EP0082742B1 - Method and device for magnetographic copying - Google Patents

Description

The present invention relates to a magnetographic printing method which makes it possible to obtain images in two colors 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 images of characters for example, on a printing medium also consisting of a strip. or a sheet of paper. In these printing machines, of a type analogous to that which has been described and represented 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 latent magnetic image on the surface of a magnetic recording element generally in the form of a rotating drum or an endless belt, this latent image consisting 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 applied only 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 two 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 printing medium, it is practically impossible to prevent even very slight shifts occurring between the different parts of images thus printed. , which, of course, detracts from the sharpness 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 US Pat. No. 3,965,478, 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 image portions whose hue corresponds to the areas elementary elements of large dimensions have a definition, that is to say a degree of finesse, less good than cells whose hue 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.

The present invention overcomes 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 a relatively short time, high quality two-color images, while requiring only two developers having the same particle size and the same magnetic characteristics.

• - magnétiser d'abord la surface de l'élément d'enregistrement pour former des points magnétisés présentant des intensités d'aimantation d'amplitudes différentes et de polarités magnétiques opposées, les points magnétisés destinés à la formation des parties d'image qui, sur le support, doivent apparaître en l'une desdites couleurs ayant tous des intensités d'aimantation de même amplitude J_i et de même sens, la polarité magnétique de ces points étant opposée à celle des autres points magnétisés, ces autres points magnétisés ayant tous des intensités d'aimantation de même amplitude J₂ telle que J2< J↑,
• -à déposer sur cette surface un premier révélateur pulvérulent dont la teinte est celle de la première desdites couleurs;
• -à éliminer ce premier révélateur de cette surface, à l'exception des points magnétisés dont l'intensité d'aimantation est égale à J_i,
• - à déposer sur cette surface un second révélateur pulvérulent dont la teinte est celle de la seconde desdites couleurs ayont cependant les mêmes propriétés physiques que le premier révélateur,
• - à éliminer en partie ce second révélateur de cette surface tout en soumettant cette dernière à l'action d'un champ magnétique constant orienté en sens inverse de l'intensité d'aimantation J₁, l'amplitude de ce champ magnétique ayant une valeur telle que les intensités d'aimantations des points magnétisés se trouvent modifiées et prennent pratiquement la même valeur, de sorte que ce second révélateur est éliminé de cette surface, à l'exception des points magnétisés dont l'intensité d'aimantation était égale à J₂, chacun de ces derniers points étant ainsi recouvert d'une seule couche dudit second révélateur, tandis que chacun des autres points magnétisés se trouve recouvert d'une seule couche dudit premier révélateur,
• - et à effectuer, enfin, un transfert de toutes ces couches de révélateurs sur le support d'impression.

More specifically, the invention relates to a magnetographic printing 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 dots constituting a magnetic latent image, then depositing on this surface a powdery developer established to remain applied only on the magnetized points of said surface and thus form a powder image, and finally transferring this powder image on a printing medium, said method being characterized in what, to make it possible to obtain an image in two previously chosen colors, it consists of:

• - first magnetize the surface of the recording element to form magnetized points having magnetization intensities of different amplitudes and opposite magnetic polarities, the magnetized points intended for the formation of the image parts which, on the support, must appear in one of said colors all having magnetization intensities of the same amplitude J _i and of the same direction, the magnetic polarity of these points being opposite to that of the other magnetized points, these other magnetized points all having magnetization intensities of the same amplitude J ₂ such that J2 <J ↑,
• to deposit on this surface a first powdery developer, the color of which is that of the first of said colors;
• -to eliminate this first developer from this surface, with the exception of magnetized points whose magnetization intensity is equal to J _i ,
• - depositing on this surface a second powdery developer, the color of which is that of the second of said colors, however, have the same physical properties as the first developer,
• - to partially eliminate this second developer from this surface while subjecting the latter to the action of a constant magnetic field oriented in the opposite direction to the magnetization intensity J ₁ , the amplitude of this magnetic field having a value such that the magnetization intensities of the magnetized points are modified and take practically the same value, so that this second developer is eliminated from this surface, with the exception of the magnetized points whose magnetization intensity was equal to J ₂ , each of these latter points thus being covered with a single layer of said second developer, while each of the other magnetized points is covered with a single layer of said first developer,
• - And to carry out, finally, a transfer of all these layers of developers on the printing medium.

