GB2133319A

GB2133319A - Development of electrostatic images

Info

Publication number: GB2133319A
Application number: GB08331063A
Authority: GB
Inventors: Dr Gabor Forgo; Karl Moser
Original assignee: Elfotec AG
Current assignee: Elfotec AG
Priority date: 1982-11-25
Filing date: 1983-11-22
Publication date: 1984-07-25
Also published as: GB8331063D0; DE3243705C1; FR2536876A1

Abstract

Electrostatic latent images on a photoconductive carrier (9) are developed by means of a magnetic brush (3) consisting of a rotating magnetic core (4) surrounded by a non-magnetizable sheath (5). The amplitude and frequency of a bias voltage applied to the brush is determined such that as a result of the co-operation between the magnetic and electrical alternating fields, a cloud of toner is obtained which is limited in its dimensions, consists of vibrating particles and whose thickness is smaller in a radial direction than a spacing (d1) between the carrier (9) and the magnetic brush (3), and, further the transport of the toner particles which takes place exclusively to the image regions of the carrier, is caused by the electrostatic field arising from the charge image on the carrier. The frequency of the alternating field may be 5-12 kHz. The metering gap (d2) is approx. half the spacing (d1). <IMAGE>

Description

SPECIFICATION A method of, and an apparatus for producing images by electrophotography This invention relates to a method of producing images by electrophotography, in which an electrostatic latent image is produced on a photoconductive intermediate image carrier, to develop this latent image toner is deposited using a magnetic brush for an insulating, magnetic, single-component toner which employs a rotating magnetic core surrounded by a non-magnetizable sheath, on the image regions of the intermediate image carrier while said carrier is moved past the magnetic brush, leaving a spacing to the brush, which brush lies, with respect to the intermediate image carrier, at a bias voltage consisting of a high-frequency alternating voltage with a direct voltage level, and finally the toner image which is obtained is transferred from the intermediate image carrier to a receiver sheet and is fixed thereon.

US-PS-4, 4,121,931 discloses a method of producing images by electrophotography in which the thickness of the covering of toner on the rotating sheath is predetermined by means of a stripping device which defines a metering gap and, in order to charge the toner, a direct voltage is applied between the intermediate carrier and the sheath and/or the stripping device.

The main disadvantage of the known method is that it is difficult to develop fine lines and, due to the polarizability of the toner, this toner is widely deposited where there is a difference between the potential of the charge image and the developing electrode, so that there is always the risk of forming 2 disturbed background.

US-P S 4,299,900 discloses a developing process for magnetic, insulating single-component toners, the process being carried out with a toner layer thickness on the magnetic brush which is smaller than the developing gap. Whereas the sheath of the magnetic brush rotates, a stationary magnetic core is used, so that the thickness of the toner layer on the magnetic brush may be adjusted with respect to a stationary pole by means of a magnetic stripper. The toner is charged triboelectrically. According to one embodiment of this known process, a low-frequency alternating voltage of 800 V may be applied between the intermediate image carrier and the magnetic brush, the frequency of said voltage amounting to 200 Hz.

Disadvantages of this known method are, in particular, the use of a very thin layer of toner on the magnetic brush, the tolerance requirements which result therefrom and, cbnsequently, a difficult production. A prerequisite for an efficient method is the observance of a very precise relationship between the thickness of the toner layer and the developing gap, and the developing gap is to be smaller than or equal to 0.3 mm.

It is known from US-P S 3,893,418 which describes a method which is also termed a "jumping method" that high-freuqency square pulses may be applied between the toner brush and the intermediate image carrier in order to loosen the toner particles of a micro-field toner brush having a non-magnetic single-component toner. In order to obtain, on the one hand, an improvement in the line copies and, on the other hand, an improvement in the half-tone region, a combination of two highfrequency pulse sequences having frequencies of from 2 to 5 kHz and from 1 8 to 22 kHz is used.

A disadvantage of this method is the complicated and expensive structure of the micro-field toner brush, since a non-magnetic toner has to be used. In general, an expensive apparatus is required which is also critical with respect to liability to interference.

DE-OS 3,102,600 discloses a developing method for magnetic latent images which operates with a varying electrical alternating field in the developing gap. This electrical alternating field is so powerful even in the areas without a magnetic latent image, that the toner particles constantly pass over the developing gap so that they are transferred to the image-free areas of the latent image carrier as well as to the areas which have a magnetic image, thus resulting in the build-up of a background fog in the image-free areas. The transfer back of the toner particles forming this fog is to be effected by the anti-phase part of the alternating voltage which is applied.

However, due to the forces which are effective at close range, the toner particles which form the background fog are bound so strongly to the surface of the latent image carrier that they are only partially transferred back. If the toner particles which are retained are transferred to a receiver sheet, a background is produced on the copy which impairs the quality thereof. If the toner particles are not transferred to the receiver sheet, they have to be removed from the latent image carrier by a cleaning procedure which leads to an increase in the consumption of toner per copy, which is undesirable with respect to the economy of the copying process, or a complex return device is required which also entails the problem of changing the composition of the supply of toner.

