Developing device, image forming apparatus and process cartridge including replenishment openings

A developing device for developing a latent image formed on an image carrier of the present invention is generally made up of a developing section and a developer storing section. The developing section includes a developer carrier for conveying a one-component type developer deposited thereon, a feeding member for feeding the developer to the developer carrier, and a conveying member for conveying the developer toward the feeding member while agitating it. The developer storing section replenishes a developer stored therein to the developing section in a direction including at least a horizontal direction component. The developing section and developer storing section are formed with a plurality of openings in the lengthwise direction such that the replenishment of developer from the developer storing section to the developing section and the return of the developer from the latter to the former are executed via the openings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a developing device for use in a copier, facsimile apparatus, printer or similar image forming apparatus, an image forming apparatus using the same and a process cartridge.

2. Description of the Background Art

A developing device of the type using only toner grains, i.e., a one-component type of toner not containing carrier grains is extensively used. This type of developing device is configured to charge the toner grains in a casing by agitating them and then convey the charged toner grains to a toner carrier. The toner grains thus deposited on the toner carrier are regulated in amount by a metering member and then brought to a developing zone where the toner carrier and an image carrier face each other at the shortest distance. At the developing zone, the toner grains are transferred from the toner carrier to the image carrier to thereby develop a latent image formed on the image carrier.

While the toner grains deposited on the toner carrier should ideally be entirely charged to preselected polarity, some of such toner grains are, in practice, not fully charged. Should the toner grains with short charge be conveyed via the metering member, they would fly away from the toner carrier and would thereby smear the inside of the image forming apparatus as well as paper sheets or similar recording medium.

Further, toner and members that strongly rub against the toner, e.g., the toner carrier, a toner feeding member and the metering member vary with the elapse of time, and so does the toner electrifying ability. As a result, it soon becomes difficult for such members to electrify each other. In addition, it is likely that additives present on the surfaces of the individual toner grains are separated from or buried in the surfaces of the toner grains and fail to exhibit a fluidity enhancing function and an electrification control function expected thereof. Consequently, the ratio of toner grains with short charge to the entire toner grains is apt to increase, and toner grains of opposite polarity and short charge are also apt to increase. If such toner grains are conveyed to the developing zone by the toner carrier, then they smear the background of an image and thereby lower the quality of the image.

In light of the above, various devices have heretofore been proposed to control stress to act on toner grains or to discharge a developer from a developing device before the developer is critically deteriorated. Japanese Patent Laid-Open Publication No. 2002-333764, for example, proposes to obviate, e.g., the blow-off or the blocking of toner grains ascribable to the excessive replenishment from a toner replenishing or storing section to a developing section by returning excessive part of the toner grains to the toner replenishing section. At this instant, part of toner grains with short charge is also returned to the toner replenishing section because of the replacement of toner grains, so that image quality is maintained at a certain acceptable level.

However, the system proposed by the above document has some problems left unsolved. For example, toner grains replenished from the toner replenishing section are soon returned to the replenishing section or not smoothly returned to the replenishing section with the result that excess toner grains increase dynamic torque to an unusual level.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a developing device capable of smoothly discharging toner grains from a developing section, an image forming apparatus using the same and a process cartridge.

A developing device for developing a latent image formed on an image carrier of the present invention is generally made up of a developing section and a developer storing section. The developing section includes a developer carrier for conveying a one-component type developer deposited thereon, a feeding member for feeding the developer to the developer carrier, and a developer conveying member for conveying the developer toward the feeding member while agitating it. The developer storing section replenishes a developer stored therein to the developing section in a direction including at least a horizontal direction component. The developing section and developer storing section are formed with a plurality of openings in the lengthwise direction such that the replenishment of developer from the developer storing section to the developing section and the return of the developer from the latter to the former are executed via the openings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1of the drawings, an image forming apparatus embodying the present invention is shown in a front view and implemented as a printer by way of example. As shown, the printer includes a photoconductive belt or image carrier1passed over a plurality of support rollers and caused to turn clockwise, as indicated by an arrow A inFIG. 1. Arranged around the photoconductive belt1are a charger3, an optical writing unit4, and four developing units or devices5Bk (black),5C (cyan),5M (magenta) and5Y (yellow). An intermediate image transfer belt or intermediate image transfer body6and a belt cleaner7are also located in the vicinity of the photoconductive belt1. An OPC (Organic PhotoConductor) layer, not shown, is formed on the surface of the intermediate image transfer belt. Let the photoconductive belt1and intermediate image transfer belt6be simply referred to as belts1and6, respectively, hereinafter.

