GB2121323A - Developing electrostatic latent images - Google Patents

Abstract

Image development apparatus comprises a magnetic brush developer roll (3) consisting of a co-rotating shell (31) and cylindrical magnet (32) with a non-magnetic guide plate (7) positioned over and spaced from a part of the periphery of the developer roll away from the development region W. A toner transporting roll (2), having fixed magnets (21), conveys toner from a sump in the bottom of vessel (1) to the exterior of the guide plate where the toner is attracted onto the outside of the plate to be metered at a controlled rate onto the surface of the shell (31) round a tip (73) of the plate. A scraper blade (71) having openings (72) removes the toner from the shell (31) after development but allows some of that toner to pass between the shell and the guide plate. A doctor blade (74) controls the height of toner on the roll (2). <IMAGE>

Description

SPECIFICATION Apparatus for developing electrostatic latent images by applying magnetic toner This invention relates to a magnetic-brush development process for electric pattern images.

In a magnetic-brush development process, developer powder, which includes magnetic material, stored in a developer vessel is conveyed to a development zone and attracted to a magnetic-brush roll. Image-bearing material, positioned adjacent the magnetic-brush roll, may for example be a highly resistive polyester sheet, photoconductive selenium, an electrically insulating film overlying a layer of photoconductive cadmium sulphide disposed in an insulating binder, a thin film of polyvinylcarbazole or poly-N-vinylcarbazole, or a layer formed of the mixture of photoconductive zinc oxide and an insulating resin binder, as is known in the art.

A developer powder is supplied from the developer vessel through a gap of predetermined size onto the magnetic-brush roll and, according to the rotation of the magnetic-brush roll, the developer powder rotates or tumbles along the roll to the development zone. At least at the development zone, the developer powder forms a magnetic-brush on the roll and the magneticbrush rubs the surface of the image-bearing material to cause the toner material of the developer powder to adhere to electric pattern images on the surface. For the purposes of this application, electric pattern images include electrostatic images, capacitive images, and electrically conductive images. For the convenience of explanation in this specification, the latent electrostatic images will be used.

For image developing, there has previously been used an admixture of ferromagnetic carrier particles and toner particles. The ferromagnetic carrier particles are resin-coated-iron beads and the toner particles are a mixture of pigment and binder. The carrier particles and the toner particles are triboelectrically charged to opposite polarities by blending them. The materials of the carrier particles and the toner particles are selected to cause a charge on the toner which is opposite to the charge of the electrostatic latent image on the image-bearing material. The admixture is stored in the developer vessel, in which the toner particles adhere to the surfaces of the carrier particles by the triboelectric charge, and is then conveyed on the surface of the magnetic-brush roll as the roll rotates.

The admixture forms a magnetic-brush at the development zone, and when the magnetic-brush rubs the latent image, the toner particles adhere to the latent image by the electrostatic attraction force between the charge of the latent image and the charge of the toner, but the carrier particles remain on the magnetic-brush roll by the magnetic attraction force between the carrier and the roll.

After the development the admixture, less the adhered toner, returns to the developer vessel and is replenished with new toner.

On the other hand, a single component or monocomponent magnetic toner has been used in magnetic-brush image developing and has the advantage that it is not necessary to use carrier particles and therefore mixing is unnecessary.

Although such a magnetic toner is referred to as "single component" or "monocomponent", the name does not mean that the toner consists of oniy one component, but that the toner comprises mainly one kind of particles composed of fine magnetic particles, organic binder, pigment, carbon black and flow agents. No so-called "carrier" is required.

Developing apparatus for developing electrostatic latent images by employing a single component magnetic toner is disclosed, for example, in British Patent Specifications Nos.

1,493,280 and 2,018,1 64A.

The apparatus as disclosed in British Patent Specification No. 1 ,493,280 is arranged in such a manner that a magnetic developing roll, including a cylindrical permanent magnetic member contained coaxially in the shell thereof, may be placed adjacent to a photoconductor. The developer container, in which a single component magnetic toner is stored, has an opening positioned near above said shell of the magnetic developing roll so that the toner may be supplied onto the shell from said container. As the shell is rotated, the toner on the shell is conveyed towards the photoconductor. After having been conveyed on the shell the toner is accumulated at a position upstream of the spot where the magnetic developing roll is adjacent to the photoreceptor, forming a toner reservoir.

