MAGNETIC BRUSH APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES

Abstract: In most electrographic copiers using magnetic brush development (14), the brush's applicator roll (26) is located in a fixed position relative to the surface of the insulating member (12) so that the brush nap of developer material is always maintained in contact with the surface. However, in certain instances, it is desirable to selectively prevent the brush nap from contacting such surface. Heretofore, only mechanical techniques have been used to selectively control the application of developer by magnetic brushes. Unfortunately, such techniques complicate the magnetic brush construction (14) and exhibit a relatively slow response time in selectively establishing and suppressing a brush nap suitable for charge pattern development. Such a slow response limits the operational speed of teh reproduction apparatus. In order to solve this problem, the magnetic brush (14) of the present device utilizes one or more electromagnets (32) which, when energized and de-energized, respectively establish and suppress a nap of developer between the brush and a developable charge pattern.

Description: MAGNETIC BRUSH APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES This invention relates generally to electrographic development, and more particularly to an improved magnetic brush apparatus having a selectively energizable electromagnet for controlling the brush nap used to effect development of an electrostatic image. In the electrographic process for making reproductions of input information, an electrostatic charge pattern is formed on the surface of an insulating member in image-wise configuration corresponding to the information to be reproduced. The charge pattern is developed by applying developer material to such pattern to form a visible image. The visible developer material image is then either transferred to a receiver member and fixed to such member, or fixed to the insulating member itself. In commercial high speed electrographic equipment, a common mechanism for developing electrostatic charge patterns is a so-called "magnetic brush". A typical magnetic brush includes one or more permanent magnets located within a rotatably driven applicator roll. Developer material comprises, for example, a mixture of finely divided pigmented thermoplastic marking particles (toner) held to the surface of ferromagnetic particles (carrier) by electrostatic charges created by triboelectrification. The carrier particles, with the attached toner particles 4 are held on the applicator roll in a bristle-like formation by the magnetic fields of the. ^ magnets to form a brush nap. Of course, developer material of the type comprised solely of marking particles which exhibit magnetic properties (referred to as single component developer) is also suitable. The developer material is then brought into contact with the electrostatic charge pattern by brushing the nap bristles across the surface of the insulating member. When the developer material contacts the surface of the insulating member, the forces associated with the electrostatic charge pattern strip the toner from the brush nap and apply it to the charge pattern to effect development. In most electrographic copiers using magnetic brush development, the brush's applicator roll is located in a fixed position relative to the surface of the insulating member so that the brush nap of developer material is always maintained in contact with the surface. However, in certain instances, it is desirable to selectively prevent the brush nap from contacting such surface. For example, in certain monochromatic (i.e. black-and- white) applications, it is desirable to quickly disengage the brush nap from the insulating member (after developing a charge pattern) to prevent the development of electrostatic charges outside the charge pattern. Also, in multicolor applications where a plurality of brushes, each bearing toner of a different color, are used to develop related (i.e. color separated) electrostatic charge patterns, it is desirable that the brush nap of each brush contact and develop only that image area which it is intended to develop, otherwise, toner particles of one color may deposit in image areas meant for development by the other colors. Heretofore, only mechanical techniques have been used to selectively control the application of developer by magnetic brushes. Such techniques include, for example, the use of movable skieves for controlling nap height, and the relocation of permanent magnets within the O FI . IFO applicator roll to control nap formation. See, for example, the disclosure of U.S. 3,570,453. Unfortunately, such development control techniques complicate the magnetic brush construction and are 5 subject to failure due to the contaminating nature of the toner particles. Furthermore, they exhibit a relatively slow response time in selectively establishing and suppressing a brush nap suitable for charge pattern development. Such a slow -_Q response limits the operational speed of the reproduction apparatus. The object of this invention is to provide a magnetic brush development apparatus in which the brush nap can be established and suppressed at a 5 substantially faster rate than the mechanical apparatus of the prior art. The