US 3911865 A
Weakly charged toner particles are removed from a developer mix prior to developing an electrostatic latent image by bringing the developer in contact with a biased roller. Also, toner that is charged to the wrong polarity may be removed from the developer mix by a biased roller prior to developing the electrostatic latent image. The removal of the toner particles from the developer prior to presenting the developer to an electrostatic latent image substantially reduces background when developing the latent image.
Claims available in
Description (OCR text may contain errors)
Oct. 14, 1975 United States Patent [191.
Volkers Hudson. 118/637 Berlier et a1. 117/17 5 1/1974 Urbanek et a1.
3,662,71 l 5/1972 3,754,962 8/1973 3,783,826 118/620 3,818,864 6/1974 Bickmore............................ 118/637 1 TONER PICKOFF APPARATUS  lnventor: Stewart William Volkers,
 Assignee: Xerox Corporation, Stamford,
 Filed: Mar. 30, 1973  Appl. No.: 346,485
developer mix prior to developing an electrostatic 1atent image by bringing the developer in contact with a biased roller. Also, toner that is charged to the wrong 12 Claims, 2 Drawing Figures polarity may be removed from the developer mix by a biased roller prior to developing the electrostatic latent image. The removal of the toner particles from the developer prior to presenting the developer to an 1 37 electrostatic latent image substantially reduces background when developing the latent image.
References Cited UNITED STATES PATENTS US. Patent Oct. 14, 1975 a 7 III I .I I I I I I --anavp conapn--anaan TONER PICKOFF APPARATUS DESCRIPTION OF THE INVENTION In developing electrostatic latent images, a developer material. comprising toner particles and carrier particles. is presented to the electrostatic latent image for developing the same. The toner particles are triboelec trically charged through a mixing and tumbling action with the carrier particles present in the developer mix. Normally, however there are some toner particles which carry a low charge and some toner particles which are charged to the wrong polarity. The quantity of improperly charged toner particles increases with the age ofthe developer due to carrier impaction. It has been found that the background development level is a function of the concentration of improperly charged toner particles in the developer with background becoming worse with an increase in this type of toner. The reasons are not fully understood, but some general observations may be made. When positive-to-positive reproduction is desired, and the electrostatic latent image carries a positive charge, then the carrier will have a positive charge and the toner a negative charge. Normally, the electrostatic latent image may have a charge of +800 volts and the background will have a charge of around +200 volts and the toner will carry negative charge.
One explanation of the above phenomenon is that assuming the electrostatic latent image carries a positive charge of +800 volts and the background a charge of +200 volts, the toner particles of low negative charge will be attracted by both the background charge and the image charge while the more negatively charged toner will be preferentially strongly attracted by the high charge on the image. It is also felt that toner particles of low charge and charged to the wrong polarity are mechanically deposited on the background. When a development electrode is utilized, development of background areas is suppressed whereby toner charged to the correct polarity is repelled from the background areas, but the toner charged to the wrong polarity is developed on the background.
It is therefore an object of this invention to eliminate the above disadvantages and thereby suppress background development by withdrawing low or weakly charged toner particles from the developer prior to presenting the developer to an electrostatic latent image.
lt is a further object of this invention to eliminate the above disadvantages and thereby suppress background by withdrawing toner particles, which are charged to the wrong polarity, from the developer prior to presenting the developer to an electrostatic latent image.
A still further object of this invention is to suppress development of selected lower contrast electrostatic images by removing lower charged toner particles from the developer prior to presenting the developer to an electrostatic image. i
Other objects of the invention will become apparent from the following description with reference to the drawings wherein:
FIG. 1 is a view of the development system utilizing the principles of this invention; and
FIG. 2 is a view ofa magnetic brush development system utilizing the principles of this invention.