• - des moyens de calibrage et d'inversion de courant interposés entre les têtes magnétiques et la source d'impulsions pour, d'une part inverser sélectivement le sens du courant des impulsions envoyées par ladite source, d'autre part permettre à chacune de ces impulsions d'être ajustée sélectivement à l'une de deux valeurs d'amplitude prédéterminées et former ainsi sur la surface d'enregistrement une image magnétique latente dont les points magnétisés présentent des intensités d'aimantation d'amplitudes différentes et de polarités magnétiques opposées, les points magnétisés destinés à la formation des parties d'image qui, sur le support, doivent apparaître en l'une desdites couleurs ayant tous des intensités d'aimantation de même amplitude J₁ et de même sens, la polarité magnétique de ces points étant opposée à celle des autres points magnétisés, ces autres points magnétisés ayant tous des intensités d'aimantation de même amplitude J₂ telle que J₂<J₁,
• - un premier dispositif de retouche placé, par rapport au sens de déplacement de la surface, en aval dudit dispositif applicateur et établi pour retirer ledit premier révélateur des points magnétisés dont l'intensité d'aimantation est inférieure à J_i,
• - un second dispositif applicateur disposé entre le poste de transfert et le premier dispositif applicateur pour déposer un second révélateur pulvérulen sur ladite surface d'enregistrement, ce second révélateur comprenant des particules dont la teinte, est celle de l'autre desdites couleurs ayant cependant les mêmes propriétés physiques que le premier révélateur, ledit second révélateur ne restant appliqué que sur les points magnétisés de ladite surface,
• - et un second dispositif de retouche placé en aval dudit second dispositif applicateur et comprenant un dispositif générateur de champ magnétique pour appliquer un champ magnétique constant à ladite surface d'enregistrement, ce champ étant orienté en sens inverse de l'intensité d'aimantation J₁, l'amplitude de ce champ étant établie pour modifier les amplitudes des intensités d'aimantation des points magnétisés et les rendre pratiquement égales lorsque ces points magnétisés passent devant ledit second dispositif de retouche, de sorte que, après ce passage, chacun des points magnétisés dont l'intensité d'aimantation était égale à J₂ est revêtu d'une seule couche du second révélateur alors que chacun des points magnétisés dont l'intensité d'aimantation était égale à J₁ est recouvert d'une seule couche du premier révélateur, ces deux révélateurs formant ainsi une image de poudre en deux couleurs sur la surface d'enregistrement.

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 surface in a direction perpendicular thereto to form on this surface a set of magnetized points constituting a latent magnetic image, drive means for causing a relative displacement between the recording element and the magnetic heads, a source of pulses established for selectively sending electrical pulses to said heads, an applicator device making it possible to deposit a powdery developer on said recording surface, this developer remaining applied only to the points magnetized from this surface to form a powder image, and a transfer station ert for transferring said image of powder onto a printing medium, said machine being characterized in that, said developer, said first developer, comprising particles whose hue is one of the two colors previously chosen, it further comprises :

• - means for calibrating and reversing current interposed between the magnetic heads and the pulse source for, on the one hand selectively reversing the direction of the current of the pulses sent by said source, on the other hand allowing each of these pulses to be selectively adjusted to one of two predetermined amplitude values and thus form on the recording surface a latent magnetic image, the magnetized points of which have magnetization intensities of different amplitudes and of opposite magnetic polarities, the magnetized points intended for the formation of the image parts which, on the support, must appear in one of said colors all having magnetization intensities of the same amplitude J ₁ and of the same direction, the magnetic polarity of these points being opposite to that of the other magnetized points, these other magnetized points all having magnetization intensities of the same amplitude J ₂ such that J ₂ <J ₁ ,
• a first retouching device placed, with respect to the direction of movement of the surface, downstream of said applicator device and established to remove said first developer from magnetized points whose magnetization intensity is less than J _i ,
• - a second applicator device disposed between the transfer station and the first device applicator for depositing a second powder developer on said recording surface, this second developer comprising particles whose color is that of the other of said colors, however having the same physical properties as the first developer, said second developer only remaining applied on the magnetized points of said surface,
• - And a second retouching device placed downstream of said second applicator device and comprising a magnetic field generating device for applying a constant magnetic field to said recording surface, this field being oriented in the opposite direction to the magnetization intensity J ₁ , the amplitude of this field being established to modify the amplitudes of the magnetization intensities of the magnetized points and make them practically equal when these magnetized points pass in front of said second retouching device, so that, after this passage, each of the points magnetized whose magnetization intensity was equal to J ₂ is coated with a single layer of the second developer while each of the magnetized points whose magnetization intensity was equal to J ₁ is covered with a single layer of the first developer, these two developers thus forming a powder image in two colors on the recording surface.

• - Les figures 1A à 1F: 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 à la figure 2,
• - La figure 5: est une vue montrant la disposition de points magnétisés qui ont été formés sur l'élément d'enregistrement pour constituer l'image magnétique latente d'un caractère,
• - La figure 6: montre des courbes illustrant la façon dont varie la force d'attraction magnétique exercée par chaque point magnétisé, avant et après l'application du champ magnétique constant qui est utilisé dans le procédé de l'invention;
• - et la figure 7: représente la structure du poste de transfert 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 by way of nonlimiting example and with reference to the appended drawings in which:

• - Figures 1A to 1F: illustrate the different phases of the magnetographic printing process according to the invention;
• - Figure 2: shows an embodiment of 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 represented in FIG. 2,
• FIG. 5: is a view showing the arrangement of magnetized points which have been formed on the recording element to constitute the latent magnetic image of a character,
• - Figure 6: shows curves illustrating how varies the magnetic attraction force exerted by each magnetized point, before and after the application of the constant magnetic field which is used in the method of the invention;
• - And Figure 7: shows the structure of the machine transfer station shown in Figure 2.

