COLOR ELECTROPHOTOGRAPHIC METHOD

Abstract: In a color electrophotographic process wherein color toners are developed by toner flying under D.C. electric field, electrostatic capacitance of photo­ conductor (29) is selected under 170 pF/cm², so that undesirable discharging through the toner layer is prevented, and color contamination due to the discharging is prevented thereby providing clear color image printing.

Description: The invention relates to a color electrophotographic process and apparatus as indicated in the precharacterising portions of claims 1 and 18. Such a color electrophotographic process and apparatus are used for color copying, color printing or the like. A color electrophotographic apparatus has been proposed, wherein a plurality of different color toner images are formed on a photoconductor performing a plurality of sequential steps. These steps include electric charging, exposing a color toner image to light and developing. The resulting color toner images are then simultaneously transferred to a recording medium in order to obtain a composite color picture. FIG. 1 is a schematic cross-sectional side view of such a conventional color electrophotographic apparatus using a conventional process, which is e.g. disclosed in Japanese published unexamined patent application Sho 60-95456. This color electrophotographic apparatus comprises a photoconductor 1 which is made of selenium-tellurium (Se-Te) and which rotates in the clockwise direction, a corona charger 2 which electrically charges the photoconductor 1, a laser beam scanner 3, developers 4, 5, 6 and 7 which contain yellow, magenta, cyan and black toners respectively, an image recording paper 8, an eraser lamp 9, a corona transfer means 10, a fuser 11, a cleaning blade 12 and another eraser lamp 13 for resetting the surface potential of the photoconductor 1 to the previous state. FIG. 2 shows the detailed construction of the developers 4, 5, 6 or 7. They each comprise a two-component developer in a developer container 14 (hereinafter developer) which contains a mixture of a positively charged toner 20 and of a magnetic carrier, a rotary developing sleeve 15 which is made of aluminum or a similar non-magnetic metal, a magnet roller 16, a layer thickness control blade 17 for controlling the thickness of the layer on the developing sleeve 15, a scraper 18 for scraping off the developer 14 after completion of the developing, a rotary blade 19 for stirring up the developer 14, toner 20 to be supplied, a toner supplying roller 21 and an electric power source 22 for producing a toner flying potential which a D.C. potential superposed by a high voltage alternating potential. For setting up the developer in a developing state, the toner carrier 15 is connected to the electric power source 22. For setting up the developer in a non-developing state, the toner carrier 15 is electrically floated or grounded, or a negative D.C. potential is applied thereto. In the following a color electrophotographic process using the above apparatus is described. First, an electrostatic negative latent image for a yellow toner is formed (in which the surface potential of the photoconductor is decreased along the line-image by the image exposure by positive electric charging of the photoconductor 1 by the corona charger 2 and by scanning the charged surface exposing it to light signals from the laser scanner 3 corresponding to the yellow image. A yellow toner image is formed on the photoconductor 1 by reverse development of the electrostatic latent image from negative to positive by the developer 4 which contains the yellow toner. In this case, only the developer 4 containing the yellow toner is connected to the electric power source 22, and other developers 5, 6 and 7 are adjusted to the non-developing state which will be described later. After development by the yellow toner, the yellow electrostatic latent image is erased by irradiating whole photoconductor 1 by the eraser lamp 13. By similarly performing the processes of electric charging, image exposing, developing and light-erasing of the electric charge toner images of yellow, magenta, cyan and black are formed on the photoconductor 1. After finishing the formation of all the toner images of four colors, the electrostatic latent images are erased by the eraser lamp 9, and the toner images are electrostatically transferred on a plain (ordinary) paper 8 by the corona charger 10. The toner images transferred on the plain paper 8 are fixed by application of heat from the heated fuser 11. After electrostatically transferring the toner images, the remaining toners on the photoconductor 1 are cleaned up by the cleaning blade 12. Then one cycle of color image printing is over and the photoconductor 1 is prepared for next image formation. When applying the above process a second toner image is formed on the photoconductor 1, which accordingly already bears a first toner image, a second toner (e.g. cyan toner) is deposited on the first toner image irrespective of non-irradiation of signal light, thereby to producing undesirable color mixing. The extent of such an undesirable color mixing (resulting in impure colors) increases in proportion to the thickness of the first toner layer. Hence a color image having a high color density, requiring a specially thick toner layer, is not obtainable. Researches have been made for finding the cause for the undesirable color mixing and the following was found out: (1) the voltage across the first toner image layer on the photoconductor increases in proportion to the electrostatic capacitance of the photoconductor used, (2) when the electrostatic capacitance of the photoconductor is above an electrostatic value, the voltage across the first toner image exceeds the discharge threshold voltage, hence resulting in undesirable discharging and therefore the surface potential of the above-mentioned toner image part is irregularly lowered, (3) accordingly, the selection of appropriate values of the electrostatic capacitances of the photoconductor and of the toner layer is important for obtaining clear colors having a high density without any undesirable mixing of color. When the charge potentials of the photoconductor for each image forming cycle (for each printing of four colors) and the developer bias potentials for the developing of each cycle respectively are selected equal each other, the density of colors made by composition of more than two kinds of toners, such as red and green, becomes low, and furthermore the hue of the composite color becomes unstable in the conventional apparatus. After-exposure potentials at the time after recharging of the photoconductor at the part with toner deposition and at the part without toner deposition were examined, and the following was confirmed: Even when a sufficient light exposure is made, till the photoconductor discharges the residual potential, the after-exposure surface potential at the part with toner deposition is higher than that at the part without toner deposition, by the extent corresponding to the electrostatic charge of the toner. Consequently, a first potential difference between the exposed part where the toner has already been attached and the non-exposed part where no toner has been attached becomes smaller than a second potential difference between the exposed part where no toner has been attached and the non-exposed part where no toner has been attached. Therefore, when the above-mentioned latent image of the exposed part is developed