JPH0227668B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention relates to an image adjustment method in an electrophotographic copying machine, and more particularly to an image adjustment method that includes preventing background smear on a developed image by changing the value of a bias voltage applied to a development electrode. (Prior Art) As is well known, in electrophotography, an electrostatic latent image corresponding to an original to be copied is formed by charging and exposing a photoreceptor having a photoconductive layer surface. This is developed with a developer called a toner that is charged with a polarity opposite to the charged polarity to obtain a visible image. When the charged surface of the photoconductor is exposed to the original image, the charge in the dark part of the document, that is, the part corresponding to the image area, remains almost unchanged, and the charge in the bright part of the document, that is, the part corresponding to the background part, is transferred to the photoconductor. Since the discharge occurs through the conductive substrate, a charge pattern corresponding to the original image is formed there.
This is an electrostatic latent image. Although the potential of the background portion of this electrostatic latent image decreases due to discharge, it does not become zero and remains slightly. Therefore, when the developer is supplied from the developing device, the toner adheres not only to the image area but also to the background area, thereby staining the background area. For this reason, a conductive member called a developing electrode is placed close to and facing the surface of the photoreceptor, and a bias voltage slightly higher than the potential of the latent image background part is applied to it, so that the toner adheres to this developing electrode instead of the background part. It is generally practiced to suppress the development of the background portion. However, once the developing bias voltage is set, it is not always possible to obtain a good image without background stains. This is because when the photoreceptor and the developer deteriorate due to repeated use, the background potential and the amount of charge of the toner change, which naturally changes the effectiveness of the bias voltage. Therefore, in the 1970s,
Publication No. 46334 proposes a technique in which the bias voltage is increased in relation to the increase in the number of copies, focusing on the fact that the degree of deterioration of the developer is proportional to the increase in the number of copies. Furthermore, Japanese Patent Laid-Open No. 110046/1983 proposes a technique for similarly adjusting the bias voltage, focusing on the fact that the degree of fatigue of the photoreceptor is similarly proportional to the increase in the number of copies. Furthermore, Tokukai Akira
Publication No. 53-98841 states that the degree of fatigue of the photoconductor is not only proportional to the number of copies, but is also affected by the amount of exposure, copy size, amount of charge, etc., so the amount of change in each of these is also counted and integrated. Therefore, a technique has been proposed that measures the fatigue level of the photoreceptor. Among such conventional technologies, the above-mentioned Japanese Patent Application Laid-open No. 1983-
The technique proposed in Japanese Patent No. 110046 is outstanding in that it uses the rest time of the copying machine, that is, the time when the photoreceptor is left in a dark place, as one of the parameters when controlling the bias voltage. This is because if the photoreceptor is left in a dark place, the fatigue of the photoreceptor will recover depending on the length of time the photoreceptor is left. Therefore, if this parameter is not taken into account, even though the photoreceptor has recovered from fatigue after being left in the dark for a long time, the bias voltage applied when the photoreceptor is fatigued will be applied to the developing electrode. Therefore, the bias voltage is too high, resulting in a decrease in the density of the developed image and poor reproducibility of poor originals such as low-density originals or low-contrast originals. For this reason, in the technique described in the above-mentioned publication, the developing bias voltage is lowered in accordance with the time the photoreceptor is left in a dark place. However, it has been empirically known that even with this method, copies with smeared backgrounds can still be produced when the copying machine starts operating. (Object of the Invention) Therefore, an object of the present invention is to provide an electrophotographic image adjustment method that includes preventing background smudges on copies that appear mainly when a copying operation is started after a copying machine is stopped. The image adjustment method according to the present invention focuses on the fact that the longer the copying machine is stopped, the more the initial copying background smudge becomes, and the greater the developer fatigue, the more pronounced this background smudge becomes, and it disappears after a certain amount of copying. It is characterized by determining the developing bias voltage using the copying machine down time and developer fatigue level as parameters, and gradually lowering the developing bias voltage according to the number of copies after the copying machine resumes operation. Hereinafter, the present invention will be explained with reference to the accompanying drawings. (Structure of the Invention) The graph in FIG. 1 shows the relationship between the copying machine down time and the degree of copy background staining at the beginning of the copying operation, and plots the measurement results using a copying machine in which the photoreceptor surface potential is kept constant. This is what I did. As is clear from this figure, the longer the copier down time is, the
The degree of initial copy background staining is large, and the number of copies until the copy background staining becomes good is increasing. Further, as shown in FIG. 2, when the rest time is the same under normal usage conditions, the greater the developer fatigue, the greater the degree of scumming. In this case, developer fatigue is measured by the number of copies made with the same developer contained in the developing device. Therefore, from these two graphs, it can be seen that the longer the copying machine is stopped, the larger the initial copy background smear is, and the greater the developer fatigue, the more pronounced this background smudge is, and it disappears after a certain amount of copying has been performed. That is understood. The inventor of the present invention considered how such a phenomenon occurs and found that when the copying machine is stopped, the amount of charge on the toner in the developer decreases compared to when the copying machine is in continuous operation, as shown in Figure 3. It was speculated that it was from. When the developer is a two-component system consisting of toner and carrier, the toner is charged to the opposite polarity to the charge polarity of the electrostatic latent image due to frictional mixing with the carrier, but the copying machine is stopped and the developing device is stopped. In this case, the developer is not stirred, and the charge on the toner is thought to gradually disappear due to the surrounding atmosphere. When the amount of charge of the toner decreases, the selective adhesion to the latent image area decreases, and more toner adheres to the background area of the latent image, resulting in background smearing. However, this background smudge gradually disappears because the toner charge amount increases as the copying operation starts and the developer in the developing device is stirred. When the developer becomes fatigued, the amount of toner charge decreases rapidly during rest periods.
Furthermore, since recovery of the toner charge amount at the start of operation is delayed, the above-mentioned phenomenon appears to be noticeable. Based on the above considerations, in order to prevent the initial background smearing of copies, it is necessary to detect the down time of the copying machine and the degree of fatigue of the developer, and increase the bias voltage value according to the increase in these factors. It was concluded that the bias voltage value should be gradually lowered according to the number of copies after the copying machine is stopped. Based on this conclusion, we conducted an experiment by adding the bias increment shown in Table 1 to the specified bias value, and as shown in Figure 4, we obtained good copies with almost no initial background smudge. It was done.

