EP0328428B1 - Device for intermittently applying particles of a pulverulent toner to the recording medium of a magnetographic printer - Google Patents

Abstract

The invention relates to a device for intermittently applying particles of a powdered developer to the recording surface of a magnetographic printer. This device includes a reservoir (17) containing developer particles, a transport element (23) for placing these particles in the vicinity of the surface of a magnetic drum (11), a deflector (24) disposed between this transport element and the drum to apply the particles to the drum surface, and a squeegee (45) disposed between the deflector and the transfer station (H), and actuated by an electromagnet (EA) for selectively pulling away the particles that have been deposited on the drum surface. The invention is applicable to magnetographic printers.

Description

The present invention relates to a device for intermittently applying particles of a powder developer to the recording surface of a magnetographic printer.

Magnetic printing machines are known which, in response to the reception of electrical signals from a control unit, make it possible to form images, such as images of characters for example, on a printing medium generally consisting of a strip or a sheet of paper. In these printing machines, which are analogous to those which have been described and represented in the patents of the United States of America Nos. 3,161,544 and 4,072,957, the printing of the images is carried out by first forming, at from the received signals, 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 recording element being coated with a layer of material magnetic. This magnetic latent image is then developed, that is to say made visible, using a powdery developer which, consisting of particles of thermoplastic resin containing magnetic particles and pigments, is only attracted by the regions of the recording element on which the latent image has been recorded, this developer then forming a powder image on the surface of this element. After which, this powder image is transferred to the print medium.

In order to allow the formation of the latent magnetic image on the surface of the recording element, these machines are provided with a recording member, called a transducer, which has one or more magnetic recording heads near which moves this record item. Each of these heads generates, each time it is excited for a short time by an electric current of suitable intensity, a magnetic field which has the effect of creating, on the surface of the recording element which scrolls past these heads, magnetized domains of small dimensions, these domains, practically punctual, being generally designated by the name of magnetized points.

All of these magnetized points constitute the magnetic latent image. The surface portion of the recording element which thus passes in front of each head is usually referred to as an information recording track, the recording element generally comprising several tracks which can be subjected to the recording, either individually during successive recording operations, or simultaneously during a single operation.

Magnetic printing machines have already been produced in which the transducer has as many magnetic heads as there are tracks on the recording element, these heads being arranged next to each other and being aligned in a transverse direction to the direction of movement of the recording element. Since, in these machines, each track is associated respectively with each of the transducer heads, the recording of a latent image on the recording element is carried out during a single movement of movement of this element the along its endless journey. These machines are thus capable of operating at a high printing rate, this rate being able to reach, for example, a hundred pages per minute. However, for certain applications, such a high rate is not always necessary, so that we can then be satisfied with a magnetographic printer machine less efficient, but also much less expensive, equipped with a transducer comprising a number magnetic heads significantly less than the number of tracks of the recording element. This is how a magnetographic printing machine is known, which has been described in US Patent No. 4,072,957 and in which the transducer has only one magnetic head, this head being mounted so as to be able to move along a magnetic recording drum, in a direction parallel to the axis of rotation of this drum.

In this machine, the recording of a latent magnetic image is carried out track by track, the recording of the information in the track located opposite this head being carried out during a complete revolution of the drum, this head being, at the end of this lap, moved to be brought opposite the next track and allow this last track to be recorded in turn. Under these conditions, the recording of this latent image on the drum is carried out in as many revolutions of the drum as there are tracks on this drum. The revelation of this latent image, that is to say the deposition of developer particles on the drum, is only undertaken when the formation of this image on this drum is finished. This operation is carried out by means of an applicator device, of known type, which, in the machine described in the aforementioned American patent No. 4,072,957, comprises a magnetic cylinder mounted on an axis parallel to the axis of rotation of the drum. , this cylinder, placed close to the surface of the drum, being arranged so as to be in contact with developer particles contained in a reservoir placed below the drum. Thus, when this magnetic cylinder rotates, the developer particles which are entrained by this rotating cylinder are brought in the vicinity of the surface of the drum and, attracted by the magnetized points of this surface, are deposited on the surface portions on which the latent image was formed. The particles thus deposited then pass in front of a transfer roller which is normally biased in abutment against the surface of the drum and are then transferred to a sheet of paper which is engaged, at this time, between the drum and this transfer roller.

