FR2625574A1 - Device for eliminating contamination due to electric charges acquired by a printing support in a magnetographic printing machine - Google Patents

Abstract

1. The invention relates to a device for eliminating contamination due to electric charges acquired by a printing support in a magnetographic printing machine. 2. This device, consisting of an alternating-current corona discharge device 31, is located near the face 50 of the printing support 22 which has been in contact with the magnetic layer 12 of the recording element 10, and near the point H where this support leaves this layer. The value of the AC voltage applied to the electrode 38 of this device depends on the distance separating it from the printing support, on the electrical resistivity of this support and on the strength of the electric field applied between the recording element 10 and the transfer roller 24. 3. Application to magnetographic printers.

Description

DEVICE FOR ELIMINATING POLLUTION DUE TO ACQUIRED ELECTRIC CHARGES
BY A PRINTING MEDIUM IN A MAGNETOGRAPHIC PRINTING MACHINE.

The present invention relates to a device for eliminating pollution due to electrical charges acquired by a printing medium in a magnetographic printing machine.

In modern equipment used for data processing, more and more printing machines are used in which the printing of characters is carried out without using the impact of types of relief printing on a sheet of paper. Receiving paper. This is how we know t magnetographic printing machines, of the kind which has been described and represented in the French patent application published under No. 2,305,764 (this application corresponding to the patent of the States - United of Iberia N "3,945,343), and in which the printing of the images is carried out by first forming, from electrical signals delivered by a control unit, a latent magnetic image on the surface of a recording element comprising a conductive metallic surface coated with a magnetic recording layer, this latent image consisting of a set of magnetized zones which, practically punctual, are usually designated under the name of magnetized points.

This latent image is then developed, that is to say made visible, using a powder developer which, consisting of thermoplastic resin particles containing magnetic particles, is only attracted by the magnetized points of the recording element, thereby forming a powder image on the surface of this element. After which, this powder image is transferred to a printing medium constituted by a sheet or a strip of paper. To this end, this printing medium is engaged between the recording element and a transfer roller which, consisting of a conductive cylinder coated with a layer of elastomer, is normally biased in abutment against the surface of this recording element, the region where this printing medium is thus engaged constituting the transfer station. The conductive core of this recording element being electrically connected to ground and the particles constituting the powder image formed on this element being, before arriving at the transfer station, charged at a given electrical polarity, the transfer of this image of
powder on the printing medium is produced, in a known manner, by
bringing the conductive cylinder of the transfer roller to an electrical potential of polarity opposite to that of these particles. Under these conditions, the particles of this powder image are subjected,
when they arrive in the transfer station, to the action of an electric field which, acting against the magnetic force which keeps these particles applied on the surface of the recording element, forces these particles to pass from this surface onto the print medium.

The printing medium, onto which the developer particles are thus transferred, generally exhibits a high electrical resistivity. It even happens, in certain cases, that this resistivity is of the order of
1014 ohms-centimeter. As a result, this printing medium, when it arrives in the electric field of the transfer station, tends to charge so as to create an electric field of opposite direction and thus generate, on its face which is in contact with the elastomer layer of the transfer roller, electric charges of the same sign as that of the potential of the cylinder of this roller, while electric charges, of opposite sign to that of the potential of this cylinder, appear on the face of the layer of elastomer which is in contact with the printing medium. Since this layer of elastomer is not a perfect insulator, the electric charges which have appeared on its face in contact with the printing medium are attracted by the cylinder of the transfer roller and, managing, at least in part, to pass through this layer, flow through this cylinder. Under these conditions, when the print medium leaves the recording element and the transfer roller, this print medium appears provided with electrical charges. of the same sign as that of the cylinder potential of this roller, which remain
"trapped" inside the texture of this printing medium, this printing medium thus being charged with an electrical polarity of sign opposite to that of the image charges located on the surface of the recording element.

