CH641576A5 - PROCESS FOR DEVELOPING A LATENT ELECTROSTATIC IMAGE AND APPARATUS FOR IMPLEMENTING IT. - Google Patents

Description

The present invention relates to a method for developing a latent electrostatic image and an apparatus for its implementation.

There are already a number of methods for developing latent electrostatic images. The latter can be formed by charging a photoconductive surface, then by exposing the charged surface to light or to ionizing radiation intended to make the photoconductive material which is present in the areas exposed to light or to radiation conductive. , thereby neutralizing the areas which receive light. The latent electrostatic image can be formed on a dielectric sheet, by excitation of styles intended to form the desired electrostatic pattern on the dielectric sheet. The development of the latent electrostatic image was first carried out by electric attraction of fine particles towards the regions of the latent image formed on the photoconductive surface, preferably using particles carrying electric charges whose polarity is opposite to that of the charges of the latent image. These particles are usually of the dry type, and the name xerography has therefore been used for this process. Subsequently, it was found that particles could be suspended in a dielectric liquid and that the latent electrostatic image could be developed by immersion of the latter in the liquid. When a dry powder has been used as a developing agent, it has usually been transferred to plain paper or another backing sheet, before being fused. This operation requires heating and the fine particles are then formed from a hot-melt resin. When a support sheet, for example of paper, is coated with a photoconductive material, for example of zinc oxide, no transfer of the electrostatic image is necessary after its development, this last operation being carried out with a developing liquid containing particles of a toning agent or toner.

When the technique progressed, it was found that it was convenient for the latent electrostatic image to be developed by a liquid agent, the developed image then being transferred to plain paper, so that the fusion of an image developed by a powder no longer needed. The developing liquid usually contains a hydrocarbon vehicle, for example of the Isopar-G type or the like, in which particles of turning agent intended to make the latent image visible are dispersed. The immersion of the photoconductive surface carrying the latent electrostatic image in a bath of a liquid development agent requires wetting of the entire surface of the photoconductor. The amount of liquid developer remaining on the developed image can be largely removed by a scraper roller. However, there still remains a small amount of liquid agent on the entire photoconductive surface. This remaining liquid, with the developed image, is then transferred to paper or another medium, and

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should be dried on the paper. This operation causes a constant evaporation of the small amount of liquid vehicle transferred from the photoconductive surface to the support paper. This evaporation is undesirable for the following reasons. First, the need for drying the sheet not only consumes a certain amount of energy, but also takes a time such that the copying speed is limited by this operation. In addition, the ratio of the liquid vehicle to the turning agent is constantly changing. When a battery of electrophotographic copiers is used in a small chamber, the evaporation of the liquid vehicle in this chamber also poses problems.

Note that it is desirable that the latent electrostatic image can be developed by applying a developing liquid only in the region of the image and not on the photoconductive surface on which the latent electrostatic image has been formed. If this characteristic could be obtained, since the area forming the image only corresponds to 5 to 7% of the total surface in the case of current copies, the potential evaporation would be reduced correspondingly. If a roller, for example a printing roller, is coated with a developing agent and if this roller can touch the photoconductor carrying the latent electrostatic image, the photoconductor and the latent image are covered with the developing liquid. There then remains development liquid on the zones which do not form the image as well as on the zones representing the image, and the contrast between the zones forming the image and the other zones constituting the background, on the conductive surface, is reduced, so that the copy obtained is not satisfactory. It was then realized that, if the roller had hollow parts and if the developing liquid used was more concentrated, that is to say if the ratio of the particles of tackifier to the liquid vehicle was increased so that the developing liquid contains more solids, such an intaglio printing roller would place the developer on the latent electrostatic image without leaving developing liquid on the regions which do not form the image. A scraper blade could then remove the developer liquid from the protrusions, the recessed portions carrying the developer liquid. It is found that wiping the protruding regions so that they are clean is more difficult than can be expected. The wiping blade does not remove all of the liquid from the protruding areas, so that threads can form and the resulting image is not clear. In addition, this formation of threads increases as the copying speed increases, so that the number of copies that can be made during a given time is limited.

We have also sought to remedy the drawbacks of using an application roller in contact with the drum, by separating this roller from the photoconductive surface and by attracting the development liquid from the application roller to the photoconductive drum, under the action of the electrostatic field of the latent electrostatic image. These attempts were not satisfactory because the developed image is neither clear nor free from defects, given the unpredictable behavior of the developing liquid when it arrives on the photoconductor.

The invention aims to create a process which does not have the drawbacks and defects which have hitherto prevented the satisfactory use of the rollers for applying a developing liquid.

U.S. Patent No. 3084043 describes the use of a belt having protrusions forming ridges and hollows. A layer of conductive ink is applied to the belt which is then brought into contact with the photoconductor carrying the latent electrostatic image.

U.S. Patent No. 3,232,190 describes the passage of a film carrying particles of a dry toning agent close enough to a latent electrostatic image for the dry particles to jump across the space and allow the development of the image. This process requires the melting or fixing of the tackifying particles after transfer to the support sheet.