In this applicator device, the magnetic cylinder is driven in rotation, not continuously, but only during one rotation revolution of the drum, according to the formation of a latent image on this drum. This avoids that, during the periods of latent image formation where no sheet of paper is engaged between the drum and the transfer roller, developer particles are not deposited on this drum and do not subsequently stain the transfer roller. However, this applicator device, which operates intermittently and makes it possible to apply developer particles to the drum without causing clouds of particles capable of producing pollution inside the machine, is not entirely satisfactory because the drum is at a very short distance from the magnetic cylinder, and the magnetized points which have been formed on this drum are then necessarily subjected, when they pass in front of this cylinder, to the action of the magnetic fluxes generated by this cylinder and thus risk being greatly altered or even erased.

Admittedly, this drawback could be remedied by using an applicator device which has been described in French Patent No. 2,408,462, this device comprising, on the one hand, a reservoir disposed below the recording element and containing developer particles, on the other hand, a transport element arranged to bring these particles in the vicinity of the surface of this recording element, this device further comprising a fixed deflector interposed between this surface and this transport element for collect the particles transported by the latter, this deflector being arranged so as to form with this surface a trough, of substantially prismatic shape in which the particles thus collected accumulate. The particles thus accumulated eventually come into contact with this surface and by being entrained by the latter in the direction of the edge of the prism constituting the bucket, the particles entrained beyond this edge remaining only applied to the magnetized points. formed on this surface.

This applicator device, which does not cause any alteration of the magnetized points and which does not cause any pollution inside the machine, has the disadvantage of not ensuring good revelation of the latent images when the transport element of which it is provided is driven, not continuously, but intermittently.

The present invention overcomes these drawbacks and proposes a device which, mounted in a magnetographic printer in which the recording of each latent image is carried out during several successive turns of movement of the recording element along its path without end, allows to apply, intermittently, developer particles on the surface of this recording element, and this without causing pollution and without disturbing the latent images which have been formed on this recording element.

More specifically, the present invention relates to a device for intermittently applying particles of a powder developer to the recording surface of a magnetographic printer, this surface being driven in displacement along a predetermined closed path enabling it to pass by a post of transfer where the developer which has been deposited on this surface is transferred to a printing medium, this applicator device comprising a member designed to permanently apply developer particles to said recording surface, the recording surface cooperating with a latent image recording device established to form on this surface a latent image during several successive turns of movement of this surface, this applicator device being characterized in that it further comprises a particle eliminator device disposed along said path, downstream of the point of application of the particles on this surface by said applicator device, between this point of application and said transfer station, this eliminator device being established for removing the developer particles which are on this surface, except during the last of said successive turns of movement of this surface.

• . La figure 1 représente une machine imprimante magnétographique équipée d'un dispositif applicateur de particules réalisé selon l'invention,
• . La figure 2 est une vue montrant la structure de l'organe d'enregistrement, ainsi que les organes de commande d'actionnement du dispositif applicateur de particules équipant la machine représentée sur la figure 1,
• . La figure 3 est une vue en coupe montrant le détail de réalisation d'une portion du dispositif applicateur de la machine représentée sur la figure 1,
• . La figure 3A est une vue à grande échelle destinée à montrer le profil de l'arbre d'actionnement de la raclette qui fait partie du dispositif applicateur qui est illustrée sur la figure 3,
• . La figure 4 est un schéma du circuit de commande utilisé pour commander le positionnement de la raclette faisant partie du dispositif applicateur représenté sur la figure 3.

The invention will be better understood and other objects 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:

• . FIG. 1 represents a magnetographic printing machine equipped with a particle applicator device produced according to the invention,
• . FIG. 2 is a view showing the structure of the recording member, as well as the actuation control members of the particle applicator device fitted to the machine shown in FIG. 1,
• . FIG. 3 is a sectional view showing the detail of embodiment of a portion of the applicator device of the machine shown in FIG. 1,
• . FIG. 3A is a large-scale view intended to show the profile of the squeegee actuation shaft which is part of the applicator device which is illustrated in FIG. 3,
• . FIG. 4 is a diagram of the control circuit used to control the positioning of the squeegee forming part of the applicator device shown in FIG. 3.

The printing machine which has been schematically represented in FIG. 1 is a machine which performs the printing of sheets of paper which are successively extracted, in a known manner, from a supply magazine 10.

