EP0082741B1 - 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 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 received signals, a latent magnetic image on the surface of a magnetic recording element generally having the shape of a rotating drum or an endless belt, this latent 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 process 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 method has been proposed which has been described in US Patent No. 3,965,478 and which consists in forming on the surface of the recording element a plurality of elementary magnetized 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 whose particles 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, 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 large developer particles, but also smaller particles, which naturally causes color change.

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 a relatively short time, high-quality color images. while requiring only 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 la même polarité magnétique, mais des intensités d'aimantation différentes, ces points ayant des tailles différentes, de valeurs L" L₂, L₃..., Lp telles que L_I<L₂<L₃< ... <Lp, chacune de ces valeurs étant associée à l'une respective des p susdites couleurs, les points magnétisés destinés à la formation des images ou parties d'image qui, sur le support, doivent apparaître en une même couleur ayanttous la même taille et la même intensité d'aimantation,
• - à déposer ensuite, sur cette surface un premier révélateur pulvérulent dont la teinte est celle d'une première desdites couleurs,
• - à éliminer ce premier révélateur des points magnétisés dont la taille est supérieure à L₁,
• - à répéter cette opération de dépôt sur cette surface avec, successivement, chacun de (p-1) autres révélateurs pulvérulents de teintes différentes mais de mêmes caractéristiques, physiques, chacune de ces operations 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 la taille est supérieure à, respectivement, L" L₃, ... L_P-1, de sorte que, après la dernière opération de dépôt, chaque point magnétisé de taille L, est recouvert de p couches superposées desdits p révélateurs, chaque point magnétisé de taille L₂ est recouverte de (p-1) couches superposées desdits (p-1) autres révélateurs, et ainsi de suite, chaque point magnétisé de taille Lp étant alors uniquement recouvert d'une seule couche du p ième révélateur,
• - et à effectuer enfin un transfert pratiquement 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 consisting in magnetizing the surface of a magnetic recording element, in a direction perpendicular to this surface so as to forming a set of magnetized points constituting a latent magnetic 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 that, in order to obtain on this medium an image in p previously chosen colors, p being an integer at least equal to 2, it consists of:

• - Magnetize first the surface of the recording element to form magnetized points having the same magnetic polarity, but different magnetization intensities, these points having different sizes, of values L "L ₂ , L ₃ . .., Lp such that L _I <L ₂ <L ₃ <... <Lp, each of these values being associated with a respective one of the aforementioned p colors, the magnetized points intended for the formation of the images or parts of image which, on the support, must appear in the same color having all the same size and the same intensity of magnetization,
• - then depositing on this surface a first powder developer, the color of which is that of a first of said colors,
• - eliminating this first developer of magnetized dots whose size is greater than L ₁ ,
• - repeating this deposition operation on this surface with, successively, each of (p-1) other powdery developers of different colors but with the same physical characteristics, each of these deposition operations being immediately followed, with the exception of the last deposition operation, of a developer elimination operation, the magnetized points concerned by this elimination being, during each respective of these (p-2) elimination operations, those whose size is greater than, respectively , L "L ₃ , ... L _P-1 , so that, after the last deposition operation, each magnetized point of size L, is covered with p superimposed layers of said p revealers, each magnetized point of size L ₂ is covered with (p-1) superimposed layers of said (p-1) other developers, and so on, each magnetized point of size Lp then being only covered with a single layer of the p th developer,
• - And finally to carry out a practically total transfer of all these layers of developers on the printing medium in order to form on the latter a powder image made up of a plurality of punctual spots, each of these spots resulting from the transfer of the layers 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 générateurs de fréquences placées sous la contrôle de la source d'impulsions et établis pour, en réponse à chacune des impulsions délivrées par cette source, exciter sélectivement les têtes magnétiques et permettre à chacune de ces têtes d'être excitée par l'un de p courants continus périodiques de même intensité, ces p courants ayant des fréquences respectives différentes de valeurs f_i, f₂, ..., f_P et former ainsi sur la surface d'enregistrement une image magnétique latente dont les points magnétisées présentent la même polarité magnétique, mais des intensités d'aimantation différentes, ces points ayant des tailles différentes de valeurs L₁, L₂, ..., Lp telles que L,<L₂< ... Lp, chacune de ces valeurs étant associée à l'une respective des p susdites couleurs, les points magnétisés destinés à la formation des parties d'images qui, sur le support doivent apparaître en une même couleur ayant tous la même taille,
• -(p-1) autres dispositif applicateurs répartis le long du trajet suivi par la surface d'enregistrement au cours de sont 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 réspective desdites couleurs autres que la première couleur ces p révélateurs ayant cependant les même 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 la taille est supérieure à L" le deuxième dispositif de retouche étant établi pour retirer le deuxième révélateur des points magnétisés dont la taille est supérieure à L₂, 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:

• - frequency generator means placed under the control of the pulse source and established for, in response to each of the pulses delivered by this source, selectively excite the magnetic heads and allow each of these heads to be excited by the one of p periodic continuous currents of the same intensity, these p currents having respective frequencies different from values f _i , f ₂ , ..., f _P and thus forming on the recording surface a latent magnetic image whose magnetized points have the same magnetic polarity, but different magnetization intensities, these points having different sizes of values L ₁ , L ₂ , ..., Lp such that L, <L ₂ <... Lp, each of these values being associated with a respective one of the p above colors, the magnetized points intended for the formation of the parts of images which, on the support must appear in the same color all having the same size,
• - (p-1) other applicator devices distributed along the path followed by the recording surface during their 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 for tint one perspective of said colors other than the first color these p developers having however the same physical characteristics,
• - (p-1) retouching devices each placed downstream, relative to the direction of movement of the surface, of each respective of said devices applicators, with the exception of the last, the first of these retouching devices being established to remove the first developer of the magnetized dots whose size is greater than L ″ the second retouching device being established to remove the second developer of the magnetized dots whose the size is greater than L ₂ , 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 selon 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 les images magnétiques latentes de deux caractères différents, 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 à 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 non-limiting example, and with reference to the appended drawings in which:

• Figures 1A to 1G; illustrate the different phases of the magnetographic printing process according to the invention,
• FIG. 2: represents an embodiment of a printing machine which implements the printing method according to 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,
• Figure 4: shows a diagram of the electrical circuits used to control the different magnetic recording heads of the machine shown in Figure 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 images of two different characters, 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. 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 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 cell 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 closure 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 respectively below the poles 15 and 16 form a closed magnetic circuit. It should be noted here, although in the case illustrated in FIG. 3, the poles 15 and 16 are located near the surface of the magnetic layer 12, another arrangement could be adopted in which these two poles are placed in contact with 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 located 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 field 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 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 two sets of magnetized zones in each of which the magnetized zones A are arranged in a rectangular matrix comprising seven rows and five columns. In the first of these two together, the magnetized zones are distributed inside the rectangular matrix so as to constitute the image of the character "U". In the second set, the magnetized zones are distributed, inside the rectangular matrix, so as to constitute the image of the character "H". We can notice, in fig. 5, that the magnetized zones of these two sets are in the form of rectangles all having the same longitudinal dimension M, but that the size (that is to say the transverse dimension L ₁ , of the magnetized zones of the first set is greater than that L ₂ of the magnetized zones of the second set. However, it should be noted that the size of the magnetized zones is such that it is less than the spacing pitch P of the rows and columns of the rectangular matrix. the magnetizations presented by two neighboring magnetized zones have 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 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 p colors, previously chosen, p being an integer at least equal to 2, the following method is used, the different of which phases 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 whose magnetizations are all oriented in the same direction but have different values. 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. It can be noted that, according to a characteristic of the present invention, these three areas A1, A2 and A3 do not all have the same size, that is to say the same dimension, the size L ₂ of the magnetized area A2 being, in the example illustrated by FIG. 1A, greater than the size L, of the magnetized zone A1, but less than the size L ₃ of the magnetized zone A3. However, it can be seen, in FIG. 1A where the north (N) and South (S) magnetic polarities have also been represented, as well as the magnetizations J ₁ , J ₂ , J ₃ of these magnetized zones, that these three zones A1 , A2 and A3 have magnetizations which are all oriented in the same direction, but which have different values, each of the magnetizations of these zones being in fact represented by an arrow whose length is proportional to the value of this magnetization. It should be noted that, in the most general case where the magnetized zones of the recording element are intended to form, on the printing medium, images comprising p different colors the sizes of these magnetized zones are established, d 'a way which will be indicated later, to be equal to one of p values of different sizes L "L ₂ , L ₃ ...., Lp each of these p values corresponding to one respective of the p colors of However, the magnetized zones which are intended to form on the support of images or parts of image having the same color are all of the same size and have the same intensity of magnetization. Thus, for example, the magnetized zones A1, A2, and A3 which have been shown in Fig. 1A and which have different sizes are intended to form on the printing paper three punctual spots of different colors. magnetized A1 is intended to form a ta che spot red, that the magnetized area A2 is intended to form a spot spot black and that the magnetized area A3 is intended to form a spot spot yellow. It will be considered, on the other hand, that in the example illustrated by FIG. 1A, the size L ₂ of the magnetized zone A2 is substantially equal to 1.6, L "L, being the size of the zone A1 and that the size L ₃ of the magnetized zone A3 is substantially equal to 2L _1. To form these three magnetized zones A1, A2 and A3, three identical recording heads of the type of that shown in FIG. 3 can be used, and excite these three heads with each of three electric currents of the same intensity, these currents flowing in the same direction, but periodically, at respective frequencies f _i , f ₂ , f ₃ such as the size L ₂ of the magnetized zone formed by the head which is excited by the current of frequency f ₂ is equal to 1.6 times the size L, of the magnetized zone formed by the head which is excited by the current of frequency f ₁ , is that the size L ₃ of the magnetized zone formed by the head which is excited by the current of frequency f ₃ is equal at twice this size L ,. It is known, in fact, that the smaller the frequency of the current used to excite this head, the smaller the size of a magnetized area formed by a magnetized head on the recording element.

