LU81179A1 - DEVELOPMENT OF ELECTROSTATIC LATENT IMAGES USING A LIQUID TURNING PRODUCT AND A DEVELOPING ELECTRODE - Google Patents

Description

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The present invention relates to a method and a device for developing latent electrostatic images using a liquid toning product and a developing electrode.

5 'It is known to form an electrostatic image on the surface of an electrophotographic member first applying a uniform electrostatic charge to the surface and then exposing the surface to a light pattern; then, the latent electrostatic image thus formed is developed by a toning product before it has deteriorated.

The two most common types of cornering products commonly used for developing latent images 1. electrostatic are liquid products and dry powder products. Liquid toning products, which seem necessary for high spatial resolution, are essentially formed by small pigmented particles, called toning particles, dispersed in an insulating liquid. When a liquid toning product is brought into contact with a surface containing an electro-static latent image, for example by immersing the surface in a chamber containing a liquid toning product, the toning particles in the liquid migrate towards the surface and are deposited on the parts bearing the image by a phe-► nomene known as electrophoresis. v25 If one wishes to reproduce a continuous tone image or an image containing a solid area, a developing electrode is usually used to assist in the developing operation. The development electrode consists, at the base, of a plane and electrically 30 'plate - conductive placed close to the surface carrying the image, and parallel to it. The developing electrode changes the field configuration of the electrostatic image and increases the field in space over areas of uniform charge. The developing electrode further intensifies the electric field near the image bearing surface. Normally, the developing electrode is electrically short-circuited by convection of electrophotographic organ. It has been found that special 2 * effects can be obtained, such as an inversion of tone or a reduction in haze, by establishing a difference in direct current potential between the developing electrode and the electrophotographic member during the development process.

: We found that liquid toning products! tended to generate space-stable "convection cells" during the development process when used with a development electrode. These cells 10 of convexioni, generally of spherical shape, are formed! * over a range of field strengths and have the effect of subtracting I * · parts of the surface of the electrophoto organ- •> i. ’Graph at contact with bend particles. By 't \ 1 following the convection cells, an optical density pattern! 15 of cellular shape is superimposed on the motil of the image and it is visible in the developed image. The size of the convexioni cells that form depends on the force

Idu champ ', of the volume concentration of the turning particles and the space between the electrophotographic surface "20 and the developing electrode. The range of field strengths over which the convection cells form can vary depending on the composition of the particular toning liquid product. It is believed that such convection cells! (* are formed as a result of hydrodynamic instability of! * 25 liquid toning products.

The production of these convection cells. can be avoided by performing the development operation at field strengths - outside the range at which cells tend to form. This can be achieved: I 30 by changing the space between the electrophoxographic organ j · and the developing electrode because the field strength varies directly with the voltage on the photoconductive surface of the electrophotographic organ and conversely with the distance between the developing electrode and the electrophotographic member. This means is not satisfied because most electrophotographic organs require! a certain range of field strengths to produce good <3 continuous tone images or good images containing solid areas, this range being in the range or comprising at least part of the range of field forces where the cells of convection ..

Thus, it is necessary to find a more feasible technique to prevent the formation of such convection cells in order to promote uniform deposition of the turning particles in areas of uniform charge density during the development of the electrostatic images. 10 ticks.

* Accordingly, the present invention provides a method for developing an electrostatic latent image "formed on the surface of an electrophotographic member using a liquid toning product formed of toning particles suspended in an insulating liquid and a developing electrode. , and according to the invention, an alternating bias voltage is applied between the developing electric and the electrophotographic member through the liquid toning product during the developing operation.

Furthermore, the present invention provides a device for practicing this method comprising a developing electrode, a turning chamber between the electrode and the electrophotographic member 5 for receiving a turning suspension formed of turning particles. suspended in an electrically insulating liquid, the development electrode being spaced from the electrophotographic member and, according to the invention, an alternating voltage source is connected between the development electrode and the electrophotographic member to generate between them, an alternating bias voltage.

The invention will be better understood, and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the schematic / appended drawings given solely by way of example illustrating r- * Φ * * / k%!

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* 1%! i several embodiments of the invention, v in | which: - Figure 1 is a schematic sectional view of a device for developing an electrostatic latent image on the surface of an electrophotographic organ according to the teachings of the invention, some of the parts not being " good proportions for ease of representation j - Figure 2 is a photomicrograph magnified Ί0 ten times, showing the cell density pattern formed on an image developed with a liquid toning product and a developing electrode as a result of the production! 'tion of convection cells in the toning product; liquid during the development process; and!' 15 - - Figure 3 is mie photomicrograph magnified ten times showing how the cell motif above indicated is greatly reduced by applying the teachings of the invention.

