DE2054080A1 - Device for the electrostatic charge of toner particles - Google Patents

Description

Patent attorneys Dipl.-Ing. F. Weickmann, 2054080

Dipl.-Ing. H. Weickmann, D1PL.-PHYS. Dr. K. Fincke Dipl.-Ing. F. A. Weιckmann, Dipl.-Chem. B. Huber

8 MUNICH 27, DEN

MÖHLSTRASSE 22, CALL NUMBER 48 3921/22

XEROX CORPORATION, Xerox Square
Rochester, Ν.Ύ. 14603 / USA

Device for the electrostatic corona charging of toner particles

The invention relates to a device for the electrostatic corona charging of toner particles, in particular in front of a electrostatic image transfer of a toner image onto a copy sheet. Toner particles are in the non-image areas of a developed toner image charged so that they do not come with the toner particles of the image area parts are transferred to the copy sheet.

In the electrophotographic process, a recording medium is coated with a layer of photoconductive insulating material conductive pad used to generate latent electrostatic images of information to be reproduced. The record carrier is first and then evenly charged electrostatically on its photoconductive surface with a light pattern of the information to be reproduced exposed. The surface parts of the recording medium hit by the light lose their charge, while the remaining parts of the surface remain unaffected. This creates a latent electrostatic image on the recording medium generated whose configuration corresponds to the light pattern. The-

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This latent image can then be created by applying toner to the Record carriers are developed. The toner comes in fine applied in distributed form and can be made of synthetic resins or art ^ Resin mixtures exist in which a pigment, for example carbon black, is dispersed. The toner is attracted and bound in those surface areas of the recording medium, which have retained their initial charge, i.e. in the image area parts, whereby a toner image is formed which is on a copy sheet or another image carrier and fixed thereon to form a permanent reproduction of the original can be.

So that the toner particles can be attracted to the latent electrostatic image of the recording medium, they are before being applied to the recording medium, it is triboelectrically charged with a polarity that corresponds to the polarity of the charge of the image surface parts of the recording medium is opposite. A method of applying the toner particles to the Recording medium consists in the fact that the toner is mixed with a coarse carrier material and this mixture over the Record carrier is cascaded. As a carrier material, grains of glass or sand can be used with a Material are coated that has a distance from the toner in the triboelectric series, so that a triboelectric Charge is generated between the toner and the carrier material when the two are mixed together. During the mixing, the toner is attracted to the carrier particles, after which both together over the 'recording medium cascaded. In the image area parts, the toner is then separated from the carrier particles and on the recording medium bound, revealing the electrostatic latent image.

During the development process, toner particles also happen to be in the parts of the background that do not belong to the image deposited. This unwanted build-up of toner

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is caused by uncontrolled clouds of toner particles that accompany the flow of developer material when it passes over the Record carrier is cascaded. Furthermore, toner particles are occasionally deposited in these surface areas, the one Carry a neutral charge or a charge opposite to the charge favorable for development. Toner clouds are generated when carrier particles provided with toner hit the surface of the recording medium. The impact forces between the The recording medium and the carrier particles as well as the toner separate the toner particles from the carrier particles and effect the generation of a cloud-like mass which is found in all areas of the recording medium, including those not related to the image belonging surface parts, can deposit. This condition is exacerbated when working with a high concentration of toner will.

The deposition of toner in areas that do not belong to the image of the recording medium can be reduced somewhat if the tendency of the carrier particles, the toner, is exploited to be removed from the recording medium. This cleaning effect occurs when the carrier particles are not completely covered Toner particles are covered and so on the recording medium cascaded. They then attract toner particles that are bound to the recording medium by weak electrostatic forces are, for example, the toner particles bound in the surface parts that do not belong to the image. The cleaning process however, it is normally not sufficient to remove all of the toner particles in the base surfaces, especially if one high concentration of toner is used to develop the latent image.

To prevent the toner particles not belonging to the image Surface parts of the recording medium are also transferred during the image transfer of the generated toner image, the recording medium can be exposed to a corona discharge device that generates a low corona current between

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see the image development and the image transmission generated. By Apply a polarity of the discharge device opposite to that of the toner particles in the image area parts of the recording medium, the toner particles are in the surface parts of the recording medium that do not belong to the image charged to the same polarity as that of the discharge device, making it possible to use only the toner of the image area parts to be transferred to the copy sheet. If a corona discharge wire is used for this purpose, which is made of, for example If there is a platinum alloy, the movement of the corona wire occurs relative to the surface parts that do not belong to the image a tendency of the low corona current, the toner particles in these areas to one polarity opposite of those of the toner particles in the image area parts. Then the corona discharge wire moves on the image surface parts passes, its emissions are either repelled or by corona suppression around the image surface parts led around so that they remain unaffected. Corona suppression is a phenomenon in which positive corona emissions be repelled from the photoconductive surface by the stronger electrostatic field of the image surface parts. Although this procedure, when performed with a corona wire, will reduce background toner on the copy sheets made possible, it has been shown that the results obtained are not always satisfactory.