The thus formed reservoir permits turbulence in the toner, and this tubulence is accompanied by the rotation of the magnet within the shell in a direction opposite to the direction of rotation of the shell.

In British Patent Specification 2,018,1 64A, the permanent magnet contained in the magnetic developing roll is so rotated that the magnetic toner may be extracted from the toner vessel onto the shell to proceed toward the photoreceptor.

Since the shell rotates in the same sense as the magnet, the shell acts to convey the magnetic toner in a direction counter to that of the magnetic action. Accordingly two currents of toner are generated, one current being driven adjacent the toner layer on the shell by the rotation of the inner permanent magnet from the toner vessel to the photoreceptor and the other current being driven adjacent the bottom of the toner layer by the rotation of the shell itself from the photoreceptor to the toner vessel. When this second current of toner has reached a doctor blade spaced from the toner vessel, the toner amount supplied from the toner vessel to the shell is reduced and accordingly this lower current is also weakened, then the supplied amount of toner becomes constant.

Compared with the admixture developer of ferromagnetic carrier particles and toner particles, the single component magnetic toner has rather a tendency to agglomerate. Since the admixture developer includes carrier particles in addition to toner, the bond between toner particles themselves is weakened, so agglomeration cannot easily occur.

Though internally the single component magnetic toner contains fine magnetic particles, e.g. magnetite, the surfaces of the toner particles are covered substantially with resin. This resin must be a fixable resin because the toner must be fixed onto a sheet of electro-photographically sensitive paper after development, or onto a sheet of plain paper onto which the developed image has been transferred. For this purpose, the resin should generally be a pressure fixing resin sensitive to reduced pressures as compared with those often used, or a resin having a melting point adjacent 1 000C. Therefore in the single component magnetic toner wherein toner particles are always in contact with each other, agglomeration occurs not only during storage and transportation, but inside the copying machine.

The particle size of widely-usable single component magnetic toner is selected within a diameter range of from 5 to 50 ium. Such particles, when agglomerated in clumps of from tens to thousands, occasionally grow to several millimeters. If toner particles agglomerated in such a way are employed, they will be squeezed between the shell of the magnetic developing roll and the doctor blade, thus often causing toner blocking which interrupts the toner-supply to the developing zone between the magnetic developing roll and the photoreceptor. Even if blocking does not occur, the surface of the photoreceptor will be smeared, thereby marring the quality of the developed image.

The object of the present invention is to propose development apparatus for use with magnetic toner which is of poor fluidity and likely to aggiomerate, but so as to achieve stable flow without such difficulties as toner blocking.

Accordingly the present invention provides apparatus for developing electrostatic latent images formed on an image-bearing material by applying magnetic toner, the apparatus comprising: (a) a magnetic developing roll positioned adjacent the image-bearing material to form a developing gap therewith, wherein the magnetic developing roll includes a cylindrical rotatable shell of non-magnetic material and a rotatable permanent magnet member positioned coaxially within the shell, the permanent magnet member having a plurality of adjacent axially extending magnetic poles of alternating polarity on the peripheral surface thereof, both the shell and the permanent magnet member being driven for rotation in the same direction for advancing the magnetic toner to the developing gap on the shell surface; (b) a toner vessel for storing magnetic toner, the vessel having the magnetic developing roll in its upper portion; (c) a rotatable tonertransporting roll positioned in the vessel, the toner-transporting roll having magnetic poles on its surface for conveying magnetic toner from the vessel towards the magnetic developing roll, the magnetic developing roll being positioned between the toner-transporting roll and the image-bearing material; (d) a guide plate extending along the surface of the shell spaced from and covering a part of the surface of the magnetic-brush developer roll opposite to that adjacent the image-bearing material, the guide plate being positioned to receive on its surface facing away from the magnetic-brush developer roll magnetic toner from the toner-transporting roll, by the magnetic attraction force of the developing roll, and to advance the magnetic toner on its surface to an end edge by the rotating magnetic field due to the magnetic developing roll; and (e) a non-magnetic scraper positioned on the shell surface downstream of the developing gap, the scraper having at least one opening therethrough to allow scraped magnetic toner to be transported to the spacing between the magnetic developing roll and the guide plate.

It is preferable in this invention that a toner reservoir space is positioned between the guide plate and the toner transporting roll.