Referring to FIG. 1, it shows an electrophotographic organ, I 20 generally identified by the reference 11. The organ 11 comprises a photoconductive layer 13 and a conductive substrate 15 · The substrate 15 comprises a conductive layer 17 and a base or carrier "! 9 - The photoconductive layer 13 is formed by high frequency sputtering of cadmium sulfide and the conductive substrate 15 is a layer of tin-indium oxide on a polyester plastic base.

! A developing electrode 21 is provided for! help develop an electrostatic latent image on the surface? of the photoconductive layer 13. The electrode 21 is placed above the electrophctographic member 11 and a certain amount of a development product 23 of the liquid toning type is placed in the space between the electrophotographic member 11 and the de-JJ velorpeinent electrode 21. The liquid toning product 23 which is confined in a chamber (not shown) comprises toning particles 25 suspended in a non-conductive liquid (electrically insulating 27). The particular 5 fl toning liquid product used depends on the characteristics of the particular electrophotographic organ. A preferred liquid toning product comprises a pulverized carbon and resin powder dispersed in an isoparaffinic 5-hydrocarbon fraction marketed by Exxon Company in Houston, Texas, USA, under the name of "Isopar".

It is believed that the alternating bias voltage eliminates the hydrodynamic instability of the liquid toning product which produces the convection cells 10 by interrupting or disturbing the otherwise unidirectional movement of the toned charged particles toward the photoconductive surface. These interruptions or disturbances cause changes in direction of the forces exerted by the particles on the liquid in which they are suspended.

The AC bias waveform is preferably symmetrical in shape. The range of frequencies and amplitudes of the alternating polarization over which the formation of convection cells is either prevented or reduced to a remarkable point, depends on the particular toning fluid employed, the distance between the electrophotographic member and the development electrode and maximum charge voltage on the electrophotographic organ.

'25 If the frequency of the AC polarization is either too high or too low, the resulting changes in the direction of movement of the bending particles will have insufficient effect on the direction of movement of the insulating liquid and therefore will not prevent the 30 production of convection cells. If the amplitude of the * alternating polarization is too low, this will not effect the movement of the turning particles controlling the movement of the insulating liquid. If the amplitude of the AC polarization is too high, this will prevent the production of the convection cells but will also prevent the turning particles from settling on the surface of the gold! electrophotographic gane.

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Preferably, the amplitude at single alternation of the polarization in alternating current must be equal,, ’at least to the maximum charge voltage on the photoconductive surface. If the amplitude does not reach this level, j 5 the bias voltage cannot. Sufficiently interrupt the movement of the turning particles to prevent the production of the convection cells in the areas of the image at the highest tensions. The effectiveness of alternative polarization also depends on its waveform.

For a given frequency and amplitude, a slot is more effective in preventing the formation of cells. convection than a sinusoidal wave, and a sine wave is more efficient than a triangular wave. It has been found, however, that any waveform can be used to prevent the formation of convection cells! appropriately adjusting the frequency and / or amplitude of the alternating current.

For any combination of the electrophotographic member, the charging voltage, the liquid toning product and the physical arrangement, there is a conbi-! optimal amplitude, frequency and waveform of polarization in alternating current, l being able to prevent the formation of convection cells | which are believed to be responsible for the annoying cellular optical density pattern 25 in uniformly charged image areas. The optimal combination can be determined in each application case.

j In the present case, an alternating current source 29 is connected between the developing electrode 2! 2Q and the conductive layer 17 to produce a bias i in alternating current between the two elements. The polarized! ! tion in alternating current is applied between these two? elements to prevent the formation of convexio cells in the liquid toning product. The AC bias voltage is applied at least during the period of time that the liquid toning product is placed between the two organs during the process of developing the electrostatic image.

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7 b The invention will be better described with reference to the examples which follow.

Exempte_l · i i ... · An electrophotographic organ containing a! 5 photoconductive layer of cadmium sulphide pulverized at high frequency was uniformly charged by corona effect at a negative surface potential · of 25 volts, spokes in j contact with a grain-free rag, exposed to a | light pulse of xenon discharge then developed; , 10 for a period of the order of 15 seconds using i * a toning product comprising a pulverized powder of carbon and resin dispersed in Isopar, and a developing electrode parallel to the photoconductive surface of the organ electrophotographic and spaced * 15 from it by 0.25 mm. During the whole development operation, the electrode and the electrophotographic organ were both kept at ground potential. The developed image contained a large cell density pattern overlaid on the image pattern. A photomicrograph of the developed image is illustrated in Figure 2.

Example_2.

Example 1 was repeated except that during the development operation, a symmetrical alternating and sinusoidal polarization having a peak-to-peak amplitude - 25 from 50 volts and a frequency of 44 Hz and a DC level of 0 volts was applied between the development electrode and the electrophotographic organ. The significant reduction in the cell density pattern is illustrated in Figure 3.