magnetic brush of the invention is characterized by one or more electromagnets which, when energized and de-energized, respectively establish and suppress a 0 n a P of developer between the brush and a developable charge pattern. The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment 5 presented below. In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which: 0 Fig. 1 is a schematic view of an assembly for developing a plurality of related electrostatic charge patterns on the surface of an insulating member with the improved magnetic brush developer apparatus according to this invention; 5 Fig. 2 is a side elevational view, partly in section, of the improved magnetic brush developer OMPI apparatus according to this invention, with the electromagnets energized; and Fig. 3 is a side elevational view, partly in section, similar to Fig. 2, with the ^ electromagnets deenergized. Referring now to the accompanying drawings, a developer assembly 10 having a plurality of magnetic brushes 14 is schematically shown in Fig. 1. While the assembly 10 is illustratively 10 described as having a plurality of magnetic brushes for use in a multicolor electrographic process to respectively develop a plurality of related electrostatic charge patterns on the surface of an insulating member 12, the invention is of course 5 applicable to a monochrome process in which only a single magnetic brush developer apparatus is employed. The insulating member 12 is, for example, a moving composite web including an insulative layer, 0 a grounded conductive layer, and a photoconductive layer. As the member 12 is moved, the surface of its insulative layer is uniformly electrostatically charged. The charge is then selectively reduced by imagewise exposure of the photoconductive layer to • * color separation images of input information to be reproduced, such as a multicolor document. Such exposure forms spaced related electrostatic charge patterns (e.g. ]_, I G , L) on the surface of the member 12 corresponding to the color separation 0 images respectively. Additionally, an electrostatic charge pattern (e.g. I BK ) of any black portion of the input information may also be formed on the surface. In order to reproduce the input information utilizing the subtreetive color 5 reproduction process, the charge patterns are developed with pigmented marking particles (toner) which are complementary to the primary colors (and with black marking particles if a charge pattern corresponding to black information is formed) by the assembly 10. As mentioned above, developer assembly 10 includes a plurality of magnetic brushes 14 located adjacent to the member 12. Each brush has a housing 16 which serves as a reservoir for the pig ented toner particles and ferromagnetic particles (carrier). In the illustrative example of Fig. 1, the left-most housing contains cyan toner particles, the left-center housing contains magenta toner particles, the right-center housing contains black toner particles, and the right-most housing contains yellow toner particles. Each housing 16 contains a pair of brush structures 18. Since the brush structures are of identical construction, only one such structure will be described. Referring to Figs. 2 and 3, stationary cylindrical cores 20, a non-magnetic material such as aluminum, are mounted in the housing 16. Stationary magnetic pole pieces 22 are adhesively bonded to the cores 20. A plurality of permanent magnets 24, of rubber-bonded barium ferrite strips for example, are fixed on the periphery of the pole pieces 22. Applicator rolls 26, rotatably mounted in the housing 16 concentrically with respective cores 20, surround the magnet arrangement in the field of the permanent magnets. The applicator rolls 26 are hollow cylinders or tubes of non-magnetic material, such as aluminum, with a roughened peripheral surface. Augers 28 and 30 circulate the carrier particles and toner particles so that the toner particles develop a triboelectric charge which causes such particles to adhere to the carrier particles. As the applicator rolls 26 rotate, the carrier particles (with adhering toner particles) are held on the roughened peripheral surfaces of the applicator rolls by the magnetic fields of the permanent magnets 24 and are moved with such members into juxtaposition with the member 12. The carrier and toner particles establish a brush nap in the form of bristles extending radially from the 10 applicator rolls in the magnetic fields. The strength of the magnetic fields and the relative spacing between the member 12 and the apparatus 14 insure that the brush nap bristles do not contact the surface of such member. " Electromagnets 32 are mounted in the respective cores 20 of the brush structures 18, within the applicator rolls 26, in the area where the applicator rolls are in close proximity to the member 12 (the development zone). The 20 electromagnets are electrically coupled to a power source PS through respective switches S. When the electromagnets are energized, the field strengths of the electromagnets 32 enhance the formation of the brush nap bristles. That is, the electromagnets 25 cause the brush nap bristles to immediately stand up on the surface of the rotating applicator rolls in the development zone to a height where the bristles contact the surface of the member 