FIG. 1A shows, in enlarged section, a magnetic recording element 10, of known type, which can be used for implementing the method of the invention. It will be considered, in the example described, 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 of a material having a high magnetic permeability, such as iron or mild steel, this support being coated with a layer 12 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 one or more recording heads 13 of the type which has been represented in FIG. 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 poles 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 directly above 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 a recording pole 15 is very small compared to that D of the flux closing pole 16. Under these conditions, if an electric current is sent to winding E intensity

I, this current creates, inside the magnetic core 14, a magnetic flux whose line of average force is represented by the line of dashed lines 17. In the portion of the magnetic layer 12 which is in the region 100 of the recording pole 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 takes place transversely. In this portion, the magnetic field created by the head 13 causes the appearance of a practically point magnetized zone usually called magnetized point, this magnetized zone 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 in the region 101 of the flux closure pole 16, because the width of this pole 16 is much greater than that of the recording pole 15, the value of the magnetic field created by the head 13 is much less than that of the saturation field of the magnetic layer 12, so that this closing pole flux 16 can neither cause the formation of a magnetized zone 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 latent 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 rows 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 transverse 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 image of the recording element is then developed by depositing on the surface of the magnetic layer 12 a powdery developer comprising finely divided particles each formed from an organic thermoplastic resin in which have incorporated a pigment and magnetic particles. 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 whose 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 both of two previously chosen colors, the following method is used, the different phases of which will now be described with reference to Figures 1A to 1E.

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 magnetization intensities of different amplitudes and magnetic polarities opposite. In FIG. 1A, only two of these zones, referenced A1 and A2, 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 and J ₂ of the areas A1 and A2, each of these magnetizations being represented by an arrow whose length is proportional to the value of this magnetization. It can then be seen in FIG. 1A, that the magnetizations J ₁ and J ₂ of the zones A1 and A2 have different values and that they are oriented in opposite directions to each other, so that the magnetic polarity that presents zone A1 on the surface of the recording element 10, that is to say here a south polarity (S), is opposite to that which zone A2 has on this same surface, that is to say say here a north polarity (N). It can also be seen that, in the example illustrated in FIG. 1A, the magnetization J _{1 of the} zone A1 is greater than the magnetization J ₂ of the zone A2. In general, the magnetized zones which are intended to form, on the printing medium, images or parts of images which must appear in one of the two colors chosen have magnetizations which all have the same value J ₁ and which are all oriented in the same direction. However, the magnetization of these zones is oriented in the opposite direction to the magnetizations of the other magnetized zones which are intended to form, on this support, images or image parts which must appear in the other color. The magnetizations of these other zones all have the same value J ₂ , J ₂ being different from J ₁ . Thus the areas A1 and A2 which have been shown in FIG. 1A and which have magnetizations of different values and of opposite directions, are intended to form on the printing paper two punctual spots of different colors. We will consider, for example, that the magnetized area A1 is intended to form a point spot of red color and that the magnetized area A2 is intended to form a point spot of black color. To form the two magnetized zones A1 and A2, it is possible to use two identical recording heads, of the type which has been represented in FIG. 3, and to excite these two heads with currents of different intensities and such that the direction in which circulates the current in the winding of the first head is inverse to that in which the current flows in the winding of the second head.

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 tint of which is one of the two colors 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 revealing that on these areas magnetized, for example by arranging the recording element so that, when this developer is applied to this element, each developer particle is subjected, on the one hand to the action of the attraction force magnetic exerted by the magnetized zones, on the other hand to the action of a gravity force oriented in opposite direction to this magnetic attraction force, each of these magnetized zones is then coated, as shown in FIG. 1B, of a layer 18 of the first developer, the thickness of this layer being all the greater the higher the magnetic attraction force exerted by the magnetized zone on which this layer is deposited. It should be pointed out that the force with which each of the developer particles which have been deposited on the same magnetized zone is attracted depends not only on the value of the magnetization J of this zone and on the distance h which separates each particle. of this zone, but also of the physical characteristics of this developer, such as the particle size state and the percentage of magnetic particles of this developer. For a given developer, the magnetic force exerted on a developer particle which has been deposited on a magnetized area is all the greater the higher the value of the magnetization J of this area. Furthermore, this magnetic attraction force is all the smaller the greater the distance h which separates this particle from this zone, the variations of this magnetic force Fm as a function of this distance h being illustrated by the curves which have have been shown in FIG. 6. In this FIG. 6, the dashed line curve 81 represents the variations, as a function of h, of the magnetic attraction force exerted by the magnetized zone A1, of magnetization J _i . Likewise, the dashed line curve 82 represents the variations, as a function of h, of the magnetic attraction force exerted by the magnetized zone A2, of magnetization J ₂ . 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 of the magnetized zones A1 and A2, a value of the distance h for which this force of gravity F _G is equal to the force of magnetic attraction exerted by each of these zones, this particular value thus determining the thickness of the developer layer remaining on this area. Since, in the example described, the magnetic force exerted at a given point by zone A1 is greater than that exerted at this point by zone A2, the particular value e _o for which the magnetic force exerted by A1 is equal to F _G is greater than that e ' _o for which the magnetic force exerted by A2 is equal to F _G. FIG. 6 shows that, in the example described, e _o is substantially equal to 87 μm while e ′ _o is substantially equal to 77 μm, these values determining the respective thicknesses of the layers 18 of first developer which have been deposited on the magnetized zones A1 and A2.