This machine comprises a recording element which is constituted, in the example described, by a drum 11 provided with a magnetic recording surface 12. This drum 11, which is mounted so as to be able to rotate around an axis horizontal 13, is rotated, in the direction indicated by arrow F, by an electric motor (not shown). The recording of the information on this drum is carried out by a recording device 14, the structure of which will be described a little later. We will consider that this device comprises several magnetic heads. Each of these heads generates, each time it is excited for a short time by an electric current, a variable magnetic field, which has the effect of creating, on the cylindrical surface 12 of the drum which passes in front of these heads, zones magnetized 15, practically pointwise, all of these zones constituting a magnetic latent image corresponding to an image to be printed. These magnetized zones then pass in front of an applicator device 16 which is arranged below the drum 11 and which makes it possible to apply to the cylindrical surface of this drum particles of a powdery developer contained in a reservoir 17. The structure of this device applicator will be described in detail a little later. The developer particles which are thus applied by this device to the drum adhere, in principle, only to the magnetized areas thereof and then form a powder coating on the surface 12 of the drum. A retouching device 18, in front of which this image then passes, makes it possible to remove the developer particles which have adhered elsewhere than on the magnetized zones of the drum, as well as the particles which are found in excess in these zones. It should be noted here that the developer which is thus deposited on the surface 12 of the drum consists of fine particles of thermoplastic resin containing magnetic particles and pigments, this resin being capable of melting when subjected to a heat source and thus fix itself on a sheet of paper onto which this developer has been transferred. After which, the developer particles which, after passing in front of the retouching device 18, remain on the drum 11 are normally transferred, almost entirely, onto a sheet of paper 19 which, after having been extracted from the magazine 10, is applied, by means of a transfer roller 20, against the surface of the drum 11. The region H where this roller 20 comes to contact the surface of this drum, when no sheet is engaged between this roller and this drum, constitutes the transfer station. It is in this station that the transfer takes place, onto a sheet of paper which is engaged between the drum 11 and the roller 20, of the image of powder which has been formed on the surface of this drum. The developer particles which, when this transfer is carried out, still remain on the surface of the drum are then removed by means of a cleaning device 21. The magnetized zones which have passed past the cleaning device 21 then pass in front of a cleaning device. erasing 22, which allows the portions of the drum 11 which have thus been demagnetized by the latter device to be able to be magnetized again when they appear in front of the recording device 14.

The structure of the recording device which equips the machine shown in FIG. 1 is illustrated in FIG. 2. If one then refers to this latter figure, it can be seen that the axis 13 around which the drum 11 rotates is supported at its ends, by two vertical support plates 30 and 31 made integral with one another by means of a transverse connection plate 32. The plates 30 and 31 further support a guide bar 33 which is arranged parallel to the axis 13 of the drum 11. A carriage 34, slidably mounted on this bar 33, can be driven in displacement, step-by-step, in a direction parallel to the axis 13 of the drum, by means of a threaded rod 35 secured to the drive shaft of an electric motor 36 itself fixed on the vertical plate 30. On the carriage 34 are mounted magnetic heads T1, T2, T3, ..., Tn which, arranged at intervals regular, are placed so that they are near immediate mite from the surface 12 of the drum 11.

When the motor 36 is excited, these magnetic heads, driven by the carriage 34, move simultaneously in a direction parallel to the axis 13 of the drum 11. The assembly formed by the carriage 34 and the magnetic heads T1, T2, T3 , ..., Tn can thus move between two positions boundaries, one of which LG is shown in solid lines in FIG. 2 and the other of which LD is shown in dashed lines in this same figure. The surface portion of the drum 11 which passes in front of each of these heads, when the carriage 34 is stationary, is usually designated by the name of track. In FIG. 2, these tracks, which are circular, have been represented in dashes and designated by references such as P11, P12, ..., P16, P21, ..., P26, P31, ... Pn1, .. ., Pn6. For reasons of clarity of the drawing, these tracks have been shown in FIG. 2, in positions relatively spaced from one another. It should however be noted that, in reality, these tracks are very close to one another and that, in the example described, the distance which separates two neighboring tracks is of the order of a hundred micrometers. For recording information on the drum 11, these tracks are used in groups of six tracks, each of these groups being associated respectively with each of the magnetic heads of the recording device 14. This is how the six tracks P11 to P16 are intended to receive the information which is recorded by means of the head T1. Similarly, the six tracks P21 to P26 are intended to receive the information which is recorded by means of the head T2, and so on for the following groups of tracks. The heads T1, T2, T3, ..., Tn are positioned on the carriage 34 in such a way that, when this carriage is immobilized in its limit position LG (on the left in FIG. 2), each of these heads is in opposite the first of the tracks of the group with which it is associated. Thus, in this position, the head T1 is in front of the track P11, the head T2 is in front of the track P21, etc., and the last head Tn is in front of the track Pn1. It is then possible, during the same rotation turn of the drum 11, to record information simultaneously in the tracks P11, P21, P31, ..., Pn1. A clock disk D, fixed to the shaft 13 of the drum 11, is provided with a slot which allows, at each revolution of this drum, to let pass, for a brief instant, a light beam emitted by a light source L and sent to a PH photocell. Each time this slot lets through this light beam, that is to say each time that the drum 11 has completed one revolution, this cell PH delivers an electrical signal to an electrical control circuit, of known type (not shown) , which is established to then control the momentary excitation of the motor 36 and, consequently, the displacement very fast, by one step, from carriage 34. The signal which is sent by this cell PH when the recording in tracks P11, P21, P31, ..., Pn1 is finished, has the effect of bringing the heads T1 , T2, T3, ..., Tn opposite, respectively, tracks P12, P22, P32, ..., Pn2. It is then possible, during a second revolution of rotation of the drum 11, to record information simultaneously in tracks P12, P22, P32, ..., Pn2. The electrical signal which, at the end of this second round, is sent by the PH cell, again causes a displacement of one step of the carriage 34, which brings the heads T1, T2, T3, ..., Tn into opposite tracks P13, P23, P33, ..., Pn3, these tracks being those which, in FIG. 2 are located immediately to the right of tracks P12, P22, P32, ..., Pn2. It is then possible, during a third rotation turn of the drum, to record information simultaneously in these tracks P13, P23, P33, ..., Pn3.

