EP0315509B1 - Method for fixing a toner powder put on a sheet, and device for fixing this toner according to this method - Google Patents

Description

The present invention relates to a method for fixing a powdery developer deposited on a sheet, and to an implementation device.

Such a method finds more particularly, although not exclusively, its application in electrostatic or magnetographic printing machines in which individual sheets of paper are, after having been coated with a powdery developer product transferred from a recording medium, advanced one after the other to pass into a fixing device responsible for causing the momentary fusion of this developer and thus allow it to be permanently fixed on these sheets.

The present invention also relates to a fixing device in which this method is implemented.

In modern equipment which is used for data processing, fast printers are used more and more in which the printing of the characters is carried out without for that calling upon the impact of types of relief printing on a sheet of receiving paper. These printing machines, called non-impact, comprise a recording element constituted most often by a rotary drum or an endless belt, on the surface of which it is possible to form, by electrostatic or magnetic means, sensitized areas, also called images. latent, which correspond to the characters or images to be printed. These latent images are then developed, that is to say made visible, using a powdery developer which, deposited on the recording element, is only attracted by the sensitized zones thereof. , thus forming a powder image on the surface of this element. After which, this recording element is brought into contact with a sheet of paper in order to allow the developer particles constituting this powder image to be transferred onto this sheet to be definitively fixed there.

These printing machines are capable, when fed sheet by sheet, of carrying out the printing of sheets of paper, on one of their faces, at a relatively high printing rate, this rate can indeed reach, for example, one hundred pages per minute and even more. As a result, the quantity of paper which is printed by these machines in a given time is relatively large, in comparison with that which is printed, during the same time, by printing machines of the impact type.

In order to reduce the mass of paper which is printed by these machines at high rate of printing, machines have been produced capable of printing each sheet of paper on its two faces. This is how machines are known of the type which has been described in French patent No. 2,119,656 (this patent corresponding to the United States patent No. 3,697,171) and in which , to allow the printing of a sheet on its two faces, a first powder image is formed on the recording element, this image then being transferred to an intermediate element, then a second powder image is formed on the recording element, and finally applying this recording element and this intermediate element on the two sides of this sheet, in order to cause the simultaneous transfer of these two images on these two faces.

However, since the second powder image can only be formed on the recording element when the first powder image has been transferred to the intermediate element, the time required for the formation of these two images and their transfer simultaneous on both sides of a sheet is almost double that which would be necessary for the formation of a single image of powder on the recording element and the transfer of this single image on one of the faces of this leaf.

Consequently, in order to allow the time necessary for the formation of each of the first powder images on the recording element and for the transfer of each of them to the intermediate element, it is necessary that the sheets to be printed which are continuously advanced to pass between the recording element and the intermediate element, succeed one another at intervals such that any two successive sheets are separated from each other by a distance at least equal to the length of a leaf.

Furthermore, since, in order to guarantee a relatively high printing rate, these sheets are driven at high speed, it is necessary that the fixing device through which each sheet passes after having received the powder images on its two faces is equipped of a heating element sufficiently powerful to cause the fusion of the developer particles which cover this sheet, during the brief passage of the latter in this device. Given the high speed of leaf drive, we are therefore led to provide, to cause this fusion, either a very powerful heating element, or a heating element of moderate power, but long enough to allow, by increasing the duration of passage of each sheet in the fixing device, to obtain the same effects as those produced with a very powerful heating element. The use of a very powerful heating element has the drawback, however, of generating significant heating which is detrimental to the proper functioning of the machine, whereas the use of a heating element of moderate power, but very long, has the disadvantage of considerably increasing the size of the machine.

The present invention remedies these drawbacks and proposes a method allowing: developer particles deposited on sheets advanced at high speed to be definitively fixed on these sheets, while using for this fixing only a relatively short heating element and moderate power.

More specifically, the present invention relates to a method for fixing a powdery developer deposited on at least one of the faces of a printing sheet, comprising subjecting said face to the action of radiation capable of causing momentary fusion of this developer, this process being characterized in that it consists in first bringing this sheet so that a first edge of the sheet is in front of a radiation source temporarily immobilized in a rest position, and then to implement two successive phases, one of which consists in keeping this sheet stationary and in moving this source parallel to this sheet, at a first constant speed, in a direction perpendicular to said edge, but over a length less than the distance which separates this edge from a second edge opposite to said first edge, and the other of which consists in moving this source in the opposite direction and simultaneously moving said sheet in the same direction as this source and at a constant speed greater than said first speed, the movement of said source, during this other phase, being carried out at a second speed whose value is numerically equal at the speed of movement of the sheet minus the value of said first speed.

