Patent Publication Number: US-2011064430-A1

Title: Image forming method and determination method of contrast potential

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior U.S. Patent Application 61/242,986 filed on Sep. 16, 2009, and 61/245,007 filed on Sep. 23, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to an image forming method and a determination method of a contrast potential 
     BACKGROUND 
     In an electrophotographic image forming apparatus, a photoconductive surface as an image carrier is charged and an electrostatic latent image is formed by exposure. A developer including a charged toner is supplied to the electrostatic latent image, and after development, a developed image is transferred to a transfer medium such as paper. 
     In the electrophotographic process as stated above, in order to print a high quality image, it is necessary to keep the charging amount of the developer to be constant. However, in a long non-operation period or during storage under high humidity environment, the developer is discharged and the charging amount is reduced. Thus, there arises a problem that at printing immediately after such storage, a desired image density or gradation can not be obtained. 
     Then, before the printing after the storage, calibration is performed in which a specified latent image is formed on a photoreceptor, and an image formation condition for image density or the like is optimized. However, there is a problem that when continuous printing is performed after the calibration, by the agitation of the developer the absolute value of the charging amount of the developer is increased, and the image density is gradually reduced. 
     Thus, in order to replace a part of the toner whose charging amount is reduced and in which the charging amount change is liable to occur, forcible toner consumption (refresh) to forcibly consume the toner is performed before the printing after the storage. After the execution of the forcible toner consumption, the calibration is again performed, so that a stable image can be obtained in the printing after that. 
     Before the execution of the forcible toner consumption, it is necessary to perform calibration in order to adjust the toner consumption amount to a specific amount. However, there is a problem that by the calibration before or after the execution of the forcible toner consumption, time elapsed before the printing after the storage becomes possible becomes long. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structure of an image forming apparatus in an embodiment of the invention; 
         FIG. 2  shows a structure of an image forming section in the embodiment of the invention; 
         FIGS. 3 ,  6  and  8  are flowcharts of image formation in the embodiment of the invention; 
         FIG. 4  shows a relation between a development contrast potential and a relative humidity in the embodiment of the invention; 
         FIG. 5  shows an estimation method of a development contrast potential in the embodiment of the invention; 
         FIGS. 7A and 7B  show tables of toner replacement amounts corresponding to respective parameters in the embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present embodiment of the invention, an example of which is illustrated in the accompanying drawing. 
     Hereinafter, embodiments will be described with reference to the drawings. 
     Embodiment 1 
       FIG. 1  shows a structure of an image forming apparatus  10  in this embodiment. As shown in the drawing, a sensor  11  to acquire an environmental value such as a relative humidity, a clock  12  to acquire a non-operation time or an operation time as needed, and a counter  13  to acquire a developer life (the number of processed sheets for an inputted developer) are provided. These are connected to a memory  14  which stores the environmental value such as the relative humidity, the developer life, and a correlation between these and an image formation condition such as a development contrast potential (difference between a toner attachment side electrostatic latent image potential and a development potential). 
     The memory  14  is connected to an arithmetic unit  15  to obtain an image formation condition or the like. The arithmetic unit  15  is connected to an image forming section  16  which includes an exposure device, a photoreceptor, a developing device and the like and forms an image under the obtained image formation condition. 
     Incidentally, in the image forming apparatus as stated above, the image forming section can be constructed as shown in  FIG. 2 . As shown in  FIG. 2 , a secondary transfer roller  18   b  to transfer an image on an intermediate transfer belt  17  onto a transfer medium P and image forming units  20   Y ,  20   M ,  20   C  and  20   K  are arranged along a conveyance direction (arrow direction) of the intermediate transfer belt  17 . 
     The image forming units  20   Y ,  20   M ,  20   C  and  20   K  respectively includes photoreceptors  21   Y ,  21   M ,  21   C  and  21   K . Further, charging devices  22   Y ,  22   M ,  22   C  and  22   K  having charging members as charging units and gap adjustment mechanisms and developing rollers as developing members are provided around the respective photoreceptors. The image forming units  20   Y ,  20   M ,  20   C  and  20   K  further include developing devices  23   Y ,  23   M ,  23   C  and  23   K  containing developers made of respective color toner particles of yellow, magenta, cyan and black and carrier particles, primary transfer rollers  24   Y ,  24   M ,  24   C  and  24   K  as transfer units, and cleaner units  25   Y ,  25   M ,  25   C  and  25   K . These are respectively arranged along the rotation directions of the corresponding photoreceptors  21   Y ,  21   M ,  21   C  and  21   K . 
     The respective primary transfer rollers  24   Y ,  24   M ,  24   C  and  24   K  are arranged inside the intermediate transfer belt  10 , and nip the intermediate transfer belt  17  in cooperation with the corresponding photoreceptors  21   Y ,  21   M ,  21   C  and  21   K . Exposure devices  26   Y ,  26   M ,  26   C  and  26   K  are respectively arranged so that exposure points are formed on the outer peripheral surfaces of the photoreceptors  21   Y ,  21   M ,  21   C  and  21   K  between the charging devices  22   Y ,  22   M ,  22   C  and  22   K  and the developing devices  23   Y ,  23   M ,  23   C  and  23   K . The secondary transfer roller  18   b  is arranged outside the intermediate transfer belt  17  so as to contact therewith. 
