Image forming device

Abstract

To prevent a reduction in the fixation strength of a toner image to a sheet.SOLUTION: An image forming apparatus (1) comprises a control unit (7). When the number of printing sheets included in a print job is equal to or more than a predetermined number, the control unit (7) starts feeding paper with a paper feeding unit (2) after the lapse of a first time from when temperatures detected by temperature detection members (46A, 46B) reach a first temperature, and when the number of printing sheets included in the print job is less than the predetermined number, starts feeding paper with the paper feeding unit (2) after the lapse of a second time shorter than the first time from when the detection temperatures reach a second temperature equal to or lower than the first temperature.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an image forming apparatus. [Background technology]

[0002] Patent Document 1 discloses an image forming apparatus including a control unit that controls a paper feed unit to feed a recording material to a nip portion of a fixing device based on temperature information output from a temperature sensor that detects the temperature of a fixing roller. The image forming apparatus sets a paper feed permission temperature, which is the temperature of the fixing roller that triggers the operation of the paper feed unit to start image formation. The image forming apparatus also sets a paper feed permission temperature when the number of images to be formed is less than a predetermined number lower than the paper feed permission temperature when the number of images to be formed is equal to or greater than the predetermined number. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-231803 Summary of the Invention [Problem to be solved by the invention]

[0004] In the fixing device of the image forming apparatus disclosed in Patent Document 1, a belt is heated by a heater whose longitudinal direction is the width direction of the belt, and a nip is formed by sandwiching the belt between the heater and a pressure roller. This means that a belt and heater with low thermal capacity are used. Therefore, in the image forming apparatus, when the number of images formed on a sheet increases, the temperature of the nip is likely to decrease, which may result in an insufficient fixation strength of the image on the recording material. The present disclosure aims to suppress a decrease in fixation strength of the toner image on the sheet. [Means for solving the problem]

[0005] In order to solve the above problems, an image forming apparatus according to a first aspect of the present disclosure includes a paper feed unit that feeds sheets, a process unit that forms a toner image on the sheet fed by the paper feed unit, a fixing unit that heats and fixes the toner image formed on the sheet by the process unit, and a control unit, wherein the fixing unit includes a heater having a substrate and a resistance heating element, an endless belt that includes an inner circumferential surface that contacts the heater and rotates around the heater, a pressure member that forms a nip portion by sandwiching the belt between itself and the heater, and a temperature detection member that detects the temperature of the heater, and the control unit is configured to In this case, the control unit executes a process including a power supply process that starts supplying power to the heater, and a paper feeding process that starts paper feeding by the paper feeding unit after a predetermined time has elapsed since the detected temperature of the temperature detection element reaches a predetermined temperature, and in the paper feeding process, if the number of printed sheets included in the print job is equal to or greater than a predetermined number, the control unit starts paper feeding by the paper feeding unit after a first time has elapsed since the detected temperature reaches a first temperature, and if the number of printed sheets included in the print job is less than a predetermined number, the control unit starts paper feeding by the paper feeding unit after a second time has elapsed, the second time being shorter than the first time, since the detected temperature reaches a second temperature that is equal to or lower than the first temperature.

[0006] When the number of printed sheets is equal to or greater than a predetermined number, the control unit starts feeding paper after a first time has elapsed since the detected temperature reached a first temperature. The first time is longer than a second time, which is the time from when the detected temperature reached the first temperature to when the paper feeding unit starts feeding paper when the number of printed sheets is less than a predetermined number. As a result, when the number of printed sheets is equal to or greater than a predetermined number, heat accumulates in the fixing unit, so that a drop in temperature in the fixing unit can be suppressed even when a low-heat-capacity belt and heater are used, and a decrease in the fixing strength of the toner image on the sheet can be suppressed.

[0007] A second aspect of the present disclosure is the image forming apparatus of the first aspect, wherein the second temperature may be lower than the first temperature. Compared to when the number of printed sheets is equal to or greater than a predetermined number, when the number of printed sheets is less than a predetermined number, printing can be completed before the temperature of the fixing unit drops, even if paper feeding is started when the temperature of the fixing unit is lower. Therefore, by starting paper feeding when the second temperature, which is lower than the first temperature, is reached, the first print out time (FPOT) can be shortened.