An object of the present invention is to improve the method of the initially-defined type using an insulating, magnetic, single-component toner having fine toner particles which preferably have a diameter in the range of from 5 to 1 5 microns, such that when the charge image is being developed, thin lines do not become narrower, and the toner is prevented from being transferred to the image-free areas of the intermediate image carrier, so that it is unnecessary for the toner to be returned and an effective surface covering, a clean background and a good marginal definition are ensured.

According to the present invention by determining the amplitude and the frequency of the bias voltage so that as a result of the co-operation between the magnetic alternating field and the electrical alternating field, a cloud of toner is formed which is limited in its dimensions and consists of vibrating particles, and whose thickness is smaller in a radial direction than the spacing between the intermediate image carrier and the magnetic brush, and the transport of the toner particles which takes place exclusively to the image areas is at least caused substantially by the electrostatic field arising from the charge image on the intermediate image carrier.

The measures employed according to this invention result in an interaction between electrical and magnetic forces, which provides surprisingly advantageous copying results.

The magnetic alternating field which forms along the circumference of the sheath and also in the developing gap produces, together with the high-frequency alternating voltage at the magnetic brush which also increases the mobility of the toner particles which, as such, flow badly, a cloud of very highly excited particles which is defined in its extent by the intentionally selected vibration amplitudes.

The toner particles which are required to develop the charge image may be drawn out of this cloud of particles which are extremely mobile, but which do not cross over the developing gap, as a result of the electrostatic forces which arise from the charge image carrier on the intermediate image carrier, optionally being supported by a tribo-electric charging of the toner particles, and the toner particles may then be deposited on the intermediate image carrier.

Since on the one hand, the toner particles do not cross over the developing gap in the image-free areas, but, on the other hand, the low electrostatic forces arising from fine charge image lines suffice for transferring toner particles to the intermediate image carrier in a very defined manner, it is possible for thin lines to be developed in a faultless manner, i.e. without being narrowed, and also for disturbing background effects to be eliminated in the image-free areas, so that a return of particles is unnecessary.

The magnetic core and an electrically conductive and non-magnetizable sheath which surrounds the core and carries the toner particles are preferably driven in opposite directions.

An advantageous apparatus for implementing the present method comprises a magnetic brush to which a toner storage container is assigned and which consists of a multipole magnetic core and a sheath, a doctor which defines a metering gap, an intermediate image carrier which supports a photoconductor layer and together with the magnetic brush defines a developing gap, and a generator for producing high-frequency voltage which is superimposed on a direct voltage for use as a bias voltage, and this apparatus is distinguished in that the metering gap defined by the metering doctor is about half the size of the developing gap between the magnetic brush and the photoconductor layer of the intermediate image carrier, and the magnetic core is connected to a rotary drive.

An essential factor in connection with all embodiments of an apparatus for implementing the method is that the critical parameters are not mechanical, but are of an electrical nature, and the electrical parameters may be predetermined very precisely without a particular expense, thus resulting in a complete arrangement which is particularly favourable in terms of production and is also extremely reliable in operation.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 schematically shows an embodiment of an apparatus for implementing the method according to the present invention, and Fig. 2 schematically shows a further embodiment.

According to Fig. 1, a storage container 1 contains an insulating, magnetic single-component toner 2, the specific resistance of which amounts to at least 1013 Qcm. In particular, the particles of this single-component toner have dimensions of from 5 to 1 5 microns.

A magnetic brush which is generally indicated by reference numeral 3 is located in the outlet region of the storage container 1. This magnetic brush consists of a multipole magnetic core 4 and an electrically conductive, non-magnetizable sheath 5 which may preferably have an insulating covering having a layer thickness in the range of from about 2 to 20 microns.

In the embodiment shown, the magnetic core 4 is driven in a clockwise direction, and the sheath 5 is driven in an anti-clockwise direction.

During operation, a layer of toner particles forms on the sheath 5, the thickness of which is predetermined by a metering doctor 6 which consists in particular of iron or brass.

A generator 7 produces a high-frequency alternating voltage which is superimposed on a direct voltage and is applied to the magnetic brush, that is, to the sheath 5.

The intermediate image carrier is formed by a photoconductor layer 9 applied to a drum 8 which, in the embodiment shown, is moved in an anti-clockwise direction.

A developing gap d1 is adjusted between the photoconductor layer 9 and the magnetic brush 3 and is preferably about twice the size of the metering gap d2 selected between the magnetic brush 3 and the doctor 6.

The operational parameters for the apparatus shown are preferably selected as follows: Speed of sheath 5: 80 r.p.m.

Direction of rotation of sheath 5: Anti-clockwise Outer diameter of sheath 5: 32.2 mm Speed of core 4: 700 r.p.m.