by applying the second toner (e.g. the magenta toner) thereto, the deposition amount of the second toner (e.g. of the magenta toner) on the layer of the first toner (e.g. of the yellow toner) becomes smaller than that of the non-deposition part (wherein there is no first toner layer) of the first toner (e.g. of the yellow toner). Thereby the density of the composite color is lower than expected. Furthermore, the surface potential of the part with toner deposition varies depending on the deposition amount of the deposited first toner (e.g. of the yellow toner). As can be seen from the above, the problem involved with conventional color electrographic processes and apparatusses is the varying deposition amount of toner on the photoconductor layer, which results in an instable color density resp. Undesirable color mixing. US-A-4 407 917 discloses an image synthesizing and copying process and apparatus as stated in the precharacterising portion of claim 1. According to this method a base member consisting of an aluminum plate and of a Selenium layer and of further photoconductive layers. The thickness of the Selenium layer is 50 µm. According to this process two photoconductive layers are formed one on top of the other, which have different spectral sensitivities. The photoconductor is charged to a first polarity and then reversely charged to the extent that latent electrostatic images can be formed on the surface of the photoconductor. After exposure the synthesized latent electrostatic images are developed by two types of toners which are charged in the opposite polarities. In EP-A-143 535 a multiplex image reproducing process using the flying toner development is described. The object underlying the invention is to provide an electrophotographic process and apparatus which have no undesirable color mixing and which allow producing a color image having a high color density. This object is solved by a process and by an apparatus having the features of claims 1 and 18 respectively. Advantageous examples of the process according to the invention are indicated in the subclaims. Applying the process and apparatus in accordance with the invention results in a color image having a high density and a good quality without any undesirable color contamination, contrary to the results achieved up to the invention. This, is a consequence of the cognition that the parameters of the electrophotographic process have to be applied not independently, but dependent on each other. In particular, the electrostatic capacitance of the toner layer and its specific dielectric constant have to be defined depending on the electrostatic capacitance of the photoconductor. The electrostatic capacitance of the photoconductor layer should be 170 pF/cm² or below. Then the voltage across the toner image layer can be supressed to a low value, thereby enabling prevention of undesirable discharging at the toner image layer part. As well, the electrostatic capacitance of the photoconductor should be 20 pF/cm² or higher so that the photoconductor retains a sufficiently high electrostatic charge to achieve clear electrostatic latent images. When the specific dielectric constant of the toner is above 3, the charged voltage across the toner layer on the photoconductor has such a low level that undesirable discharging in the toner layer is prevented. The above selection of parameters allows making the surface potential of the toner image part very small and undesirable deposition of toner of a color not needed can be prevented. The invention allows increasing the contrast potential (potential difference) between the exposed part and the non-exposed part of the image of the previously deposited toner layer so that the color density of the composite color can be increased. This is due to the fact that the second toner can be sufficiently deposited. Use of the flying toner development allows further uniform developing without any edge effect, i.e. the thickness of the toner layer at the edge part of the image essentially is the same as at the central part. In the following the invention is further elucidated explaining advantageous examples of the invention in conjunction with the drawings. In the drawings FIG. 1 is a cross sectional side view of essential parts of a conventional color electrophotographic apparatus. FIG. 2 is a cross sectional side view of the essential parts of a developer of the apparatus of FIG. 1. FIG. 3 is a sectional side view of essential parts of the developer to be used according to the present invention. FIG. 4 is a sectional side view of an example of a first example of the color electrophotographic apparatus in accordance with the present invention. FIG. 5 is a sectional side view of another example of the color electrophotographic apparatus in accordance with the present invention. FIG. 6A is a sectional side view of another example of the colour electrophotographic apparatus in accordance with the present invention. FIG. 6B is a sectional side view of another example of the color electrophotographic apparatus in accordance with the present invention. FIG. 6C is a sectional sided view of another example of the color electrophotographic apparatus in accordance with the present invention. FIG. 7 is a sectional side view of showing an example of a corona charger to be used in the example shown in FIG. 6A. FIG. 8 is a sectional side view of another example of a corona charger to be used in the example shown in FIG. 6C. DESCRIPTION OF THE PREFERRED EMBODIMENTS It is found that in case that the electrostatic capacitance of the photoconductor layer used in the color electrophotographic apparatus is below 170 pF/cm², the voltage across the toner image layer can be suppressed to a low level when the photoconductor having one or more toner image layers is recharged. This allows prevention of undesirable discharging at the toner image layer part. The electrostatic capacitance is preferably 20 pF/cm² or higher, since a photoconductor having a capacitance of less than 20 pF/cm² cannot retain enough electrostatic charge to make clear electrostatic latent images. When the specific dielectric constant of the toner is above 3, the charged voltage across the toner layer on the photoconductor assumes such a low level that undesirable discharging in the toner layer is prevented. By the above-mentioned selections, the surface potential of the toner image part can be made very small. and undesirable deposition of toner of an unnecessary color can be prevented. In general, though depending on the way of developing, the thickness of the toner layer at the edge part of the image is liable to be larger than at the central part of the image. This is the result of the known edge effect which is particular to the electrophotographic method, and therefore the discharging is more liable at the edge part than the central part. Accordingly, the developing method of toner flying under a DC electric field, which can realize uniform developing without any edge effect, is used for embodying the present invention. When the charged potential of the photoconductor is raised as the sequence comprising the charging, light-exposuring and developing goes to the next sequence, the contrast potential (potential difference) between the exposed part and the non-exposed part of the image on the previously deposited toner layer can be increased in comparison with the conventional example. In such a case, the color density of the composite color can be increased, since the secondary