The recording element 10 having been magnetized in the manner just indicated, a first powdery developer is then deposited on the surface of this element, the color of which is one of the 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 one arranges to leave this developer remaining that on these magnetized areas, for example by arranging the recording element so that, at the time of the application of developer on this element, each particle of developer is subjected, on the one hand to the action of force of magnetic attraction exerted by the magnetized areas, on the other hand to the action of a gravity force oriented in opposite direction to this magnetic attraction force, each of these magnetized areas is then coated, as shown in the FIG. 1B, of a layer 18 of first developer, the thickness of this layer being all the more important as the magnetic attraction force exerted by the magnetized zone on which this layer is deposited is higher. 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 geometrical dimensions of this zone, as well as of the physical characteristics of this developer, such as the granulometric state and the percentage in magnetic particles of this developer.

It has moreover been established that the magnetic force which is exerted on a particle of developer placed near the center of a magnetized zone was all the greater the smaller the size of this magnetized zone. When this particle's moves away from the center of this zone, in a direction perpendicular to the surface of this zone, this magnetic force decreases as the distance h which separates this particle from this center increases, the variations of this force magnetic F _m as a function of this distance h being illustrated by the curves which have been represented in FIG. 6. In this figure, the curve in dashed lines 71 represents the variations, as a function of h, of the magnetic force exerted by an area magnetized small, such as zone A1, the curve in solid lines 72 represents the variations, as a function of h, of the magnetic force exerted by a magnetized zone of medium size, such as zone A2, and the curve in lines 73, represents the variations, as a function of h, of the magnetic force exerted by a large magnetized area, such as the area A3. FIG. 6 shows only the three curves corresponding to the three sizes L "L ₂ and L ₃ of the zones which are intended to form, on the printing medium, an image comprising, in the example described, three different colors. However, we can easily understand that, in the most general case, there are as many curves as colors in the image to be printed, that is to say p curves if this image has p colors, each of these p curves corresponding to one of the p size values L "L ₂ , L ₃ , ... Lp of the magnetized zones. FIG. 6 also shows the value F _G of the force of gravity mentioned above, this force being exerted on each particle of developer during the application of this developer on the surface of the element d 'recording. We then see that there exists, for each magnetized area 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 area, this particular value thus determining the thickness of the developer layer remaining on this area. Thus, in the example described, this particular value, for each of the magnetized zones A1, A2 and A3 is substantially the same because, for large values of h, the three curves 71, 72 and 73 which are represented in Figure 6 are very similar. In the example illustrated in FIG. 6, this particular value, indicated by e _o , is close to 100 μm, this value thus being that of the layer of first developer deposited on each of the magnetized zones A1, A2 and A3.

