = 33 pm.

The magnetized zones A1, A2 and A3 having thus been coated with a layer 18 of first developer, the recording element 10 is then subjected to a retouching operation intended, not only to remove the residual particles of first developer remaining in outside the magnetized zones of the recording element 10, but also to remove all the particles of first developer on the magnetized zones whose magnetization is less than the value of the strongest magnetization, these magnetized zones being, in the example described, the zones A2 and A3 whose respective magnetizations J ₂ and J ₃ each have a value lower than the magnetization J ₁ of the zone A1. During this retouching operation, which can be carried out by electrostatic or pneumatic magnetic means (suction or blowing of air), each particle of developer remaining on the surface of the recording element 10 is subjected to a constant force of value. F ₁ , acting against the magnetic force F _m which keeps each particle applied to the magnetized area on which it has been deposited. On the diagram of FIG. 6, the value F ₁ of this force has been represented, this value being chosen so that the straight line of ordinate F ₁ only intersects the curve representing the variations as a function of the distance h, of the magnetic force exerted by the magnetized zones having the strongest magnetization, this curve being, in the example described, curve 60. In the diagram of FIG. 6, the abscissa of the point of intersection has also been designated by e ₁ I ₁ of curve 60 with the straight line of ordinate F ₁ . It can thus be seen that, for the developer particles which have been deposited on each of the magnetized zones of stronger magnetization such as A1 and which are situated at a distance less than e ₁ , the magnetic force exerted by this zone is greater than the force retouching F ,, so that these particles will remain in this area. On the contrary, for the particles which have been deposited on each of the magnetized zones of stronger magnetization and which are situated at a distance greater than e ₁ , the magnetic force exerted by this zone is less than the retouching force F ₁ , so that these particles will be removed from this area. Consequently, there will remain, on each of the magnetized zones of stronger magnetization (such as A1) a layer of first developer having practically a thickness equal to e ₁ . As for the particles of first developer which have been deposited on each of the other magnetized zones (such as A2 and A3), since the magnetic force exerted by each of these zones is always less than the retouching force F ₁ , these particles will be totally eliminated from these areas. Under these conditions, only the magnetized zones of stronger magnetization (such as A1) will appear coated, as can be seen in FIG. 1C, with a layer of first developer of thickness e ₁ .

The developer depositing and retouching operations which have just been described are then repeated with a second powdery developer, the color of which is one of the p colors chosen, but other than that of the first developer. In the example described, it will be considered that this second powdery developer is black in color. The deposition of this second developer is carried out under the same conditions as those which have been described for the deposition of the first developer, so that, when this deposition is completed, each of the magnetized zones of the recording element 10 is coated, as shown in FIG. 1D, of a layer 19 of second developer. On the magnetized zones of stronger magnetization, such as the zone A1, this layer 19 is thus superimposed on the layer 18 of the first developer. After which, the recording element 10 is subjected to a second retouching operation, similar to that described above, but whose constant value force F ₂ is less than the force of the first retouching operation. On the diagram of FIG. 6, the value F ₂ of this force has been represented, this value being chosen so that the line of ordinate F ₂ intersects at points 1 ₂ and 1 ′ _{2 only the} two curves 60 and 61 which represent, respectively, one, the variations, as a function of the distance h, of the magnetic force exerted by the magnetized zones of stronger magnetization, that is to say of magnetanta tion J _i , the other the variations, as a function of h, of the magnetic force exerted by the magnetized zones whose magnetization has in the following successive magnetization values J ₁ , J ₂ , J ₃ ..., Jp, the value J ₂ . We then see, in FIG. 6, that there exists a value e ₂ of the abscissa h for which the magnetic force exerted by each of the magnetized magnetization zones J, is equal to the force F ₂ , and that it there is also a value e ' ₂ of the abscissa h for which the magnetic force exerted by each of the magnetized magnetization zones J ₂ is equal to this force F ₂ . Without repeating all the detailed explanations which have been given above, it is understood that this value e ₂ represents, as can be seen in FIG. 1E, the total thickness of the heaps formed by the superposition of layers 18 and 19 on each magnetized zones of magnetization J ₁ , and that this value e ′ ₂ represents the thickness of the layer 19 of the second developer on each of the magnetized zones of magnetization J ₂ .