In operation, the charger3is applied with a high-tension voltage for uniformly charging the surface of the belt1. An image signal processor, not shown, converts color image data, e.g., a color image signal received from a computer to an optical writing signal and sends the optical writing signals to the optical writing unit4. The optical writing unit4controls a laser or light source in accordance with the optical writing signal, thereby scanning the belt1with a laser beam. As a result, a latent image representative of a Bk (black), a C (cyan), an M (magenta) or a Y (yellow) image is formed on the belt1. The developing units5Bk through5Y, each storing fresh toner or developer of a particular color and charged to opposite polarity to the latent image, each develop one of the above latent images Bk, C, M and Y corresponding in color thereto to thereby form a toner image of a particular color.

At a position where the belts1and6contact each other, the toner image is transferred from the belt1to the belt6by a charge opposite in polarity to the toner image applied to the belt6. Such steps of forming a toner image and transferring it from the belt1to the belt6are repeated four consecutive times for completing a composite four- or full-color image on the belt6. The color image thus formed on the belt6is transferred to a paper sheet or similar recording medium selectively fed from a sheet cassette40or a manual feed tray41via a roller pair42by a transfer roller43. The paper sheet, carrying the color image thereon, is conveyed to a fixing unit represented by a fixing roller pair44and has the color image fixed thereby.

Reference will be made toFIG. 2for describing the developing units5Bk through5Y unique to the illustrative embodiment. Because the developing units6Bk through5Y are identical in configuration with each other except for the color of toner or developer to use, let the following description concentrate on the developing unit5Bk by way of example. The developing unit5Bk of the illustrative embodiment uses a one-component type developer, i.e., toner.

As shown inFIG. 2, the developing unit5Bk is generally made up of a developing section11and a toner cartridge or developer storing section12. The developing section11is provided with durability high enough to withstand repeated use while the toner cartridge12is bodily replaced when run out of toner. The developing section11includes a casing13formed with an opening facing the belt1, a developing roller or toner carrier14, and a feed roller or feeding member15for feeding toner to the developing roller14. The developing section11further includes an agitator or conveying member17for conveying toner present in the casing13toward the feed roller15and a doctor roller or metering member16.

The developing roller14is partly exposed to the outside via the opening mentioned above and is rotated counterclockwise, as viewed inFIG. 2, at a preselected linear velocity. In this condition, the developing roller14conveys toner deposited thereon to a developing position where the developing roller14and belt1contact each other, causing the toner to develop a latent image formed on the belt1. In the illustrative embodiment, the developing roller14is formed of metal. The feed roller15is pressed against the developing roller14by preselected pressure at a position above the axis of the developing roller14and is also rotated counterclockwise, as viewed inFIG. 2. The surface of the feed roller15is formed of foam polyurethane. The agitator17is rotated counterclockwise, as viewed inFIG. 2, for feeding toner toward part of the feed roller15between the feed roller15and the doctor roller16. The agitator17may be formed of polypropylene or similar soft, elastic material so as to surely convey the toner in close contact with the inner wall of the casing13.

The doctor roller16is positioned below the feed roller15and rotated clockwise, as viewed inFIG. 2, opposite in direction to the developing roller14at the position where the doctor roller16and developing roller14face each other. In this condition, the doctor roller16regulates a toner layer formed on the developing roller14to preselected thickness while electrifying the toner in contact therewith. The toner, charged on the surface of the developing roller14, develops a latent image formed on the belt1. In the illustrative embodiment, the surface of the doctor roller16is formed of urethane rubber coated with resin. A cleaning blade, not shown, is held in contact with the doctor roller16in order to clean part of the doctor roller14moved away from the position where the doctor roller16faces the developing roller14.

In the illustrative embodiment, a spring or biasing means, not shown, constantly presses the doctor roller16against the developing roller14in order to absorb changes in the outside diameter of the developing roller14and that of the doctor roller16ascribable varying ambient conditions including humidity. Inlet seals, not shown, are positioned on the inner periphery of the casing13in such a manner as to contact the developing roller14at their edges, preventing the toner from leaking via gaps between the developing roller14and the casing13.

