Electrophotographic developing apparatus

Abstract: Electrophotographic developing apparatus comprising a carrier of latent image (11) formed on a photoconductive layer (20) superposed to a conductive layer (19), a carrier of developing material (14) juxtaposed to the latent image carrier (11) at a developing zone for transferring the developing material on the photoconductive layer (20) in a configuration corresponding to the latent image and a transfer station in which a printing support (44) is brought in contact with the photoconductive layer (20) for transferring the developing material from the photoconductive layer (20) to the printing support (44), the distribution of the developing material on the photoconductive layer (20) being enhanced by subjecting the photoconductive layer, in a zone extending between the developing zone and the transfer station, to an alternate electrical field which enables migration of developing material particles from the outside of the latent image to the inside of the latent image.

Description: The present invention relates to a developing apparatus for use in electrophotographic copying machines and electrophotographic printers. It is known that in copying and printing machines of the electrophotographic type, an electrostatic latent image holder, generally consisting in a conductive cylinder coated with a layer of photoconductive material, is juxtaposed, along a generatrix of the cylinder, to a developing material carrier, it too generally shaped as a cylinder. The two cylinders may be in contact or spaced apart with a predetermined gap therebetween, generally in the order of 50 to 500um. The two cylinders rotate in the opposite direction generally with the same peripheral speed. In several implementations however they rotate with a differing peripheral speed and/or in the same direction. A thin layer of powder developing material, known as "toner", suitably electrized by triboelectric effect, is formed on the surface of the developing material carrier, hereinafter designated as developing roller. The toner, which generically has magnetic properties, adheres to the developing roller, due to magnetic fields, suitably generated on the developing roller surface and due to Van Der Waals forces which acts between the toner granules and the developing roller, in spite of an electrical potential applied to the developing roller and relative to ground, of the same polarity of the electrical charge acquired by the toner, due to the triboelectric effect. This charge is generally negative. The conductive cylinder of the latent image carrier, which in the following will be shortly designated as OPC, due to the extensive use of Organic Photo Conductive materials for its implementation, is generally grounded. An electrical charge, generally negative, is formed on the OPC surface by means of an electrostatic charge generator. This electrical charge lowers the surface potential to a predetermined value, for example -680V as to ground. The OPC generatrixes, duly electrized, reach, due to the OPC rotation, an exposing station where the OPC is selectively exposed to an electromagnetic radiation. In the exposed zones the photoconductive material looses its electrical charge and its electrical potential drops virtually to OV (In practice to about 50V). The several OPC generatrixes so exposed, then reach the developing station where the toner particles, negatively charged and immersed in the electrical field formed by the differing potential of the developing roller and the OPC, are attracted on the OPC in the OPC zones where it has been discharged at OV. In the zones where the OPC is charged (-680V) the electrical field opposes to the toner migration from the developing roller. Continuing its rotation the OPC carries the toner particles, selectively located on its surface, in a transfer zone or transfer station, where the OPC contacts, along one of its generatrixes, a printing support (generally a paper sheet) which is fed with the same speed of the OPC. In the transfer station the printing support is interposed between OPC and an electrostatic charge generator which charges the printing support with a positive charge. The positive polarization is sufficing to attract the toner from the OPC to the printing support where the toner adheres and is subsequently fixed in permanent way in a fixing station. This process, conceptually very simple in practice, does not produce perfect results, corresponding to the desired ones. The toner transfer from the developing roller to the OPC occurs not only in the zones where it is required, but at some extent also in the zones where it is not desired, with a "background" effect which hampers the quality of the images which can be obtained. This is due both to the impossibility of obtaining a sharp change of the electrical field at the borders of the latent image, both to the impossibility of charging in a uniform way the several toner particles. It must be assumed that statistically a certain number of particles is weakly charged, not charged at all or electrically charged with the opposing polarity. The adherence of electrically neutral particles to the OPC rather than to the toner carrying drum, cannot be controlled by means of electrical fields and excapes to the control. Even in case of weakly charged particles, the