Cleaning Apparatus for an Electrostatic Recording Machine

The present invention relates to a cleaning apparatus for an electrostatic recording machine--such, for example, as an image transfer type electrophotographic copier--and particularly to an improvement in an apparatus used for both developing and cleaning.

In an image transfer type electrophotographic copying machine, an original to be copied is illuminated by light, the light reflected from the original is projected onto a photosensitive substance, and an electrostatic latent image is thereby formed on the photosensitive substance. This electrostatic latent image is then developed by a developing agent containing toner to produce a visible or toner image, the visible image being thereafter transferred onto a transfer paper and then fixed, whereby a copy image of the original is formed on the transfer paper. In an electrophotographic process of this type, toner remains on the surface of the photosensitive substance even after the image transfer process has been completed. Since this toner residue will cause stains to appear on an image produced in the next succeeding electrophotographic process, it must be removed from the surface of the photosensitive substance prior to further copying. An apparatus furnished for this purpose is commonly referred to as a cleaning apparatus, of which a variety of systems--including those using blades, feathers and magnetic force--are already known.

In a copying apparatus utilizing developer powder of a two-component system containing toner and carrier, the prior art teaches that development and cleaning may be carried out by the same device to simplify the copier structure. Such an arrangement is certainly advantageous in providing simplified structure and enabling efficient use of the developing toner without needless waste.

However, it has been found that an increased content of toner relative to carrier content results in decreased cleaning effectiveness, and further that the self-cleaning capability of the toner is successively reduced due to fatigue of the developer toner.

With a view to eliminating the foregoing disadvantage, the so-called two-revolution-one-copy type copying machine has been developed. This type of machine uses a single component developer comprising magnetic toner (and coloring agent if required) contained in resin, and also uses a revolving drum which produces a single copy for each two full revolutions thereof.

FIG. 1 illustrates a schematic construction of this type of copying machine, wherein a movable original table 2 is provided on the upper surface of a copying machine body 1. An PG,4 original 3 placed on original table 2 is illuminated by an exposure lamp 4, and the light reflected from original 3 is directed by a mirror 5 and a lens 6 onto a photosensitive layer 7a laminated about the surface of a revolving drum 7. Successively disposed about photosensitive layer 7a are a charging electrode 8 for uniformly charging photosensitive layer 7a, an electric neutralizing lamp 9 for neutralizing residual electric charge remaining on photosensitive layer 7a, a developing and cleaning apparatus 10 for both developing the electrostatic latent image formed on photosensitive layer 7a and removing residual toner from the photosensitive layer, an image transfer electrode 11 for causing the toner image formed as a result of development to be transferred onto a transfer paper P, and a separation apparatus 12 for separating transfer paper P from photosensitive layer 7a. Transfer paper P is stored in a paper feeding tray 13 which is provided in machine body 1, is fed by a feed roller 15 synchronized with the copying operation, and is further conveyed to the image transfer position by a paper feeding roller 14. Separating apparatus 12 separates the transfer paper from photosensitive layer 7a after completion of image transfer, the paper P thereafter being conveyed to a fixing apparatus 16 to be heat fixed and then delivered to a tray 17 on the outside of the machine.

In this type of machine, drum 7 begins its rotation at the commencement of the copying operation. During the first rotation of the drum its photosensitive layer 7a is electrically charged and receives an electrostatic latent image by light exposure of the original, the latent image is developed to a toner image which is then transferred onto transfer paper, and the paper is separated from the photosensitive layer. During the next or second rotation of the drum, the residual electric charge is removed from photosensitive layer 7a, and toner remaining on the drum surface is cleaned therefrom. Thus, one copying operation is completed by two full rotations of the drum.