However, from the time the print media separates from the
surface of the recording element, we observe, for a given value of 1 spacing between this support and this element and by virtue of the law of
Paschen, the generation of electrical aromas, which are manifested by electrical charges, of opposite sign to that of the potential of the cylinder of the transfer roller, which are then attracted by the "trapped" charges of the printing medium. which are thus of the same sign as those carried by the developer particles which have been transferred to the printing medium, therefore tend, when arriving on this medium, to expel the developer particles which are on the latter, which has the effect of causing, in the area of separation of the printing medium and the recording element, the formation of a cloud of developer particles. The particles thus expelled are entrained by the current of air which normally circulates along the recording element when the latter is set in motion, and will then deposit on different parts of the machine, thus creating real pollution .

Since the pollution caused by the deposition of these developer particles is due to the very strong electrification undergone by the printing medium following its passage in the transfer station, we therefore thought, to eliminate this pollution, to use a device for discharging the print medium of the type which has been described and represented in the British patent application published under the number 2 127 230, this device comprising an electrode supplied with an alternating electric voltage and disposed near of the transfer station, on the side of the face of the print medium which is opposite to that which is brought into contact with the surface of the recording element. This discharge device effectively made it possible, when it was thus mounted in a printing machine, the recording element of which was provided with an electrostatic or photoconductive recording layer, to effectively de-electrify sheets of paper which, having an electrical resistivity of the order of 1010 ohm-meters, left the transfer station, this de-electrification having the effect not only of facilitating the separation of these sheets from the surface of the recording element, but also of avoiding practically any pollution inside the machine. However, contrary to all expectations, these results could not be obtained when this same device was mounted in the same way in a printing machine equipped with a magnetic recording element. and using, as printing medium, a continuous paper strip having an electrical resistivity close to 1012 ohm-meters.

More specifically, the invention relates to a device for eliminating pollution due to electrical charges acquired by the printing medium in a magnetographic printing machine, this machine comprising a recording element formed of a conductive core electrically connected to ground and coated with a magnetic recording layer, and a transfer roller pressed against this layer to allow charged developer particles which have been deposited on this layer to be transferred to a printing medium engaged between this roller and this magnetic layer, said transfer roller being formed from a conductive cylinder coated with a layer of elastomeric material, and brought to a given electrical potential such as the electric field caused by the potential difference between said core and said force cylinder said charged particles to pass over said printing medium, said pollution elimination device being cons with an alternating current corona discharge device and being characterized in that it is disposed on the side of the face of the printing medium which has been in contact with said magnetic layer, and near the point where this medium d impression separates from this layer.

The invention will be better understood and other objects, details and advantages thereof will appear better in the following description, given by way of nonlimiting example, and with reference to the appended drawings in which s - Figure 1 represents a magnetographic printing machine equipped with a pollution eliminator device established according to the invention.

FIG. 2 is a detail view, showing, in an enlarged manner, the part, located in the vicinity of the transfer station, of the machine shown in FIG. 1, FIG. 3 is a view, in longitudinal section, of the pollution elimination device which equips the machine shown in the figure - Figure 4 is a photo showing the degree of pollution caused, after a certain time, by the deposit of developer particles on the cleaning nozzle which equips the machine shown in FIG. 1, when the pollution eliminator device is not used during the operation of the machine,
- Figure 5 is a schematic view showing the region in which the wire of the pollution elimination device must be positioned, - Figure 6 shows fragments of states printed on the printing medium when the pollution elimination device is in operation , - Figure 7 shows fragments of printed states obtained on the printing medium when a relatively short time has passed, this. time being counted from the moment the pollution eliminator stops working.

The printing machine which has been shown diagrammatically in FIG. 1 comprises a recording element which is constituted, in the example described, by a magnetic drum 10. This drum, formed of a conductive cylindrical core 11 coated with a layer magnetic recording 12 itself made of a magnetically hard material such as the nickel-cobalt-phosphorus alloy, is mounted so as to be able to rotate about a horizontal axis 13. The rotation of this drum, in the direction indicated by arrow F, is provided by an electric motor (not shown).

