Patent Publication Number: US-9417566-B2

Title: Image forming apparatus and system operable in ghost-suppression mode

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus which uses an electrophotographic image forming method, and also, an image formation system comprising an image forming apparatus, and a controlling device (control section) which is driven by a printer driver in an information processing peripheral device (apparatus). 
     In recent years, electrophotographic image forming apparatuses which are capable of forming multicolor images or full-color images, have come into general use. Regarding the configuration of a color image forming apparatus, there is an image forming apparatus of the so-called tandem type. In the case of an image forming apparatus of the tandem type, multiple photosensitive members which are different in the color of the image to be formed thereon, are aligned in tandem, and multiple toner images, different in color, formed on the peripheral surfaces of the photosensitive drums are sequentially layered upon an intermediary transferring member, or a sheet of recording medium borne by a recording medium bearing member. 
     As a method for charging the photosensitive drums of an electrophotographic image forming apparatus, a charging method which charges an electrophotographic photosensitive member by placing a charging member in contact with, or in the adjacencies of, the peripheral surface of the photosensitive member, and applying electrical voltage to the charging member, has come into wide use, because of its merits that this charging method makes it possible to reduce in voltage the electrical power source for the charging device, and also, is relatively small in the amount of ozone generation. Among various charging methods which are basically the same as the above described charging method, the so-called “DC charging method” which applies only DC voltage to the charging member to charge a photosensitive member, is advantageous in terms of operational cost as well as initial cost, compared to the so-called “AC charging method” which applies a combination of DC and AC voltages to the charging member to charge a photosensitive member, for the following reason. That is, “DC charging method” is small in the amount of electrical discharge between a charging member and a photosensitive member, and therefore, is smaller in the amount by which the peripheral surface of the photosensitive member is shaved, than the “AC charging method”. Thus, “DC charging method” can contribute to extending a photosensitive member in service life. Further, the former is advantageous over the latter, in that the former does not require an AC power source. 
     However, “DC charging method” is inferior to “AC charging method” in terms of the uniformity in the electrical charge of the peripheral surface of the photosensitive member, because, unlike the AC charging method, “DC charging method” cannot make the peripheral surface of a photosensitive member uniform in potential. More concretely, the so-called “transfer ghost” is more likely to occur when “DC charging method” is used, than when the “AC charging method” is used. “Transfer ghost” is such a phenomenon that as a photosensitive member is rotated, the portions of the peripheral surface of the photosensitive member, which did not have toner when they are in the transferring section, become different in surface potential level from the portions of the peripheral surface of the photosensitive member, which had toner when they are in the transferring section, after the transfer of the toner in the transferring section. This nonuniformity in surface potential level is not eliminated in the charging section; that is, the peripheral surface of the photosensitive member remains nonuniform in potential level even after it is charged by the charging member. In other words, even after the charging of the photosensitive member, the photosensitive member remains nonuniform in potential level. Consequently, images, which suffer from unwanted nonuniformity in density, are outputted. 
     There is disclosed in Japanese Laid-open Patent Application 2002-189400 a technology for preventing the occurrence of the transfer ghost. According to this patent application, after the transfer process, the peripheral surface of a photosensitive member is exposed to light by a pre-exposing device to reduce the surface potential level of the photosensitive member to virtually 0 V, that is, to make the peripheral surface of the photosensitive member virtually uniform in potential level at roughly 0 V, in order to minimize the peripheral surface of the photosensitive member in nonuniformity in terms of potential level. A transfer ghost is more conspicuous in a case where a certain area of the peripheral surface of a photosensitive member is greater in the amount of toner than the other area, since toner functions as electrical resistor in the transferring section. For example, in the case of an image forming apparatus of the tandem-type, a transfer ghost which is generated in the downstream transferring section, in terms of the direction in which the sheet of recording medium is conveyed, as a toner image of the secondary color, which was formed on a transferring medium in the transferring section of the upstream image forming section, is likely to be conspicuous, in particular, in a case where a toner image of the secondary color is a solid image made up of two toners which are different in color. For example, a solid red image made up of a combination of a yellow monochromatic image and a magenta monochromatic image is likely to generate a transfer ghost in a half-tone area of an image, which is cyan or black in color, and which is formed in the downstream image forming sections. 
     Here, as described above, the occurrence of a transfer ghost such as the above described one can be prevented by discharging the peripheral surface of a photosensitive member with the use of a charging means such as a pre-exposing device. In such a case, however, not only the charge current necessary to charge the areas of the peripheral surface of the photosensitive member, which did not have toner in the transferring section, increase, but also, the charge current necessary to charge the areas of the peripheral surface of the photosensitive member, which had toner, increases. Consequently, the peripheral surface of a photosensitive member becomes more susceptible to shaving. Thus, this method is likely to reduce a photosensitive member in service life. Further, in order to provide an image forming apparatus with a charging removing means such as a pre-exposing device, a space is necessary between the cleaning section for cleaning the photosensitive member and the charging section, or between the transferring section and charging section. Thus, it is likely that the image forming apparatus has to be increased in size by the amount equal to the size of the space. Moreover, it is likely that the image forming apparatus has to be increased in cast by the amount necessary to provide the apparatus with a charge removing means such as the pre-exposing device. 
     Therefore, an image forming apparatus which uses a DC charging method, and yet, can prevent the occurrence of a transfer ghost without being provided with a charge removing means such as a pre-exposing device, is desired. 
     Further, normally, a transfer ghost is likely to occur only under a certain condition such as the above-described ones (certain area of multicolor image has uniform in color and has secondary color). Ordinarily, therefore, image forming apparatuses are not primarily set to prevent the occurrence of a transfer ghost. 
     SUMMARY OF THE INVENTION 
     Thus, it is sometimes desired that the setting for preventing the occurrence of a transfer ghost is added to the normal settings for the apparatus. Therefore, it is desired that an image forming apparatus can be simply switched in operational setting to the one for preventing the occurrence of a transfer ghost, as necessary. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention. 
         FIG. 2  is a schematic sectional view of a combination of the charge roller and photosensitive drum in the first embodiment, and shows the laminar structure of the charge roller and that of the photosensitive drum. 
         FIG. 3  is a drawing which shows the operational sequence of the image forming apparatus in the first embodiment. 
         FIG. 4  is a block diagram of the image formation system in the first embodiment, and shows the configuration of the system. 
         FIG. 5  is a schematic drawing of an example of the display portion of the control panel of the image forming apparatus in the first embodiment. 
         FIG. 6  is a flowchart for describing the operation of the image forming apparatus in the first embodiment. 
         FIG. 7  is a schematic drawing of an example of a combination of the touch screen (touch panel) of the printer driver of the information processing peripheral device, and the control screen (touch panel) of the image forming apparatus, in the first embodiment. 
         FIG. 8  is a flowchart for describing the operational sequence of the image forming apparatus in the second embodiment of the present invention. 
         FIG. 9  is a flowchart for describing the operational sequence of the image forming apparatus in the third embodiment of the present invention. 
         FIG. 10  is a schematic drawing of an example of an image having a transfer ghost. 
         FIG. 11  is a schematic drawing for describing the mechanism of the occurrence of a transfer ghost. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, the image forming apparatuses and image formation systems, which are in accordance with the present invention, are described in detail with reference to the appended drawings. 
     Embodiment 1 
     1. Overall Structure and Operation of Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of the image forming apparatus  100  in the first embodiment of the present invention. The image forming apparatus  100  has multiple image forming sections, more specifically, the first, second, third, and fourth image forming stations SY, SM, SC and SK, which form yellow (Y), magenta (M), cyan (C) and black (K) monochromatic images, respectively. These four stations are aligned with preset intervals. 
