1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine and the like performing an image formation by an electrophotographic system or an electrostatic recording system.
2. Related Background Art
FIG. 19 is a schematic block diagram showing one example of an image forming apparatus utilizing the conventional electrophotographic system.
This image forming apparatus comprises an electrphotographic photosensitive member (hereinafter referred to as a photosensitive drum) of a drum type as an image bearing member, around the periphery of which are disposed a charging roller 2, an exposing apparatus 3, a developing apparatus 4, a transfer roller 5 and a cleaning blade 6.
The photosensitive drum 1 is constituted in such a manner that an OPC photosensitive member layer 1b is formed on the outer surface of a conductive drum base 1a comprising, for example, aluminum and the like, and is rotationally driven by a drive apparatus (not shown) in the arrow direction (clockwise) at a predetermined process speed (a circumferential speed). The drum base 1a of the photosensitive drum 1 is grounded to a ground (GND).
The charging roller 2 as contact charging means is formed in a roller shape with a resistantly adjusted elastic layer 2b disposed on a metal core 2a and is rotated in the arrow direction (anticlockwise direction). The charging roller 2 contacts the surface of the photosensitive drum 1 by a predetermined pressing force, and by a charging bias (a bias in which an alternating voltage is superposed on a direct-current voltage) applied from a charging power source 170, the photosensitive drum 1 is charged with a predetermined potential of a negative polarity in the present conventional example.
Regarding the exposing apparatus 3, a modulated laser light according to the time series digital image signal of the image information to be inputted is outputted from a laser output portion (not shown) and performs an image-exposure-L on the surface of the photosensitive drum 1 so that an electrostatic latent image is formed on the surface of the photosensitive drum 1 charged by the charging roller 2 according to the image information.
The developing apparatus 4 allows a developer (for example, a magnetic one component insulating toner t in the present conventional example) borne on a developing sleeve 4a to be adhered on the electrostatic latent image formed on the photosensitive drum 1, and visualizes the latent image as a toner image by a reversal-developing. The non-magnetic developing sleeve 4a which is rotated at a speed approximately equal to that of the photosensitive drum 1 is arranged with a minute gap portion (for example, below 200 xcexcm) disposed against the photosensitive drum 1 and magnetically confines the toner t on the surface of the developing sleeve 4a by a magnetic force of a magnetic roll 4b fixed and involved in the interior of the developing sleeve 4a. The toner t magnetically confined on the developing sleeve 4a is regulated to a predetermined layer thickness by a layer thickness regulating blade 4c. 
The developing sleeve 4a is applied with a developing bias in which the alternating voltage in a square wave shape is superposed on the direct-current voltage from a developing power source 180. In this way, an electrical field is generated in the gap portion between the developing sleeve 4a and the photosensitive drum 1, and a toner t on the surface of the developing sleeve 4a flies. On this occasion, the portion not subjected to the image-exposure-L in the surface (OPC photosensitive member layer 1b) of the photosensitive drum 1 is charged with a negative polarity. The surface potential thereof is reduced lower than the direct-current voltage of the negative polarity of a developing power source 180 and exerts a force on the toner t in a direction to push it back to the developing sleeve 4a. Accordingly, by the alternating electrical field of the gap portion generated by the alternating voltage of the developing power source 180, even when the toner t flies, it is pushed back to the interior of the developing apparatus 4.
On the other hand, since the portion subjected to the image-exposure-L in the surface (OPC photosensitive member layer 1b) of the photosensitive drum 1 is attenuated in a negative charge, that portion becomes higher than the direct-current voltage of the negative polarity of the developing power source 180 (the absolute value of the negative voltage of that surface portion is reduced lower than the absolute value of the direct-current voltage of the developing power source 180), and exerts a force on the toner t to pull it back to the photosensitive drum 1. Accordingly, by the alternating electrical field of the gap portion generated by the alternating voltage of the developing power source 180, the toner t flies and the flied toner t is adhered on the surface of the photosensitive drum 1.
In this way, the portion subjected to the image-exposure-L in the surface of the photosensitive drum 1 is adhered with the toner t and the portion not subjected to the image-exposure-L in the surface of the photosensitive drum 1 is not adhered with the toner t, so that the toner t is adhered on the electrostatic latent image formed by being subjected to the scan-exposure-L, whereby the toner image is formed.
The transfer roller 5 as contact transfer means is formed in a roller shape with a resistantly adjusted elastic member layer 5b disposed on a metal core 5a and is rotated in the arrow direction (counterclockwise direction).
The transfer roller 5 forms a transfer nip portion by contacting the surface of the photosensitive drum 1 with a predetermined pressing force and transfers the toner image formed on the surface of the photosensitive drum 1 in the transfer nip portion by a transfer bias (a bias of the (positive) polarity reverse to the toner t) applied from a transfer power source 190 to a transfer material P such as a paper and the like.
Next, the image forming operation by the above described image forming apparatus will be described.
When the image forming is effected, the photosensitive drum 1 is rotationally driven by a drive apparatus (not shown) in the arrow direction at a predetermined circumferential speed and is charged with a predetermined potential by the charging roller 2 applied with a negative polarity charging bias from the charging power source 170. With the image-exposure-L according to the image information from the exposing apparatus 3 given to the charged photosensitive drum 1, the potential on photosensitive drum 1 is reduced in the potential of the portion subjected to the image-exposure-L, and the electrostatic latent image is formed according to the inputted image signal.