The recording of the information on this drum is carried out by a magnetic recording member 14 comprising several heads arranged one next to the other and aligned parallel to the axis of rotation 13 of the drum, these heads being kept pressed against the external surface. 15 of this drum.

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 15 of the drum, magnetized zones -16 practically punctual, all of these areas constituting a magnetic latent image corresponding to an image to be printed. These magnetized zones 16 then pass in front of a developer applicator device 17. which is arranged below the drum 10 and which makes it possible to apply to the surface 15 of this drum particles of a powdery developer contained in a reservoir 18. particles, which are formed of thermoplastic resin in which magnetic particles have been incorporated, adhere, in principle, only to the magnetized zones of the drum 10, so that these zones, after passing in front of the applicator device 17, appear coated a layer of developer, thus forming deposits 19 on the surface 15 of the drum. These deposits 19 then pass in front of a retouching device 20 which has the role of eliminating the developer particles which have adhered elsewhere than on the magnetized areas 16, as well as the particles which are found in excess in these areas. which, after passing in front of the retouching device 20, remain on the drum, then pass in front of an electrical charging device 21, which has the effect of charging, at an electrical polarity which will be indicated below, the resin constituting the particles of developer parading in front of this device.

After which, the particles thus loaded are transferred, almost entirely, onto a printing medium consisting of a continuous strip of paper 22 which is applied against the surface 15 of the drum by means of a transfer roller 23, the transfer of these particles taking place in a region H usually called transfer station. The transfer roller 23, which is normally biased against the drum 10 by an elastic element of known type (not shown), consists of a metal cylinder 24 mounted on an axis parallel to the axis of rotation 13 of the drum , this cylinder being coated with a layer 25 of elastomeric material.

As can be seen in FIG. 1, this cylinder 24 is electrically connected to the negative terminal (-) of a DC voltage source R, the positive terminal (+) of this source being itself set to masee.

On the other hand, the axis of rotation 13 and the conductive core 11 of the drum 10 are also electrically connected to ground. Furthermore, the charging device 21 is established so as to positively charge the resin constituting the developer particles which pass in front of it.

Under these conditions, it is understood that the developer particles which have thus been charged by this charging device 21 are subjected, when they arrive at the transfer station H, to the action of an electric field oriented in the direction going from the drum 10 to the transfer roller 23, this field having the effect of forcing the developer particles which adhere to the drum to pass over the paper strip 22 in contact with which they have been brought in. The residual developer particles which, when this transfer is carried out, are still on the drum 10 and are then removed by means of a cleaning device comprising, on the one hand, a suction nozzle 26 disposed above the transfer roller 23 and intended to collect the largest part of the developer particles remaining on the surface of the drum, on the other hand a brush 27 disposed downstream of the nozzle 26 and intended to perfect the cleaning carried out by the latter. The magnetized zones which have passed in front of the cleaning device then pass in front of an erasing device 28, which allows the portions of the drum 10 which have been demagnetized by this latter device to be able to be magnetized again when they appear before the registration body 14.

As can be seen in Figures 1 and 2, the suction nozzle 26 is placed near the transfer station H, so as to be able to collect, as soon as possible, the developer particles which, at the outlet of this post, remain on the magnetic layer 12 of the drum. In order to prevent these particles from sticking, by electrostatic effect, to sticking to the external walls of this nozzle, it is made of an electrically conductive material and is connected to ground. Under these conditions, the nozzle 26 is brought to the same electrical potential as the core 11 of the drum, that is to say here to a positive potential, so that the developer particles which, after their passage in the transfer station, remain charged at the same polarity as this nozzle does not, theoretically, tend to stick to the bottom wall 29 (Figure 2) of this nozzle.

The printing strip 22 on which the developer particles which have been transferred to the transfer station H are applied has a high electrical resistivity and a very low level of water content.