[0008] A third aspect of the present disclosure is the image forming apparatus of the first or second aspect, wherein the first temperature may be a fixing temperature that is a target temperature for thermal fixing by the fixing unit. Since paper feeding starts a predetermined time after the fixing temperature that is the target temperature for thermal fixing by the fixing unit is reached, it is possible to further suppress a decrease in fixing strength of the toner image on the sheet when the number of printed sheets is equal to or greater than a predetermined number.

[0009] A fourth aspect of the present disclosure is the image forming apparatus of any one of the first to third aspects, wherein the second temperature may be a temperature that is a lower limit of a temperature range at which the fixing unit can fix the sheet when the sheet reaches the nip portion. Because paper feeding starts after the temperature reaches a temperature at which the sheet can be fixed when the sheet reaches the nip portion, the toner image can be fixed to the sheet without causing fixing problems.

[0010] A fifth aspect of the present disclosure is an image forming apparatus according to any one of the first to fourth aspects, wherein the control unit may execute a process including a speed change process, before starting paper feeding by the paper feeding unit, to make the sheet conveying speed in the fixing unit faster when the number of printed sheets included in the print job is less than a predetermined number than the conveying speed when the number of printed sheets included in the print job is equal to or greater than a predetermined number.

[0011] By increasing the sheet conveyance speed in the fixing unit, the number of times the pressure member surface passes through the heating unit per unit time before the sheet reaches the nip increases, so the pressure member surface can be heated quickly and efficiently, and fixing defects can be suppressed. Therefore, even if paper feeding is started when the temperature of the fixing unit is lower when the number of printed sheets is less than the specified number, fixing defects can be suppressed compared to when the number of printed sheets is greater than the specified number.

[0012] A sixth aspect of the present disclosure is the image forming apparatus of any one of the first to fifth aspects, wherein the control unit may execute a process including a temperature change process that changes the first temperature depending on at least one of a type and a size of a sheet included in the print job. This can appropriately suppress a decrease in fixing strength of a toner image to the sheet depending on the type and size of the sheet. [Effects of the Invention]

[0013] According to one aspect of the present disclosure, it is possible to suppress a decrease in the fixing strength of a toner image on a sheet. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view showing a configuration of an image forming apparatus according to a first embodiment of the present disclosure. [Figure 2] 2 is a block diagram showing a configuration of a main part of the image forming apparatus shown in FIG. 1. FIG. [Figure 3] 2 is a perspective view of a fixing unit included in the image forming apparatus shown in FIG. 1. FIG. [Figure 4] FIG. 4 is a cross-sectional view of the fixing unit shown in FIG. [Figure 5] 4 is a plan view showing a holder and a temperature detection member included in the fixing unit shown in FIG. 3. FIG. [Figure 6] 3 is a flowchart showing an example of processing performed by a control unit shown in FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0015] [Embodiment 1] <Overall Configuration of Image Forming Apparatus 1> 1 is a cross-sectional view showing the configuration of an image forming apparatus 1 according to a first embodiment of the present disclosure. Although the image forming apparatus 1 is a monochrome printer, the scope of application of the present disclosure is not limited to monochrome printers and is also applicable to color printers. In this embodiment, a laser printer that forms an image on a sheet S using toner will be described as an example of the image forming apparatus 1.

[0016] 1, image forming apparatus 1 includes a paper feed unit 2, a process unit 3, a fixing unit 4, a paper discharge roller 5, and a paper discharge tray 6. Paper feed unit 2 feeds sheets S, and includes a paper feed tray 21, a pickup roller 22, a transport roller 23, and a registration roller 24. The sheets S may be recording paper, an OHP (Over Head Projector), an envelope, cardboard, or the like.

