Patent Publication Number: US-9851659-B2

Title: Transfer device, non-transitory computer readable medium, image forming apparatus, and transfer method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-030082 filed Feb. 19, 2016. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a transfer device, a non-transitory computer readable medium, an image forming apparatus, and a transfer method. 
     SUMMARY 
     According to an aspect of the invention, a transfer device includes: 
     a transport belt that contacts with a photoreceptor including a surface on which a toner image is formed, and transports a recording medium; 
     a transfer roll that transfers the toner image formed on the surface of the photoreceptor to the recording medium while pressing the recording medium to the photoreceptor through the transport belt; 
     an adjusting unit that adjusts a pressing force of the transfer roll against the recording medium; and 
     a controller that controls the adjusting unit so that a first pressing force is stronger than a second pressing force, wherein 
     the first pressing force is the pressing force after the recording medium starts to pass through a transfer position of the transfer roll and is moved by a predetermined distance, and 
     the second pressing force is the pressing force when the recording medium starts to pass through the transfer position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram showing a configuration example of an image forming apparatus; 
         FIG. 2  is a schematic view showing an example of a transporting state of paper in a case where a pressing force of a transfer roll against a photoreceptor is weakened; 
         FIG. 3  is a schematic view showing an example of a transporting state of paper in a case where a pressing force of the transfer roll against the photoreceptor is strengthened; 
         FIG. 4  is a graph showing an example of a smear generation rate and a peeling failure occurrence rate in a case where a pressing force of the transfer roll against the photoreceptor is changed; 
         FIG. 5  is a perspective view showing an example of a schematic configuration of the transfer roll and the adjusting device; 
         FIG. 6  is a front view showing an example of a schematic configuration of the transfer roll and the adjusting device; 
         FIG. 7  is a diagram showing an example of a state of an eccentric cam, in a case where a pressing force of the transfer roll against the photoreceptor is set to a first pressing force; 
         FIG. 8  is a diagram showing an example of a state of an eccentric cam, in a case where a pressing force of the transfer roll against the photoreceptor is set to a second pressing force; 
         FIG. 9  is a diagram showing a configuration example of main parts of an electric system of the image forming apparatus; 
         FIG. 10  is a flowchart showing an example of a flow of a transfer process; 
         FIG. 11  is a schematic view showing a relationship between a position of paper and a pressing force; and 
         FIG. 12  is a diagram showing an example of a shape of the eccentric cam. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments for realizing the invention will be described in detail with reference to the drawings. In addition, the same reference numerals are used for constituent elements performing the same operations and functions in all of the drawings and the description will be omitted for the overlapped parts. 
     Overall Configuration 
     First, a configuration of an image forming apparatus  10  according to the exemplary embodiment will be described with reference to  FIG. 1 . The image forming apparatus  10  includes an image forming apparatus main body  12 , and a paper feeding unit  14  in which plural sheets of paper P which is an example of a recording medium are laminated and accommodated in a bundle form, in a lower portion of the image forming apparatus main body  12 . 
     A pickup roll  16  which is in pressure contact with the paper P and transfers the paper P from the paper feeding unit  14  is disposed right above the paper feeding unit  14  on a front end side (right end side in  FIG. 1 ). 
     A transport roll pair  24  which nips and transports the paper P and a registration roll pair  25  for skew correction of the paper P are disposed on the downstream side of the pickup roll  16 . A first transporting path  22  which is extended from the gap between the transport roll pair  24  and curved upwards, and extends to the gap between the registration roll pair  25  is formed between the transport roll pair  24  and the registration roll pair  25 . 
     The paper P transported from the paper feeding unit  14  passes through the first transporting path  22  and is temporarily stopped by the registration roll pair  25 . Then, the paper P is loaded on a transport belt  26  which will be described later at a predetermined timing and transported to a gap formed with a photoreceptor  30  and a transfer roll  38  which rotate. 
     A position sensor  47  which detects presence and absence of the paper P transported through the transport belt  26  is installed on the upstream side of the transfer roll  38  in a transporting direction of the paper P, to oppose the transport belt  26 . 
     Hereinafter, the gap formed with the photoreceptor  30  and the transfer roll  38  is referred to as a “transfer gap  36 ”. 
     The transport belt  26  is an endless belt containing at least a material such as rubber having elasticity of expanding and contracting in the transporting direction of the paper P. The transport belt is, for example, stretched using a tension roll  27  disposed on the upper portion of the image forming apparatus main body  12  and a tension roll  29  disposed on the lower portion thereof. The transport belt  26 , for example, rotates in a predetermined direction (arrow A direction) and transports the paper P, due to the rotation of one of the tension roll  27  and the tension roll  29 . 
     A blade  45  is installed so as to be in contact with the surface of the transport belt  26 , ahead of a position where the transport belt  26  is folded back toward a direction opposite to the transporting direction of the paper P through the tension roll  27 . The blade  45  removes a toner attached to the surface of the transport belt  26 . 
     Meanwhile, a process cartridge  28 K corresponding to a black color is, for example, disposed to oppose the transport belt  26  in a horizontal direction. The process cartridge  28 K includes a photoreceptor unit  35 , and a developing device  64  including a developing roll  86 . 
     The photoreceptor unit  35 , for example, includes the photoreceptor  30 , a charging device  32 , a finger  41 , a cleaning device  39 , and a charge eliminating device  43 . 
     The charging device  32 , for example, contains a charge corotron and, for example, negatively charges the surface of the photoreceptor  30 . 
     An exposure device  34  is installed beside the process cartridge  28 K. The exposure device  34  emits laser to the photoreceptor  30  which is charged by the charging device  32  to remove the negative charges, and forms a positive electrostatic latent image corresponding to an image desired to be formed by a user (user image) on the photoreceptor  30 . 