The recording of the information in the following tracks is carried out in the same manner as that which has just been described, the carriage 34 being moved by one step, in the direction of its limit position LD, at the end of each of the rotations of the drum 11. It will then be understood that, with the recording device which is shown in FIG. 2, the recording of a latent magnetic image on the drum requires six complete rotations of the drum. Naturally, during the first five of these six turns, the erasing device 22 is disabled, so as not to erase the portions of latent image which have already been recorded on the drum. This erasing device is only reactivated when the powder image corresponding to this latent image has been transferred to a sheet of paper. As soon as the erasing device 22 has been reactivated, a new latent image can be recorded on the drum 11, this recording being carried out either by moving the carriage 34, step by step from its limit position LD in the direction of its limit position LG, that is to say beforehand bringing this carriage back to its limit position LG and then moving it step by step, in the direction of its limit position LD.

We will now describe, with reference to FIGS. 1 and 3, the structure of the particle applicator device which makes it possible to form the powder image corresponding to the latent image which has been recorded on the drum. during six successive turns of this drum. As can be seen in FIG. 1, this applicator device comprises, on the one hand, a transport element 23 which takes developer particles from the reservoir 17 to bring them in the vicinity of the surface 12 of the drum, on the other hand a fixed deflector 24 which is interposed between the transport element 23 and the drum 11 to collect the particles transported by this element 23 and apply them to the surface of this drum.

The transport element 23 consists, in the example described, of a magnetic cylinder whose axis of rotation 25 is parallel to the axis 13 of the drum 11 and can rotate in two bearings (not shown) which are respectively provided with lateral faces 26 and 27 of the reservoir 17.

The deflector 24, which is shown on a large scale in FIG. 3, is a part made of a non-magnetic material and fixed to the two lateral faces of the reservoir 17. This part has a flat face 40 limited by a first and a second edge 41 and 42 parallel to the axes 13 and 25. The deflector 24 is arranged in such a way that, on the one hand, its first edge 41 is in close proximity to the surface 12 of the drum and that, on the other hand, if one designates by G the generator of the drum where the plane P of the face 40 intercepts the surface 12 of this drum, this plane P forms with the normal plane in G on the surface of the drum an angle A whose value is less than forty five degrees. The distance which separates this generarice G from the first edge 41 of the deflector is always very small. Thus, in the example described, this distance is substantially equal to one millimeter. Furthermore, in the example described, the width of the face 40 is of the order of one centimeter.