[0017] The paper feed tray 21 is a tray on which sheets S are placed before printing. The pickup roller 22 feeds the sheets S one by one from the paper feed tray 21. The transport roller 23 transports the sheets S fed by the pickup roller 22 to the registration roller 24. The registration roller 24 aligns the leading edge of the sheet S and then transports the sheet S to the process unit 3.

[0018] The process unit 3 forms a toner image on the sheet S fed by the paper feed unit 2, and includes a photosensitive member 31, a charger 32, an exposure unit 33, a developing unit 34, a transfer unit 35, and a cleaning unit 36.

[0019] The photoreceptor 31 has a base tube made of a conductive metal such as aluminum and a photosensitive layer provided on the outer surface of the base tube. The photoreceptor 31 is an organic photoconductor (OPC) and is a photosensitive drum having a positively charged photosensitive layer whose image formation potential has a positive polarity. As the photoreceptor 31 rotates, the photosensitive layer is charged by a charger 32 and exposed to laser light from an exposure device 33, forming an electrostatic latent image on the surface. The photoreceptor 31 may also be a photosensitive belt having a photosensitive layer provided on the outer surface of a metal circular belt.

[0020] The charger 32 charges the surface of the photoconductor 31, and is, for example, a charging roller that charges the photoconductor 31 by contacting it, or a scorotron charger that charges the photoconductor 31 without contacting it. The exposure unit 33 exposes the surface of the photoconductor 31 that has been charged by the charger 32. The exposure unit 33 has, for example, multiple light-emitting elements arranged in a row along the direction of the rotation axis of the photoconductor 31, and forms an electrostatic latent image on the surface of the photoconductor 31 by emitting light in accordance with print data.

[0021] The developing unit 34 supplies developer to the electrostatic latent image formed on the surface of the photoreceptor 31, and includes, for example, a developing roller and a developer container containing developer. The transfer unit 35 transfers the developer image formed on the surface of the photoreceptor 31 to the sheet S, and is, for example, a transfer roller. The cleaning unit 36 ​​removes residual developer and paper dust adhering to the photoreceptor 31 from the photoreceptor 31, and is, for example, a cleaning roller.

[0022] The fixing unit 4 heats and fixes the toner image formed on the sheet S by the process unit 3. Details of the fixing unit 4 will be described later. The sheet S on which the toner image has been fixed by the fixing unit 4 is discharged by the paper discharge rollers 5 to the paper discharge tray 6. The paper discharge tray 6 is a tray onto which the sheet S is discharged and placed after printing.

[0023] 2 is a block diagram showing the main configuration of the image forming apparatus 1 shown in FIG. 2, the image forming apparatus 1 includes a control unit 7, a communication unit 8, and an energization circuit 9. The control unit 7 is a controller that controls each unit of the image forming apparatus 1, such as the paper feed unit 2, the process unit 3, the fixing unit 4, the communication unit 8, and the energization circuit 9. The control unit 7 includes an ASIC (Application Specific Integrated Circuit) 71 and a storage unit 72. The control unit 7 executes various processes to cause the image forming apparatus 1 to perform printing processes and associated processes.

[0024] The control unit 7 may include a processor such as a CPU (Central Processing Unit). In this case, a control program that realizes a print control method may be stored in the storage unit 72. The processor of the control unit 7 may operate in accordance with the control program stored in the storage unit 72, thereby executing the print process in the image forming apparatus 1.

[0025] The control unit 7 may also include a computer-readable recording medium storing a control program. The recording medium may be a "non-transitory tangible medium" such as a ROM (Read Only Memory), a magnetic disk, a card, a semiconductor memory, a programmable logic circuit, or the like.

[0026] Furthermore, a RAM (Random Access Memory) or the like on which the control program is developed may be used as the recording medium. The control program may be supplied to the computer via any transmission medium capable of transmitting the control program, such as a communication network or broadcast waves. The storage unit 72 is a memory from which information can be read and written. The storage unit 72 is a RAM or NVM (Non-Volatile Memory). Predetermined information such as the number of pages to be printed is stored in the storage unit 72.