     The developing roll  86  is installed in the developing device  64  so as to contact with the photoreceptor  30  and the negatively-charged toner which is supplied from the inner portion of the developing device  64  is attached to the surface thereof. The developing roll  86  attaches the toner attached to the surface, to the electrostatic latent image formed on the photoreceptor  30  in accordance with the rotation of the photoreceptor  30 . Accordingly, the electrostatic latent image on the photoreceptor  30  is developed and a toner image is formed. In the exemplary embodiment, the color of the toner is assumed to be black in the description, unless otherwise noted regarding the color. 
     The finger  41  is, for example, attached to the photoreceptor unit  35  so that one end thereof contacts with the surface of the photoreceptor  30 . When the paper P passes through the transfer gap  36 , the paper P is nipped between the photoreceptor  30  and the transfer roll  38  together with the transport belt  26 , and in this case, the paper P may be peeled from the transport belt  26  to which the paper P is electrostatically adsorbed and the paper may be attached to the photoreceptor  30 , due to a force of the transfer roll  38  pressing the paper P against the photoreceptor  30 , that is, a pressing force. 
     The finger  41  peels the paper P attached to the photoreceptor  30  from the photoreceptor  30  and returns to the upper portion of the transport belt  26 . 
     In the same manner as the finger  41 , the cleaning device  39  is attached to the photoreceptor unit  35 , for example, so that one end thereof contacts with the surface of the photoreceptor  30 , and removes residual toner attached to the photoreceptor  30  after the toner image is transferred to the paper P. 
     The charge eliminating device  43  removes the residual charges on the photoreceptor  30 , after the toner image is transferred to the paper P. An erase lamp which emits light to the photoreceptor  30  to remove the residual charges on the photoreceptor  30  is an example of the charge eliminating device  43 . 
     Meanwhile, a second transporting path  23  which introduces the paper P transported with the transport belt  26  to an exit unit  20  is formed on the downstream side of the transport belt  26  in the paper transporting direction. A fixing device  40  which fixes the toner image transferred to the paper P, to the paper P, a transport roll pair  42  which transports the paper P while nipping the paper, and an exit roll pair  44  which outputs the paper P to the exit unit  20  are disposed on the second transporting path  23 . 
     The fixing device  40  includes a heat source, and performs heating while pressurizing the paper P to which the toner image is transferred, to fix the toner image to the paper P. The fixing device  40  is installed so that a distance between the fixing device  40  and the transfer roll  38  is as short as possible, in order to reduce the size of the image forming apparatus  10 . Accordingly, with respect to the same paper P, the fixing of the transferred toner image may be performed by the fixing device  40 , while performing transfer of the toner image by the transfer roll  38 . 
     A third transporting path  46  for returning the paper P switched back by the exit roll pair  44  to the registration roll pair  25  is formed on a position opposing the transport belt  26 , for example. 
     A transport roll pair  48  which transports the paper P while nipping the paper is disposed on the third transporting path  46 , and the paper P in which an image is formed on one image formation surface of the paper P is switched back by the exit roll pair  44  to be introduced to the third transporting path  46 , at the time of duplex printing. The paper P is transported to the third transporting path  46  by the transport roll pair  48 , and the paper P is transported to the registration roll pair  25  again by a reverse roll  49 , after reversing the image formation surface so that a new toner image is transferred to another image formation surface to which the toner image is not transferred yet. 
     Image Formation Operation 
     In a case of forming an image on the paper P in the image forming apparatus  10  according to the exemplary embodiment, first, the paper P extracted from the paper feeding unit  14  is transported to the registration roll pair  25  by the transport roll pair  24  and is transported to the transport belt  26  after being subjected to skew correction by the registration roll pair  25 . 
     The paper P is transported to the transfer gap  36  in accordance with the movement of the transport belt  26 . 
     The paper P transported to the transfer gap  36  is nipped by the photoreceptor  30  and the transfer roll  38 , and the photoreceptor  30  on which the toner image is formed is pressed against the paper P. At this time, the toner image is more easily transferred to the paper P, by applying a transfer bias voltage having polarity opposite to the charging polarity of the toner to the transfer roll  38  from a power supply for transfer (not shown). Since the toner image on the photoreceptor  30  is transferred to the paper P in the transfer gap  36 , the transfer gap  36  is an example of a transfer position. 
     The paper P to which the toner image is transferred by the transfer roll  38  is transported to the fixing device  40  and the transferred toner image is fixed onto the paper P by the fixing device  40 . 
     In a case of forming a user image on one surface of the paper P (hereinafter, referred to as “simplex printing”), after fixing the transferred toner image to the paper P by the fixing device  40 , the paper P is discharged to the exit unit  20  by the exit roll pair  44 . 
     Meanwhile, in a case of forming a user image to both surfaces of the paper P (hereinafter, referred to as “duplex printing”), after forming the user image on one surface of the paper P, the paper P is switched back by the exit roll pair  44 . Accordingly, the image formation surface of the paper P is reversed and transported to the third transporting path  46 . 
     In addition, the paper P is transported again to the registration roll pair  25  from the third transporting path  46  and passes through the transfer gap  36 , again. Accordingly, the toner image corresponding to the user image is transferred to the other image formation surface of the paper P where the toner image is not transferred yet, by the transfer roll  38 . 
     Details of Transfer Operation 
     Next, the transfer operation of the toner image to the paper P performed by the transfer roll  38  will be described in detail with reference to  FIG. 2  and  FIG. 3 . In  FIG. 2  and  FIG. 3 , the members necessary for the description are selected and shown from the members relating to the transfer, and thus, the charging device  32 , the developing roll  86 , the cleaning device  39 , and the charge eliminating device  43  installed around the photoreceptor  30 , for example, are omitted. 
     First, in the image forming apparatus  10  according to the exemplary embodiment, as shown in  FIG. 2  and  FIG. 3 , a position of a rotation shaft  51  of the transfer roll  38  in a transporting direction of the paper P shown by an arrow  80  is disposed to be different from a position of a rotation shaft  30 A of the photoreceptor  30 . Specifically, the position of the rotation shaft  51  of the transfer roll  38  is disposed on the downstream side of the position of the rotation shaft  30 A of the photoreceptor  30  in the transporting direction of the paper P. 
