Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent an image defect with a simple configuration.SOLUTION: An image forming apparatus comprises: a first conveyance path that guides a sheet to a conveyance rotating body pair from the opposite side of the rotation axis of a transfer roller with respect to a nip line of the conveyance rotating body pair; a second conveyance path that guides a sheet to the conveyance rotating body pair from the same side as the rotation axis with respect to the nip line; and a control unit. The control unit can execute a mode including: first conveying processing of conveying a sheet toward a reverse conveying unit using the conveyance rotating body pair through the first conveyance path, a transfer nip part, and a fixing nip part, without transferring a toner image to the sheet at the transfer nip part; second conveying processing of conveying a sheet toward the second conveyance path using the reverse conveying unit; and third conveying processing of transferring a toner image to a sheet at the transfer nip part.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus for forming an image on a sheet. [Background technology]

[0002] Conventionally, an image forming apparatus has been disclosed that includes a rotary developing device, a photoconductor on whose surface a toner image is formed by the rotary developing device, and an intermediate transfer device that transfers the toner image formed on the photoconductor and transfers the toner image to a sheet (see Patent Document 1). The intermediate transfer device has an intermediate transfer belt onto which a toner image is primarily transferred, and the toner image on the intermediate transfer belt is transferred to the sheet by a secondary transfer roll at a secondary transfer position. In such an image forming apparatus, if a gap occurs between the sheet and the intermediate transfer belt immediately before the secondary transfer position, discharge in the gap may cause image defects such as blank areas in the toner image transferred to the sheet.

[0003] Therefore, the image forming apparatus described in Patent Document 1 is provided with a sheet guide member that presses the sheet against the intermediate transfer belt. The sheet guide member is biased toward the intermediate transfer belt by a pressing spring, and the biasing force of the pressing spring is changed according to the type of sheet or temperature and humidity.

[0004] Also, Patent Document 2 discloses an image forming apparatus in which a standby conveying roller pair conveys a sheet to a transfer nip between an intermediate transfer belt and a secondary transfer roller. The sheet conveying speed by the standby conveying roller pair is set faster than the sheet conveying speed at the transfer nip, and the sheet is bent between the standby conveying roller pair and the transfer nip. A sheet regulating member is provided between the standby conveying roller pair and the transfer nip to regulate the sheet from bending toward the opposite side to the intermediate transfer belt. This allows the sheet to be pressed against the intermediate transfer belt, and prevents a gap that may cause discharge from being generated between the sheet and the intermediate transfer belt. The difference in sheet conveying speed between the standby conveying roller pair and the transfer nip is changed according to the type of sheet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-064748 A [Patent Document 2] JP 2009-204642 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, the image forming apparatus described in Patent Document 1 requires the provision of a complex, large sheet guide member that varies the pressing force on the sheet, which results in an increase in size and cost of the image forming apparatus.

[0007] In addition, in the image forming apparatus described in Patent Document 2, since there is a difference in the sheet conveying speed between the standby conveying roller pair and the transfer nip, the intermediate transfer belt forming the transfer nip and the sheet rub against each other, which may cause the toner image to be rubbed, resulting in image defects.

[0008] SUMMARY OF THE PRESENT EMBODIMENTS In view of the above, an object of the present invention is to provide an image forming apparatus capable of suppressing image defects with a simple configuration. [Means for solving the problem]

[0009] The present invention relates to an image forming apparatus, comprising: an image forming section having an image carrier carrying a toner image, an intermediate transfer body onto which the toner image on the image carrier is primarily transferred, and a transfer roller which forms a transfer nip with the intermediate transfer body and which performs a second transfer of the toner image on the intermediate transfer body to a sheet at the transfer nip; a pair of transporting rotors which transport a sheet in a sheet transport direction toward the transfer nip; a fixing section disposed downstream of the transfer nip in the sheet transport direction, the fixing section having a heating unit and a pressure roller having an elastic layer including a foam which forms a fixing nip with the heating unit; a first transport path which is disposed upstream of the pair of transporting rotors in the sheet transport direction and guides a sheet to the pair of transporting rotors from the opposite side of the rotation axis of the transfer roller with respect to a nip line of the pair of transporting rotors; a second conveying path that guides the sheet from the same side to the pair of conveying rotors; a reverse conveying section that conveys the sheet that has passed through the first conveying path, the pair of conveying rotors, and the transfer nip section to the second conveying path; and a control section that controls the image forming section, the pair of conveying rotors, and the reverse conveying section, wherein the control section is capable of executing operations including a first conveying process that conveys the sheet toward the reverse conveying section via the first conveying path, the transfer nip section, and the fixing nip section without transferring a toner image to the sheet at the transfer nip section, a second conveying process that conveys the sheet toward the second conveying path by the reverse conveying section after the first conveying process, and a third conveying process that transfers a toner image to the sheet at the transfer nip section after the second conveying process, and the sheet is conveyed in the operations such that a surface that faces the pressure roller in the first conveying process faces the intermediate transfer body in the third conveying process. Effect of the Invention

[0010] According to the present invention, it is possible to suppress image defects with a simple configuration. [Brief description of the drawings]

[0011] [Figure 1]1 is an overall schematic view showing a printer according to a first embodiment. [Diagram 2] FIG. [Diagram 3] FIG. 4 is a cross-sectional view showing a pair of registration rollers, an intermediate transfer belt, and a secondary transfer roller. [Figure 4] 1A is a cross-sectional view showing a state in which the sheet is in good contact with the intermediate transfer belt, and FIG. 1B is a cross-sectional view showing a state in which the sheet is floating above the intermediate transfer belt. [Diagram 5] FIG. 4 is a cross-sectional view showing a state in which thick paper is transported through a first transport path. [Figure 6] FIG. 11 is a cross-sectional view showing a state in which thick paper is transported through a second transport path. [Figure 7] FIG. 2 is a block diagram showing a control block of the printer. [Figure 8] 11 is a flowchart showing a single-sided printing mode. [Figure 9] 11 is a flowchart showing a double-sided printing mode. [Figure 10] 11 is a flowchart showing a discharge suppression printing mode. [Figure 11] FIG. 4 is a cross-sectional view showing a discharge area near a transfer nip in a low humidity environment. [Figure 12] 10 is a flowchart showing a discharge suppressing printing mode according to a second embodiment. [Figure 13] FIG. [Figure 14] FIG. 4 is a cross-sectional view showing a discharge area near a transfer nip in a low humidity environment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] <First embodiment> [Overall structure] First, a first embodiment of the present invention will be described. A printer 100 as an image forming apparatus according to the first embodiment is an in-line (tandem) full-color laser beam printer. As shown in FIG. 1, the printer 100 includes a sheet feeding section 60, an image forming section 50, a fixing unit 20, and a reverse conveying section 70.

[0013] When an image formation command is output to the printer 100, an image forming operation is started by the image forming unit 50 based on image information input from an external computer or the like connected to the printer 100. The image forming unit 50 includes four process cartridges 1a, 1b, 1c, and 1d that form images of four colors, yellow (Y), magenta (M), cyan (C), and black (Bk), exposure devices 5a, 5b, 5c, and 5d, and an intermediate transfer belt 8. The exposure devices 5a, 5b, 5c, and 5d correspond to the photosensitive drums of the process cartridges 1a, 1b, 1c, and 1d.

[0014] The four process cartridges 1a, 1b, 1c, and 1d have the same configuration except for the colors of the images they form. Therefore, only the image forming operation of the process cartridge 1a will be described, and descriptions of the process cartridges 1b, 1c, and 1d will be omitted.

[0015] The exposure device 5a corresponding to the process cartridge 1a irradiates a laser beam toward the photosensitive drum 2 of the process cartridge 1a based on the input image information. At this time, the photosensitive drum 2 is pre-charged by the charging roller 4, and an electrostatic latent image is formed on the photosensitive drum 2 by irradiating the photosensitive drum 2 with the laser beam. The photosensitive drum 2 as an image carrier is rotated at a predetermined process speed by a motor (not shown). Thereafter, the electrostatic latent image is developed by the developing roller 3, and a yellow (Y) toner image is formed on the photosensitive drum 2.

[0016] Similarly, magenta (M), cyan (C) and black (Bk) toner images are formed on the photosensitive drums of the process cartridges 1b, 1c and 1d. The toner images of each color formed on each photosensitive drum are transferred to the intermediate transfer belt 8 by primary transfer rollers 9a, 9b, 9c and 9d.