The transport element 23 has a direction of rotation, indicated by the arrow R, such that it drives the developer particles towards the face 40 of the deflector. The second edge 42 of this deflector is practically in contact with the surface of the transport element, so that the particles which are entrained by this transport element are mostly stopped in passing by the deflector 24 and then come s '' accumulate in a bucket 43, of substantially prismatic shape, delimited by the cylindrical surface 12 of the drum and by the face 40 of the deflector 24. The direction of rotation F of the drum 11 is such that, when the quantity of particles accumulated in this trough 43 is sufficient to reach the first edge 41 of the deflector, the particles which are located near the surface 12 of the drum are entrained in the direction of the generatrix G which in a way constitutes the edge of the bucket 43. A portion of these particles is then applied to the magnetized zones 15 of the drum. The particles which are thus entrained by the drum are not stopped in passing by the deflector 24 owing to the fact that the latter does not touch the surface of the drum and that it therefore leaves, between its first edge 41 and the drum, a narrow opening, the width of which is however sufficient to allow the developer particles entrained by the drum to exit from the bucket 43. The developer particles which, applied to the magnetized zones of the drum, exit from the bucket 43, continue to adhere to these areas and thus make visible the image which is to be printed, while those which leave the bucket 43 without being retained by the drum fall back into the reservoir 17. Since the distance which separates the edge 41 of the deflector from the surface 12 of the drum is very small, the number of particles which also exit from the bucket 43 is relatively small, so that the particles which, not retained by the drum, fall back into the tank 17 are few and do not form clouds of particles liable to pollute the machine.

According to the invention, the applicator device 16 further comprises, as shown in FIG. 1, a squeegee 45 which is placed between the deflector 24 and the transfer station H and which is actuated by an electromagnet EA so as to be brought, either in a first position in which it is in contact with the surface 12 of the drum and then stops in passing the developer particles which, leaving the bucket 43, have remained applied to this surface, or in a second position in which it is moved away from this surface and then allows the particles which have been deposited thereon to pass.

The particles which are stopped by the squeegee 45, when the latter is in its first position, eventually fall back into the reservoir 17. However, in order to prevent some of these particles from reaching to slip between the squeegee and the drum and thus continue to adhere to the surface of the drum, it is necessary that this squeegee is applied to this surface with sufficient force, the value of which also depends on the size of the particles and the force that keeps these particles on the surface of the drum.

In an advantageous embodiment which is illustrated in FIG. 3, this squeegee is in the form of a flexible blade comprising, on the one hand, a fixed part 46 intended to allow this blade to be firmly attached to a plate fixed transverse 47 constituting the reservoir 17, on the other hand a free part 48 terminated by an edge 49 which, parallel to the axes 13 and 25, is applied to the surface 12 of the drum, this edge thus coming to contact this surface along a generator K of the drum. This flexible blade 45 is positioned so that its end portion which is close to the edge 49 forms, with the half-tangent T to this surface 12 at the contact point K and oriented in the direction of movement of the drum, an angle t whose the value remains between ten and forty five degrees.

E représentant la valeur du module d'élasticité du matériau constituant la lame. On voit donc que si on choisit, pour réaliser cette lame, un matériau présentant des propriétés élastiques suffisantes, on pourra déterminer les valeurs de la largeur a, de l'épaisseur e et de la flexion f qu'il convient d'adopter pour obtenir une force P dont la valeur est au moins égale à 2,5 N/dm.So that, practically, none of the particles which are stopped by this blade can slip between this blade and the surface of the drum, it is necessary that this blade is applied to this surface with sufficient force. For the type of particles which is used in the example described, it has thus been found that the force P exerted, per unit of length on the edge 49 of the blade in contact with the drum should be at least equal to 2, 5 N / dm. Now, if we designate by b the length, by a the width and by e the thickness of the free part 48 of the blade (this length b corresponding to the length of the edge 49 of this blade), we know that when this blade is subjected to a bending such that the edge 49 of this blade is displaced by a distance f relative to its original position, the force P which is exerted, per unit of length, on this edge 49 has the expression: $$\text{P =}\frac{\text{Ee³f}}{\text{4a³}}$$

E representing the value of the modulus of elasticity of the material constituting the blade. We therefore see that if we choose, to make this blade, a material with sufficient elastic properties, we can determine the values of the width a, the thickness e and the bending f that should be adopted to obtain a force P whose value is at least equal to 2.5 N / dm.