U.S. Patent No. 3,383,209 describes the use of an applicator roller having grooves for applying a conductive ink to a latent electrostatic image. The applicator roller is in contact with the photoconductor, and the development is carried out by selective wetting.

U.S. Patent No. 3,486,922 describes the development of a latent electrostatic image using a conductive and aqueous developing liquid. The application roller is in contact with the surface carrying the electrostatic image. The surface is hydrophobic, so that only the image is developed, the areas forming the background not being, since they form a surface which drives away water.

U.S. Patent No. 3,560,204 describes the use of an acidic or basic water-soluble dye as an auto-fixing ink. The surface carrying the latent electrostatic image passes near a rotating drum having an input layer, but not in contact with it. There is a field between the applicator drum and an electrode placed behind the image, so that an unsupported column of ink, having the shape of the image, jumps across the space and forms a inked image. This patent also indicates that non-polar liquids, for example toluene or a nitrile of silicone, can be used and that they do not jump as quickly on the other side of space as the solutions with aqueous base.

U.S. Patent No. 3,667,428 describes the use of an applicator roller having a smoothly patterned surface for transferring developing material onto a latent electrostatic image. The application surface has a doctor blade for removing excess developer or developer. This is described as containing polar and non-polar liquids and dry powder electroscopic materials. The applicator roller is in contact with the photoconductive surface.

U.S. Patent No. 3772012 describes a reverse development process. The operation is carried out by polarization of the development agent application device practically at a potential equal to the maximum potential of the surface forming the image. Development is carried out by placing the application surface close enough to the surface of the electrostatic image so that the polar developing liquid is attracted from the hollow parts of the application surface to the image surface, with a negative image configuration.

U.S. Patent No. 3,839,032 describes the use of application rollers disposed in contact with a photoconductive drum carrying a latent electrostatic image so that a developing liquid is applied to and develops that image.

U.S. Patent No. 3,886,900 describes the use of an application roller for dynamically forming a pond or meniscus so that a developing liquid is applied to a latent electrostatic image.

U.S. Patent No. 3,974,554 describes the use of a roller for applying a fluid for developing electrostatic latent images, formed with a scroll design having grooves and protrusions of convex shape. , the roller being in contact with the latent electrostatic image.

U.S. Patent No. 3,991,711 describes a roller for applying an electrically conductive ink to an electrostatic image, the roller being formed from an electrically conductive elastomer and being placed in contact of the photoconductive surface.

U.S. Patent No. 4021586 describes an applicator roller for alternately entering and exiting a developing liquid bath. A coating electrode placed in the bath is intended to cause the electrodeposition of particles of a bath turning agent on the roller when the latter is in the bath. The roller is then removed from the developing agent and placed in contact with the photoconductive surface carrying the latent electrostatic image which is mounted on a rotating drum. A scent is intended to reduce the quantity

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of liquid vehicle contained in the deposit of toning agent of the applicator roller before the contact of the latter with the photoconductive surface.

The invention is defined as stated in claims 1 and 9.

The invention implements the phenomenon of electrophoresis which appears on the photoconductor during the transfer of the developing liquid from the applicator roller to the surface of the photoconductor and after this transfer. There is no column of developing agent precisely corresponding to the shape of the latent electrostatic image. Small amounts of developing fluid, roughly corresponding only to the fields of the latent electrostatic image, leap across the space, and electrophoresis continues after the developing fluid has left this space and reached the photoconductive surface. This post-transfer electrophoresis allows clearly delimited images to be obtained, with the application of small quantities of developing liquid for short periods to the latent electrostatic image. In this way, an apparatus intended for implementing the invention can operate quickly and can therefore form a number of copies, for a given time, greater than that which can be obtained with known copiers. In addition, the copies formed are sharp and practically dry, since the entire surface of the photoconductor has not been wetted by the developing liquid. The quantity of liquid carried by the applicator roller can be adjusted by forming, in this roller, hollow parts having a predetermined capacity. The applicator roller can be wiped off with a doctor blade so that the quantity of liquid is leveled in the hollow parts of the roller. However, it is found that it is advantageous that a smooth applicator roller is used in combination with a scraper roller which rotates in the direction of rotation of the applicator roller. The variation of the speed of the scraper roller rotating in the opposite direction allows the adjustment of the volume of the liquid carried by the applicator roller between close limits. Light liquid hydrocarbons are normally dielectric. A polar compound or charge directing agent must be added to the developing liquid so that the turning agent particles are charged, or the manufacture of these particles themselves must be such that they have the proper polarity and, in addition, that the developing liquid is made conductive in the desired manner so that the developing liquid jumps across the space. This conductivity must not reach a value such that the developing liquid partially discharges the latent electrostatic image, after its transport.

The electrophoresis subsequent to this postponement or to this transport is facilitated by a developing electrode whose role is to increase the fields of the latent electrostatic image. The image can also be made even sharper before being transferred to a support sheet such as plain paper, by fully illuminating the image developed using light as described in the United States patent application. America N ° 958979 filed November 9, 1978 by Benzion Landa. This global illumination by light not only facilitates movement towards the support sheet, but also increases the density of the image carried by this sheet. In a variant, the electrophoresis after transport can be increased by a corona discharge carried out on the revealed image with a polarity opposite to that of the latent electrostatic image.