     Since a transfer bias voltage is applied to the transfer roll  38  when transferring the toner image to the paper P, discharging may occur between the photoreceptor  30  and the transfer roll  38 , in some cases, before the paper P is nipped in the transfer gap  36 . 
     When discharging occurs between the photoreceptor  30  and the transfer roll  38  before the paper P is nipped in the transfer gap  36 , nonuniformity of density may occur in the toner image of the photoreceptor  30  due to influences of the discharging, and quality of the image formed on the paper P may be deteriorated. 
     However, since discharging properties change by disposing the position of the rotation shaft  51  of the transfer roll  38  to the downstream side of the position of the rotation shaft  30 A of the photoreceptor  30  in the transporting direction of the paper P, the discharging occurring between the photoreceptor  30  and the transfer roll  38  before the paper P is nipped in the transfer gap  36  is prevented and a deterioration in quality of the image is prevented, compared to a case where the positions of the rotation shaft  30 A of the photoreceptor  30  and the rotation shaft  51  of the transfer roll  38  in the transporting direction of the paper P are set to be the same. 
     The transfer roll  38  shown in  FIG. 2  shows a state where the transfer roll  38  is disposed to be separated from the photoreceptor  30  within a range that the toner image is transferred to the paper P, so as to weaken a pressing force of the transfer roll  38  against the photoreceptor  30 , compared to the transfer roll  38  shown in  FIG. 3 . 
     As the image forming apparatus  10  according to the exemplary embodiment, in a case where a material having elasticity is used in the transport belt  26 , the transport belt  26  expands and contracts along the transporting direction of the paper P, when the transport belt  26  is stretched and moved in the transporting direction of the paper P due to the rotation of the tension roll  27 . Adhesiveness between the paper P and the photoreceptor  30  decreases in accordance with the weakening of the pressing force of the transfer roll  38  against the photoreceptor  30 , but in accordance with a decrease in adhesiveness between the paper P and the photoreceptor  30 , a position deviation (hereinafter, referred to as “slip”) of the paper P due to the expansion and contraction of the transport belt  26  easily occur in the transfer gap  36  and nonuniformity of density may be generated in the toner image due to rubbing of the paper P and the photoreceptor  30  due to the slip, for example. 
     The nonuniformity of density due to the slip of the paper P (hereinafter, this phenomenon is referred to as “smear”) is easily generated, for example, in a case of transferring a halftone image, which expresses shades by adjusting an adhesion area of a toner on the paper P to the paper P along a width direction of the paper P. Here, the width direction of the paper P is a direction orthogonal to the transporting direction of the paper P and is specifically a direction along a rotation shaft direction of the photoreceptor  30 . 
     Particularly, in a case of transferring the halftone image having density of 50% to the paper P, portions to which the toner is transferred and portions to which the toner is not transferred are alternately expressed at even intervals, and accordingly, when the slip of the paper P occurs and the paper P is rubbed with the photoreceptor  30 , a width of a line or the like expressed by the toner changes to cause a deterioration in quality of an image. 
     Therefore, when the transfer roll  38  is disposed to be closer to the photoreceptor  30 , compared to the case of  FIG. 2 , in order to the strengthening of the pressing force of the transfer roll  38  against the photoreceptor  30 , the adhesiveness between the paper P and the photoreceptor  30  increases, and thus, occurrence of the slip in the transfer gap  36  is prevented. 
     However, by doing so, the paper P is easily attached to the photoreceptor  30  due to the pressing force, in accordance of the strengthening of the pressing force of the transfer roll  38  against the photoreceptor  30 . That is, an angle between the paper P and the transport belt  26  (hereinafter, referred to as a “peeling angle”) increases in accordance with the strengthening of the pressing force of the transfer roll  38  against the photoreceptor  30 , and as shown in  FIG. 3 , peeling failure which is a phenomenon that the paper P is wound around the photoreceptor  30  occurs. 
     In a case where the peeling failure occurs, since a downstream end, in the transporting direction, of the paper P (hereinafter, referred to as a front end of the paper P) which is wound around the photoreceptor  30  collides with the finger  41 , the paper P is peeled from the photoreceptor  30  and returns to the upper portion of the transport belt  26 . 
     As described above, the finger  41  is disposed so as that the one end thereof contacts with the surface of the photoreceptor  30 . However, since residual toner after transfer is present on the surface of the photoreceptor  30 , the residual toner is attached to the end of the finger  41  contacting with the photoreceptor  30 . Accordingly, dirt due to the residual toner attached to the finger may be attached to the front end of the paper P which is peeled from the photoreceptor  30  by the finger  41 . 
       FIG. 4  is a graph showing a generation rate of a deterioration in image quality (hereinafter, referred to as a “smear generation rate”) and a peeling failure occurrence rate in a case where a pressing force of the transfer roll  38  against the photoreceptor  30  is changed. A graph  81  shows a front surface peeling failure occurrence rate, a graph  82  shows a back surface peeling failure occurrence rate, and a graph  83  shows a smear generation rate, respectively. 
     Here, the “front surface peeling failure occurrence rate” indicates an occurrence rate of peeling failure when executing simplex printing and the “back surface peeling failure occurrence rate” indicates art occurrence rate of peeling failure when a toner image is transferred to one image formation surface (back surface) of the paper P in which the toner image is already transferred to the other image formation surface, at the time of duplex printing. 
     A horizontal axis of  FIG. 4  indicates linear pressure which is an example of an index showing a pressing force of the transfer roll  38  against the photoreceptor  30 , and a vertical axis on the right side of  FIG. 4  indicates the smear generation rate and a vertical axis on the left side thereof indicates the peeling failure occurrence rate, respectively. The linear pressure becomes strong towards the right side of the horizontal axis of  FIG. 4 , and the smear generation rate and the peeling failure occurrence rate increase upwardly in the vertical axis of  FIG. 4 . 