[0017] The toner image transferred to the intermediate transfer belt 8 is then transported to a secondary transfer roller 15, which serves as a transfer roller, by the intermediate transfer belt 8 which rotates in the direction of the arrow Z. Note that the image forming operation for each color is performed at a timing to overlap the upstream toner image which has been primarily transferred onto the intermediate transfer belt 8.

[0018] The intermediate transfer belt 8 as an intermediate transfer body is wound around the assist roller 11 as the second roller, the drive roller 12 as the first roller, and the tension roller 13. In other words, the intermediate transfer belt 8 is stretched around the assist roller 11, the drive roller 12, and the tension roller 13. The intermediate transfer belt 8 rotates in the direction of the arrow Z when the drive roller 12 is rotated by a motor (not shown). The drive roller 12 has a high friction rubber layer on its surface to drive the intermediate transfer belt 8, and this rubber layer has conductivity with a volume resistivity of 10.5 Ωcm or less. The drive roller 12 is disposed so as to sandwich the intermediate transfer belt 8 together with the secondary transfer roller 15. That is, the drive roller 12 faces the secondary transfer roller 15, and the intermediate transfer belt 8 is disposed between the drive roller 12 and the secondary transfer roller 15. The intermediate transfer belt 8 is sandwiched between the drive roller 12 and the secondary transfer roller 15. Between the intermediate transfer belt 8 and the secondary transfer roller 15, a transfer nip portion SN is formed.

[0019] The tension roller 13 applies a total tension of about 60 N to the intermediate transfer belt 8, and rotates following the intermediate transfer belt 8. The assist roller 11 regulates the angle of the intermediate transfer belt 8 so that the sheet can enter the transfer nip portion SN while contacting the intermediate transfer belt 8.

[0020] The assist roller 11, drive roller 12, and tension roller 13 are each grounded via a resistive element having the same resistance value. In this embodiment, the resistance value of the resistive element provided in the printer 100 is either 1 GΩ, 100 MΩ, or 10 MΩ. The resistance value of each rubber layer of the assist roller 11 and drive roller 12 is sufficiently small relative to any of 1 GΩ, 100 MΩ, and 10 MΩ, so that electrical effects can be ignored.

[0021] The secondary transfer roller 15 is composed of an elastic roller having a volume resistivity of 107 to 109 Ωcm and a rubber hardness of 30° (Asker C hardness scale). The secondary transfer roller 15 is configured to press against the drive roller 12 via the intermediate transfer belt 8 with a total pressure of approximately 39.2 N. The secondary transfer roller 15 rotates following the intermediate transfer belt 8. Furthermore, a voltage of -2.0 to 7.0 kV can be applied to the secondary transfer roller 15 from a transfer power source (not shown).

[0022] In parallel with the above-mentioned image forming operation, a sheet stored in a cassette 16 serving as a sheet storage unit disposed at the bottom of the printer 100 is fed by a pickup roller 17. The sheet fed by the pickup roller 17 is conveyed by a conveying roller pair 18 to a registration roller pair 19 serving as a conveying rotary body pair. The sheet includes various types of sheets such as paper and envelopes, plastic films such as overhead projector sheets (OHP), and cloth, including plain paper, cardboard, coated paper, and the like.

[0023] The sheet is corrected for skew by hitting the nip portion of the registration roller pair 19, which is in a stopped state. Then, the full-color toner image on the intermediate transfer belt 8 is transferred at the transfer nip portion SN to the first surface of the sheet conveyed at a predetermined conveying timing by the registration roller pair 19 by the secondary transfer bias applied to the secondary transfer roller 15. Residual toner remaining on the intermediate transfer belt 8 is collected by a cleaner 75.

[0024] The sheet onto which the toner image has been transferred is applied with a predetermined heat and pressure by the fixing unit 20, so that the toner is melted and fixed (fixed). The sheet that has passed through the fixing unit 20 is discharged onto the discharge tray 51 by the pair of discharge rollers 23 of the reverse conveying section 70.

[0025] The reverse conveying section 70 has a pair of discharge rollers 23, and a discharge sensor 30 and a reversing guide 24 provided between the fixing unit 20 and the pair of discharge rollers 23. The discharge sensor 30 can detect the position of the rear end of the sheet, and is composed of, for example, a flag that moves when pressed by the sheet, and an optical sensor that detects the movement of the flag. The reversing guide 24 guides the sheet reversed by the pair of discharge rollers 23 to a duplex conveying path 27 described later.

[0026] When forming images on both sides of a sheet, first, the sheet is conveyed in a first direction D1 toward the outside of the machine by the fixing unit 20 and the discharge roller pair 23. Based on the fact that the trailing end of the sheet conveyed in the first direction D1 is detected by the discharge sensor 30, the discharge roller pair 23 reverses its rotation after the trailing end of the sheet passes a predetermined position. As a result, the sheet is conveyed by the discharge roller pair 23 in a second direction D2 opposite to the first direction D1, and switches back. The switched back sheet is guided to the double-sided conveying path 27 by the reversing guide 24, and is conveyed again to the registration roller pair 19 by the conveying roller pairs 25 and 26. Note that a switching member that can be moved between a first position and a second position may be provided between the discharge sensor 30 and the discharge roller pair 23. The switching member guides the sheet conveyed by the fixing unit 20 to the discharge roller pair 23 at the first position. Also, the switching member guides the sheet reversed by the discharge roller pair 23 toward the reversing guide 24 at the second position. The switching member is moved from the first position to the second position based on the fact that the discharge sensor 30 detects the trailing end of the sheet conveyed in the first direction D1.

[0027] Then, a toner image is transferred to the second surface of the sheet at the transfer nip portion SN. The toner image is fixed by the fixing unit 20, and the sheet having the images formed on the first and second surfaces is discharged onto the discharge tray 51 by the pair of discharge rollers 23.

[0028] [Fuser unit] Next, the fixing unit 20 will be described in detail with reference to Fig. 2. As shown in Fig. 2, the fixing unit 20 has a heating unit 21 and a pressure roller 22. The heating unit 21 has a heater 21a made of a plate-shaped ceramic heater, a holder 21b that holds the heater 21a, and a fixing film 21c that incorporates the heater 21a and the holder 21b. In addition, a thermistor 21d that controls and adjusts the temperature of the heater 21a is provided on the opposite side of the heater 21a to the pressure roller 22.

[0029] The pressure roller 22 is in pressure contact with the heater 21a and the holder 21b via the fixing film 21c to form a fixing nip portion N. In the present embodiment, the pressure roller 22 is biased toward the heating unit 21, but the heating unit 21 may be biased toward the pressure roller 22.

[0030] The pressure roller 22 has an iron core 22a, an elastic layer 22b made of foamed silicone rubber provided on the core 22a, and a release layer 22c provided on the elastic layer 22b. The elastic layer 22b is located between the core 22a and the release layer 22c including the surface of the pressure roller 22 that contacts the sheet.

[0031] If the pressure roller 22 has a large heat capacity and a large thermal conductivity, the heat on the surface is easily absorbed into the interior, and the surface temperature is less likely to rise. In other words, a material with as low a heat capacity and a low thermal conductivity as possible and with a high heat insulating effect can shorten the time required for the surface temperature of the pressure roller 22 to rise. For this reason, in this embodiment, the thermal conductivity of the foamed silicone rubber as the foam constituting the elastic layer 22b of the pressure roller 22 is 0.11 to 0.16 W / m·K, which is lower than that of solid rubber, which has a thermal conductivity of about 0.25 to 0.29 W / m·K. In addition, the specific gravity related to the heat capacity is about 1.05 to 1.30 for solid rubber, while it is about 0.75 to 0.85 for foamed silicone rubber, and the elastic layer 22b also has a low heat capacity.

[0032] When the sheet passes through the fixing nip N, the amount of heat applied from the fixing unit 20 differs between the surface of the sheet that contacts the heating unit 21 (hereinafter referred to as the first contact surface) and the surface that contacts the pressure roller 22 (hereinafter referred to as the second contact surface). Specifically, the amount of heat applied from the heating unit 21 to the first contact surface is greater than the amount of heat applied from the pressure roller 22 to the second contact surface. As a result, the first contact surface contracts more than the second contact surface due to heat, and the sheet curls to fit the outer circumferential surface of the heating unit 21. In addition, compared to the case where solid rubber with a relatively large heat capacity is applied to the elastic layer 22b of the pressure roller 22, the amount of curl of the sheet increases when foamed silicone rubber with a low heat capacity is applied to the elastic layer 22b as in this embodiment.