Referring to FIG. 1, a drum 10 has a photosensitive surface 12 thereon bearing an electrostatic latent image. The drum 10 is rotated in a clockwise direction by a motor 14 outlined in block form. A developer housing 16 is located adjacent the drum and contains a toner reservoir 18 having a supply of toner 20 therein and a developer sump having a supply of developer 22 therein. The developer comprises carrier and toner particles so selected that a triboelectric attraction exists between them causing the two particles to cling together and acquire an opposite charge. Each carrier has numerous toner particles attracted thereon allowing them to be transferred into contact with the photosensitive surface where the greater electrostatic attraction of the latent image will overcome the triboelectric attraction between the two developer components causing toner to be stripped off the carrier and electrostatically bonded to the charged image to effect development thereof. A bucket conveyor 24 is rotatably mounted in the housing 16 and is arranged to convey developer 22 from the developer sump upwards and deposit the same onto a chute 26. A motor 28 shown in block diagram, is positively connected to a pulley 30 to rotate the pulley and thereby the bucket conveyor 24. A toner pickoff rotatable biased roller 32 is located at the bottom of the chute 26 and has a motor 34 (shown in block diagram) positively connected thereto for rotating the same in a clockwise direction. The roller 32 collects toner particles thereon in a manner to be described hereafter. A biased development electrode 33 is provided adjacent the roller 32 to develop toner particles onto the roller 32. A combined scraper and guide member 36 is located in the housing and is tangentially disposed relative to the surface of the roller 32 to engage and scrape off any toner particles collected on the roller 32 and guide the same back into the developer sump without engaging the photosensitive surface.
Assuming an image of a positive charge, when it is desired to withdraw weakly charged negative toner, the roller 32 has a positive voltage V applied thereto and the development electrode 33 has either a negative or less positive voltage V applied thereto. The difference in the voltage potentials V, and V is usually such as to produce an attractive field which is equal to or slightly greater than the attractive field in the background areas of the latent image.
When developer passes over the rotating roller 32, the attractive field between the roller 32 and electrode 33 will be insufficient to strip toner from its corresponding carrier if the triboelectric attraction therebetween is greater than the field due to the potential between the roller 32 and electrode 33. However, if the triboelectric attraction between the toner and its associated carrier is less than the attractive force due to the potential between the roller 32 and electrode 33 then the toner will be stripped from its carried and deposited upon the roller 32. Positive charged particles will not develop on the roller 32 since it is at a higher potential than the electrode 33. Any positive toner which is attracted to the electrode 33 will be physically removed therefrom by passing developer.
Assuming an image of a positive charge, when it is desired to withdraw particles charged to the wrong polarity (positive), the roller 32 has a negative voltage V, applied thereto and the development electrode 33 has either a positive or less negative voltage V applied thereto. The difference in the voltage potentials V and V is usually such as to produce a field which will attract the positive charge particles to the roller 32 as the roller 32 rotates. The negative charged particles will not develop on the roll 32 since the roll 32 is at the lower potential compared to the electrode 33. Any negative toner which is attracted to the electrode 33 will be physically removed by and/or attracted to the passing developer.
AS the roller 32 rotates, the scraper 36 engages the roller 32 to remove any toner particles deposited thereon. The toner particles fall by gravity over the scraper guide 36 back into the developer sump without engaging the photosensitive surface 12 wherein the toner particles will be mixed with the carrier particles to become triboelectrically correctly charged and redistributed by the bucket conveyor over the chute 26 and past the roller 32 and onto the photosensitive surface to develop the latent image thereon.
The connections between the motors and their associated members to be driven thereby have been omitted for clarity and it should be understood that such connections can be any type wellknown in the art. The latent image is imposed on the photosensitive surface 12 and the developed latent image is transferred from the photosensitive surface to a copy paper by wellknown xerographic techniques as for instance described in US. Pat. No. 3,336,905.