[0027] The communication unit 8 has a communication interface and performs two-way data communication with devices external to the image forming apparatus 1 via a wired or wireless network. The energization circuit 9 has a switching element such as a triac. The energization circuit 9 switches between an energized state in which current is supplied from an AC power source (not shown) to the heater 41 of the fixing unit 4 and a non-energized state in which no current is supplied, by turning the switching element on or off in accordance with instructions from the control unit 7.

[0028] <Configuration of Fixing Unit 4> Fig. 3 is a perspective view of the fixing unit 4 included in the image forming apparatus 1 shown in Fig. 1, and Fig. 4 is a cross-sectional view of the fixing unit 4 shown in Fig. 3. Fig. 5 is a plan view showing a holder 43 and temperature detection members 46A and 46B included in the fixing unit 4 shown in Fig. 3. As shown in Figs. 3 to 5, the fixing unit 4 includes a heater 41, a belt 42, the holder 43, a pressure roller 44, a stay 45, and temperature detection members 46A and 46B.

[0029] As shown in Fig. 4, the heater 41 is a heat source and has a plate shape extending in the longitudinal direction of the fixing unit 4. The heater 41 has a substrate 41A and a resistance heating element 41B. The substrate 41A has a plate shape extending along the longitudinal direction of the fixing unit 4. The resistance heating element 41B is provided on the substrate 41A along the longitudinal direction of the fixing unit 4.

[0030] The longitudinal direction of the fixing unit 4 is parallel to the rotation axis of the pressure roller 44 and perpendicular to the conveyance direction of the sheet S in the nip N (described later). In other words, the longitudinal direction of the fixing unit 4 is the width direction of the sheet S passing through the nip N.

[0031] In addition, wiring and terminals for supplying power to the resistance heating element 41B are also provided on the substrate 41A. A ceramic substrate is used as the substrate 41A. That is, the heater 41 is a ceramic heater. In this case, the heater 41 has a low heat capacity. The heater 41 may be a steel heater in which a metal substrate, more specifically a steel substrate, is used as the substrate 41A.

[0032] The belt 42 is an endless belt that includes an inner circumferential surface 42A that contacts the heater 41 and rotates around the heater 41. The belt 42 is a cylindrical member with a low heat capacity. The belt 42 is made of a single-layer or multi-layer film that has heat resistance, releasability, strength, durability, etc.

[0033] The holder 43 is a member that supports the heater 41 and rotates and holds the belt 42, and is made of a heat-resistant and insulating resin material. The holder 43 has a support surface 43A that supports the heater 41. The support surface 43A supports the heater 41 while facing the back surface of the heater 41, i.e., the surface of the substrate 41A opposite to the resistance heating element 41B. The heater 41 is disposed with the surface on which the resistance heating element 41B is provided facing the inner circumferential surface 42A of the belt 42.

[0034] The pressure roller 44 is an example of a pressure member, and forms a nip N by sandwiching the belt 42 between itself and the surface of the heater 41. Both ends of the pressure roller 44 are supported by a pair of side frames via bearings. The pressure roller 44 is driven to rotate by a motor included in the image forming apparatus 1, and the belt 42 is rotated in accordance with the rotation of the pressure roller 44.

[0035] Stay 45 is a member for increasing the rigidity of holder 43, and is disposed on the opposite side of support surface 43A of holder 43. If holder 43 has high rigidity, stay 45 can be omitted.

[0036] The pressure member is not limited to pressure roller 44, and may be composed of, for example, a pressure belt and a nip forming member. That is, an elastic pad, a resin or metal pressure plate, or the like is disposed as the nip forming member within the space surrounded by the pressure belt. The nip forming member presses against the surface of heater 41 via belt 42 and the pressure belt to form nip N.

[0037] As shown in FIG. 5, the temperature detection members 46A and 46B are provided to face the back surface 43B of the holder 43, which is opposite the support surface 43A. Holes 43C and 43D are formed in the back surface 43B of the holder 43. The temperature detection member 46A contacts the back surface of the heater 41 through the hole 43C. The temperature detection member 46B contacts the back surface of the heater 41 through the hole 43D. The temperature detection members 46A and 46B detect the temperature of the heater 41. The temperature detection members 46A and 46B are thermistors, but may be other temperature detection members such as thermostats.