These deposition and retouching operations are repeated as many times as there are colors in the image to be printed. Thus, in the example described where this image has three colors, there is therefore deposited, on each of the magnetized areas of the recording element, a third powder developer whose color differs from that of the two developers previously deposited. . In the example described where the number of colors is equal to three, this third powder developer is therefore the last to be deposited on the recording element 10. It will be considered here that this third powder developer is yellow. When this deposition is completed, each of the magnetized zones of the element 10 appears coated, as shown in FIG. 1F, with a layer 20 of third developer, this layer 20 being, on the magnetized zones whose magnetization is greater at D ₃ (that is to say on the areas A1 and A2), superimposed on the layer 19 of the second developer. After which, the recording element 10 is subjected to a third retouching operation, similar to those carried out previously, the value F ₃ of the force applied during this third operation being less than that, F ₂ , put implemented during the second retouching operation. On the diagram of FIG. 6, the value F ₃ of this retouching force has been represented, this value being chosen so that the line of ordinate F ₃ intersects all the curves representative of the variations, as a function of h, of the forces magnetic exerted by the magnetized zones. It should however be pointed out here that, since in the example described, the third developer is the last to be deposited on the magnetized areas of the recording element 10, this last retouching force is intended, for a part to remove the developer particles that remain outside the magnetized areas of the recording element 10, on the other hand, to limit the thicknesses of the third developer layers which have been deposited on these areas. This last retouching operation which differs from the previous retouching operations by the fact that it does not cause a total elimination of the particles which are on some of the magnetized zones, is therefore not a particle elimination operation on certain magnetized zones of the recording element 10. In the example illustrated by FIG. 6, it is then understood that the thickness of the total layer of the three developers which remains on each magnetization zone J, is given by abscissa e ₃ from point 1 ₃ where the line of ordinate F ₃ intersects the curve 60, that, likewise, the thickness of the total layer of two developers which remains on each magnetization zone J ₂ is given by abscissa e ' ₃ from point l' ₃ where the ordinate line F ₃ intersects the curve 61, and the thickness of the layer of third developer which remains on each magnetization zone J ₃ is given by the abscissa e " ₃ from point 1" ₃ where the line of ordinate F ₃ intersects the line 62.

It should also be noted that the powdery developers which are used in the process of the invention have practically the same physical properties, in particular the same particle size state, the coercive field, the same magnetization at saturation, the same density and the same melting point. , so that the magnetic force F _m that each of the magnetized zones exerts on each of the particles, whatever they are, located at the same distance h, varies only as a function of the value of the magnetization of this zone. When the last developer has been deposited on the recording element and the last retouching operation has been carried out, a strip of paper 21 intended for printing is then brought, as shown in FIG. this recording element 10, either in contact with this recording element 10, and the transfer then takes place on this strip of paper of the developer layers which are located on the magnetized zones of this element 10. This transfer can moreover be carried out in a known manner, either by application of pressure, or by magnetic or electrostatic means. However, the conditions for this transfer are established in such a way that all of the developer layers are transferred almost entirely onto the paper strip 21. Thus in the example illustrated in FIG. 1G, the layer 20 of third developer which is found on the area A3 is transferred to the strip 21 where it then forms a heap which, consisting of this third developer, has a yellow color. Likewise, the layers 19 and 20 of the second and third developer, which were superimposed on the area A2, are found on the paper by forming a pile 22 in which the layer 19 of the second developer then covers the layer 20 of the third developer. Finally, the layers 18, 19 and 20 of the first, second and third developers, which were superimposed on the area A1, are found on the paper by forming a pile 23 consisting of the layer 18 of the first developer which covers the layer 19 of the second developer, lacquer, in turn covers layer 20 of the third developer.