Thus, in the example described, this strip has an electrical resistivity practically equal to 1012 ohm-meters and a water content rate of less than 8 t. By way of comparison, it will be indicated that the elastomeric material constituting the layer 25 of the transfer roller has an electrical resistivity generally between 106 ohm-meters and 109 ohm-meters. Furthermore, the voltage V of the voltage source R is established at a value such that the electric field E which is exerted on the developer particles passing through the transfer station reaches a high amplitude, of the order of 2.106 volts per meter, this value of the field making it possible to overcome the action of the magnetic force which keeps these particles applied to the surface of the drum. However, by crossing this transfer station, the strip of paper 22 acquires, due to its very high resistivity electric and by virtue of a phenomenon which has been explained above, a very high electric charge whose polarity is of the opposite sign to that of the charges located on the surface of the conductive core 11 of the drum. Thus, in the example described, the strip 22 acquires a very strong negative charge. Under these conditions, when this strip separates from the drum, it occurs, in the separation zone Z (see FIG. 2) located immediately downstream from the transfer station, under Paschen's law and for a determined value of l spacing between the strip 22 and the surface 15 of the drum, electrical aromas which have the effect of driving out a significant portion of the particles which have been transferred onto the strip of paper 22. It was then found that, contrary to all expectations, the particles thus expelled were applied, in particular, to the bottom wall 29 of the nozzle 26, and this despite the fact that this wall is brought to the same electrical potential as the core 11 of the drum. This can be seen in the photo in FIG. 4 which shows the mass 30 formed by the accumulation of developer particles on the wall 29 of the nozzle 26 after half an hour of operation of the machine, this wall being shown, in this photo, in the position it occupies when, after stopping the machine, it is moved away from the surface of the drum by the operator, for cleaning. elsewhere observed that, if this wall was not frequently cleaned, the constitutive particles of the pile 30 ended, as a result of the weighting of this pile, by detaching from the wall 29 to fall back on the paper strip 22 and thus causing a real smearing of this strip.

However, the printing machine which is shown in FIGS. 1 and 2 is equipped with an anti-pollution device which, when it is put into action, eliminates the drawback indicated above. This device, which is shown in longitudinal section and is designated by the general reference 31 in FIG. 3, comprises a screen 32 which, made of aluminum, has a U-shaped section, and two insulating supports 33 and 34 which are fixed at the two opposite ends of this screen. This device further comprises a spiral spring 35, one of the ends of which is fixed to the support 34 by means of a screw 36, this screw also serving to secure the fixing of an electrical connection lug 37. end of the spring 35 is attached to one end of a metal wire 38, of small diameter, this wire being wound, at its other end, around a pin 39 engaged in a housing 40 of the support 33 and kept fixed to the by means of a locking screw 41. The screen 32 is connected to a terminal of an AC voltage source, the other terminal of this source being connected to the connection lug 37.

As can be seen with reference to FIGS. 2 and 3, the anti-pollution device 31, which is an electrical discharge device with a corona effect, is fixed to the bottom wall 29 of the nozzle 26, so that the wire 38 is oriented parallel to the axis of rotation 1.3 of the drum 10, that is to say transversely to the direction of movement T of the paper strip 22. This wire 38, which is made of a material such that the tungsten, the golden tungsten or the stainless steel, is moved away from the screen 32 by a distance of the order of six to seven millimeters and it is brought to an alternating electric potential of sufficient value to cause an electric shock corona effect. It should be noted here that the minimum value Vc of the electrical voltage which must be applied to the wire 38 to obtain this effect is all the higher the greater the wire has this diameter. Thus, for example, for tungsten wires having diameters of 40, 60, .n and 100 m, the minimum values of the alternating electric voltages necessary to trigger corona discharges are of the order, 2700 volts, 3100 volts and 3800 volts respectively. Furthermore, the device 31, which is located on the side of the face 50 of the strip of paper which has been brought into contact with the surface 15 of the drum, is positioned further so that the wire 38 is located, as shown in Figure 5, in the area
Z for separating the paper and the drum, this zone, located after the transfer station, being limited by the cylindrical surface 15 of the drum and the face 50 of the paper strip 22 which has been in contact with this cylindrical surface, this zone being further limited by the cylindrical surface (Q) admitting for axis the generator to the drum passing through H and for radius a length L corresponding to the distance, from H, beyond which the scent phenomena caused by l electrification of paper cease to occur. Practically this length L is of the order of 25 mm.

