Abstract:
Disclosed is an image forming apparatus capable of preventing generation of an excessive difference in potential between a cleaning member and an intermediate transfer member to thereby increase the service life of the intermediate transfer member.  
     The image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control the potential gradient between the cleaning member and the intermediate transfer member.

Description:
FIELD OF THE INVENTION AND RELATED ART STATEMENT  
         [0001]    The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, a facsimile, or a multifunction apparatus formed by combining them, and in particular, to an improvement in a cleaning technique for an image forming apparatus.  
           [0002]    Up to now, electrophotographic (electrostatic transfer type) image forming apparatuses, such as copying machines and printers, have been widely known. In such image forming apparatuses, a toner image is transferred to a recording sheet by a final transfer member, and then the toner image is fixed to thereby realize on the recording sheet the toner image as a permanent image. The portion of the toner remaining on the transfer member without being transferred to the recording sheet has to be removed by a cleaning device in the image forming apparatus. As a method for removing such residual toner, there has been proposed a technique according to which a metal cleaning roll is caused to abut the transfer member, thereby making the residual toner to adhere to the cleaning roll by electrostatic force (See, for example, JP 6-59586 A and JP 6-35340 A). Further, there has also been proposed a technique according to which a metal cleaning blade is caused to abut against a metal cleaning roll to prevent the cleaning blade from being turned up (See, for example, JP 2000-142310).  
           [0003]    However, these conventional techniques have the following problem.  
           [0004]    The metal cleaning roll exhibits a relatively low electrical resistance value, so that the surface potential thereof is quickly changed in response to a variation in the bias to be applied thereto. Whereas, an intermediate transfer member which is in contact with the cleaning roll exhibits a relatively high electrical resistance value, so that even if the bias to be applied varies, the surface potential thereof is only changed transitionally. Thus, an excessive difference in potential can be instantaneously generated between the cleaning roll and the intermediate transfer member. As a result, a discharge stress is imparted to the intermediate transfer member, thereby deteriorating or damaging the surface of the intermediate transfer member.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0005]    The present invention has been made in view of the above-mentioned technical problem, and provides an image forming apparatus capable of preventing generation of an excessive difference in potential between a cleaning member and an intermediate transfer member, thereby making it possible to elongate the service life of the intermediate transfer member.  
           [0006]    According to an aspect of the present invention, an image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control a potential gradient between the cleaning member and the intermediate transfer member, in which when switching the intermediate-transfer-member-bias to be applied to the intermediate transfer member from a first intermediate-transfer-member-bias to a second intermediate-transfer-member-bias and switching the cleaning-member-bias to be applied to the cleaning member from a first cleaning-member-bias to a second cleaning-member-bias, respectively (substantially at the same time), the control unit makes the switching start timing for the intermediate-transfer-member-bias earlier than the switching start timing for the cleaning-member-bias.  
           [0007]    More specifically, the control unit can be constructed such that during the transition of the intermediate-transfer-member-bias from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the switching of the cleaning-member-bias is started. Further, the control unit can also be constructed such that after the transition of the intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the switching of the cleaning-member-bias is started.  
           [0008]    According to another aspect of the present invention, the image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control the potential gradient between the cleaning member and the intermediate transfer member, in which when switching the intermediate-transfer-member-bias to be applied to the intermediate transfer member from a first intermediate-transfer-member-bias to a second intermediate-transfer-member-bias and switching the cleaning-member-bias to be applied to the cleaning member from a first cleaning-member-bias to a second cleaning-member-bias, respectively (substantially at the same time), the control unit effects the switching of the cleaning-member-bias stepwise.  
           [0009]    More specifically, the stepwise switching of the cleaning-member-bias may be effected in two stages; it is possible to effect switching from a first cleaning-member-bias to a transition cleaning-member-bias and then effect switching from an intermediate cleaning-member-bias to a second cleaning-member-bias. The stepwise switching of the cleaning-member-bias may be effected in three stages; it is possible to effect switching from a first cleaning-member-bias to a first transition cleaning-member-bias and then effect switching from the first transition cleaning-member-bias to a second transition cleaning-member-bias and then effect switching from the second transition cleaning-member-bias to a second cleaning-member-bias.  