The toner cartridge or developer storing section12includes agitators27aand27brotatable counterclockwise, as viewed inFIG. 2, to replenish fresh toner or developer to the developing section11. The agitators27aand27b, like the agitator17, each may be formed of polypropylene or similar soft, elastic material so as to surely convey the fresh toner while being rotated in elastic contact with the inner periphery of the toner cartridge12.

The fresh toner is replenished from the toner cartridge12to the developing section11via a toner or developer replenishing opening20, which is formed in both of the toner cartridge12and casing13. The toner replenishing openings20of the toner cartridge12and casing13align with each other when the toner cartridge12is mounted to the developing unit5Bk. In the illustrative embodiment, the mean amount of toner to be replenished from the toner cartridge12to the developing section11is selected to be greater than the mean amount of toner to be transferred from the developing roller14to the belt1during continuous operation. This configuration, however, is apt to cause an excessive amount of toner to be replenished to the developing section11and bring about toner blow-off or toner blocking. Also shown inFIG. 2are a check valve23for selectively closing the toner inlet20of the developing section11and a seal24for allowing the developing section11and toner cartridge12to closely contact each other.

As shown inFIG. 3, a toner or developer discharging opening is formed in order to discharge the toner from the developing section11for thereby obviating the blow-off or similar undesirable occurrence. In the illustrative embodiment, the toner discharging opening is implemented as toner returning openings21(only one is visible) for returning the toner from the developing section11to the toner cartridge12.

We conducted a series of experiments and found that the toner could not be smoothly returned from the developing section11to the toner cartridge12or that the toner was immediately returned from the former to the latter after replenishment, depending on the position of the toner returning openings21. When the toner was not smoothly returned, torque necessary for driving the developing section11increased to an unusual level. Also, when the toner was immediately returned after replenishment, deteriorated toner could not be replaced with the fresh toner. The position of the toner replenishing openings20and that of the toner returning openings21are the key to the solution of the above problems.

FIG. 4shows a positional relation between the toner replenishing openings20and the toner returning openings21shown inFIGS. 2 and 3, respectively. As shown inFIGS. 1 through 4, the toner replenishing opening20is located at a position where the force of the agitator27, which is disposed in the toner cartridge12for moving the toner toward the developing section11, strongly acts. In the illustrative embodiment, the toner replenishing opening20is positioned at substantially the center in the axial direction of the developing roller14(i.e., the lengthwise direction) and extends over about one-third of the entire length of the roller14. On the other hand, the toner returning openings21are located at a position where the force of the agitator27mentioned above acts little and at a higher level than the toner replenishing opening20. More specifically, the toner returning openings21are shifted from each other in the lengthwise direction so as not to be directly vertically above the toner replenishing opening20. While the toner returning openings21should ideally not overlap the toner replenishing opening20in the lengthwise direction, as shown inFIG. 4, the former may slightly overlap the latter so long as the expected function is achievable.

In the developing unit5described above, the force of the agitator27disposed in the toner cartridge12and rotated to move the toner is not directly imparted to the toner return openings21. Therefore, part of the toner positioned above the toner returning openings21is surely returned to the toner cartridge12by the rotation of the agitator17disposed in the developing section11. This is successful to protect the developing section11from the blow-off or the blocking of toner ascribable to the excessive replenishment of toner to the developing section11.

Assume that the height of the toner returning openings21above the bottom of the developing unit5is hB, that the height of the toner replenishing opening20above the bottom of the developing unit5is hS, and that a height corresponding to a toner-end level Sin the developing section11is hO. Then, there should preferably be satisfied a relation:
hB>hO>hS

Further, assume the shortest distance between the circumference of the locus of rotation of the agitator27and the toner replenishing openings20, the shortest distance between the circumference of locus of rotation of the agitator17disposed in the developing section11and the toner replenishing openings20, the shortest distance between the circumference of the former and the toner returning openings21, and the shortest distance between the circumference of the latter and the toner returning openings21. Then, the circumference of the agitator27should preferably be closer to the toner replenishing opening20than the circumference of the agitator17while the latter should preferably be closer to the toner returning openings21than the former.

Moreover, it is preferable to form projections at opposite ends of the toner replenishing opening20of the toner cartridge21or to form projections22, not shown, at opposite ends of each toner returning opening21of the developing section11, so that each other's rotating forces are prevented from being imparted to the other via the opening20or the openings21.