It has been found that, during the operation of the device 31, the wire 38 is subjected to vibrations and risks, if it is placed too close to the paper strip 22, of catching the developer particles which had been transferred onto this band. These same particles were also likely to interfere with the normal operation of the emission control system and thus adversely affect its effectiveness. To eliminate these risks, the wire 38 is kept tensioned under a mechanical tension whose value is determined experimentally so as to significantly reduce the amplitude of these vibrations, the adjustment of the tension of the wire to this value being carried out by means of the spring 35 and to the pin 39. Thus in the example described where the wire 38 is made of tungsten and has a diameter of seventy microns, the tension to which this wire is subjected is practically equal to four newtons. In addition, the wire 38 is positioned, as seen in Figure 5, so that the distance between it and the paper strip 22 is at least equal to a minimum value f below which the operation of the anti -pollution may be disturbed. It has been found, experimentally, that this minimum value f is practically equal to 4 mm.

Furthermore, when the anti-pollution device is operating, it is necessary to prevent the surface 15 of the magnetic drum from being reached by the ions generated by this device, these ions having the consequence, when they arrive on the magnetic layer of the drum, to cause an oxidation of this layer and thus to modify the magnetic properties thereof. To eliminate this risk, it is therefore necessary that the wire 38 is spaced from the surface 15 of the drum by a sufficient distance at least equal to a limit value d from which this magnetic layer practically ceases to be degraded. It has been found, experimentally, that this limit value d is substantially equal to 10 mm.

Ultimately, in order to satisfy all the requirements which have been set out above, it is important that the wire 38 of the anti-pollution device is placed in a region of the space limited essentially by three surfaces, namely the surface (Q ) of the cylinder having for its axis the generator of the drum 10 passing through H and for radius the aforementioned length L, the plane (P) situated at the aforementioned distance f from the portion of the strip 22 which, leaving the transfer station, is driven in displacement in the direction T, and the cylindrical surface (M) which, outside the drum, is located at the aforementioned distance d from the surface 15 of this drum. In the plan of Figure 5, this region is in the form of a small triangle ABC which, in this figure, has been hatched.

The alternating electric voltage which is applied to the wire 38 is adjusted to a value which is all the greater the greater the distance which separates this wire from the strip of paper 22, the greater the electrical resistivity of this strip of paper. high and that the value of the electric field between the drum 10 and the transfer roller 23 is higher. It should however be noted that this tension is always at least equal to the.

limit value Vc which was mentioned above, this limit value being that from which it is possible to obtain an electrical discharge with a corona effect. In the example described, the paper strip 22 has an electrical resistivity substantially equal to 1012 ohm-meters and the electric field established between the drum and the transfer roller is of the order of 2.106 volts per meter. Under these conditions, this strip electrifies very strongly when it crosses the transfer station. To de-electrify this strip, the anti-pollution device described above was used, this device comprising a wire 38 of seventy microns in diameter and producing electrical discharges with corona effect when the alternating electrical voltage applied to this wire is at less equal to 3200 volts. We first positioned this device so that the wire 38, while remaining located in the region represented by the triangle ABC in FIG. 5, is 6 mm from the paper strip 22. Then, after having applied to the wire 38 an alternating electrical voltage substantially equal to 3300 volts, the printing machine was put into operation and the state presented by the paper strip 22 was observed when it had been printed. FIG. 6 shows fragments of reports which have been printed, under these conditions, on the paper strip, these reports being noted after the machine has been operating for three hours without having undergone the slightest cleaning. see in FIG. 6, these printed states show practically no smearing, this effect resulting from the fact that no pollution took place during this period of operation of the machine. Examination of the wall 29 of the cleaning nozzle at the end of this period moreover made it possible to note that, practically, no particle of developer had been deposited on this wall during this period. This printing operation was also repeated after having previously positioned the wire 38 at a distance of 10 mm from the paper strip and then having applied thereto an alternating electrical voltage of 4200 volts. The results which have been obtained in this case are similar to those which are illustrated in FIG. 6. These printing operations have at last been repeated, but leaving the wire 38 voluntarily off. The results obtained in this case, after the machine has been operating for only half an hour, are illustrated in Figure 7. If we then compare the printed states shown in Figure 7 to those which are shown in Figure 6, we see that the states which are obtained when no electrical voltage is applied to the wire have many traces of smearing, these traces coming from a relatively significant pollution of the machine.