[Table] FIG. 5 schematically shows, as a block diagram, an electric circuit for carrying out the image adjustment method of the present invention. The agent usage number detection circuit 11 counts the number of copies after a new developer is put into the developing device one by one, and the bias control circuit 12
send that signal to. Therefore, when replacing the developer, be sure to turn on the reset switch to clear the count. Copy number detection circuit 1
3 similarly counts the number of copies one by one, but it is cleared when the copying machine stops, and when the copying machine starts operating, it counts one copy again and sends the signal to the bias control circuit 12. The downtime detection circuit 14 detects the time from when the main switch of the copying machine is turned off and the copying machine comes to rest, or after the motor of the developing device stops and developer agitation stops, until the copying machine is started again. The time is detected by a time meter or the like, converted into digital data, and then inputted to the bias control circuit 12. Note that this circuit 14
The copy machine operates using a backup battery or the like even after the main switch of the copying machine is turned off. Upon receiving these signals, the bias control circuit 12 performs necessary processing to set an appropriate bias voltage value, and applies it to the developing roller 17, which also serves as a developing electrode in the developing device 16, through the bias power supply 15. The developer 18 housed in the developing device 16 is composed of toner and carrier, and as it is stirred by an impeller 19, the toner is tribo-electrified to a polarity opposite to that of the electrostatic latent image on the photoreceptor 20. be done. The polarity of the bias voltage applied to the developing roller 17 is opposite to the charging polarity of the toner, and is lower than the image area potential of the electrostatic latent image and slightly higher than the background area potential.
Therefore, the toner adheres to the image area of the electrostatic latent image, but does not adhere to the background area, thereby preventing background smearing. If the bias voltage is too high, the image density will decrease or the reproducibility of poor originals will deteriorate, and if the bias voltage is too low, scumming will occur. In this invention, an appropriate bias value is set based on three factors: copying machine down time, developer fatigue level, and number of copies, achieving good reproduction of poor originals and effective prevention of scumming. . The operation of the bias control circuit 12 is performed by a microcomputer as shown in FIG. 6, which is used to control the copying machine. That is,
Central processing unit CPU is read-only memory
The microprograms written in the ROM are sequentially read out and necessary input/output operations are controlled while executing predetermined sequence processing. During this time, necessary data exchange between the central processing unit CPU and external devices, that is, each device of the copying machine, takes place.
This is done through the input/output control unit IOC and the interface circuit IF. Signals inputted to the central processing unit CPU through the interface circuit IF include a start signal, a print signal, a detection signal from the rest time detection circuit 14, and a detection signal from the developer usage number detection circuit 11 that indicates the degree of fatigue of the developer. , a detection signal from the copy number detection signal circuit 3, a timing signal, etc. The central processing unit CPU outputs bias control signals and other signals in response to these input signals. Each detection circuit 11, 13,
The detection signal from 14 is a random access memory.
Once stored in RAM, it is input to the central processing unit CPU and taken out as appropriate. (Effects of the Invention) As described above, according to the image adjustment method of the present invention, the developing bias value is set using the down time of the copying machine and the degree of developer fatigue as parameters, and this value is set when the copying machine is started again. Since the number of copies is gradually decreased according to the number of copies, it is possible to almost completely prevent the initial background staining of copies, and it is also possible to excellently reproduce poor originals. Of course, the image adjustment method according to the present invention can be used in conjunction with other known image adjustment methods. Furthermore, it is of course possible to use a one-component developer consisting only of toner as the developer.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the degree of scumming and the number of copies until it disappears during different lengths of copying machine down time, and Figure 2 is a graph showing the degree of scumming and the relationship between the level of scumming and the number of copies until it disappears when the copier has different levels of fatigue. FIG. 3 is a graph showing the relationship with the copying machine down time; FIG. 3 is a graph showing the change in toner charge amount when the copying machine is in continuous operation and when the copying machine is stopped; FIG. 4 is FIG. 1 showing the effects of the present invention. 5 is a block diagram schematically showing an electric circuit for carrying out the present invention, and FIG. 6 is a block diagram schematically explaining the operation of the bias control circuit shown in FIG. be. 17...Developing roller (developing electrode), 20... Photoreceptor.

Claims (1)

[Scope of Claims] 1. A means for developing an electrostatic latent image formed on a photoreceptor with a developer on a developing electrode, and adjusting the image by changing the value of a bias voltage applied to the developing electrode. In an electrophotographic copying machine, the electrophotographic copying machine is equipped with means for detecting a downtime of the copying machine, and increasing the bias voltage as the time becomes longer; and detecting a degree of fatigue of the developer, and increasing the bias voltage as the time increases. and gradually decreasing the increased bias voltage value in accordance with the number of copies from when the copying machine resumes operation.