     By the way, in this embodiment, the four stations SY, SM, SC and SK are practically the same in structure and operation, although they are different in the color of the toner they use. Hereafter, therefore, unless they need to be differentiated, they are described together; suffixes Y, M, C and K which indicate the color of the image they form are eliminated. Further, in a case where it is necessary for the first, second, third, and fourth stations, and the components thereof, to be separately described, they may be provided with suffixes “Y”, “M”, “C” and “K” which correspond to the color of the toner images they form. 
     The station S has a photosensitive drum  1 , as an image bearing member, which is an electrophotographic rotatable photosensitive member (photosensitive member). Further, the station S has the following processing devices, which are disposed in the adjacencies of the peripheral surface of the photosensitive drum  1 . The first one is a charge roller  2 , as a charging means, which is a charging member in the form of a roller. The second is an exposing device  3  as an exposing means. The next is a developing device  4  as a developing means. The next is a primary transfer roller  5 , as a primary transferring means, which is the primary transferring member in the form of a roller. The last one is a drum cleaning device  6  as a means for cleaning the photosensitive drum  1 . 
     The image forming apparatus  100  has also an intermediary transfer belt  7  as an intermediary transferring member, which is in the form of an endless belt. The intermediary transfer belt  7  is disposed so that it opposes the corresponding photosensitive drum  1  in each station S. It is wrapped around a combination of multiple belt supporting rollers, more specifically, a driver roller  71 , a tension roller  72 , and a belt backing roller  73  (which opposes secondary transfer roller), being thereby suspended (supported), and also, being provided with a preset amount of tension. The abovementioned primary rollers  5  are disposed on the inward side of the loop which the intermediary transfer belt  7  forms, and also, are disposed so that they oppose the corresponding photosensitive drums  1 , one for one. Each primary transfer roller  5  is pressed against the corresponding photosensitive drum  1 , with the placement of the intermediary transfer belt  7  between itself and the photosensitive drum  1 , forming thereby the primary transferring section T 1  (primary transfer nip), in which the intermediary transfer belt  7  contacts the photosensitive drum  1 . Moreover, on the outward side of the loop which the intermediary transfer belt  7  forms, the secondary transfer roller  8  is disposed, as a secondary transferring means, which is the secondary transferring member which is in the form of a roller. The secondary transfer roller  8  remains pressed toward the belt backup roller  73  with the placement of the intermediary transfer belt  7  between itself and the belt backing roller  73 , forming thereby the secondary transferring section T 2  (secondary transfer nip) in which the secondary transfer roll contacts the intermediary transfer belt  7 . Further, on the outward side of the loop which the intermediary transfer belt  7  forms, the belt cleaning device  30  as a means for cleaning the intermediary transfer belt  7 , is disposed so that it opposes the driver roller  71 . 
     In this embodiment, the photosensitive drum  1  is 30 mm in diameter, 330 mm in length in terms of the direction parallel to its rotational axis. It is a negatively chargeable organic member (OPC). Referring to  FIG. 2 , the photosensitive drum  1  comprises: an aluminum cylinder  1   p  (as electrically conductive substrate); an undercoat layer  1   q  which is coated on the peripheral surface of the aluminum cylinder for preventing optical interference and improving the photosensitive drum  1  in terms of adhesion between the aluminum cylinder  1   p  and the layer on the undercoat layer  1   q ; and an optical charge generation layer  1   r  coated on the undercoat layer; a charge transfer layer is coated on the optical charge generation layer  1   r , listing from the inward side of the photosensitive drum  1 . Normally, the photosensitive drum  1  is rotationally driven by a driving device (unshown) at a process speed (peripheral velocity) of 210 mm/sec in the direction indicated by an arrow mark R 1  in the drawing. 
     As the photosensitive drum  1  is rotationally driven, the peripheral surface of the photosensitive drum  1  is uniformly charged by the charge roller  2  to preset a polarity (negative in this embodiment) and a preset potential level. During this process, a charge bias (charge voltage) is applied to the cleaning roller  2  from a charge voltage power source  20  (high voltage power source circuit). The charge voltage power source has a DC voltage generation circuit  21  and a DC voltage amplification circuit  22 . In this embodiment, the DC voltage to be applied to the charge roller  2  in each station S is generated by voltage generation circuit  21 , with which each station S is provided. The amount of the DC voltage to be applied to the cleaning roller  22  of each station S is adjusted by the DC voltage amplification circuit  22 , with which each station S is provided. In this embodiment, a DC charging method is employed as the method for charging the photosensitive drum  1 , as described above. In this embodiment, the charge bias is −1300 V of DC voltage, and a potential level Vd of an unexposed point of the peripheral surface of the photosensitive drum  1  is −700 V in the developing position. 
     In this embodiment, the cleaning roller  2  is 230 mm in length in terms of the direction parallel to the axial line of the cleaning roller  2 . Referring to  FIG. 2 , the cleaning roller  2  comprises: a metallic core  2   p  (supporting member) and three layers, more specifically, an undercoat layer  2   q , an intermediary layer  2   r , and a surface layer  2   s , which are coated in layers in the listed order on the peripheral surface of the metallic core  2   q . The undercoat layer  2   q  is formed of foamed sponge for reducing charging noises. The surface layer  2   s  is a protective layer provided to preventing the occurrence of leak even if the photosensitive drum  1  has defects such a pinholes. More concretely, the specifications of the cleaning roller  2  in this embodiment are as follows: 
     Metallic core  2   p : a piece of round stainless rod which is 6 mm in diameter 
     Undercoat layer  2   q : foamed EPDM in which carbon particles were dispersed, and which is 0.5 g/cm 3  in specific gravity, 10 2 -10 9 Ω in volume resistivity, and 3.0 mm in thickness 
     Intermediary layer  2   r : NBR rubber in which carbon particles were dispersed, and which is 10 2 -10 5 Ω in volume resistivity, and 700 μm in thickness 
     Surface layer  2   s : fluorine resin in which stannic oxide particles and carbon particles were dispersed, and which is 10 7 -10 10 Ω in volume resistivity, 1.5 μm in surface roughness (10 point surface roughness Ra in JIS, and 10 μm in thickness. 
     The charge roller  2  is kept pressed toward the rotational axis of the photosensitive drum  1  by a pair of compression springs  2   t , in such a manner that a preset amount of contact pressure is maintained between the charge roller  2  and photosensitive drum  1 , forming thereby a charging nip a, that is, the area of contact between the photosensitive drum  1  and charge roller  2 . Further, the charge roller  2  is rotated by the rotation of the photosensitive drum  1  in the direction indicated by an arrow mark R 2  in the drawing. In this embodiment, the overall volume resistivity of the charge roller  2  is 1.0×10 5 Ω. 
     The charged photosensitive drum  1  is scanned by (exposed to) a beam of light projected by the exposing device  3  while being modulated according to the information of the image to be formed. In this embodiment, the exposing device  3  is a laser beam scanner which employs a semiconductor laser. The exposing device  3  outputs a beam of laser light while modulating the beam with the image formation signals inputted from a host processing device such as an image reading device. The beam of laser light scans the charged peripheral surface of the photosensitive drum  1 . Consequently, an electrostatic latent image (electrostatic image) which reflects the inputted image formation signals, is effected upon the peripheral surface of the photosensitive drum  1 . In this embodiment, as a given charged point of the peripheral surface of the photosensitive drum  1  is illuminated with the beam of laser light, its potential level V 1  becomes −200 V. 
     The electrostatic latent image formed on the peripheral surface of the photosensitive drum  1  is developed (into visible image) by the developing device  4  which uses toner as developer. The developing devices  4 Y,  4 M,  4 C and  4 K contain yellow, magenta, cyan and black toners, respectively. Each developing device  4  has a driver roller, as a developer bearing member, which conveys toner to an area of development where the developing device  4  opposes the photosensitive drum  1 . To the development roller, a development bias (development voltage) which is a combination of a DC voltage (Vdc) and an AC voltage (Vac) is applied. More concretely, in this embodiment, the development bias is an alternating voltage which is a combination of −500 V of DC voltage and AC voltage which is 1800 V in peak-to-peak voltage and 8 kHz in frequency. In this embodiment, a combination of the exposing device  3  and developing device  4  makes up a toner image forming means which forms a toner image on the charged peripheral surface of the photosensitive drum  1 . 
     The toner image formed on the photosensitive drum  1  is transferred (primary transfer) by the function of the primary transfer roller  5 , in the primary transferring section T 1 , onto the intermediary transfer belt  7  which is rotationally driven in the direction indicated by an arrow mark R 3  in the drawing. During this process, a primary transfer bias (primary transfer voltage) which is DC voltage and is opposite in polarity from the polarity (normal polarity) of the toner is applied to the primary transfer roller  5  from a primary transfer voltage power source  51 . In this embodiment, the primary transfer bias is set so that the amount of the primary transfer current which flows to the primary transfer roller  5  (primary transferring section T 1 ) during the primary transfer becomes roughly 20 μA. 