toner can be sufficiently deposited on the part comprising deposited toner. As a photoconductor, which is used in the present invention, an ordinary electrophotographic photoconductor, wherein photo-electric conductive materials such as amorphous selenium, arsenic selenide, CdS, ZnO, amorphous silicon or organic photoconductive material is coated on an electrically conductive material, can be used; and especially the photoconductors comprising a photo-sensitive layer having an electrostatic capacitance of 20--170 pF/cm² range are preferable. Selection of the electrostatic capacitance of the photoconductor layer within the above-mentioned range can be made by regulating the thickness of the photoconductor layer. For example, in case of a selenium photoconductor the thickness should be 35--90 µm, in case of an arsenic selenide photoconductor the thickness should he 60--90 µm, and in an organic photoconductor the thickness should be 15--50 µm. Though both the normal developing method and the reversal developing method can be used in the present invention, the reversal developing method, wherein polarity of the toner is not reversed in the recharging, is especially preferable. For preventing that the potential across the toner layer becomes too high when the photoconductor bearing the toner image layer thereon is recharged, the thickness of the toner layer thickness to be formed on the photoconductor by thickness to be formed on the photoconductor by development for one color is preferably limited within the range of 5--30 µm in the developing process Any toner can be used as far as it has the specific dielectric constant of 3 or higher and more preferably 4 or higher. Especially for the color toner of subtractive color mixing method, non-magnetic toners which is splendid in transparency is preferable. Average particle size of the toner is preferably smaller the better, in order to increase the electrostatic capacitance of the toner layer. Accordingly, it is preferably 10 µm or smaller, and more preferably 6 µm or smaller. The dielectric constant of the toner being 3 or higher can e.g. be obtained by using a polymer resin having the specific dielectric constant of 3 or higher, or by dispersing and mixing an inorganic dielectric material having the specific dielectric constant of 3 or higher in the toner composition. Furthermore, a dielectric constant of 4 or higher can be obtained by mixing an inorganic dielectric material having the specific dielectric constant of 4 or higher in the toner composition. The word "toner" herein means toner compositions to be used in the conventional electrophotographic process, that is defined as the composition made by dispersing the pigment and charge control agent in a binder resin, and subsequently the surface of the toner powder or toner particle is coated by an outer additive such as silica. As a polymer resin having the specific dielectric constant of 3 or higher, epoxy resin, melamine resin, phenol resin, alkyd resin, polyester resin, or the like, are used as binder resin, or these resins are used together with another resin bonding agent. As the inorganic dielectric material, a white fine powder which has a specific dielectric constant as large as possible and a grain size smaller than 1 µm is usable. The following materials may be used: barium sulfate, whose specific constant ε is 11.4 alumina, whose specific constant ε is 9.3--11.3 barium titanate, whose specific constant ε is 250--4500 titanium oxide, whose specific constant ε is 90-- 170 silicon dioxide, whose specific constant ε is 4.5. These dielectric materials are used singly or two or more of them together, depending on the necessity. The specific dielectric constant can be adjusted arbitrarily by changing the amount of the dielectric materials contained in the toner composition. For instance, by mixing, 10--40 weight parts of barium sulfate or alumina with 100 weight parts of the toner composition, the specific dielectric constant of the toner can be made to 3--6. When 0.1--5 weight parts of titanium oxide are added to the above toner composition, the specific dielectric constant becomes 4--200; when 1--5 weight parts of titanium oxide are added to the above toner composition, the specific dielectric constant becomes 3--10; when silicon dioxide of 20--50 weight parts are contained in the toner composition, the specific dielectric constant becomes 3--4. As has been described, a toner flying development under a DC electric field, wherein a DC voltage is provided across the toner carrier and the photoconductor, is used according to the invention. A known toner flying development can be used, wherein a toner carrier carrying a thin layer of toner thereon is provided with a small gap for not touching the photoconductor in front of the toner, and a voltage is applied between the toner carrier and the photoconductor to make the toner on the toner carrier fly to the photoconductor. An example of a developing method in accordance with the toner flying development under a DC electric field using a non-magnetic toner is described with reference to FIG. 3. As shown in FIG. 3, for forming a thin layer of the non-magnetic toner on the surface of the toner carrier 25, which is made of a metal cylinder of aluminum or stain-less steel, the toner carrier 25 and the fur-brush roller 26, which is made of carbon-containing resin or of fine metal wires, are rotated in the respective directions marked by arrows in FIG. 3. The non-magnetic toner 24 is charged by rubbing of the toner particles at each other by rotation of the fur-brush roller 26, and by electrostatically sticking them to the toner carrier 25, which is held at a small gap to the photoconductor 29 in order not to touch it. The gap between the toner holder 25 and the photoconductor 29 is preferably less than 300 µm, and more preferably 50--200 µm. Then the surface of the toner absorbed on the toner holder 25 is regulated to make the layer thickness uniform by the rubber blade 27, and a thin layer of toner of 20--50 µm thickness is formed on the toner holder 25. The fur-brush roller 26 can be electrically floated or grounded. The amount of the toner to be supplied to the toner holder 25 can be electrically controlled by adjusting DC voltage applied between the fur-brush roller 26 and the toner holder 25. Thus the thin layer of the toner is formed. <CONCRETE WORKING EXAMPLE 1> A first concrete working example is elucidated with reference to FIG. 4. A photoconductor drum 30 with a diameter of 100 mm and having on its surface an amorphous Se-Te photoconductor (wherein the thickness of the photoconductor was 60 µm and its electrostatic capacitance was 92 pF/cm²) were revolved at a surface speed of 75 mm/sec. Then the surface of the photoconductor drum was charged to +800 V by a corona charger 31 (wherein the corona voltage was +7 KV and voltage of the grid 32 was +850 V). Then, the charged surface was exposed to light from a light emitting diode array 33 of 7 µW/dot output and 670 nm wavelength, through a self-focusing rod lens array 34. Thus the charged surface was exposed to the signal light corresponding to the yellow image, for producing a latent image. Then, the surface having the latent image was positioned to oppose the surface of the developer rollers 35, 37 and 38. The developer rollers 35, 37 and 38 had a diameter of 16 mm and were rotating at a surface speed of 75 mm/sec in the same direction with respect to the opposing surfaces of the photoconductor 30. The first