           [0010]    Further, it is possible to set the transition cleaning-member-biases (the first transition cleaning-member-bias and the second transition cleaning-member-bias) to be smaller than the second intermediate-transfer-member-bias.  
           [0011]    Further, the stepwise switching of the cleaning-member-bias is effected such that the variation in the cleaning-member-bias increases gradually. For example, it is possible to effect setting such that the following inequality holds good: |(transition cleaning-member-bias)−(first cleaning-member-bias)&lt;|(second cleaning-member-bias)−(transition cleaning-member-bias)|, and that the following inequality holds good: |(first transition cleaning-member-bias)−(first cleaning-member-bias)&lt;|(second transition cleaning-member-bias)−(first transition cleaning-member-bias)&lt;|(second cleaning-member-bias)−(second transition cleaning-member-bias)|.  
           [0012]    Further, it is also possible to combine the first and second aspects of the invention. That is, according to another aspect of the present invention, the image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member, and which is in contact with the image bearing member; a final transfer member which is in contact with the intermediate transfer member; a cleaning member which is in contact with the intermediate transfer member; and a control unit which applies an intermediate-transfer-member-bias to the intermediate transfer member and applies a cleaning-member-bias to the cleaning member which is in contact with the intermediate transfer member to thereby control the potential gradient between the cleaning member and the intermediate transfer member, in which when switching the intermediate-transfer-member-bias to be applied to the intermediate transfer member from a first intermediate-transfer-member-bias to a second intermediate-transfer-member-bias and switching the cleaning-member-bias to be applied to the cleaning member from a first cleaning-member-bias to a second cleaning-member-bias, respectively (substantially at the same time), the control unit makes the switching start timing for the intermediate-transfer-member-bias earlier than the switching start timing for the cleaning-member-bias, and effects switching of the cleaning-member-bias stepwise.  
           [0013]    Examples of the situation in which switching of the intermediate-transfer-member-bias to be applied to the intermediate transfer member from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias and switching of the cleaning-member-bias to be applied to the cleaning member from the first cleaning-member-bias to the second cleaning-member-bias are effected respectively (substantially at the same time) include the time when power is turned on, transition from a standby mode to an image formation mode, transition from the image formation mode to a cleaning mode, transition from the cleaning mode to the image formation mode, transition from the cleaning mode to the standby mode, and transition from a normal-polarity cleaning mode to a reversed-polarity cleaning mode.  
           [0014]    Further, the second intermediate-transfer-member-bias and the second cleaning-member-bias are of the same polarity, and when the following inequality: |(second intermediate-transfer-member-bias)|&lt;|(second cleaning-member-bias)| holds good, in general, discharge is likely to occur between the cleaning member and the intermediate transfer member, so that application of the present invention to such a case is particularly effective.  
           [0015]    Further, taking into account the fact that discharge between the cleaning member and the intermediate transfer member is likely to occur when the potential of the cleaning member is higher than the potential of the intermediate transfer member, it is desirable to perform control such that during switching from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the relationships: −Δ(lower limit) (0&lt;Δ(lower limit))&lt;(cleaning-member-bias)−(intermediate-transfer-member-bias) &lt;Δ(upper limit) (0&lt;Δ(upper limit)), and Δ(upper limit)&lt;Δ(lower limit) are satisfied. Further, it is desirable to perform control such that during switching from the first intermediate-transfer-member-bias to the second intermediate-transfer-member-bias, the relationship: (cleaning-member-bias)&lt;(intermediate-transfer-member-bias) is satisfied.  
           [0016]    Further, generally speaking, when the electrical resistance of the cleaning member is lower than the electrical resistance of the intermediate transfer member which is in contact with the cleaning member, discharge is likely to occur between the cleaning member and the intermediate transfer member, so that application of the present invention to such a case is particularly effective. Specifically, as the cleaning member, it is possible to adopt a metal cleaning roll. In this case, an arrangement may be adopted in which a metal cleaning blade is caused to abut against the cleaning roll.  