The magnetized areas of the recording element having thus been coated with a layer 18 of first developer, this recording element 10 is then subjected to a retouching operation intended, not only to remove the residual particles of first developer remaining outside the magnetized zones of this element, but also to remove all the particles of first developer on all the magnetized zones which, such as A2, have the smaller of the two magnetization values J ₁ and J ₂ . During this retouching operation, which can be carried out by electrostatic or pneumatic 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 , which is exerted 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 F ₁ being chosen so that the straight line of ordinate F ₁ only intersects the curve 81. In FIG. 6, we have also designated by the abscissa of the point of intersection I ₁ of the curve 81 with the straight line of ordinate F _i . It can thus be seen that, for the developer particles which have been deposited on each of the magnetized zones of greater magnetization (such as Al) and which are situated at a distance less than e, the magnetic force exerted by this zone is greater than the retouching force F ,, so that these particles will remain in this area. On the contrary, for the particles which, on each of the magnetized zones of greatest magnetization, are located at a distance greater than e _i , the magnetic force exerted by this zone is less than the retouching force F ₁ , so that these particles will be eliminated from this area. Consequently, on each of the magnetized zones of greater magnetization (such as A1), there will remain 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 magnetized zones of weaker magnetization (such as A2), since the magnetic force exerted by each of these magnetized zones is always less than the retouching force F _i , these particles will be completely removed from these areas. Under these conditions, only the magnetized zones of stronger magnetization (such as Al) will appear coated, as can be seen in FIG. 1C, with a layer of first developer of thickness e ₁ .

When the retouching operation which has just been described is finished, a second powdery developer is then deposited on the surface of the recording element, the color of which is of the other of the two colors chosen, that is to say ie black in the example described. The filing 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 the figure 1 D, 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 is subjected to a second retouching operation, similar to that described above, but the force of which, of constant value F ₂ is less than the force F ₁ of the first retouching operation. This second retouching operation is carried out in the presence of a constant magnetic field, of value H, produced by a magnetic field generating device 56, (FIG. 1E) of known type. The magnetic field H which is produced by this device 56 is oriented in the opposite direction to the direction of the larger of the two magnetizations of the magnetized zones, that is to say in the opposite direction to the direction of the magnetization J _i . Under these conditions, under the effect of this magnetic field H, the magnetization of the magnetized zones of greater magnetization is weakened, while that of the other magnetized zones, such as zone A2, is reinforced. This then results in a reduction in the magnetic attraction force exerted by each of the magnetized zones whose magnetization has been weakened, while the attraction force exerted by each of the magnetized zones whose magnetization has been strengthened is increased . The value of the magnetic field H is chosen in such a way that the magnetic force exerted by each magnetized zone of weakened magnetization then becomes equal to that exerted, under the same conditions, by each magnetized zone of reinforced magnetization, the variations of this force magnetic, as a function of the distance h, being, on the diagram of FIG. 6, represented by the curve in solid lines 80. On the diagram of FIG. 6, the abscissa of the point of intersection has been designated by e ₂ 1 of this curve 80 with the straight line of ordinate F ₂ , F ₂ being the value of the force of the second retouching operation. It is then seen, in FIG. 6, that for the developer particles which have been deposited on each of the magnetized zones and which are situated at a distance greater than e ₂ , the magnetic force exerted by this zone is less than the retouching force F ₂ , so that these particles will be eliminated from this area. On the contrary, for each of the particles which, on each zone, are located at a distance less than e ₂ , the magnetic force exerted by this zone is greater than the force F ₂ , so that these particles will remain on this zone. Consequently, on each of the magnetized zones of the recording element 10, there will remain a layer of developer having practically a thickness equal to e ₂ . Given that, as can be seen in FIG. 6, this thickness e ₂ is smaller than that e ₁ which was mentioned above, each of the magnetized zones which, originally, had the strongest magnetization will appear coated a layer 18, of thickness e ₂ , of the first developer, while each of the magnetized zones which, originally, had the weakest magnetization will appear coated with a layer 19, of thickness e ₂ , of the second developer. This is shown in Figure 1 E where the magnetized area A1 is covered with a layer 18. of the first developer and where the magnetized area A2 is covered with a layer 19 of the second developer. It should also be noted here that, with the exception of their color, these two developers have practically the same physical properties, in particular the same particle size state, the same saturation induction, the same density and the same melting point.