It is desirable that the scraper blade contacts the shell surface tangentially.

More preferably, the toner transporting roll is holiow.

In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a schematic cross-sectional view of an embodiment of apparatus of this invention; FIGURE 2 is a plane view of a scraper blade usable in the apparatus shown in Figure 1; and FIGURE 3 is a simplified cross-sectional view illustrating the principle of this invention.

Referring now to Figures 1 to 3, the magnetic single component toner 12 is stored in the toner vessel 1. The toner transporting roll 2 is rotatably arranged in the lower part of the interior of toner vessel 1. The magnetic developing roll 3 partially projects from the opening 13 near the top of the toner vessel 1 so as to be positioned close to the surface of the photoreceptor, in this case a photoconductor drum 4, at a development zone W.

The magnetic developing roll 3 includes a rotatable non-magnetic, cylindrical shell 31 and a rotatable permanent magnet member incorporated coaxially in the shell 31. The rotatable permanent magnet member includes a shaft 33 coaxial with the shell 31 and a cylindrical permanent magnet 32 secured around the shaft 33. The permanent magnet 32 has on its peripheral surface a plurality of adjacent axially extending magnetic poles N, S of alternating polarity. In the drawing, N and S mean a north pole and a south pole, respectively. The shaft 33 of the magnetic developing roll 3 isjournalled at both ends with the shell 31 by bearing members, and the shell 31 and the shaft 33 are independently motor-driven.

The shell 31 and the permanent magnet 32 rotate independently in the same direction at different speeds. For purposes of illustration the shaft 33 and the permanent magnet 32 rotate clockwise, i.e. in the direction shown by an arrow x and the shell 31 rotates in the direction shown by an arrow y i.e. in the same direction as that of the magnet.

A guide plate 7, covering at least a quarter of the periphery of shell 31, and spaced therefrom with a gap d3, is on the side of magnetic developing 3 opposite to the photoreceptor 4. Downstream of the developing zone, that is, the zone W where the photoreceptor 4 comes closest to roll 3, is a scraper blade 71 having one end in contact with the surface of shell 31; scraper blade 71 is fastened to the downstream end of the guide plate 7.

As shown in Figure 3, the scraper blade 71 has a plurality of openings 72 therealong. The upstream portion 73 of the guide plate 7 is tapered, while the downstream end of the guide plate 7 is formed as a second scraper blade 74 of knife edge type. The term "upstream" and "downstream" used here indicate positioning relative to the clockwise-rotating surface of the shell 31 of the magnetic developing roll 3.

The toner transporting roll 2 in the bottom of toner vessel 1 comprises a rotatable cylindrical roll having a pair of permanent magnets 21 such as rubbery magnets or piastic magnets.

Magnetic toner 12, contained in a toner supply cartridge 5, is supplied from the opening 51 into the toner vessel 1 by rotation of the cartridge 5 to invert it.

The functioning of the above described developing apparatus will be explained with reference to Figure 3. When the toner transporting roll 2 is rotated in the direction shown by arrow R, a mass 121 of magnetic toner on the surface of the permanent magnet 21 is conveyed anticlockwise to approach the guide plate 7. In this case the toner near the surface of toner mass 1 21 is gradually transferred onto the guide plate 7 by the magnetic attraction of magnetic developing roll 3 and then conveyed in the direction of arrow E by the rotation of permanent magnet 32 in the direction of arrow X.

In this case, as the rotation of the tonertransporting roll 2 effects continuous supply of magnetic toner onto the guide plate 7, a relatively large quantity 122 of magnetic toner is adsorbed and accumulated on the guide plate 7. However, as mentioned above, the quantity of toner transported by the rotation of permanent magnet 32 is limited, so such a large quantity 122 of magnetic toner is conveyed bit by bit at a constant mass rate in the direction of arrow E along guide plate 7 and finally the toner mass 123 is entirely supplied from the upper (upstream) position 73 to shell 31. When the magnetic toner has been transferred from the guide plate 7 onto the shell 31 of magnetic developing roll 3, the magnetic toner is stirred and the toner layer on the shell becomes turbulent. This turbulence releases the adherence between the toner particles, to enhance the fluidity of toner.

