GB2044138A - Electrophotographic development apparatus - Google Patents

Abstract

In an electrographic apparatus of the type having a brush-type applicator on which is formed a nap containing toner with the nap contacting the electrostatic image-bearing surface of a flexible web (16) during image development, a back-up roller (40) is pivotably mounted about axes A and B to have two degrees of freedom of movement and in response to changes in the nap thickness moves to a position which compensates for such nap thickness changes by maintaining relatively constant pressure between the nap and the web (16) across the web (16). <IMAGE>

Description

SPECIFICATION Electrophotographic development apparatus which compensates for toner nap variations The present application relates to electrography, and more specifically to magnetic brush developing apparatus.

Magnetic brush developing apparatus which brushes a nap of developer across a moving surface carrying an electrostatic image is known. Uniform development of the image is impeded by variations in the size of the nap as the brush moves.

U.S. Patent 4,013,041 (U.K. Patent Specification No. 1,541,585) discloses apparatus in which the image carrying surface is a flexible web and two back-up rollers are spaced to permit a pronounced floating action of the web being contacted with the nap. This structure improves development uniformity over other prior art.

It is the object of the invention to provide means to further improve the uniformity of development of an electrostatic image on a flexible web.

According to the present invention there is provided electrophotographic development apparatus for developing an electrostatic image carried on a surface of a flexible web using a magnetic brush, comprising a back-up roller positioned to press against the surface of the web opposite the magnetic brush, and a supportforthe roller which permits two degrees of freedom of movement of the roller relative to the web.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view in side elevation, partly in section, of an electrographic copier apparatus embodying the present invention; Figure 2 is an enlarged perspective detail of the back-up roller support assembly forming part of the apparatus of Figure 1; and Figures 3a, 3b, and 3c are schematic views showing how the back-up roller is mounted to rotate freely about both axes A and B.

In Figure 1 an electrographic copier apparatus 10 having various stations of an electrographic apparatus is illustrated schematically. An information bearing medium, such as a document 13, is illuminated by radiation from flash lamps 14. Such radiation is reflected from the document 13 and projected by a lens 15 onto a photo-conductor, shown as a web 16 which is uniformly charged at a charging station 20, to selectively dissipate the charge and form an electrostatic image. The path of the web 16 is defined by rollers 5 to 9. The roller 5 is driven by a drive mechanism 12 shown for simplicity to include a motor-pulley arrangement.

The apparatus 10 further includes a magnetic brush development station 22 at which the moving electrostatic image is contacted with finely divided charged toner particles that adhere to the web surface in a configuration defined by the electrostatic image on the web 16, a transfer station 24 in which toner particles are transferred in the image configuration to a receiving surface of a copy sheet 27 on which it can be permanently fused and a cleaning station (not shown) in which residual toner is removed from the photoconductive layer so that it can be reused. In certain known modifications of this same system, one or more of these stations may be eliminated.

The magnetic brush development station 22 includes a magnetic brush member 30 which is rotated in a clockwise direction, as shown by the arrow, in a magnetic field produced by stationary magnets (not shown) which are located inside the member 30. The brush member 30 is effective to transport in bristle formation a mixture of ferromagnetic carrier particles and toner particles across the image bearing surface of the web 16 so that toner particles will contact and then remain on charged portions of image areas on the surface of the web 16. Since a mixture having a high toner concentration would tend to clog, a continuous control mixing paddlewheel 31 has been provided to uniformly blend the mixture. An example of a magnetic brush development station is more fully disclosed in U.S. Patent No. 3,543,720 (U.K. Patent Specification No.

1,265,901).

A rotatable, back-up roller 40 has two degrees of freedom of pivotal movement about orthogonal axes A and B. The roller 40 under the influence of gravity is urged into contact with a non-imaging surface 1 6b of the web by movement about the axis A, to compensate for parallel spacing variations between the nap on the magnetic brush 30 and an image bearing surface 16a of web 16. As best shown in Figure 3a, if for example, there is a sudden increase in the amount of developer in the nap, the increased nap thickness causes the flexible web 16 to move above its normal path oftravel. Upward movement of the web 16 causes the roller to pivot upwardly about axis A.

Pivotal movement about axis B compensates for across web tilt errors and is shown in Figure 3c. By across web tilt errors is meant that the nap thickness may be different along the length of the brush member 30. Thus, if the nap is thicker adjacent the further left-hand edge of the web 16 (see Figure 2), than it is adjacent the nearer, right-hand edge, the roller 40 will rotate in a clockwise direction about axis B as viewed in Figure 2. Since the roller can pivot freely about both axes A and B, it compensates for changes in thickness of the nap on the magnetic brush, and maintains relatively constant pressure along the nip between the nap and the web.

Figure 3b shows several of the positions of the roller 40 caused by variations in the nap contour. As shown in dotted line at 40', the axis B may be tilted upwardly a distance Dat the roller 40 by the roller 40 moving upwardly about axis A to position B' where it is also free to pivot about axis B in either clockwise or counterclockwise directions, 40'a or 40'b respectively.

Referring now to Figure 2, a support for gimballing the roller 40 to have the required two degrees of freedom of rotation is shown in detail. A support plate 50 is located under the web 16 and is fixed by screws 52 to a housing 27. Integrally formed on the plate are two spaced upstanding support members 54. Fixedly secured in the members 54 is a shaft 56.

The axis of the shaft 56 defines the axis A. A first bracket 58 is pivotably mounted on the shaft 56. The back-up roller 40 is rotatably mounted in end flanges 60 of a second bracket 62. The axis of a second shaft 64 defines the axis B and interconnects the brackets 58 and 62. The shaft 64 is fixed to the bracket 62 and pivotably mounted in the bracket 58.

The various elements of the support are selected so that the roller 40 presses against the surface 1 6a of the web 16 opposite the magnetic brush to maintain a relatively constant pressure at the nip between web and nap along the length of the brush member 30 and thereby provide uniform development of electrostatic images. In response to a change in the nap contour, the roller 40 moves relative to the web to a position to maintain the pressure between the web and nap relatively constant.

The back-up roller may also be provided with another degree of freedom so that it may rotate about an axis orthogonal to the non-image bearing surface 16a of the web to prevent it from interfering with the action of a web tracking apparatus, for example a web tracking apparatus of the type disclosed in U.S. Patent 3,974,952. (U.K. Patent Application no. 1,515,535).

The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the scope of the claims. For example, the developer has been described as a two component (toner and carrier) developer, but the invention may be practiced with a single component developer. The electrostatic image has been described as being produced electrophotographically, but other well-known methods of forming electrostatic images may be used.

Claims (5)

1. Electrophotographic development apparatus for developing an electrostatic image carried on a surface of a flexible web using a magnetic brush, comprising a back-up roller positioned to press against the surface of the web opposite the magnetic brush, and a support for the roller which permits two degrees of freedom of movement of the roller relative to the web.

2. Apparatus as claimed in claim 1, wherein the support permits first pivotal movement of the roller around an axis parallel to the roller and spaced therefrom and second pivotal movement of the roller about an axis parallel to the direction of motion of the web.

3. Apparatus as claimed in claim 2, wherein the first pivotal movement in the direction towards the web is under the influence of gravity.

4. Apparatus as claimed in claim 2 or 3, wherein the support further permits movement of the roller about an axis orthogonal to the web.

5. Apparatus substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.