JP2018193150A - Paper conveyance mechanism, image forming device - Google Patents

Abstract

To precisely control conveyance speed of paper.SOLUTION: In this configuration, since the paper P is driven and conveyed on both sides from both a pressing roller 10 and a belt 20, slippage hardly occurs as compared with the conventional paper conveyance mechanism driven only by the belt. Therefore, the conveyance speed can be precisely controlled. In the case where the conveyance speed is faster than the reference value, it is necessary to lower the rotational speed of the pressing roller 10 and perform control to increase the drive speed of the belt 20 in order to suppress slippage and bring the conveyance speed closer to the reference value. In the case where the actually measured conveyance speed is slower than the reference value, it is only necessary to increase the rotational speed of the pressing roller 10 and to lower the drive speed of the belt 20.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a paper transport mechanism that transports paper using a driven belt, and an image forming apparatus using the same.

  In an image forming apparatus such as a printer, for example, after a toner patterned and carried on a photosensitive drum is transferred to a sheet, the toner pattern is fixed on the sheet using a fixing heating roll or the like. Is output. In such a process, the paper is curved (curled). If the sheet is curled in this way and then transported, problems such as the sheet being jammed in the transport path or the sheet being distorted in the output tray will occur. For this reason, a mechanism for correcting the roll of paper is used.

  On the other hand, in order to convey the paper, a mechanism using a plurality of rollers and a belt stretched over the rollers is used. In the configuration described in Patent Document 1, a pressing roller pressed in a direction to correct the curl of the paper is used, and when the paper is conveyed between the pressing roller and an elastically deformed belt, The sheet is deformed in the direction opposite to the curl by the pressing roller. At this time, the belt is placed on a roller (drive roller) that is located at a distance from the pressing roller and is driven to rotate, is driven at a predetermined speed, and the paper is conveyed along the movement of the belt. For this reason, the curl is corrected when the sheet is conveyed by this mechanism. The degree of curl correction is determined by the degree of curvature applied to the paper of the pressure roller during the conveyance, and the degree of curvature is determined by the amount of protrusion of the pressure roller toward the paper and the belt, that is, the position of the pressure roller. A pressing roller is provided so that this position can be adjusted and is fixed when the paper is conveyed. Note that the pressing roller is not driven to rotate and is driven when the paper is moved. For this reason, the sheet is moved by a belt driven by a driving roller.

  In this configuration, although the paper is driven by the belt, in reality, a slight slip occurs between the paper and the belt, so that the paper conveyance speed is the belt driving speed (in the linear portion of the belt). Smaller than (movement speed). In particular, this situation varies depending on the thickness of the paper and the position of the pressure roller. For example, when the paper is thick or the protrusion of the pressure roller is large, the slip is small and the paper transport speed is large, and when the paper is thin or the protrusion of the pressure roller is small, the slip is large and the paper The conveyance speed is reduced.

  For this reason, in the technique described in Patent Document 1, the belt driving speed (rotation speed of the driving roller) is adjusted according to the above situation so that the paper transport speed is constant. This suppresses the occurrence of a paper jam or the like.

Japanese Patent No. 3879379

  In the configuration described in Patent Document 1, it is assumed that there is slippage between the belt and the paper. Although the slip or the relationship between the belt driving speed and the sheet conveying speed has the above-described tendency, the slipping condition varies depending on the condition of the interface between the belt and the sheet. In particular, since there are various types of paper, the situation of this interface is not constant, and it is difficult to precisely control the conveyance speed regardless of the type of paper.

  That is, a paper transport mechanism capable of precisely controlling the paper transport speed has been desired.

  This invention is made | formed in view of such a condition, and it aims at providing the technique which can solve the said subject.