In this developer apparatus, the shell 31 is never supplied with a large quantity of toner at one time, but toner is supplied to the shell at a constant rate. On the other hand when the surplus magnetic toner, which is not transferred from the transporting roll 2 to the guide plate 7 but is retained on magnet 21, comes to the second scraper blade 74 it is recirculated to the sump in the bottom of toner vessel 1.

Then the toner on the next permanent magnet 21 is transferred to the guide plate 7 by the magnetic attraction of permanent magnet 32 of the magnetic developing roll 3. Thus, the toner attracted onto the guide plate 7, as mentioned above, is conveyed bit by bit to the guide plate without fail.

In this case, in order to transfer the toner from the toner transporting roll 2 to the guide plate 7 it is necessary that the magnetic force of permanent magnet 21 should be weaker than that of magnetic developing roll 3, but the value of magnetic force of permanent magnet 21 may be determined depending on the magnetic force of magnetic developing roll 3 and on the distance between guide plate 7 and the transportation roll 2.

The magnetic toner 1 2 supplied successively onto the shell 31 is conveyed in the arrowed direction F (Figure 3) by the rotation of the permanent magnet 32 in the sense of arrow X, and is blocked by the presence of photoreceptor drum 4 so that a build-up of toner may occur upstream of the gap between the photoreceptor and the magnetic developing roll 3. Also the shell 31 is rotated in the direction of arrow Y so that the toner positioned near the shell 31 may be drawn back with shell 31 in the same sense of rotation and so the thickness t of the toner in a reservoir mass 1 24 may be substantially constant as shown in the drawings.When the ratio of the rate of rotation of shell 31 to that of permanent magnet 32 is maintained constant at a value less than about 1/20, the thickness t of toner reservoir mass 124 will be held substantially constant so that the width of development may be maintained constant to ensure stable development.

Furthermore, as shown in the drawings, the toner contained in the toner reservoir mass 1 24 is always circulated anti-clockwise and part of it passes through the gap d2 whilst the rest of the toner is transferred clockwise in the same sense as the rotation of shell 31.

When the toner has reached the end of the toner reservoir mass 1 24, the toner joins with the toner being conveyed in the direction of arrow F and is conveyed back towards gap d2.

Toner having passed through gap d2 is conveyed toward the direction of arrow G, by the rotation of permanent magnet 32, and part of it is scraped off the magnetic developing roll 3 by the scraper blade 71 positioned downstream of gap d2 to be recollected in the toner vessel 1. The rest of the toner passes through the openings 72 of scraper blade 71 and is conveyed again along the shell 31 towards gap d2. In this way the toner, after use in image development, is partially and continuously scraped down; there will therefore never be excessive toner on the shell 31, and it is so easy to maintain the toner quantity constant in the reservoir mass 124 so the undesirable growth of the toner reservoir mass may be effectively prevented.Though the toner, after being used in image development, is temporarily blocked by the scraper blade 71, the openings 72 allow the continued flow of toner without applying any undesirable mechanical force to the toner.

Moreover even if the toner reservoir mass 124 grows towards the guide plate 7, the gap d3 between the internal surface of the guide plate 7 and the exterior of the shell 3 is selected as to be equal to or larger than the thickness t of the toner reservoir mass 124 so that the guide plate 7 may, without applying undesirable mechanical force to the toner, prevent the reservoir of toner passing anti-clockwise round the magnetic developing roll 3 from blocking. Since the scraper blade 71 has at least one special opening 72, the toner reservoir mass 124 will, even if it grows far enough clockwise to reach the scraper blade 71, be scraped down into the toner vessel 1 through the opening 72, thus preventing the toner reservoir mass 124 from growing as far as the image developing zone.

As hereinbefore mentioned the developing apparatus is arranged so that despite the peripheral guide plate 7 and the scraper blade 71 around the shell 31, toner displacement is not interrupted, regardless of these plates; this prevents toner blocking from occurring.

Furthermore, as the scraper blade 71 has at least one opening 72, the undesirable growth of the toner reservoir mass is prevented.

If there is not such a guide plate 7 or scraper blade 71, the toner quantity on the shell 31 will increase and peripheral extent of the toner reservoir mass 124 will grow in the direction of rotation of the shell and one end of the reservoir will reach the developing zone. Then, because the transportation power will act strongly on the toner in the developing zone, the soft agitation of the toner will be prevented by rotation of toner particles accompanying the rotation of the permanent magnet 32, to influence the development adversely.