Direction of rotation of core 4: Clockwise Diameter of core 4: 29.5 mm Magnet: 10 pole, 650 gauss, measured on the sheath Developing gap: d,=0.4 mm detering gap: d2=0.25 mm, metering doctor made of iron Alternating voltage: f=1 0 kHz, U=1 500 Vpp, sinusoidal Direct voltage: U=+60 V Photoconductor 9: Selenium Photoconductor speed: v=1 2 cm/sec Direction of rotation of photoconductor: Anti-clockwise.

Perfect copies having an effective surface covering, a clean background, a good marginal definition and no narrowing of thin lines were obtained with an apparatus of this type and by applying the specified parameters.

The embodiments shown in Fig. 2 differs from the apparatus described with reference to Fig. 1 in that the core 4 and the sheath 5 respectively rotate in the opposite directions, and in various parameters.

The parameters were selected as follows for the apparatus according to Fig. 2: Speed of sheath 5: 71.5 r.p.m.

Direction of rotation of sheath 5: Clockwise Speed of core 4: 900 r.p.m.

Direction of rotation of core: Anti-clockwise Magnet: 10 pole, 650 gauss, measured on the sheath Developing gap: d,=0.4 mm Metering gap: d2=O.2 mm, metering doctor made of brass Alternating voltage: f=6 kHz, U=1400 Vpp, sinusoidal Direct voltage: U=+60 V Photoconductor 9: Selenium Photoconductorspeed: v=12 cm/sec Direction of rotation of photoconductor: Anti-clockwise.

Perfect copies within the sense of the present invention were also obtained with this embodiment.

Claims

1. A method of producing images by electrophotography, in which an electrostatic latent image is produced on a photoconductive intermediate image carrier, to develop this latent image toner is deposited using a magnetic brush for an insulating, magnetic single-component toner which employs a rotating magnetic core surrounded by a non-magnetizable sheath, on the image regions of the intermediate image carrier while said carrier is moved past the magnetic brush, leaving a spacing to the brush which brush lies, with respect to the intermediate image carrier, at a bias voltage consisting of a high-frequency alternating voltage having a direct voltage level, and finally, the toner image which is obtained is transferred from the intermediate image carrier to a receiver sheet and is fixed thereon, wherein the amplitude and frequency of the bias voltage are determined so that as a result of the cooperation between the magnetic alternating field and the electrical alternating field, a cloud of toner is formed which is limited in its dimensions and consists of vibrating particles and whose thickness is smaller in a radial direction than the spacing between the intermediate image carrier and the magnetic brush, and the transport of the toner particles which takes place exclusively to the image regions, is at least caused substantially by the electrostatic field arising from the charge image on the intermediate image carrier.

2. A method according to claim 1, wherein the magnetic core and an electrically conductive, nonmagnetizable sheath which encloses the core and carries the toner particles are driven in opposite directions.

3. A method of producing images by electrophotography substantially as herein described with reference to either of Figs. 1 and 2 of the accompanying drawings.

4. An apparatus for producing images by electrophotography, comprising a magnetic brush which is assigned to a storage container of toner and consists of a multipole magnetic core and a sheath, a doctor defining a metering gap, an intermediate image carrier which supports a photoconductor layer and together with the magnetic brush defines a developing gap, and a generator to produce a highfrequency voltage which is superimposed on a direct voltage for use as a bias voltage, wherein the metering gap defined by the metering doctor is about half the size of the developing gap between the magnetic brush and the photoconductor layer of the intermediate image carrier, and the magnetic core is connected to a rotary drive.

5. An apparatus according to claim 4, wherein the magnetic core has at least eight poles of alternating polarity.

6. An apparatus according to claim 4 or 5 wherein, the conductive sheath which surrounds the magnetic core is provided with an insulating covering.

7. An apparatus according to claim 6, wherein the layer thickness of the covering is in the range of from about 2 to 20 microns.

8. An apparatus according to any of claims 4 to 7, wherein the metering doctor consists of magnetizable or magnetic material.

9. An apparatus according to any of claims 4 to 7, wherein the metering doctor consists of nonmagnetic material.

10. An apparatus according to any of claims 4 to 9, wherein the frequency of the alternating voltage is in the range of from about 5 to 12 kHz.

11. An apparatus according to any of claims 4 to 10, wherein the direct voltage level is below 100V.

1 2. An apparatus according to claim 11, wherein the direct voltage level is in the range of from 20 to 80 V.

13. An apparatus according to claim 12, wherein the direct voltage level is about 60 V.

14. An apparatus according to any of claims 4 to 13, wherein the high-frequency alternating voltage is at least substantially sinusoidal, and is superimposed on the direct voltage.

1 5. An apparatus according to any of claims 4 to 14, wherein the peak-to-peak value of the highfrequency alternating voltage is greater than 1,000 V.

16. An apparatus for producing images by electrophotography substantially as herein described with reference to either of Figs. 1 and 2 of the accompanying drawings.