     Here, in this embodiment, the primary transfer bias is controlled so that it remains stable at a preset level during the primary transfer process. More concretely, in this embodiment, the primary transfer bias is controlled so that the amount by which electrical current flows to the primary transfer roller  5  (primary transferring section T 1 ) during the pre-rotation period, which will be described later, becomes a stable target value. Further, it is based on this target current value that the target value for the primary transfer bias for the primary transfer is obtained. Thus, during the primary transfer, the primary transfer bias is controlled so that it becomes stable at its target value. The target value for the primary transfer bias may be the value of the voltage applied to make the transfer current stable at a preset level, or a value obtained with the use of a mathematical expression, based on the value for making the transfer current stable, a lockup table, or the like. This type of control is well-known as ATVC in the field of printing device, and therefore, is not described in detail here. 
     For example, during an image forming operation for forming full-color images, four monochromatic toner images, different in color, are formed on the photosensitive drums  1  in the four stations SY, SM, SC and SK, one for one, and are sequentially transferred in layers onto the intermediary transfer belt  7  in the primary transferring sections T 1 . Consequently, the four monochromatic toner images, different in color, which are for forming a full-color image, are layered on the intermediary transfer belt  7 . 
     The toner images formed on the intermediary transfer belt  7  are transferred (secondary transfer) onto a sheet of recording medium such as recording paper, by the function of the secondary transfer roller  8 , in the secondary transferring section T 2 . During this process, the secondary transfer bias (secondary transfer voltage) which is DC voltage and is opposite in polarity from the toner charge (normal polarity of charged toner) is applied to the secondary transfer roller  8  from a secondary transfer voltage power source  81 . Each sheet P of recording medium is conveyed to the secondary transferring section T 2  by a pair of conveyance rollers  11 , etc., of a recording medium feeding device with such a timing that it arrives at the secondary transferring section T 2  at the same time as the toner images on the intermediary transfer belt  7 . 
     After the transfer of the toner images onto the sheet P of recording medium, the sheet P is separated from the intermediary transfer belt  7 , and is conveyed to a fixing device  9  as a fixing means. The fixing device  9  has a fixation roller  9   a  and a pressure roller  9   b  which form a fixation nip between themselves. It applies heat and pressure to the sheet P and the toner images thereon, by conveying the sheet P through the fixation nip while keeping the sheet P and the images thereon pinched by the fixation roller  9   a  and pressure roller  9   b . Thus, the toner images are melted and mixed. Then, as they cool down, they become fixed to the sheet P. After the fixation of the toner images to the sheet P, the sheet P is discharged from the main assembly of the image forming apparatus  100 . 
     The toner (primary transfer residual toner) remaining on the peripheral surface of the photosensitive drum  1  after the primary transfer is removed from the peripheral surface of the photosensitive drum  1  by the drum cleaning device  6 , and is recovered. Moreover, the toner (secondary transfer residual toner) remaining on the outward surface of the intermediary transfer belt  7  after the secondary transfer is removed from the outward surface of the intermediary transfer belt  7  by the belt cleaning device  30 , and is recovered. 
     By the way, in this embodiment, the image forming apparatus  100  is not provided with a discharging means, such as a pre-exposing device, which is to be disposed on the downstream side of the primary transferring section T 1  and on the upstream side of the charging section in which the peripheral surface of the photosensitive drum  1  is charged by the charge roller  2 , in terms of the rotational direction of the photosensitive drum  1 . 
     2. Operational Sequence 
       FIG. 3  is a drawing of the operational sequence of the image forming apparatus  100 . 
     a. Initialization Rotation Period (Preparatory Multiple Rotation Period) 
     The initialization period is an operational period which occurs immediately after the image forming apparatus  100  is started up (startup period, startup operation period, warm-up period). In the initialization period, as the electric power source of the image forming apparatus  100  is turned on, the photosensitive drum  1  begins to be rotationally driven, and operations for preparing preset processing devices for image formation, such as starting up the fixing device  9  (increasing temperature of fixing means of fixing device  9  to preset level), are carried out. 
     b. Preparatory Rotation Period for Printing (Pre-Rotation Period) 
     The preparatory period for printing is a period between when a print signal (signal for starting image formation) is inputted into the image forming apparatus  100  and when the operation for actually forming images is started. In a case where a print signal is inputted during the initialization rotation period, the preparatory process for printing is carried out as soon as the initial rotation process is completed. In a case where no print signal is inputted during the initialization rotation period, the driving of the main motor is temporarily stopped after the completion of the initialization rotation process, the rotational driving of the photosensitive drum  1  is stopped, and the image forming apparatus  100  is kept on standby until a print signal is inputted. Then, as a print signal is inputted, the preparatory rotation process for printing is carried out. 
     c. Printing Process (Image Formation Process) 
     The printing process corresponds to the very period in which a toner image is formed on the photosensitive drum  1 , the toner image is transferred onto a sheet P of recording medium, the toner image is fixed to the sheet P. To describe in detail, the charging, exposing, developing, primary transferring, secondary transferring, and fixing processes in the printing process are different in the timing with which they are carried out. In the continuous printing mode, the above-described printing process is repeatedly carried out by a number of times which corresponds to a preset print count (n=3 in  FIG. 3 ). 
     d. Sheet Interval 
     The sheet interval is one of the periods which occur when the image forming apparatus  100  is in the continuous printing mode. It is a period between when the trailing edge of a sheet P of recording medium passes the transferring section and when the leading edge of the following sheet P of recording medium arrives at the transferring section. That is, it is a period in which no sheet P is in the transferring section. 
     e. Post-Rotation Period (Process) 
     The post-rotation process corresponds to the period which follows the outputting of the last print (last sheet P of recording medium) in the printing process. It corresponds to the period in which the photosensitive drum is rotationally driven for a while for the post-rotation operation after the completion of the printing process. 
     f. Standby Period 
     As the preset post-rotation is ended, the driving of the main motor is stopped, and the rotational driving of the photosensitive drum  1  is stopped. Then, the image forming apparatus  100  is kept on standby until the next print signal is inputted. In a case where only a single print needs to be made, the image forming apparatus  100  is put through the post-rotation process after the outputting of the single print. Then, it is put on standby. If a print signal is inputted while the image forming apparatus  100  is kept on standby, the image forming apparatus  100  begins to be rotated to be prepared for the very process for printing. 
     The above-described printing process c corresponds to the image formation period, whereas the above described initial rotation process a, printing pre-rotation process b, sheet interval d, and post-rotation process e correspond to the periods in which no image is formed. By the way, an operational sequence which is initiated by a printing signal to form an image on a single sheet P of recording medium, or images on two or more sheets P of recording medium, and which comprises the above descried printing preparation rotation process, printing process, sheet interval, post-rotation process, etc., may be referred to as an image outputting operation (job). 
     3. Control Sequence 
       FIG. 4  is a block diagram of an image formation system which comprises the image forming apparatus  100 , and a personal computer  300  (which hereafter may be referred to simply as “PC”), and which is controlled by a printer driver. 
     The image forming apparatus  100  has a printer engine  110  in its main assembly. The printer engine  110  is one of the primary structural components for forming an image on a sheet P of recording medium, and outputting the formed image. It comprises each of the above described image forming sections S, intermediary transfer belt  7 , fixing device  9 , etc. Further, the image forming apparatus  100  has a controlling section  120  in its main assembly. The controlling section  120  controls the entirety of the image forming operation of the image forming apparatus  100 . Further, image forming apparatus  100  has a control panel  200 , through which an instruction to start an image outputting operation, and settings for the image outputting operation, are inputted, and on which information is displayed. Further, the image forming apparatus  100  is provided with an unshown image reading device (image scanner). 
     The image forming apparatus  100  is in connection to a PC  300  as an information processing peripheral device. In this embodiment, the PC  300  is connected to the image forming apparatus  100  through a LAN cable  302 , interface of the image forming apparatus  100 , and interface  320  of the PC  300 , in such a manner that communication is possible between the PC  300  and image forming apparatus  100 . By the way, it is not mandatory that the image forming apparatus  100  and PC  300  are connected to each other by wire. That is, the image forming apparatus  100  and PC  300  may be in connection to each other by wireless communicating means. 
