JPS59211069A - image forming device - Google Patents

Abstract

PURPOSE:To eliminate difference of an image density due to a difference of a copying speed by varying two developing characteristics in accordance with the copying speed, and making a Do-Dc curve roughly constant. CONSTITUTION:Frequency of AC bias voltage applied between a photosensitive body and a developer carrier body by using a bias power source whose AC frequency is variable is changed in accordance with a copying speed. That is to way, when a developing characteristics is changed by varying the AC bias voltage and frequency to 1,000Vp-p, 1,000Hz against 100mm./sec copying speed, to 1,000Vp-p, 800Hz same against 200mm./sec same, and to 1,000Vp-p same against 300mm./sec same, a variation ratio of an image density against an original density Do becomes nearly equal in any copying speed, and difference of an image density due to a difference of a copying speed is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus, and particularly to a developing device, in which an image bearing member and a developer carrier are opposed to each other while maintaining a developing gap, and which performs development while applying an alternating electric field to the developing gap. The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a magnetic recording machine, etc., which has an image forming speed of two or more stages.

FIG. 1 shows an outline of a variable magnification electrophotographic copying machine as an image forming apparatus, in which 1 indicates a surface insulating layer la, a photoconductive layer 1
b, a photoreceptor whose basic structure is a three-layer structure consisting of a conductive substrate 1c. This photoreceptor l has a first corona discharger 2.
A latent image corresponding to the original image is formed by irradiating the image of the original 12 via the DC corona from the second corona discharger 3 and the AC corona from the second corona discharger 3 and the lens 11, and by further irradiating the entire surface by the lamp 4. The latent image on the photoreceptor is then developed with toner by a developer 5, and the developed image is transferred to a transfer material 6 under the electric field of a discharger 7. The transfer material 6 is separated from the photoreceptor by a separation roller 8 and reaches a fixing device (not shown), where the toner is fixed and a copy is completed. On the other hand, residual toner on the photoreceptor l is removed by a cleaning device 9, and the photoreceptor l is reused.

In the developing section, a thin layer of developer (toner) is carried on a developer carrier 50 and the carrier 50 is rotated.

Then, an alternating current voltage (an alternating current with a direct current component superimposed may be used) is applied as a bias voltage to the carrier 50.
Developing is carried out by causing developer to fly back and forth between photoreceptor 0 and photoreceptor l. In other words, in the process of this reciprocating flight, the developer adheres to the image area of the electrostatic latent image (the area where the developer should adhere), and the non-image area of the electrostatic latent image (the background area where the developer should not adhere). The developer is drawn back to the carrier 50 from the area 2), and a background area free of developer adhesion can be obtained. In this way, a developed image with good contrast and gradation can be obtained.

In this electrophotographic copying machine, the lens 11 is placed at the solid line position,
When copying in various modes such as normal size, reduction, enlargement, etc. by placing it at the 11' position, 11'' position, etc. indicated by the two-dot chain line,
It is extremely difficult to perform reduced copying and enlarged copying at the same copying speed as full-size copying. Therefore, it is conceivable to set the copying speed according to each copying mode, but by changing the photoreceptor speed corresponding to the selected magnification, the copying speed changes, and the charging time and exposure time for the photoreceptor also change. For this reason, for example, as shown in FIG. 2, there is a song that expresses the relationship between the exposure amount and the surface potential of the photoreceptor! 112 a to 2C are copying speeds (curve 2a is 100 mm/see, curve 2b is 200 mm/see)
/sec, 3c is 300 mm/sec), and the obtained image has a different density depending on the copying speed.

Conventionally, as a method for improving this, changing the exposure width and exposure position depending on the copying speed has been carried out, but satisfactory results have not always been obtained.

The present invention has been proposed in view of the above, and in an electrophotographic copying machine having two or more copying speeds, by changing the relationship between the photoreceptor surface potential and image density during development according to the copying speed, The purpose is to eliminate differences in image density due to differences in copy speed.

The drawings will be explained in detail below. FIG. 3 shows an AC power source 1 connected between a developer carrier 50 in a developing device 5 and a photoreceptor 1 in which the copying speed is constant and the jumping developing method disclosed in, for example, Japanese Unexamined Patent Publication No. 18859/1985 is adopted. This is a diagram showing the relationship between the photoreceptor surface potential and the density of the copied image when the frequency of the AC bias voltage applied is changed.
3c is 4°0H2), the higher the value, the sharper the change in image density with respect to the surface potential, that is, the steeper the curve becomes. In FIG. 4, the AC bias voltage and its frequency are kept constant at 100 OVTI-P and 400 Hz, and the copying speed is set at 100 mm/sec, for example.
This is a diagram showing the relationship between the original density and the copy image density when changing the speed in three steps: 200 mm/sec and 300 mm/sec.
sec, same is 200 mrn/sec, same is C
(300 mm/sec), the faster the change in the copy image density with respect to the original density, that is, the steeper the curve rises. Further, when the alternating current bias voltage and its frequency were set to 100 OV pp and 800 Hz and the copying speed was changed in the same manner as above, the same results as shown in the fourth figure were obtained.

The present invention has been made by paying attention to the phenomena shown in FIGS. 3 and 4, and uses an AC frequency variable bias power supply 10 to control the AC bias voltage applied between the photoreceptor and the developer carrier. It is characterized by changing the frequency according to the copying speed. That is, for a copying speed of 100 mm/sec, the AC bias voltage and frequency are 1000 Vp-p and 1000 H.
z, the same IQOOV for the same 200 mm/sec
p-p, 800H2, and 300mm/
1000VT+-Tl for sec, 400
Hz and the development characteristics, the rate of change in the density of the copied image with respect to the original density Do (curves 5a to 5C in FIG. 5) becomes approximately equal at any copying speed, as shown in FIG. This reduces the difference in image density due to the difference in image density.

As described above, the present invention changes the development characteristics according to the copying speed and controls the D'o-Dc curve to be approximately constant. This has the effect of eliminating the difference and making it possible to always obtain copies with the same image density.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the image forming apparatus of the present invention, Fig. 2 is a graph showing the relationship between the surface potential V of the photoreceptor and the exposure amount E, and Fig. 3 is a graph showing the relationship between the surface potential V of the photoreceptor and the image density due to jumping development. 4 is a graph showing the relationship between document density DO and image density Dc in the conventional image forming apparatus. FIG. 5 is a graph showing the relationship between document density DO and image density Dc in the image forming apparatus of the present invention.
3 is a graph showing the relationship between o and image density DC. 1 is a photoreceptor, 5 is a developing device, and 10 is a variable AC power source. Patent applicant Canon Co., Ltd.-443 = JP-A-59-211069 (4)

Claims (1)

[Claims] (1) An image that has a developing device in which an image carrier and a developer carrier are opposed to each other while maintaining a development gap, and develops while applying an alternating electric field to the development gap, and has image forming speeds of two or more stages. An image forming apparatus characterized in that the frequency of the alternating electric field is changed according to each of the image forming speeds.