developer 35 bore a yellow toner layer of an average particle size of 10 µm and a layer thickness of 30 µm and it was charged with +3 µC/g at its surface. The developing gap between the surface of the toner on the developer roller and the photoconductor was 150 µm. Then, a DC developing bias voltage of +700 V was applied from the DC power source 36, and the yellow toner flew from the developing roller 35 to the latent image part of the photoconductor 30 and was deposited thereon. The thickness of the toner layer deposited on the photoconductor 30 was about 10 µm. Thereafter, the yellow-developed photoconductor 30 was driven to move in front of the magenta developer 37 and subsequently to the cyan developer 38, both impressed with a bias voltage of +850 V. After thus passing three or four developers of different colors, the toner images consisting of three or four toner layers of different patterns on the photoconductor 30 were subject to erasing by an eraser lamp 40, for the whole surface area of the photoconductor 30. Thereafter, the color toner images were transferred on to a paper 42 by actuating a transfer charger 41, and then the paper 42 was detached from the photoconductor 30 by a peel-off charger 43. Finally, the color toner images on the paper 42 were fixed by fusing. After transferring the toner on the photoconductor 30 to the paper 42, the charge on the surface of the photoconductor 30 was discharged by an eraser 44, and the remaining toner on the photoconductor 30 was removed by actuating a cleaning device 45 and the apparatus was made ready for next color copying. The color image on the paper thus obtained had a high density of 1.7 at maximum density, and an image of good quality was obtained without any undesirable color contamination. <CONCRETE WORKING EXAMPLE 2> In place of the Se-Te photoconductor used in the concrete working example 1, an arsenic selenide photoconductor wherein the (thickness of the photoconductor layer was 90 µm and the electrostatic capacitance was 104 pF/cm²) was used, and the other conditions were the same as in the case of concrete working example 1. Then the obtained printed color image on the paper had such a high density of 1.5 at maximum density, and an image of good quality image was obtained without any undesirable color contamination. <CONCRETE WORKING EXAMPLE 3> An organic photoconductor containing azo pigment as the photoconductor (wherein the thickness of the photoconductor layer was 30 µm, and the electrostatic capacitance was 80 pF/cm²) was used as the photoconductor, and the other conditions were the same as in the case of actual working example 1, and color printing was made. The obtained color image print had such a high maximum density of 1.7 and there was no color contamination. <CONCRETE WORKING EXAMPLE 4> Three kinds of positive-charge-use color toners of yellow (Y), magenta (M) and cyan (C) were prepared. (1) The following compositions were mixed for 2 hours at 150 °C and then cooled, ground and sieved to obtain a yellow toner of 5--15 µm (the average particle size was 10 µm). The specific dielectric constant of the obtained toner was about 7: Dielectric material: Barium titanate (ε : 2500) 1g Pigment: C.I. pigment yellow #12 25g Binding resin: Styrene-acryl resin 464g Change control gent: Amino styrene resin 10g. (2) The following compositions were mixed for 2 hours at 150 °C and then cooled, ground and sieved to obtain a magenta toner of 5--15 µm (the average particle size was 10 µm). The specific dielectric constant of the obtained toner was about 7: Dielectric material: Barium titanate (ε : 2500) 1g Pigment : C.I. pigment red #5 30g Binding resin: Styrene-acryl resin 454g Change control gent: Amino styrene resin 15g. (3) The following compositions were mixed for 2 hours at 150 °C and then cooled, ground and sieved to obtain a cyan toner of 5--15 µm (the average grain size was 10 µm). The specific dielectric constant of the obtained toner was about 7: Dielectric material: Barium titanate (ε: 2500) 1g Pigment: C.I. pigment blue #15 25g Binding resin: Styrene-acryl resin 464g Change control gent: Amino styrene resin 10g. Next, by using the above-mentioned three kinds of toner, a color picture was made by means of the apparatus shown in FIG. 5. The apparatus of FIG. 5 comprises a scorotron charger 47 (the corona voltage was +7 KV, the grid voltage was +850), LED array 48 (the output was 7 µW/dot, the wavelength was 670 nm), a self-focusing rod lens array 49, developers 50, 51 and 52 containing respective toners of yellow, magenta and cyan respectively, an eraser lamp 53, a corona charger 54 for toner transferring, an A.C. eraser 55 for detaching plain paper 56, and a cleaning brush 57, around a photoconductor drum 46 made by vapor depositing Se-Te (the thickness of the photoconductor layer was 60 µm, electrostatic capacitance was 92 pF/cm²), in this order. The developer 50, 51 or 52 is the same that elucidated with reference to FIG. 3. As the toner carrier, an aluminium drum having a roughened surface was used, for the fur-brush roller in the developer, a brush made by implanting carbon-containing (specific resistance of 10⁶ Ωcm) rayon fibers implanted on an aluminium pipe was used. The charge on the respective toners born on respective toner carrier 50, 51 or 52 at the operation of the developer was 2--5 µC/g. The gap between the photoconductor 46 and the toner carrier was about 150 µm. Next, forming of an image on the photoconductor is elucidated. The photoconductor 46 was rotated in the direction shown by arrow 46a at a surface speed of 100 mm/sec., and by means of the Scorotron charger 47, the photoconductor 46 was charged to +800 V. Subsequently, by means of a LED array 48, a yellow image signal was scan-exposured, and a negative electrostatic latent image of +800 V at non-exposed part and +40 V at exposed part were formed. After exposing to the yellow image, the photoconductor 46 was passed in front of the three developers 50, 51 and 52, and reversal-development was carried out by the yellow toner. The thickness of the developed yellow toner image was about 12µm. The data of respective developers are as follows: (1) In the yellow developer 50: Voltage impressed on the toner carrier: +750 V, Voltage impressed on the fur-brush: +850 V, Thickness of the toner layer on the toner carrier: about 40 µm. (2) In the magenta and cyan developers 51 and 52: Voltage impressed on the toner carrier: grounded, Voltage impressed on the fur-brush: grounded, Thickness of the toner layer on the toner carrier: about 40 µm. After development, the photoconductor holding the Y toner image was irradiated by the eraser lamp 53, thereby to light erase the electrostatic latent image, and the photoconductor was again charged by the Scorotron charger 47. The surface potential of the photoconductor 46 was +800 V at both parts of existence and non- existence of the toner. Next, an electrostatic negative latent image was formed by scanning exposure of the magenta image signal by the LED array 48. The surface potential of the exposed parts in the region having no yellow toner was +40 V, and the surface potential of the exposed parts with yellow Y toner was 160 V. After exposing to the magenta image, the photoconductor 46 was passed in front of the developers 50, 51 and 52, and the reversal-development was carried out by magenta toner. The thickness of the obtained composite toner image was about 12 µm at the part having only magenta toner, and 21 µm at the part where yellow toner and