It has moreover been observed, at the end of this short period of operation of the machine> the presence, on the wall 29 of the cleaning nozzle, of a particularly dense mass of developer particles, such as that which is shown on the photo of of figure 4.

It should also be noted that the frequency of the alternating electrical voltage which is applied to the wire 38 'of the anti-pollution device which has just been described is adjusted as a function of the speed of movement of the paper strip 22, this frequency having to in fact be at least equal to a lower limit value NL which is all the higher the faster the speed of movement of this strip. Thus, for example, in the case where the strip of paper is driven at a speed close to thirty centimeters per second, this lower limit value is of the order of 40 Hz. In this case, we adopt , for convenience, the frequency of 50 HZ. Similarly, in the case where the belt drive speed is 1.5 m / s, this lower limit value is of the order of 250 Hz and we can then choose, for practical reasons, the frequency of 400 Hz.

Claims (9)

Claims  1.A device for eliminating pollution due to electrical charges acquired by a printing medium in a magnetographic printing machine, this machine comprising a recording element (10) formed of a conductive core (11) electrically connected to ground and coated with a magnetic recording layer (12), and a transfer roller (23) pressed against this layer (12) to allow charged developer particles which have been deposited on this layer to be transferred onto a printing medium (22) engaged between this roller and this magnetic layer, said transfer roller being formed of a conductive cylinder (24) coated with a layer of elastomeric material (25), and brought to a given electrical potential such that the electric field caused by the potential difference between said core (11) and said cylinder (24) forces said charged particles to pass over said printing medium (22), said pollutant elimination device tion consisting of an AC corona discharge device (31) and being characterized in that it is disposed on the side of the face (50) - of the printing medium which has been in contact with said magnetic layer, and near the point (H) where this print medium separates from this layer. 2. Device according to claim 1, characterized in that it comprises a corona discharge generating electrode constituted by a wire (38) extending in a direction transverse to the direction of movement (T) of the printing medium. 3. Device according to claim 2, characterized in that the wire (38) is positioned in such a way that the distance which separates it from the portion of the print medium having been in contact with the magnetic layer (12) is at least equal at 4 mm. 4. Device according to claim 2, characterized in that the wire (38) is positioned in such a way that the distance which separates it from the magnetic layer (12) is at least equal to 10 mm. 5. Device according to claim 2, characterized in that the wire (38) is positioned in such a way that the distance which separates it from the point (H) where the printing medium (22) leaves the magnetic layer (12) is at more equal to 25 mm. 6. Device according to any one of claims 3 to 5, characterized in that the alternating electric voltage which is applied to the wire (38) is adjusted to a value which is all the greater as the distance which separates this wire from the portion of the printing medium having been in contact with the magnetic layer is larger. 7. Device according to any one of claims 3 to 5, characterized in that the alternating electrical voltage which is applied to the wire (38) is adjusted to a value which is all the greater as the electrical resistivity presented by the support printing is higher. 8. Device according to any one of claims 3 to 5, characterized in that the alternating electric voltage which is applied to the wire (38) is adjusted to a value which is all the greater as the value of the electric field between l the recording element (10) and the transfer roller (23) is stronger. 9. Device according to any one of claims 2 to 8, charac-se in that the frequency of the electric voltage applied to the wire (38) is at least equal to a limit value (NL) which is all the greater the speed of movement of the print media is greater.