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 size is greater than the smallest of the sizes of the magnetized zones, these magnetized zones being, in the example described, the zones A2 and A3 whose respective sizes L ₂ and L ₃ are each greater than the size L, of the zone A1. During this retouching operation, which can be carried out by magnetic, electrostatic or pneumatic means (suction or blowing of air), each developer particle 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, of this force has been represented, this value being chosen so that the straight line of ordinate F, intersects only the curve representing the variations as a function of the distance h of the force magnetic exerted by the magnetized zones having the smallest size, this curve being, in the example described, the curve 71. In the diagram of FIG. 6, the abscissa of the point of intersection l has also been represented by e ₁ , of curve 71 with the straight line of ordinate F ,. We can see that, for developer particles which have been deposited on each of the smaller magnetized zones (such as A1) and which are located at a distance less than e _i , the magnetic force exerted by this zone is greater than the retouching force F _i , so that these particles will remain in this area. On the contrary, for the particles which have been deposited on each of the smaller magnetized zones and which are located at a distance greater than e ₁ , the magnetic force exerted by this zone is less than the retouching force F, so that these particles will be removed from this area. Consequently, on each of the smaller magnetized zones, there will remain a layer of first developer having practically a thickness equal to e _i . 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 _i , these particles will be totally eliminated from these areas. Under these conditions, only the smaller magnetized zones (such as A1) will appear coated, as can be seen in FIG. 1C, with a layer of first developer of thickness e ,.

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 areas of smaller size, such as the area 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 strength, of constant value F ₂ , is less than the strength 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 71 and 72 which represent, respectively, one the variations, as a function of the distance h, of the magnetic force exerted by the magnetized zones of smaller size (that is to say of size L,), the other the variations , as a function of h, of the magnetic force exerted by the magnetized zones whose size has, in the series of successive size values L ₁ , L ₂ L ₃ , ..., Lp, the value L 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 zones of size L, is equal to the force F ₂ , and that there exists also a value e ' ₂ of the abscissa h for which the magnetic force exerted by each of the magnetized zones of size L 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 pile constituted by the superposition of layers 18 and 19 on each magnetized areas of size L ₁ , and that this value e ′ ₂ represents the thickness of the layer 19 of the second developer on each of the magnetized areas of size L 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 powdery developer is therefore the last to be deposited on the recording element 10. It will be considered here that this third powdery 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 size is less than L 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 , brought into operation. work during the second retouching operation. In the diagram of FIG. 6, the value F ₃ of this retouching force has been shown. It should be noted 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 retouching force is intended only on the one hand to limit the thickness of the third developer layers which have thus been deposited, on the other hand to remove the developer particles which remain outside the magnetized zones of the recording element 10, and no longer as retouching forces previous, to remove all of the particles that are on some of these magnetized areas. Consequently, the value F ₃ of this retouching force can be relatively low, while still remaining sufficient to remove the particles remaining outside the magnetized zones. This is why, this last retouching operation; unlike the previous retouching operations, is not a particle removal operation on some of the magnetized areas of the recording element and that it is therefore not, strictly speaking, part of the process of the present invention.