[0033] The amount of curl of the sheet also depends on the amount of heat of the heating unit 21. For example, by controlling the amount of heat of the heating unit 21, the amount of curl of the sheet can be adjusted.

[0034] The release layer 22c of the pressure roller 22 is made of perfluoroalkoxy resin (PFA). The release layer 22c is made of, for example, a tube that covers the elastic layer 22b or paint that is applied to the elastic layer 22b, but in this embodiment, it is made of a highly durable tube that covers the elastic layer 22b. In addition to PFA, the release layer 22c may be made of fluororesins such as polytetrafluoroethylene (PTFE) and perfluoroethylene-propylene copolymer (FEP), or fluororubber or silicone rubber that has good releasability.

[0035] The fixing film 21c of the heating unit 21 is rotated by the pressure roller 22. The sheet is heated and pressurized by the heating unit 21 and the pressure roller 22 in the fixing nip portion N, and the toner image on the sheet is fixed (bonded) to the sheet. The sheet is also transported to the discharge roller pair 23 by the heating unit 21 and the pressure roller 22.

[0036] [Contact between sheet and intermediate transfer belt] Next, the contact between the sheet and the intermediate transfer belt 8 will be described with reference to Fig. 3 to Fig. 4(b). Fig. 3 is a cross-sectional view showing a pair of registration rollers 19, the intermediate transfer belt 8, and the secondary transfer roller 15. Fig. 4(a) is a cross-sectional view showing a state in which the sheet S is in good contact with the intermediate transfer belt 8, and Fig. 4(b) is a cross-sectional view showing a state in which the sheet S is floating above the intermediate transfer belt 8. The arrow (S) in Figs. 4(a) and (b) indicates the movement trajectory of the sheet S.

[0037] As shown in FIG. 3, the intermediate transfer belt 8 is formed with a belt tension portion 8a stretched between the assist roller 11 and the drive roller 12. The belt tension portion 8a is a portion located upstream of the drive roller 12 and downstream of the assist roller 11 in the rotation direction of the intermediate transfer belt 8 (the direction of the arrow Z in FIG. 1). The nip line 19a of the pair of registration rollers 19 intersects with the belt tension portion 8a at point P. Point P is located upstream of the transfer nip portion SN in the sheet conveying direction CD. The sheet S conveyed by the pair of registration rollers 19 abuts against the belt tension portion 8a. The nip line 19a is a straight line perpendicular to a straight line connecting the center 19bC of the drive roller 19b and the center 19cC of the driven roller 19c that constitute the pair of registration rollers 19. The pair of registration rollers 19 forms a nip portion 19N as a conveying nip portion. The nip line 19a passes through a nip portion 19N of a pair of registration rollers 19 that sandwich the sheet S and transport it in the sheet transport direction CD. The nip line 19a is a tangent to at least one of the driving roller 19b and the driven roller 19c. When the deformation amount of the driving roller 19b and the driven roller 19c is sufficiently small, the nip line 19a is the same as a common tangent to the driving roller 19b and the driven roller 19c. When the sheet S contacts the intermediate transfer belt 8, in order to suppress discharge between the sheet S and the intermediate transfer belt 8, it is preferable that the angle between the surface of the intermediate transfer belt 8 and the sheet S is small. Therefore, the angle θ between the belt tension portion 8a and the nip line 19a on the sandwiching angle side is preferably 30 degrees or less.

[0038] The sheet S, which is fed from the cassette 16 and conveyed by the conveying roller pair 18, is guided by the first conveying path 18a to the nip portion 19N of the registration roller pair 19. The first conveying path 18a is curved, and the sheet S passing through the first conveying path 18a also assumes a curved posture along the first conveying path 18a.

[0039] In addition, the first conveying path 18a is joined by a second conveying path 26a, which is a part of the double-sided conveying path 27 (see FIG. 1). The second conveying path 26a is a downstream portion of the double-sided conveying path 27 in the sheet conveying direction CD, and is curved. The second conveying path 26a joins the first conveying path 18a at a joining portion 65 located upstream of the nip portion 19N in the sheet conveying direction CD, and the sheet S passing through the second conveying path 26a assumes a curved posture along the second conveying path 26a.

[0040] The first conveying path 18a guides the sheet S in a curved position to the nip portion 19N of the registration roller pair 19 from the same side as the intermediate transfer belt 8 with respect to the nip line 19a of the registration roller pair 19. More specifically, the first conveying path 18a guides the sheet S to the nip portion 19N of the registration roller pair 19 from the opposite side of the rotation shaft 15a of the secondary transfer roller 15 with respect to the nip line 19a of the registration roller pair 19. In other words, the first conveying path 18a guides the sheet S to the nip portion 19N of the registration roller pair 19 from the same side as the rotation shaft 12a of the drive roller 12 with respect to the nip line 19a of the registration roller pair 19. Further, the second conveying path 26a guides the sheet S in a curved posture from the side opposite to the intermediate transfer belt 8 with respect to the nip line 19a of the registration roller pair 19, i.e., the same side as the secondary transfer roller 15, to the nip portion 19N of the registration roller pair 19 (see FIG. 6). More specifically, the second conveying path 26a guides the sheet S to the nip portion 19N of the registration roller pair 19 from the same side as the rotation shaft 15a of the secondary transfer roller 15 with respect to the nip line 19a of the registration roller pair 19.

[0041] The second conveying path 26a and the intermediate transfer belt 8 are disposed close to each other so that when the sheet S is guided by the second conveying path 26a, the leading edge of the sheet S abuts against the intermediate transfer belt 8 in a state where a part of the sheet S is curved by the second conveying path 26a. In this embodiment, the second distance L2 between the nip portion 19N and the second conveying path 26a is shorter than the first distance L1 between the nip portion 19N and the transfer nip portion SN. Also, the second distance L2 between the nip portion 19N and the second conveying path 26a is shorter than the distance between the point P and the nip portion 19N. As a result, the leading edge of the sheet S reaches the intermediate transfer belt 8 and the nip portion 19N in a state where a part of the sheet S is curved along the second conveying path 26a.

[0042] The sheet S, which has been conveyed to the registration roller pair 19 via the first conveying path 18a, is conveyed along the nip line 19a by the registration roller pair 19. As shown in FIG. 4A, the sheet S abuts against the belt tension portion 8a at, for example, point P. Then, the sheet S enters the transfer nip portion SN while contacting the belt tension portion 8a. In FIG. 4A, a gap 6a is formed upstream of the transfer nip portion SN in the sheet conveying direction CD and between the sheet S and the secondary transfer roller 15.

[0043] Also, as shown in FIG. 4(b), there are cases where the sheet S is transported by the transfer nip portion SN without contacting the belt tension portion 8a at point P. At this time, a gap 6a is formed between the sheet S and the secondary transfer roller 15 as in the case of FIG. 4(a), and a gap 6b is formed between the sheet S and the belt tension portion 8a. Thus, in FIG. 4(a) and (b), the region where the gaps 6a and 6b are formed is defined as the discharge region 6. The discharge region 6 is the region from point P to the transfer nip portion SN in the sheet transport direction CD.

[0044] 4(a), in a state where the sheet S and the belt tension portion 8a are in contact with each other in the discharge region 6, discharge occurs mainly in the gap 6a between the sheet S and the secondary transfer roller 15. The discharge in the gap 6a contributes to the transfer of the toner image from the belt tension portion 8a to the sheet S, and good transferability is obtained.

[0045] 4(b), when the sheet S and the belt tension portion 8a are not in contact with each other in the discharge region 6, discharge occurs in the gap 6b between the belt tension portion 8a and the sheet S. Discharge in the gap 6b may cause a toner image to not be transferred well, resulting in a so-called blank area, which may cause a defective image.

[0046] In the configuration of this embodiment, the nip line 19a of the pair of registration rollers 19 crosses the belt tension portion 8a. Therefore, as shown in FIG. 4(a), the sheet S conveyed by the pair of registration rollers 19 comes into contact with the belt tension portion 8a before entering the transfer nip portion SN, and is sent into the transfer nip portion SN while being in contact with the belt tension portion 8a. Therefore, the toner image can be transferred to the sheet S satisfactorily.