The characteristics and advantages of the invention will be better understood on reading the description which follows of exemplary embodiments and with reference to the appended drawings in which:

- fig. 1 is a schematic section of a copier intended for the implementation of the method of the invention;

- fig. 2 is an elevation in partial section, along the arrows 2 of FIG. 1, representing the applicator roller and the scraper roller;

- fig. 3 is an elevation in partial section, similar to FIG. 2, showing the applicator roller driven by the photoconductive drum, the scraping roller and the assembly comprising a wiping blade being removed;

- fig. 4 is an enlarged schematic perspective of part of the surface of an applicator roller having recessed parts

5 formed by knurling;

- fig. 5 is an enlarged partial section along line 5-5 of FIG. 1;

- fig. 5a is a section similar to FIG. 5, showing circumferential grooves forming hollow parts on the applicator roller;

- fig. 6 is a plan view of the part shown in FIG.

5;

- fig. 6a is a plan view of the part shown in FIG.

5a;

is - fig. 7 is an enlarged plan view showing grooves formed axially in the applicator roller and constituting the hollow parts, and

- fig. 8 is a schematic section of a copier, similar to FIG. 1, intended for the implementation of an alternative method of ac-

2o centuation of the phenomenon of electrophoresis during the development of the latent electrostatic image.

More specifically, as indicated in the drawings, a conductive drum 2 having a layer 4 of a photoconductive material is mounted, by means of discs 6 provided with openings, on a shaft 8 on which the discs are held by a key 10 so that the drum rotates with the shaft counterclockwise as indicated by arrow A in fig. 1. The photoconductive drum and the assembly placed around it are mounted in a light-tight casing (not shown) in a known manner. The shaft 8 can be grounded or polarized at any DC voltage desired by a source not shown, in known manner. A corona discharge device 12 is intended to apply a charge to the photoconductor 4 when the latter passes in front of the device. When the photoconductor is based on selenium, the charge has a positive polarity. When the photoconductive material is based on zinc oxide bound by a resin or is an organic photoconductive material, for example polyvinylcarbazole or the like, the charge is negative.

When the photoconductor has received a charge, it passes in front of the objective 14, mounted in the housing and intended to project an image which must be reproduced on the charged photoconductive surface. As a photoconductor is an insulator in the dark and a conductor in the light, the projection of the pattern of light and shadow representing the image on the photoconductor allows the illuminated regions to conduct the charges in the photoconductor to the conductive drum. 2 then, via the discs 6, towards the shaft 8. A latent electrostatic image is thus formed on the surface of the photoconductor 4. The image has a high potential of the order of 800 to 1000 V beyond from that of tree 8, although this value can vary between very distant limits. A reservoir 16 formed of an insulating material is intended to contain a developing liquid 18. An applicator roller 20 which can be formed from a conductive resin such as Buna N is mounted near the photoconductor. Buna N, as is known, is a syn-55 thetic rubber formed by copolymerization of butadiene and acrylonitrile. The applicator roller can be formed from synthetic resin or metal. When it is made of electric synthetic resin, the thin lines of the latent electrostatic image are developed, but not the spread regions. In the case of a metallic roller, although the development takes place on wide areas forming the image, arcs can appear in image areas of high intensity and can give copies with white lines. horizontal at the locations of the arches. Buna N is convenient in practice, because its conductivity can be precisely regulated and because it resists hydrocarbon oils which are the liquid vehicles used for the toning agents used according to the invention.

The surface of the roller may be smooth or textured, i.e. it may have recessed portions intended to contain a

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certain volume of developing fluid. Fig. 4 represents a knurled surface, FIGS. 5 and 6 a surface having cavities, FIGS. 5a and 6a a roller having circumferential grooves concentric with the axis of the roller, and FIG. 7 axial grooves parallel to the axis of the roller, as indicated by the references 20 ', 20 "and 20"'.

The conductivity of the roll is important and optimum results are obtained when the surface of the roll is smooth and polished and when the material of the roll has a resistivity of between 107 and 109 μ cm. Optimal characteristics are obtained with minimal breakdown when the resistivity of the material of the roll is between 10 8 and 5108 fì cm. The resistivity of the roll is important, as described in more detail below.

The space separating the applicator roller from the photoconductive surface, at their point of greatest approximation, indicated by the facing arrows bearing the reference B, has a width of between 38 and 100 | i. Good characteristics are obtained when the space separating the roller from the surface is approximately 50 | A. The liquid is separated from the image by 12 to 75 | i.

According to the method of the invention, the developing liquid jumps from the applicator roller to the latent electrostatic image due to the presence of the electrostatic fields of the image. In regions where the fields are intense, breakdown may occur and may cause a discharge of the latent electrostatic image along the axis of closest approximation of the roller and the photoconductive surface. It is noted that the appearance of such a breakdown can be reduced considerably by using a slightly conductive roller, as indicated above. The conductivity can be regulated by loading Buna N rubber, or another synthetic resin resistant to hydrocarbon oils, by disseminated carbon particles which give a certain conductivity to the material which forms the roll.