The paper transport mechanism of the present invention is a paper transport mechanism that transports paper using a belt that can be elastically deformed and moves in the length direction, and protrudes to curve the belt toward the belt side. A pressing roller which is installed and is rotationally driven so that the surface on the side in contact with the belt moves in the same direction as the moving direction of the belt with the paper sandwiched between the belt and the belt; The moving speed of the surface in contact with the paper is set larger than the moving speed of the surface of the pressing roller in contact with the paper.
The paper transport mechanism of the present invention is characterized in that the amount of protrusion of the pressing roller toward the belt can be adjusted.
In the paper transport mechanism of the present invention, a reference value of the paper transport speed is set, and the moving speed of the surface of the pressure roller according to the magnitude relationship between the actually measured transport speed and the reference value, and the belt The drive speed is controlled.
The paper transport mechanism of the present invention decreases the moving speed of the surface of the pressing roller and increases the driving speed of the belt when the actually measured transport speed is recognized to be higher than the reference value. Control is performed.
The paper transport mechanism of the present invention increases the moving speed of the surface of the pressing roller and decreases the driving speed of the belt when the actually measured transport speed is recognized to be lower than the reference value. Control is performed.
In the paper transport mechanism of the present invention, a reference value of the transport speed of the paper is set, and the belt does not slip so that no slip occurs between the paper transported at the transport speed that is the reference value and the belt. A driving speed is set, and a moving speed of the surface of the pressure roller is set so that no slip occurs between the sheet conveyed at the conveyance speed that is the reference value and the surface of the pressure roller. It is characterized by that.
In the paper transport mechanism of the present invention, the driving speed of the belt and the moving speed of the surface of the pressing roller are set according to the thickness or type of the paper.
The image forming apparatus of the present invention is characterized in that the paper transport mechanism is used to form and output an image on the paper.

  With the above configuration, it is possible to precisely control the sheet conveyance speed.

It is a figure which shows the structure of the paper conveyance mechanism which concerns on embodiment of this invention. It is a figure which expands and shows the condition of a press roller periphery in the paper conveyance mechanism which concerns on embodiment of this invention.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the structure of a paper transport mechanism 1 according to an embodiment of the present invention. This paper transport mechanism 1 is used in an image forming apparatus having a function of forming and outputting (printing) an image on paper P. At this time, the paper transport mechanism 1 is used to correct the curl of the paper P in the image forming apparatus, and the curl is corrected when the paper P is transported by the paper transport mechanism 1. Actually, the material constituting the paper P is not limited to paper, and the paper P may be any deformable medium on which an image can be formed. For this reason, the paper P may be a film made of a resin material, such as an OHP sheet. Hereinafter, such a case is included as the paper P.

  The degree of correction of the paper P can be adjusted by the set position of the pressing roller 10 (the amount of protrusion to the paper P side) as in the technique described in Patent Document 1. In addition, a driving roller 11 that is rotationally driven and whose rotational speed can be adjusted, and a driven roller 12 that is separated from the driving roller 11 are used, and elastic deformation is caused between the driving roller 11 and the driven roller 12. A possible belt 20 is stretched over. The belt 20 is driven by the rotation of the driving roller 11, and the driven roller 12 functions as a guide for the belt 20. It is assumed that no slip occurs between the belt 20 and the surface of the driving roller 11 and the surface of the driven roller 12. For this reason, the driving speed of the belt 20 (the moving speed of the belt 20 in the linear portion in FIG. 1) is adjusted by controlling the rotational speed of the driving roller 11.

  Further, in FIG. 1, a tension roller 13 is also provided that contacts the belt 20 from the inside of the annular belt 20 and contacts the belt 20 so as to apply pressure to the outside so as to apply tension to the belt 20. The rotation axes of the pressing roller 10, the driving roller 11, the driven roller 12, and the tension roller 13 in FIG. 1 are perpendicular to the paper surface at the respective centers. Further, the driving roller 11 and the pressing roller 10 are driven to rotate counterclockwise and clockwise in the drawing, respectively, and the surface of the belt 20 moves counterclockwise as a whole in the drawing. The driven roller 12 and the tension roller 13 are not driven to rotate, but are driven as the belt 20 moves. Since the positions of the rotation axes of the driving roller 11 and the driven roller 12 are fixed, the belt 20 can be driven stably. At this time, tension is applied to the belt 20 by the tension roller 13, and the belt 20 is It can be in a state without slack. Further, since the belt 20 can be elastically deformed, the belt 20 is driven in a state without slack regardless of the setting position of the pressing roller 10.

  FIG. 1 shows the situation when the paper P is transported. At this time, the paper P is sandwiched between the belt 20 and the pressing roller 10, and the belt 20 moves from the left side to the right side in the figure. To do. At this time, the moving speed of the surface of the belt 20 in contact with the paper P is determined by the driving speed of the belt 20 (moving speed of the linear portion of the belt 20 in FIG. 1), and this driving speed is determined by the rotational speed of the driving roller 11. On the other hand, in the paper transport mechanism 1, the pressing roller 10 is also driven to rotate clockwise in FIG. The moving speed of the surface of the pressing roller 10 in contact with the paper P is determined by the rotating speed of the pressing roller 10.