[0047] However, when the sheet S has high rigidity, such as thick paper, the sheet S may be influenced by its own posture and may not easily come into contact with the belt tension portion 8a. Hereinafter, a sheet with high rigidity will be referred to as thick paper, and the basis weight will be 81.4 g / cm. 2 For example, a cardboard with a basis weight of 81.4 g / cm 2 ~300g / cm 2 In this embodiment, the sheet has a basis weight of 150 g / cm. 2 The cardboard SG was fed.

[0048] [Posture of cardboard when passing through the first transport path] Fig. 5 is a cross-sectional view showing the state in which the thick sheet SG is conveyed on the first conveying path 18a. Note that the arrow (SG) in Fig. 5 indicates the movement trajectory of the thick sheet SG. As shown in Fig. 5, the thick sheet SG conveyed on the first conveying path 18a by the conveying roller pair 18 is sent to the registration roller pair 19 in a curved posture.

[0049] 5, that is, toward the secondary transfer roller 15 side, with respect to the nip line 19a. This is because the stiffness of the curved thick paper SG causes the thick paper SG to try to return from its curved position to its normal, straight position, with the nip portion 19N of the registration roller pair 19 as a fulcrum. The greater the stiffness of the sheet (including thick paper) being conveyed, the greater the deviation toward the arrow X side with respect to the nip line 19a tends to be.

[0050] When the thick paper SG is conveyed shifted toward the arrow X side in this way, it may be difficult for the thick paper SG to contact the belt tension portion 8a of the intermediate transfer belt 8 before the discharge area 6. Then, as shown in FIG. 4(b), a gap 6b is formed between the belt tension portion 8a and the thick paper SG (sheet), causing image defects due to discharge in the gap 6b.

[0051] [Posture of cardboard when passing through the second transport path] Fig. 6 is a cross-sectional view showing the state in which the thick sheet SG is conveyed through the second conveying path 26a. The arrow (SG) in Fig. 6 indicates the movement trajectory of the thick sheet SG. As shown in Fig. 6, the thick sheet SG conveyed through the second conveying path 26a by the conveying roller pair 26 is sent to the registration roller pair 19 in a curved position.

[0052] 6, that is, toward the intermediate transfer belt 8 side, with respect to the nip line 19a. This is because the stiffness of the curved thick paper SG causes the thick paper SG to try to return from its curved position to its normal, straight position, with the nip portion 19N of the registration roller pair 19 as a fulcrum. The greater the stiffness of the sheet (including thick paper) being conveyed, the greater the deviation toward the arrow Y side with respect to the nip line 19a tends to be.

[0053] Furthermore, as described above, the thick paper SG passing through the second conveying path 26a passes through the fixing unit 20, is conveyed to the double-sided conveying path 27 by the reversing conveying section 70, and reaches the second conveying path 26a. The thick paper SG is curled at the fixing nip N of the fixing unit 20, and the curl follows the curved shape of the second conveying path 26a because the front and back of the thick paper SG are reversed at the reversing conveying section 70. In other words, the thick paper SG is curled more closely to the shape of the second conveying path 26a by the fixing unit 20, which functions as a curl imparting section and a fixing section.

[0054] As a result, the thick paper SG conveyed by the registration roller pair 19 is conveyed with a shift in the direction of the arrow Y with respect to the nip line 19a, and is surely brought into contact with the belt tension portion 8a at a position upstream of the transfer nip portion SN in the sheet conveying direction CD. Since the thick paper SG enters the transfer nip portion SN while contacting the belt tension portion 8a, there is no gap 6b shown in FIG. 4(b) between the thick paper SG and the belt tension portion 8a. Therefore, even for thick paper SG with high rigidity, image defects can be suppressed and the toner image can be transferred satisfactorily. In this way, the sheet conveyed from the second conveying path 26a to the registration roller pair 19 tends to be conveyed while being in contact with the belt tension portion 8a more than the sheet conveyed from the first conveying path 18a to the registration roller pair 19.

[0055] [Control Block] Fig. 7 is a block diagram showing the control blocks of printer 100 according to this embodiment. As shown in Fig. 7, printer 100 includes a control unit 90. Control unit 90 has a CPU 91, a ROM 92, and a RAM 93. Various programs are stored in ROM 92, and CPU 91 reads and executes the programs stored in ROM 92. RAM 93 is used as a work area for CPU 91.

[0056] The input side of the control unit 90 is connected to the discharge sensor 30, the thermistor 21d, and the environment sensor 31. In the present embodiment, the discharge sensor 30 is disposed downstream of the fixing unit 20 in the sheet transport direction CD, but is not limited thereto. In other words, the discharge sensor 30 may be disposed anywhere as long as it can come into contact with the sheet transported on the transport path.

[0057] The environment sensor 31 is disposed inside the printer 100 and detects the ambient humidity. That is, the environment sensor 31 detects the temperature inside the printer 100 and the temperature around the cassette 16.

[0058] Also, the output side of the control unit 90 is connected to a conveying motor M1, a belt driving motor M2, a reversing motor M3, a heater 21a, and a separation mechanism 96. The conveying motor M1 drives the pickup roller 17, the conveying roller pairs 18, 25, 26, the registration roller pair 19, and the pressure roller 22. The belt driving motor M2 drives the driving roller 12 for rotating the intermediate transfer belt 8. The reversing motor M3 rotates the discharge roller pair 23 forward and backward. The separation mechanism 96 separates the photosensitive drums of the four process cartridges 1a, 1b, 1c, 1d from the intermediate transfer belt 8.

[0059] The drive source of each roller is not limited to the above-mentioned configuration. For example, a clutch mechanism or a forward / reverse mechanism (not shown) may be provided to reduce the number of motors. For example, each of the rollers described above may be driven by only one motor.

[0060] In addition, an operation unit 95 including a liquid crystal panel, physical buttons, etc. is connected to the control unit 90. Through the operation unit 95, the user can specify each print mode, which will be described later.

[0061] [Explanation of each print mode] Next, each printing mode according to the present embodiment will be described with reference to Fig. 1 and Fig. 8 to Fig. 10. Fig. 8 is a flowchart showing each process in the single-sided printing mode. Fig. 9 is a flowchart showing each process in the double-sided printing mode. Fig. 10 is a flowchart showing each process in the discharge suppression printing mode.

[0062] The control unit 90 is connected to a host computer (such as a personal computer) not shown, and a user can select one of a single-sided printing mode, a double-sided printing mode, and a discharge suppression printing mode, which will be described later, via the host computer or the operation unit 95. The control unit 90 operates the printer 100 according to the printing mode selected by the user.

[0063] [Single-sided printing mode] First, the single-sided printing mode will be described. As shown in Fig. 1 and Fig. 8, when the single-sided printing mode is started, the control unit 90 brings each of the photosensitive drums of the four process cartridges 1a, 1b, 1c, and 1d into contact with the intermediate transfer belt 8 (step S10). Then, each of the photosensitive drums and the intermediate transfer belt 8 are driven.

[0064] Next, the control unit 90 increases the temperature of the heater 21a until it reaches a predetermined target temperature (step S11). The temperature of the heater 21a is adjusted to the target temperature (adjusted temperature) by the thermistor 21d. In this embodiment, the adjusted temperature of the heater 21a in the single-sided printing mode and double-sided printing mode is 190° C. Note that step S11 may be performed before step S10, or may be performed simultaneously with step S10.

[0065] Then, the control unit 90 executes the image forming operation described above (step S12). That is, the exposure device 5a irradiates the photosensitive drum 2 of the process cartridge 1a with laser light based on the input image information. At this time, the photosensitive drum 2 is pre-charged by the charging roller 4, and an electrostatic latent image is formed on the photosensitive drum 2 by irradiating it with the laser light. The photosensitive drum 2 is rotated at a predetermined process speed by a motor (not shown). Thereafter, the electrostatic latent image is developed by the developing roller 3, and a yellow (Y) toner image is formed on the photosensitive drum 2.

[0066] Similarly, magenta (M), cyan (C) and black (Bk) toner images are formed on the photosensitive drums of the process cartridges 1b, 1c and 1d. The toner images of each color formed on each photosensitive drum are transferred to the intermediate transfer belt 8 by primary transfer rollers 9a, 9b, 9c and 9d. The image forming operation in step S12 may be performed before step S11 or simultaneously with step S11.