According to another characteristic of the invention, a device for implementing the method according to the main claim is characterized in that it includes the characteristics of claim 5.

• . La figure 1 représente une machine imprimante magnétographique équipée d'un disposititf de fixation réalisé selon l'invention,
• . La figure 2 représente un diagramme montrant les déplacements linéaires relatifs des feuilles et de la source de rayonnement dans la machine imprimante représentée sur la figure 1, dans le cas où la vitesse de déplacement des feuilles est double de celle de la source de rayonnement,
• . La figure 3 représente un diagramme montrant les déplacements linéaires relatifs des feuilles et de la source de rayonnement, selon un premier mode d'éxécution du procédé et dans le cas où la vitesse de déplacement des feuilles est triple de celle de la source de rayonnement,
• . La figure 4 représente un diagramme montrant les déplacements linéaires relatifs des feuilles et de la source de rayonnement, selon un second mode d'exécution du procédé et dans le cas où la vitesse de déplacement des feuilles est triple de celle de la source de rayonnement.

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 fixing device produced according to the invention,
• . FIG. 2 represents a diagram showing the relative linear displacements of the sheets and of the radiation source in the printing machine represented in FIG. 1, in the case where the speed of displacement of the sheets is double that of the radiation source,
• . FIG. 3 represents a diagram showing the relative linear displacements of the sheets and of the radiation source, according to a first mode of execution of the method and in the case where the speed of displacement of the sheets is three times that of the radiation source,
• . FIG. 4 represents a diagram showing the relative linear displacements of the sheets and of the radiation source, according to a second embodiment of the method and in the case where the speed of displacement of the sheets is three times that of the radiation source.

The structure and operation of the fixing device which equips the printing machine shown in Figure 1 are conditioned by the mode in which the sheets of paper are advanced in this machine to be printed. This is why, before describing this fixing device, we will first give details relating to the constitution of this machine.

The printing machine which has been schematically represented in FIG. 1 is a printing machine which performs the printing of sheets of paper successively extracted from a supply magazine 10. This machine comprises a recording element which is constituted, in the 'example described, by a magnetic drum 11. The drum 11, which is mounted so as to be able to rotate about a horizontal axis 12, is driven in rotation, in the direction indicated by the arrow F, by an electric motor (not represented). The recording of information on this drum is carried out by a recording member 13 comprising several magnetic recording 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 surface of the drum which passes in front of the recording member, practically point magnetized zones, all of these zones constituting a latent magnetic image corresponding to an image to be printed. These magnetized zones then pass in front of a developer applicator device 14 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 15. The particles of developer which are thus applied to the drum adhere, in principle, only on the magnetized areas thereof and then form on the surface of the drum a powder image. A retouching device 16, 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 of the drum 11 consists of magnetic particles coated with a thermoplastic resin which, as will be seen below, is capable of melting, when it is subjected to a heat source, and thus to be fixed on a sheet of paper on which this developer has been transferred. The developer particles which, after passing in front of the retouching device 16, remain on the drum 11, then pass in front of a charging device 17 which has the role of positively charging the resin constituting the developer particles, at the moment when these particles pass in front of him. After which, these particles are normally transferred, almost entirely, onto a sheet of paper 18 which, after having been extracted from the magazine 10, in a manner which will be indicated below, is applied, by means of a transfer roller 19, against the surface drum 11.

This transfer roller 19 consists of a metal cylinder 20 coated, on its cylindrical surface, with a layer 21 of elastic material formed of a polyurethane rubber. This cylinder 20 is mounted so as to be able to rotate about a horizontal axis 22 which is positioned so that the elastic layer 21 of the roller 19 remains constantly stressed in application against the cylindrical surface of the drum 11. The region H where the roller transfer 19 thus comes into contact with the surface of the drum 11 constitutes the transfer station.