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 same coercive field, 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 size of this zone. When the last developer has been deposited on the recording element and the last retouching operation has been carried out, then, as shown in FIG. 1G, a strip of paper 21 intended to be printed, is in the immediate vicinity of this recording element 10 is in contact with this recording element 10 and the transfer is then made, onto this strip of paper, of the layers of developer which are located on the magnetized zones of this element 10. This transfer can be 'elsewhere be performed, in known manner, either by applying pressure, or by magnetic or electrostatic means. However, the conditions of 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 was on zone A3 is transferred to the strip of paper 21 where it then forms a heap which, made up 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 , which, 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 magnetic drum 10 similar to that which has been described and shown in 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 beside 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 frequency generator assembly 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 stud switches K1 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 the a respective one 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 Bn. 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 source G1 of periodic direct current of frequency f _i , while the input terminal 1 of each second switches L-1 to Ln is connected to the negative (-) terminal of this source G1. The input terminal 2 of each of the first switches K-1 to Kn is connected to the positive terminal (+) of a second source G2 of periodic direct current of frequency f ₂ , while the input terminal 2 of each second switches L-1 to Ln is connected to the negative (-) terminal of this source 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 source G3 of periodic direct current, of frequency f ₃ while the input terminal 3 of each of the second switches L-1 to Ln is connected to the negative (-) terminal of this source 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 applies to the movable contact blades of switches 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, 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 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 printing a character is obtained by selectively exciting seven different times, five contiguous heads taken from among all of the heads magnetic 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 _G and t ₇ , on five corresponding outputs If at Sn of the pulse source 26. Thus, for example, that to form, by means of the magnetic heads 13-1 to 13-5 the latent magnetic image necessary for the printing of the character "G", the pulse source 26 delivers, 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 and S5, and finally at time t ₇ a pulse on each of its outputs S2 to S4.

The switches K-1 to K-n and L-1 to L-n are intended to determine the size of the magnetized areas on the drum 20, this size determining the color of the spot spot which will subsequently be formed on the paper by each of the magnetized areas. To this end, 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 periodically in the winding of the head associated with this portion of the circuit, when the relay contact which is in series with this winding is closed, is that delivered by the source G1, this current having a frequency f,. If these two switches are placed in position 2, the current which circulates periodically, when this relay contact is closed, is that delivered by the source G2, this current having the frequency f ₂ . Finally, if these two switches are placed in position 3, the current which circulates periodically in this winding, when this relay contact is closed, is that delivered by the source G3, this current having the frequency f 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 periodic direct current of frequency f ₃ flows momentarily in the closed circuit constituted by the generator G3, the switch Kn in position 3, the winding At 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 magnetized zone which, having a size L ₃ relatively small, is practically punctual, this zone having a magnetization J which remains after the disappearance of the periodic current of frequency f ₃ in the winding En.

It should also be noted that the sources of currents G1, G2 and G3 are established to deliver periodic direct currents of the same intensity I, but of respective frequencies f ₁ , f ₂ and f ₃ such that, during all the time when the contact of relay CB which is associated with each of the heads is closed, these frequency currents f _i , f ₂ and f3 are capable of forming on the surface of the drum 10 magnetized zones whose size is equal to L ₁ , L _{2 respectively} and L ₃ , the size L ₂ being greater than the size L ₁ , but less than the size L ₃ . It is understood, under these conditions that, if one wants, for example, to obtain on the drum 10, a latent magnetic image whose magnetized zones have a size L ₁ , it suffices, before the magnetic heads necessary for the formation of this image are excited, move the corresponding switches -K and L to position 1. Similarly, if we want the magnetized areas of this image to have a size L ₂ , it suffices, before these heads are excited, to place these switches in position 2. Finally, if we want the magnetized areas of this image to have a size L 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 moreover, be performed either manually by the operator before any printing operation, or entirely automatically by actuation means, of known type, excited by the same control unit as that which controls the operation of the pulse source 26. It is even possible, depending on the case and application, to place some of the switches K-1 to Kn and L-1 to Ln in 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 frequency generator assembly 27 which has been represented in FIG. 2 is constituted, in the example described, by 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 current sources G1 to G3, all these elements being connected together as illustrated in FIG. 4. It should however be noted that , although the frequency generator assembly which is represented in FIG. 4 only comprises three current sources and switches comprising only three positions, this generator assembly, when used in a machine intended for printing of images in p colors, includes p current sources G1, G2, ..., Gp, of respective frequencies f ₁ , f ₂ , ..., f _P and switches K-1 to Kn and L-1 to Ln each comprising p positions. In this case, the frequencies f _i , f ₂ , ..., fp of the respective current sources G _i , G ₂ , ..., Gp are adjusted so as to form magnetized zones of respective sizes L ₁ , L ₂ , ..., Lp as we have; L ₁ <L ₂ <... <Lp.