The object of the invention is therefore to create a device which has better electrostatic charging of the toner particles in particular in the surface parts of an electrophotographic toner image that do not belong to the image.

A device of the type mentioned is designed to solve this problem according to the invention in such a way that a knife-like discharge electrode is provided,

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which has a cross-section with an extremely small width compared to the height.

With this device, a stronger corona flow can be applied to the surface parts of the recording medium that do not belong to the image are supplied than is possible when using a corona wire, at the same time the image area parts not be affected. Therefore, the toner is set to one polarity in the area parts not belonging to the image opposite to that of the toner, the image surface parts are better charged, and the transfer of this toner can be better be prevented. Furthermore, a discharge device according to the invention is more stable and cannot break off or tear, as is possible with a wire. The device according to the invention can consist of the generally used conductive Substances exist and can easily be brought into the given shape.

The discharge electrode is knife-like and is at a threshold voltage for the emission of charge carriers operated. As a result of the better corona emission with this electrode, the toner is in the areas that do not belong to the image Surface parts are charged more evenly and more completely, so that no background drawings in a transferred toner image occur more.

The apparatus of the present invention is used in precharge prior to transferring a toner image onto a copy sheet. The knife-like electrode has a cross section whose width is extremely small compared to its Height is. The electrode is operated with the threshold voltage in such a way that a corona current occurs when the charge carriers are emitted between the developed recording medium and the electrode arises, which does not affect the toner particles in the Image belonging area parts of the developed toner image

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charges evenly without affecting the toner in the image areas. The polarity of the electrode voltage is opposite those of the toner particles in the image area parts, and those on the toner particles in the non-image The charge applied to belonging area parts is also opposite to that of the toner particles in the image area parts. Since the toner particles in the image area parts and in the area parts not belonging to the image are opposite to each other Have polarity after exposure to this pre-charge, only the toner particles in the image area parts are transferred to a copy sheet, creating background drawings on the Reproduction should be avoided.

The invention is described below using an exemplary embodiment shown in the figures. Show it?

Fig. Ί a cross section of a corona discharge device according to the invention and

Figure 2 shows a developed electrostatic latent image upon which the apparatus of the invention is applied.

A discharge device according to the invention can be used for pre-charging of toner images are used prior to image transfer, leaving an even charge on the toner particles is applied by non-image areas of a developed electrostatic image. This charge has a polarity opposite to the polarity of the toner particles in the image area parts, so that only the latter toner particles be transferred to a copy sheet in the subsequent image transfer.

Corona discharge devices are used in electrophotographic applications Method used for charging an electrostatic recording medium. In doing so, the corona discharge device

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arranged near the recording medium and switched on a voltage, which generates a corona emission at the discharge electrode. A wire is normally used as the electrode, which generates a stream of charge carriers on the recording medium and a charge of the recording medium on a Causes equilibrium tension. This stress is a function of shape and cross section for a given device the electrode of the discharge device, the medium between the electrode and the recording medium, the distance between the electrode and the recording medium, the potential difference between the electrode and the Recording media and many other factors. If the recording medium or part of the recording medium has a voltage below the equilibrium voltage, the recording medium thus picks up corona emissions, which also gives these areas the Gweight stress reach. On the other hand, if the equilibrium voltage is equal to or less than the voltage of the recording medium, so it does not absorb any corona emissions and remains electrically unchanged. This latter case arises when the potential difference between the recording medium and the electrode of the corona discharge device for generation an emission flow to the recording medium is not sufficient. In this situation, the emissions run from the discharge electrode in the direction of the recording medium, however, the areas of equilibrium are removed from the latter s tension repelled. A grounded or pre-stressed shield is usually arranged to cover the Partially surrounds discharge wire to absorb emissions, when the equilibrium voltage is reached on the recording medium, so that the electrode continues to operate emissions can be generated even if the recording medium does not accept them. Will no shield is used, the recording medium is brought to the equilibrium voltage, after which the electrode no further

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Emissions generated more as long as the potential difference is less remains than it is necessary to generate emissions.