It is in this station that the transfer takes place, onto a sheet of paper 18 which is engaged between the drum 11 and the roller 19, 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, are still on the drum 11 are then removed by means of a cleaning device 23. The magnetized zones which have passed past the cleaning device 23 then pass in front of a cleaning device 23. erasure 24, which allows the portions of the drum 11 which have thus been demagnetized by this latter device to be able to be magnetized again when they represent themselves in front of the recording member 13.

As can be seen in FIG. 1, the sheets of paper 18 which are successively engaged between the drum 11 and the transfer roller 19 to be printed come from the magazine 10 in which they have been previously stored. The extraction of the sheets from this store is carried out by means of an extraction device 25 of known type comprising a roller 26 secured to a journal axis in a bearing fixed to one end of a lever 27 pivotally mounted about an axis 28, the other end of this lever being articulated at the end of a sliding rod 29 integral with the movable frame of an electromagnet 30. The roller 26 is rotated, in the direction of the arrow G, by an appropriate drive device (not shown). Under these conditions, it is understood that, when the electromagnet 30 is momentarily excited, the lever 27 pivots and forces the roller 26 to penetrate inside the magazine 10 and thus come into contact with one of the stored sheets. in this store. The roller 26 then forces this sheet out of the magazine and to engage between drive rollers 31. The sheet of paper which is advanced by these rollers 31 is guided in its movement by guide plates (not shown) and it then ends up engaging in the transfer station H between the drum 11 and the transfer roller 19. The developer particles which, deposited on the surface of the drum, come into contact with this sheet are then transferred almost entirely to this last, this transfer being facilitated by the fact that, the drum 11 being electrically connected to the positive terminal (+) of a DC voltage source and the cylinder 20 being connected to the negative terminal (-) of this same source, these particles, which have been positively charged by the charging device 17, are subjected to the action of an electric field when they arrive in the transfer station and are then requested to pass drum 11 on the sheet of paper 18. This sheet 18 which is now clamped between the drum 11 and the transfer roller 19 is thus driven by the rotation of this drum. However, after passing through the transfer station, this sheet is detached from the surface of the drum by a separating member 32 and, taken up by a conveyor belt 33, it then passes through a fixing device 34, which, by heating, melts the resin surrounding the magnetic particles of the developer which has been transferred to this sheet and thus causes the permanent fixation of this developer.

After passing through the fixing device 34, this sheet, driven by the belt 33, passes through a cooling device 35 before being finally placed in a receiving box 36.

In the printing machine which is represented in FIG. 1, the drum 11 has a diameter such that the length of the circumference of this drum is substantially equal to or very slightly greater than the length of sheets 18 to be printed. Under these conditions, the image of powder which is formed during a rotation of the drum can be fully transferred to one of the faces of the same sheet, provided that the excitation of the electromagnet 30 which causes the extraction of this sheet from the magazine 10 occurs at a precise time t such that the front edge of this sheet arrives at the transfer station H a little before the front edge of this powder image arrives at this same station . By forming a powder image during each rotation of the drum, it is then possible, by exciting the electromagnet 30 at times t, t + T, t + 2T, t + 3T, etc ... ( T designating the machine cycle, that is to say the time taken by the drum 11 to complete a revolution), to transfer each of these images to only one of the faces of the sheets 18 which have been extracted from the magazine 10 to those moments. However, the printer which is represented in FIG. 1 is established so as to transfer these powder images on the two faces of the sheets which are successively extracted from the magazine 10. For this purpose, the electromagnet 30 is not excited at times t, t + T, t + 2T, t + 3T, etc ..., but only at times t + T, t + 3T, t + 5T, etc.

Since, under these conditions, no sheet of paper is engaged between the drum 11 and the transfer roller 19 during the whole time when the first image of powder formed on this drum passes in front of the transfer station H, this first powder image is transferred, almost entirely, on the external surface of the elastic layer 21 of this transfer roller. This first powder image is intended, as will be seen below, to be applied to the reverse side of the sheet which will be extracted from the magazine 10 at time t + T. It should also be indicated that the magnetic drum 11 and the transfer roller 19 have the same diameter, so that this first image of powder can, when it has been transferred to this roller, take up entirely on the external surface of the layer 21.