           [0017]    Further, the present invention is applicable not only to a monochrome image forming apparatus, but also to a multicolor image forming apparatus. That is, it is possible to adopt a construction which has, as the image bearing member, plural image bearing members for different colors and, as the intermediate transfer device, a single intermediate transfer member. Further, it is also possible to adopt a construction which has, as the image bearing member, plural image bearing members for different colors and, as the intermediate transfer device, a first upstream side intermediate transfer member which is in contact with a part of the plural image bearing members, a first downstream side intermediate transfer member which is in contact with a part of the remaining ones of the plural image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member.  
           [0018]    More specifically, the latter type of image forming apparatus may have, as the image bearing member, four image bearing members for yellow, magenta, cyan, and black, and may have, as the transfer device, a first upstream side intermediate transfer member and a first downstream side intermediate transfer member which are respectively in contact with two of the four image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member.  
           [0019]    In accordance with the present invention, it is possible to provide an image forming apparatus capable of preventing generation of an excessive difference in potential between the cleaning member and the intermediate transfer member and of extending the service life of the intermediate transfer member. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Preferred embodiments of the present invention will be described in detail based on the following figures, wherein:  
         [0021]    [0021]FIG. 1 is a schematic sectional view of a full color printer according to an embodiment of the present invention;  
         [0022]    [0022]FIG. 2 is a main-part sectional view of a full color printer according to an embodiment of the present invention;  
         [0023]    [0023]FIG. 3 is a block diagram illustrating the construction of a potential gradient control system of a full color printer according to an embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is a timing chart illustrating the operation of a potential gradient control system of a full color printer according to an embodiment of the present invention;  
         [0025]    [0025]FIG. 5 is a graph illustrating the bias switching operation of a full color printer according to Embodiment 1;  
         [0026]    [0026]FIG. 6 is a graph illustrating the bias switching operation of a full color printer according to Embodiment 2;  
         [0027]    [0027]FIG. 7 is a graph illustrating the bias switching operation of a full color printer according to Embodiment 3; and  
         [0028]    [0028]FIG. 8 is a graph illustrating the bias switching operation of a full color printer according to Embodiment 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    Preferred embodiments of the present invention will now be described in detail.  
         [0030]    [0030]FIG. 1 shows a tandem type full color printer (image forming apparatus) according to an embodiment of the present invention. FIG. 2 is shows a main image forming portion of the full color printer (image forming apparatus) shown in FIG. 1.  
         [0031]    This full color printer  1  is roughly composed of an image forming portion, an intermediate transfer device, a final transfer roll  40 , a fixing device  6 , and a sheet feeding portion.  
         [0032]    The image forming portion is composed of four image forming units  1 Y through  1 K for yellow (Y), magenta (M), cyan (C), and black (K), and an exposure device  12 . The image forming units  1 Y through  1 K are respectively composed of four photosensitive drums (image bearing members)  10 Y through  10 K, charging rolls (contact type charging members)  11 Y through  11 K which are respectively in contact with the photosensitive drums  10 Y through  10 K, developing devices  12 Y through  12 K respectively opposed to the photosensitive drums  10 Y through  10 K, and brush rolls  13 Y through  13 K which are respectively in contact with the photosensitive drums  10 Y through  10 K.  
         [0033]    Regarding the arrangement of the members around each photosensitive drum  10 , the charging roll  11 , the developing device  12  (the developing sleeve of the developing device), a first intermediate transfer roll (described below), and the brush roll  13  are arranged around the photosensitive drum  10  from the upstream side to the downstream side with respect to the rotating direction of the photosensitive drum  10 .  
         [0034]    A DC voltage of approximately −840 V is applied to the photosensitive drums  10 Y through  10 K by the charging rolls  11 Y through  11 K, whereby the drums are uniformly charged to approximately 300 V; when electrostatic latent images are written thereto by the exposure devices  15 , the surface potential thereof is reduced to approximately −60 V.  
         [0035]    Each of the developing devices  12 Y through  12 K is a magnetic-brush-contact, two-component development type developing device equipped with a developing roll, a developer amount regulating member, a developer carrying member, and an auger for carrying and agitating developer. The amount of developer regulated by the developer amount regulating member and carried to the developing portion is approximately 30 to 40 g/m 2 ; at this time, the charge amount of the toner existing on the developing roll is approximately −20 to −30 μC/g. An AC+DC developing voltage is applied to these developing devices  12 Y through  12 K to execute development; this developing voltage is composed of an AC component of approximately 4 kHz and 1.6 kVpp, and a DC component of approximately −230 V.  