The developer layers which have thus been transferred onto the paper strip 21 are then subjected to a fixing operation, this operation being carried out at a temperature making it possible to bring the three developers to the viscous, but non-liquid state, which prevents the various developers constituting the heaps 22 and 23 from mixing. Under these conditions after this fixing operation, the layer 20 of the pile 22 is masked by the layer 19 which then forms on the paper a spot spot having the shade of the second developer, that is to say black in the example. described. Likewise, the layers 19 and 20 of the pile 23 are hidden by the layer 18 which then forms on the paper a spot spot having the color of the first developer, that is to say red in the example described. Finally, the single layer 20 of the third developer forms, when it is fixed to the paper, a point spot which has the yellow tint of this third developer.

FIG. 2 shows a magnetographic printing machine which performs color printing according to the printing method which has just been described. The machine which is shown in this figure comprises a magnetic recording element in the form of a similar magnetic drum 10. that which has been described and represented in the aforementioned French patent No. 2,402,921, this drum being driven in rotation, in the direction indicated by the arrow R, by an electric motor 25. The magnetization of the magnetic layer of this drum is provided by a set of n magnetic heads 13-2 to 13-n arranged one next to the other and aligned parallel to the axis of rotation of the drum. These heads, which are of the type which has been represented in FIG. 3, are selectively excited by electrical pulses sent by a source of pulses 26 and applied to the windings of these heads via a calibration device current 27, the structure of which has been shown in detail in FIG. 4.

Referring to FIG. 4, it can be seen that each of the respective windings E-1 to En of magnetic heads 13-1 to 13-n is connected, at one end, to the movable contact blade of the a respective one of n first stud switches K-1 to Kn and, at the other end, to the movable contact blade of a respective one of n second stud switches L-1 to Ln, via l '' a respective of n contacts CB-1 to CB-n. Each of the contacts CB-1 to CB-n is controlled by a respective one of n relay coils B-1 to B-n. FIG. 4 shows that, in the example described, each of the pad switches K-1 to Kn and L-1 to Ln has three pads or input terminals designated, in the figure, by 1, 2 and 3. The input terminal 1 of each of the first switches K-1 to Kn is connected to the positive (+) terminal of a first current generator G1, while input terminal 1 of each of the second switches L-1 to Ln is connected to the negative (-) terminal of this generator G1. The input terminal 2 of each of the first switches K-1 to Kn is connected to the positive (+) terminal of a second current generator G2, while the input terminal 2 of each of the second switches L-1 to L-2 is connected to the negative (-) terminal of this generator G2. Finally, the input terminal 3 of each of the first switches K-1 to Kn is connected to the positive (+) terminal of a third current generator G3, while the input terminal 3 of each of the second switches L -1 to Ln is connected to the negative (-) terminal of this generator G3. Figure 4 shows that the movable contact blades of switches K-1 and L-1 are mechanically coupled so that they can be placed simultaneously on the same input terminal. The same is true for the movable contact blades of switches K-2 and L-2, ..., K-n and L-n. As can be seen in FIG. 4, the relay coils B-1 to Bn can be excited by electrical pulses supplied on the outputs S1 to Sn of the pulse source 26, each of these coils B-1 to Bn being, for this purpose, connected to a respective one of the outputs S1 to Sn by means of a respective one of n conductors W1 to Wn.