     The PC  300  has a main assembly  310  as the primary structural component. The main assembly  310  may be an ordinary computer which comprises a computing device and storage sections. It is operated (controlled) by a basic operating system (OS). Further, the PC  300  has a display  301  as an information displaying section, such as an LCD display, an inputting section  304  such as a keyboard, a mouse, etc. Further, the PC  300  contains optional application software  311  (application program) such as a word processor, which operates on the basic OS. Further, the PC  300  contains a printer driver  312  (driver program) which operates on the basic OS. The printer driver  312  controls the image forming apparatus  100  by transmitting commands (information regarding image to be formed, information regarding settings for image forming operation) related to an image outputting operation, to the controlling section  120 . 
     With the presence of the above described system, not only is the image forming apparatus  100  enabled to function as a copying machine to read an original and create a copy of the read original, but also, function as a printer to form an image which reflects the information inputted from the PC  300  regarding an image to be formed. 
     In this embodiment, the controlling section  120  functions as a toner amount adjusting means which reduces the amount by which toner is adhered to the peripheral surface of the photosensitive drum  1  per unit area, to effect the secondary color, in the upstream image forming section(s), to a value which is less than the standard value. Further, the controlling section  120  functions as an electric current adjusting means which increases the amount by which the primary transfer current is supplied to the primary transfer roller(s) in the downstream image forming section(s), to a value which is more than the standard value. 
     4. Transfer Ghost 
       FIG. 10  is a schematic drawing of an image having transfer ghosts. It shows a phenomenon that in a case where an image is formed on a sheet of recording paper by forming first a solid red R image by layering yellow (Y) and magenta (M) solid toner images on the sheet P of recording paper, and then, layering a solid cyan (C) halftone image on the same sheet P of recording paper, the portion of the solid cyan (C) halftone area of the resultant image, which corresponds to the portion of the peripheral surface of the photosensitive drum  1 , which corresponded to the formation of the solid red R image, appears less in density. 
     This phenomenon is described further with reference to  FIG. 11 . Part (a) of  FIG. 11  is a schematic drawing of the image forming sections SY, SM, SC and SK, and shows their structure. Part (b) of  FIG. 11  shows the relationship between the surface potential level (which hereafter may be referred to as “post-transfer potential level”) of a given point of the peripheral surface of the photosensitive drum  1 C in the station C, after the passage of the given point through the primary transfer section T 1 C, and that after charging by charge roller  2 C (which hereafter may be referred to as “post-charging potential level”). The vertical axis of part (b) of  FIG. 11  is graduated so that the greater (in absolute value of negative potential level) the peripheral surface of the photosensitive drum  1  is, the higher the peripheral surface of the photosensitive drum  1  is in potential level. 
     Referring to part (a) of  FIG. 11 , as the solid red R image formed by the station SY and SM is conveyed by the intermediary transfer belt  7  through the primary transfer section T 1 C of the station SC, the portion of the peripheral surface of the photosensitive drum  1 C, which corresponds to the portion of the intermediary transfer belt  7 , which has the solid red R image, becomes higher in potential level (negatively) compared to the portion of the peripheral surface of the photosensitive drum  1 C, which does not correspond in position to the solid red R image on the intermediary transfer belt  7 , as shown in part (b) of  FIG. 11 , for the following reason. That is, the toners, of which the solid red R image is formed, function as an electrical resistor, reducing thereby the amount by which the primary transfer current is flowed by the primary transfer voltage through this portion of the peripheral surface of the photosensitive drum  1 . Thus, this portion of the peripheral surface of the photosensitive drum  1  does not fully reduce in potential. After this portion of the peripheral surface of the photosensitive drum  1 C comes out of the primary transferring section T 1 C, it is charged by the charge roller  2 C. However, the hysteresis of the difference in potential level between the portion of the peripheral surface of the photosensitive drum  1 C, which corresponds in position to the red R image on the intermediary transfer belt  7 , and the other portion of the peripheral surface of the photosensitive drum  1 C, slightly remains even after the peripheral surface of the photosensitive drum  1 C is charged by the charge roller  2 C. This difference in potential level between the two portions makes the corresponding two portions of the resultant image different in density (tone), creating an effect of the presence of the ghost of the red R image. This is the “transfer ghost”. 
     As described above, a “transfer ghost” is such a phenomenon that is attributable to the phenomenon that in the case of an image forming apparatus of the so-called tandem type, the toner transferred onto the intermediary transfer belt  7  in the upstream stations S in terms of the moving direction of the intermediary transfer belt  7  makes the peripheral surface of the downstream photosensitive drum  1  nonuniform in the post-transfer potential level (that is, the pre-charging potential level). Thus, the greater the amount by which toner is borne by the intermediary transfer belt  7  in the upstream stations S, the more conspicuous the resultant transfer ghost. Therefore, an image of the red R color, which is the secondary color, formed of yellow (Y) and magenta (M) toners, for example, has an effect upon the portions of the final image, which have cyan (C) and/or black (K) color. Similarly, a green (G) image, that is, an image of the secondary color, which is formed of yellow (Y) and cyan (C) toners, or a blue (B) image, that is, an image of the secondary color, which is formed of magenta (M) and cyan (C), are likely to have an effect upon a black (K) image. 
     Table 1 shows the relationship among the maximum amount (%) of toner on the area of an image, which has the secondary color, transfer ghost, and chroma of the secondary color. Here, an example of the maximum amount of toner of the secondary color R area is the sum of the amount by which yellow (Y) toner was borne by the intermediary transfer belt  7  in the station SY, or the first station, and the amount by which magenta (M) toner was borne by the intermediary transfer belt  7  in the station SM, or the second station. In this embodiment, the maximum amount of toner for red R color, or the secondary color, was set to 1.0 (mg/cm 2 ), which corresponds to 200% in Table 1. Regarding image evaluation in terms of transfer ghost, test images such as those shown in  FIG. 10  were outputted, and were visually (subjectively) evaluated. “◯” in Table indicates that the transfer ghost did not occur, and “Δ” indicates that the transfer ghost occurred, but is not problematic in severity in practical terms. “X” indicates that the transfer ghost which was problematic in severity occurred. As for the evaluation of the images in terms of the chroma of the secondary color, test images such as those shown in  FIG. 10  were outputted, and were visually (subjectively) evaluated. “◯” in Table 1 indicates that the images had satisfactorily high chroma, and “Δ” indicates that the images were slightly inferior in chroma, but were not problematic in practical terms. “X” indicates that the images were so low in chroma that they were problematic in practical terms. The maximum amount of toner for the secondary color R can be adjusted by adjusting the amount by which the photosensitive drum  1  is exposed by the exposing device  3  to form monochromatic yellow (Y) and magenta (M) toner images. 
     Further, Table 2 shows the relationship among the amount of the primary transfer current, transfer ghost, and coarseness which is described later, in this embodiment. Here, the primary transfer current is the primary transfer current in the station SC, or the third station in terms of the moving direction of the intermediary transfer belt  7 , and the first transfer current in the station SK, or the fourth station S. As for the evaluation in terms of coarseness, test images such as the one in  FIG. 10  were outputted, and were visually (subjectively) evaluated. “◯” in Table 2 indicates that the images did not appear coarse, and “Δ” indicates that the images appeared somewhat coarse, but were not appear coarse enough to be problematic in practical terms. “X” indicates that the images appeared coarse enough to be problematic. 
     In the case of the image forming apparatus  100  in this embodiment, the normal setting (default setting, referential setting), that is, the pre-adjustment setting, for the maximum amount of toner for red R color (secondary color) was 200%, and the amount (target current value) of the primary transfer current for the station SC for color (C), and that for the station SK for color (K) was 20 μA. The values in Table 2 were those obtained by making adjustments so that the amount of the primary transfer current in the station SC for color (C) and that in the station SK for color (K) became 20 μA. By the way, the results shown in Table 2 were those obtained by making adjustment so that the maximum amount by which toners were to be borne for color R or the secondary color. By the way, these normal settings are for an environment which is normal in temperature and humidity. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 2ry Color max. 
                 Transfer 
                 Chroma of 
               