12 (see Fig. 2). During such contact, an electrostatic charge pattern 30 on the moving member 12 has an attraction for the toner particles of a magnitude which overcomes the attraction of the carrier particles for the toner particles. Accordingly, toner particles are attracted from the carrier particles to the JJ electrostatic charge pattern to develop the pattern and form a visible image. When the electromagnets 32 are de-energized, the brush nap bristles in the development zone immediately collapse out of contact with the member 12 (see Fig. 3). Thus, even if the member 12, passing over the rotating applicator members 26, bears an electrostatic charge pattern, such pattern will not be developed. Of course, other locations for the electromagnets can accomplish the desired result of this invention. For example, the electromagnet could be located adjacent to the applicator roll on the opposite side of the member 12. Such electromagnet would have a field strength on energization to alternatively establish a sufficient brush nap on the applicator member to contact the member 12 in the development zone, or to suppress the brush nap and thereby prevent contact with the member 12. Referring again to Fig. 1, a sensor 34 detects electrostatic charge patterns on the insulating member 12. Such detection may be indirectly accomplished, for example, by sensing timing marks associated with respective charge patterns. The sensor 34, which may be of the type disclosed in U.S. Patent No. 4,025,186 issued May 24, 1977 in the name of Hunt, Jr. et al for example, produces signals indicative of the position of the respective patterns. Such signals are transmitted to a control and timing unit 36 operatively coupled to the switches for the electromagnets 32 of the magnetic brush developer apparatus 14. The unit 36 includes a microprocessor, for example, which selectively actuates the switches in timed relation to the passage of a pattern to be developed by a particular developer apparatus. For example, when the electrostatic charge pattern _ (corresponding to the red color separation image of the input information) on the moving member 12 travels through the developer assembly 10, it is detected by the sensor 34 which sends an appropriate signal to the unit 36. After a time interval which places such pattern adjacent to the magnetic brush developer apparatus having cyan toner particles (left-most apparatus in Fig. 1), the unit 36 energizes the electromagnets of such apparatus to immediately stand the brush nap bristles up on its applicator member into contact with the web to develop such pattern. Of course, the remaining electromagnets remain de-energized as such pattern passes thereover so that no development occurs. In a similar manner, charge pattern ϊ„ corresponding to the green color separation image is detected and developed only with magenta toner particles, charge pattern I corresponding to the black image is detected and developed only with black toner particles, and charge pattern Ig corresponding to the blue color separation image is detected and developed only with yellow toner particles. Thus, the related patterns on the web 12 are developed only with respective pigmented toner particles to form spaced visible toner particle images complementary to the color separation images of the input information to be reproduced. Subsequently the developed images may be transferred in accurate register and fixed to a receiver sheet, to make the desired multicolor reproduction. With the improved magnetic brush developer apparatus of this invention, the selective energization of the electromagnets of such apparatus eliminates the need for mechanical magnetic brush control arrangements found in the prior art. The control of the brush nap bristles provided by the electromagnets prevents contamination or degradation of developed images by toner particles of adjacent brush naps. Further, the brush nap bristles are substantially immediately available for development so that there is no need to provide a time delay period in the development process to allow an effective brush nap to be established.

Claims: Claims : 1. A magnetic brush apparatus for developing an electrostatic charge pattern on an insulating member (12) by contacting the member with particulate magnetic developer material, said apparatus including an applicator roll (26) and magnetic means (24) which cooperate in transporting such developer material in juxtaposition with such member, characterized in that said magnetic means comprises a selectively energizable electromagnet (32) for establishing a developer nap on said applicator roll which contacts the member to apply developer material to such charge pattern, and means (36, 5) for selectively energizing said electromagnet. 2 * The apparatus of Claim 1 wherein said electromagnet is positioned within said applicator roll and such insulating member. 3. The apparatus of Claim 2 wherein said energizing means (36, 5) includes means (34) for detecting an electrostatic charge pattern on the insulating member when such pattern is in juxtaposition with said applicator roll.

Inventors: JOSEPH BRIAN JOHN, WILLIAMS JAMES ORA, JOSEPH, BRIAN, JOHN, WILLIAMS, JAMES, ORA
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1983
Main classification: G03G15
IPC codes: G03G15