It should also be noted that the formation, during a first cycle of the drum 11, of the first image of powder on this drum is followed by the production of a second image of powder on this same drum, this production occurring during 'a second drum cycle. This second powder image is intended to be applied to the front face of the sheet extracted from the magazine 10 at time t + T. During the formation of the second powder image on the drum 11, the first powder image which is now on the roller 19 is driven by the rotation of this roller and it passes in front of a cleaning brush 37 which is normally applied to the surface of the layer 21, but which is separated from this layer by known means (not shown) for the entire duration of the passage of the first image of powder in front of this brush. After which, this first image passes in front of a second charging device 38 which has the role of making the particles constituting this first image lose the positive electric charge that they had received from the charging device 17 and of making them acquire an electric charge now. negative. The position occupied by this first image on the layer 21 is such that the corresponding regions of this image and of the second powder image approach the transfer station H at the same time. However, just before these two images come into contact. from one another, the sheet of paper 18 which has been extracted from the magazine 10 at the instant t + T engages between the drum 11 and the roller 19. Since the particles of the second powder image have been positively charged by the device 17 and that those of the first powder image have been negatively charged by the device 38, all these particles are subjected, when they arrive at H, to the electric field established between the drum 11 and the roller 19 and, under the effect of this field, are forced to pass, almost entirely, onto the sheet of paper 18, which is then printed simultaneously on its two faces. As indicated above, this sheet is, after being passed between the drum 11 and the roller 19, taken up by the belt 33 and, driven by the latter, it passes through the fixing device 34 before being finally deposited in reception box 36.

From what has been explained above, it is understood that the sheets of paper 18 which are extracted from the magazine 10 at times t + T, t + 3T, t + 5T, etc. and which are driven continuously by the rollers 31 and drum 11 follow one another at intervals such that any two successive sheets are separated from each other by a distance M which is slightly greater than the length L of a sheet. It will be considered that, in the example described, each of these sheets has a length L of 297 mm and that the interval M which separates two successive sheets driven continuously is equal to 331 mm.

As can be seen in FIG. 1, the fixing device 34 comprises a support 40 pivotally mounted on a horizontal axis 41 coupled to the drive shaft of an electric motor 42.

This electric motor 42, in two directions of travel, is of known type. It will be considered that this engine is of the type which was incidentally described in the patent application filed by the Applicant on December 20, 1984 and published under No. 2,575,111 (this application corresponding to the patent of the United States of America N ° 4.657.416), this motor, of alternating current type, comprising two inductor windings wound in opposition so that, when one of the windings is energized in alternating current, the motor turns in one direction, whereas when l The other winding is excited, this motor turns in the opposite direction and at the same speed.

This motor 42, by rotating, pivots the support 40 around its axis 41, the movement of this support being limited by two stops constituted by two electrical contacts 43 and 44. The support 40 normally occupies a rest position P₀ (shown in dashed lines in FIG. 1) in which he keeps the contact 43 pressed, this rest position being that where the support 40 is the farthest from the transfer station H.

The support 40 can be moved by the motor 42 to come and occupy a limit position P₁ in which it keeps the contact 44 pressed, this limit position being that where this support is closest to the transfer station H. We also see, on Figure 1, that the conveyor belt 33 is an endless belt which can be driven in displacement, in the direction indicated by the arrow, by an electric motor 45. This belt, which is mounted on pulleys, passes, during of its movement, on a guide plate 46, cylindrical shape, the axis of curvature of which is located on the pivot axis 41 of the support 40, this plate extending at least over the entire portion of the path followed by the support 40 when the latter is moved between its positions P₀ and P₁.

The support 40 is provided with two sources of thermal radiation 47 and 48 arranged on either side of 1 assembly constituted by the plate 46 and the curved portion of the belt 33, at equal distance from this assembly. Thus, when a sheet, leaving the transfer station, is engaged between this plate and this curved portion, this sheet, driven by the belt 33, passes between the two sources of radiation 47 and 48. In reality, the belt 33 is double and consists of two belt elements engaging the side edges of this sheet. Similarly, the guide plate 46 consists of two plate elements on which these two lateral edges of the sheet are applied. Under these conditions, the belt 33 and the plate 46 cannot stop the radiation which, coming from the sources 47 and 48, is sent to the parts of the sheet on which the two images of powder have been transferred.