[0067] Then, the control unit 90 feeds the sheet S from the cassette 16 in accordance with the transfer timing of the toner image transferred to the intermediate transfer belt 8 (step S13). Note that the feeding of the sheet S may be performed in parallel with the image forming operation in the above-mentioned step S12.

[0068] Next, the sheet S passes through a first conveying path 18a (see FIG. 3) and is conveyed to a transfer nip portion SN by a pair of registration rollers 19. A full-color toner image on the intermediate transfer belt 8 is transferred to a first surface of the sheet S at the transfer nip portion SN (step S14).

[0069] The sheet onto which the toner image has been transferred is applied with a predetermined heat and pressure by the fixing unit 20, so that the toner is melted and fixed (fixed) (step S15). The sheet S that has passed through the fixing unit 20 is discharged onto the discharge tray 51 by the pair of discharge rollers 23 of the reverse conveying section 70 (step S16).

[0070] [Duplex printing mode] Next, the double-sided printing mode will be described. As shown in Figures 1 and 9, when the single-sided printing mode is started, steps S10 to S15 are executed as described in the single-sided printing mode. Steps S10 to S15 are the same as in the single-sided printing mode, so their description will be omitted.

[0071] The control unit 90 is configured not to transport a sheet S having a length less than a predetermined length in the double-sided printing mode. In other words, a sheet S having a length less than a predetermined length is not permitted to be transported to the second transport path 26a. On the other hand, the smallest sheet among the sheets permitted to be transported to the second transport path 26a is defined as the smallest sheet. The control unit 90 can obtain the size of the sheet S to be used based on an input via the operation unit 95. The control unit 90 can also obtain the size of the sheet S to be used based on the output of a detection unit that detects the size of the sheet S (not shown).

[0072] Here, as shown in FIG. 3, the first distance L1 between the nip portion 19N and the transfer nip portion SN is shorter than the length obtained by subtracting the first distance L1 from the length of the minimum sheet in the sheet conveying direction CD. As a result, when the leading edge of the sheet S reaches the transfer nip portion SN, the length of the portion of the sheet S located on the upstream side of the nip portion 19N in the sheet conveying direction CD is longer than the length of the portion located on the downstream side of the nip portion 19N. In addition, the second conveying path 26a is disposed close to the nip portion 19N of the registration roller pair 19, and the second distance L2 between the nip portion 19N and the second conveying path 26a is shorter than the first distance L1. As a result, when the leading edge of the sheet S reaches the transfer nip portion SN, the length of the portion of the sheet S curved along the second conveying path 26a is longer than the length of the portion located on the downstream side of the nip portion 19N in the sheet conveying direction CD.

[0073] In this embodiment, the smallest sheet is A5 size, and the length of the smallest sheet in the sheet conveying direction CD is 210 mm. The first distance between the nip 19N and the transfer nip SN is preferably 50% or less of the length of the smallest sheet, and more preferably 25% or less of the length of the smallest sheet. The distance between the nip 19N and the point P is shorter than the first distance L1 between the nip 19N and the transfer nip SN.

[0074] The sheet S with an image formed on its first surface is conveyed in a first direction D1 toward the outside of the machine by the pair of discharge rollers 23. The control unit 90 reverses the pair of discharge rollers 23 after the rear end of the sheet conveyed in the first direction D1 passes a predetermined position based on the detection of the rear end of the sheet by the discharge sensor 30. As a result, the sheet S is conveyed in a second direction D2 opposite to the first direction D1 by the pair of discharge rollers 23 and switches back. That is, the sheet S is conveyed in a reversed state (step S17). The switched back sheet is guided by the reversing guide 24 to the double-sided conveying path 27, and is conveyed again to the pair of registration rollers 19 by the pairs of conveying rollers 25 and 26. At this time, the sheet S passes through the second conveying path 26a.

[0075] Then, a toner image is transferred to the second surface of the sheet at the transfer nip portion SN (step S18). The sheet with the toner image transferred to the second surface is applied with a predetermined heat and pressure by the fixing unit 20, so that the toner is melted and fixed (fixed) (step S19). The sheet S that has passed through the fixing unit 20 is discharged to the discharge tray 51 by the discharge roller pair 23 of the reverse conveying section 70 (step S16).

[0076] [Discharge suppression print mode] Next, the discharge suppression printing mode will be described. In the following, for example, a case where thick paper SG is fed in the discharge suppression printing mode will be described. As shown in FIG. 1 and FIG. 10, when the discharge suppression printing mode is started, the control unit 90 separates the photosensitive drums of the four process cartridges 1a, 1b, 1c, and 1d from the intermediate transfer belt 8 by the separation mechanism 96 (step S20). The separation mechanism 96 separates the intermediate transfer belt 8 from the photosensitive drums by, for example, moving downward a holder that holds the tension roller 13 and the primary transfer rollers 9a, 9b, 9c, and 9d. Then, the control unit 90 drives the intermediate transfer belt 8 by the belt drive motor M2.

[0077] Next, the control unit 90 increases the temperature of the heater 21a until it reaches the first temperature (step S21). The heater 21a is adjusted to the first temperature by the thermistor 21d. The first temperature may be the same as or different from the target temperature for the single-sided printing mode or the double-sided printing mode. In this embodiment, the first temperature, which is the adjusted temperature of the heater 21a in step S21, is set to 120°C, which is lower than the adjusted temperature for fixing the toner image (for example, 190°C). In this way, by setting the adjusted temperature of the heater 21a to the first temperature, which is lower than the target temperature for fixing the toner image, energy saving can be achieved. However, it is preferable that the first temperature is a temperature at which the fixing film 21c slides smoothly. Also, step S21 may be performed before step S20, or may be performed simultaneously with step S20.

[0078] Next, the control unit 90 feeds the thick paper SG in the cassette 16 (step S22). Step S22 may be performed simultaneously with energization of the heater 21a. Note that step S22 is performed before the image forming operation by the image forming unit 50 (step S27). The thick paper SG is conveyed to the transfer nip portion SN by the registration roller pair 19 via the first conveying path 18a. The thick paper SG is conveyed toward the fixing unit 20 by the registration roller pair 19 and the transfer nip portion SN, but when passing through the transfer nip portion SN, the toner image is not transferred to the thick paper SG. That is, when the thick paper SG passes through the transfer nip portion SN for the first time, the transfer voltage is not applied to the secondary transfer roller 15. Also, after the thick paper SG passes through the transfer nip portion SN for the first time, the toner image formed on the intermediate transfer belt 8 is conveyed to the transfer nip portion SN.

[0079] Then, the thick sheet SG is curled in the fixing nip N of the fixing unit 20 (step S23). At this time, the thick sheet SG curls along the outer circumferential surface of the heating unit 21 due to the surface temperature difference between the heating unit 21 and the pressure roller 22. The surface temperature difference between the heating unit 21 and the pressure roller 22 is preferably in the range of 20°C to 50°C.

[0080] In this embodiment, as described above, the heater 21a is adjusted to the first temperature, for example, 120° C. At this time, the surface temperature of the fixing film 21c reaches 110° C., and the surface temperature of the pressure roller 22 is about 70° C., but conditions such as the temperature setting of the heater 21a and the surface temperature difference between the heating unit 21 and the pressure roller 22 may be appropriately changed depending on the type of sheet and environmental conditions.

[0081] The thick sheet SG curled by the fixing unit 20 is reversed and conveyed by the reverse conveying section 70 in the same manner as in step S17 described above (step S24). The reversed and conveyed (switched back) thick sheet SG is guided by the reverse guide 24 to the double-sided conveying path 27, and is conveyed again to the registration roller pair 19 by the conveying roller pair 25, 26. At this time, the sheet S passes through the second conveying path 26a.

[0082] In parallel with step S24, the control unit 90 causes each of the photosensitive drums of the four process cartridges 1a, 1b, 1c, and 1d to come into contact with the intermediate transfer belt 8 (step S25). Then, each of the photosensitive drums and the intermediate transfer belt 8 are driven.

[0083] Furthermore, the control unit 90 increases the temperature of the heater 21a until it reaches a predetermined target temperature. The target temperature of the heater 21a in step S26 is higher than the first temperature in step S21. The temperature of the heater 21a is adjusted to the target temperature (adjusted temperature) by the thermistor 21d (step S26). The adjusted temperature of the heater 21a in step S26 is set to 190° C. Note that step S26 may be performed before steps S24 and S25, or may be performed simultaneously with steps S24 and S25.