  When the paper P is conveyed, guides 31 and 32 are provided before and after the pressing roller 10, respectively. Therefore, in FIG. 2, the paper P is properly extended before and after the pressing roller 10. For example, the paper P is bent. It is suppressed that the sheet is conveyed in the conveyed state. The guide 31 is provided with a paper sensor 33 that optically detects the presence or absence of the paper P. It can be recognized by the paper sensor 33 that the paper P is in a state immediately before it contacts the pressing roller 10. Further, the guide 32 is provided with a transport speed sensor 34 that optically recognizes an actual moving speed (transport speed) of the transported paper P. In general, the thickness of the paper P is negligibly thin compared to the size of each part in FIG. 1 and the belt 20 is elastic, so that the paper P is not between the belt 20 and the pressure roller 10 ( During non-conveyance), the belt 20 and the pressing roller 10 are in direct contact.

  FIG. 2 is an enlarged view showing a situation in the vicinity of the pressing roller 10 when the paper P is conveyed in FIG. Since the paper P is transported so that the curl of the paper P is opposite to the curvature of the paper P and the belt 20 in FIG. 2, the curl can be corrected at the time of transport. At this time, the position of the pressing roller 10 can be adjusted in the range indicated by the arrow A1 as described above, and the curvature radii of the belt 20 and the paper P in FIG. 2 vary depending on the position. Specifically, when the position of the pressing roller 10 is on the upper left side in FIG. 2, the radius of curvature is small, and when it is on the lower right side, the radius of curvature is large. In the former case, the degree of curl correction of the paper P is strong, and in the latter case, this degree is weak. In any case, the state in which the belt 20 and the paper P are curved so as to have a convex shape toward the upper left side in the figure is maintained. For this reason, the paper P is pressed by the pressing roller 10 from the lower right side in the drawing and is pressed by the belt 20 from the upper left side in the drawing. As described above, the belt 20 is formed of an elastic body, and the paper P can be deformed at least in a narrow range. Therefore, the belt 20 and the paper P are in this form regardless of the setting position of the pressing roller 10, and the arrow A2, It can move in the direction indicated by A3.

  In this configuration, the paper P is driven and transported on both sides from both the pressing roller 10 and the belt 20, and therefore, slippage is less likely to occur compared to a conventional paper transport mechanism driven only by the belt. For this reason, a conveyance speed can be controlled precisely. For this reason, the curl correction of the paper P can be stably performed, and thus the curl correction ability is enhanced. For this reason, the paper transport mechanism 1 includes a CPU (control unit: not shown) that adjusts the rotational speed of the pressing roller 10 and the driving speed of the belt 20 (the rotational speed of the driving roller 11). Below, the content of the control which this control part performs is demonstrated.

  First, when the conveyance speed of the paper P is set to a preset value, the surface of the pressure roller 10 for preventing slippage between the paper P and the pressure roller 10 and between the paper P and the belt 20. The moving speed and the driving speed of the belt 20 will be described below. The moving speed of the surface of the pressing roller 10 and the driving speed of the belt 20 are individually controlled, and these relationships depend on the thickness of the paper P.

  In FIG. 2, the sheet P is in contact with the pressing roller 10 and the belt 20 in a region (nip region) where the range of the rotation angle of the pressing roller 10 is between B and C. At this time, since the paper P is curved in the direction opposite to the curl, the curl is corrected. Since the belt 20 and the paper P are elastically deformed, the paper P can be conveyed. Since the deformation amount of the paper P at this time is small, it can be considered that the thickness tp of the paper P and the thickness tb of the belt 20 are uniform.