         [0036]    The intermediate transfer device is equipped with a first upstream side intermediate transfer roll (first upstream side intermediate transfer member)  20   a  which is in contact with the photosensitive drums  10 Y and  10 M, a first downstream side intermediate transfer roll (first downstream side intermediate transfer member)  20   b  which is in contact with the photosensitive drums  10 C and  10 K, a second intermediate transfer roll  30  which is in contact with the two first intermediate transfer rolls  20   a  and  20   b , and a toner sensor  8  which detects optically and in a non-contact fashion the presence and density of a toner image on the second intermediate transfer roll  30 .  
         [0037]    Further, the first upstream side intermediate transfer roll  20   a  is equipped with a first upstream side cleaning device (cleaning device)  21   a . This first upstream side cleaning device  21   a  is equipped with a metal (stainless steel) cleaning roll (cleaning member)  210   a  which is in contact with the first upstream side intermediate transfer roll  20   a , a cleaning blade  211   a  abutting the cleaning roll  210   a , an intermediate transfer brush roll  213   a  which is in contact with the intermediate transfer roll  20   a  in the vicinity of the upstream side of the cleaning roll  210   a  with respect to the rotating direction of the first upstream side intermediate transfer roll  20   a , and a cleaner housing  212   a  which accommodates the cleaning roll  210   a , the cleaning blade  211   a , and the intermediate transfer brush roll  213   a.    
         [0038]    Similarly, the first downstream side intermediate transfer roll  20   b  is equipped with a first downstream side cleaning device (cleaning device)  21   b . This first downstream side cleaning device  21   b  is equipped with a metal (stainless steel) cleaning roll (cleaning member)  210   b  which is in contact with the first downstream side intermediate transfer roll  20   b , a cleaning blade  211   b  abutting the cleaning roll  210   b , an intermediate transfer brush roll  213   b  which is in contact with the intermediate transfer roll  20   b  in the vicinity of the upstream side of the cleaning roll  210   b  with respect to the rotating direction of the first downstream side intermediate transfer roll  20   b , and a cleaner housing  212   b  which accommodates the cleaning roll  210   b , the cleaning blade  211   b , and the intermediate transfer brush roll  213   b    
         [0039]    The second intermediate transfer roll  30  is equipped with a second cleaning device (cleaning device)  31 . This second cleaning device  31  is equipped with a metal (stainless steel) cleaning roll (cleaning member)  310  which is in contact with the second intermediate transfer roll  30 , a cleaning blade  311  abutting the cleaning roll  310 , a brush roll  313  which is in contact with the intermediate transfer roll  30  in the vicinity of the downstream side of the cleaning roll  310  with respect to the rotating direction of the second intermediate transfer roll  30 , and a cleaner housing (accommodating member)  312  accommodating the cleaning roll  310 , the cleaning blade  311 , and the brush roll  313 .  
         [0040]    Regarding the arrangement of the members around the first upstream side intermediate transfer roll  20   a , the photosensitive drum  10 M, the photosensitive drum  10 Y, the second intermediate transfer roll  30 , the intermediate transfer brush roll  213   a , and the cleaning roll  210   a  are arranged around the first upstream side intermediate transfer roll  20   a  from the upstream side to the downstream side with respect to the rotating direction of the first upstream side intermediate transfer roll  20   a . Further, regarding the arrangement of the members around the first downstream side intermediate transfer roll  20   b , the photosensitive drum  10 K, the photosensitive drum  10 C, the second intermediate transfer roll  30 , the intermediate transfer brush roll  213   b , and the cleaning roll  210   b  are arranged around the first downstream side intermediate transfer roll  20   b  from the upstream side to the downstream side with respect to the rotating direction of the first downstream side intermediate transfer roll  20   b . Further, regarding the arrangement of the members around the second intermediate transfer roll  30 , the first upstream side intermediate transfer roll  20   a , the first downstream side intermediate transfer roll  20   b , the toner sensor  8 , the final transfer roll  40 , the cleaning roll  310 , and the brush roll  313  are arranged around the second intermediate transfer roll  30  from the upstream side to the downstream side with respect to the rotating direction of the second intermediate transfer roll  30 .  