[0084] Then, the control unit 90 executes the image forming operation (step S27) in the same manner as in step S12 described above. The thick sheet SG conveyed by the registration roller pair 19 via the second conveying path 26a is in a curved position as shown in Fig. 6. The thick sheet SG is influenced by the curved position of the thick sheet SG itself and is conveyed shifted toward the arrow Y side in Fig. 6, i.e., toward the intermediate transfer belt 8, with respect to the nip line 19a.

[0085] The thick paper SG is curled in step S23, and the curl follows the curved shape of the second conveying path 26a because the thick paper SG is turned over in the reversing conveying section 70. That is, the thick paper SG is curled by the fixing unit 20, so that the curl follows the shape of the second conveying path 26a.

[0086] As a result, the thick sheet SG reliably contacts the belt tension portion 8a at a position upstream of the transfer nip portion SN in the sheet conveying direction CD, and enters the transfer nip portion SN while contacting the belt tension portion 8a.

[0087] A full-color toner image on the intermediate transfer belt 8 is transferred to a first surface of the sheet S at the transfer nip portion SN (step S28). The sheet onto which the toner image has been transferred is applied with a predetermined heat and pressure by the fixing unit 20, so that the toner is melted and fixed (fixed) (step S29). The sheet S that has passed through the fixing unit 20 is discharged onto the discharge tray 51 by the discharge roller pair 23 of the reverse conveying section 70 (step S30).

[0088] The discharge suppression printing mode as such a mode and operation includes a first conveying process, a second conveying process, and a third conveying process, which will be described below. The first conveying process includes steps S21, S22, and S23, and is a process in which the sheet is conveyed toward the reverse conveying section 70 via the first conveying path 18a, the transfer nip section SN, and the fixing nip section N without transferring a toner image to the sheet at the transfer nip section SN. The second conveying process includes step S24, and is a process in which the sheet is conveyed toward the second conveying path 26a by the reverse conveying section 70 after the first conveying process. The third conveying process includes steps S26, S27, S28, and S29, and is a process in which a toner image is transferred to the sheet at the transfer nip section SN after the second conveying process. In the discharge suppression printing mode, the sheet S is conveyed so that the surface of the sheet S facing the pressure roller 22 in the first conveying process faces the intermediate transfer belt 8 in the third conveying process. In other words, the reverse conveying section 70 and the second conveying path 26a are disposed so that the surface of the sheet S facing the pressure roller 22 in the first conveying process faces the intermediate transfer belt 8 in the third conveying process. In the discharge suppression mode, the control section 90 controls the heater 21a so that the temperature of the heater 21a in the first conveying process is different from the temperature of the heater 21a in the third conveying process. More specifically, the control section 90 controls the heater 21a so that the temperature of the heater 21a in the third conveying process, i.e., the target temperature in step S26, is higher than the temperature of the heater 21a in the first conveying process, i.e., the first temperature in step S21.

[0089] For example, immediately before the discharge suppression printing mode, a large number of sheets are printed, causing the pressure roller 22 to heat up, which may make it difficult to create a difference in surface temperature between the fixing film 21c and the pressure roller 22. In this case, the control unit 90 may determine the time until the fixing unit 20 cools down based on the detection result of the thermistor 21d, and may delay the feeding of the sheet until the fixing unit 20 cools down.

[0090] [Check the effect] To confirm the effect of the discharge suppression printing mode of this embodiment, printing was performed in an environment with a room temperature of 15°C and a humidity of 10%, and the occurrence of image defects due to discharge in the gap 6b (see Figure 4(b)) was confirmed.

[0091] The sheet used has a basis weight of 80 g / cm 2 and plain paper with a basis weight of 150 g / cm 2 The sheets were A4 size thick paper. The plain paper and thick paper were left in the above environment for 48 hours before use. Printing was performed on the plain paper and thick paper in single-sided printing mode and discharge suppression printing mode, and the occurrence of image defects was evaluated visually. The experimental results are shown in Table 1 below.

[0092] [Table 1] As shown in Table 1 above, even in single-sided printing mode, image defects caused by discharge did not occur with plain paper, but with thick paper, image defects caused by discharge in gap 6b (see Figure 4(b)) occurred. On the other hand, in discharge-suppressed printing mode, even with thick paper, image defects caused by discharge in gap 6b (see Figure 4(b)) did not occur, and good transferability was achieved.

[0093] As described above, for thick paper with high rigidity, image defects can be suppressed by executing the discharge suppression printing mode. This is because the sheet transported to the transfer nip SN via the second transport path 26a in the discharge suppression printing mode is closer to the intermediate transfer belt 8 than the sheet transported to transfer via the first transport path 18a in the single-sided printing mode.

[0094] In addition, the pressure roller 22 of the fixing unit 20 has an elastic layer 22b made of foamed silicone rubber, and thus has a characteristic of low heat capacity. This makes it easy to create a difference in surface temperature between the fixing film 21c and the pressure roller 22, and makes it easy to curl the thick paper SG (sheet) passing through the fixing unit 20 along the outer circumferential surface of the heating unit 21. By curling the thick paper SG in this way, the thick paper SG can be transported closer to the intermediate transfer belt 8, and image defects caused by discharge in the gap 6b (see FIG. 4(b)) can be more effectively suppressed.

[0095] In this embodiment, the thick paper SG can be moved toward the intermediate transfer belt 8 with a simple configuration and at low cost, thereby suppressing image defects.

[0096] <Second embodiment> Next, a second embodiment of the present invention will be described, which is configured by changing the first temperature of the first embodiment to a second temperature in the discharge suppression printing mode. Therefore, the same configuration as the first embodiment will be omitted from the illustrations or will be described by using the same reference numerals in the drawings.

[0097] However, if the printer or sheet is left in a low-humidity environment for a long period of time, the resistance of each component of the printer and the sheet increases, and the discharge area formed near the transfer nip SN may become wider. Figure 11 is a cross-sectional view showing a discharge area 7 near the transfer nip SN in such a low-humidity environment. The discharge area 7 is wider in the sheet conveying direction CD than the discharge area 6 described in Figures 4(a) and (b). The arrow (SG1) in Figure 11 indicates the movement trajectory of the thick paper SG1 in the discharge suppression printing mode of the first embodiment in a low-humidity environment.

[0098] Upstream of the transfer nip portion SN in the sheet conveying direction CD, a gap 7a is formed between the thick sheet SG1 and the secondary transfer roller 15. Discharge in the gap 7a contributes to the transfer of the toner image from the belt tension portion 8a to the sheet S.

[0099] On the other hand, a gap 7b is formed upstream of the transfer nip portion SN in the sheet conveying direction CD, between the thick paper SG1 and the belt tension portion 8a. Discharge in the gap 7b may cause a toner image to be transferred poorly, resulting in a so-called blank area, which may cause a defective image.

[0100] Thus, even if the discharge suppressing printing mode of the first embodiment is performed on thick paper SG1, the discharge area (7) becomes wider in a low humidity environment, and image defects may occur.

[0101] Therefore, the control unit 90 (see FIG. 7) of this embodiment is capable of executing a discharge suppressing print mode shown in FIG 12. The discharge suppressing print mode of this embodiment shown in FIG 12 differs from the discharge suppressing print mode of the first embodiment shown in FIG 10 only in step S31.

[0102] In step S31, the control unit 90 increases the temperature of the heater 21a until it reaches the second temperature. The heater 21a is adjusted to the second temperature by the thermistor 21d. In the present embodiment, the second temperature, which is the adjusted temperature of the heater 21a in step S31, is set to 210°C, which is higher than the adjusted temperature (e.g., 190°C) for fixing the toner image.

[0103] The surface temperature difference between the heating unit 21 and the pressure roller 22 is preferably about 40° C. to 50° C. In the present embodiment, as described above, the heater 21a is adjusted to the second temperature, for example, 210° C. At this time, the surface temperature of the fixing film 21c reaches 190° C., and the surface temperature of the pressure roller 22 is about 140° C. Conditions such as the temperature setting of the heater 21a and the surface temperature difference between the heating unit 21 and the pressure roller 22 may be appropriately changed depending on the type of sheet and environmental conditions.

[0104] In this way, by setting the controlled temperature of the heater 21a to the second temperature higher than the target temperature for fixing the toner image, it is possible to impart a stronger curl to the sheet in step S23. Note that step S31 may be performed before step S20, or may be performed simultaneously with step S20.