An alternative embodiment of the present invention will be described with reference toFIG. 5hereinafter. As shown, in the illustrative embodiment, the toner returning openings21each are elongate in the up-and-down direction while the toner replenishing opening20is absent around the top or the bottom of the toner return opening21. In the illustrative embodiment, the agitator disposed in the toner cartridge12is replaced with a plurality of blades29mounted on a rotary shaft28, but absent around the toner returning openings21. More specifically, the blades29are separate from each other in the axial direction of the shaft28and shifted in position from each other in the circumferential direction of the shaft28. For example, three blades29may be mounted on the shaft28at positions angularly shifted from each other by 120°.

In the developing device5described above, the force of the agitator27in rotation is not directly imparted to the toner returning openings21, so that part of the toner positioned above the toner returning opening21of the developing section11is surely returned to the toner cartridge12by the agitator17. This is also successful to protect the developing section11from the blow-off or the blocking of toner ascribable to the excessive replenishment of toner to the developing section11.

In the illustrative embodiment, too, there should preferably hold a relation:
hBT>hO>hS>hB
where hBTdenotes the distance or height between the top of each toner return opening21and the bottom of the developing unit5.

FIG. 6shows another alternative embodiment of the present invention. As shown, in the illustrative embodiment, the toner returning opening21is not formed in the wall of the developing section11that faces the toner cartridge12. More specifically, assuming that the above wall of the developing section11is a front wall, then the toner returning opening21is formed in the side wall of the developing section11. In this configuration, the toner present in the developing section11is not returned to the toner cartridge12, but is delivered to a waste toner container31via the toner returning opening21.

In the illustrative embodiment, the toner, once used in the developing section11and therefore deteriorated, is discarded and replaced with fresh toner little by little. It is therefore possible to insure stable image quality over a long period of time by controlling the deterioration of the toner in the developing section11. It is to be noted that the waste toner container31may be implemented as a single container removably mounted to the developing section11or as a partitioned spaced formed in the toner cartridge12and therefore removable integrally with the toner cartridge12, as desired.

In the illustrative embodiments shown and described, the mean amount of toner to be replenished from the toner cartridge12to the developing section11is selected to be greater than the mean amount of toner to be transferred from the developing roller14to the belt1during continuous operation, as stated earlier. More specifically, assume that fresh toner is replenished in a greater amount than toner deposited on and therefore consumed by the photoconductive element when a latent image with a small image area ratio is repeatedly developed. For example, assume that even when the area ratio of an image is 1%, toner corresponding in the amount of consumption to 3% is replenished to the developing section11while excess toner is discharged from the developing section11via the toner discharging openings21. Then, toner in the developing section11can be replaced with fresh toner little by little without increasing loads on the photoconductive drum or the cleaner. To further reduce wasteful toner replenishment, the amount of replenishment may be sensed and controlled or the amount of toner to be deposited on the photoconductive element may be calculated and subject to feedback control.

In any one of the illustrative embodiments, an arrangement may be made such that the toner or developer discharged via the toner returning openings21is classified into reusable toner and toner to be discarded by toner or developer classifying means. That is, toner deteriorated in the developing section11and then discharged does not have to be entirely discarded, but may be classified by a conventional filter, gravity, electrostatic force or the like so as to return reusable part of the toner, thereby reducing the amount of waste toner. More specifically, in the illustrative embodiment shown inFIG. 6, the waste toner container or waste developer containing section31may be replaced with toner or developer classifying means configured to discard only the toner heavily damaged while returning reusable toner to the toner cartridge12. In this case, the classifying means may be configured to classify toner collected by the belt cleaner7also in order to minimize the amount of waste toner and effectively use limited resources and, moreover, to prevent image quality from being lowered.

The developing device of any one of the illustrative embodiments may be used as a cleanerless system in which it plays the role of the belt cleaner7at the same time. In this configuration, the toner present in the developing section11is replaced with fresh toner little by little, protecting images from degradation ascribable to the collected toner.

In accordance with the present invention, the developing device uses toner grains on which additives are deposited in an amount of one-tenth of the mean volumetric grain size of the toner inclusive or below. Therefore, even when the additives are separated from or buried in the cores of the toner grains, replenished toner can sufficiently make up for the loss of additives. In addition, the additives separated from the cores move upward in the developing section11and then discharged, so that an adequate amount of additives can be stably maintained in the developing section11to thereby extend the life of the toner.