         [0041]    Each of the first intermediate transfer rolls  20   a  and  20   b  is formed by providing a silicone rubber layer on a metal pipe, and forming thereon a high release coating layer; while the acceptable resistance value thereof normally ranges from 10 5  to 10 9  Ω, in this example, it is approximately 10 8  Ω. This electrical resistance value is higher than that of the cleaning rolls  210   a ,  210   b , and  310 . And, the requisite surface potential for transferring toner images from the photosensitive drums  10 Y through  10 K to the first intermediate transfer rolls  20   a  and  20   b  normally ranges from approximately +250 through 500 V, and an optimum potential value can be set according to the toner charging condition, the ambient temperature, the humidity, etc.  
         [0042]    Like the first intermediate transfer rolls  20   a  and  20   b , the second intermediate transfer roll  30  is formed by providing a silicone rubber layer on a metal pipe and forming thereon a high release coating layer; while the acceptable resistance value thereof normally ranges from 10 8  to 10 12  Ω, in this example, it is approximately 10 11  Ω (That is, it exhibits a resistance value higher than that of the first intermediate transfer rolls  20   a  and  20   b ). And, the requisite surface potential for transferring toner images from the first intermediate transfer rolls  20   a  and  20   b  to the second intermediate transfer roll  30  normally ranges from approximately +600 through 1200 V, and an optimum potential value can be set according to the toner charging condition, the ambient temperature, the humidity, etc.  
         [0043]    The final transfer roll  40  is formed by providing an urethane rubber layer on a metal pipe and providing thereon a resin coating layer; while the acceptable resistance value thereof normally ranges from 10 6  to 10 9  Ω, in this example, it is approximately 10 8  Ω (That is, it exhibits a resistance value lower than that of the second intermediate transfer roll  30 ). And, the transfer voltage to be applied to this final transfer roll  40  in order to transfer a toner image from the second intermediate transfer roll  30  to the sheet (recording sheet) normally ranges from approximately +1200 through 5000 V, and an optimum voltage value can be set according to the ambient temperature, the humidity, the kind of sheet S (the resistance value thereof, etc.), etc. In this example, the constant current system is adopted, and approximately +6 μA is applied under normal temperature and normal humidity to obtain a substantially appropriate final transfer voltage of approximately +1600 to 2000 V.  
         [0044]    Further, (unlike the first intermediate transfer rolls  20   a  and  20   b  and the second intermediate transfer roll  30 ), the final transfer roll  40  is not caused to abut against the cleaning roll (cleaning member). Further, (exclusive of the time when replacing the image forming unit and inclusive of the times when the apparatus is in the image formation mode, the process control mode, and the cleaning mode), the final transfer roll  40  comes into contact with the second intermediate transfer roll  30 , and requires no special retracting mechanism or the like.  
         [0045]    The surface roughness (Rz) of the final transfer roll  40  may be 20 [μm(Rz)] or less, for example, 10 [μm(Rz)], and the surface roughness (Rz) of the first and second intermediate transfer rolls  20   a ,  20   b , and  30  may be 10 [μm(Rz)] or less, for example, 1 [μm(Rz)]. Further, the final transfer roll  40  exhibits a higher degree of surface roughness (Rz) than the first and second intermediate transfer rolls  20   a ,  20   b , and  30 . It is desirable that the surface roughness of these rolls be not more than the average grain size of the toner forming the toner image.  
         [0046]    In the fixing device  6 , a heating roll  62  and a pressurizing roll  61  are held in press contact with each other to form a fixing nip. Arranged in the heating roll  62  is a halogen lamp (not shown) serving as the heat source; at the time of fixing, the surface of the heating roll  62  is heated to a predetermined fixing temperature. Further, on the downstream side of the fixing nip with respect to the direction in which the sheet S is transported, there are arranged fixing/discharge roll pairs  63   a  and  63   b.    
         [0047]    The sheet feeding portion is formed along the transport path (indicated by the dotted line) P for the sheet S extending from the sheet feeding tray  50  to the discharge tray  70 . The sheet feeding tray  50  accommodates plural sheets S, and from the sheet feeding tray  50  to the downstream side of the transport path, there are sequentially arranged a roll pair formed by a pick-up roll  51   a  and a retarding roll  51   b , a pair of transport rolls  52   a  and  52   b , a pair of registration rolls  53   a  and  53   b , and (on the downstream side of the final transfer roll  40  and the fixing device  6 ) a pair of discharge rolls  54   a  and  54   b.    
