AN ELECTROPHOTOGRAPHIC APPARATUS

Abstract: An electrophotographic apparatus is provided with an electrically conductive voltage control bar (4) between the photoconductor drum or belt (1) and the developer or developer/cleaner roller (2). The voltage control bar prevents toner filming on roller (2), and makes it possible for that roller to be made of an insulating material such as molded plastics.

Description: This invention relates to an electrophotographic apparatus. US -A- 3,884,572 discloses an electrophotographic apparatus having a roller which mechanically rubs toner from the surface of the photoconductor. The roller is made of material which may be somewhat insulating, but the roller is used exclusively as a cleaner. Its purpose is to replace fibre brush or fur brush cleaners commonly used at the time of that invention. US -A- 4,478,510 discloses a system for placing toner on a substrate, pel by pel, by shooting toner from a developer roller through an opening in a strip. This is done with an electric field between the developer roller and electrodes in the strip to draw the toner off the roller. GB -A- 2,065,557 discloses a monocomponent cleaning system with a rotating magnet assembly rather than the conventional stationary one. The system is similar to two component magnetic brush cleaning systems but for monocomponent toner. JP -A- 60,130,778 discloses a developing device having a wire between the photoconductor and the magnet roller. This wire acts as a developer mechanical stirring mechanism. JP -A- 57,198,470 discloses a developing device having a charged grid between the photoconductor and the magnet roller. This charged grid is used to reduce image degradation through dirt and provide finer resolution. The voltage on the magnet roller is used to determine the fundamental relation of electric potential between the roller and the grid. Electrophotographic systems have employed magnetic brush development systems in various forms over the years. It is currently almost the only means by which toner is applied to the photoconductor. It basically consists of a metal roller surrounding a set of magnets. The rotation of the roller and magnetic field together transports the developer mix against the photoconductor, allowing the toner to transfer where the voltage between the roller and the photoconductor is appropriate. The present invention seeks to provide an improved electrophotographic apparatus. An electrophotographic apparatus comprising a photoconductor drum or belt, a developer or developer/cleaner roller spaced from said drum or belt and a developer mix of toner and carrier in the form of chains provided in the space between said drum or belt and said roller, said space being chosen so that said chains do not simultaneously contact both said drum or belt and said roller, is characterised, according to the invention, by a development electrode in the form of a conductive bar connected to a voltage supply with a tapered leading edge and positioned between said drum or belt and said roller so as to intercept, in operation, said chains of developer mix. We separate the two functions of the developer roller - transport of the mix and transfer of the toner - by placing a stationary conductive bar (development electrode) between the roller and the photoconductor. Application of the present invention leads to several advantages over the conventional system. The bar is used as the developer or development/cleaning electrode (two-cycle process). The bar is an electric conductor placed between the magnetic cleaning roller and the photoconductor. It should be sufficiently wide so that the carrier chains do not simultaneously touch the roller and the photoconductor. Since the bar acts as the development electrode in place of the roller, the bar-photoconductor separation should be comparable to the original roller-photoconductor separation in the absence of the bar. The development electrode has a potential applied to it with respect to the photoconductor. For the simplest situation, the magnetic cleaner roller and the development electrode in the form of a bar are electrically connected. The use of the bar frees the roller from the burden of maintaining the proper potential. In this fashion, two needs for the roller are circumvented; (1) development electrode voltage control and (2) precise fabrication. It is no longer necessary for the roller to be fabricated to precise dimensions because the previous requirement for uniform development has now been transferred to the bar. Roller runout affects the mix volume only very slightly, because the bar has a slight averaging effect on the amount that gets to the nip. For these reasons it is now possible to make the magnetic cleaning roller of an insulating material. In particular, such an insulating material can be molded plastics. The ease and cost of producing the roller are therefore markedly reduced compared with corresponding machined metal parts. The use of an insulating material for the magnetic cleaning roller can lead to charge build-up on the roller. Excessive charge accumulation is eliminated by brushing a metal strip against the roller. This strip is to control the charge rather than toner filming. It is thus seen that application of the present invention makes possible the use of an inexpensive insulating material instead of an expensive metal conductor - and it also permits the use of a roller not necessarily machined to close tolerances. The bar is also advantageous in a two-cycle electrophotographic process. Conventionally the developer/cleaner roller has a propensity to film during the cleaning cycle because the process of removing toner from the photoconductor develops toner on the roller. This toner can then smear on the roller, which is very undesirable. Such toner film development on the magnetic roller is prevented by the use of the bar. During the entire electrophotographic cycle, in the situation where the bar and the roller are electrically connected or in the situation where the roller is made of electrically insulating material and has a conductive strip to eliminate charge accumulation, no electric field exists between the bar and the roller, and no toner develops on the roller during the cleaning cycle. In the situation where the bar and the roller are not electrically connected and the roller is not made of electrically insulating material, the voltage between the roller and the bar can when necessary, be adjusted to prevent toner accumulation. Because the large electric field is confined to the region between the bar and the photoconductor, any toner that would have developed on the roller without the bar during the cleaning deposits instead on the bar. How the invention can be carried out will now be described by way of example, with reference to the accompanying diagrammatic drawing which represents a side view, not to scale, of an electrophotographic apparatus embodying the invention. As may be seen by inspection of the drawing, a photoconductor drum or belt 1 is separated from a magnetic cleaning roller 2. Toner carrier mix 3 in the form of chains leaves the photoconductor roller and moves towards the magnetic cleaning roller 2. An electrically conductive development electrode in the form of a bar 4 is inserted into the space between the photoconductor drum or belt 1 and the cleaning roller 2. Towards the bottom of the cleaning roller a metal discharge strip 5 brushes against it if the roller needs to be discharged. In order to illustrate the advantages of applying the invention, a development electrode in the form of a stainless steel bar 8 mm wide and 0.5 mm thick, spanning the length of the developer roller, was used on a developer-cleaner in a prototype with the photoconductor drum. In the absence of the bar, the magnetic roller was found to be covered with toner after a minute of operation with development/cleaning voltage alternately applied. With the bar, absolutely no toner was seen on the roller after ten minutes of such operation. The bar itself was also clean, probably from the much larger relative motion of the mix in the region of high electric field. Toner was developed on the photoconductor during the development cycle, showing that sufficiently large fields existed between the bar and the photoconductor. The mix was found to flow in the bar photoconductor region as if the bar were absent. The leading edge of the bar, however, had to be sufficiently sharpened to separate the mix into the two regions.

Claims: An electrophotographic apparatus comprising a photoconductor drum or belt (1); a developer or developer/cleaner roller (2) spaced from said drum or belt (1) and; a developer mix of toner and carrier (3) in the form of chains provided in the space between said drum or belt (1) and said roller (2), said space being chosen so that said chains do not simultaneously contact both said drum or belt (1) and said roller (2), said apparatus characterised by further comprising a development electrode in the form of a conductive bar (4) connected to a voltage supply with a tapered leading edge and positioned between said drum or belt (1) and said roller (2) so as to intercept, in operation, said chains of developer mix. An apparatus as claimed in claim 1, wherein said electrode (4) is electrically connected to said roller (2). An apparatus as claimed in claim 1, wherein said roller (2) is made of an electrically insulating material. An apparatus as claimed in claim 3, wherein said roller (2) is of moulded plastics. An apparatus as claimed in claim 3 or claim 4, wherein a conductive strip (5) is arranged to brush against said roller (2) to eliminate charge accumulation.

Inventors: BEARDSLEY, GARY SCOTT, LEE, MICHAEL HEI-WO
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: G03G15
IPC codes: G03G15, G03G21