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 thickness of which is substantially equal to 100 μm. It will be 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 particles of developer from 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 constitute a trough in which the particles collected by the deflector accumulate, this deflector leaving between it and the drum has a very small opening, of the order of 1 millimeter, through which the particles which have come to bear against the surface of this drum pass. 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 particles of sizes L ₂ and L 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 shown in French patent No. 2,141,435 and that it is adjusted so as to leave no residue on the areas of size L "than a layer of first developer having a thickness close to 30 μm. The magnetized zones of the drum 10 which have passed past the retouching device 41 then pass before a second applicator device 42, of a type similar to that of the first applicator device, this second applicator device making it possible to deposit on the drum 10 particles of a second powdery developer which, being black in color, in the example described, is contained in a reservoir 50. This second applicator device 42 is established for deposit a layer of second developer, on the one hand on each of the magnetized areas of sizes L ₂ and L ₃ , the thickness of this layer being substantially equal to 100 μm, on the other hand on each of the layers are already deposited from the first developer, the total thickness of the layers thus superimposed of two developers on the areas of size L, being substantially equal to 100 μm. After which, the magnetized zones of the drum 10 pass in front of a second retouching device 42, similar to the first retouching device 41 and disposed 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 particles from the second developer which have been deposited on the magnetized zones of size L _3, is set so as to leave on the waist regions L _2, a layer of second developer having a thickness in the region of 25 μm and, on the L size zones, a composite layer comprising two superposed layers of first and second developer, the thickness of this composite layer being substantially equal to 40 μm. 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 zones of size L ₃ , the thickness of this layer being substantially equal to 100 μm, as well as on each of the layers of second developer already deposited on each of the areas of size L, and L ₂ , the total thickness of the three layers thus superimposed on the areas of size L, being close of 100 μm, while the total thickness of two layers thus superimposed on the areas of size L ₂ is close to 100 μm. The magnetized areas of the drum which have been thus 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 areas and to limit the thicknesses of the developer layers deposited on magnetized areas. In the example described, this third retouching device 52 is adjusted so that the areas of size L ₃ which have passed in front of this device 52 appear coated with a layer of third developer whose thickness is close to 40 μm, while the areas of size L 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 50 μm. Finally, the areas of size L, which have 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 57 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 45 can be adjusted by known means (not shown) so as to cause a quasi-total transfer of all the layers of developer which remain on the drum 10 after passing in front of the retouching device 52. The value F _T of this force is, as shown in FIG. 6, greater than cell F, which was mentioned above, so that, in the end, almost all of the composite layer which was on each of the magnetized areas of size L is transferred to the paper, thus forming punctual spots having a thickness of the order of 55 μm and having the red color of the first developer. Likewise, almost all of the composite layer which was on each of the magnetized zones of size L ₂ is transferred to the paper, thus forming point spots having a thickness close to 45 μm and having the black color of the second developer. Finally, almost all of the layer of third developer which was on each of the magnetized zones of size L ₃ is transferred to the paper, thus forming punctual spots, of yellow color, having a thickness close to 37 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 fixation 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 punctual task having the shade of the second developer. Likewise, each of the heaps of developers such as 23, when cooled on paper, forms a punctual 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 implemented in the context of the claims which follow.