         [0048]    [0048]FIG. 3 is a block diagram illustrating the potential control system of this full color printer  1 . According to the situation the color printer  1  is in, that is, based on the fact as to whether the printer is ready for the printing sequence (the image formation mode) or the cleaning sequence (the cleaning mode), the potential control portion (control unit)  9  controls voltages V( 11 ), V( 20 ), V( 210 ), V( 30 ), V( 310 ), and V( 40 ) respectively applied to the charging roll  11 , the first intermediate transfer rolls  20   a  and  20   b , the cleaning roll  210 , the second intermediate transfer roll  30 , the cleaning roll  310 , and the final transfer roll  40 , with the result that according to the situation the full color printer  1  is in, an appropriate potential gradient is formed between the charging roll  11 , the first intermediate transfer rolls  20   a  and  20   b , the cleaning roll  210 , the second intermediate transfer roll  30 , the cleaning roll  310 , and the final transfer roll  40 .  
         [0049]    [0049]FIG. 4 is a timing chart showing the values of the voltages applied to the charging roll  11 , the first intermediate transfer rolls  20   a  and  20   b , the cleaning roll  210 , the second intermediate transfer roll  30 , the cleaning roll  310 , and the final transfer roll  40  in the printing preparation sequence, the printing sequence, and the cleaning sequence. As can be clearly seen from this timing chart, the portions of the timing chart encircled by ellipsoids indicate the points in time at which the biases applied to the intermediate transfer rolls  20   a ,  20   b , and  30  and the biases applied to the cleaning rolls  210  and  310  are simultaneously switched, that is, during the transition from the standby state to the printing sequence and the transition from the normal polarity cleaning to the reverse polarity cleaning in the cleaning sequence.  
         [0050]    During the transition from the standby state to the printing preparation sequence, the bias (the intermediate-transfer-member-bias) applied to the second intermediate transfer roll  20  is switched from V 1 ( 30 )=0 [V] (the first intermediate-transfer-member-bias) to V 2 ( 30 )=+1000 [V] (the second intermediate-transfer-member-bias). During the transition from the standby state to the printing preparation sequence, the bias to be applied to the cleaning roll  310  (the intermediate-transfer-member-bias) is switched from V 1 ( 310 )=0 [V] (the first cleaning-member-bias) to V 2 ( 310 )=+1400 [V] (the second cleaning-member-bias). Here, V 2 ( 30 ) and V 2 ( 310 ) are of the same polarity (positive), and the relationship: V 2 ( 30 )&lt;V 2 ( 310 ) holds true.  
         [0051]    In the following, different operation modes for the bias switching control at the transition from the standby state to the printing preparation sequence according to embodiments of the present invention will be described.  
         [0052]    Embodiment 1 FIG. 5 is a graph illustrating the potential gradient control operation for the full color printer  1  of this embodiment. In the graph, the horizontal axis indicates time [sec], and the vertical axis indicates potential [V]. The solid line indicates the variation with time of the surface potential of the second intermediate transfer roll  30 , and the alternate long-and-short dashed line indicate the variation with time of the surface potential of the cleaning roll  310 . Here, when the bias to be applied to the second intermediate transfer roll  30  is switched at the time point t1 from V 1 ( 30 )=0 [V] to V 2 ( 30 )=+1000 [V], the surface potential of the second intermediate transfer roll  30  increases transiently, and attains V 2 ( 30 ) at the time point t1′. When, at the time point t2, the bias to be applied to the cleaning roll  310  is switched from V 1 ( 310 )=0 [V] to V 2 ( 310 )=+1400 [V], the surface potential of the cleaning roll  310  immediately attains V 2 ( 310 ).  
         [0053]    In this embodiment, the potential control portion  9  controls the bias switching timing such that the time point t2 is after the time point t1′. By thus controlling the bias switching timing, no great difference in potential is generated (instantaneously) between the second intermediate transfer roll  30  and the cleaning roll  310 , making it possible to extend the life (service life) of the second intermediate transfer roll  30 . In particular, between the time point t1 and the time point t1′, the surface potential of the second intermediate transfer roll  30  is always higher than that of the cleaning roll  310 , so that the discharge preventing effect is enhanced.  