The structure of the pulse source 26 will not be described here for the reason that this structure is of known type. It will be considered here that, in the example described, the structure of this source 26 is similar to that of the recording control device which has been described and represented in French patent No 2 443 335. It will simply be recalled that in the case where the machine which is represented in FIG. 2 is used to print characters made up of dots located inside a rectangular matrix comprising seven lines and five columns, the lines of this matrix extending in a parallel direction at the axis of rotation of the drum 10, the latent magnetic image necessary for the printing of a character is obtained by selectively exciting, on seven different occasions, five contiguous heads taken from the set of magnetic heads 13-1 to 13-n, this excitation being carried out by means of pulses delivered, at successive instants t _i , t ₂ , t ₃ , t ₄ , t _s , t _s , and t ₇ , on five corresponding of the outputs S1 to Sn of the pulse source 26. This is thus, for example, that to form, by means of the magnetic heads 13-1 to 13-5, the latent magnetic image necessary for the impression of the character "G", the source of pulses 26 delivers, at the instant t ₁ a pulse on each of its outputs S2 to S4, at time t ₂ a pulse on each of its outputs S1 to S5, at time t ₃ a pulse on its output S5, at time t ₄ a pulse on each of its outputs S1, S2, S3, and S5, at time t ₅ a pulse on each of its outputs S1 and S5, at time t ₆ a pulse on each of its outputs S1 and S5 and finally at time t ₇ a pulse on each of its outputs S2 to S4.

The switches K-1 to Kn and L-1 to Ln are intended to determine the amplitude of the magnetizations of the magnetized zones on the drum 10, this amplitude conditioning the color of the spot spot which will subsequently be formed on the paper by each of the zones magnetized. For this purpose, the first switches K-1 to Kn, the second switches L-1 to Ln, the relay contacts CB-1 to CB-n and the windings E-1 to En of magnetic heads are distributed as shown in the Figure 4, so as to constitute n portions of circuits C-1, C-2, ..., Cn each associated with a respective one of the n heads 13-1 to 13-n, each of these portions comprising, in series , one for the first switches K-1 to Kn, one for the windings E-1 to En, one for the relay contacts CB-1 to CB-n and one for the second switches L -1 to Ln.

In the case where the two switches of the same circuit portion are placed in position 1, that is to say when the movable contact blade of these two switches is placed on the input terminal 1, the current which circulates in the winding of the head associated with this circuit position, when the relay contact which is in series with this winding is closed, is that delivered by the generator G1, this current having an intensity I ₁ . If these two switches are placed in position 2, the current flowing in this winding, when this relay contact is closed, is that delivered by the generator G2, this current having an intensity 1 ₂ . Finally, if these two switches are placed in position 3, the current which circulates in this winding when this relay contact is closed, is that delivered by the generator G3, this current having for intensity 1 ₃ .

Thus for example, if, at the moment when a pulse is delivered on the output Sn of the source 26, the two switches Kn and Ln are placed in position 3, this pulse causes, when arriving on the coil Bn, the momentary excitation of this coil. The coil Bn then closes its contact CB-n for a short time, so that a direct current of intensity 1 ₃ flows momentarily in the closed circuit constituted by the generator G3, the switch Kn in position 3, the winding In of the head 13-n, the closed contact CB-n and the switch Ln in position 3. Therefore, this magnetic head 13-n forms on the surface of the drum 10, a practically punctual magnetized zone, this zone having a magnetization J ₃ which remains after the disappearance of the current of intensity 1 ₃ in the winding En.

It should also be noted that the current generators G1, G2 and G3 are set up to deliver currents of respective intensity I ₁ , 1 ₂ and 1 ₃ such that the magnetized zones formed on the surface of the drum 10 by these currents have magnetizations respective values Ji, J2, J ₃ predetermined.