Each of the magnetized zones of the recording element 10 being thus coated with a layer of one or the other of the two developers, there is then brought, as shown in FIG. 1F, a strip of paper 20, intended to be printed, in the immediate vicinity of this recording element and this recording element 10 is subjected to the action of a constant magnetic field produced by a magnetic field generating device 21, analogous to the device 56 of which we have spoken upper. By immediate proximity is meant that the distance which separates the paper strip 20 from the surface of the recording element 10 is at most equal to one millimeter. The magnetic field which is produced by the device 21 has the same amplitude H and the same direction as that which is produced by the device 56 so that, under the action of this field, the magnetic force exerted by each of the magnetized zones of the recording element 10 varies, as a function of the distance h, in the manner which, in the diagram of FIG. 6, is illustrated by the curve in solid lines 80. While the recording element 10 remains thus subjected to the action of the magnetic field produced by the device 21, the paper strip 20 is then brought into contact with this recording element, then each of the particles of one or the other developer which are found is subjected on this element to the action of a force, called transfer, which has the effect of urging each of these particles towards the paper strip and thus transferring almost all of these particles onto this strip. This transfer operation can moreover be carried out either by pressing the strip of paper 20 on the recording element 10, or by using electrostatic means. However, we will consider that, whatever the means used. to carry out this transfer operation, the transfer force used during this operation maintains a constant value F _T. In the diagram of FIG. 6 the value F., of this force has been represented. It should be noted here that the value FT of this transfer force is chosen so as to allow all or almost all of the particles on the element 10 to be transferred to the paper strip 20. It is of course possible to use a transfer force whose value F _T is such that the line of ordinate F _T is above the curve 80 and does not intersect this curve . Under these conditions, all of the developer particles are transferred onto the paper strip 20. However, in a preferred embodiment of the method of the invention, the value F _T of the transfer force is chosen so that the ordinate line F _T intersects the above-mentioned curve 80 at a point G whose abscissa e ₃ is close to zero. We will consider, for example, that the abscissa e ₃ of this point G is substantially equal to 3 μm. Under these conditions, almost all of the developer particles which were on the recording element 10 are transferred to the paper strip 20, so that at the end of the transfer operation, there remains, on each of the zones of the recording element 10, a layer of developer whose thickness e ₃ is practically negligible. This way of doing things prevents residual particles of first developer which, after the first retouching operation, have not been removed from the magnetized areas of weaker magnetization (such as A2) are not transferred to the paper strip. 20. As a result, as can be seen in FIG. 1F, the layer 19 which has been deposited on each of the magnetized zones of weakest magnetization of the recording element 10 forms, when it is transferred in almost all on the paper strip 20, a small pile 34 comprising only particles of the second developer. Likewise, the layer 18 which has been deposited on each of the magnetized zones with the strongest magnetization of the element 10 forms, when it is transferred almost entirely on the paper strip 20. a small pile 35 comprising only particles of the first developer.

The developer layers which have thus been transferred to the paper strip 20 are then subjected to a fixing operation intended to definitively fix these layers on the paper. This operation is carried out at a temperature which makes it possible to bring the two developers constituting these layers, at least in the viscous state. This temperature can be chosen, preferably, so as to cause the fusion of two developers, without however presenting the risk of ignition or carbonization of the paper. Advantageously, the developers used can be of the type which have been described in the French patent application which was filed on March 20, 1980 by the Applicant and published under No. 2,478,839. These developers have the advantage of melting at a temperature below 140 ° C. Under these conditions, after this fixing operation, each of the heaps 34 forms on the paper a punctual spot having the shade of the second developer, that is to say black in the example described, while each of the heaps 35 forms on the paper a spot spot having the color of the first developer, that is to say red in the example described.

FIG. 2 shows a magnetographic printing machine which performs printing in two colors according to the printing process which has just been described. The machine which is shown in this figure comprises a magnetic recording element in the form of a magnetic drum 10 similar to that which has been described and shown in the aforementioned French patent No. 2,402,921, this drum being driven in rotation, in the direction indicated by arrow R, by an electric motor 25. The magnetization of the magnetic layer of this drum is ensured by a set of n magnetic heads 13-1 to 13-n arranged one next to the other , and aligned parallel to the axis of rotation of the drum, these heads being placed near the magnetic surface of this drum. These heads, which are of the type which has been shown 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 and current inversion 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 of its ends, to the movable contact blade of the '' a respective one of n first reversing contacts K-1 to Kn, and, at the other end, to the movable blade of one respective of n second reversing contacts L-1 to Ln, via one respective of n relay contacts CB-1 to CB-n. Each of the relay contacts CB-1 to CB-n is controlled by a respective one of n relay coils B-1 to Bn. FIG. 4 shows that each of the reversing contacts K-1 to Kn and L-1 to Ln has two positions designated by the numbers 1 and 2. Position 1 of each of the reversing contacts K-1 to Kn is connected to the positive terminal (+) of a first current generator G1, while the position 1 of each of the change-over contacts L-1 to Ln is connected to the negative terminal (-) of this first generator G1. Position 2 of each of the change-over contacts K-1 to Kn is connected to the negative (-) terminal of a second current generator G2, while position 2 of each of the change-over contacts L-1 to Ln is connected to the positive (+) terminal of this second generator G2. FIG. 4 shows that the movable contact blades of the reversing contacts K-1 to Kn are mechanically coupled so that they can be placed simultaneously in the same position. It is the same for the movable contact blades of the change-over contacts K-2 and L-2, ......., Kn and Ln. 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, in fact, connected to a respective one of the outputs S1 to Sn through the 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 that, in the example described, this pulse source has a structure 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 rows and five columns, the rows of this matrix s extending in a direction parallel to 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 among all of the magnetic heads 13-1 to 13-n, this excitation being carried out by means of pulses delivered at successive instants t ₁ , t ₂ , t ₃ , t ₄ , t _s , t ₆ and t ₇ , on five corresponding outputs S1 to Sn of the pulse source 26. Thus, for example, to form, by means of the magnetic heads 13-1 to 13-5, the latent magnetic image necessary for printing the character "G ", the pulse source 26 delivered, at time 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 to S5, at time t ₆ a pulse on each of its outputs S1 to S5, and finally at time t, a pulse on each of its outputs S2 to S4.