The amount of developing fluid that jumps across the space is large. It is noted that the volume of the development liquid carried by the applicator roller must be between 1.5 and 4.7 mm3 / cm2. Such a value can be obtained by adjusting the hollow parts when the applicator roller is textured. Such a textured roller is simply wiped off by a not shown scraper blade, in a well known manner, so that the quantity of liquid remaining in the hollow parts of the roller is leveled. In this way, it is certain that a predetermined quantity of developing liquid is present on the applicator roller and can be transported to the other side of the space delimited between the roller and the photoconductive surface during the development of the image. latent electrostatic carried by this surface.

Preferably, the applicator roller is driven so that it moves at the same speed as the surface of the photoconductor. Perfect speed equality is not necessary. It is important that the relative movement of the roller and the surface of the photoconductor is regular and not variable. The roller can be driven at a speed slightly higher than that of the photoconductive surface carrying the latent electrostatic image. If this speed is too high, however, the electrostatic image can be submerged. If the roller is driven too slowly, the image tends to not be revealed enough.

As shown in fig. 1, a scraper roller 22 is arranged so that it rotates with a shaft 24 driven by a belt or chain 26, itself driven by a pulley or a sprocket 28 with chain which is controlled by a motor 30 at speed variable. The direction of rotation of the scraper roller 22 is the same as that of the applicator roller 20. A scraper blade 32 removes excess revealing fluid from the scraper roller and keeps it clean. The scraper roller is kept at a distance between 25 and 125 n from the applicator roller 20, by rings 34. The quantity of liquid carried by the applicator roller, as indicated above, must be between 1.5 and 4.7 mm3 / cm2. This quantity of liquid is easily dosed by varying the speed of rotation of the scraper roller and by varying the space C between the scraper rollers and the applicator. A small space allows a relatively slow speed of rotation of the scraper roller; a relatively large space between the two rollers requires a greater speed of rotation of the scraper roller. The latter can rotate at various speeds under the control of the variable speed motor 30. At high speeds, the volume of developing liquid carried by the applicator roller is so small that the latent electrostatic image carried by the photoconductive surface is little revealed and very dry. When the speed of the scraper roller is too low, the images obtained are very wet. The speed should be such that the proper volume of developing fluid is applied to the scraper roller. This speed is between 100 and 200 rpm and depends on the value of the space B separating the applicator roller from the photoconductive surface and from the particular developing liquid which is used. The smooth surface applicator roller, combined with a variable speed scraper roller, is a very flexible combination and has a number of advantages. The amount of toning agent carried by the applicator roller can be easily adjusted. A wiping blade 36, pressing against the surface of the applicator roller, keeps the latter in clean form. When using a textured roller, the volume of developing liquid is fixed and no adjustment of the surface of the roller can be made according to variations in the developing liquid. In addition, the recessed portions of a textured roller tend to accommodate deposits which change the volume capacity of the applicator roller.

As shown in fig. 1, the applicator roller can be driven by a variable speed motor 38, by means of a belt or chain 40 driven by a pulley or a chain wheel 39 so that it rotates at a speed such that the relative displacement of the surface of the applicator roller and the surface of the photoconductor is almost zero. When the applicator roller is driven faster, a greater amount of developing liquid is presented for the development of the latent electrostatic image. When the applicator roller rotates slightly more slowly, the amount of developing fluid presented and able to jump across the space for the development of the latent electrostatic image is lower. When using a textured roller, the adjustment of the speed of the applicator roller allows adjustments according to variations in the developing liquid and changes in the capacity of the hollow parts, due to the collection of deposit therein. this. The applicator roller is biased at a voltage higher or lower than that of the mass by a potentiometer identified by the general reference 42 and intended to transmit a biasing voltage via a conductor 44, to a shaft 46 with which the applicator roller turns. Since a relative speed of movement between the surface of the applicator rollers and that of the zero-value photoconductive drum is usually optimal, the construction shown in FIG. 3 easily allows this characteristic to be obtained. The applicator roller 20 has flanges 21 at both ends. These flanges bear against the photoconductor 4 and they are fixed to the roller so that they rotate with it. The roller is mounted so that it rotates with the shaft 46, and it is insulated from ground so that the bias voltage applied to the shaft 46 can be adjusted by the potentiometer 42. It is noted that the friction between the flanges 21 and the surface of the photoconductor on which they bear causes the rotation of the roller, so that the relative displacement of the surface of the applicator roller and the surface of the photoconductor carried by the drum 2 is practically zero.

As shown in fig. 1, the applicator roller 20 is shown in a manner which allows its rotation independent of that of the photoconductive surface. The shaft 46 of the applicator roller carries end discs 50 which are fixed to the inner rings 52 of ball bearings, the outer rings 54 of which bear against the photoconductive surface, as indicated with reference to FIGS. 1 and 2. The shaft 46 passes through the bearings 47 which are supported with play by the frame 51 of the copier, and it carries a pulley or a pinion 53 with chain intended to cooperate with the belt or the chain

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40. Springs 49, mounted between the bearings 47 and the frame, push the outer rings 54 against the surface 4. As indicated previously, the relative speed of the applicator roller and the photoconductive surface can be adjusted by varying the speed of the motor. 38..