[0038] <Processing of the control unit 7> Fig. 6 is a flowchart showing an example of the processing of the control unit 7 shown in Fig. 2. As shown in Fig. 6, when the control unit 7 receives a print job instructing printing on N sheets S (S1), it determines whether N is equal to or greater than 1 and equal to or less than 3 (S2). N is the number of sheets S to be printed included in the print job, and is a natural number. If the control unit 7 determines that N is equal to or greater than 1 and equal to or less than 3 (S2: YES), the control unit 7 executes a paper feed process that starts paper feed at the second paper feed permission temperature (S3).

[0039] Step S3 will be specifically described below. The control unit 7 executes a power supply process to start supplying power to the heater 41 by supplying a current to the heater 41 using the energization circuit 9. As a result, power is supplied to the resistance heating element 41B of the heater 41, and the temperature of the heater 41 rises.

[0040] After the control unit 7 executes the power supply process, the control unit 7 starts feeding the sheet S by the sheet feeding unit 2 a second time after the temperature detected by at least one of the temperature detection members 46A, 46B reaches the second sheet feed permission temperature. The control unit 7 continues to supply power to the heater 41 until the second time has passed since the temperature detected by at least one of the temperature detection members 46A, 46B reaches the second sheet feed permission temperature. The control unit 7 also starts feeding the sheet S by rotating the pickup roller 22, the conveyance roller 23, and the registration roller 24 using a motor (not shown).

[0041] The second time is a time shorter than the first time, which will be described later, and the first and second times indicate periods of time. The second paper feed permission temperature is a temperature lower than the first paper feed permission temperature, which will be described later. Note that the second paper feed permission temperature may be a temperature that is the lower limit of the temperature range at which fixing is possible in the fixing unit 4 when the sheet S reaches the nip N. This allows the sheet S to start feeding after it reaches a temperature at which fixing is possible when it reaches the nip N, so that the toner image can be fixed on the sheet S without causing fixing problems.

[0042] If the control unit 7 determines that N is 4 or more (S2: NO), the control unit 7 determines whether N is 4 or more and 50 or less (S4). If the control unit 7 determines that N is 4 or more and 50 or less (S4: YES), the control unit 7 executes a paper feed process that starts paper feed at the first paper feed permission temperature (S5). The first paper feed permission temperature and the second paper feed permission temperature are temperatures included in the temperature range at which fixing can be performed in the fixing unit 4 when the sheet S reaches the nip portion N.

[0043] Step S5 will be described in detail below. The control unit 7 executes the power supply process. After the control unit 7 executes the power supply process, the control unit 7 starts feeding the sheet S by the sheet feeding unit 2 after a second time has elapsed since the temperature detected by at least one of the temperature detection members 46A and 46B reached the first sheet feed permission temperature. The control unit 7 continues to supply power to the heater 41 until the second time has elapsed since the temperature detected by at least one of the temperature detection members 46A and 46B reached the first sheet feed permission temperature.

[0044] If the control unit 7 determines that N is 51 or greater (S4: NO), the control unit 7 executes a paper feed process that starts paper feed a first time after the temperature detected by at least one of the temperature detection members 46A, 46B reaches the first paper feed permission temperature (S6). The control unit 7 continues to supply power to the heater 41 until the first time has passed since the temperature detected by at least one of the temperature detection members 46A, 46B reaches the first paper feed permission temperature.

[0045] After the processing of step S3, S5, or S6, the control unit 7 causes the process unit 3 to form a toner image on the sheet S, and the fixing unit 4 to heat and fix the toner image formed on the sheet S. Then, the control unit 7 causes the sheet discharge rollers 5 to discharge the sheet S onto the discharge tray 6. The control unit 7 continues to supply power to the heater 41 until the fixing unit 4 has completed fixing the toner image on the sheet S.