  When no slip occurs between the pressing roller 10 and the paper P in the nip region in FIG. 2, the moving speed of the surface of the pressing roller 10 is equal to the moving speed of the surface of the paper P on the pressing roller 10 side. Therefore, when the radius of the pressing roller 10 in FIG. 2 is r and the angular velocity around the center O of the pressing roller 10 is ω, the moving speed Vrt = r × ω. On the other hand, if no slip occurs between the belt 20 and the paper P in this region, the moving speed of the belt 20 on the paper P side is equal to the moving speed of the paper P on the belt 20 side. Furthermore, in order for the pressing roller 10, the sheet P, and the belt 20 to move together in the nip region, the belt 20 and the sheet P need to move around the center O of the pressing roller 10 at the same angular velocity ω. Therefore, the movement speed Vrt1 on the belt 20 side of the paper P is as follows when ω is eliminated from the above relationship, and this Vrt1 becomes equal to the movement speed of the surface of the belt 20 on the paper P side.

  Further, the conveyance speed Vpf of the paper P is a movement speed of the paper P outside the nip region. Vpf is equal to the moving speed at the central portion in the thickness direction of the paper P in the nip region, and is as follows similarly to the above Vrt1.

  Further, the driving speed Vb of the belt 20 is a moving speed outside the nip region of the belt 20, and is equal to the moving speed at the center in the thickness direction in the nip region as described above. .

  Since the belt 20 is at the outermost side and the surface of the pressing roller 10 is at the innermost side as viewed from the center of rotation O, and the paper P is between them, Vb> Vpf> Vrt is always satisfied. Here, since the radius r of the pressing roller 10 and the thickness tb of the belt 20 are determined according to the apparatus configuration, they can be considered constant. Further, the transport speed Vpf of the paper P is set according to the image forming apparatus in which the paper transport mechanism 1 is used. On the other hand, the thickness tp of the paper P varies depending on the type of the paper P. For this reason, in the paper transport mechanism 1, the moving speed Vrt and the angular speed ω of the surface of the pressing roller 10 are determined from the required transport speed Vpf of the paper P and the thickness tp of the paper P by the equation (2), and ( By determining the moving speed Vb (rotational speed of the driving roller 11) of the belt 20 by the equation (3), the paper P can be transported at the transport speed Vpf in a state where there is no slip according to the thickness of the paper P. it can. As a result, the paper P can be transported with high transport speed at the above Vpf.

  In addition, the paper P is sandwiched between the belt 20 and the pressure roller 10 when the paper P is conveyed, but the belt P does not exist between the belt 20 and the pressure roller 10 when the paper P is not conveyed. And the pressure roller 10 are in direct contact with each other and are driven together. In this state, it is preferable to suppress these abrasions without causing slippage between them. For this purpose, it is preferable to drive these so that the surface of the belt 20 on the side in contact with the pressing roller 10 and the surface of the pressing roller 10 move at the same speed. In this case, the moving speed Vb0 of the belt 20 is equal to that when tp = 0 in the expression (3), and is as follows.

  That is, when the angular speed ω of the pressure roller 10 (the moving speed Vrt of the surface of the pressure roller 10) is not changed between the state in which the paper P is transported and the state in which the paper P is not transported, the driving speed of the belt 20 is set to the speed of the paper P. Switching may be made so that Vb is the above when transported and Vb0 is when not transporting. Generally, r >> tb, and in this case, Vb> Vb0. That is, when transporting the paper P, it is necessary to increase the driving speed of the belt 20 (increase the rotational speed of the driving roller 11). This switching can be performed at the timing when the paper P is detected by the paper sensor 33 in FIG.

  The results calculated for each of the actual speeds with the above settings will be described. Here, the radius r of the pressing roller 10 is 5 mm (diameter 10 mm), the thickness tb of the belt 20 is 0.3 mm, and the conveyance speed Vpf of the paper P is 800 mm / s. This Vpf corresponds to 150 sheets / min for A4 size paper (landscape). The thickness tp of the paper P was set to three types: 80 μm (plain paper), 160 μm (thick paper), and 50 μm (thin paper). Since tp << tb << r, the moving speed Vrt of the surface of the pressing roller 10 in this case is slightly smaller than Vpf (800 mm / s), and the rotating speed of the pressing roller 10 is 25 rps (Rotation Per Second). ) The driving speed Vb (during conveyance) and Vb0 (during non-conveyance) of the belt 20 are slightly larger than Vpf. Actually, for these three types of paper P, the result of calculating the moving speed Vb during conveyance of the belt 20 and the moving speed Vb0 during non-conveyance when the moving speed Vrt, Vrt of the surface of the pressing roller 10 is constant. Is shown in Table 1. Under the conditions shown in Table 1, no slip occurs between the pressure roller 10 and the belt 20 during non-conveyance, and between the paper P and the pressure roller 10 during conveyance of the paper P, the paper P and the belt 20 No slip occurs between the two.