The operation of the fixing device which has just been described will now be explained with reference to the diagram in FIG. 2 in which the movements of the sheets and of the support 40 have been shown, respectively, in solid lines and in dashes, the sheets and the support being supposed to move, in this figure, along a rectilinear path, for reasons of clarity of the description. It will be considered that, initially, the support 40 is in the rest position P₀ and that the belt 33 is driven continuously by the motor 45. It can be seen, with reference to FIG. 2, that the sheet of paper which is extracted from store 10 at time t + T is moved by the rollers 31, the drum 11 and the belt 33 at a constant speed V₁ equal to:

The value of t is such that the front edge BV of this sheet arrives practically, at time 3T, at a point A located directly above the sources 47 and 48 when the support 40 is in the rest position. A detector C is arranged along the path of this sheet, upstream of point A, relative to the direction of movement of the sheet, and at a distance from A equal to the length of this sheet.

This detector C is established to deliver an electrical pulse each time the rear edge BR of a sheet driven by the belt 33 passes in front of it. This pulse, transmitted to a control circuit (not shown) of the motors 42 and 45, has the effect of de-energizing the motor 45 and exciting the motor 42, the latter then causing the support 40 to move to its limit position P₁. As a result of the motor 45 being de-energized, the belt 33 stops and stops driving the sheet which is in contact with it. However, as a result of the inertia of the mechanical parts of the machine and the delays in the electrical circuits, the edge BV of this sheet comes to a stop slightly downstream of point A, about one centimeter from it.

The fastening device which is shown in Figures 1 and 2 is established so that, when the motor 42 is energized, the support 40 is moved by this motor, from its position P₀ to its position P₁, at a speed constant V₂ equal to half of that V₁ at which the sheet which is now immobilized was driven by the belt 33. The power of the radiation sources 47 and 48 is such that, when these sources are displaced at this speed V₂ in front of the sheet thus immobilized, the radiation from these sources is sufficient to ensure the fixation of the developer which has been transferred to this sheet, without however causing excessive heating of this sheet. On the other hand, the electrical contact 44 is positioned in such a way that the displacement N undergone by the support 40 to go from its position P₀ to its position P₁ is equal to:

c'est-à-dire à l'instant 4T. Le support 40, en arrivant à cet instant à sa position P₁ , enfonce le contact électrique 44, ce qui a pour effet d'exciter le moteur 45 et de faire tourner en sens inverse le moteur 42. Dans ces conditions, le support 40 revient vers sa position de repos P₀ à la même vitesse V₂ que celle à laquelle il était déplacé pour aller vers sa position P₁ , si bien que ce support 40 atteint sa position de repos à l'instant 5T. A cet instant, le support 40 enfonce le contact électrique 43, ce qui provoque la désexcitation du moteur 42 et, par suite, l'immobilisation du support 40 dans sa position de repos jusqu'à ce que ce support se trouve à nouveau entraîné du fait du passage, devant le détecteur C, du bord arrière de la feuille de papier suivante. Par ailleurs, lors du retour du support 40 vers sa position de repos, le moteur 45, qui est à nouveau excité, entraîne la feuille qui, entre les instants 3T et 4T, se trouvait immobilisée dans le dispositif de fixation, cette feuille étant alors déplacée à la vitesse V₁ afin de passer sous le dispositif de refroidissement 35 et d'être finalement éjectée dans la case de réception 36. Pendant le mouvement de retour du support 40 vers sa position de repos P₀, cette feuille se trouve ainsi animée, par rapport au support 40, d'une vitesse relative égale à V₁-V₂, c'est-à-dire, puisque V₁ = 2V₂, égale à V₂.Since this support 40 is driven by the motor 42 as soon as the rear edge BR of the sheet of paper has passed in front of the detector C, that is to say at the instant 3T, we then see that this support 40 reaches its limit position P₁ at the instant:

that is to say at time 4T. The support 40, arriving at this time at its position P₁, depresses the electrical contact 44, which has the effect of exciting the motor 45 and turning the motor 42 in opposite directions. Under these conditions, the support 40 returns towards its rest position P₀ at the same speed V₂ as that at which it was moved to go towards its position P₁, so that this support 40 reaches its rest position at time 5T. At this instant, the support 40 depresses the electrical contact 43, which causes the motor 42 to become de-energized and, consequently, the support 40 to be immobilized in its rest position until this support is again driven from the makes passage, in front of detector C, of the rear edge of the next sheet of paper. Furthermore, when the support 40 returns to its rest position, the motor 45, which is again excited, drives the sheet which, between times 3T and 4T, was immobilized in the fixing device, this sheet then being moved at speed V₁ in order to pass under the cooling device 35 and to be finally ejected into the receiving box 36. During the return movement of the support 40 to its rest position P₀, this sheet is thus animated, by relative to the support 40, with a relative speed equal to V₁-V₂, that is to say, since V₁ = 2V₂, equal to V₂.