The reversing contacts K-1 to Kn and L-1 to Ln and the current generators G1 and G2 are intended to determine the direction of orientation and the amplitude of the magnetizations of the magnetized zones on the drum 10; this direction and this amplitude conditioning the color of the spot which will be subsequently formed on the paper by each of these magnetized zones. For this purpose, the first reversing contacts K-1 to Kn, the second reversing contacts 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 as shown in FIG. 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 respective of the first change-over contacts K-1 to Kn, one respective of the windings E-1 to En, one respective of the relay contacts CB-1, to CB-n , and a respective one of the second change-over contacts L-1 to Ln. In the case where the two change-over contacts of the same circuit portion are placed in position 1, the current which circulates in the winding of the head associated with this circuit portion, during all the time when the relay contact which is in series with this winding is closed, is that delivered by the generator G1, This current, which has an intensity I ₁ , then flows in this winding in the direction which, in FIG. 4, is indicated by the arrow I ₁ . It will be considered, in the example described, that, in the case where the current which circulates in this winding has for intensity I ₁ and for direction that indicated by this arrow 1 ₁ , the magnetization of the magnetized zone which is formed on the drum 10 by the head provided with this winding is oriented in the direction indicated by the arrow J ₁ in Figure 1A and has the same amplitude as that shown in this Figure 1A. Likewise, in the case where the two reversing contacts of this portion of the circuit are placed in position 2, the current which circulates in the winding of the head associated with this portion of the circuit, during all the time when the relay contact which is in series with this winding is closed, is that delivered by the generator G2. This current, which has an intensity 1 ₂ lower than that I ₁ of the current delivered by the generator G1, then flows in this winding in the direction which, in FIG. 4, is indicated by the arrow 1 ₂ . In the example described, we will consider. that, in the case where the current which circulates in this winding has for intensity 1 ₂ and for direction that indicated by this arrow 1 ₂ , the magnetization of the magnetized zone which is formed on the drum 10 by the head provided with this winding is oriented in the direction indicated by the arrow J ₂ in FIG. 1A and has the same amplitude as that shown in this FIG. 1A.

Since the intensity and the direction of current flow in each winding are determined by the position of the change-over contacts of the portion of the circuit which is associated with this winding, we can see that by properly positioning the change-over contacts K-1 at Kn and L-1 at Ln before the source 26 delivers pulses on its outputs, we will obtain on the drum 10, when these pulses are sent, magnetized zones whose magnetization will be oriented in the desired direction and will have the desired amplitude. Thus, for example, if it is desired to obtain on the drum 10, by means of the heads 13-1 to 13-5, a latent image having the configuration of a character of the type of that illustrated by FIG. 5, this latent image being such that the magnetized zones which constitute it have a magnetization oriented in the direction of the arrow J1, it suffices, before the magnetic heads 13-1 to 13-5 are excited, to place the reversing contacts K-1 to K- 5 and L-1 to L-5 in position 1. If on the contrary, it is desired that these magnetized zones have a magnetization oriented in the direction of the arrow J2, it suffices, before the heads 13-1 to 13-5 do not are excited, place the reversing contacts K-1 to K-5 and L-1 to L-5 in position 2. The positioning of the reversing contacts K-1 to Kn and L-1 to Ln on one or the other of positions 1 and 2, can be carried out, either manually by the operator before any printing operation, or fully automatically, the changeover contacts K-1 to Kn and L-1 to Ln being, in the latter case, controlled by actuation means of known type, excited by the same control unit as that which controls the operation of the pulse source 26. It should also be noted that, depending on the case and applications , some of the reversing contacts K-1 to Kn and L-1 to Ln can be placed in position 2 while the other reversing contacts are placed in position 1, which allows, when printing a line of characters by example, to obtain characters printed in one of the two colors, while the other characters of this line are printed in the other color.

It should be noted that the current calibration and reversing device 27 which has been represented in FIG. 2 is constituted, in the example described, by the assembly comprising the generators G1 and G2, the reversing contacts K-1 to Kn and L-1 to Ln, the coils B-1 to Bn and the contacts CB-1 to CB-n, all these elements being connected together as illustrated in FIG. 4.

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. It will be considered, in the example described, 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, 1 thickness of which is, on the magnetization zones J ₁ , close to 87 μm and, on the magnetization zones J ₂ , close to 77 pm. Preferably this applicator device 40 is of the type of cees 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 of the tank 49 to the vicinity of the surface of the drum 10, on the other hand a deflector interposed between this element and the drum to constitute a trough in which the particles collected by the deflector come to accumulate, this deflector leaving between it and the drum a very small opening of the order of 1 millimeter through which pass particles which have come to be applied 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 retouching device 41 which on the one hand makes it possible to remove the particles of developer remaining on the drum 10 outside the magnetized zones, on the other hand to remove the first developer on the magnetized areas of magnetization J ₂ . The retouching device 41 which is used for this operation can be of the 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 shown in French patent N ° 2,411,435 and that it is adjusted so as to leave subsist on each of the zones. magnetized with magnetization J ₁ of the drum 10 a layer of first developer whose thickness is practically equal to 40 μm. The magnetized zones of the drum 10 which have undergone this retouching operation then pass in front of a second applicator device 42, of a type similar to that of the first applicator device, this second applicator device 42 making it possible to deposit particles on the drum 10 of a second powder developer which, being black in the example described, is contained in a reservoir 50. This second applicator device is established for, on the one hand, depositing on each of the magnetized magnetization zones J ₂ of the drum 10 a layer second developer having a thickness substantially equal to 77 µm, on the other hand depositing on each of the already deposited layers of first developer a second layer of second developer, the thickness of this second layer being of the order of 50 µm. Since, in the example described, the thickness of the layer of first developer is practically equal to 40 μm, the total thickness of the two layers is therefore of the order of 90 μm. The magnetized zones of the drum 10 which are then coated with these layers of developers then pass in front of a magnetic field generating device 56 and in front of a second retouching device 43 similar to the retouching device 41. The magnetic field generating device 56 is made up, in the example described, by a permanent magnet and it is arranged so that the magnetic field which it generates is oriented in the opposite direction to the magnetization J _i . This device 56 is also established so that the amplitude H of this magnetic field meets the conditions which have been explained in detail above. The second retouching device 43, in cooperation with this magnetic field generating device 56, makes it possible to reduce the thicknesses of the developer layers which are then located on the magnetized zones of the drum 10, and it is adjusted so that, after the the retouching operation carried out by this device 43, each of the magnetized zones of the drum is coated with a layer having a thickness practically equal to 35 μm, this layer then comprising only particles of first developer when it is on a zone d magnetization J ₁ and only particles of second developer when it is on a magnetization zone J ₂ .