         [0054]    Embodiment 2  
         [0055]    [0055]FIG. 6 is a graph illustrating a potential gradient control operation for the full color printer  1  according to this embodiment. The items that are the same as those of Embodiment 1 are indicated by the same reference numerals, and a description thereof will be omitted.  
         [0056]    In this embodiment, the bias switching timing is controlled such that the time point t2 is after the time point t1 and before the time point t1′. By thus controlling the bias switching timing, no great difference in potential is (instantaneously) generated between the second intermediate transfer roll  30  and the cleaning roll  310 , making it possible to extend the life (service life) of the second intermediate transfer roll  30 .  
         [0057]    Embodiment 3  
         [0058]    [0058]FIG. 7 is a graph illustrating a potential gradient control operation for the full color printer  1  according to this embodiment. In this embodiment, when at the time point t1 the bias to be applied to the cleaning roll  310  is switched from V 1 ( 310 )=0 [V] to Vm( 310 )=+400 [V] (V 1 ( 310 )&lt;Vm( 310 )&lt;V 2 ( 310 )), the surface potential of the cleaning roll  310  immediately attains Vm ( 310 ). Further, when at the time point t3 the bias to be applied to the cleaning roll  310  is switched from Vm( 310 ) [V] to V 2 ( 310 )=+1400 [V], the surface potential of the cleaning roll  310  immediately attains V 2 ( 310 ). Here, the relationship: |Vm( 310 )−V 1 ( 310 )|&lt;|V 2 ( 310 )−Vm( 310 )| holds true. The items that are the same as those of Embodiment 1 are indicated by the same reference numerals, and a description thereof will be omitted.  
         [0059]    In this embodiment, the potential control portion  9  controls the bias switching timing such that the time point t3 is after the time point t1′. By thus controlling the bias switching timing, no great difference in potential is (instantaneously) generated between the second intermediate transfer roll  30  and the cleaning roll  310 , making it possible to extend the life (service life) of the second intermediate transfer roll  30 .  
         [0060]    Embodiment 4  
         [0061]    [0061]FIG. 8 is a graph illustrating a potential gradient control operation for the full color printer  1  according to this embodiment. In this embodiment, when at the time point t4 the bias to be applied to the cleaning roll  310  is switched from V 1 ( 310 )=0 [V] to Vm( 310 ), the surface potential of the cleaning roll  310  immediately attains Vm( 310 ). Further, when at the time point t5 the bias to be applied to the cleaning roll  310  is switched from Vm( 310 ) [V] to V 2 ( 310 ), the surface potential of the cleaning roll  310  immediately attains V 2 ( 310 ). The items that are the same as those of Embodiments 1 and 3 are indicated by the same reference numerals, and a description thereof will be omitted.  
         [0062]    In this embodiment, the potential control portion  9  controls the bias switching timing such that the time point t4 is after the time point t1 and before the time point t1′. Further, the potential control portion  9  controls the bias switching timing such that the time point t5 is after the time point t1′. By thus controlling the bias switching timing, no great difference in potential is (instantaneously) generated between the second intermediate transfer roll  30  and the cleaning roll  310 , making it possible to extend the life (service life) of the second intermediate transfer roll  30 . In particular, between the time point t1 and the time point t1′, the surface potential of the second intermediate transfer roll  30  is always higher than that of the cleaning roll  310 , so that the discharge preventing effect is enhanced.  
         [0063]    While in Embodiments 1 through 4, described above, the bias switching timing at the time of transition from the standby state to printing sequence is controlled, the bias switching timing at the time of transition from the normal polarity cleaning to the reversed polarity cleaning during the cleaning sequence can also be controlled in the same manner. Further, while in Embodiments 1 through 4, described above, the present invention is applied to the bias switching control for the second intermediate transfer roll  30  and the cleaning roll  310  in contact therewith, it is naturally also possible to apply the present invention to the bias switching control for the first intermediate transfer rolls  20   a  and  20   b  and the cleaning rolls  210   a  and  210   b  in contact therewith.