     As described above, since occurrence of the slip in the transfer gap  36  is prevented in accordance of an increase in linear pressure of the transfer roll  38 , the smear generation rate decreases. Meanwhile, since the peeling angle of the paper P become large, the peeling failure occurrence rate increases. 
     As shown with the graph  81  and the graph  82 , the back surface peeling failure occurrence rate tends to increase with respect to each linear pressure of the transfer roll  38 , when the front surface peeling failure occurrence rate and the back surface peeling failure occurrence rate are compared to each other. This is may be because the paper P is more easily attached to the photoreceptor  30  in a case of transferring a toner image to the back surface, compared to a case of transferring a toner image to the front surface, due to influences applied when transferring the toner image to the paper P which has been heated once by the fixing device  40  and bending of the paper P due to the attachment of the toner to the surface. 
     Adjustment Mechanism of Pressing Force of Transfer Roll 
     Next, an adjusting device  59  which adjusts a pressing force of the transfer roll  38  of the image forming apparatus  10  according to the exemplary embodiment will be described with reference to  FIG. 5  to  FIG. 8 . 
       FIG. 5  is a perspective view showing an example of a schematic configuration of the transfer roll  38  and the adjusting device  59 ,  FIG. 6  is a front view showing an example of a schematic configuration of the transfer roll  38  and the adjusting device  59 , when the transfer roll  38  and the adjusting device  59  is seen in a direction shown with an arrow  84  of  FIG. 5 . 
     As shown in  FIG. 5  and  FIG. 6 , the adjusting device  59 , for example, includes a rotation shaft  55 , an eccentric cam  52 , a light shielding plate  54 , and optical sensors  56 A and  56 B and has a structure in which the eccentric cam  52  and the light shielding plate  54  are attached to the rotation shaft  55  which rotates in a direction of an arrow  58  by a motor (not shown). 
     The eccentric cam  52  has a shape in which one end of a linear portion of two semicircular body having different diameters is, for example, overlapped with an end of a linear portion of the other semicircular body. Accordingly, in the eccentric cam  52 , distances between a position through which the rotation shaft  55  penetrate (hereinafter, referred to as the “center of eccentric cam  52 ”) and peripheries of the eccentric cam  52  are different. 
     The eccentric cam  52  is attached to the rotation shaft  55  so as to contact with a bearing  50  which is attached to the rotation shaft  51  with the transfer roll  38  and supports the rotation shaft  51 , and the eccentric cam  52  rotates in a direction of the arrow  58  in accordance with the rotation shaft  55 . 
     In  FIG. 5 , among the ends of the eccentric cam  52 , a portion at which the distance from the center of the eccentric cam  52  is longest is shown as a first pressing position  52 A, and a portion at which the distance from the center of the eccentric cam  52  is shortest is shown as a second pressing position  52 B. 
     The rotation shaft  55  of the adjusting device  59  is, for example, attached to the image forming apparatus main body  12  so as not to move in a direction of an arrow  57 , whereas the rotation shaft  51  of the transfer roll  38  is, for example, attached to the image forming apparatus main body  12  so as to move in a direction along the arrow  57 . Accordingly, when the rotation shaft  55  rotates and the first pressing position  52 A of the eccentric cam  52  comes into contact with the bearing  50 , the transfer roll  38  is pressed upwards of the arrow  57  and a pressing force of the transfer roll  38  against photoreceptor  30  is strengthened. 
     Meanwhile, when the rotation shaft  55  rotates and the second pressing position  52 B of the eccentric cam  52  comes into contact with the bearing  50 , the transfer roll  38  moves downwards of the arrow  57  due to the weight of the transfer roll  38 , for example. Accordingly, a pressing force of the transfer roll  38  against photoreceptor  30  is weakened, compared to a case where the first pressing position  52 A of the eccentric cam  52  comes into contact with the bearing  50 . 
       FIG. 7  is a diagram showing an example of a positional relationship between the transfer roll  38  and the eccentric cam  52  in a case where the first pressing position  52 A of the eccentric cam  52  comes into contact with the bearing  50 , and  FIG. 8  is a diagram showing an example of a positional relationship between the transfer roll  38  and the eccentric cam  52  in a case where the second pressing position  52 B of the eccentric cam  52  comes into contact with the bearing  50 . 
     Since a distance L 1  between the rotation shaft  51  of the transfer roll  38  and the rotation shaft  55  of the adjusting device  59  in  FIG. 7  is longer than a distance L 2  between the rotation shaft  51  of the transfer roll  38  and the rotation shaft  55  of the adjusting device  59  in  FIG. 8 , the pressing force of the transfer roll  38  against the photoreceptor  30  in a case where the first pressing position  52 A of the eccentric cam  52  comes into contact with the bearing  50  becomes stronger than the pressing force of the transfer roll  38  against the photoreceptor  30  in a case where the second pressing position  52 B of the eccentric cam  52  comes into contact with the bearing  50 . 
     Hereinafter, the pressing force of the transfer roll  38  against the photoreceptor  30  in a case where the first pressing position  52 A of the eccentric cam  52  comes into contact with the bearing  50  is referred to as a “first pressing force” and the pressing force of the transfer roll  38  against the photoreceptor  30  in a case where the second pressing position  52 B of the eccentric cam  52  comes into contact with the bearing  50  is referred to as a “second pressing force”. 
     The two U-shaped optical sensors  56 A and  56 B each including two projections  561  and  562  are provided around the rotation shaft  55  so that openings formed with the projection  561  an the projection  562  oppose the rotation shaft  55 , and the two light shielding plates  54  are attached to the rotation shaft  55  so as to pass through the openings of the optical sensors  56 A and  56 B. 