control action exerted by the electrical fields is at some extent unadequate. To overcome these limitations it has been proposed (and described in several patents among which US Pat 3,866,574 is mentioned) to apply an alternate voltage to the developing roller, in addition to the biasing DC voltage so as to generate a pulsing electrical field between OPC and developing roller. Several frequences and amplitudes have been proposed, thus achieving some enhancement of the printed images in terms of contrast, resolution and background reduction. An apparatus with the features of the preamble of claim 1 is described for example in document US-A- 4 701 042 Basically two explanations have been given for these results: the pulsing electrical field should cause a vibration of the toner particles which make easier their detaching from the developing roller even if the particles are weakly charged. The pulsing field (at the extreme an alternate field) should cause a particles rebound from the OPC to the developing roller with the consequence of a collision among particles and detaching of a greater toner amount from the developing roller. Whatever the explanation may be, the achieved results are limited. The present invention overcomes these limitations and provides a apparatus where the background effect is minimized and the image resolution is enhanced at an extreme level. In addition the efficiency of the process is improved and the toner amount which is wasted is reduced to a minimum. These advantages are achieved with an electrophotographic developing apparatus where an alternate voltage is applied between ground and the conductive cylinder of the OPC instead of applying it between ground and developing roller. This arrangement, apparently should not change in relevant way the performances of the apparatus, relative to the apparatuses of the prior art, because the electrical field which is produced between OPC and developing roller, (if locally considered as flat armatures of a capacitor) only depends on the voltage existing between the two elements. De facto, this arrangement changes the distribution of the electrical field around the whole OPC cylinder, subjecting the toner particles laid down thereon to pulsed forces of attractive and repulsive nature which in the unexposed have an intensity sufficing to enable the migration of the charged particles from the unexposed zones to the exposed ones. This migration is caused by the attractive component of the electrical field, tangent to the OPC surface, due to the differing status of electrical surface bias. Since the background effect is nearly completely avoided it must be concluded that the neutral particle are dielectrically biases by the pulsing electrical field and even if subjected to an attractive force are somehow allowed to migrate in the zone having an higher potential. Some induced triboelectric effect cannot be excluded. A further advantage occurs in the transfer zone where the electrical field locally reaches a high strength and where the printing support, positively biased, is subjected to a pulsing force which causes its vibration. The vibration, percievable as noise, must produce some triboelectric effect which electrizes uncharged particles too and some mechanical effect of capture, in addition to the electrical one, so that all the toner particles which are present on the exposed OPC zones are transferred on the printing support. The features and the advantages of the invention will result more clearly from the following description of a preferred embodiment of the invention and from the enclosed drawings where: Figure 1 shows in qualitative way the electrical field generated by a developing roller biased by an alternate voltage according to the prior art. Figure 2 shows in qualitative way the electrical field generated by an OPC drum biased by an alternate voltage applied between OPC and ground, in accordance with the present invention. Figure 3 shows in schematics a preferred form of embodiment for the apparatus of the invention. Figure 4 shows the electrical state of a portion of the OPC drum in the apparatus of fig. 3. Figures 5 and 6 show the electrical field acting in two zones, respectively unexposed and exposed, of the OPC portion of fig. 4. Figures 7 and 8 show alternative embodiments for one detail of the apparatus shown in fig. 3. For a better understanding of the invention, Figure 1 shows in qualitative way the electrical field generated by a developing roller 1, biased by an alternate voltage (produced by generator 2) applied between developing roller 1 and ground, when the conductive cylinder 3 of the OPC 4 is grounded. In the zone where the two cylinders are juxtaposed, a strong alternate electrical field is established between the two elements. On the remainder of the developing roller surface a lesser strong, radial alternate electrical field is established. The remainder of the OPC surface is immersed in an electrical field directed tangentially to the OPC surface (zone 5,6) or null (zone 7) owing to the shielding effect of the OPC itself. This is the electrical field distribution which occurs "grosso modo" in the prior art developing apparatuses. Figure 2 shows in qualitative way the electrical field generated by the conductive cylinder 3 of the OPC 4, when biased by an alternate