[0105] As in the first embodiment, the discharge suppression printing mode of this embodiment includes a first conveying process, a second conveying process, and a third conveying process, which will be described below. The first conveying process includes steps S31, S22, and S23, and is a process in which the sheet is conveyed toward the reverse conveying section 70 via the first conveying path 18a, the transfer nip section SN, and the fixing nip section N without transferring a toner image to the sheet at the transfer nip section SN. The second conveying process includes step S24, and is a process in which the sheet is conveyed toward the second conveying path 26a by the reverse conveying section 70 after the first conveying process. The third conveying process includes steps S26, S27, S28, and S29, and is a process in which a toner image is transferred to the sheet at the transfer nip section SN after the second conveying process.

[0106] In the discharge suppression mode, the control unit 90 controls the heater 21a so that the temperature of the heater 21a during the first transfer process is different from the temperature of the heater 21a during the third transfer process. More specifically, the control unit 90 controls the heater 21a so that the temperature of the heater 21a during the third transfer process, i.e., the target temperature in step S26, is lower than the temperature of the heater 21a during the first transfer process, i.e., the first temperature in step S31.

[0107] Fig. 13 is a cross-sectional view showing the reverse conveying section 70. The dashed line (SG1) in Fig. 13 shows the curled state of the thick paper SG1 during reverse conveying in step S24 in the discharge suppression printing mode of the first embodiment. Also, the solid line (SG2) shows the curled state of the thick paper SG2 during reverse conveying in step S24 (see Fig. 12) in the discharge suppression printing mode of the second embodiment. The thick papers SG1 and SG2 are the same type as the thick paper SG in the first embodiment.

[0108] By setting the temperature of the heater 21a of the fixing unit 20 to a second temperature (e.g., 210°C) higher than the first temperature (e.g., 120°C), the curl imparted to the thick sheet of paper SG2 becomes stronger. The curl is a curl that follows the curved shape of the second conveying path 26a because the thick sheet of paper SG2 is turned over in the reversing conveying section 70.

[0109] Fig. 14 is a cross-sectional view showing the discharge area 7 near the transfer nip SN in a low-humidity environment. The arrow (SG2) in Fig. 14 shows the movement trajectory of the thick paper SG2 in the discharge suppression printing mode (see Fig. 12) of the second embodiment in a low-humidity environment.

[0110] As shown in Fig. 14, the thick sheet SG2 conveyed by the registration roller pair 19 is conveyed with a greater deviation from the nip line 19a in the direction of the arrow Y than in the first embodiment. Therefore, the thick sheet SG2 reliably contacts the belt tension portion 8a at a position upstream of the transfer nip portion SN in the sheet conveying direction CD. Then, the thick sheet SG2 enters the transfer nip portion SN while contacting the belt tension portion 8a, so there is no gap 7b shown in Fig. 11 between the thick sheet SG2 and the belt tension portion 8a. Therefore, even with the thick sheet SG2, which has a high rigidity in a low humidity environment, image defects can be suppressed and the toner image can be transferred satisfactorily.

[0111] In addition, the thick sheet SG2 that was strongly curled in step S23 in Fig. 12 is given a curl in the opposite direction to the curl given in step S23 in step S29. Therefore, the thick sheet SG2 is discharged outside the apparatus with the curl reduced.

[0112] [Check the effect] To confirm the effect of the discharge suppression printing mode of this embodiment, printing was performed in an environment with a room temperature of 15°C and a humidity of 10%, and the occurrence of image defects due to discharge in the gap 7b (see Figure 11) was confirmed.

[0113] The sheet used has a basis weight of 200 g / cm 2 The cardboard used was A4 size. The cardboard was left in the above environment for 48 hours. Printing was performed on the cardboard in a single-sided printing mode, the discharge suppression printing mode of the first embodiment (hereinafter referred to as the first discharge suppression printing mode), and the discharge suppression printing mode of the second embodiment (hereinafter referred to as the second discharge suppression printing mode). The occurrence of image defects was evaluated visually. The experimental results are shown in Table 2 below.

[0114] [Table 2] As shown in Table 2 above, in the case of thick paper left for a long time at room temperature of 15°C and humidity of 10%, image defects caused by discharge in gap 7b (see FIG. 11) occurred in the single-sided printing mode and the first discharge suppressing printing mode. On the other hand, in the second discharge suppressing printing mode, even with thick paper left for a long time at room temperature of 15°C and humidity of 10%, image defects caused by discharge in gap 7b (see FIG. 11) did not occur, and good transferability was achieved.

[0115] As described above, in the discharge suppression printing mode of the present embodiment, in step S31, the heater 21a is adjusted to the second temperature, which is higher than the target temperature for fixing the toner image. Then, in this state, in step S23, the thick paper SG2 passes through the fixing nip N, so that a stronger curl can be imparted to the thick paper SG2. This makes it possible to suppress image defects caused by discharge in the gap 7b (see FIG. 11) even if the discharge area (7) expands in a low-humidity environment.

[0116] In this embodiment, the thick paper SG can be moved toward the intermediate transfer belt 8 with a simple configuration and at low cost, thereby suppressing image defects.

[0117] <Other embodiments> In any of the above-described embodiments, the heater 21a of the fixing unit 20 is in direct contact with the fixing film 21c, but is not limited thereto. For example, the heater 21a may be configured to be in contact with the fixing film 21c via a sheet material having high thermal conductivity, such as an iron alloy or aluminum.

[0118] In addition, in any of the above-described embodiments, the thick paper SG is curled by the fixing unit 20, but this is not limited to the above. For example, a curling mechanism for curling the thick paper SG may be provided separately from the fixing unit 20. For example, the curling mechanism is composed of a rubber roller and a sponge roller.

[0119] In addition, in any of the above-described embodiments, the pressure roller 22 has the elastic layer 22b made of foamed silicone rubber, but is not limited to this. For example, the elastic layer 22b may be made of foamed rubber mainly composed of chloroprene rubber or ethylene propylene rubber.

[0120] In addition, in any of the above-described embodiments, the discharge-reducing printing mode is described as a mode in which printing is performed only on one side of a sheet, but is not limited to this. For example, after printing on one side of a sheet, printing may be performed on both sides of the sheet by performing steps S17 to S19 and S16 in FIG.

[0121] Also, the discharge suppression printing mode of the first embodiment and the discharge suppression printing mode of the second embodiment may be selectable. That is, the control unit 90 may be configured to be able to execute both the discharge suppression printing mode of the first embodiment and the discharge suppression printing mode of the second embodiment. Also, the control unit 90 may automatically select either the discharge suppression printing mode of the first embodiment or the discharge suppression printing mode of the second embodiment based on the detection result of the environment sensor 31. Also, in any of the above-mentioned embodiments, the discharge suppression printing mode is executed on thick paper, but is not limited to this. For example, the discharge suppression printing mode may be executed on coated paper or plain paper.

[0122] In addition, in any of the above-described embodiments, the sheet is reversed by the discharge roller pair 23, but this is not limiting. For example, the sheet may be reversed by a roller group consisting of three rollers, or may be reversed by a roller pair that only performs reversal.

[0123] The image forming apparatus includes printers, copiers, facsimiles, and multifunction machines, and refers to an apparatus that forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from an original. In addition, an image forming apparatus may be connected to accessories such as an optional feeder, an image reader, and a sheet processing device in addition to a main body having an image forming function, and the entire system to which such accessories are connected is also a type of image forming apparatus.