In practice, it is arranged so that the amplitude of the bending f undergone by the flexible blade is at most equal to half the width a of this blade, this arrangement allowing this flexible blade, when the latter is produced. in one of the materials usually chosen to constitute elastic strips, to remain in the field of elastic deformations. However, the material which is used to make this flexible blade must not have too great a hardness so as not to risk damaging the surface of the drum on which this blade is applied. It has been found that, so that the flexible blade remains within the limit of elastic deformations and does not cause any degradation of the surface condition of the drum, the material used to constitute this blade must have a modulus of elasticity E at least equal to 300 daN / mm² and a hardness at most equal to 600 Vickers. Thus, for example, this flexible blade can be a polyethylene terephthalate blade, the latter material, usually designated by the name of "Mylar" (registered trademark), having a modulus of elasticity practically equal to 480 daN / mm², the free part of this blade having a width a practically equal to 8 mm and a thickness e practically equal to 0.2 mm. The force P which is exerted, per unit of length, on the edge 49 of this blade, when the latter is subjected to a bending f equal to half the width a of the blade, that is to say here at 4 mm, then has the value: $$\text{P =}\frac{\text{480 x (0.2) ³ x 4}}{\text{4 x (8) ³}}\text{= 7.5 x 10⁻³ daN / mm = 7.5 N / dm}$$

The flexible blade can also be a stainless steel blade having a modulus of elasticity practically equal to 25,000 daN / mm², the free part of this blade having a width a practically equal to 8 mm and a thickness e practically equal to 0.05 mm. The force P which is exerted, per unit of length, on the edge 49 of this blade, when the latter is subjected to a flexion f equal to half the width of the blade, that is to say 4 mm, then has the value: $$\text{P =}\frac{\text{25,000 x (0.05) ³ x 4}}{\text{4 x (8) ³}}\text{= 6.1 x 10⁻³ daN / mm = 6.1 N / dm}$$

In order that the flexible blade is not subjected to too rapid wear due to its friction on the surface of the drum, it is necessary that the force with which this blade is applied to this surface is not too high. Tests have shown that, in order to obtain moderate wear of the blade, the force P exerted, per unit of length, on the edge 49 of this blade should not practically exceed the value of 20 N / dm.

The applicator device 16 also comprises, as can be seen in FIG. 3, an actuating member which makes it possible to separate the squeegee 45 from the surface 12 of the drum and thus to allow the particles which have been applied to remain on this surface. using the deflector 24.

This actuating member is formed by a rod 50 which is arranged parallel to the axes 13 and 25 and which can pivot in two bearings (not shown) fixed on the lateral faces 26 and 27 of the reservoir 17, and a lever 51 which is mounted at one end of the rod 50 and whose arm is articulated at the end of a sliding rod 52 secured to the movable armature of an electromagnet EA, this electromagnet itself being even fixed on one of the lateral faces of the reservoir 17. When this electromagnet EA is not excited, this lever 51 occupies a first position, called the rest position, which, in FIG. 3, has been shown in lines interrupted. When, on the contrary, this electromagnet EA is excited, the lever 51 occupies a second position, called the working position, which, in FIG. 3, has been shown in dashed lines. In the embodiment which is illustrated in FIG. 3, in which the squeegee 45 is constituted by a flexible blade, the rod 50 is machined so as to have, in its middle part and over a length at least equal to the length of the blade, a flat face 53 which, as can be seen in FIG. 3A, passes through the axis 54 of the rod and is limited by two edges 55 and 56.

This rod 50 is positioned so that its middle part, which is thus of semi-cylindrical shape, is between the free part 48 of the blade 45 and the surface 12 of the drum, and that, when the lever 51 and placed in rest position, the edge 55 of this middle part is as close as possible to this free part 48 without however contacting it, as can be seen in FIG. 3A. Under these conditions, this middle part of the rod 50 does not risk, when the lever 51 is in the rest position, to modify the value of the force with which the edge 49 of the blade 45 is applied to the surface of the drum.

If, now, the electromagnet EA is excited, the lever 51 comes into the working position and makes the rod 50 pivot by an angle w in the direction which is indicated by the arrow in FIG. 3A.

In this case, the middle part of this rod comes to occupy a position which, illustrated in dashed lines in FIG. 3A, forms an angle w with the original position, and the edge 55 of this middle part, which comes to rest on the free part 48 of the blade 45, forces this free part to flex more and to move away from the surface 12 of the drum.

We will now describe, with reference to FIG. 4, the circuit for controlling the excitation of the electromagnet EA. This circuit includes manual and relay contacts intended to be used under the conditions which will be described. In FIG. 4, each relay contact is designated by the same reference as that of the winding which controls it, but preceded by the letter C. A contact, normally closed when the coil of the relay which controls it is not energized, is represented in this figure by a black triangle. The relays shown in this figure are normally supplied by direct current oris between two terminals (+) and (-), the terminal (-) being earthed.