c'est-à-dire, à 157 mm. Ainsi, dans l'exemple décrit, la valeur de ce déplacement est très voisine de celle de la moitié de la longueur des feuilles.We can also observe that, in the example described, where each of the sheets of paper has a length L of 297 mm and where the interval M separating any two successive sheets supposed to be driven continuously is equal to 331 mm, the displacement N undergone by the support 40 to go from one of its positions to the other is equal to:

that is to say, at 157 mm. Thus, in the example described, the value of this displacement is very close to that of half the length of the sheets.

From the explanations which have just been given, it can therefore be seen that the fixing of the powder images deposited on a sheet of paper is carried out according to a method which consists in first bringing this sheet into the fixing device, such that the front edge of this sheet comes to rest practically opposite the sources of thermal radiation 47 and 48, the latter being momentarily immobilized in a rest position P₀, then move these sources in front of this sheet, at a first speed constant V₂, towards the rear edge of the sheet, but over a length N less than the length of the sheet, then bring these sources back to the rest position and simultaneously move this sheet in the same direction as these sources, at a constant speed V₁ greater than the first speed V₂, the return of the sources to the rest position taking place at a speed whose value is numerically equal to l with difference V₁ - V₂. In the example described where the speed of movement of the sheet is twice that of movement of the sources, the length N of the path followed by these sources to go from one position to the other is practically equal to half the length of the sheet, and the return of the sources to the rest position is accomplished at the same speed as the outward journey, that is to say at the speed V₂.

k étant un nombre positif donné supérieur à 1, cette position limite P₁ est située à une distance N de la position de repos, telle que l'on ait pratiquement :

In a more general embodiment of the process of the invention, the speed V₁ of movement of the sheets may very well not be equal to twice that V₂ at which the sources are moved to go from their rest position P₀ to their limit position P₁. In the general case where the speed V₁ of movement of the sheets is equal to:

$\text{V₁ = kV₂,}$

k being a given positive number greater than 1, this limit position P₁ is located at a distance N from the rest position, such that there are practically:

Under these conditions, from the moment when the detector C detects the passage of the rear edge of a sheet engaged in the fixing device, the sources 47 and 48 are displaced, at speed V₂, from their rest position P₀ at this position P₁, the time taken by these sources to complete this displacement being equal to:

de sorte que le temps mis par ces sources pour revenir en position de repos est égal à :

When they return to the rest position, the speed at which these sources are driven is equal to:

$\text{V′₂ = V₁ - V₂ = V₂ (k - 1)}$

so that the time taken by these sources to return to the rest position is equal to:

Thus, the time required for fixing the powder images formed on a sheet is equal to:

de sorte que cet élément devrait présenter une longueur R telle que l'on ait :

On voit donc que cette longueur est notablement supérieure à celle :

qui, dans le dispositif de l'invention, est nécessaire au déplacement des sources de rayonnement.Given that L and V₂ are constant, one thus sees that this time of fixing is constant, ie that it is independent of the speed of displacement of the sheets. Ultimately, the method of the present invention makes it possible to obtain an excellent fixing of the powder images on the sheets of paper, regardless of the speed at which these sheets are driven. This process is all the more advantageous since it requires for its implementation only a fixing device having a reduced size and comprising a heating element of moderate power. Indeed, in the case where one would use, for fixing the powder images formed on sheets advanced continuously at the speed kV₂, a heating element of the same power as that fitted to the device of the invention, but fixed , these sheets should be subjected to the action of this heating element during the time:

so that this element should have a length R such that we have:

We therefore see that this length is significantly greater than that:

which, in the device of the invention, is necessary for the displacement of the radiation sources.