Claims (4)

1. A magnetographic printing process consisting in magnetising the surface of a magnetic recording element in a direction perpendicular to this surface, in such a manner as to form a group of magnetised dots forming a latent magnetic image, in thereupon depositing on this surface a pulverulent developer arranged to remain adhering only on the magnetised dots of said surface and thus to form 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 permit producing on this carrier an image in p previously selected colours, p being an integer equal to at least 2, it consists in:

- initially magnetising the surface of the recording element to form magnetised dots of identical magnetic polarity but having different intensities of magnetisation, these dots having different sizes, of values L₁, L₂, L₃ ... Lp such that L₁<L₂<L₃... <Lp, each of these values being associated with a corresponding one of the p aforesaid colours, the magnetised dots intended to form images or parts of images which are to appear in an identical colour on the carrier having the same size and the same intensity of magnetisation,

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

-eliminating this first developer from the magnetised dots having a greater size than L₁,

- repeating this depositing operation on this surface with, successively, each of the (p-1) other pulverulent developers of different shades but of identical physical characteristics, each of these depositing operations being followed immediately, except in the case of the last depositing operation, by an operation for eliminating the developer, the magnetised dots affected by this elimination being, during each corresponding one of these (p-2) eliminating operations, those of which the size is greater than, respectively, L₂, L₃ ...L_p-1, so that after the final depositing operation, each magnetised dot having the size L, is coated with p superposed layers of the said p developers, each magnetised dot of the size L₂ is coated with (p-1) superposed layers of the said (p-1) other developers and so on in sequence, each magnetised dot of the size Lp then being coated only by a single layer of the p-th developer;

-and finally performing a practically total transfer of all these layers of developer on to the print carrier so that a powder image comprising a plurality of punctiform spots may be formed on the same, each of these spots resulting from the transfer of the layers of developers, which were deposited on each corresponding one of the said magnetised dots and having the colour of the developer which, just before the transfer operation had been in direct contact at this point with the surface of the recording element.

2. A 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 to form thereon a group of magnetised dots (A) constituting a latent magnetic image, driving means (24) 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 selective transmission of electrical pulses to the said heads, and an applicator device (4) by means of which a pulverulent developer may be deposited 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 that of a first one of p previously selected colours, it also comprises:

- means (27) for generating frequencies placed under the control of the source of pulses (26) and organised for selectively energising the heads (13-1 to 13-n) in response to each of the pulses supplied by this source, and enabling each of these heads to be energised by one of the p periodic direct currents of identical intensity, these p currents having different respective frequencies of the values f" f₂ ... fp, thereby producing on the recording surface a latent magnetic image of which the magnetised dots (such as A1, A2, A3) have the same magnetic polarity but different intensities of magnetisation (J₁, J₂, J₃...), these dots having different sizes of such values L_i, L₂...L_P that L₁<L₂... <Lp, each of these values being allocated to a corresponding one of the p aforesaid colours, the magnetised dots intended to form the parts of images which are intended to appear on the print carrier (21) in one and the same colour or having the same size,

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

- (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) except for the final one (44), the first of these retouching devices (41) being arranged to remove the first developer (18) from the magnetised dots (such as A2, A3) having a size greater than L" the second retouching device (43) being arranged to remove the second developer (19) from the magnetised dots (such as A3) having a size greater than L₂ and so on in sequence,

- and a transfer device (45) situated downstream of the last applicator device (44) for practically total transfer on to the print carrier (21) of the different layers of developers (18,19,20) which cover the magnetised dots of the surface when these dots are moved past in front of the said last applicator device.

3. A printing machine according to claim 2, characterised in that, each magnetic head comprising a coil (E) wound around a core (14), the frequency generator means (27) comprise:

- circuit portions (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 contact (such as CB-1) connected in series with the coil (such as E-1) 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 (26) of pulses;

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

-and p sources of periodic direct current (G1, G2, G3 ...) having the respective frequencies f_i, f₂...f_P the first (G1) of the said current sources 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 sources 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, the frequencies f_i, f₂... fp of these p sources of current being so organised that when the relay contactors (CB-1 to CB-n) are closed selectively, in response to the pulses transmitted by the source (26) of pulses, for forming magnetic areas of the respective sizes L₁, L₂ ... Lp on the recording surface.

4. A printing machine according to claim 2 or 3, characterised in that it also comprises a fixing device (46) arranged along the path followed by the print carrier and downstream of the transfer station (45), this fixing device being adjusted in such a manner that the particles of developer which, whilst deposited on this carrier pass in front of the said fixing device, are exposed to a softening action but not to a melting action.

Images