     In the image forming apparatus constructed as stated above, first, a toner image is formed by the image forming unit  20 Y. In synchronization with the timing of the toner image of yellow formation in the image forming unit  20   Y , the same process is performed also in the image forming units  20   M ,  20   C  and  20   K . The toner images of magenta, cyan and black formed on the photoreceptors of the image forming units  20   M ,  20   C  and  20   K  are also successively primarily transferred onto the intermediate transfer belt  17 . 
     The transfer medium P is conveyed from a cassette (not shown), and is sent by an aligning roller (not shown) in synchronization with the timing to the toner image on the intermediate transfer belt  17 . 
     A bias (+) of reverse polarity to the charging polarity of the toner is applied to the secondary transfer roller  18   b  by a power source (not shown). As a result, the toner image on the intermediate transfer belt  17  is transferred onto the transfer medium P by a transfer electric field formed between the intermediate transfer belt  17  and the secondary transfer roller  18   b . A fixing device (not shown) for fixing the toner transferred on the transfer medium P is arranged, and a fixed image is obtained by causing the transfer medium P to pass through the fixing device. 
     Incidentally, although the description is made on the example in which the image forming units are arranged in color order of yellow, magenta, cyan and black, the color order is not particularly limited. Besides, only the black image formation unit may be used. 
     By using the image forming apparatus having the structure as stated above, image formation is performed as shown in, for example, a flowchart of  FIG. 3 . 
     A parameter value at the time of final image formation and an image formation condition are previously acquired. The image formation is performed under a specified image formation condition (ACT  1 - 1 ), the sensor  11  acquires an environmental value, such as a relative humidity, at the time of final image formation, and the counter  13  acquires a parameter value, such as a developer life, at the time of final image formation. The acquired environmental value, together with the image formation condition, such as a development contrast, at the time of final image formation, is stored in the memory  14  in the image forming apparatus  10  (ACT  1 - 2 ). Thereafter, a power source of the image forming apparatus  10  is turned OFF. 
     After being left standing for a week, the image forming apparatus is returned from the sleep state by turned ON the power source thereof (ACT  1 - 3 ). Before image formation is again performed, the present relative humidity is similarly acquired and is stored in the memory  14  in the image forming apparatus (ACT  1 - 4 ). 
     Here,  FIG. 4  shows a relation between a development contrast potential as an image formation condition and a relative humidity. As shown in  FIG. 4 , in the initial developer, the development contrast potential to obtain a specified image contrast is proportional to the relative humidity. In addition, the value of a life developer as the developer after 20 k processing is shown. Although the development contrast potential to the relative humidity is reduced from that of the initial developer, the inclination is almost the same. That is, irrespective of the life, the inclination of the variation of the development contrast potential to the relative humidity is almost constant. 
     Accordingly, in the arithmetic unit  15 , as shown in  FIG. 5 , a development contrast potential “A” to a stored relative humidity “a” at the time of final image formation is plotted, and on a straight line passing through the plotted point and having the inclination shown in  FIG. 4 , an image contrast potential “B” corresponding to the stored present relative humidity “b” is obtained. In this way, without performing after-mentioned calibration, the optimum development contrast potential as the execution condition is estimated (ACT  1 - 5 ). 
     Next, at the estimated image contrast potential, forcible toner consumption (refresh) is performed in the image forming section  16  (ACT  1 - 6 ). A consumed amount of toner is newly supplied, so that partial replacement of toner in the developing device to supply the developer is performed. 
     After completion of the forcible toner consumption, and before image formation, the calibration is performed (ACT  1 - 7 ). Here, a specified latent image is formed on the photoreceptor and is developed, the density (attachment amount) is measured on the secondary transfer belt, and the image formation condition such as an image contrast potential, a charging bias voltage, or an exposure intensity is optimized. The image formation is performed in the image forming section  16  under the optimized image formation condition (ACT  1 - 8 ). 
     In this way, since the execution condition of the forcible toner consumption is estimated before the image formation, the forcible toner consumption can be performed without calibration, and a time elapsed before the start of image formation can be shortened. Further, in the subsequent continuous image formation, a change of image density or the like is suppressed, and stable image formation can be performed. 
     Incidentally, in this embodiment, although the relative humidity having a relatively high influence on the toner charging amount is mentioned as the environmental value, another condition such as a temperature may be used. Besides, when a variation of the environmental value such as the relative humidity is small, the execution condition of the forcible toner consumption may be obtained based on another parameter having an influence on the toner charging amount. 
     For example, the execution condition may be obtained based on a non-operation time before image formation. The non-operation time can be obtained by the clock  12 . Further, the execution condition may be obtained based on a developer life. The developer life can be acquired by the counter  13 . 