c'est-à-dire sensiblement égal 2T/3. Ainsi le retour du support 40 en position de repos s'effectue à une vitesse V′₂ qui est supérieure à la vitesse V₂.By way of example, FIG. 3 shows the diagram showing the relative linear displacements of the sheets and of the support 40 in the case where the speed V₁ of movement of the sheets is three times that of V₂ at which this support is moved to go from its rest position P₀ to its limit position P₁. For reasons of simplification, we have omitted to represent in this figure the sheet feed magazine as well as the drive members of the leaves extracted from this magazine. For the same reasons, it has been assumed, in this figure, that the rest position P₀ of the support 40 is exactly in line with the front edge BV of the sheets which come to rest in the fixing device at times 3T, 5T, 7T, etc .... It can then be seen, with reference to FIG. 3, that the sheet whose front edge comes into line with this position P₀ at time 3T is first immobilized in the fixing device and that, during this immobilization, the support 40 is moved, from its position P₀ to its position P₁, at constant speed V₂. This speed V₂ which is that at which the support 40 is normally moved, when the sheet is immobilized, to cause the developer to be fixed on this sheet without thereby causing unnecessary overheating of this sheet, is very slightly greater than L / 2T. The displacement of the support 40, in front of the sheet thus immobilized, is carried out over a length N = 2 / 3L and for a time t₁ equal to 2/3 L / V2, that is to say substantially equal to 4/3 T. When the support 40 reaches its limit position P₁, this sheet is again driven in displacement at the speed V₁ = 3V₂, while the support 40 is returned to its rest position P₀ at the speed V′₂ = 2V₂, this return being carried out for a time equal t₂ equal to:

that is to say substantially equal to 2T / 3. Thus the return of the support 40 to the rest position takes place at a speed V′₂ which is greater than the speed V₂.

Ultimately, the method of the invention is characterized in that it consists in first bringing each sheet so that the front edge BV of this sheet is opposite the radiation source 47 or 48 temporarily immobilized in the position rest P₀, and then to implement two successive phases, one of which (for example the first) consists in keeping this sheet stationary and in moving this source in front of this sheet at constant speed V₂, over a length N less than the length L of the sheet, and of which the other phase (that is to say here the second) consists of moving this source in the opposite direction, and simultaneously moving this sheet in the same direction as this source, at the speed V₁> V₂, the movement of the source, during this other phase, taking place at the speed V′₂ = V₁ - V₂.

The order in which these two phases are implemented can also be reversed. In this case, the limit positions P₀ and P₁ of the radiation source are also reversed. An example of this way of doing is illustrated in FIG. 4, in which it can be seen that, from the moment when the front edge BV of a sheet comes into line with the radiation source temporarily immobilized in the position of rest P₀, this source is driven, in the same direction as the sheet, at the speed V′₂ = V₁ - V₂, while the sheet continues its movement at the speed V₁. This first phase ends when the rear edge BR of this sheet arrives opposite the position P₀, that is to say when the source has traveled the distance N indicated above. From this moment there, the second phase is implemented, this phase consisting in keeping the sheet stationary and in moving this source in front of this sheet, in the opposite direction to the movement made during the first phase and at the constant speed V₂ .

Claims (8)

1. A method for fixing a pulverulent developer deposited on at least one of the faces of a printing sheet (18), consisting in subjecting said face to the action of a radiation capable of causing the momentary fusion of this developer, this method being characterised in that it consists in firstly bringing this sheet such that a first edge (BV) of the sheet is situated facing a radiation source (47 or 48) temporarily fixed in a position of rest (P₀), and in then putting into operation two successive phases, one of which consists in keeping this sheet fixed and in moving this source parallel to this sheet, at a first constant speed (V₂ ), following a direction perpendicular to said edge, but over a length (N) shorter than the distance which separates this edge (BV) from a second edge (BR) opposite said first edge, and the other phase of which consists in moving this source in the opposite direction and in simultaneously moving said sheet in the same direction as this source and at a constant speed (V₁ ) greater than said first speed, the movement of said source, during this other phase, being carried out at a second speed (V'₂), the value of which is numerically equal to the speed of movement of the sheet decreased by the value of said first speed (V₂ ).
2. A method according to Claim 1, characterised in that the phase during which the sheet is kept fixed is only undertaken when the phase during which the source and this sheet are moved simultaneously is completed.
3. A method according to Claim 1, characterised in that the movement of the radiation source, from its position of rest, is carried out over a length practically equal to half the distance separating the first and second edges of the sheet.
4. A device for fixing, according to the method of Claim 1, a pulverulent developer deposited on at least one of the faces of a printing sheet (18), this device comprising at least one radiation source (47 or 48) capable of causing the momentary fusion of this developer when said sheet is subjected to the radiation from this source, and a conveyor element (33) coupled to a motor (45) and set up to move said sheet in front of this source when the sheet is introduced into this device, this device being characterised in that it further comprises:

   - a mobile support (40) on which is fixed the radiation source (47), this support being coupled to a second motor (42) and being set up so as to be moved along the path which the sheet follows when it is entrained by the conveyor element (33), this support normally occupying a position of rest (P₀ ),

   - a detector (C) arranged along said path and set up to deliver a signal when the front edge (BV) of the sheet which is entrained by the conveyor element arrives opposite the source (47) temporarily fixed in its position of rest, this signal causing, on the one hand, the stoppage of the conveyor element and the momentary fixing of the sheet, and on the other hand causing the movement of the mobile support and of the source, at a first constant speed (V₂ ), in the direction of the rear edge (BR) of the sheet,

   - and a release member (44) set up to deliver a signal when this support has covered a predetermined length (N), shorter than the distance (L) separating the front edge (BV) from the rear edge (BR) of the sheet, this latter signal causing, on the one hand, the excitation of the second motor (42) to bring the mobile support and the source back to the position of rest (P₀ ), and on the other hand causing the excitation of the first motor (45) to entrain the sheet in the same direction as that of the return of said support and at a constant speed (V₁ ) higher than said first speed, the second motor being set up to return said support at a second speed (V'₁ ), the value of which is numerically equal to the speed (V₁ ) of movement of the sheet decreased by the value of said first speed (V₂ ).
5. A device for fixing, according to the method of Claim 1, a pulverulent developer deposited on at least one of the faces of a printing sheet (18), this device comprising at least one radiation source (47 or 48) capable of causing the momentary fusion of this developer when said sheet is subjected to the radiation from this source, and a conveyor element (33) coupled to a motor (45) and set up to move said sheet in front of this source when this sheet is introduced into this device, this device being characterised in that it further comprises:

   - a mobile support (40) on which is fixed the radiation source (47), this support being coupled to a second motor (42) and being set up so as to be moved along the path which the sheet follows when it is entrained by the conveyor element (33), this support normally occupying a position of rest (P₀ ),

   - a detector (C) arranged along said path and set up to deliver a signal when the front edge (BV) of the sheet which is entrained by the conveyor element arrives opposite the source (47) temporarily fixed in its position of rest, this signal causing the excitation of the second motor (42) to entrain this support in the same direction as this sheet, at a constant speed (V'₂ ), the value of which is numerically equal to the speed (V₁ ) of movement of the sheet decreased by the value of a first constant speed (V₂ ), lower than the speed (V₁ ) of movement of the sheet, this first speed (V₂) being that at which the support is to be moved to obtain the fixing on the sheet which is assumed to be immobilised,

   - and a release member (44) set up to deliver a signal when this support has covered a predetermined length (N) shorter than the distance (L) separating the front edge (BV) from the rear edge (BR) of the sheet, this latter signal causing, on the one hand, the de-excitation of the first motor (45) to stop the movement of the sheet, and on the other hand the excitation of the second motor (42) to return the mobile support and the source into the position of rest, this second motor being set up to return this support at a speed equal to said first speed (V2).
6. A fixing device according to any one of Claims 4 and 5, characterised in that it further comprises a second release member (43) arranged so as to be actuated by the support (40) when the latter is returned into the position of rest, this second member being set up to deliver a signal each time it is actuated by this support, this signal causing the de-excitation of the second motor (42) and, consequently, the immobilisation of the mobile support and of the source in the position of rest.
7. A fixing device according to any one of Claims 4 to 6, characterised in that the mobile support (40) is articulated about an axis (41).
8. A fixing device according to Claim 7, characterised in that the conveyor element (33) is constituted by a continuous belt which, in its part situated along the path followed by the mobile suport (40), comes to apply itself on a fixed curved element (46), the centre of curvature of which is situated on the axis of articulation (41) of said mobile support.

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