It is understood, under these conditions, that if it is desired, for example, to obtain on the drum 10 a latent magnetic image in the magnetized zones having a magnetization J ₁ , it suffices, before the magnetic heads necessary for the formation of this image do not are excited, place the corresponding switches K and L on position 1. Similarly, if we want the magnetized areas of this image to have a magnetization J ₂ , it suffices, before these heads are excited, to place these switches in position 2. Finally, if we want the magnetized zones of this image to have a magnetization J ₃ , it suffices, before these heads are excited, to place these switches in position 3. The positioning of the switches K-1 to Kn and L-1 to Ln can also be carried out, either manually by the operator before any printing operation, or entirely automatically by actuation means of known type excited by the same unit of controls that which controls the operation of the pulse source 26. It is even possible, according to the cases and applications, to place some of the switches K-1 to Kn and L-1 to Ln on a determined position, for example, in position 2, while other switches are placed in another position. This arrangement allows, when printing a line of characters for example, to obtain characters printed in one color, while the other characters of this line are printed in another color.

It should be noted that the current calibration device 27 which has been shown in FIG. 2 consists, in the example described, of the assembly comprising the relay coils B-1 to Bn and their contacts CB -1 to CB-n, the switches K-1 to Kn and L-1 to Ln, and the generators G1 to G3, all these elements being connected together. As illustrated in FIG. 4. It should however be noted that , although the calibration device which is represented in FIG. 4 comprises only three current generators and switches comprising only three positions, this calibration device, when it is used in a machine intended for printing images in p colors, includes p current generators G1, G2, ..., Gp and switches K-1 to Kn and L-1 to Ln each comprising p positions. Under these conditions, the intensities 1 ₁ , I ₂ , 1 ₃ , ..., I _P delivered by each respective of the p generators G1, G2, G3, ..., Gp are adjusted so as to cause the formation, in the 'recording element, of magnetized zones of respective magnetizations J _i , J ₂ , J ₃ , ..., Jp.

Returning now to FIG. 2, it can be seen that the printing machine produced according to the invention also comprises a first applicator device 40, of known type, which makes it possible to apply to the surface of the drum 10 particles of a first powdery developer contained in a reservoir 49. In the example described, it will be considered that this first developer is red in color. This first applicator device 40 is established for depositing on each of the magnetized zones of the drum 10 a layer of first developer, the thick of which its thickness is substantially equal to 100 μm for the magnetized zones of magnetization J ₁ , to 65 μm for the magnetized zones of magnetization J ₂ and to 33 μm for the magnetized zones of magnetization J ₃ . It is considered that this applicator device 40 is preferably of the type of those which have been described and represented in French patents No. 2,408,462 and 2,425,941, this device comprising, on the one hand, a rotating magnetic element. which brings the developer particles from the reservoir 49 to the vicinity of the surface of the drum, on the other hand a deflector interposed between this element and the drum to form a trough in which the particles collected by the deflector accumulate, this deflector leaving between it and the drum a very small opening, of the order of 1 millimeter, through which pass the particles which have come to bear against the surface of this drum. The magnetized zones of the drum 10 which have thus been coated with a layer of first developer then pass in front of a first retouching device 41 intended to remove the particles of first developer which have been deposited on the magnetized zones of magnetizations J ₂ and J ₃ . This retouching device can be of the magnetic, electrostatic or pneumatic type. It will be considered that, in the example described, this retouching device 41 is of the type which has been described and represented in French patent No. 2,411,435 and that it is adjusted so as not to allow it to remain, on the magnetization zones J ₁ , that a layer of first developer having a thickness close to 20 μm. The magnetized zones of the drum 10 which have passed past the retouching device 41 then pass in front of a second applicator device 42 of a type similar to that of the first applicator device, this second device making it possible to deposit on the drum 10 particles of a second developer pulverulent which, being black in color, in the example described, is contained in a reservoir 50. This second applicator device 42 is established for depositing a layer of second developer, on the one hand on each of the magnetized zones of magnetizations J ₂ and J ₃ , the thickness of this layer being substantially equal to 65 μm for the magnetization zones J ₂ and to 33 μm for the magnetization zones J ₃ , on the other hand on each of the layers already deposited with the first developer , the total thickness of the layers thus superimposed of the two developers on the magnetization zones J ₁ being substantially equal to 100 μm. After which, the magnetized zones of the drum 10 pass in front of a second retouching device 43 similar to the first retouching device 41 and arranged downstream of the second applicator device 42 relative to the direction of travel of the drum. This second retouching device 43 which is intended to remove the particles of second developer which have been deposited on the magnetized areas of magnetization J ₃ , is adjusted so as to leave only the areas of magnetization J ₂ remaining, a layer of second developer having a thickness close to 23 μm, and, on the magnetization zones J ₁ , a composite layer comprising two superposed layers of first and second developer, the thickness of this composite layer being substantially equal to 40 pm. The magnetized zones of the drum 10, which have passed past the second retouching device 43 then pass in front of a third applicator device 44 which, being of a type analogous to the preceding ones, makes it possible to deposit on the drum 10 particles of a third powdery developer, yellow in color, contained in a tank 51. This third applicator device 44 is established for depositing a layer of third developer on each of the magnetization zones J ₃ , the thickness of this layer being substantially equal to 33 μm, as well as on each of the layers of second developer already deposited on each of the magnetization zones J ₁ and J ₂ , the total thickness of the three layers thus superimposed on the magnetization zones J ₁ being close to 100 μm, while the total thickness of the two layers thus superimposed on the magnetization zones J ₂ is close to 65 μm. The magnetized zones of the drum which have thus been coated then pass in front of a third retouching device 52 which is intended essentially to remove the developer particles which remain on the drum outside the magnetized zones. In the example described, this third retouching device 52 is adjusted so that the magnetization zones J ₃ which have passed in front of this device 52 appear coated with a layer of third developer whose thickness is close to 20 μm while the magnetization zones J ₂ which have passed in front of this device 52 appear coated with a composite layer comprising two superposed layers of second and third developer, the thickness of this composite layer being substantially equal to 40 μm. Finally, the magnetization zones J ₁ , which passed in front of this device 52 appear coated with a composite layer comprising three superposed layers of three developers, the thickness of this composite layer being substantially equal to 60 μm.