magenta toner were superposed. At the not exposed part on the yellow toner part there was no undesirable deposition of the magenta toner. The data of the respective developers were as follows: (1) In the yellow and the cyan developers 50 and 52: Voltage applied to the toner carrier: +750 V, Voltage impressed on the fur-brush: +550 V, Thickness of the toner layer on the toner carrier: 0. (2) In the magenta developer 51: Voltage impressed on the toner carrier: +750 V, Voltage impressed on the fur-brush: +850 V, Thickness of the toner layer on the toner carrier: about 40 µm The photoconductor 46 was again erased by light irradiation by the A.C. eraser 55 and then charged by the scorotron charger 47. The surface potential of the photoconductor 46 was +800 V irrespective of the existence and non-existence of the toner. Next, the photoconductor 48 was light-scanned with a cyan image signal by the LED array. The surface potential of the exposed part on the part without toner was +40 V. The surface potential of the exposed part on a single layer of yellow or magenta toner was +160 V. The surface potential of the exposed part on the double layers of the yellow and magenta toners superposition was +220 V. Next, the photoconductor 46 was passed in front of the three developers 50, 51 and 52, which were set in the below-mentioned respective conditions, and the latent image was reversal-developed by the cyan toner. Then, there was no cyan toner deposited on the non-exposed parts on either of and both of the yellow and magenta toner deposited parts. The data of the respective developers were as follows: (1) In the yellow and magenta developers 50 and 51: Voltage applied to the toner carrier: +750 V, Voltage applied to the fur-brush: +550 V, Thickness of the toner layer on the toner carrier; 0. (2) In the cyan developer 52: Voltage applied to the toner carrier: +750 V, Voltage applied to the fur-brush: +850 V, Thickness of the toner layer on the toner carrier: about 40 µm. Next, after wholly irradiating the surface of the photoconductor 46 by the eraser lamp 53, the toner images on the photoconductor 46 were transferred to the plain paper 56 by means of the corona charger 54 (the corona voltage was 5.5 kV). Subsequently, the plain paper 56 was detached from the photoconductor 46 by the A.C. charger 55. Thereafter, the toner image transferred to the plain paper 56 was fused by a known fuser (not shown) and a stable color printing was obtained. Then a very small amount of the remaining toner on the photoconductor 46 was removed by a cleaning brush 57 and the apparatus was prepared for copying the next image. As a result of the above-mentioned sequence of the color printing, and even in spite of the recharging of the photoconductor bearing already a toner image, the hitherto-observed undesirable lowering of the photoconductor potential at the part where another toner had already been deposited did not take place. Resultantly, a clear color print without color contamination was obtained. <COMPARISON EXAMPLE 1> A comparison toner was prepared by removing the barium titanate from the toner composition as described in respect of the actual working example 1. The specific dielectric constant of the three kinds of toner thus obtained was selected about 2.2, respectively. When a color print was made using this comparison example toner, and applying the similar process as elucidated in the actual working example 4, the cyan toner was undesirably deposited on the part which was to become red (where only the yellow toner and the magenta toner only were to be deposited). Therefore a clear red was not obtained. At that time, after re-charging the photoconductor at the part developed by the magenta toner, the surface potential of the part where the yellow toner and the magenta toner were superposed (rendering the total thickness of the toner layer about 24 µm) was measured, and the surface potential was about 400 V. <CONCRETE WORKING EXAMPLE 5> In place of the Se-Te photoconductor used in the concrete working example 4, an arsenic selenide photoconductor (the thickness of the photoconductor layer was 60 µm and the electrostatic capacitance was 156 pF/cm²) was used, and the other conditions were the same as those of the concrete working example 4, for making a color printed image. The printed color image on the paper had a similar clear color, and no color contamination was observed. <COMPARISON EXAMPLE 2> Using the toner described in the COMPARISON EXAMPLE 1, color printed images were made by the same process as described in the CONCRETE WORKING EXAMPLE 5. The obtained printed color image had considerable color contaminations such that the magenta and the yellow toners were undesirably deposited on the parts to be represented as yellow, and the yellow toner was undesirably deposited on the parts to be represented as magenta and red. At that time, after re-charging the photoconductor at the part bearing one kind of toner, the surface potential of the part where the toner was deposited (the layer thickness was about 12 µm) was measured, and the surface potential was about 400 V. Furthermore, after re-charging the photoconductor at the part bearing two kinds of toners, the surface potential of the part where the two kinds of toners were superposedly deposited (the toner layer thickness was about 24 µm) was measured, and the surface potential was about 250 V. <CONCRETE WORKING EXAMPLE 6> Another concrete working example is elucidated with reference to FIG. 6B. The developers 58, 59 and 60 are non-contact type non-magnetic single-component developers wherein the toners are made to fly by a DC electric field, and they respectively comprise aluminum-made developing rollers 61, 62 and 63. On each surface thereof a thin layer of toner is formed by blades 64. The developers 58, 59 and 60 contain yellow toner, magenta toner and cyan toner, respectively, and these toners have insulating properties. The developing rollers 61, 62 and 63 are disposed around the surface of the photoconductor 65 with a specified developing gap to the surface of the photoconductor 65. The respective developer comprises a distance control mechanism which controls the gap between the developing roller 61, 62 or 63, and the photoconductor drum 65, in such a manner that each roller is moved close to the photoconductor when developing is made and it is removed from the photoconductor when no developing is made. The details of the data of the developer, of the developing conditions and of the physical property of the toners are as follows: Details of the developer and of the developing conditions: Diameter of the developing roller: 16 mm, Peripheral speed of the developing roller: 150 mm/sec, Toner layer thickness on the developing roller: 30 µm, Direction of rotation of the developing roller: opposite to the direction of photoconductor 65, Developing gap during the development: 150 µm, Developing gap when the developers is not developing: 70 µm. Physical properties of toner: Charge of the toner: +3 µC/g, Average particle size: 10 µm. A photoconductor drum 65 of 100 mm diameter having Se-Te photoconductor was rotated at a peripheral speed of 150 mm/sec. and the surface of the photoconductor was charged by a corona charger 66 (corona voltage: +7 KV) to a surface potential of +800 V. Then, by irradiating an LED array 67 of 670 nm wavelength and 7 µmW/dot output, a negative yellow signal light was irradiated on the photoconductor surface 65 through a rod lens array 68, in order to produce an