The magnetized zones of the drum 10 which have undergone this second retouching operation then arrive in the immediate vicinity of a strip of paper 20 onto which almost all of the two developers which have been deposited on the surface of the drum 10 will be transferred. To this end, the machine which has been shown in FIG. 2 comprises a transfer station which, in the example described, comprises a roller 45 over which the paper strip 20 passes. The roller 45 is a pressure roller which makes it possible to apply the paper strip 20 to the drum 10 with a determined value force which in general does not exceed 600 newtons per linear meter. In the example described, this force has been adjusted by springs which have not been shown in the drawings, for reasons of simplification. In the more particularly advantageous embodiment which has been illustrated in FIG. 7, the transfer station further comprises a guide roller 44 which is disposed upstream of the roller 45 with respect to the direction of travel of the drum and the strip of paper, and which makes it possible to bring the paper strip 20 in the immediate vicinity of the surface of the drum 10, a little before this strip is applied against this surface. FIG. 7 indeed shows that the point T where the strip 20 comes into contact with the drum 10 is located between the rollers 44 and 45. The machine shown in FIG. 2 also has a second magnetic field generating device 21 which is arranged at the level of the transfer station, that is to say in the immediate vicinity of the roller 45.

In the example described, this device 21 is constituted by a permanent magnet, but it should be noted however that this magnet could be replaced by any other equivalent device, for example by a magnetic induction coil excited by a direct current. This second generator device 21 is established so that the magnetic field H that it produces is oriented in the same direction and the same amplitude as the magnetic field generated by the first magnetic field generator device 56. Under these conditions, when transfer, the magnetization intensities of the magnetized zones subjected to the action of this magnetic field all take the same value, which makes it possible to transfer onto the paper strip 20, layers all having the same thickness. Preferably, the force with which the roller 45 presses the strip of paper 20 on the drum 10 is adjusted in such a way that the layers of first and second developers which were deposited on the magnetized zones of the drum 10 are transferred almost entirely on the strip of paper 20.

In the embodiment illustrated in FIG. 7, the second magnetic field generating device 21 is preferably placed between the guide roller 44 and the aforementioned point T, but close to this point T. It has in fact been found that this arrangement allows to improve the efficiency of the transfer and the quality of the image formed on the paper during this transfer.

The machine which has been shown in FIG. 2 also includes a developer fixing device 46 under which the paper strip 20 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, on the strip of paper 20. It should be noted here that this device fixing 46 is adjusted so that these developers undergo a clear melting without causing an ignition or even a deterioration of the paper strip 20, the melting temperature of these developers being, in the example described, lower than 140 ° C. . Under these conditions, each pile as 34 forms when it cools down on the paper a spot spot having the shade of the second developer, while each pile as 35 forms, when cooling on this paper, a point spot having the shade of the first 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 able to be magnetized when they become then present in front of the set of 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 implemented in the context of the claims which follow.

Claims (9)

1. A magnetographic printing process consisting in magnetising the surface of a magnetic recording element along a direction perpendicular to this surface, in such a manner as to form a group of magnetised dots establishing a latent magnetic image, in thereupon depositing on to this surface a pulverulent developer arranged to remain adhering only to the magnetised dots of the said surface and thereby forming a powder image, and in finally transferring this powder image on to a print carrier, the said process being characterised in that in order to enable producing on this carrier an image in two previously selected colours, it consists in:

-initially magnetising the surface of the recording element to form magnetised dots having magnetisation intensities of different amplitudes and opposite magnetic polarities, the dots intended to form image portions which on the carrier are intended to appear in one of the said colours all having magnetisation intensities of identical amplitude J₁ and of identical direction, the magnetic polarity of these dots being opposite to that of the other magnetised dots, these other magnetised dots all having magnetisation intensities of identical amplitude J₂ such that J₂<J_l,

-depositing on to this surface a first pulverulent developer of which the shade is that of a first one of the said colours,

-eliminating this first developer from this surface except from the magnetised dots of which the magnetisation intensity is equal to J₁,

-depositing on to this surface a second pulverulent developer of which the shade is that of the second of the said colours, whilst however having the same physical properties as the first developer,

- partly eliminating this second developer from this surface whilst exposing this latter to the action of a constant magnetic field oriented in the direction opposite to the magnetisation intensity J_i, the amplitude of this magnetic field having a value such that the magnetisation intensities of the magnetised dots are modified and assume practically the same value, so that this second developer is eliminated from this surface except from the magnetised dots of which the magnetisation intensity was equal to J₂, each of these latter dots thus being coated with only one layer of the said second developer, whereas each of the other magnetised dots is coated with only one layer of the said first developer,

- and finally performing a transfer of all these layers of developer on to the print carrier.