The magnetized zones of the drum 10 which have undergone all these deposition and retouching operations are then brought into contact with a strip of paper 21 which is applied to the drum 10, as shown in FIG. 2, under the action exerted by a pressure roller 45. The force with which the strip 21 is applied against the drum 10 by the pressure roller 41 can be adjusted, by known means, (not shown) so as to cause a total transfer of all the layers of developer which remain on the drum 10 after the passage in front of the retouching device 52. The value F _t of this force is, as shown in FIG. 6, greater than that F _{1 of} which we have spoken above, so that, ultimately, the whole of the composite layer which was on each of the magnetized magnetization zones J _i is transferred to the paper, thus forming point spots having a thickness of the order of 60 μm and having the red color of the first developer. Likewise, the whole of the composite layer which was on each of the magnetized magnetization zones J ₂ is transferred to the paper, thus forming spot spots having a thickness close to 40 μm and having the black color of the second developer. Finally, the entire layer of third developer which was on each of the magnetized zones of magnetization J ₃ is transferred to the paper, thus forming spot spots, of yellow color, having a thickness close to 20 μm _.

The machine which has been shown in FIG. 2 also includes a developer fixing device 46 under which the paper strip 21 passes when the transfer operation which has just been described has been carried out. This fixing device 46 which is constituted, in the example described, by an electrically heated element, is intended to permanently fix the developers which have been transferred to the paper strip 21. It should be noted here that this fixing device 46 is adjusted so that these developers undergo practically no fusion, but only a sufficient softening to ensure their fixing on the paper. Under these conditions, no mixing of colors is likely to occur in the heaps of developers which, such as 22 and 23, comprise several layers of developers of different shades. Thus, each of the heaps of developers, such as 22, forms, when cooled on the paper, a point spot having the color of the second developer. Likewise, each of the heaps of developers, such as 23, forms, when cooled on the paper, a point spot having the color of the first developer. Finally, each of the heaps which have only a single layer of third developer forms, when cooled on the paper, a punctual spot having the color of this third developer.