The tendency of the recording medium in the area of the corona discharge device assuming an equilibrium tension acts in only one direction. The emissions have no effect on those areas of the recording medium whose potential is equal to or greater than the equilibrium voltage is. However, if the charge is unequal from surface element to surface element, as is the case with an electrostatic one, for example latent image is the case, those areas of the image which are below the equilibrium tension are brought to the equilibrium voltage, after which they are no longer influenced. The emissions are therefore selective on those areas of the recording medium that have a lower charge. It is known that when a small corona current is applied to a developed recording medium before the image transfer of the toner image onto a copy sheet, the toner in those not belonging to the image Surface parts of the recording medium is not transferred to the copy sheet together with the toner image. By operating of the corona discharge device with opposite polarity of the toner particles in the image area parts cannot Area parts belonging to the image to a polarity opposite to that of the toner particles in the image area parts to be charged. This will be done without much deterioration of those already present on the toner particles in the image area parts Charges reached. By such a charge before the image transfer and the polarity difference generated by it between the toner particles in the image area parts and the area parts not belonging to the image it is only possible transferring the toner particles in the image area parts to the copy sheet.

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In operation, the discharge device used for pre-charging Moved over the developed recording medium, the corona emissions not causing the toner in the image Charging areas belonging to the equilibrium potential. The fact that the toner particles of different charge and so that different potentials of positive and negative polarity are distributed in these areas, they are accordingly charged by the generated flow of charge carriers. A potential opposite to that of the toner particles in the image area parts can in this way on all toner particles in the area parts not belonging to the image regardless of their initial charge. The toner particles in the image area parts remain during this Process unaffected, as they cause a corona suppression. Corona suppression consists in repulsion the corona emissions from the recording medium due to the stronger electrostatic fields of the image surface parts. By this phenomenon has no effect on the image surface parts of the recording medium and / or the corona discharge the toner particles distributed therein.

If a corona discharge wire is used to generate the corona current, it is operated in such a way that only a small amount Corona current is transmitted to the recording medium in the surface parts not belonging to the image and that the corona emissions repelled or bent over in the image surface parts. In this way, the toner remains in the image area axes unaffected, because of the low corona current, the toner is not in the; areas belonging to the picture, however not very quickly and not heavily charged when the corona wire is moved past the recording medium. Will be a Rness edge-like electrode operated at threshold voltage, the pre-charging is carried out in a "deterrent process" carried out. This means that the men3orartir; formed electrode a pieringon corona

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emits a relatively higher corona current when it faces the parts of the image area, however, when it is not in relation to the image facing parts of the area. As a result, the toner in the image area parts is not affected by the pre-charging, as is the case with the wire electrode, but the strength and speed with which the toner gets into the not surface parts belonging to the image to a uniform charge opposite to that of the toner in the image surface parts is brought is much improved.

In Fig. 1 is a corona discharge device according to the invention shown in cross section, it is in operation close to a developed electrostatic recording medium the type shown in Fig. 2 is arranged. It has been found that the corona discharge device shown in FIG when operated with the threshold voltage for corona emission, it is very suitable for pre-charging prior to image transfer is. The electrode 11 is knife-like, i.e. it has a long and narrow cross-section in the direction of the Recording medium 21 out. It is partially surrounded by metal shields 12 and 13, which are used to hold charge carriers which are emitted from the top edge of the electrode and from a rigid electrode holder. The shields are held above the recording medium and, depending on the length of the electrode, carry one or more Electrode holder ^ 14. The holders can be of any suitable Insulation material that is sufficiently rigid to keep the electrode in a fixed position relative to the shields to keep.

The corona discharge device is arranged close to the developed surface of a recording medium so that its slim electrode cross-section is oriented towards the recording medium, as shown in the figures. The electrode can be made of any suitable material,

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which is suitable for the emission of charge carriers, for example Made of stainless steel material, it should be extremely thin compared to its height, as can be seen in the cross section. It is arranged in the longitudinal direction parallel to the surface to be charged and emits charge carriers over its entire length in a plane perpendicular to the plane of the drawing. Their cross-section is shown perpendicular to their length. Me cross-sectional area has a height that is perpendicular to the surface of the recording medium 21 is measured, and a width which runs parallel to the recording medium 21. It has been shown that an electrode with a cross-sectional height on the order of 2.5 cm and a width on the order of 0.025 mm is to be used preferably. These electrode dimensions are only an example of the cross-sectional ratios and can be changed within a wide range as long as the cross-sectional width is extremely small compared to the cross-section height. A stainless steel electrode with a cross-sectional height of approx. 2.5 cm and a cross-sectional width of approx. 0.05 mm was used as a further exemplary embodiment. partially surrounded with 22 mm long shields placed on each side of the electrode so that the electrode protruded approx. 4.8 mm below the shields. This arrangement was then reversed at the threshold voltage with one polarity that of the toner particles in the image area parts operated for corona emission. It turned out that close the highly charged areas of a recording medium have a low corona current and near the areas of lower charge a relatively higher corona current was generated. With this kind of charge, the toner particles remained in the image area parts or the areas of high charge unchanged, while the toner particles in the non-image areas or the Low charge areas to a uniform polarity opposite to that of the toner particles in the image area parts have been charged. As already described, the dimensions of the discharge device are only given as an example

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the general design is given and are not intended to limit the invention in any way.