The developing apparatus of the present invention is arranged in an experimental electro photographic duplicator of the pressure fixing or fusing type wherein a selenium drum of 120 mm outer diameter and rotating at a peripheral speed of 150 mm/sec is employed as photoreceptor. The shell 31 comprises a stainless steel sleeve of 32 mm outer diameter, and the permanent magnet comprises a cylindrical Ba ferrite magnet of 29 mm outer diameter with twelve symmetrically magnetized poles which present a magnetic flux density of 600 gauss at the shell, the shell and the permanent magnet being rotated at 70 rpm and 150 rpm respectively, the gap d2 and gap d3 being 0.3 mm and 1.6 mm, respectively. The toner transporting roll is rotated at 20 rpm and its permanent magnet comprises a rubbery magnet having a magnetic flux density of 300 gauss.The toner has particle diameters within a range from 5 to 30 Xtim, and a pressurefixable magnetic toner having a volume resistivity of 1 x 1014 Q.cm is employed. The resistivity of the toner is determined under a weight load of about 200 g and in a D.C. electric field of 400 v/cm after it is built up to a thickness of from 10 to 20 mm in a fluorocarbon polymer cylinder having an inner diameter of 3.05 mm.

After copying tests with 30,000 sheets of paper under the above-mentioned conditions, it was revealed that the obtained images were of high quality and had a clear background, the image density being higher than 1.2, the degree of resolution being more than 6.3 lines/mm. Blocking of toner was not observed.

As mentioned above, the apparatus of the present invention can serve to ensure the following effects: (1) Even when using a magnetic toner of mediocre fluidity, the developing apparatus does not suffer toner blocking, but can afford stable transportation of toner and excellent developments results.

(2) Regardless of having a simple mechanism, the apparatus permits correct control of the supply quantity of magnetic toner onto the shell, and is excellent in practicality, utility and reliability.

The guide plate 7 and scraper blade 71 are preferably formed of metal.

Claims (8)

1. Apparatus for developing electrostatic latent images formed on an image-bearing material by applying magnetic toner, the apparatus comprising: (a) a magnetic developing roll positioned adjacent the image-bearing material to form a developing gap therewith, wherein the magnetic developing roll includes a cylindrical rotatable shell of non-magnetic material and a rotatable permanent magnet member positioned coaxially within the shell, the permanent magnet member having a plurality of adjacent axially extending magnetic poles of alternating polarity on the peripheral surface thereof, both the shell and the permanent magnet member being driven for rotation in the same direction for advancing the magnetic toner to the developing gap on the shell surface; (b) a toner vessel for storing magnetic toner, the vessel having the magnetic developing roll in its upper portion; (c) a rotatable tonertransporting roll positioned in the vessel, the toner-transporting roll having magnetic poles on its surface for conveying magnetic toner from the vessel towards the magnetic developing roll, the magnetic developing roll being positioned between the toner transporting roll and the imagebearing material; (d) a guide plate extending along the surface of the shell spaced from and covering a part of the surface of the magnetic developing roll opposite to that adjacent the image-bearing material, the guide plate being positioned to receive on its surface facing away from the magnetic developing roll magnetic toner from the toner transporting roll, by the magnetic attraction force of the developing roll, and to advance the magnetic toner on its surface to an end edge by the rotating magnetic field due to the magnetic developing roll; and (e) a non-magnetic scraper positioned on the shell surface downstream of the developing gap, the scraper having at least one opening therethrough to allow scraped magnetic toner to be transported to the spacing between the magnetic developing roll and the guide plate.

2. Apparatus according to claim 1 , wherein said guide plate is non-magnetic.

3. Apparatus according to claim 1 or 2, wherein said scraper blade has a plurality of closely spaced openings extending in a rectilinear array parallel to its scraper edge.

4. Apparatus according to any one of claims 1 to 3, and further comprising a toner reservoir space positioned between the guide plate and the toner transporting roll.

5. Apparatus according to any one of claims 1 to 4, wherein the scraper blade tangentially contacts the shell surface.

6. Apparatus according to any one of claims 1 to 5, wherein the toner transporting roll is hollow.

7. Apparatus according to claim 6, wherein the toner transporting roll has rubber magnets on its peripheral surface.

8. Image development apparatus substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.