Toner grains to be replenished from the toner cartridge12to the developing section11may be higher in the ratio of additives deposited thereon than toner grains to be set in the developing section11for the first time. This prevents the additives of the toner grains present in the developing section11from being buried in the cores and maintains the fluidity of the toner grains relatively high, thereby insuring the desirable electrification of the toner while maintaining friction between the toner and members contacting it relatively low. Consequently, it is possible to reduce torque required of the developing device and insure high image quality at the same time.FIG. 7compares the developing unit of the present invention and a conventional developing unit as to the variation of torque. As shown, the present invention successfully controls the rise of torque at the beginning of use.

The developing device of the present invention, effecting contact development with a one-component type developer, should preferably satisfy a relation:
0<|VD|−|VB|<|VD−VL|<400V
where VDdenotes the potential of the image carrier in the dark, VLdenotes a potential after exposure, and VBdenotes a bias voltage for development. This relation allows development to be effected with a potential difference of 400 V or below for thereby reducing electrostatic hazards to the photoconductive element and therefore extending the life of the photoconductive element.

Further, the developing device of the present invention is capable of extending the life of toner stored in the developing section11. Therefore, as shown inFIG. 8specifically, if at least the image carrier and developing unit are constructed into a single process cartridge PC removable from the apparatus body, then the durability of the toner is further extended to increase the interval between consecutive times of maintenance while reducing the frequency of replacement of the process cartridge PC, thereby enhancing efficient maintenance and operation. The process cartridge may further include charging means, developing means, cleaning means and so forth, if desired. This is also successful to achieve the above advantages.

In any one of the illustrative embodiments, a plurality of openings are formed in the lengthwise direction of the developing unit. This prevents the amount of developer in the developing unit from being irregular in the lengthwise direction and therefore obviates irregular image density. Moreover, it is possible to prevent the developer from accumulating at opposite lengthwise ends of the developing section, further enhancing the above advantages.

Specifications of the developing device in accordance with the present invention are as follows. The developing roller was provided with a diameter of 26 mm and a surface formed of polyester-melamine resin and carbon black. The surface layer of the developing roller had electric resistance of 4 LogΩ and surface roughness of 2.6 μm Rz.

The feed roller was provided with an outside diameter of 16 mm, made of foam polyurethane and carbon black, ion-conductive material or similar conductive material, and provided with electric resistance of 6 LogΩ. The feed roller bit into the developing roller by 0.5 mm.

The regulating roller or metering member was provided with an outside diameter of 14 mm, a surface made of fluoric resin and carbon black, ion conductive material or similar conductive material, electric resistance of 4 LogΩ to 7 LogΩ, hardness of 75° in JIS (Japanese Industrial Standard) K6253 scale, and surface roughness of 2.6 μm Rz. With this configuration, the regulating roller was rotated either intermittently or constantly, as the case may be.

As for black toner, 100 parts by weight of low molecular weight, polyester resin, 6 parts by weight of carbon black and one part by weight of high molecular weight, charge control agent were kneaded by a conventional screw kneader, rolled by cooling, roughly pulverized, finely pulverized by a jet mill and finally classified by an air-stream type classifier to obtain toner grains having a volume-mean grain size of 6.7 μm and a number-mean grain size of 6.1 μm. Magenta toner, cyan toner and yellow toner each were produced in the same manner as the black toner except for the color of a pigment.

Subsequently, 0.3 part by weight of silica was added to 100 parts of weight of each toner and then mixed together by a Henschel mixture for 1 minute. Thereafter, 0.3 part by weight of titania was added to the toner and then mixed together for 1 minute, and then 1.0 part of weight of silica was further added and mixed for 1 minutes. The mean amount of charge measured on the surface of the developing roller was 30 μC/g.

The photoconductive element was measured to have a mean potential (VL) of −50 V in a black, solid portion and a mean potential (Vd) of −500 V in a background portion.

In summary, it will be seen that the present invention provides a developing device using a dry one-component type developer and capable of reducing damage to toner to thereby extend the life of the developing device and toner. Further, the developing device of the present invention obviates the unusual rise of dynamic torque ascribable to erroneous toner-end sensing or excess toner present in a toner storing section, and implements toner recycling while protecting toner from deterioration. It is therefore possible to reduce required torque and realize high-quality development at the same time.