     It is preferable that the estimation of the execution condition of the forcible toner consumption is performed at every start of image formation. Besides, the estimation may be set to be performed when the non-operation time or the developer life exceed a specified value. 
     Embodiment 2 
     In this embodiment, although the same image forming apparatus as that of embodiment 1 is used, there is a difference that an execution condition of forcible toner consumption is stepwise set based on a variation amount of a parameter. For example, image formation is performed as shown in a flowchart of  FIG. 6 . 
     Similarly to embodiment 1, image formation is performed under a specified image formation condition (ACT  2 - 1 ), the sensor  11  acquires an environmental value, such as a relative humidity, at the time of final image formation, and the clock  12  acquires the date and time of the final image formation. They are stored in the memory  14  in the image forming apparatus  10  (ACT  2 - 2 ). Thereafter, the power source of the image forming apparatus  10  is turned OFF. 
     After being left standing for one week, the image forming apparatus is returned from the sleep state by turning ON the power source thereof (ACT  2 - 3 ). Before image formation is again performed, for example, the present relative humidity and the date and time are similarly acquired and are stored in the memory  14  in the image forming apparatus (ACT  2 - 4 ). 
     In the arithmetic unit  15 , a humidity change as a variation of a parameter value and a non-operation time are calculated from the relative humidity and the date and time at the time of final image formation and those at the present time, which are stored in the memory  14  (ACT  2 - 5 ). The necessity of execution of the forcible toner consumption is determined for each of the parameters (ACT  2 - 6 ). For example, when the humidity change is a specified value or more, or the non-operation time is a specified time or more, it is determined that the execution is necessary. 
     With respect to the parameter for which it is determined that the execution is necessary, a toner replacement amount as an execution condition is obtained (ACT  2 - 7 ). The toner replace amount can be obtained from tables as shown in  FIGS. 7A and 7B , each of which includes the toner replacement amount corresponding to each parameter value and is previously stored in the memory  14 . 
     The toner replacement amounts for the respective parameters are compared with each other, and a larger one (higher level) is selected, so that the toner replacement amount is determined (ACT  2 - 8 ), and the forcible toner consumption is executed (ACT  2 - 9 ). At this time, for example, an image contrast potential is made constant, a specified pattern such as a solid fill is formed on A3 transfer media the number of which corresponds to the toner replacement amount, and a specified amount of toner is discharged. A consumed amount of developer is newly supplied, so that the toner in the developing device to supply the developer is partially replaced. 
     After the forcible toner consumption is completed, similarly to embodiment 1, calibration is performed before image formation (ACT  2 - 10 ). Then, the image formation is performed in the image forming section  16  under the optimized image formation condition (ACT  2 - 11 ). 
     As described above, the suitable forcible toner consumption can be easily performed by obtaining the execution condition from the tables of the toner replacement amounts corresponding to the values of the respective parameters. In the subsequent continuous image formation, the change of image density or the like is suppressed, and the stable image formation can be performed. 
     In embodiments 1 and 2, it is preferable that the forcible toner consumption is automatically performed at every start of image formation in order to maintain the picture quality. Besides, the forcible toner consumption may be performed by forming an image pattern corresponding to a previously set replacement toner amount at a specified time. For example, this is effective for a case where after a small number of sheets are intermittently printed for a long period, continuous printing is performed with high picture quality. 
     Embodiment 3 
     In this embodiment, although the same image forming apparatus as that of embodiment 1 is used, there is a difference that not an execution condition for forcible toner consumption, but an image formation condition updated during image formation is estimated. 
     Similarly to the estimation of the execution condition of the forcible toner consumption of embodiment 1, as shown in a flowchart of  FIG. 8 , image formation is performed under the initial image formation condition of a development contrast potential or the like (ACT  3 - 1 ). At the time of start of image formation, as initial parameter values, an initial environmental value such as a relative humidity is acquired by the sensor  11 , an initial date and time is acquired by the clock  12 , and an initial developer life is acquired by the counter  13  (ACT  3 - 2 ), and the values are stored in the memory  14 . 
     When a specified time elapses or a specified number of sheets are processed from the start of image formation, parameter values such as an environmental value, a date and time, and a developer life are similarly acquired (ACT  3 - 3 ). Similarly to embodiment 1, the arithmetic unit  15  estimates the presently optimum image formation condition based on the correlation between the image formation condition and each parameter, the initial image formation condition and initial parameter values, which are previously obtained and stored in the memory  14 , and the acquired parameter values (ACT  3 - 4 ). 
     The image formation condition in the image forming section  16  is updated to the estimated condition, and the image formation is continued (ACT  3 - 5 ). 
     Even in the image formation such as continuous printing, the environmental value such as the relative humidity and the developer life are changed. Besides, the toner charging amount is changed also by the image formation time. Thus, until now, calibration is performed at intervals of a specified time or a specified number of sheets. However, for the calibration, it is necessary to once stop the apparatus for a certain time. 
     According to the embodiment, the respective parameter values are acquired during the image formation, and the image formation condition can be updated based on these parameter values. Thus, the image formation can be continued without stopping the apparatus for a long time. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omission, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.