voltage (produced by generator 2) applied between cylinder 3 and ground, when the developing roller 1 is grounded. In the zone where the two cylinders are juxtaposed a strong alternate electrical field is establishes, similar to the one fig. 1, but having a more radial distribution, relative to the OPC axis, than in the case of fig. 1. The remainder of the OPC surface is immersed in an electrical field which is much lesser strong but still radial, relative to the OPC axis. This is the electrical field distribution which occurs, "grosso modo" in the developing apparatus of the invention. The difference between the electrical fields which are generated in the two cases, neglected in the common way of conceiving the electrical field generated between two armatures as the result of a voltage applied between the two armatures, hence independent of the potential of the two armatures as to ground and the surrounding environment, is the only possible explanation of the results achieved by the present invention. Figure 3 shows in schematics a preferred embodiment for the apparatus of the invention. In Fig. 3 a developing unit 10 is juxtaposed to an OPC device 11 in form of rotating drum. The developing unit 10 comprises a tone reservoir 12 for toner 13 and a developing roller 14 in conductive material. The developing roller rotates in the direction of arrow 15, at a predetermined speed, in the order of 5 cm/sec. A thin toner layer 17, having a thickness imposed by a control blade 16, in the order of 50 um is drawn from reservoir 12, adheres to the surface of roller 14 and is brought towards the developing zone. The adherence of the toner particles to the roller is assured by Van Der Waals forces and, in case of magnetic toner, by magnetic fields suitably generated with known means. The toner which adheres to roller 14 is negatively charged by triboelectric effect. The developing roller 14 is electrically biased at a negative potential in the order of -300,-500V as to ground, by a DC voltage generator 18 connected between roller 14 and ground. The OPC device 11 comprises a cylinder 19, in conductive material, coated with a layer 20 of photoconductive material and rotates in the direction of arrow 21. At the developing zone or station the OPC surface is spaced apart form the roller 14 surface by a gap having the same order of magnitude of the toner layer 17 thickness or slightly greater. A corotron located upstream of the developing station and consisting in an ionization wire 21 (negatively biased at a level in the order of -2KV,-5KV by means of a voltage generator 22) and a grid shield 23, charges in a uniform way the surface of the photoconductive layer with negative charges. The electrical potential of the surface charge is controlled by a DC voltage generator 24, which applies a voltage in the order of -700V between the grid shield 23 and the conductive cylinder 19 of the OPC device. In this way the OPC surface is charged at a potential of -700V relative to the potential of cylinder 19. The so charged OPC surface is selectively exposed to an electromagnetic radiation 25, controlled by an image generator 26, at an exposure station located downstream of the corotron and upstream of the developing station. In the exposed zones the photoconductive material allows the electrical charges at the surface to discharge on the conductive cylinder 19. In these zones the OPC surfaces takes a substantially nul potential as to the potential of cylinder 19. According to the invention the cylinder 19 is electrically biased by a generator 27 of AC voltage in the order of 200-500V peak; connected between cylinder 19 and ground. The frequency of the AC voltage may be selected within very broad limits, with a lower limit which essentially depends on the developing speed, say the peripheral speed of the OPC. The upper limit seems related to the size and the mass of the toner particles in inverse relation. In practice, with a toner formed by particles having a size in the order of 10 um and a toner bulk density (before powdering of the material) of 0,6 Kg/dm³, all frequences comprised between 100 and 1500 Hz provide satisfactory results. The surprising and unexpected results which are obtained by this biasing may be explained with reference to figures 4,5 and 6, which show the electrical state of an OPC portion downstream of the developing station (At the developing station the effect of the alternate OPC biasing is substantially the same of an alternate biasing of the developing roller and is not considered here). Figure 4 shows a portion 30 of the OPC which comprises an unexposed zone 31, hence with a negative surface charge (of -700V relative to the conductive cylinder 19), and an exposed zone 32 which has been discharged and on which toner particles lay down. For purpose of simplification it may be assumed that the negative charge of the toner particles does not change in substantive way the electrical fields generated by the external biasing and by the OPC polarization. With this assumption and using the principle of the cumulation of effects, it is possible to consider in qualitative way the electrical fields which affect the OPC portion 30 and its surface. An AC bias of 400V applied to the conductive cylinder 19 generates a radial field shown by arrows 33,34. In the zone 31 the radial field