               
                 amount (%) 
                 ghost 
                 2ry color 
               
               
                   
               
             
            
               
                 200 
                 X 
                 ◯ 
               
               
                 180 
                 X 
                 ◯ 
               
               
                 160 
                 X 
                 Δ 
               
               
                 140 
                 ◯ 
                 X 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 1ry transfer 
                 Transfer 
                   
               
               
                 current (μA) 
                 ghost 
                 Coarseness 
               
               
                   
               
             
            
               
                 20 
                 X 
                 ◯ 
               
               
                 22 
                 X 
                 ◯ 
               
               
                 24 
                 X 
                 Δ 
               
               
                 26 
                 Δ 
                 X 
               
               
                 28 
                 ◯ 
                 X 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, as the maximum amount of toners for color red R, which is the secondary color, the amount of toners which function as electrical resistor in the primary transferring station T 1  in the station SC, and that in the station SK, reduces. Thus, it becomes easier for the primary transfer current to flow when the primary transfer bias is applied. Consequently, it becomes less likely for a transfer ghost to appear, or a transfer ghost is likely to be less conspicuous even if it appears. However, as the maximum amount of toners for color R which is the secondary color is reduced, the area of the image, which has color R, reduces in chroma. 
     Further, referring to Table 2, as the stations SC and SK for the colors C and K, respectively, are increased in the amount of the primary transfer current, it becomes easier for the primary transfer voltage to flow even with the presence of the toners between the photosensitive drum  1  and intermediary transfer belt  7 . Thus, it is less likely for a transfer ghost to appear, or a transfer ghost is likely to be less conspicuous even if it appears. However, as the stations SC and SK for colors C and K, respectively, are increased in the amount of the primary transfer current, images are likely to suffer from an image defect called “coarseness”, the extent of which is proportional to the amount by which the primary transfer current is increased. “Coarseness” is an image defect attributable to a phenomenon that after toner is transferred from the photosensitive drum  1  onto the intermediary transfer belt  7 , it is reversed in polarity (made positive) in the primary transferring section T 1 , and then, is returned (transferred back) to the photosensitive drum  1 . 
     Decreasing the maximum amount of toners for color R which is the secondary color, and increasing the primary transfer current in the stations SC and SK (which hereafter will be referred to as “downstream stations S”) for colors C and K, respectively, are both effective to prevent the occurrence of a transfer ghost, or reducing the level of conspicuousness at which a transfer ghost appears. However, employing only one of the above described tactics made it difficult to achieve both objectives, that is, prevention of the occurrence of a transfer ghost, or minimization of the level of conspicuousness at which the transfer ghost appears, and prevention of the occurrence of the above described image defects (reduction in chroma of secondary colors, and coarseness). 
     Table 3 shows the relationship among the maximum amount of toners for the secondary color, amount of the primary transfer current, transfer ghost, chroma of the secondary color, and coarseness, which occurred when the maximum amount of toners for the secondary color was reduced, and the amount of the primary transfer current in the downstream stations S was increased. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     As will be evident from Table 3, for the purpose of preventing a transfer ghost from appearing in an image such as the one shown in  FIG. 10 , for example, it is desired to reduce the maximum amount of toners for the secondary color from 200% to 160%, and increase the primary transfer current in the downstream stations S from 20 μA to 24 μA. With the employment of these tactics, it is possible to prevent the occurrence of a transfer ghost, or make as low as possible the level of conspicuousness at which a transfer ghost appears, without excessively reducing the secondary color R in chroma, and also, without raising the level of coarseness at which an image will appear. 
     By the way, Tables 1 and 3 are related to the maximum amount of toners for the secondary color R (combination of primary colors Y and M). However, the maximum amount of toners for the secondary colors G (combination of primary color Y and C) and B (combination of primary colors M and C) is also related to the occurrence of a transfer ghost in the stations SK for the color K as was described previously. In this embodiment, therefore, for the purpose of reducing the maximum amount of toners for the secondary color in a transfer ghost prevention mode which is described later, the maximum amount of toners for other secondary colors, that is, colors G and B, is also reduced like the above described maximum amount of toners for the secondary color R. 
     4. Transfer Ghost Suppression Mode 
     The image forming apparatus  100  is configured so that it can carry out an image outputting operation in a transfer ghost suppression mode, in which it is reduced in the maximum amount of toners for the secondary color, and is increased in the amount of the primary transfer current in the downstream stations S, based on the results of the above described research. In particular, this embodiment is described with reference to a case where when a user makes the image forming apparatus  100  carry out an image outputting operation (copying operation) by inputting a printing command through the control panel  200  with which the main assembly of the image forming apparatus  100  is provided, the user makes the image forming apparatus  100  carry out the image outputting operation in a transfer ghost suppression mode by inputting a mode setting command through the control panel  200 . In the case of the image forming apparatus  100  in this embodiment, in a case where a user wants to output images which are likely to suffer from a transfer ghost, or the user found a transfer ghost in the outputted images, the user can make the image forming apparatus  100  carry out the image outputting operation in a transfer ghost suppression mode, if the user wishes. 
     In this embodiment, in the transfer ghost suppression mode, the maximum amount of toners for the secondary color is reduced from the normal setting, which is 200%, to 160%, and the primary transfer current (target current amount) for the downstream stations S is increased from the normal setting, which is 20 μA, to 24 μA. 
     Next, referring to  FIG. 5 , the control panel  200  of the image forming apparatus  100  is described. Part (a) of  FIG. 5  is a schematic external view of the control panel  200 . The control panel  200  has a start button  201  for making the image forming apparatus  100  start an image outputting operation (copying operation) based on the inputted information. It has also a display  202  (touch panel) which functions as an information inputting section as well as an information displaying section. A user is allowed to input various settings for an image outputting operation through the control panel  200  by making selections by pressing (touching) the buttons displayed on the display  202 . 
     Part (b) of  FIG. 5  shows an example of the initial screen of the display  202 . Referring to part (b) of  FIG. 5 , the initial screen has a button for displaying various buttons which a user can use to choose various settings for an image outputting operation. Part (c) of  FIG. 5  shows an example of a screen which has the buttons for selecting various settings. Referring to part (c) of  FIG. 5 , this screen has a transfer ghost suppression mode button  204  (which hereafter may be referred to as “mode selection button”) for placing the image forming apparatus  100  in a transfer ghost suppression mode, or moving the image forming apparatus  100  out of a transfer ghost suppression mode. As a user places the image forming apparatus  100  in a transfer ghost suppression mode with the use of the mode setting button  204 , it becomes possible for the image forming apparatus  100  to carry out an image outputting operation in a transfer ghost suppression mode such as the above described one. The mode setting button  204  is an example of a means for activating both the toner amount adjusting means and current amount adjusting means during an image outputting operation. A user can make the image forming apparatus  100  carry out an image outputting operation in a transfer ghost suppression mode by placing the image forming apparatus  100  in a transfer ghost suppression mode with the use of the mode setting button  204 , shown in part (c) of  FIG. 5 , and then, pressing (touching) the start button  201 , shown in part (a) of  FIG. 5 . 
       FIG. 6  is a flowchart for describing the operation of the image forming apparatus  100  in this embodiment. Part (a) of  FIG. 6  shows the operational sequence to be followed by a user, and part (b) of  FIG. 6  shows the operational sequence which is followed by the control section  120  of the image forming apparatus  100 . By the way,  FIG. 6  primarily shows the steps which are related to the procedure for placing the image forming apparatus  100  in a transfer ghost suppression mode. That is,  FIG. 6  does not show many of other steps (optional steps). 
     Referring to part (a) of  FIG. 6 , a user is to output an image with the use of the image forming apparatus  100  (S 101 ). If the user determines that a transfer ghost has occurred (S 102 ), the user is to choose a transfer ghost suppression mode with the use of the control panel  200  (S 103 ), and make the image forming apparatus  100  output the same image for the second time (S 104 ). More specifically, the user is to press (touch) the mode selection screen button on the display  202  of the control panel  200 , and press (touch) the mode setting button  204  on the display  202  of the control panel  200 . On the other hand, if the user determines that a transfer ghost did not occur to the first image outputted by the image forming apparatus  100 , the user is not required to do anything. By the way, if the user determines in advance that the image to be outputted is such an image that is likely to cause a transfer ghost, the user is allowed to put the image forming apparatus  100  in a transfer ghost suppression mode prior to the outputting of the first image, in order to make the image forming apparatus  100  carry out an image outputting operation in a transfer ghost suppression mode from the very beginning. 