     Among the two projections  561  and  562  of the U-shaped optical sensors  56 A and  56 B, a light emitting element such as a light emitting diode which emits light to the projection  562  is attached to the projection  561  and a light receiving element such as a phototransistor which detects light is attached to the projection  562 . 
     As shown in  FIG. 5 , the light shielding plate  54  has a defective part  85  which is formed when a part of a disk is chipped in an arch shape. Accordingly, in a case where the light shielding plates  54  rotate in accordance with the rotation of the rotation shaft  55  and the defective parts  85  of the light shielding plates  54  pass through the openings of the optical sensors  56 A and  56 B, the optical sensors  56 A and  56 B may detect light emitted from the light emitting element attached to the projection  561  of the optical sensors  56 A and  56 B using the light receiving element attached to the projection  562 . Meanwhile, in a case where the portion other than the defective parts  85  of the light shielding plates  54  pass through the openings of the optical sensors  56 A and  56 B, light emitted to the optical sensors  56 A and  56 B from the light emitting element attached to the projection  561  of the optical sensors  56 A and  56 B is shielded by the light shielding plates  54 , and accordingly, the light may not be detected by the light receiving element attached to the projection  562 . 
     Here, for convenience of description, among the two light shielding plates  54  attached to the rotation shaft  55 , one light shielding plate  54  is shown as the “light shielding plate  54 A” and the other light shielding plate  54  is shown as the “light shielding plate  54 B”. In a case where it is not necessary to distinguish the two light shielding plates  54 , these are simply shown as the “light shielding plate  54 ”. 
     As shown in  FIG. 5 , the light shielding plate  54 A is attached to the rotation shaft  55  so that the first pressing position  52 A of the eccentric cam  52  comes into contact with the bearing  50 , in a case where the defective part  85  of the light shielding plate  54 A passes through the opening of the optical sensor  56 A. Meanwhile, the light shielding plate  54 B is attached to the rotation shaft  55  so that the second pressing position  52 B of the eccentric cam  52  comes into contact with the bearing  50 , in a case where the defective part  85  of the light shielding plate  54 B passes through the opening of the optical sensor  56 B. In the light shielding plate  54 , the shape of the defective part  85  is determined so as not to detect the light from the optical sensors  56 A and  56 B at the same time. 
     Therefore, a case where the light is detected by the optical sensor  56 A where the light is shielded by the light shielding plate  54 A indicates that the first pressing position  52 A of the eccentric cam  52  is positioned to come into contact with the bearing  50 , and a case where the light is detected by the optical sensor  56 B where the light is shielded by the light shielding plate  54 B indicates that the second pressing position  52 B of the eccentric cam  52  is positioned to come into contact with the bearing  50 . 
     Next, constituent elements of an electric system of the image forming apparatus  10  according to the exemplary embodiment will be described with reference to  FIG. 9 . 
     As shown in  FIG. 9 , the image forming apparatus  10  according to the exemplary embodiment includes a central processing unit (CPU)  130  which is an example of a controller which controls overall operations of the image forming apparatus  10 , and a read only memory (ROM)  132  in which various programs and various parameters are stored in advance. The image forming apparatus  10  further includes a random access memory (RAM)  134  which is used as a work area when executing various programs by the CPU  130 , and a non-volatile memory  136  such as a flash memory. A timer unit which stores current time is, for example, embedded in the CPU  130 . 
     In addition, the image forming apparatus  10 , for example, includes a communication line interface (I/F) unit  138  which transmits and receives image data to and from an external apparatus connected to a communication line. The image forming apparatus  10  includes an operation display  140  as an example of a notification unit which receives instruction with respect to the image forming apparatus  10  from a user and notifies a user of various information items relating to an operation state of the image forming apparatus  10 . The operation display  140 , for example, includes display buttons for expressing the reception of an operation instruction by executing a program; or a display in which a touch panel is provided in a display surface on which various information items are displayed, and hardware keys such as numeric keys or a start button. 
     The image forming apparatus  10  includes various transport motors  142  for rotating various rolls such as the pickup roll  16 , the transport roll pair  24 , the registration roll pair  25 , the tension rolls  27  and  29 , the transport roll pairs  42  and  48 , the exit roll pair  44 , and the reverse roll  49  which are used in the transportation of the paper P. 
     The image forming apparatus  10  includes an adjusting motor  144  which rotates the rotation shaft  55  of the adjusting device  59 , the optical sensors  56 A and  56 B, and the position sensor  47  which detects whether or not the paper P before a toner image is transferred is loaded on the transport belt  26 . 
     The image forming apparatus  10  includes an image forming unit  146  which includes the process cartridge  28 K or the like which is a configuration member for forming a toner image on the surface of the photoreceptor  30 , and the fixing device  40  which fixes a toner image to the paper P by drying and pressing the toner image transferred to the paper P. 
     Each unit of the CPU  130 , the ROM  132 , the RAM  134 , the memory  136 , the communication line I/F unit  138 , the operation display  140 , the transport motor  142 , the adjusting motor  144 , the optical sensors  56 A and  56 B, the position sensor  47 , the image forming unit  146 , and the fixing device  40  is connected to each other through a bus  148  including an address bus, a data bus, and a control bus. 
     With the configuration described above, the image forming apparatus  10  according to the exemplary embodiment accesses the ROM  132 , the RAM  134 , and the memory  136  and transmits and receives of communication data to and from an external apparatus through the communication line I/F unit  138 , by using the CPU  130 . The image forming apparatus  10  acquires various instruction information items through the operation display  140  and displays various information items with respect to the operation display  140 , by using the CPU  130 . The image forming apparatus  10  controls the rotation of the transport motor  142  and the adjusting motor and controls the image forming unit  146  and the fixing device, by using the CPU  130 . The image forming apparatus  10  acquires each of detection information of the paper P detected by the position sensor  47  and information relating to the received light detected by the optical sensors  56 A and  56 B, by using the CPU  130 . 