is overlapped with the field generated by the OPC polarization charges (-700V) so that the potential of the space surrounding zone 31 may be represented in its extreme conditions by diagrams A,B of figure 5. The potential of the space surrounding zone 32 is represented in its extreme conditions by diagrams C,D of Fig. 6. A generic toner particle P (Fig.4), negatively charged and located at the surface of zone 31, is therefore subjected to a repulsive force of variable amplitude which tends to push it away from the surface, opposing to the non electrostatic forces (Van Der Waals forces) which retain it at the surface. This repulsive force provides a relative mobility to the particle. In zone 31,32 a tangential electrical field due to the presence of electrical charge in zone 31 and to the missing of electrical charges in zone 32 overlaps with the radial field generated by voltage generator 27 and by the charges in zone 31. Therefore a tangential force acts on particle P in addition to the repulsive one. This tangential force tends to pull particle P towards zone 32. Based on the experimental result it must be concluded that, even if the repulsive force is modest, its repeated application on a particle such as P, for the whole time period required by OPC portion 30 to move from the developing station to the transfer station, allows particle P to move towards zone 32 owing to the tangential force acting thereon. If particle P is electrically neutral the following considerations may be developed. When particle P is immersed in an electrical field, it is subjected to dielectric polarization, hence to a radial pulsing force. In view of the experimental results it must be concluded that the continued and repeated application of a pulsed force causes a mechanical oscillation of the particle on the OPC surface. This oscillation induces at some extent a triboelectric effect so that the particle, initially neutral, charges negatively and behave as already described. As a consequence most of the particles, which are transferred by the developing roller on the OPC surface in unexposed zones, migrate in the exposed zones, the more they are close to the borders of the exposed zones. Therefore the apparatus of the invention enables to obtain highly contrasted images having highly defined edges and a substantive background reduction. Further advantages are provided by the invention as claimed. It has been observed that the electrical AC biasing of the OPC provides further advantages in terms of toner transfer from the OPC to the printing support. Compared with conventional electrophotographic systems where some toner always remains on the OPC, all the toner present on the exposed zones of the OPC in transferred to the printing support leaving the OPC perfectly clean. To explain this result the transfer mechanism is briefly explained. With reference to figure 3 the transfer station comprises a corotron 40 facing the OPC drum 11. The corotron comprises a ionizing wire 41, electrically biased at an high positive potential in the order of +3+5 KV by a voltage generator 42 and a grid shield 43 ground connected. A printing support 44 is brought in contact with the OPC at the transfer station and is fed, interposed between corotron 40 and OPC11, at a speed equal to the peripheral OPC speed. Corotron 40 diffuses positive electrical charges on the printing support, which is electrized, thus generating a strong electrical field between printing support and OPC. This field detaches the toner particles (negatively charged) from the OPC and attract them onto the printing support for subsequent fixing in a fixing station. It is clear that this action is exerted on electrically charged particles, not on the neutral ones. However the alternate biasing of cylinder 19 as already indicated with references to Fig.4, imparts to the neutral particles a dielectric polarization subjecting them to a pulsed attractive force and to oscillations which cause their electrical charging due to triboelectric effect. The phenomenom is exalted, in the exposed zones, by the presence of contiguous particles, mostly charged, hence subjected to attractive and repulsive forces with consequent relative displacement among charged particles and neutral ones and related friction. The consequence is that at the transfer zone all the particles present in the exposed zones of the OPC are electrically charged and subjected to the transfer electrical field. It is further noted that for correct insertion of the printing support between OPC and corotron 40, shield 43 is provided with a conductive guiding blade 45 juxstaposed to the OPC surface at a distance in the order of 2-3 mm from OPC at the printing support input and at a distance in the order of 0,5 mm at the output. In this zone the electrical field generated by the alternate biasing of the OPC is particularly strong, at a level such that owing to such field the printing support, ionized by charge migration from the zone facing the corotron grid to the zone interposed between OPC and guiding blade, vibrates causing a noise at the frequency of the alternate biasing. In such zone two effects cumulate each to the other. On one side, the electrical field is so strong that the triboelectric effect and the particle migration are increased. On the other side, the printing support