electrostatic latent image. The contrast potential of the latent image was 750 V. Reversal-developing of the above-mentioned latent image was made by a yellow developer 58, which was turned to the developing state by applying a +700 V voltage to the developing roller 61. Then, the photoconductor 65 was passed in front of the magenta developer 59 and the cyan developer 60 both in the non-developing state, and the yellow toner image was formed. After the development, the whole surface of the photoconductor 65 was irradiated by an eraser lamp (not shown in Fig. 6B), thereby to erase the latent image. Next, again by the corona charger 66 (corona voltage: +7.3 KV), the photoconductor 65 was charged to +850 V. Then, by the LED array 67 the photoconductor 65 was exposed to signal light of the magenta image signal, thereby to produce the electrostatic latent image for the magenta image. The surface potential of the exposed part formed on the previously formed yellow toner image layer was +100 V, and the contrast potential of the above-mentioned latent image was 750 V. Subsequently, the photoconductor 65 was passed in front of the yellow developer 58 in the non-developing state, and further in front of the magenta developer 59 wherein the developing roller 62 was applied with +800 V, and in front of the cyan developer 60 in the non-developing state, thereby to produce a toner image of the magenta color. After the development, the latent image was erased by irradiating the whole surface of the photoconductor by the eraser lamp. Next, again by the corona charger 66 (corona voltage: +7.5 KV), the photoconductor 65 was charged to +950 V. Then, the photoconductor 65 was exposed to signal light of the cyan image signal, by the LED array 67 thereby to produce the electrostatic latent image for the cyan image. The surface potential of the exposed part formed on the previously formed magenta toner image layer was +100 V. The surface potential of the exposed part formed on the layer of images made by superposing the yellow toner and magenta toner was +200 v. The contrast potential of the above-mentioned latent image was 750 V. Subsequently, the photoconductor 65 was passed in front of the yellow developer 58 in the non-developing state, the magenta developer 59 in the non-developing state and the cyan developer 60 in the developing state where the developing roller 63 was impressed with +900 V to realize the developing state, thereby to form a cyan toner image. The color toner image thus obtained on the photoconductor 65 was transferred to plain paper by means of the transfer charger 70, and thereafter the color image of the toners was fixed by fusing. Then, the surface of the photoconductor 65 was erased by the eraser lamp 69, and the photoconductor 65 was cleaned by pressing a revolving fur-brush 72 thereon. As a result, a clear color image which has a color density of composite color of red, green and blue was above 1.5, and the color density of the three color superposed part of the yellow toner, the magenta toner and the cyan toner was above 1.7, and there was no undesirable color contamination. <COMPARISON EXAMPLE 3> Using the apparatus described in the COMPARISON EXAMPLE 1, color printed images were made by setting the condition such that the initial potential of the photoconductor for each cycle of printing for one color was +800 V, and the bias for developing of the developer was +700 V. Then, the contrast potential in the yellow cycle was 750 V, in the magenta cycle 700 V and in the cyan cycle 600 V. The color density of a composite color of red, green and blue was 1.2. The color density for the three color superposition of yellow, magenta and cyan was 1.3. <CONCRETE WORKING EXAMPLE 7> A color image print was made by an apparatus shown in FIG. 6A where a Scorotron charger 73 shown in FIG. 7 is used in place of the corona charger 66 of the CONCRETE WORKING EXAMPLE 6. The Scorotron charger of FIG. 7 has grid electrodes 78. Rotating the photoconductor drum 65 at a peripheral speed of 150 mm/sec., the charging was made by the Scorotron charger 73 (corona voltage: +7 KV, grid voltage 750 V), in order to to obtain a surface potential of +800 V. Next, by driving the LED array 67 of 670 nm wavelength and 7 µW/dot output, the photoconductor 65 was exposed to the negative yellow image signal light through the rod lens array 68, in order to make a latent image. The contrast potential of the latent image was 750 V. Reversal-development was made by the yellow developer 58, which was turned to the developing state by applying +700 V to its developing roller 61. Then, the photoconductor 65 bearing the yellow toner thereon was passed in front of the magenta developer and the cyan developer which were the in non-developing state. Thus a yellow toner image was made. After the yellow development, the latent image on the photoconductor surface 65 was erased by irradiating the whole surface by the eraser lamp 69. Next, the photoconductor 65 was again charged by the corona charger 73 (corona voltage: +7 KV, grid voltage 800 V), in order to raise the surface potential of the photoconductor 65 to 750 V. Thereafter, the photoconductor 65 was exposed to the magenta image signal light by the LED array 67, thereby to make the electrostatic latent image of magenta. The surface potential of the exposed part formed on the yellow toner image was +100 V, and the contrast potential of the latent image was 750 V. Then, the photoconductor 65 was passed in front of the yellow developer 58 in the non-developing state, the magenta developer 59 which was turned to the developing state by application of +800 V to the developing roller 62 and the cyan developer 60 in the non-developing state, thereby to produce a magenta toner image. And thereafter, the whole photoconductor surface was irradiated by the eraser lamp 69 in order to erase the latent image. Next, the photoconductor 65 was again charged by the corona charger 74 (corona voltage: + 7 KV, grid voltage 900 V), in order to raise the surface potential of the photoconductor 65 to 750 V. Thereafter the photoconductor was exposed to cyan image signal light by the LED array 67, thereby to make the electrostatic latent image of cyan. The surface potential of the exposed part formed on the magenta toner image was +200 V, and the contrast potential of the latent image was 750V. Then, the photoconductor 65 was passed in front of the yellow developer 58 and the magenta developer 59 which were in non-developing state, and the cyan developer 60 which was turned to the developing state by application of +900 V to the developing roller 62, thereby to produce a cyan toner image. After transferring the color toner image made on the photoconductor 65 to the plain paper 71 by means of a transfer charger 70, the transferred toner image was fixed by fusing. After removing the toner image, the surface of the photoconductor 65 was erased of charge by the eraser 69, and further the surface of the photoconductor 65 was cleaned by pressing the fur-brush 72 thereon. The resultant color image print had a color density of 1.5 or higher of composite color of red, green and blue. The color density for the three color superposition of yellow, magenta and cyan was 1.7 or higher. <CONCRETE WORKING EXAMPLE 8> A color image print was made by using an apparatus shown in FIG. 6B. This apparatus is similar to that used for the CONCRETE WORKING EXAMPLE 6, but excludes the eraser. By rotating the photoconductor drum 65 at a peripheral speed of 150 