2. A printing process according to claim 1, characterised in that the transfer of the layers of developer on to the print carrier is performed in the presence of a second magnetic field which has the same direction and the same amplitude as the first magnetic field applied during the operation for elimination of the second developer.

3. A magnetographic printing machine for carrying out the process according to claim 1, comprising a recording element (10) provided with a magnetic recording surface, a plurality of magnetic heads (13-1 to 13-n) controlled by means of electrical pulses and arranged so that in response to these pulses, they magnetise the said recording surface in a direction perpendicular to the same, so that a group of magnetised dots (A) constituting a latent magnetic image is formed on this surface, driving means (25) for causing a relative displacement between the recording element and the magnetic heads, a source (26) of pulses for selective transmission of electrical pulses to the said heads, an applicator device (40) whereby it is possible to deposit 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, and a transfer station (45) for transferring the said powder image on to a print carrier (20) the said machine being characterised in that the said developer, referred to as the first developer, comprising particles of which the shade is that of one of the colours previously selected, it also comprises:

- current calibrating and inverting means (27) interposed between the magnetic heads (13-1 to 13-n) and the source (26) of pulses, for the purpose on the one hand of selectively reversing the direction of the current of the pulses transmitted by the said source, and on the other hand of permitting each of these pulses to be adjusted selectively to one of two predetermined amplitude values and of thus forming on the recording surface a latent magnetic image of which the magnetised dots have magnetisation intensities of different amplitudes and of opposite magnetic polarities, the magnetised dots intended to form portions of images which are scheduled to appear on the carrier in one of the said colours all having magnetisation intensities of identical amplitude J₁ and of identical direction, the magnetic polarity of these dots being opposite to that of the other magnetised dots, these other magnetised dots all having magnetisation intensities of identical amplitude J₂ such that J₂<J,,

- a first retouching device (41) positioned, with respect to the direction of displacement of the surface, downstream from the said applicator device (40) and arranged to remove the said first developer from the magnetised dots of which the magnetisation intensity is lower than J_i,

- second applicator device (42) situated between the transfer station (45) and the first applicator device (40) for depositing a second pulverulent developer on the said recording surface, this second developer comprising particles of which the shade is that of the other of the said colours whilst however having the same physical properties as the first developer, the said second developer remaining applied only on the magnetised dots of the said surface,

- and a second retouching device (43) placed downstream of the said second applicator device (42) and comprising a magnetic field generator device (56) for applying a constant magnetic field to the said recording surface, the amplitude of this field being established so as to modify the amplitudes of the magnetisation intensities of the magnetiseci dots and to make all these practically equal when these magnetised dots pass in front of the said second retouching device (43), so that after this traversal, each of the magnetised dots of which the magnetisation intensity was equal to J₂ is coated with a single layer of the second developer, whereas each of the magnetised dots of which the magnetisation intensity was equal to J₁ is coated by a single layer of the first developer, these two developers thus forming a powder image in two colours on the recording surface.

4. A printing machine according to claim 3, characterised in that the magnetic field generator device (56) is formed by a permanent magnet.

5. A printing machine according to either of the claims 3 and 4, characterised in that, each magnetic head comprising a coil (E) wound around a core (14), the current calibrating and reversing means (27) comprise:

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

+ a relay contact (such as CB-1) connected in series with the coil of the associated head 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 having two positions (1, 2) 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) having two positions (1, 2) 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 contact (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,

- and two current generators (G1, G2), the first (G1) of the said current generators having its positive terminal (+) connected to the first (1) of the two positions of each of the n first switches (K-1 to K-n) and its negative terminal (-) connected to the first (1) of the two positions 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 two positions of each of the n second switches (L-1 to L-n) and its negative terminal (-) connected to the second (2) of the two positions of each of the n first switches (K-1 to K-n), each of these two generators being arranged so that when the relay contacts (CB-1 to CB-n) are closed selectively in response to the pulses transmitted by the source (26), it delivers the respective one of two currents of the respective intensities 11, 12, these intensities 11 and 12 being set to engender the formation in the recording element of magnetised dots having the corresponding magnetisations J₁ and J₂.

6. A printing machine according to any one of claims 3 to 5, characterised in that it also comprises a second magnetic field generator device (21) situated at the level of the transfer station (45) for application of a constant magnetic field to the recording surface during its passage in front of this transfer station, this magnetic field being oriented in the same direction and having the same amplitude as the magnetic field generated by the first magnetic field generator device (56).

7. A printing machine according to claim 6, characterised in that the second magnetic field generator device (21) comprises a permanent magnet.

8. A printing machine according to any one of claims 3 to 7, characterised in that the recording element being formed by a magnetic drum (10), the transfer station comprises on the one hand a presser roller (45) for applying the print carrier

(20) against this drum, on the other hand a guiding roller (44) situated upstream of the presser roller to permit the said carrier to be placed in contact with this drum at a point (T) situated between these two rollers, and in that the second magnetic field generator device (21) is situated substantially at the level of this point.

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