If a magnetic brush type combination developing and cleaning apparatus is utilized in such an image transfer type electrophotographic copying machine, there may arise a danger of incomplete cleaning operation. Still referring to FIG. 1, the magnetic brush type combination developing and cleaning apparatus 10 comprises a rotatable sleeve 10b which is partially immersed in developer D contained in a developer container 10a, and a group of fixed magnets 10c are arranged within sleeve 10b. Sleeve 10b is disposed adjacent photosensitive layer 7a. In such a magnetic brush type developing and cleaning apparatus, development occurs when tufts or ears of the developer, which are formed on the surface of sleeve 10b by sleeve rotation, lightly contact an electrostatic latent image formed on photosensitive layer 7a and are electrostatically attracted thereto. Thus, a gap on the order of 1-5 mm (1.0-1.5 mm in the case of single component developer; 1-5 mm in the case of two component developer) should be provided between photosensitive layer 7a and sleeve 10b to enable formation of the ears or tufts of developer. During cleaning, on the other hand, residual toner on photosensitive layer 7a is attracted by the magnetic attraction force of magnets 10c within sleeve 10b, the residual toner being thereby separated and removed from the surface of layer 7a. The cleaning function may accordingly be weakened due to the presence of the gap. Moreover, the thickness of sleeve 10b may adversely affect the magnetic cleaning function. As a consequence, where copying machines utilize a single magnetic brush type developing and cleaning apparatus, there is a possibility that the cleaning effect might not be satisfactorily obtainable.

An electrophotographic copying apparatus of the dry toner image transfer type is proposed in U.S. Pat. No. 2,918,808 wherein a blade means is disposed anterior of a magnetic brush such that its forward end portion is brought into pressing engagement with the surface of the photoreceptor at least during its second revolution to aid cleaning by the magnetic brush. In this case, however, the material and contact pressure force of such a blade means must be carefully selected so as not to affect the electric charge impressed on the photoreceptor during its first revolution; alternatively, the forward end portion of the blade means should be moved out of engagement with the surface of the photoreceptor during the first revolution and into pressing engagement therewith during the second revolution.

The present invention is intended to remedy the foregoing problems, and provides an improved magnetic brush type developing and cleaning apparatus incorporating a cleaning means arranged just upstream of the magnetic brush developing and cleaning position or zone at which the rotating sleeve defines a predetermined gap with respect to the photosensitive member encircling the drum.

FIG. 1 is a schematic, constructural depiction of a two-revolution-one-copy type image transfer electrophotographic copying machine to which the present invention is applicable; and

FIG. 2 is a diagrammatic view of the principal part of a developing and cleaning apparatus constructed in accordance with the teachings of the invention.

FIG. 2 is a diagrammatic illustration of a developing and cleaning apparatus in accordance with the invention, and wherein the reference numerals which are the same as in FIG. 1 indicate the same or corresponding constructural parts and elements.

A magnetic bar 20 extending parallel to the widthwise direction of revolving drum 7 (orthogonal to the FIG. 2 drawing) is disposed in the housing of apparatus 10 above or upstream of the developing and cleaning position A at which the predetermined gap between sleeve 10b and photosensitive layer 7a is defined. Magnetic bar 20 is so arranged as to extend over and close to photosensitive layer 7a. The gap between magnetic bar 20 and the photosensitive layer 7a is preferably as narrow as possible, taking into account possible peripheral irregularities and eccentricity of revolving drum 7, and may, as a practical matter, be set on the order of approximately 0.5-1 mm. The trough shaped container 10d, which is fitted within developer container 10a, is a toner supplier.

As noted, magnetic bar 20 is so set that it extends over and close to photosensitive layer 7a. Toner remaining on photosensitive layer 7a is thus initially magnetically absorbed and removed by magnetic piece 20 when drum 7 begins its second revolution in a copying operation, and any subsequently remaining toner is further swept and removed from layer 7a both by the magnetic absorption force of magnets 10c in revolving sleeve 10b of the conventional developing and cleaning apparatus, and also by the ears of developer which are formed on sleeve 10b. Magnetic piece 20 is further arranged so that the toner removed thereby may fall into the developing and cleaning apparatus when magnetic piece 20 can no longer retain it. In addition, it would be convenient to automatically return a portion of the toner removed by magnetic piece 20 into container 10a or toner supplier 10d.

As described, the present invention enables a two-revolution-one-copy type electrophotographic copying machine using a magnetic brush type developing and cleaning apparatus to effectively provide a reliable and adequate cleaning operation and effect. This is accomplished by placing a magnetic member or piece on the upper reaches of the developing and cleaning apparatus, upstream of the developing and conventional cleaning position. It should be recognized and understood that the present invention can be applied not only to an electrophotographic copying machine utilizing a revolving drum such as shown in the preceding example, but also to an electrophotographic copier incorporating an endless belt.

Although the foregoing description referred specifically to a magnetic brush type developing and cleaning apparatus, the present invention can likewise be applied to a magnetic brush type cleaning apparatus without limitation to the aforementioned type of apparatus.