     Next, an operation of the image forming apparatus  10  in a transfer operation will be described with reference to  FIG. 10 . 
       FIG. 10  is a flowchart showing an example of a flow of a transfer process which is executed by the CPU  130 , when transferring a toner image formed on the surface of the photoreceptor  30  to the paper P by the transfer roll  38 . A program (transfer program) regulating the transfer process is, for example, installed in the ROM  132  in advance. 
     A toner image is already formed on the surface of the photoreceptor  30 , and in the adjusting device  59 , the second pressing position  52 B of the eccentric cam  52  comes into contact with the bearing  50 , and accordingly, the transfer roll  38  is set to press the photoreceptor  30  with the second pressing force. 
     First, in Step S 10 , the CPU  130  acquires an output value of the position sensor  47  and determines whether or not a front end of the paper P is detected on the transport belt  26 . 
     The position sensor  47 , for example, outputs “0” as an output value, in a case where the paper P is not present in a detection range of the position sensor  47  and outputs “1” as an output value, in a case where the paper P is present therein. Accordingly, the CPU  130  may determine that the front end of the paper P is detected, in a case where the output value of the position sensor  47  changes from “0” to “1”. The output value output by the position sensor  47  is merely an example, and for example, the position sensor may output “1”, in a case where the paper P is not present in a detection range of the position sensor  47  and may output “0”, in a case where the paper P is present therein. 
     The negative determination of the determination process in Step S 10  indicates a state where the paper P is not transported to the transport belt  26 , and accordingly, the process returns to the process in Step S 10  and the detection of the front end of the paper P is continued. 
     Meanwhile, in a case of the positive determination of the determination process in Step S 10 , the time when the front end of the paper P is detected (hereinafter, referred to as “detection time of paper P”) is acquired from the timer unit included in the CPU  130 , the time is stored, for example, in a predetermined area of the RAM  134 , and the process proceeds to Step S 20 . 
     The detection time of the paper P may be acquired from an external apparatus such as a time server connected to the communication line via the communication line I/F unit  138 , for example. 
     In Step S 20 , the CPU  130  calculates a movement distance of the paper P from the detected position of the front end of the paper P by the position sensor  47 , using a transport speed of the transport belt  26  and an elapsed time from the detection time of the paper P stored in the RAM  134  in Step S 10 . 
     Specifically, the CPU  130  acquires the current time from the timer unit included in the CPU  130 , acquires the detection time of the paper P from the RAM  134 , and calculates the elapsed time from the detection time of the paper P. The CPU  130  calculates a distance from the detected position of the front end of the paper P by the position sensor  47  to the front end of the transported paper P by multiplying the calculated elapsed time by the transport speed of the transport belt  26 . 
     In a case where the transport speed of the transport belt  26  changes depending on information relating to the image formation such as the type of the paper P classified by a thickness of the paper P or the size of the paper P, the transport speed of the transport belt  26  may be calculated from the information relating to the image formation which is set in the operation display  140  by a user, for example. In a case where the transport speed of the transport belt  26  is a fixed value, the transport speed may be stored in a predetermined area of the memory  136 , for example. 
     In parallel with the process in Step S 20 , the transfer of a toner image formed on the surface of the photoreceptor  30  to the paper P transported to the transfer gap  36  is started by the transfer roll  38 . As described above, when the paper P starts to pass through the transfer gap  36 , the second pressing position  52 B of the eccentric cam  52  contacts with the bearing  50 , and accordingly, the pressing force of the transfer roll  38  against the photoreceptor  30  is set to be smaller than that in a case where the first pressing position  52 A of the eccentric cam  52  contacts with the bearing  50 . 
     In Step S 30 , the CPU  130  determines whether or not the movement distance of the paper P calculated in Step S 20  satisfies a regulated distance. 
     This regulated distance is, for example, set to a distance between the detected position of the front end of the paper P by the position sensor  47 , and a position where the front end of the paper P is moved from the transfer gap  36  by the predetermined distance L after the front end of the paper P starts to pass through the transfer gap  36 . The regulated distance is, for example, set in a predetermined area of the memory  136  in advance and the CPU  130  acquires the regulated distance from the memory  136 . 
     In a case of negative determination of the determination process in Step S 30 , the process returns to the process in Step S 20  and the processes in Steps S 20  and S 30  are repeated until the movement distance of the paper P calculated in Step S 20  satisfies the regulated distance. 
     By performing the control described above, the transfer is performed with the second pressing force which is a pressing force weaker than the first pressing force, from the time when the front end of the paper P is started to be pressed in the transfer gap  36  by the transfer roll  38  until the time when the paper P is moved in the transporting direction of the paper P by the distance L. Accordingly, compared to a case where the transfer is performed with the first pressing force, a range of the paper P which is moved upwards to the upstream side of the front end of the paper P in the transporting direction of the paper P by the distance L is hardly attached to the photoreceptor  30 , and the peeling failure occurrence rate decreases. 
     The distance L is set so that the peeling failure occurrence rate in a case where the paper P is pressed with the second pressing force becomes equal to or less than the predetermined peeling failure occurrence rate, and the distance L is determined in advance by experiment performed by a real machine of the transfer roll  38  or a computer simulation or the like based on the design specification of the transfer roll  38 . Here, as long as a peeling failure occurrence rate is equal to or less than the predetermined peeling failure occurrence rate, the peeling failure occurrence rate is assumed as a peeling failure occurrence rate with which it is assumed that there is no problems in the image forming operation of the image forming apparatus  10 . 
     Meanwhile, in a case of the positive determination of the determination process in Step S 30 , that is, in a case where the front end of the paper P is moved from the transfer gap  36  by the predetermined distance L or longer, the process proceeds to Step S 40 . 
     In Step S 40 , the CPU  130  controls the adjusting motor  144  to rotate the rotation shaft  55  of the adjusting device  59 . 
     In Step S 50 , the CPU  130  acquires the output value of the optical sensor  56 A to determine whether or not the light is detected by the optical sensor  56 A. 