  In FIG. 2, since the distance from the center O of the belt 20 varies depending on the thickness of the paper P, there is no slip between the paper P and the pressure roller 10 in the nip region, and these are completely interlocked. Even if the rotation speed of the pressing roller 10 is constant, the conveyance speed of the paper P varies according to its thickness. Specifically, the transport speed is high when the paper P is thick, and the transport speed is low when the paper P is thin. In Table 1, since the conveyance speed is constant (800 mm / s), the moving speed Vrt of the surface of the pressing roller 10 (the rotational speed of the pressing roller 10) is compared with the case where plain paper is used as the paper P. Therefore, it is set low when the paper P is thick and high when the paper P is thin.

  On the other hand, when the paper P is thick, the difference between the moving speed of the surface of the pressing roller 10 and the moving speed of the surface of the belt 20 is large, and when the paper P is thin, the difference between these moving speeds is small. For this reason, in Table 1, the driving speed of the belt 20 when the transport speed is constant (800 mm / s) is higher when the paper P is thicker than when the plain paper is used as the paper P. When it is thin, it is set low. That is, according to the thickness tp of the paper P, the belt 20 and the pressing roller 10 can be driven under the condition that no slip occurs with the paper P.

  On the other hand, for this purpose, it is necessary to recognize the thickness tp of the paper P. However, in practice, it may be difficult to accurately recognize tp. In such a case, an operation for precisely controlling the conveyance speed by suppressing slip will be described below. Here, although tp cannot be recognized correctly, it is assumed that the standard value of tp is known and the difference from this standard tp is unknown. As a standard value of tp, for example, the case of plain paper (80 μm) in Table 1 can be used.

  In this case, the pressing roller 10 and the belt 20 are driven under the conditions for plain paper in Table 1. In this case, as described above, when the paper P is plain paper, the conveyance speed is 800 mm / s. In the following control, this conveyance speed is used as a reference value.

  When the paper P is thicker than plain paper (in the case of thick paper in Table 1), under this driving condition, as described above, the transport speed is higher than the reference value of 800 mm / s. This transport speed can be recognized by the transport speed sensor 34 in FIG. That is, when the transport speed is faster than the reference value, it can be estimated that the actually transported paper P is thicker than the plain paper. In this case, in order to suppress the slip and bring the conveyance speed close to the reference value, the rotational speed of the pressing roller 10 is decreased and the driving speed of the belt 20 is increased corresponding to the case of thick paper in Table 1. Control may be performed. In this case, if the amount of decrease in the rotational speed of the pressing roller 10 and the amount of increase in the driving speed of the belt 20 are set to be small in advance, at least the actual conveyance speed is controlled by this control within a range that does not fall below the reference value. It can be close to the value.

  Conversely, when the actually measured conveyance speed is slower than this reference value, it can be estimated that the actually conveyed paper P is thinner than the plain paper. In this case, contrary to the above, corresponding to the case of the thin paper in Table 1, the rotation speed of the pressing roller 10 may be increased and the driving speed of the belt 20 may be decreased. In this case, if the increase amount of the rotation speed of the pressing roller 10 and the decrease amount of the driving speed of the belt 20 are set to be small in advance, at least the actual conveyance speed is controlled by this control to prevent slipping and exceed the reference value. It can be close to the reference value in a non-existing range. Alternatively, when the actually measured conveyance speed is higher than the reference value as described above, the rotation speed of the pressing roller 10 is decreased and the driving speed of the belt 20 is increased, and the actually measured conveyance speed is higher than the reference value. When it is low, the conveyance speed can be brought close to the reference value by combining the control for increasing the rotation speed of the pressing roller 10 and decreasing the driving speed of the belt 20.

  That is, even when the actual thickness tp of the paper P is unknown, the control unit performs actual conveyance when a predetermined standard condition (for example, driving conditions when plain paper is used in Table 1) is adopted. The speed is measured by the transport speed sensor 34, and the transport speed is fed back to finely adjust the rotational speed of the pressing roller 10 and the driving speed of the belt 20 as described above, thereby controlling the transport speed while suppressing slippage. Can be approached.