The scraper roller 22 is similarly kept at a distance from the applicator roller, by a ball bearing, the outer rings of which form the rings 34. The inner rings 33 of the ball bearing are mounted on end portions of reduced diameter of the roller, fixed to the shaft 24 of the scraper roller, this shaft protruding from the housing 51 and carrying a pulley or a sprocket 23 with chain. The motor shaft 30 drives a pulley or a chain sprocket 28. The belt or chain 26 is disposed between the member 23 carried by the shaft 24 and the member 28 carried by the motor shaft. The variable speed motor 30 is intended to rotate the scraper roller 22 in the same direction as the applicator roller, at any desired speed.

As shown in fig. 1, the reservoir 16 has a pipe 15 connected to a reserve of developing liquid (not shown) so that this liquid is transmitted to the reservoir. A level adjustment device, not shown and of known type, is intended to maintain the level of the developing liquid in the reservoir at a predetermined height. As shown in fig. 1, the applicator roller 20 is mounted so that its lower part plunges into the developing liquid placed in the reservoir, and it is intended to transport the liquid from the reservoir to the adjacent latent electrostatic image, carried by the photoconductor as described above .

The composition of the developing fluid is important. When it is too conductive, it causes a partial discharge of the latent electrostatic image. When it is not sufficiently conductive, the quantity of developing liquid which jumps across the space is not sufficient and the development is insufficient. The developing liquid used differs from known liquids in that the concentration of the toning agent is higher than that which has been used until now, and in that the liquid vehicle is a hydrocarbon oil whose vapor pressure is lower than that of the hydrocarbon liquids used in known copiers with liquid developer. Most known development fluids have Isopar-G as the liquid vehicle. It is a narrow cut of isoparaffinic hydrocarbons having an initial boiling temperature of 159 ° C and a final boiling temperature of 174 ° C. The flash point is above 38 ° C. However, this hydrocarbon evaporates completely in 2200 s when it remains exposed, according to the measurements corresponding to method No. 11 fixed by the rules of The Federation of Societies for Paint Technology, United States of America. According to the invention, it is desirable that the evaporation of liquid hydrocarbon vehicles is reduced and, consequently, an isoparaffinic hydrocarbon of higher boiling temperature is used, for example Isopar-M, the temperature range of which boiling is between 210 and 252 ° C and the vapor pressure is lower. Its flash point is 77 ° C and it takes 3500 s for only 10% to evaporate, according to the conditions of method No. 11 of The Federation of Societies for Paint Technology. It is practically a light mineral oil. Other light mineral oils, such as Marcol 52 and Marcol 62, are very refined white mineral oils, devoid of unsaturated compounds and having high chemical stability. These are odorless and tasteless liquids that meet all the criteria for internal, medical and cosmetic applications. They are manufactured by Humble Oil and Refining Company, and have a viscosity of 51 and 61 s Seybolt at 38 ° C, respectively. These oils meet the criteria of FDA regulation 124.1146. Marcol 52 oil has a density of 0.827 at 15.6 ° C and Marcol 62 oil has a density of 0.834 at the same temperature. These light mineral oils are used as a liquid vehicle in which the concentrated particles of turning agent are dispersed. The development of the image is carried out by electrophoresis, that is to say by migration of the particles of toning agent in the liquid vehicle, towards the latent electrostatic image which is a pattern of charges carried by the photoconductive surface and corresponding to the pattern of light and shadow projected on the charged photoconductor. Liquid vehicles have a very low vapor pressure and the small amount of liquid vehicle that jumps out from across the space carries the particles of bleaching agent that ensure image development by electrophoresis. The droplets of turning agent liquid jump out of the other side of the space and roughly arrange themselves according to the fields of the electrostatic image. This electrophoresis continues after the passage on the other side of the space and a small amount of liquid vehicle remains on the photoconductor and effectively allows this electrophoresis. When the developed image is transferred onto a support sheet, for example of paper, the liquid vehicle immediately disperses in the paper and, although the image is wet on the photoconductor, it is practically dry and does not spread on the paper on which it is reported, as described in more detail below. It should be noted that, although the use of liquid vehicles has been described more precisely, the use of which is advantageous, any suitable dielectric liquid which does not dissolve the particles of toning agent is also suitable.

In the development agent by electrophoresis, the amount of solid material reaches 5 to 30% of the weight of the liquid vehicle. Liquid hydrocarbons are dielectric and do not jump across the space. A polar compound, for example a charge control agent, must be added to the developing liquid so that the method can be implemented according to the invention without the applicator roller being in contact with the photoconductor. As described in U.S. Patent No. 3,669,886, these charge management agents are well known in the art. These are polar compounds, soluble in the liquid carbon-carbon vehicle, and they create an electrical charge on microscopic particles of pigment (turning agent) which are intended to migrate by electrophoresis during the development operation.