     Referring to part (b) of  FIG. 6 , as the mode setting button  204  of the control panel  200  is touched (S 201 ), the controlling section  120  of the image forming apparatus  100  changes the maximum amount of toners for the secondary color and primary transfer current amount to the values for a transfer ghost suppression mode (S 202 ). That is, it reduces the toner amount for the secondary color from the maximum amount of toners for the secondary color, which is 200%, to 160%, and increases the setting of the primary transfer current amount for the downstream stations S from 20 μA to 24 μA. Then, as the start button  201  is touched (S 203 ), the controlling section  120  makes the image forming apparatus  100  carry out an image outputting operation (S 204 ). 
     As described above, in this embodiment, the image forming apparatus  100  employs the DC charging method, and is not provided with a discharging device such as a pre-exposing device. That is, it is structured to be inexpensive, and advantageous in terms of size reduction. According to this embodiment, even if an image forming apparatus is structured like the image forming apparatus  100  in this embodiment, it is possible to suppress the occurrence of such a transfer ghost which is likely to be generated by a solid area of an image, which has the secondary color and is formed in the upstream stations S, across the halftone areas of the same image, in the downstream stations S. Further, both the setting for reducing the maximum amount of toners for the secondary color, and setting for increasing the primary transfer current, can be done at the same time. That is, it is unnecessary for a user to separately choose the two settings. In other words, the occurrence of a transfer ghost can be suppressed by a simple operation. Further, according to this embodiment, it is possible to suppress the occurrence of a transfer ghost while preventing the problem that the attempt to suppress the occurrence of a transfer ghost will likely to result in the formation of images which have less chroma, and/or coarse across its secondary color portions. 
     Embodiment 2 
     Next, another embodiment of the present invention is described. In terms of basic structure and operation, the image forming apparatus  100  in this embodiment is the same as that in the first embodiment. Therefore, its elements which are the same in function and/or structure as the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, one for one, and are not described here. 
     The first embodiment was described with reference to a case where a transfer ghost suppression mode was selected with the use of the control panel  200  with which the image forming apparatus  100  was provided. In comparison, this embodiment is described with reference to a case where the image forming apparatus  100  is made to carry out an image outputting operation in a transfer ghost suppression mode when the image forming apparatus  100  carries out an image outputting operation (printing operation) in response to a command from the PC  300 . The PC  300  is in connection with the image forming apparatus  100  so that communication is possible between two apparatuses. The PC  300  is an example of an apparatus (information processing peripheral device, etc.) which is combined with the image forming apparatus  100  to make up an image formation system  400 . The printer driver  312  of the PC  300  makes up a controlling device which transmits to the image forming apparatus  100 , the information about the image to be formed, and the information about the settings for an image outputting operation. 
     In this embodiment, in a transfer ghost suppression mode, the setting for the maximum amount of toners for the secondary color is reduced from the normal setting, which is 200%, to 160%, and the setting for the primary transfer current amount is increased from the normal setting which is 20 μA, to 24 μA, as they were in the first embodiment. 
       FIG. 7  is a schematic drawing of the image formation system  400  in this embodiment. In this embodiment, the PC  300  is in connection with the image forming apparatus  100  through a LAN cable. The PC  300  is enabled to display images across its display  301  with the use of an application software  311  ( FIG. 4 ), etc., which has been installed in the PC  300 . Further, the PC  300  is enabled to command the image forming apparatus  100  to carry out an image outputting operation (printing operation) with the use of the printer driver  312  ( FIG. 4 ) which has been installed in advance. That is, the PC  300  is enabled to transmit to the controlling section  120  ( FIG. 4 ) of the image forming apparatus  100 , the information about the image displayed across the above described display  301 , for example, and the information about the settings for an image outputting operation, through the LAN cable  302 . Further, in this embodiment, the printer driver  312  is enabled to transmit to the image forming apparatus  100 , such information as print count, recording medium size, and also, the information about the setting for a transfer ghost suppression mode, as the settings for the image outputting operation. 
       FIG. 7  is an example of a touch panel screen displayed on the display  301  of the PC  300 . Referring to  FIG. 7 , in this embodiment, the control panel screen of the printer driver  312  is provided with a button  303  (mode setting button) for setting a transfer ghost suppression mode. The mode setting button  304  is an example of the mode setting sections section which the PC  300  has and is provided by the printer driver  312 . This mode setting button  304  is for activating both the toner amount adjusting means and current amount adjusting means when images are outputted by the image forming apparatus  100 , as that in the first embodiment was. By the way, the printer driver  312  is enabled to make the display  301  of the PC  300  provide a user with multiple control screens which are different in function from the screen having the mode setting button  304 , in addition to the above described screen having the mode selection button  303 , although these screens are not shown in  FIG. 7 . These screens include a screen having buttons for selection sheet size, etc., a screen having a button for instructing the image forming apparatus  100  to start an image outputting operation, etc. 
       FIG. 8  is a flowchart of the operational sequence of the image forming apparatus  100  in this embodiment. Part (a) of  FIG. 8  is a flowchart of the operational sequence to be followed by a user, and part (b) of  FIG. 8  shows the operational sequence for the printer driver  312 . Part (c) of  FIG. 8  shows the operational sequence to be followed by the controlling section  120  of the image forming apparatus  100 .  FIG. 8  primarily shows the portions of the operational sequence, which is related to a transfer ghost suppression mode section, and does not show various optional steps in the operational sequences. 
     Referring to part (a) of  FIG. 8 , a user is to display on the display  301  the PC  300 , an image which the user want to output, with the use of the application software  311  or the like (S 301 ). Then, the user is to start up the printer driver  312  of the application software  311  (S 302 ). Then, the user is to make the image forming apparatus  100  output the image with the use of the printer driver  312  (S 303 ). Next, if the user determines that the image outputted by the image forming apparatus  100  has a transfer ghost (S 304 ), the user is to select a transfer ghost suppression mode with the use of the printer driver  312  (S 303 ), and make the image forming apparatus  100  output the same image for the second time (S 306 ). 
     Next, referring to part (b) of  FIG. 8 , as the printer driver  312  is started up from the application software  311 , it displays a control panel screen on the display  301  of the PC  300  (S 401 ). Then, as the mode setting button  304  on the control panel screen is touched (S 402 ), the printer driver  312  transmits a command for placing the image forming apparatus  100  in a transfer ghost suppression mode, to the controlling section  120  of the image forming apparatus  100 , through the LAN cable (S 403 ). Further, as the button on the control panel screen, which is for making the image forming apparatus  100  carry out the image outputting operation, is touched (S 404 ), the printer driver  312  transmits a command for making the image forming apparatus  100  start the image outputting operation, to the controlling section  120  of the image forming apparatus  100  through the LAN cable (S 405 ). 
     Next, referring to part (c) of  FIG. 8 , as the controlling section  120  of the image forming apparatus  100  receives the signal for turning on a transfer ghost suppression mode, from the PC  300 , (S 501 ), it places the image forming apparatus  100  in a transfer ghost suppression mode only for the current job (S 502 ). Then, the controlling section  120  reduces the setting for the maximum amount of toners for the secondary color from 200% to 160%, and increases the setting for the primary transfer current amount for the downstream stations S from 20 μA to 24 μA (S 503 ). Thereafter, as the image outputting operation start signal is inputted from the PC  300  (S 504 ), the controlling section  120  makes the image forming apparatus  100  carry out the image outputting operation (S 505 ). After the completion of the job, the controlling section  120  takes the image forming apparatus  100  out of the transfer ghost suppression mode (S 506 ). 