     The optical sensor  56 A, for example, outputs “0” as an output value, in a case where light is detected by the light receiving element of the optical sensor  56 A and outputs “1” as an output value, in a case where light is not detected. Accordingly, the CPU  130  may determine that light is detected by the optical sensor  56 A, in a case where “1” is acquired as the output value from the optical sensor  56 A. The output value output by the optical sensor  56 A is merely an example, and for example, the optical sensor may output “1”, in a case where light is detected by the light receiving element of the optical sensor  56 A and output “0”, in a case where light is not detected. 
     In a case of the negative determination of the determination process in Step S 50 , the process in Step S 50  is repeated until light is detected by the optical sensor  56 A. On the other hand, in a case of the positive determination, the process proceeds to Step S 60 . 
     In Step S 60 , the CPU  130  controls the adjusting motor  144  to stop the rotation of the rotation shaft  55  of the adjusting device  59 . By stopping the rotation of the rotation shaft  55  of the adjusting device  59  at the timing when the optical sensor  56 A detects light in the process in Step S 50 , the first pressing position  52 A of the eccentric cam  52  moves to a position to come into contact with the bearing  50 . That is, after the paper P is moved from the transfer gap  36  in the transporting direction of the paper P by the distance L, the transfer is performed with the first pressing force which is a pressing force stronger than the second pressing force. 
     Accordingly, compared to a case where the transfer is performed with the second pressing force, the smear generation rate in a range of the paper P from the position moved upwards to the upstream side of the front end of the paper P in the transporting direction of the paper P by the distance L, to the upstream end, in the transporting direction, of the paper P (hereinafter, “rear end of the paper P”) is decreased. 
     In Step S 70 , the CPU  130  calculates a movement distance of the paper P from the detected position of the front end of the paper P by the position sensor  47 , in the same manner as in Step S 20 . 
     In Step S 80 , the CPU  130  determines whether or not the rear end of the paper P has passed through the transfer gap  36 , by using the movement distance of the paper P calculated in Step S 70 . Specifically, the CPU  130  determines whether or not the front end of the paper P is moved from the transfer gap  36  by a length of the paper P. Here, the length of the paper P means a length thereof along the transporting direction of the paper P, and is, for example, acquired from the size of the paper P set in the operation display  140 . 
     In a case of negative determination of the determination process in Step S 80 , that is, in a case where the toner image is being transferred to the paper P by the transfer roll  38  yet, the process proceeds to Step S 70  and the processes in Steps S 70  and S 80  are repeated until the transfer to the paper P is completed. On the other hand, in a case of the positive determination of the determination process in Step S 80 , that is, in a case where the transfer of the toner image to the paper P by the transfer roll  38  is completed, the process proceeds to Step S 90 . 
     In Step S 90 , the CPU  130  controls the adjusting motor  144  to rotate the rotation shaft  55  of the adjusting device  59 . 
     In Step S 100 , the CPU  130  executes the same process as the process sin Step S 50  and determines whether or not light is detected by the optical sensor  56 B. 
     In a case of the negative determination of the determination process in Step S 100 , the process in Step S 100  is repeated until light is detected by the optical sensor  56 B. On the other hand, in a case of the positive determination of the determination process, the process proceeds to Step S 110 . 
     In Step S 110 , the CPU  130  controls the adjusting motor  144  to stop the rotation of the rotation shaft  55  of the adjusting device  59 . By stopping the rotation of the rotation shaft  55  of the adjusting device  59  at the timing when the optical sensor  56 B detects light in the process in Step S 100 , the second pressing position  52 B of the eccentric cam  52  moves to a position to come into contact with the bearing  50 . That is, the pressing force of the transfer roll  38  against the photoreceptor  30  in a case where the front end of the next paper P is transported to the transfer gap  36  is set to the second pressing force. 
     In Step S 120 , the CPU  130  determines whether or not the transfer of the toner image instructed by a user is completed. 
     In a case of negative determination of the determination process in Step S 120 , the process proceeds to Step S 10  and is on standby until the next paper P which is a transfer target of a toner image is transported onto the transport belt  26 . In a case where the front end of the paper P is detected in the process in Step S 10 , the CPU  130  executes the processes in Steps S 20  to S 120  described above, to continuously transfer the toner image to the paper P while changing the pressing force of the transfer roll  38  against the photoreceptor  30 . 
     Meanwhile, in a case of the positive determination of the determination process in Step S 120 , the transfer process shown in  FIG. 10  ends. 
       FIG. 11  is a diagram showing a relationship between a paper position of the paper P and the pressing force in a case where the transfer process shown in  FIG. 10  is executed. 
     As shown in  FIG. 11 , according to the image forming apparatus  10  according to the exemplary embodiment, a range of the paper P which is moved toward the upstream side of the front end of the paper P in the transporting direction of the paper P by the distance L is pressed by the transfer roll  38  with the second pressing force F 2  which is weaker than the first pressing force F 1  to perform the transfer of the toner image formed on the photoreceptor  30 . Meanwhile, a range of the paper P after the front end of the paper P is moved from the transfer gap  36  in the transporting direction of the paper P by the distance L is pressed by the transfer roll  38  with the first pressing force F 1  which is stronger than the second pressing force F 2  to perform the transfer of the toner image formed on the photoreceptor  30 . 
     Therefore, the image forming apparatus  10  decreases the peeling failure occurrence rate of the paper P and the smear generation rate of the paper P. 
     As the thickness of the paper P decreases, rigidity of the paper P showing a degree of bendability of the paper P in a case where a pressing force is applied to the paper P decreases. That is, as the thickness of the paper P decreases, the paper P tends to be easily attached to the photoreceptor  30  at the time of the transfer operation. Accordingly, as the thickness of the paper P decreases, it is preferable to set the distance L longer. By setting the distance L as described above, the paper P is hardly attached to the photoreceptor  30  and the peeling angle of the paper P is controlled to be in a predetermined range. 