itself exert a mechanical action of variable compression on the toner facilitating the transfer. The only drawback is noise generation, which may be completely avoided by electrically connecting shield 43 to the conductive cylinder 19, as shown in figure 7, or limited to an acceptable level by biasing shield 43 with a fraction of the biasing potential of cylinder 19. This potential, relative to ground may be easily obtained by connecting shield 43 to the intermediate point of a resistive voltage divider 46,47 connected between the output of generator 27 of fig. 3 and ground as shown in fig. 8. It is clear that a voltage dependent resistor VDR or a zener diode connected between cylinder 19 and shield 43 may be a substitute for such voltage divider. The same arrangements may be used to generate the several biasing voltages required in the apparatus, departing from the voltage generated by one or two voltage generators only (respectively a positive and a negative voltage generator). It is clear that several other changes can be made to the apparatus of the invention. Thus, even if the description of a preferred embodiment relates to a latent image carrier (OPC) and a toner carrier both in form of rotating cylinders, the invention is equally applicable in case one or both of these elements are in form other than a rotating cylinder, such as a movable belt mounted on rotating drums. The essential aspect consists in the generation of a variable electrical field perpendicular to the surface of the latent image carrier, which field acts on a relatively wide area of the carrier comprised between the development station and the transfer station. The electrical alternate biasing of cylinder 19 of the OPC is only a preferred embodiment, because it assures the generation of such variable field extending to the whole OPC surface and including the development station and the fixing station. It has been already indicated that the electrical field so generated is particularly strong in the development station and in the transfer station. It is weaker in the intermediate zone. Even in such zone the field may be strenghtened with the consequent possibility of lowering the alternate biasing voltage still achieving the same results. This strengthening of the electrical alternate field generated by the OPC may be obtained by juxtaposing to the OPC surface a conductive armature 48 located between the development station and the transfer station and electrically grounded as shown in Fig.3. Such armature is preferably located at a distance from the OPC surface comprised between 1 mm and 5 mm and extends along the OPC surface for an arc having a length comprised between 2 and 20 mm or more depending on the peripheral distance between the developing station and the transfer station.

Claims: Electrophotographic imaging apparatus comprising a latent image carrier (11) formed of a photoconductive layer (20) superimposed on a conductive layer (19), a carrier of developing material (14) juxtaposed to said latent image carrier (11) at a developing zone for transferring said developing material on said photoconductive layer (20) in a configuration corresponding to said latent image, a transfer station (40) for transferring said developing material from said photoconductive layer to a printing support (44), juxtaposed to said latent image carrier at said transfer station, and a first voltage generator (18) for applying to said developing material carrier (14) a predetermined potential with respect to ground, characterized in that it comprises: a second voltage generator (27) connected between ground and said conductive layer (19) of said latent image carrier (11) for applying to said conductive layer (19) an alternating potential having a predetermined frequency and amplitude with respect to ground, whereby a periodically variable electrical field, perpendicular to said photoconductive layer, is generated around said latent image carrier (11) at said developing zone, said transfer station (40) and therebetween. Apparatus as claimed in claim 1 where said transfer station (40) comprises an electrostatic charge generator having a discharging electrode (41) and a grid shield (43,45), said second voltage generator (27) being further connected between ground and said grid shield (43,45). Apparatus as claimed in claim 1 where said predetermined potential is comprised between -200V and -600V and said alternate potential has an amplitude comprised between 200 and 600 V peak and a frequency comprised between 100 and 1500 Hz. Apparatus as claimed in claim 1 comprising a conductive armature (48) juxtaposed to said latent image carrier (11) in a zone of said carrier extending between said developing zone and said transfer station at a distance comprised between 2 mm and 5 mm, said conductive armature being connected to ground. Apparatus as claimed in claim 1 where said transfer station (40) comprises an electrostatic charge generator having a discharging electrode (41) and a grid shield (43,45), said apparatus further comprising means (46,47) for applying to said grid shield (43,45) a fraction of said alternate potential as to ground.

Inventors: FARE CARLO, FARE', CARLO
Applicants: BULL HN INFORMATION SYST, BULL HN INFORMATION SYSTEMS ITALIA S.P.A.
Application year: 1990
Main classification: G03G15
IPC codes: G03G15