[0046] In steps S3, S5 and S6, the control unit 7 executes a paper feeding process to start paper feeding by the paper feeding unit 2 after a predetermined time has elapsed since the detected temperature of at least one of the temperature detection members 46A, 46B reaches a predetermined temperature.

[0047] Furthermore, in the paper feeding process of steps S3 and S5, if the number of printed sheets included in the print job is less than a predetermined number, control unit 7 starts paper feeding by paper feeding unit 2 a second time after the detected temperature of at least one of temperature detection members 46A, 46B reaches a second temperature that is equal to or lower than the first temperature. In embodiment 1, the first temperature is the first paper feed permission temperature, and the second temperature is the first paper feed permission temperature or the second paper feed permission temperature.

[0048] Furthermore, in the paper feeding process of step S6, if the number of printed sheets included in the print job is greater than or equal to a predetermined number, the control unit 7 starts paper feeding by the paper feeding unit 2 after a first time has elapsed since the detected temperature of at least one of the temperature detection members 46A, 46B reaches a first temperature.

[0049] As a result, when the number of printed sheets is equal to or greater than the predetermined number, the control unit 7 starts feeding paper after the first time has elapsed since the temperature detected by at least one of the temperature detection members 46A, 46B reaches the first temperature. The first time is longer than the second time, which is the time from when the temperature detected by at least one of the temperature detection members 46A, 46B reaches the first temperature to when the paper feeding unit starts feeding paper when the number of printed sheets is less than the predetermined number.

[0050] As a result, when the number of printed sheets is equal to or greater than a predetermined number, heat accumulates in the fixing unit 4, and even when using a belt 42 and heater 41 with low heat capacity, a drop in the temperature of the fixing unit 4 can be suppressed, and a decrease in the fixing strength of the toner image on the sheet S can be suppressed. When the number of printed sheets is large, the user expects that the printing process by the image forming apparatus 1 will take a certain amount of time compared to when the number of printed sheets is small, so a slight delay in the printing process will not bother the user, and print quality can be guaranteed.

[0051] The first paper feed permission temperature is the fixing temperature, which is the target temperature for thermal fixing by the fixing unit 4. As a result, in step S6, the control unit 7 starts paper feeding a predetermined time after the fixing temperature, which is the target temperature for thermal fixing by the fixing unit 4, is reached, so that when the number of printed sheets is equal to or greater than a predetermined number, it is possible to further prevent a decrease in the fixing strength of the toner image on the sheet S.

[0052] The fixing temperature is within the temperature range in which fixing can be performed in the fixing unit 4 when the sheet S reaches the nip N. If the temperature detected by the temperature detection members 46A, 46B becomes higher than the fixing temperature, the sheet S may wrinkle or hot offset may occur, affecting print quality. Hot offset refers to the toner on the sheet S melting too much or the sheet S not being printed sufficiently. By setting the first paper feed permission temperature to the fixing temperature, a decrease in fixing strength is suppressed and such problems can be prevented from occurring.

[0053] [Embodiment 2] A second embodiment of the present disclosure will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the first embodiment, and the description thereof will not be repeated. The same will also be used for the third and subsequent embodiments, and the description thereof will not be repeated.

[0054] In the second embodiment, instead of step S2 shown in FIG. 6, the control unit 7 determines whether N is 1 or more and 50 or less. If the control unit 7 determines that N is 1 or more and 50 or less, the control unit 7 executes the process of step S3. If the control unit 7 determines that N is 51 or more, the control unit 7 executes the process of step S6. The processes of steps S4 and S5 shown in FIG. 6 are not executed. In the second embodiment, the first temperature is the first paper feed permission temperature, and the second temperature is the second paper feed permission temperature. In other words, the second temperature is lower than the first temperature.

[0055] As a result, when the number of printed sheets is less than the predetermined number, printing can be completed before the temperature of the fixing unit 4 drops, even if paper feeding is started when the temperature of the fixing unit 4 is lower than the predetermined number, compared to when the number of printed sheets is equal to or greater than the predetermined number. Therefore, by starting paper feeding when the second temperature, which is lower than the first temperature, is reached, the FPOT (First Print Out Time) can be shortened.