  For example, when the paper P is long in the transport direction and the actual transport speed of the paper P is measured in the vicinity of the pressing roller 10 by the transport speed sensor 34, the above control is performed a plurality of times during the transport of one paper P. By carrying out, the conveyance speed can be brought close to the reference value. Alternatively, when many sheets of the same type of paper P are continuously processed, the conveyance speed can be made closer to the reference value as the processing proceeds by performing the above control. For example, when the paper P output from the output tray is stacked, if the conveyance speed is not appropriate, it may be difficult to stack the paper P properly. In such a case, especially when the paper P is stacked on the upper side (later), since a large number of the papers P are already stacked, the space in the output tray is narrowed, and paper jamming or the like is likely to occur. In the above configuration, since the conveyance speed for the paper P to be output later is particularly appropriate, such a situation is unlikely to occur.

  As described above, even when the thickness tp of the paper P is unknown, the transport speed is brought close to this reference value by comparing the actual transport speed of the paper P with the reference value of the predetermined transport speed. be able to. Here, the range of fluctuation of tp is actually about 2 to 1/2 times the standard value as shown in Table 1, for example, and the amount of change in the driving conditions of the pressure roller 10 and the belt 20 in the above control Therefore, the control for feeding back the actually measured transport speed is extremely effective. The rotational speed of the pressing roller 10 and the increase amount and decrease amount of the driving speed of the belt 20 in the above control can be set in advance according to the assumed range of variation in tp.

  In the above configuration, the pressing roller 10 is used to correct the curl of the paper P. However, even if it is not such a purpose, as in FIGS. 1 and 2, when a pressing roller is used that protrudes so as to curve the belt toward the belt side, it is necessary to precisely control the paper conveyance speed. The above configuration is effective. In the above example, the belt 20 is driven by the driving roller 11, but the driving method of the belt is also arbitrary.

  In particular, such a paper transport mechanism using a belt and a roller is effective in an image forming apparatus that forms and outputs (prints) an image on paper. However, it is obvious that the same configuration is effective in apparatuses other than the image forming apparatus, particularly in an apparatus that continuously processes a large number of sheets.

DESCRIPTION OF SYMBOLS 1 Paper conveyance mechanism 10 Press roller 11 Driving roller 12 Driven roller 13 Tension roller 20 Belt 31, 32 Guide 33 Paper sensor 34 Conveyance speed sensor O Center P Paper

Claims (8)

A paper transport mechanism that transports paper using a belt that is elastically deformable and moves in the length direction,
The belt is projected so as to bend toward the belt side, and the surface on the side in contact with the belt moves in the same direction as the moving direction of the belt with the paper sandwiched between the belt and the belt. A pressure roller that is driven to rotate,
The sheet transport mechanism according to claim 1, wherein a moving speed of a surface of the belt contacting the sheet is set to be higher than a moving speed of a surface of the pressing roller contacting the sheet.   The sheet conveying mechanism according to claim 1, wherein a protruding amount of the pressing roller toward the belt is adjustable. A reference value for the paper transport speed is set,
The moving speed of the surface of the pressing roller and the driving speed of the belt are controlled according to the magnitude relationship between the actually measured conveyance speed and the reference value. Paper transport mechanism.   When it is recognized that the actually measured conveyance speed is higher than the reference value, control is performed to decrease the moving speed of the surface of the pressing roller and increase the driving speed of the belt. The paper transport mechanism according to claim 3.   When it is recognized that the actually measured conveyance speed is lower than the reference value, control is performed to increase the moving speed of the surface of the pressing roller and decrease the driving speed of the belt. The paper transport mechanism according to claim 3 or 4. A reference value for the paper transport speed is set,
The belt drive speed is set so that no slip occurs between the belt and the paper transported at the transport speed that is the reference value,
The moving speed of the surface of the pressing roller is set so that no slip occurs between the sheet transported at the transporting speed that is the reference value and the surface of the pressing roller. The paper transport mechanism according to claim 1 or 2.   The paper transport mechanism according to claim 6, wherein the driving speed of the belt and the moving speed of the surface of the pressing roller are set according to the thickness or type of the paper.   An image forming apparatus using the sheet transport mechanism according to claim 1 to form and output an image on the sheet.