The machine shown in Figure 2 further comprises a cleaning device which, consisting of a brush 47 in the example described, ensures the cleaning of the parts of the surface of the drum which have passed in front of the transfer station. After this cleaning, these parts pass in front of an erasing device 48, of the electromagnetic type, which erases the latent magnetic images carried by these parts, so that these parts are again capable of being magnetized when they arise. then in front of all the magnetic heads 13-1 to 13-n.

Of course, the invention is in no way limited to the embodiments described and illustrated which have been given only by way of example. On the contrary, it includes all the means constituting technical equivalents of those described and illustrated, considered in isolation or in combination, and used in the context of the claims which follow.

Claims (4)

1. Magnetographic printing process enabling the production on a print carrier of an image in p previously selected colours, p being a whole number at least equal to two, this process consisting in magnetising the surface of a magnetic recording element along a direction perpendicular to this surface, in a manner such as to form a group of magnetised points or dots forming a latent magnetic image, these magnetised dots all having the same size and displaying the same magnetic polarity but having different intensities of magnetisation of values J1, J2, J3... Jp such that J1>J2>J3>Jp, each of these values being allocated to a corresponding one of the p colours of the said image, the magnetised dots intended for forming images or parts of an image which are intended to appear in one and the same colour on the carrier all having the same intensity of magnetisation, the said process being characterised in that it consists furthermore in:

- depositing on this surface a first pulverulent developer of which the shade is that of a first one of the said colours, this first developer subsisting on all the magnetised dots of this surface,

- thereafter eliminating this first developer from the magnetised dots of which the intensities of magnetisation are lower than J1,

- repeating this operation of producing a deposit on this surface with, successively, each of (p-1) other pulverulent developers of different shades, these p developers however having the same physical characteristics, each of these depositing operations being followed immediately, except in the case of the final depositing operation, by an operation for removal of the developer, the magnetised dots affected by this removal being, during each particular one of these (p-2) removal operations, those of which the intensity of magnetisation is lower, respectively, than J2, J3...Jp-1, so that after the final depositing operation, each magnetised dot having an intensity of magnetisation equal to J1 is covered by p superposed layers of the said developers, each magnetised dot having an intensity of magnetisation equal to J2 is covered by (p-1) superposed layers of the said (p-1) other developers, and so on in sequence, each magnetised dot of which the intensity of magnetisation is equal to Jp then being covered by a single layer only of the p-th developer,

- and performing a total transfer of all these layers of developers on to the print carrier in order to form on the latter a powder image constituted by a plurality of punctiform spots, each of these spots resulting from the transfer of the layers of developers which were deposited on each of the said corresponding magnetised dots and having the colour of the developer which had been in direct contact at this point with the surface of the recording element immediately before the transfer operation.

2. Magnetographic printing machine for carrying out the process according to claim 1, comprising a recording element (10) equipped with a magnetic recording surface, a plurality of magnetic heads (13-1 to 13-n) controlled by means of electrical pulses and organised so that in response to these pulses they magnetise the said recording surface along a direction perpendicular to this surface for the purpose of establishing thereon a group of magnetised dots (A) forming a latent magnetic image, driving means (25) for causing a relative displacement between the recording element (10) and the magnetic heads (13-1 to 13-n), a source of pulses (26) arranged for selectively transmitting electrical pulses to the said heads, and an applicator device (10) enabling depositing a pulverulent developer on the said recording surface, this developer remaining applied only on the magnetised dots of this surface to form a powder image, the said machine being characterised in that, the said developer comprising particles of which the shade is of a first of p previously selected colours, it comprises moreover:

- current calibration means (27) interposed between the source of pulses (26) and each of the heads (13-1 to 13-n) to allow each of the current pulses transmitted by this source to be adjusted in a selective manner to one of p predetermined amplitude values and thus to form a latent magnetic image on the recording surface, of which the magnetised dots (such as A1, A2, A3) have the same magnetic polarity but have intensities of magnetisation differing from the values J1, J2... Jp, such that J1>J2>Jp, each of these values being associated with a corresponding one of the aforesaid p colours, the magnetised dots intended to form image portions which are to appear in one and the same colour on the print carrier (21) all having the same intensity of magnetisation.