The charge control agent must be such that it gives a negative polarity when the photoconductor is intended to be positively charged and that it gives a positive polarity when the photoconductor is intended to be negatively charged. Charge management agents are not necessary when the pigment particles are of such a nature that they themselves conduct the conductivity of the developing liquid. It is known that the grinding with a ball mill of long chain polymers degrades them and that these materials thus acquire a charge. A charge can also be imparted to long chain polymers by dyeing selected polymers or by adding a selected comonomer during the polymerization process. The liquid vehicle remains dielectric, but the parts charged with toning agent make the developing liquid conductive as a whole. The conductivity of the developing liquid is essential for a space of given size. When the space is larger, the conductivity should be greater. For example, purely by way of nonlimiting illustration, for a space of 50 ji, a conductivity of between 50,000 and 1,000,000 pS is required. When the conductivity is too high, the latent electrostatic image partially discharges. When the conductivity is too low, the amount of developing liquid that jumps out from the other side of the space is not sufficient and the image is not properly developed.

After transporting the developing agent from the application roller to the photoconductive surface, development by electrophoresis, which started during transport, continues. The particles of virage agent, outside the charged parts of the latent electrostatic image, move towards these charged parts, inside the limits of the latent electrostatic image. This electrophoresis is increased, in an exemplary implementation of the invention, by using a field electrode 60 which is polarized by a direct current source such as a battery 62 of accumulators. The polarity of the polarization of the deve5 electrode 60

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The development is the same as that of the charge of the photoconductor, that is to say that that of the charge of the latent electrostatic image. When the photoconductor is formed from selenium, the charge is positive; when the photoconductor is of organic type or based on cadmium sulfide, the polarity is negative. It is observed that the field electrode, when it is grounded, forms a ground plane which effectively facilitates electrophoresis, even in the absence of a polarization field. The space marked by the reference D, between the field electrode and the photoconductive surface, must be as small as possible in practice. The role of the field electrode is to increase the projection of the electric field of the latent electrostatic image at a greater distance from the surface of the photoconductor. However, the space should not be so small that the electrode touches the image. As the developing liquid transported to the photoconductive drum never exceeds a thickness of 25 to 50 (i, when the electrode is placed at a distance between 300 and 380 p., There is no contact between the electrode 60 and the latent electrostatic image carried by the photoconductive drum 4. Another characteristic must be taken into account: the distance separating the surface of the photoconductive drum from the field electrode, that is to say the space D, must be less than 10 times the thickness of the photoconductor. If this distance exceeds the indicated value, the increase in resolution obtained using the field electrode is reduced. The optimal spacing is equal to the thickness of the photoconductive layer. This characteristic obviously cannot be obtained since the photoconductive material of the drum is usually very thin. The role of the field electrode is different from that of the conventional development electrode used in the development apparatus, that is to say that it is different from the role of the applicator roller 20 which acts like a developing electrode during the transport of the developing liquid from the applicator roller to the photoconductive surface. The role of developing electrodes of known type is to prevent the deposition of the toning agent on the zones which do not form the image or which form the background, on the photoconductor. This operation obviously takes place only when the developing liquid is in contact with both the electrostatic image carried by the photoconductor and the developing electrode. During the implementation of the invention, electrophoresis takes place essentially after transport. The developing electrode accentuates the electric fields which are normally weak marginal fields at the edge of the charged regions forming the latent electrostatic image. The field electrode applies a voltage corresponding to the same potential as the charged image, but to a value greater than that of the highest charge of the latent electrostatic image. The developing electrode used according to the invention creates field patterns on the surface of the photoconductor which facilitate the mobility of the particles of toning agent in the small quantity of developing liquid which has been transported on the photoconductive drum and which is found outside the drawings forming the loaded image. In this way, the electrophoresis continues on the photoconductor, quickly and efficiently, during the period following transport, and allows further development to take place in a more advanced manner than otherwise. The developing electrode 60 must be as long as possible since the electrophoresis can then continue to a greater extent and allows a sharper developed image to be obtained.

A low-power illumination lamp 60, for example of the type described in the aforementioned US patent application No. 958979, is placed so that the photoconductive surface and the developed image are fully illuminated by of light, after the image has passed electrode 60. The operation not only ensures better image clarity, as described in the aforementioned patent application, by discharging the areas forming the continuous background or not forming not the image, but also allows a more detailed transfer of the image developed on the support, for example of paper, on which the image is transferred.

The use of a mineral oil at a high boiling temperature leaves a developed wet image on the photoconductor, although the amount of liquid vehicle is small and the humidity is small. A support material such as paper 66 is transmitted by rollers 68 in front of the pressure roller 70. It can be seen that the latter allows the transfer of the wet image from the surface of the photoconductor to the support sheet 66 when it it is absorbent, for example formed from paper. This transfer is facilitated by polarization of the roller, in a manner known in the art. When the support sheet is a transparent film of a synthetic resin which is not absorbent, a corona charging device 72 can charge the back of the support sheet and can ensure the image transfer from the photoconductive surface. to the backing sheet. The polarity of this charge must be the same as that of the charge of the developed electrostatic image. When using selenium, the charge is very positive; when using cadmium sulfide or an organic photoconductor, the charge is negative. The developed image is drawn from the surface to the backing sheet. An extraction blade 74 separates the support sheet 66 from the photoconductor, the sheet being transmitted by a roller 76 to a tray or to another copy processing device, for example a collating machine or the like. A cleaning roller 78 wipes the surface of the photoconductive drum and a wiping blade 80 finishes cleaning the drum so that the surface of the photoconductive is clean.