A corona discharge device with a knife-like electrode was installed at different distances from the recording medium, Feed currents and different voltages between 100 and 900 volts, a voltage range normal for a latent image, were tested. It was found that at the threshold voltages for the corona emission, the knife-like corona device shows a steeply rising curve, if the current is plotted as a function of the voltage of the recording medium in the lower range of the voltage. this means that when the bias voltage of the recording medium increases, the corona flow through the recording medium is substantial is reduced. Furthermore, the voltage generated by the electrode remained for the upper range of the voltage of the recording medium Corona flow relatively low and constant. Since it is very convenient, in the areas of relatively high voltage, i. in the image area parts to generate a low corona current, in the areas of low voltage, i.e. in the not Areas belonging to the image to generate a higher current, the knife-like electrode is in particular for Carrying out a pre-charge before the electrostatic image transfer is suitable.

In Fig. 2 there is an electrostatic recording medium 21 shown, which consists of a photoconductive layer 22 and a conductive backing layer 25 provided for this purpose. An electrostatic latent image is created on this recording medium and according to known methods developed with toner particles. The image area parts 24 are represented by a strong toner coverage, while the Area parts 25 not belonging to the image contain only a few toner particles between the image area parts. The toner particles in the image areas adhere to the higher electrical

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static charges on the recording medium, for example initially charged to a positive voltage of 900 volts and exposed to the irradiated and not surface parts belonging to the picture was discharged to 600 volts. Then, negatively charged toner particles became negatively charged upon image development is applied to the more positively charged areas which are the image areas. However, toner particles can also be used are deposited in the non-image areas during development, which are then deposited during image transfer are also transferred and generate the unwanted background drawings on the copy sheet.

The transfer of the toner of the areas not belonging to the image is effected by the discharge device according to the invention 10 avoided. This is operated in the range of the threshold voltage for the corona emission and sprays the toner in the areas not belonging to the image with a charge whose polarity is opposite to that of the toner in the Image areas, for example, is positive. So in the image areas there is a positive charge from the toner particles recorded while the charge of the toner particles in the image area parts remains unchanged. Then it can be developed Image can be transferred onto a copy sheet using a positive polarity discharge device, the toner in the areas of the recording medium that do not belong to the image are not used as background drawings on the copy sheet transfer.

It is known to those skilled in the art that development with positive toner for negatively charged image areas or reversal development, i.e., development of the background areas with toner, the polarity of which corresponds to that of the image area parts agrees, can also be carried out with a discharge device according to the invention by changing the polarity in each case is chosen. For example, when converting

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reversal development a negative optical image for the image-wise distributed discharge of an initially negatively charged recording medium can be used, creating a latent image that does not belong to the image areas contains negative charges. The latent image is then developed with negatively charged toner and the pre-charge is turned off done with a positive knife-like electrode. the The pre-charging device does not always have to have the same polarity as the initial charge of the recording medium, but rather polarity only opposite to that of the toner particles in the image area parts.

By using a polarity opposite to that of the toner particles in the image area parts, the toner particles in the image area parts are not affected. this is due to the corona suppression, which either suppresses the current of charge carriers on the toner in the image area parts or redirects to the areas that do not belong to the image. For example, under corona suppression, the repulsion is equal positive emissions from the surface of the recording medium due to a stronger electrostatic field Understand potential on the recording medium. As a result, the corona discharge device has no effect on the Image surface parts of the recording medium and the toner particles present thereon.

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Claims (6)

Claims Device for the electrostatic corona charging of toner particles, in particular prior to an electrostatic image transfer of a toner image on a copy sheet, characterized in that a knife-like discharge electrode (11) is provided, which has a cross-section with an extremely small width compared to the height. 2. Apparatus according to claim 1, characterized in that the cross-sectional width is on the order of 0.025 mm and the cross-sectional height is on the order of 2.5 cm. 3. Apparatus according to claim 1 or 2, characterized in that the electrode (11) close to the surface to be charged (21) is arranged and transfers charge carriers in a corona flow to the surface (21) over its entire length, with its Cross-sectional height is perpendicular to the surface (21), and that the electrode (11) with a voltage in the range of a Emission of charge carriers causing threshold voltage is connected. 4. Apparatus according to claim 3, characterized in that the electrode (11) in the area facing away from the surface (21) to be charged with a grounded shield (12, 13) and is provided with a holder (14) within the shield (12, 13). 5. * Device according to one of the preceding claims, in particular for charging toner particles in the background area parts of a toner image, characterized in that the polarity of the voltage connected to the electrode (11) opposite to the polarity of the charge in the image area parts present toner particles. 109820/1892 6. Device according to one of the preceding claims, characterized by an arrangement for generating a relative movement between the electrode (11) and the surface to be charged (21). 109820/1892