- (p-1) other applicator devices (42, 44) distributed along the path followed by the recording surface during its displacement, each of these applicator devices enabling each magnetised dot of this surface to have deposited on it a layer of each corresponding one of (p-1) other pulverulent developers, each of these (p-1) developers having as its shade a corresponding one of the said colours other than the first colour, all these developers having the same physical characteristics however,

- (p-1) retouching devices (41, 43) each situated downstream with respect to the direction of displacement of the surface, of each corresponding one of the said applicator devices (40, 42) excepting the last one (44), the first of these retouching devices (41) being arranged to withdraw the first developer (18) from the magnetised dots (such as A2, A3) of which the intensity of magnetisation is lower than J1, the second retouching device (43) being arranged to remove the second developer (19) from the magnetised dots (such as A3) of which the intensity of magnetisation is lower than J2, and so on in sequence,

- and a transfer device (45) situated downstream of the final applicator device (44) for total transfer onto the print carrier (21) of the different layers of developers (18, 19, 20) which cover the magnetised dots of the surface once these dots have travelled past the said final applicator device.

3. Printing machine according to claim 2, characterised in that-each magnetic head comprising a coil (E) wound around a core (14), the current calibrating means (27) comprise:

- n portions of circuits (C-1, C-2,... C-n), each associated with the corresponding one of the n magnetic heads (13-1,13-2,... 13-n) and each comprising:

+ a relay contactor (like CB-1) connected in series with the coil (such as E-1) of the head associated with the same and actuated by means of a coil (such as B-1) energised selectively by means of the pulses transmitted by the source of pulses (26),

+ a first switch (such as K-1) comprising p input terminals (1, 2, 3...) and a moving contact blade connected to one of the extremities of the assembly formed by the said coil (E-1) in series with the said relay contactor (CB-1),

+ and a second switch (such as L-1) comprising p input terminals (1, 2, 3...) and a moving contact blade connected to the other extremity of the assembly formed by the said coil (E-1) in series with the said relay contactor (CB-1), this second switch being coupled with the first switch, in a manner such that the moving contact blades of these two switches occupy identical positions on the input terminals of corresponding numbers,

- and p current generators (G1, G2, G3...), the first (G1) of the said current generators having its positive terminal (+) connected to the first (1) of the input terminals of each of the n first switches (K-1 to K-n) and its negative terminal (-) connected to the first (1) of the input terminals of each of the n second switches (L-1 to L-n), the second (G2) of the said current generators having its positive terminal (+) connected to the second (2) of the input terminals of each of the n first switches (K-1 to K-n) and its negative terminal (-) connected to the second (2) of the input terminals of each of the n second switches (L-1 to L-n) and so on in sequence, each of these p generators being arranged so that when the relay contactors (CB-1 to CB-n) are closed selectively, in response to the pulses transmitted by the source (26), they supply the corresponding one of p currents of intensity 11, 12...lp, these intensities being adjusted in a manner such as to induce the formation in the recording element of magnetised areas having the corresponding magnetisations J1, J2, J3...Jp.

4. Printing machine according to either of claims 2 and 3, characterized in that it also comprises a fixing device (46) arranged along the path followed by the print carrier and downstream with respect to the transfer station (45), this fixing device being adjusted in a manner such that the particles of developer which, deposited on this carrier, pass in front of the said fixing device, are exposed to a softening operation but not to a melting operation.

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