We now refer to FIG. 8 which represents a variant of the apparatus according to the invention and which indicates that, after the developing liquid has been applied to the image by the roller 20 and when the image is still wet, it is subjected to an emanation of a device 82, having a polarity opposite to that of the electrostatic image so that the electrophoresis, which started after the transport of the developing liquid from the applicator roller to the latent image carried by the photoconductive surface, is accentuated. It is noted that this arrangement greatly improves the sharpness of the copy formed.

Furthermore, in the embodiment shown in FIG. 8, when the image has been developed, it is transferred to the sheet 66 of a copy material by passing this sheet 66 of the rollers 68 for positioning identified with the grip of a roller 84 for transfer and the surface 4. The roller 84 is formed from a conductive and deformable material which can for example be Buna N rubber to which carbon particles in sufficient quantity are added so that it has the desired conductivity. Any suitable device, for example a storage battery 88, constitutes a source of a bias potential of the shaft 86 of the roller 84. Under the combined action of the potential applied by the battery 88 and the pressure of the roller 84 , the developed image is transferred from the surface 4 to the sheet 66 which is then separated from the surface of the drum by the device 74 and which is transmitted to a tray or to another paper processing apparatus, in a manner well known in the art. the technique.

During operation, the photoconductive drum is rotated by a motor or another not shown drive device which rotates it in the direction of arrow A. When a copy is to be made, the drum is loaded by the device 12 of charge by corona, and it is then exposed to an image of the document, a copy of which must be made, the image being projected by the objective 14 on the surface of the photoconductor. A latent electrostatic image then forms. The applicator roller 20 rotates at such a speed that there is practically no relative displacement of the photoconductor and the applicator roller. This can be polarized to the desired value so that the quantity of developing liquid carried by the areas of the photoconductor which do not form the image is reduced during the transport of the developing liquid. This polarization, taking into account the electrophoresis process implemented after transport, is not of as great importance as in the case of known copiers using developing liquids. The part closest to the applicator roller 20 is separated from the surface of the photoconductor 4 carried by the

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drum 2 by a space whose width is preferably of the order of 50 (i. The scraping roller 22, which is wiped by a blade 32, is driven at the appropriate speed by the motor 30 and in the opposite direction, if although the volume of the development liquid carried by the applicator roller is suitable for the formation of a pleasantly developed image having the desired contrast. The applicator roller plunges into the reservoir 16 which contains the development liquid 18, the blade 36 keeping the application roller clean, this roller can, if necessary, be driven independently by the motor 38, at a speed such that the relative displacement of the application roller and the surface of the photoconductor is not zero, but such that the surface of the application roller moves faster than that of the photoconductor. The developing liquid, as described above, is formed by a liquid vehicle having a vapor pressure lower than that of ès liquid vehicles conventionally used and having a concentration of solids, relative to the liquid vehicle, which is higher than that used conventionally. The conductivity of the developing liquid is precisely adjusted by adding polar compounds or charge directing agents so that the liquid conducts enough so that a sufficient quantity jumps across the space, but it doesn’t is not conductive to the point of causing a partial discharge of the latent electrostatic image carried by the photoconductive surface. When the developing liquid has jumped from the other side of the space, development is ensured by electrophoresis after transport, although part of the development may take place during this transport. A field electrode 60 for increasing development by electrophoresis is polarized at the same polarity as the latent electrostatic image and is arranged as close as possible to the photoconductive surface without the developing liquid being able to touch the field electrode. The latter accentuates the electric fields which are normally weak marginal fields, at the edges of the charged zones, and it facilitates electrophoresis by accentuating the mobility of the particles of viral agent in the liquid vehicle, apart from the drawings charged with the 'picture.

After passing the developed image in front of the electrode 60, the regions of the background which do not form the image are discharged by the low level overall lighting lamp 64 which illuminates the whole of the developed image. The sharpness of the image is accentuated and the image transferred to the support sheet, for example of paper, may have a better density. When the backing sheet is formed from paper or other absorbent material, a slightly wet image is transferred by simple pressure. The small amount of liquid vehicle in the wet image is quickly absorbed by the paper, and the copy formed on the paper is dry without heating.

The operation of the apparatus shown in fig. 8 is the same as that of the apparatus shown in FIG. 1, until the photoconductive surface 4 leaves the applicator roller 20. After this moment, the image to which the development agent has been applied is subjected to the emanation of the device 82 which notably increases the sharpness. Then, the locating rollers 68 locate a sheet of copy material 66 in the grip of the roller 84 and the surface 4 carrying the developed image. Under the combined action of the pressure of the roller 84 and the polarization provided by the battery 88, the image is transferred from the surface 4 to the sheet which is then separated from the drum and transmitted to the user.