     As described above, according to this embodiment, not only can this image formation system provide the same effects as those obtainable by the image forming apparatus  100  in the first embodiment, but also, it can provide the following effect. That is, in this embodiment, it is possible to turn on a transfer ghost suppression mode, for each job, with the use of the printer driver  312  of the PC  300 . Therefore, it does not occur that the image forming apparatus  100  remains in a transfer ghost suppression mode after the completion of each job. Thus, it does not occur that the setting for the transfer ghost suppression mode in the preceding image outputting operation affects the following image outputting operation which is for outputting images, the pattern of which is unlikely to trigger the occurrence of a transfer ghost. That is, in this embodiment, it is unnecessary for a transfer ghost suppression mode to be turned off by a user. In other words, this embodiment can eliminate the time necessary for a user to turn off a transfer ghost suppression mode, and also, deal with the problem that a user sometimes forgets to turn off a transfer ghost suppression mode. That is, this embodiment makes it possible to make the image forming apparatus  100  operate in a transfer ghost suppression mode only for a job which is for outputting images, the pattern of which is likely to trigger the occurrence of a transfer ghost, through a simple operation. 
     Embodiment 3 
     Next, another embodiment of the present invention is described. The basic structure and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus  100  in the first embodiment. Therefore, the elements of the image forming apparatus in this embodiment, which are the same in function and/or structure as the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, one for one, and are not described here. 
     This embodiment is described with reference to a case where when the image forming apparatus  100  is made to carry out an image outputting operation (printing operation) by the PC  300 , it is made to carry out the image outputting operation in a transfer ghost suppression mode as in the second embodiment. This embodiment, however, is different from the second embodiment in that in this embodiment, the setting for the maximum amount of toners for the secondary color for the image forming apparatus  100 , and the setting for the primary transfer current amount for the downstream stations S, have been changed from the normal setting (normal settings). 
     There are cases where the setting for the maximum amount of toners for the secondary color for the image forming apparatus  100 , and/or the setting for the primary transfer current amount for the image forming apparatus  100 , have been changed in order to suppress the occurrence of other image defects than a transfer ghost. That is, it is possible that the image forming apparatus  100  will have been changed in the setting for the maximum amount of toners for the secondary color and/or primary transfer current amount, by an operator such as a user and a service person. For example, it is possible that in order to prevent the occurrence of the phenomenon that toner scatters from fine lines of an image, the image forming apparatus  100  will be reduced in the maximum amount of toners for the secondary color. Further, it is possible that in order to prevent the occurrence of the problem that the image forming apparatus  100  outputs coarse images, the coarseness of which is attributable to the retransfer of toner, which occurs in the primary transferring section, the image forming apparatus  100  will be reduced in the primary transfer current amount. 
     In this embodiment, therefore, in a case where the image forming apparatus  100  is placed in a transfer ghost suppression mode by the printer driver  312 , this transfer ghost suppression mode set by the printer driver  312  is prioritized over any operation mode set by or through the image forming apparatus  100 . 
       FIG. 9  is a flowchart for describing the operation of the image forming apparatus  100  in this embodiment.  FIG. 9  shows the operational sequence followed by the controlling section  120  of the image forming apparatus  100  in this embodiment. The operational sequence to be followed by a user, and the operational sequence followed by the printer driver  312 , are the same as those in the above described second embodiment, which are shown in part (a) of  FIGS. 8 and 8 ( b ), respectively. 
     Referring to  FIG. 9 , as the controlling section  120  of the image forming apparatus  100  receives a signal for turning on a transfer ghost suppression mode (S 601 ), it sets the image forming apparatus  100  in a transfer ghost suppression mode only for the current job (S 602 ). Then, the controlling section  120  ignores the settings of the image forming apparatus  100 , and sets the maximum amount of toners for the secondary color to 160%, and the primary transfer current amount for the downstream stations S to 24 μA (S 603 ). Thereafter, as an image outputting operation start signal is inputted from the PC  300  (S 604 ), the controlling section  120  makes the image forming apparatus  100  carry out the image outputting operation (S 605 ). Further, as the job is completed, the controlling section  120  sets the maximum amount of toners for the secondary color, and the primary transfer current amount, back to those prior to the starting of the completed job (S 606 ). 
     As described above, not only can this embodiment provide the same effects as the first embodiment, but also, can provide the following effect. That is, in this embodiment, the printer driver  312  is prioritized over the image forming apparatus  100  in terms of the setting of operational mode related to a transfer ghost. That is, the operational mode, in which the image forming apparatus  100  is, is ignored, and the operational mode for the image forming apparatus  100  is automatically switched to a transfer ghost suppression mode which is temporarily set by the printer driver  312  (maximum amount of toners for secondary color is reduced from normal setting, and primary transfer current amount for downstream stations S is increased from normal setting). Thus, this embodiment makes it possible to operate the image forming apparatus  100  in a transfer ghost suppression mode only for the users who want to suppress the occurrence of a transfer ghost, and for the images which are likely to trigger the occurrence of a transfer ghost, without the need for repeating complicated operations for setting the image forming apparatus  100 , while preventing the current setting of a transfer ghost suppression mode from affecting other users and/or images. 
     [Miscellanies] 
     In the foregoing, the present invention was described with reference to some of the preferred embodiments of the present invention. However, these embodiments are not intended to limit the present invention in scope. 
     For example, the preceding embodiments were described with reference to an image forming apparatus which forms four monochromatic toner images of yellow (Y), magenta (M), cyan (C) and black (K) colors, one for one, to effect a single multicolor (full-color) image. However, these embodiment were not intended to limit the present invention in terms of the number of toner colors and toner types. For example, the present invention is also applicable to image forming apparatuses which form only three monochromatic toner images, that is, yellow (Y), magenta (M) and cyan (C) toner images, to form a full-color (multicolor) image, and image forming apparatuses which form four monochromatic toner images of yellow (Y), magenta (M), cyan (C) and black (K) colors, one for one, and additional monochromatic toner images (including transparent image) which are different in color from these monochromatic toner images, in place of these images, to form a full-color (multicolor) image. Also in the case of such image forming apparatuses, the occurrence of a transfer ghost can be suppressed as it was in the preceding embodiment, by reducing the upstream stations S in the maximum amount of toners for the secondary color, and increasing the downstream stations in the primary transfer current amount. 
     Further, the preceding embodiments were described with reference to an image forming apparatus which forms yellow (M), magenta (M), cyan (C) and black (K) monochromatic toner images, in the listed order. However, these embodiments are not intended to limit the present invention in scope in terms of the order in which these monochromatic toner images are formed. For example, in the preceding embodiments, the primary transfer current amount was increased only for the stations SC and SK, to the primary transferring section of which the monochromatic images of primary color, which effect the secondary color, are conveyed from the upstream stations S. However, in the case of an image forming apparatus which forms monochromatic toner images in the order of yellow (Y), cyan (C), magenta (M) and black (K) toner images, it is the stations SM and SK, or the third and fourth stations, respectively, that have to be increased in the primary transfer current amount. 
     Moreover, the preceding embodiments were described with reference to an image forming apparatus of the so-called intermediary transfer type, that is, an image forming apparatus having an intermediary transferring member as the first transfer medium. However, the present invention is also applicable to an image forming apparatus of the so-called direct transfer type, which has a recording medium bearing member which bears and conveys a sheet of recording medium, in place of the intermediary transferring member in the preceding embodiments. As a recording medium bearing member, a recording medium bearing belt, which is similar in structure to the intermediary transferring member in the preceding embodiments, is used. In the case of an image forming apparatus which uses a recording medium bearing member, monochromatic toner images, different in color, are sequentially transferred in layers on to a sheet of recording medium on the recording medium bearing belt, from the photosensitive drums  1 , in the four stations S, by the function of the transfer rollers, which are similar to the primary transfer rollers in the preceding embodiments, one for one. Therefore, the toner images are fixed to the sheet of recording medium on the recording medium bearing belt, and then, the sheet of recording medium is discharged from the main assembly of the image forming apparatus. Also in the case of this type of image forming apparatus, the above described issue related to a transfer ghost, which is similar to those mentioned in the description of the preceding embodiment, will possibly occur. Therefore, even in the case of this type of image forming apparatus, the same effects as those obtainable by these preceding embodiments can be obtained by applying the present invention to the image forming apparatus, that is, by operating the apparatus in a transfer ghost suppression mode which is similar to those in the preceding embodiments. 