The toner t, which is charged with the same polarity (negative polarity) as the charged polarity of the photosensitive drum 1 by the developing sleeve 4a of the developing apparatus 4 applied with the developing bias of the same polarity of the charged polarity (negative polarity) of the photosensitive drum 1 at a developing position, is adhered on the electrostatic latent image formed on the photosensitive drum 1 and reversal-developed (visualized) as the toner image as described above.
When the toner image on the photosensitive drum 1 reaches the transfer nip portion between the photosensitive drum 1 and the transfer roller 5, in time for this timing, the transfer material P such as the paper and the like in the interior of a cassette (not shown) is conveyed to the transfer nip portion by a sheet feeding roller (not shown), and by the transfer roller 5 applied with a predetermined transfer bias of the polarity (positive polarity) reverse to the above described toner t, the toner image on the photosensitive drum 1 is transferred on the transfer material P conveyed by the transfer nip portion by an electrostatic electrical power generated between the photosensitive drum 1 and the transfer roller 5.
The transfer material P, on which the toner image is transferred, is conveyed to a fixing apparatus (not shown), and the toner image is heated, pressurized and thermally fixed on the transfer material P by a fixing nip between a fixing roller (not shown) and a pressurizing roller (not shown)and, after that, discharged outside and a series of the image forming operation is completed. A transfer residual toner remained on the photosensitive drum 1 after the above described transfer is removed and collected by a cleaning blade 6.
By the way, in the above described image forming, the transfer material P is sometimes stopped from being conveyed in front of the transfer nip portion due to a paper jam (a transfer material jam) and a sheet feeding failure. In this case, the toner image formed on the photosensitive drum 1 adheres on the surface of the transfer roller 5 instead of being transferred on the transfer material P. The toner adhered on the surface of the transfer roller 5 in this way adheres on a rear surface of the transfer material P which is conveyed to the transfer nip portion in the image forming subsequent to the canceling of the above described paper jam and sheet feeding failure, and stains it.
For this reason, in order to avoid such a malfunction, a cleaning operation of the surface of the transfer roller 5 was conventionally performed when an image forming is not effected. This cleaning operation is such that a negative direct-current voltage of the polarity reverse to the transfer time is applied to the metal core 5a of the transfer roller 5 from the transfer power source 190, so that an electrical field is generated between the photosensitive drum 1 and the transfer roller 5, thereby the toner adhered on the surface of the transfer roller 5 is transferred to the photosensitive drum 1 side and removed and collected by the cleaning blade 6. Note that, on this occasion, the developing bias is not applied to the developing sleeve 4a of the developing apparatus 4.
In order to perform such a cleaning operation, as shown in FIG. 20, the transfer power source 190 is conventionally constituted so as to output positive and negative biases. This transfer power source 190 comprises a positive bias output portion 10 for outputting a positive bias and a negative bias output portion 11 for outputting a negative bias.
The positive bias output portion 10 and the negative bias output portion 11 comprises respectively: pulse generators 12a, 12b for generating pulses; amplifiers 13a, 13b for amplifying each pulse outputted from the pulse generators 12a, 12b; block condensers 14a, 14b for preventing a direct current component of each output of the amplifiers 13a, 13b; transformers 15a, 15b for converting the voltage amplitude of an alternating current component of each output of the amplifiers 13a, 13b to a predetermined high voltage; diodes 16a, 16b for rectifying each output of the transformers 15a, 15b; and each condenser 17a, 17b and each resistor 18a, 18b. In this way, the positive bias output portion 10 and the negative bias output portion 11 are identically constituted except that a connecting polarity of each diode 16a, 16b is reversed.
In this way, in the normal image forming time, the transfer power source 190 allows the pulse generator 12a of the positive bias output portion 10 to oscillate and the pulse generator 12b of the negative bias output portion 11 to turn off oscillating, and outputs a positive polarity high voltage to the metal core 5a of the transfer roller 5 through the resistor 18a. On the other hand, in the cleaning of the above described transfer roller 5, the pulse generator 12b of the negative bias output portion 11 is allowed to oscillate and the pulse generator 12a of the positive bias output portion 10 to turn off oscillating, and a negative polarity high voltage is outputted to the metal core 5a of the transfer roller 5 via the resistor 18b. 
As described above, in order to clean the toner adhered on the transfer roller 5, the conventional image forming apparatus requires the negative bias output portion 11 for applying a voltage of the polarity reverse to the normal transfer time and this caused a problem in that a circuit configuration of the transfer power source 190 becomes complicated.
An object of the present invention is to provide an image forming apparatus capable of performing a cleaning of transfer means without complicating a circuit constitution of a transfer power source.
Another object of the present invention is to provide an image forming apparatus, comprising:
an image bearing member having a photosensitive portion on a conductive portion;
charging means for charging the image bearing member;
exposing means for exposing the charged image bearing member to form an electrostatic latent image;
developing means for developing the electrostatic latent image by a toner;
transfer means for transferring a toner image on the image bearing member to a transfer material;
image bearing member voltage applying means for applying a voltage having the same polarity as that of toner to the conductive portion of the image bearing member;
charging voltage applying means for applying a voltage to the charging means;
developing voltage applying means for applying a voltage to the developing means; and
transfer voltage applying only means for applying only a voltage having a polarity reverse to that of the toner to the transfer means,
wherein the apparatus has a cleaning mode in which, when an image forming is not effected, the voltage is applied to the image bearing member by the image bearing member voltage applying means and the voltage applied to the transfer means from the transfer voltage applying means is turned off or made lower than the voltage applied when the image forming is effected.
Still another object of the present invention will be apparent from the following description.