mm/sec., the charging was made by the charger 66 (corona voltage: +7 KV), in order to obtain a surface potential of +800 V. Next, by driving the LED array 67 of 670 nm wavelength and 7 µW/dot output, the photoconductor 65 was exposed to the negative yellow image signal light through the rod lens array 68, in order to make a latent image. The contrast potential of the latent image was 750 V. Reversal-development was made by the yellow developer 58, which was turned to the developing state by impressing +700 V on its developing roller 61. Then, the photoconductor 65 bearing the yellow toner was passed in front of the magenta developer and the cyan developer which were in the non-developing state. Thus a yellow toner image was made. Next, the photoconductor 65 was again charged by the corona charger 66 (corona voltage: +7 KV) in order to raise the surface potential of the photoconductor 65 to 850 V. Thereafter, the photoconductor 65 was exposed to magenta image signal light by the LED array 67, thereby to make the electrostatic latent image of magenta. The surface potential of the exposed part formed on the yellow toner image was +100 V, and the contrast potential of the latent image was 750 V. Then, the photoconductor 65 was passed in front of the yellow developer 58 in the non-developing state, the magenta developer 59 which is made in the developing state by application of +800 V to the developing roller 62 and the cyan developer 60 in the non-developing state, thereby to produce a magenta toner image. Next, the photoconductor 65 was again charged by the corona charger 66 (corona voltage: +7 KV), in order to raise the surface potential of the photoconductor 65 to 950 V. Thereafter, the photoconductor was exposed to cyan image signal light by the LED array 67, thereby to make electrostatic latent image of cyan. The surface potential of the exposed part formed on the magenta toner image was +100 V, the surface potential of the exposed part formed on the superposed layers of the yellow and the magenta toners was +200 V, and the contrast potential of the latent image was 750 V. Then, the photoconductor 65 was passed in front of the yellow developer 58 and the magenta developer 59 which were in non-developing state, and the cyan developer 60 which was made in the developing state by impression of +900 V on the developing roller 63, thereby to produce a cyan toner image. After transferring the color toner image made on the photoconductor 65 to the plain paper 71 by means of a transfer charger 70, the transferred toner image was fixed by fusing. After removing the toner image, the surface of the photoconductor 65 was cleaned by pressing the fur-brush 72 thereon. The resultant color image print had a color density of 1.5 or higher of the composite color of red, green and blue. The color density for the three color superposition of yellow, magenta and cyan was 1.7 or higher, and there was no color contamination. <CONCRETE WORKING EXAMPLE 9> A color image print was made by using an apparatus shown in FIG. 6C, which uses a charger 74 having three corona wires 75, 76, 77 as shown in FIG. 8. Using this charger 74, with advancing of the cycles of respective colors, the number of corona wires impressed with the corona voltage was increased and the time period to charge the photoconductor was also increased. First, by rotating the photoconductor drum 65 at a peripheral speed of 150 mm/sec., by the charger 74, with its first corona wire 75 being impressed with a corona voltage of 7 KV, the charging was made in order to obtain a surface potential of +800 V. Then, by driving the LED array 67 of 670 nm wavelength and 7 µW/dot output, the photoconductor 65 was exposed to the negative yellow image signal light through the rod lens array 68, in order to make a latent image. The contrast potential of the latent image was 750 V. The reversal development was made by the yellow developer 58, which was turned to developing state by impressing +700 V on its developing roller 61. Then, the photoconductor 65 bearing the yellow toner thereon was passed in front of the magenta developer and the cyan developer, which were in the non-developing state. Thus a yellow toner image was made. Next, the photoconductor 65 was again charged by the corona charger 74, wherein both the two corona wires 75 and 76 were charged by +7 KV corona voltage, in order to make the surface potential of the photoconductor 65 to 850 V. Thereafter, the photoconductor was exposed to magenta image signal light by the LED array 65, thereby to make the electrostatic latent image of magenta. The surface potential of the exposed part formed on the yellow toner image was +100 V, and the contrast potential of the latent image was 750 V. Then, the photoconductor 65 was passed in front of the yellow developer 58 in the non-developing state, the magenta developer 59 which was turned to the developing state by impression of +800 V on the developing roller 62 and the cyan developer 60 in the non-developing state, thereby to produce a magenta toner image. After the development, the whole photoconductor surface was irradiated by the eraser lamp 69, in order to erase the latent image. Next, the photoconductor 65 was again charged by the corona charger 74. In this case, all of the three corona wires 75, 76 and 77 were applied with the corona voltage of +7 KV, in order to raise the surface potential of the photoconductor 65 to +950 V. Thereafter, the photoconductor 65 was exposed to cyan image signal light by the LED array 67, thereby to make electrostatic latent image of cyan. The surface potential of the exposed part formed on the magenta toner image was +100 V, the surface potential of the exposed part formed on the superposed layers of the yellow and the magenta toners was +200 V and the contrast potential of the latent image was 750 V. Then, the photoconductor 65 is passed in front of the yellow developer 58 and the magenta developer 59 which are in the non-developing state, and the cyan developer 60 which was turned to the developing state by application of +900 V to the developing roller 63, thereby to produce the cyan toner image. After the development, the whole surface of the photoconductor 65 was irradiated by the eraser lamp 69, thereby to erase the latent image. The color toner image thus produced on the photoconductor 65 was then transferred to plain paper 71 by means of a transferring charger 70, and the transferred toner image was fixed by fusing. After the transferring, the surface of the photoconductor 65 was cleaned by the eraser lamp 69, and further was cleaned by pressing the fur-brush 72 thereon. The resultant color image print has a color density of 1.5 or higher of the composite color of red, green and blue. The color density for the three color superposition of yellow, magenta and cyan was 1.7 or higher, and there was no color contamination. As has been described above, according to the present invention, there was no potential lowering on the surface of the toner layer which has been previously formed on the photoconductor drum, even in the case of the re-charging for the second or third cycle of development, and images of clear color without color contamination were obtainable. Therefore, a color image of a high color density of composite color was obtainable at stable reproducibility. Furthermore, the present invention enables providing a color electrophotographic apparatus capable of easy adjustment of the color balance and which is free from color contamination.