To describe the operation of the circuit shown in FIG. 4, it will be assumed that each of the heads of the recording device has finished recording information on the first five of the six tracks associated with it, this recording having been carried out at during five successive turns of the drum 11. The magnetic heads T1, T2, T3, ..., Tn having been placed, during the fifth round, respectively in front of tracks P15, P25, P35, ..., Pn5, the electrical signal which, at the end of this fifth round, appears at the exit from the PH cell causes the carriage 34 to move one step to the right. This movement has the effect, on the one hand, of bringing the heads T1, T2, T3, ..., Tn opposite, respectively tracks P16, P26, P36, ..., Pn6 and thus allow these tracks to be recorded during a sixth turn of rotation of the drum 11, on the other hand to push in a contact KD which, like the shows Figure 2, is arranged to be actuated by the carriage 34 when the latter is brought into its limit position LD. As can be understood with reference to FIG. 4, as a result of the depressing of the contact KD, a positive voltage is applied to the input of an amplifier-diverter AD, this input being in fact connected to the terminal (+) via the KD contact.

This amplifier-drift is designed to deliver a single positive pulse at its output each time its input is brought to a positive potential. The pulse which appears at the output of this amplifier-drift AD is applied to the input of a delay element R1, which, in response to the reception of this pulse, delivers at its output a delayed pulse. The delay of this element R1 is established so that this delayed pulse appears at the output of this element only when the surface portions of the drum which passed in front of the heads T1, T2, T3, ..., Tn to l 'moment when these heads were brought in front of tracks P16, P26, P36, ..., Pn6 are about to pass under the blade 45. The delayed pulse which then appears at the output of element R1 is applied , on the one hand at the input of a second delay element R2, on the other hand to the electromagnet EA and to a relay B02.

Relay B02, energized, then closes its contact CB02 and establishes a holding circuit for itself and for the electromagnet EA, via a contact CB01, normally closed, and contact CB02. The excited EA electromagnet actuates the rod 52 and thus brings the lever 51 into the working position, which has the effect of moving the blade 45 away from the surface 12 of the drum. Under these conditions, this blade, which until then removed the developer particles which, leaving the bucket 43, remained applied on the surface of the drum, now let these particles remain on this surface, so that these particles can reach the transfer station H where they are then transferred to a sheet of paper 19 which is engaged, at that time, between the drum 11 and the transfer roller 20. The delay of the element R2 is established so that, in response to an electrical pulse applied to its input, it delivers a delayed pulse to its output after a time substantially equal to the time taken by the drum to complete one revolution. More precisely, this delayed pulse appears at the output of R2 at the moment when the powder image corresponding to the latent image formed during six successive turns of the drum has passed entirely past the blade 45. This delayed pulse is applied to a relay B01 which, energized for a short time, momentarily opens its contact CB01. Opening the contact CB01 has the effect of de-energizing the relay B02 and the electromagnet EA. As a result, the relay B02, de-energized, opens its holding contact CB02, while the electromagnet EA ceases to keep the blade 45 spaced from the surface of the drum.

From this moment, the developer particles which, after leaving the bucket 43, remain applied to the surface of the drum are stopped in passing by the blade 45 and then fall back into the reservoir 17.

The printing machine which is shown in FIG. 1 can be designed in such a way that the recording of a latent image on the drum is only carried out when the carriage 34 is moved step by step from its position LG limit towards its LD limit position.

In this case, when, at the end of its step-by-step movement, the carriage 34 has arrived at its limit position LD, this carriage is, as soon as the recording of the information in the tracks P16, P26, ... , Pn6 is finished, quickly brought back to its LG limit position in order to allow a new latent image to be recorded on the drum, the excitation of the heads of the recording device 14 being interrupted for the entire duration of the return movement of the cart.

The machine shown in Figure 1 can also be designed so that the recording of a latent image on the drum is carried out when the carriage 34 is moved step by step, either from its limit position LG, or from its limit position LD. In this case, when, at the end of its step-by-step movement, the carriage 34 has arrived at its limit position LD, this carriage is, as soon as the recording of the information in the tracks P16, P26, ... , Pn6 is finished, brought back step by step to its LG limit position, while a new latent image is formed on the drum. During the first five of the six revolutions of the drum necessary for the formation of this new image, the electromagnet EA is not excited, so that none of the developer particles which leave the trough 43 can reach transfer station H. When, as a result of its step-by-step movement, the carriage 34 reaches its LG limit position and thus brings the heads in front of tracks P11, P21, ..., Pn1, electro -EA magnet is excited at the moment when the surface portions of the drum which passed under these heads at the moment when the carriage arrived in its LG position are about to pass under the blade 45. The excitation of this electromagnet at this instant can be triggered, advantageously, by means of a contact KG which, as shown in FIG. 2, is arranged so as to be actuated by the carriage 34 at the moment when the latter reaches its limit position LG, this contact KG being mounted in derivation at the terminals of contact KD, as shown in Figure 4.