30 Exemplg_5.

Example 1 was repeated again with the exception that the electrophotographic member was charged at a negative surface potential of 10 volts, that the alternative pola-35 risaticn voltage had a peak-to-peak amplitude of | 68 volts and a frequency of -7.5 Hz and that the product of. s * 8 t i t! liquid turn was diluted by a factor of 2. The cell density pattern was completely eliminated.

! * "• JL ·? · 1 I An electrophotographic organ of the same type as j I t 5 that used in example 1 was uniformly changed by '· corona effect until a negative surface potential of 15 volts and exposed to a luminous pattern of the image . The organ was then developed for a period of the order of 15 seconds using a liquid toning product and a developing electrode parallel to the surface. photoconductive of the photoconductive member and spaced from it <0.38 mm. The liquid toning product consisted of a powdered carbon and resin powder dispersed in Isopar, but having no fixing resin as in Example 1. The table below shows the results without i application of a bias voltage then with application of a slot at different frequencies and amplitudes.

jI Frequency Peak amplitude Pattern of d- j Example of the peak-to-peak of the junction of the jer- cants alternating cells; ternative i —----------.

! I IV No bias voltage Present | I --- - -——- i 25 V 5 Hz 50 volts Eliminated i. ____! VI | 10 Hz 50 volts Eliminated: VII | 10 Hz 25 volts Eliminated! Example 7 shows that the alternative polarization effectively removes the cell density pattern even if! the peak-to-peak voltage is less than the over- potential! face on the photoconductive surface of the electro-photographic organ. It will be noted that the concept of the application I35 of an alternating bias voltage to prevent the formation of convection cells in a liquid bending product J J dispersed between two conductive and parallel members 9 can also find its use in devices for 'Electrophoresis display where a certain amount' of a liquid toning product is arranged between two parallel electrodes connected to voltage sources of different polarities.

The alternating bias voltage applied according to the teachings of the invention can be superimposed on a DC bias voltage applied for another purpose, such as a tone inversion, for example.

Of course, the invention is in no way limited to the embodiments described and shown which have only been given by way of example. In particular, it includes all the means constituting technical equivalents of the means described, as well as their combinations, if these are carried out according to the spirit and implemented in the context of the claims which follow.

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Claims (5)

1. Method for developing an electrostatic latent image jj formed on the surface of an electro-photographic organ using a liquid toning product ii · * in the form of toning particles suspended in an insulating liquid and a developing electrode characterized in that an alternating bias voltage is applied between said electrode and said electrophotographic member j, through said liquid toning product during the operation! * of development. 2. The method according to claim 1, characterized; in that the waveform of the aforementioned polarization alternating voltage is of uniform configuration. 3. Method according to any one of claims 1 or 2, characterized in that the waveform 15 of the above-mentioned alternating bias voltage is sinusoidal. \ I 4. - Method according to any one of claims 1 or 2, characterized in that the waveform 'l of the above-mentioned alternating bias voltage is rec-20 tangular. 5. Method according to any one of the preceding claims, characterized in that the amplitude! peak-to-peak of the above-mentioned alternating bias voltage represents at least twice the voltage of; 25 maximum load on the above-mentioned electrophotographic member. 6. Method according to any one of the preceding claims, characterized in that the voltage al-; The above polarization ternative is applied continuously during the development operation. ; j jq 7 * - Method according to any one of the preceding claims, characterized in that the amplitude j peak to peak of the alternating bias voltage | above is about 50 volts. / I j 8. - Method according to any one of the claims i J ver * - j ^ 5 ". '11 cations 1 α 6, characterized in that the peak-to-peak amplitude of the alternating bias voltage is of the order of 18 volts and in that the frequency is 1. of the order of 7.5 Hz. 5 9 · - Method according to any one of claims 1 to 8, characterized in that the aforementioned liquid toning product comprises a powdered spray of carbon and resin dispersed in a fraction of soparaffinic hydrocarbon. 10. - Method according to any one of claims 1 to 6 and 9, characterized in that the distance between the aforementioned development electrode and the aforementioned electrophotographic member is of the order of 0.25 mm and in that the above-mentioned alternating bias voltage has a peak-to-peak amplitude of the order of 50 volts and a frequency of the order of 44 Hz. 11. Device for practicing the method according to any one of the preceding claims of the type comprising a turning chamber between the aforementioned developing electrode and the aforementioned electrophotographic member for receiving a turning suspension formed of particles of curve in suspension in an electrically insulating liquid, said developing electrode being spaced from said member, characterized in that an alternating voltage source is connected between said developing electrode and said electrophotographic member to generate between them an alternating bias voltage. / "~ ~, ψ v