     Moreover, in the preceding embodiments, the information processing peripheral device which outputs to the image forming apparatus, the information about the image to be formed, and the information about the settings for an image outputting operation, was a personal computer. However, these embodiments are not intended to limit the present invention in terms of the information processing peripheral device. As other information processing peripheral devices, a computer of the tablet type, a smart phone, a portable telephone, a digital camera, a scanner, etc., can be listed. 
     Further, regarding the setting for the transfer current amount, the preceding embodiments were described with reference to a case in which the image forming apparatus was operated in an environment which was normal in temperature and humidity. However, some image forming apparatuses are varied in the setting for the transfer current amount, according to environmental condition, for example, whether they are operated in an environment which is low, normal, or higher in temperature and humidity. Even the image forming apparatus  100  in the preceding embodiments can be adjusted in the setting for the transfer current amount (from normal setting, including setting for transfer ghost suppression mode) according to environmental condition. Even in such a case, all that is necessary when an image forming apparatus is in the transfer ghost suppression mode is to set the image forming apparatus  100  smaller in the maximum amount of toners for the secondary color than the normal setting (default setting), and set the downstream stations higher in the primary transfer current amount than the normal setting (default setting), even if the environmental condition remains the same. 
     Further, in the transfer ghost suppression mode, an image forming apparatus may be controlled so that the setting for the maximum amount of toners for the secondary color is reduced compared to the current setting (as referential setting), and the setting for the primary transfer current amount is increased compared to the current setting (as referential setting). In the case where a user turns on the transfer ghost suppression mode after the user confirmed the presence of a transfer ghost in the outputted images, it is sometimes possible obtain good results by using the current setting as the referential setting for the adjustment. 
     The preceding embodiments were described with reference to a case where the transfer bias is controlled so that the transfer current remains stable at a preset level during an image forming operation. In such a case, as the transfer bias is applied, the areas of the peripheral surface of the photosensitive drum, which have the secondary colors, and those which do not have the secondary color, are likely to become different in potential level from each other. Therefore, the present invention is extremely effective. However, these embodiments are not intended to limit the present invention in scope. That is, even in a case where the transfer bias is controlled so that the transfer current remains stable at a preset level, and yet, a transfer ghost occurs as in the preceding embodiments, it is possible to obtain the same effects as those obtained in the preceding embodiments, by applying the present invention. 
     Moreover, in the preceding embodiments, in the transfer ghost suppression mode, only specific image forming sections (stations) among the multiple image forming sections aligned in the moving direction of transfer medium (recording medium), were increased in transfer current compared to the referential value. Further, the specific image forming sections are the third and fourth image forming sections among the four image forming sections, that is, one of the image forming sections which are not the first and second ones, for the following reason. That is, a transfer ghost is a phenomenon that occurs in the downstream image forming sections as described above. Further, as the transfer current is increased, image defects (coarseness, and the like) other than a transfer ghost sometimes occur. However, in a case where the intensity of the above described other image defects is at a tolerable level, it is possible to increase all the image forming sections in the amount of transfer current. With the use of these tactics, it is possible to simplify an image forming apparatus  100  in control, and/or to deal with an image forming apparatus, all the image forming sections of which share the same transfer current power source. 
     Further, as described above, the present invention is extremely effective to suppress the occurrence of such a transfer ghost that occurs to the images formed in the downstream image forming stations of an image forming apparatus of the so-called tandem type, and also, is attributable to the toner images of the secondary color, which are formed on a transfer medium (or recording medium) in the upstream image forming sections (stations). In comparison, as has been well known in the filed of image forming apparatuses, there are image forming apparatuses which have only a single photosensitive member and form a full-color (multicolor) image on a transfer medium (recording medium) by repeatedly transferring toner images in layers onto the transfer medium (intermediary transferring member, or sheet of recording medium borne by recording medium bearing member). Also in the case of such image forming apparatuses, there occurs that the area of the peripheral surface of the photosensitive member, which has just passed through the transferring section, is recharged to form a toner image, while the toner image of the secondary color, which was borne on the transferring member and has just been conveyed to the transferring section, is still in the transferring section, and the transferring means is supplied with transfer current. Also in such a case, it is possible that a transfer ghost which is similar to the above described transfer ghost which occurs to an image forming apparatus of the so-called tandem type, will occur to the image formed on a sheet of recording medium during the next rotation of the photosensitive member. Therefore, even in the case of these image forming apparatuses, the same effects as those obtainable by the preceding embodiments can be obtained by operating the image forming apparatuses in the transfer ghost suppression mode in accordance with the present invention, in which the amount by which toners are borne by the transferring member to form a toner image of the secondary color is reduced, and the transfer current to be supplied to the transferring section is increased. In this case, it is possible to increase only the transfer current for the transferring process for transferring an image of the third color and thereafter, in the transfer ghost suppression mode. Moreover, as has been widely known in the field of image forming apparatuses, there are image forming apparatuses which have only a single photosensitive member, form in layers multiple toner images on the single photosensitive member, and then, transferred the multiple toner images onto a sheet of recording medium all at once. Also in the case of these image forming apparatuses, there occurs that the area of the peripheral surface of the photosensitive member, which has just passed through the transferring section, is recharged to form a toner image, while the toner image of the secondary color, which was borne on the transferring member and has just been conveyed to the transferring section is still in the transferring section, and the transferring means is supplied with transfer current. Also in such a case, it is possible that a transfer ghost which is similar to the above described transfer ghost which occurs to an image forming apparatus of the so-called tandem type, will occur to the image formed on the photosensitive member during the next rotation of the photosensitive member. Therefore, even in the case of these image forming apparatuses, the same effects as those obtainable by the preceding embodiments can be obtained by operating these image forming apparatuses in a transfer ghost suppression mode in accordance with the present invention, in which the amount, by which toners are borne by the peripheral surface of the photosensitive member to form a toner image of the secondary color, is reduced, and the transfer current to be supplied to the transferring section is increased. 
     The present invention can be embodied in the form of a system, an apparatus (device), a method, a program, and/or storage medium. More concretely, the present invention is applicable to a system made up of two or more devices. It is also applicable to an apparatus made up of only a single device. By the way, the present invention includes cases where a system or an apparatus is directly or remotely supplied with a software (program) for enabling the system or apparatus to perform the functions such as those performed by the system and apparatuses in the preceding embodiments, and the system or the computer of the apparatus reads the supplied program codes, and operates according to the program codes. That is, the program codes themselves which are installed in the computer of an apparatus to make the apparatus to operate in a transfer ghost suppression mode in accordance with the present invention are also embodiments of the present invention. In other words, the present invention includes computer programs for making an apparatus operate in accordance with the present invention. As for the storage medium for supplying the programs, there are a hard disk, an optical disk, a magneto-optical disk, a nonvolatile memory, and the like, for example. Further, as a method for supplying programs, it is possible to list such a method as downloading computer program files (which include compressed ones which are automatically installed) into a storage medium such as a hard disk, from a home page. Further, not only can the above described functions in the preceding embodiments be realized as the computer makes the system or apparatus carry out the read program. That is, the present invention is embodied as an OS or the like, which operates on a computer, carries out a part (parts) or the entirety of the process based on the instruction read by the computer. Further, it sometimes occurs that a program read from a storage medium is written in a memory with which a function expansion board (daughter board) inserted in a computer, or in a function expansion unit which is in connection to a computer, is provided. In such a case, the present invention is embodied (realized) by a part (parts) or the entirety of the process which a CPU or the like with which the function expansion board or function expansion unit carries out based on the instruction of the written program. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2014-148282 filed on Jul. 18, 2014, which is hereby incorporated by reference herein in its entirety.