     In a case where the thickness of the paper P is equal to or greater than a regulated thickness, the rigidity of the paper P also increases in accordance thereto. Accordingly, it is considered that the paper P is hardly attached to the photoreceptor  30 , even when the toner image is transferred to the paper P over the front end to the rear end, while fixing the pressing force of the transfer roll  38  against the photoreceptor  30  at the first pressing force. Accordingly, in a case where the thickness of the paper P is less than the thickness (for example, a basis weight of the paper P is less than 50 [g/m2]) so as to obtain equal to or greater than the predetermined peeling failure occurrence rate, the transfer process shown in  FIG. 10  of changing the pressing force of the transfer roll  38  against photoreceptor  30  in a different part of the paper P may be executed. 
     As described above, after the toner image is transferred to the paper P by the transfer roll  38 , the toner image transferred to the paper P is fixed onto the paper P by the fixing device  40 . At this time, the fixing device  40  heats the toner image while nipping the paper P in contact portions formed with a roll pair included in the fixing device  40  to fix the toner image to the paper P. 
     In this case, when the thickness of the paper P is equal to or greater than a predetermined thickness, the paper P is difficult to be inserted between the contact portions. Accordingly, when the paper P having a thickness equal to or greater than a predetermined thickness is transported to the fixing device  40 , the front end of the paper P collides with the roll pair forming the contact portions of the fixing device  40  once, and then the paper P is drawn into the rotation of the roll pair of the fixing device  40  and interposed between the contact portions of the fixing device  40 . 
     When the paper P collides with the roll pair of the fixing device  40 , paper deviation in which the paper P is deviated to the upstream side in the transporting direction due to an impact of collision occurs. At that time, when the toner image is transferred to the paper P having the front end collided with the roll pair of the fixing device  40  by the transfer roll  38 , the paper P is rubbed with the photoreceptor  30  due to the paper deviation in the transfer gap  36 , and thus, quality of an image may be deteriorated. 
     Accordingly, in the adjusting device  59 , when the pressing force of the transfer roll  38  against the photoreceptor  30  is set to a pressing force stronger than the first pressing force at the timing when the front end of the paper P having a thickness equal to or greater than a predetermined thickness (for example, a basis weight of the paper P is equal to or greater than 100 [g/m2]) approaches the contact portions of the fixing device  40 , the paper deviation is prevented. 
     In this case, the shape of the eccentric cam  52  is changed to a shape having a point (for example, a third pressing position) where a distance from the center of the eccentric cam  52  is longer than the first pressing position  52 A, in addition to the first pressing position  52 A and the second pressing position  52 B. An optical sensor and a light shielding plate corresponding to the third pressing position are added to the adjusting device  59 . The light shielding plate added is attached to the rotation shaft  55  of the adjusting device  59  so that light is detected by the optical sensor added in a case where the third pressing position of the eccentric cam  52  comes into contact with the bearing  50  and the optical sensor added does not detect light in other cases. 
     The image forming apparatus  10  calculates a movement distance of the paper P by the same process as in Step S 20  of  FIG. 10 , and the rotation shaft  55  of the adjusting device  59  may rotate so that the third pressing position comes to contact with the bearing  50  at the timing when the front end of the paper P approaches the contact portions of the fixing device  40 . 
     As shown in  FIG. 4 , the back surface peeling failure occurrence rate tends to increase more than the front surface peeling failure occurrence rate. Accordingly, it is preferable to set the distance L in a case of transferring a toner image to the back surface of the paper P to which a toner image is transferred to one image formation surface, to be longer than the distance L used when transferring a toner image to one image formation surface. As described above, by setting the distances L from the front end of the paper P where the transfer is performed with the second pressing force to be different for each image formation surface, the peeling angle of the paper P in a case where the toner is transferred to the back surface of the paper P further decreases and the peeling angle of the paper P is controlled to be in the predetermined range. 
     Since the back surface peeling failure occurrence rate tends to increase more than the front surface peeling failure occurrence rate, the transfer process shown in  FIG. 10  may be applied to only in a case of transferring a toner image to the back surface of the paper P, in a case of performing duplex printing in the image forming apparatus  10 . 
     The shape of the eccentric cam  52  of the adjusting device  59  shown in  FIG. 7  is merely an example and there is no limitation. The shape of the eccentric cam  52  may be any shape, as long as it is a shape so as to have different distances from the position through which the rotation shaft  55  penetrates to the end of the eccentric cam  52 , or may be substantially a rhombic shape having round corners, as shown in  FIG. 12 , for example. 
     Hereinabove, the invention has been described using the exemplary embodiments, but the invention is not limited to the range described in the exemplary embodiments. Various modifications or improvement can be performed in the exemplary embodiment within a range not departing from the gist of the invention, and the modified and improved embodiments are also included in the technical scope of the invention. The order of the processes may be changed within a range not departing from the gist of the invention, for example. 
     The invention may be, for example, applied to a transfer operation of an image forming apparatus using a direct transfer belt (DTB) system of forming an image having a full-color quality. In an image forming apparatus corresponding to full-color system using the DTB system, process cartridges corresponding to each color of cyan, magenta, and yellow are disposed to oppose the transport belt  26 , in addition to the process cartridge  28 K corresponding to black. When transferring a toner image having each color to the paper P by the transfer roll, the transfer process shown in  FIG. 10  is executed. 
     In the exemplary embodiment, the transfer program has been installed in the ROM  132 , but there is no limitation. The transfer program according to the invention can be provided in a state of being recorded in a computer-readable recording medium. For example, the transfer program according to the invention can be provided in a state of being recorded in a portable recording medium such as a compact disc (CD)-ROM, a DVD (digital versatile)-ROM, or a universal serial bus (USB). In addition, the transfer program according to the invention can be provided in a state of being recorded in a semiconductor memory such as a flash memory. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.