[0124] The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0125] The disclosure of the present embodiment also includes the following configuration examples and method examples. (Configuration 1) an image forming section including an image carrier that carries a toner image, an intermediate transfer body onto which the toner image on the image carrier is primarily transferred, and a transfer roller that forms a transfer nip with the intermediate transfer body and secondarily transfers the toner image on the intermediate transfer body to a sheet at the transfer nip; a pair of conveying rotators that convey a sheet in a sheet conveying direction toward the transfer nip portion; a fixing section disposed downstream of the transfer nip portion in the sheet conveying direction, the fixing section including a heating unit and a pressure roller having an elastic layer including a foam, the pressure roller forming a fixing nip portion together with the heating unit; a first conveying path that is disposed upstream of the pair of conveying rotors in the sheet conveying direction and guides the sheet to the pair of conveying rotors from an opposite side of a rotation axis of the transfer roller with respect to a nip line of the pair of conveying rotors; a second conveying path that is disposed upstream of the pair of conveying rotary members in the sheet conveying direction and guides the sheet to the pair of conveying rotary members from the same side as the rotation shaft with respect to the nip line; a reversing conveying section that conveys the sheet that has passed through the first conveying path, the pair of conveying rotating bodies, and the transfer nip portion to the second conveying path; a control unit that controls the image forming unit, the conveying rotating body pair, and the reverse conveying unit, the control unit is capable of executing operations including a first conveying process of conveying the sheet toward the inverting conveying unit through the first conveying path, the transfer nip unit, and the fixing nip unit without transferring a toner image to the sheet at the transfer nip unit, a second conveying process of conveying the sheet toward the second conveying path by the inverting conveying unit after the first conveying process, and a third conveying process of transferring a toner image to the sheet at the transfer nip unit after the second conveying process, the sheet is transported in the operation such that a surface of the sheet facing the pressure roller in the first transport process faces the intermediate transfer body in the third transport process; 1. An image forming apparatus comprising: (Configuration 2) the second transport path and the intermediate transfer body are arranged such that, when the sheet is guided by the second transport path, a leading edge of the sheet comes into contact with the intermediate transfer body in a state where the sheet is curved along the second transport path; 2. The image forming apparatus according to claim 1, (Configuration 3) The pair of conveying rotary members forms a conveying nip portion for nipping and conveying a sheet, a second distance between the transport nip portion and the second transport path is shorter than a first distance between the transport nip portion and the transfer nip portion of the pair of transport rotating bodies; 3. The image forming apparatus according to claim 1, wherein the first and second components are arranged in a same plane. (Configuration 4) when the smallest sheet among the sheets that the control unit allows to be conveyed to the second conveying path is determined to be a minimum sheet, the first distance is shorter than a length obtained by subtracting the first distance from a length of the minimum sheet in the sheet conveying direction. 4. The image forming apparatus according to claim 3, (Configuration 5) the nip line of the pair of conveying rotary members intersects with the intermediate transfer member upstream of the transfer nip portion in the sheet conveying direction; 5. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction and a second direction. (Configuration 6) a first roller that sandwiches the intermediate transfer body together with the transfer roller; A second roller, the intermediate transfer body is disposed so as to surround the first roller and the second roller, an angle formed by a portion of the intermediate transfer body located upstream of the first roller and downstream of the second roller in a rotation direction of the intermediate transfer body and the nip line is 30 degrees or less; 6. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction and a second direction. (Configuration 7) The foam is a foam rubber. 7. The image forming apparatus according to any one of configurations 1 to 6. (Configuration 8) the fixing unit has a heater, the control unit controls the heater in the operation so that a temperature of the heater during the first transfer process is different from a temperature of the heater during the third transfer process. 8. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction and a second direction. (Configuration 9) the control unit controls the heater in the operation so that a temperature of the heater during the third transfer process is higher than a temperature of the heater during the first transfer process. 9. The image forming apparatus according to configuration 8, (Configuration 10) the control unit controls the heater in the operation so that a temperature of the heater during the third transfer process is lower than a temperature of the heater during the first transfer process. 9. The image forming apparatus according to configuration 8, (Configuration 11) a sheet storage section for storing a sheet; a feeding section that feeds the sheets stored in the sheet storage section, the control unit, in the operation, controls the image forming unit to feed a sheet by the feeding unit before forming a toner image on the image carrier. 11. The image forming apparatus according to any one of configurations 1 to 10. [Explanation of symbols]

[0126] 2: Image carrier (photosensitive drum) / 8: Intermediate transfer body (intermediate transfer belt) / 8a: Part (belt tension part) / 15: Transfer roller (secondary transfer roller) / 15a: Rotating shaft / 16: Sheet storage part (cassette) / 18a: First conveying path / 19: Pair of conveying rotating bodies (pair of registration rollers) / 19a: Nip line / 20: Curl imparting part, fixing part (fixing unit) / 21: Heating unit / 22: Pressure roller / 22b: Elastic layer / 26a: Second conveying path / 50: Image forming part / 65: Junction part / 70: Reverse conveying part / 90: Control part / CD: Sheet conveying direction / L1: First distance / L2: Second distance / SN: Transfer nip part / Z: Rotation direction (arrow direction)

Claims

1. An image forming unit having an image carrier that holds a toner image, an intermediate transfer body on which the toner image on the image carrier is primary transferred, and a transfer roller that together with the intermediate transfer body forms a transfer nip portion and secondary transfers the toner image on the intermediate transfer body to a sheet in the transfer nip portion, A pair of conveying rotating bodies that convey the sheet in the sheet conveying direction toward the transfer nip section, A fixing section located downstream of the transfer nip section in the sheet transport direction, comprising a heating unit and a pressure roller having an elastic layer containing foam that forms the fixing nip section together with the heating unit, A first conveying path is positioned upstream of the conveying rotating body pair in the sheet conveying direction and guides the sheet to the conveying rotating body pair from the opposite side of the rotation axis of the transfer roller relative to the nip line of the conveying rotating body pair, A second conveying path is positioned upstream of the pair of conveying rotating bodies in the sheet conveying direction and guides the sheet to the pair of conveying rotating bodies from the same side as the rotation axis with respect to the nip line, The inversion conveying unit conveys the sheet that has passed through the first conveying path, the pair of conveying rotating bodies, the transfer nip section, and the fixing nip section to the second conveying path, The system comprises an image forming unit, a pair of conveying and rotating bodies, and a control unit for controlling the inverting conveying unit, The control unit is capable of performing a sheet transport operation that includes: a first transport process of transporting the sheet toward the inversion transport unit via the first transport path, the transfer nip unit, and the fixing nip unit without transferring a toner image to the sheet in the transfer nip unit; a second transport process of transporting the sheet toward the second transport path by the inversion transport unit after the first transport process; and a third transport process of transferring a toner image to the sheet in the transfer nip unit after the second transport process. In the first transport process, the surface of the sheet facing the pressure roller, and in the third transport process, the surface of the sheet facing the intermediate transfer body, An image forming apparatus characterized by the following:

2. The second transport path and the intermediate transfer body are arranged such that when the sheet is guided by the second transport path, the leading edge of the sheet contacts the intermediate transfer body in a curved state along the second transport path. The image forming apparatus according to feature 1.

3. The aforementioned pair of conveying rotating bodies forms a conveying nip section that grips and conveys the sheet, The second distance between the transport nip portion and the second transport path is shorter than the first distance between the transport nip portion and the transfer nip portion of the transport rotating body pair. The image forming apparatus according to feature 1.

4. When the control unit allows the smallest sheet to be transported to the second transport path, and the smallest sheet is designated as the minimum sheet, the first distance is shorter than the length of the minimum sheet in the sheet transport direction minus the first distance. The image forming apparatus according to feature 3.

5. The nip line of the conveying rotating body pair intersects the intermediate transfer body upstream of the transfer nip portion in the sheet conveying direction. The image forming apparatus according to feature 1.

6. A first roller that sandwiches the intermediate transfer body together with the transfer roller, It further has a second roller, The intermediate transfer body is arranged so as to surround the first roller and the second roller. The angle between the portion of the intermediate transfer body located upstream of the first roller and downstream of the second roller in the rotational direction of the intermediate transfer body and the nip line is 30 degrees or less. The image forming apparatus according to feature 1.

7. The aforementioned foam is foamed rubber. The image forming apparatus according to feature 1.

8. The fixing part has a heater, The control unit controls the heater during the transport operation such that the temperature of the heater during the first transport process is different from the temperature of the heater during the third transport process. The image forming apparatus according to feature 1.

9. The control unit controls the heater during the transport operation such that the temperature of the heater during the third transport process is higher than the temperature of the heater during the first transport process. The image forming apparatus according to feature 8.

10. The control unit controls the heater during the transport operation such that the temperature of the heater during the third transport process is lower than the temperature of the heater during the first transport process. The image forming apparatus according to feature 8.

11. A seat storage section for storing the seat, The system further comprises a feeding unit for feeding the sheets stored in the sheet storage unit, The control unit, in the transport operation, controls the image forming unit to feed the sheet by the feeding unit before forming a toner image on the image carrier. The image forming apparatus according to any one of claims 1 to 10.

Images