Claims: A color electrophotographic process having a plurality of sequential electrophotographic steps of producing a plurality of color toner images for different colors, wherein each step comprises forming an electrostatic latent image on a photoconductor layer (29, 30, 46, 65), each electrostatic latent image on the photoconductor layer (29, 30, 46, 65) is developed to a toner image, these toner images accumulated on the photoconductor layer are transferred to a recording medium (42) simultaneously, the toner images transferred to the recording medium (42) are fixed thereon, characterised by selecting a photoconductor layer (29, 30, 46, 65) having an electrostatic capacitance between 20 pF/cm² and 170 pF/cm², putting a thin layer of toner having a specific dielectric constant of 3 or higher on a toner carrier (25), the surface of which is opposing the surface of the photoconductor layer (29, 30, 46, 65) with a predetermined gap that prevents touching of both surfaces, and applying a D.C. potential between the photoconductor - layer (30) and the toner carrier (25) for developing the latent, image by a process of toner flying under a D.C. electric field. A color electrophotographic process in accordance with claim 1, wherein the Photoconductor layer (29, 30, 46, 65) is a Se photoconductor layer having a layer thickness of 35 to 90 µm. A color electrophotographic process in accordance with claim 1 or 2, wherein the photoconductor layer is an arsenic selenide photoconductor layer having a layer thickness of 65 to 90 µm. A color electrophotographic process in accordance with claims 1, 2 or 3, wherein the photoconductor layer is an organic photoconductor layer having a layer thickness of 15 to 50 µm. A color electrophotographic process in accordance with any of claims 1 to 4, wherein the gap between the toner carrier (25, 61, 62, 63) and the photoconductor layer (29, 30, 46, 65) is 250 µm or smaller. A color electrophotographic process in accordance with any of claims 1 to 5, wherein the development is a reversal development. A color electrophotographic process in accordance with any of claims 1 to 6, wherein the average toner layer thickness of the uniform toner layer part made by each development is selected in the range of 5 to 30 µm for one color. A color electrophotographic process in accordance with any of claims 1 to 7, wherein the toner is a non-magnetic toner having an average particle size of 12 µm or smaller. A color electrophotographic process in accordance with any of claims 1 to 8, wherein the toner has a charge of 1 to 5 µC/g. A color electrophotographic process in accordance with any of claims 1 to 9, wherein the toner contains inorganic dielectric substance. A color electrophotographic process in accordance with claim 10, wherein the dielectric substance in one member is selected from the group consisting of barium sulfate, alumina, barium titanate and titanium oxide. A color electrophotographic process in accordance with any of claims 1 to 11, wherein the charged surface potentials of the photoconductor are raised to be higher, as the order of cycle of development advances. A color electrophotographic process in accordance with claim 12, wherein the raising of the charged potential is made by raising the potential to be applied to a corona charger for charging the photoconductor (CONCRETE WORKING EXAMPLE 6; FIG.6B). A color electrophotographic process in accordance with claim 12 or 13, wherein the photoconductor is charged by a scorotron charger (CONCRETE WORKING EXAMPLE 7; FIG. 7 and FIG. 6A). A color electrophotographic process in accordance with claim 14, wherein the raising of the charged potential is made by raising the voltage applied to the grid electrodes of the scorotron charger (CONCRETE WORKING EXAMPLE 7; FIG. 7 and FIG. 6A). A color electrophotographic process in accordance with claim 14, wherein the raising of the charged potential is made by charging with a corona charger of a constant output voltage impressed on the photoconductor for a predetermined constant time period for the respective developing cycle, and by accumulating the charges of the respective developing cycles by non-erasing of the photoconductor after completion of each developing cycle (CONCRETE WORKING EXAMPLE 8; FIG. 6B). A color electrophotographic process in accordance with claim 14, wherein the potential raising method is made by charging with a corona charger, to which a constant voltage is applied, of which charging time is increased, as the order of the developing cycle advances (CONCRETE WORKING EXAMPLE 9; FIG. 6C). A color electrophotographic apparatus comprising: a latent image forming means for forming a plurality of electrostatic latent images respectively corresponding to image signals of different colors on a surface of a photoconductor layer (29, 30, 46, 65), a plurality of developing means each having a toner carrier (25), the surface of which is opposing the surface of the photoconductor layer (29, 30, 46, 65) and respectively containing toners of different colors corresponding to the different color image signals, means for transferring the accumulated toner images made by sequential electrophotographic steps on the recording medium (42) simultaneously, and fixing means for setting the transferred toner images on the recording medium (42), characterised in that the electrostatic capacitance of the photoconductor layer (29, 30, 46, 65) is between 20 pF/cm² and 170 pF/cm², a predetermined gap is provided between the surfaces of the photoconductor layer and of the toner carrier in such a manner, that touching of the surfaces is prevented, the toners have specific dielectric constants of 3 or higher, a voltage application means is provided for applying a D.C. voltage between the photoconductor layer (30) and the toner carrier (25) for developing the latent image by toner flying under a D.C. electric field.

Inventors: TAKASHIMA, YUJI, YAMAMOTO, HAJIME
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: G03G13
IPC codes: G03G13, G03G9, G03G15