Referring to FIG. 2, the concept of this invention may be employed with magnetic brush development systems. All of the elements which are the same as those in the embodiment of FIGS. l-3 are illustrated with the same reference number only with a small a" affixed thereto. In this embodiment, a developer mix 100 comprises ferromagnetic carrier particles and toner particles located in a developer sump. A magnetic roller 102 is rotatably mounted in the sump and has a portion thereof disposed within the developer mix 100 to attract developer thereto to present the same to the latent image on the photosensitive surface 12a. A motor (shown in block form) 104 is utilized to rotate the magnetic roller 102. The toner pickoff roller 32a is located adjacent the magnetic roll 102. The roller 32a has a voltage potential V applied thereto and the magnetic brush 102 has a voltage potential V applied thereto. The potential V may be more positive or more negative than the potential on roller 32a depending upon whether it is desired to develop on the roller 32a the weakly charged particles or the particles charged to the wrong polarity, respectively. This is assuming a positively charged image. As the magnetic roll 102 rotates developer past the pickoff roll 32a, the roll 32a will attract those particles which are charged either in the wrong polarity or the weakly charged particles depending upon the field produced by the potentials V and V The particles attracted by the roller 32a are scraped off the roller by the scraper and guide 36a and are returned back to the developer sump for re-charging and re-use.
While there is illustrated a single magnetic roll, multiple magnetic rolls may be provided. Several magnetic rolls may be provided to transport developer to a developing roll or all of the rolls may be developing rolls. In the former system the pickoff roll 32a may be located adjacent the transport rolls or the first development roll to pick off toner from the developer prior to the developer being presented to the latent image. In the latter system, the pickoff roll 32a may be located adjacent the first development roll to pick off toner from the developer prior to the developer being presented to the latent image.
What is claimed is:
1. In an electrostatic processor having a photosensitive surface for carrying an electrostatic latent image, and a development system including means for transporting developer along a predetermined path leading toward and away from said surface to develop said image; said developer containing properly and improperly charged toner particles, with the properly charged particles being triboelectrically attracted to carrier particles; the improvement comprising the combination of electrically conductive means adjacent said path and upstream of said surface, and means for biasing said conductive means to create an electric field which selectively attracts at least some of the improperly charged toner particles toward said conductive means while repelling the properly charged toner particles therefrom, whereby improperly charged toner particles are removed from said developer prior to developing said image to thereby suppress background development.
2. The processor of claim 1 wherein said means for transporting said developer to said photosensitive surface comprises a chute located above the photosensitive surface and means for cascading said developer onto said chute and across said photosensitive surface, said conductive means being located within the path of the cascading developer.
3. The processor of claim 1 wherein said means for transporting said developer to said electrostatic latent image comprises a magnetic roller, said conductive means including said magnetic roller and another member adjacent said magnetic roller, with said field being established between said member and said magnetic roller.
4. The structure as recited in claim 3 wherein said other member includes a second magnetic roller, and the field between said magnetic rollers is selected to remove toner particles charged to the same polarity as said latent image.
5. The structure as recited in claim 1 wherein said field is selected to remove toner particles charged to the same polarity as said latent image from said developer.
6. The structure as recited in claim 5 wherein said conductive means includes a roller on one side of said path and an electrode adjacent said roller on the opposite side of said path, and said biasing means biases said roller and said electrode to establish said field therebetween.
7. In an electrostatic processor having a photosensitive surface for carrying an electrostatic latent image of a predetermined polarity, a development system for developing said latent image through the use of a developer comprising toner particles triboelectrically attracted to carrier particles; the combination of means for presenting said developer to said surface to develop said electrostatic latent image, and means for subjecting said developer to an electrical field prior to developing said image, said field being selected to remove from said developer relatively weakly charged toner particles having a polarity opposite the image polarity, thereby reducing the quantity of toner particles which tend to be attracted to background areas of said image by residual electrostatic charge onsaid surface.
of said field is a biased roller toward which said weakly charged toner particles are attracted.
11. The structure as recited in claim 10 wherein another terminus of said field is a biased magnetic roll, and said means for presenting developer to said surface includes said magnetic roll.
12. The structure as recited in claim 10 wherein another terminus of said field is a biased electrode. said roller and said electrode being positioned so that said developer passes therebetween.