Ultimately, whatever the mode adopted for the formation of the latent images on the drum, provided that the formation of each of these images is carried out during several successive turns of rotation of the drum (that is to say at least two turns), we therefore see that the formation of the powder image corresponding to this latent image is only undertaken during the last of these rotational turns, this powder image being transferred immediately afterwards onto a sheet of paper . Under these conditions, the transfer roller 20 does not run the risk of being smeared with developer particles during the periods when no sheet of paper is engaged between this roller and the drum 11.

Claims (10)

1. A device for intermittently applying particles of a pulverulent developer to the recording surface (12) of a magnetographic printer, this surface being driven by displacement along a predetermined closed path allowing it to pass through a transfer position (H) where the developer which has been deposited on this surface is transferred to a printing medium (19), this application device comprising a component (17, 23, 24) designed for constantly applying particles of developer to said recording surface (12), the recording surface cooperating with a latent image recording device (14) set up to form a latent image on this surface during several successive rotations in displacement of this surface, this application device being characterised in that it further comprises a device for eliminating particles (45, 50, 51, 52, EA) disposed along said path, downstream of the point (G) where the particles are applied to this surface, between this application point (G) and said transfer position (H), this elimination device being set up in order to remove particles of developer from this surface, except during the last of said successive rotations in displacement of this surface.
2. An application device according to Claim 1, characterised in that the particle elimination device comprises a scraper (45) placed between the particle application point (G) and the transfer position (H) and normally bearing against the recording surface (12) in order to remove the particles from it, and a scraper activation device (50, 51, 52, EA) set up to deflect this scraper from this surface solely during the last of said successive rotations in displacement of this surface.
3. An application device according to Claim 2, characterised in that the scraper (45) is composed of a flexible blade, one edge (49) of which is applied against the recording surface (12), this blade forming with the half-tangent (T) to this surface passing through the point of contact (K) of this blade with this surface and oriented in the direction of displacement (F) of this surface, an angle (t) the value of which is comprised between ten and forty-five degrees.
4. An application device according to any one of Claims 2 and 3, characterised in that the scraper (45) is composed of a flexible blade, one edge of which (49) is applied to the recording surface (12) with a force which, per unit of length of said edge, is equal to or greater than 2.5 N/dm.
5. An application device according to any of Claims 3 and 4, characterised in that the flexible blade (45) is made of a material having a modulus of elasticity equal to or greater than 300 daN/mm² and a hardness equal to or less than 600 Vickers.
6. An application device according to any one of Claims 2 to 5, characterised in that the activation component of the scraper (50, 51, 52, EA) comprises an element (50) which is articulated around an axis (54) and is set up so as to occupy either of two positions, this element being provided with a bearing edge (55) disposed such that, when said element is placed in its first position, this bearing edge is in the immediate vicinity of the scraper, whereas when said element is placed in its second position this bearing edge pushes away said scraper and thus obliges it to remain deflected from the recording surface.
7. An application device according to Claim 6, characterised in that the activation component of the scraper (50, 51, 52, EA) further comprises an electromagnet (EA), the mobile armature of which is coupled to the articulated element (50) in order to activate this element and allow it to be placed in one or other of its positions.
8. An application device according to any one of Claims 6 and 7, characterised in that the articulated element (50) is composed of a rod having a semi-cylindrical part limited by two edges (55 and 56), one of which constitutes the bearing edge (55).
9. An application device according to any one of Claims 1 to 8, characterised in that the component (17, 23, 24) which permanently applies particles of developer to the recording surface (12) comprises:

   - a reservoir (17) disposed below said surface (12) and containing particles of developer,

   - a transport element (23) disposed so as to bring these particles into the vicinity of this surface.

   - and a fixed deflector (24) interposed between this surface (12) and this transport element (23) to collect the particles transported by the latter, this deflector being disposed so as to form with this surface a pan (43), substantially prismatic in shape, in which the particles thus collected accumulate, these particles ending up by coming into contact with this surface and by being drawn along towards the crest (G) of said prism, the particles drawn along beyond this crest remaining applied only to the magnetised portions of this surface (12).
10. A magnetographic printing machine, characterised in that it comprises an application device for pulverulent developer according to any one of Claims 1 to 9.

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