Thus, the advantages indicated at the beginning of the description are obtained. We are thus dealing with a development method using a development liquid in a copier on plain paper, the development of the image carried by a photoconductive surface being adjusted so that only the regions charged with the image latent electrostatics are revealed practically without wetting the uncharged areas or the bottom. The proposed device includes a roller for applying the development liquid to the latent image via a space. This applicator roller comprises a device for static or dynamic adjustment of the volume of liquid presented by the roller to the latent electrostatic image which is separated from the roller by a space. The static adjuster may include a wiping blade and a textured roller having a predetermined volume capacity. The applicator roller may be a smooth roller used in cooperation with a scraper roller intended to rotate in the same direction as the applicator roller, at a speed adjusted so that the volume of liquid carried by the applicator roller is dynamically metered .

The method described here is such that it implements electrophoresis not only during the period of transport of the developing liquid from the applicator roller to the conductive surface, but essentially after this transport. The method allows the use of developing liquid whose ratio of solid materials to liquid vehicle is higher than that of liquids normally used, the liquid dielectric vehicle having a vapor pressure lower than that of vehicles used in known manner.

The conductivity of the developing liquid is controlled by the use of turning agent particles which have polar characteristics and constitute charge directing agents. The conductivity of the developing liquid composition as a whole is such that it is sufficient for the liquid to jump to the other side of the space, when using the process of the invention, but it does not reach a value such that it could cause a partial discharge 45 of the latent electrostatic image. The field electrode, or the application of a scent after development, accentuates the field of the latent electrostatic image and facilitates electrophoresis after transport. You can also use a post-developmental scent so that the electrophoresis is accentuated.

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Claims (18)

641,576 2 1. A method of developing a latent electrostatic image on an image-carrying surface, characterized in that a conductive developing liquid is transferred, which comprises particles loaded with toning agent dispersed in a dielectric carrier liquid, through a free space up to said surface to carry out a first development of the image, which is induced by the electrophoresis of the transferred particles by subjecting the image thus detected to an electric field producing a potential of the same polarity as the 'image, to obtain a second further development, and that the image thus developed is transferred to a support sheet. 2. Method according to claim 1, characterized in that the voltage of the electric field is lower than the breakdown voltage of the air. 3. Method according to one of claims 1 or 2, characterized in that the electric potential is greater than the potential of the image. 4. Method according to one of claims 1 to 3, characterized in that the free space has a thickness less than 100 | X. 5. Method according to one of claims 1 to 4, characterized in that the developing liquid has a conductivity between 50,000 and 1,000,000 pS. 6. Method according to one of claims 1 to 5, characterized in that the transfer comprises the formation on the surface of an applicator of a layer of developing liquid with a thickness of the order of 15 n . 7. Method according to one of claims 1 to 6, characterized in that the transfer comprises the formation of a layer of developing liquid on an applicator having a resistivity between IO7 and 109 fi • cm. 8. Method according to one of claims 1 to 7, characterized in that the transfer comprises the application on the back of the support sheet of a potential helping this transfer of the image from the image-carrying surface to the front of the backing sheet. 9. Apparatus for implementing the method according to claim 1, characterized by means for transferring a conductive developing liquid, which comprises charged agent of dispersing agent dispersed in a dielectric vehicle liquid, through a free space up to 'to said surface for carrying out a first development of the image, a field electrode spaced from the image-carrying surface, means for bringing the electrode to a potential of the same polarity as the image, spacing and potential being such that the resulting field is less than the dielectric strength of the air and does not cause discharge by emanation, means for subjecting the image having undergone the first development to the electric field, for carrying out a second, more advanced development of the image, and means for transferring the image thus developed onto a support sheet. 10. Apparatus according to claim 9, characterized in that the potential of the electrode is greater than that of the image. 11. Apparatus according to one of claims 9 or 10, characterized in that the thickness of the free space is less than 100 ft. 12. Apparatus according to one of claims 9 to 11, characterized in that the developing liquid has a conductivity between 50,000 and 1,000,000 pS. 13. Apparatus according to one of claims 9 to 12, characterized in that the means for transferring the development liquid comprise an applicator as well as means for forming on its surface a layer of development liquid with a thickness of order of 15 (i. 14. Apparatus according to one of claims 9 to 13, characterized in that the means for transferring the developing liquid comprise an applicator having a resistivity between 107 and 109 fi • cm. 15. Apparatus according to one of claims 9 to 14, characterized in that the transfer means comprise means for applying to the back of the support sheet a potential helping this transfer of the image from the image-carrying surface to front of the backing sheet. 16. Apparatus according to one of claims 9 to 15, characterized in that the means for transferring the developing liquid comprise an applicator roller spaced from the image-carrying surface, and that the apparatus comprises a spaced-out drying roller of the applicator roller as well as means for rotating the spin roller. 17. Apparatus according to one of claims 9 to 16, characterized in that the means for transferring the developing liquid comprise an applicator roller which comprises a pair of bandages pressing against the image-carrying surface so as to hold the roller applicator spaced from this surface. 18. Apparatus according to one of claims 9 to 17, characterized in that the means for transferring the developing liquid comprise an applicator roller as well as means for applying an electrical potential thereto.