[0056] Furthermore, when the number of printed sheets is small, users often want the printing process by the image forming apparatus 1 to be completed quickly compared to when the number of printed sheets is large. For this reason, starting paper feeding when the temperature reaches a second temperature lower than the first temperature is effective in meeting the needs of users.

[0057] In addition to the above-described configuration, the following configuration may also be used. Specifically, instead of step S5 shown in FIG. 6, the control unit 7 executes a paper feed process that starts paper feed at a third paper feed permission temperature. The third paper feed permission temperature is a temperature that is higher than the second paper feed permission temperature and lower than the first paper feed permission temperature. In this case, the first temperature is the first paper feed permission temperature, and the second temperature is either the second paper feed permission temperature or the third paper feed permission temperature.

[0058] [Embodiment 3] In the third embodiment, in step S3, the control unit 7 sets the conveying speed of the sheet S in the fixing unit 4 to a first conveying speed before the paper feed unit 2 starts feeding the sheet. The conveying speed of the sheet S in the fixing unit 4 is determined by the rotation speed of the motor that rotates and drives the pressure roller 44. In addition, in step S5, the control unit 7 sets the conveying speed of the sheet S in the fixing unit 4 to a second conveying speed before the paper feed unit 2 starts feeding the sheet.

[0059] Furthermore, in step S6, before starting paper feeding by paper feeder 2, controller 7 sets the conveying speed of sheet S in fixing unit 4 to a third conveying speed. The first conveying speed is faster than the second conveying speed, and the second conveying speed is faster than the third conveying speed. The fewer the number of print sheets included in the print job, the faster controller 7 sets the conveying speed of sheet S in fixing unit 4.

[0060] As a result of the above, before starting paper feeding by the paper feeding unit 2, the control unit 7 executes a speed change process to make the conveying speed of the sheet S in the fixing unit 4 faster when the number of printed sheets included in the print job is less than a predetermined number than the conveying speed when the number of printed sheets included in the print job is greater than or equal to a predetermined number.

[0061] By increasing the conveying speed of the sheet S in the fixing unit 4, the number of times the surface of the pressure roller 44 passes through the heating unit per unit time increases before the sheet S reaches the nip N, so the surface of the pressure roller 44 can be heated quickly and efficiently, and fixing defects can be suppressed. Therefore, even if paper feeding is started when the temperature of the fixing unit 4 is lower when the number of printed sheets is less than a predetermined number, fixing defects can be suppressed compared to when the number of printed sheets is greater than a predetermined number.

[0062] [Embodiment 4] In the fourth embodiment, in step S1 shown in FIG. 6, the control unit 7 executes a temperature change process to change the first paper feed permission temperature depending on at least one of the type and size of the sheet S included in the print job. The size of the sheet S is the length along the conveyance direction of the sheet S. The control unit 7 sets the first paper feed permission temperature higher as the size of the sheet S increases. This makes it possible to appropriately prevent a decrease in the fixing strength of the toner image on the sheet S depending on the type and size of the sheet S.

[0063] 6, the control unit 7 may further execute a temperature change process to change the second paper feed permission temperature according to at least one of the type and size of the sheet S included in the print job. The size of the sheet S is the length along the conveyance direction of the sheet S. The control unit 7 sets the second paper feed permission temperature higher as the size of the sheet S increases.

[0064] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. [Explanation of symbols]

[0065] 1. Image forming device 2 Paper feed section 3. Process Section 4 Fixing section 7 Control Unit 41 Heater 41A board 41B Resistance heating element 42 Belt 42A Inner surface 44 Pressure roller 46A, 46B Temperature sensing member N Nip S seat

Claims

1. a paper feed unit that feeds sheets; a process unit that forms a toner image on the sheet fed by the paper feed unit; a fixing unit that heats and fixes the toner image formed on the sheet by the processing unit; a control unit, The fixing unit is a heater having a substrate and a resistive heating element; an endless belt including an inner circumferential surface in contact with the heater and rotating around the heater; a pressure member that sandwiches the belt between itself and the heater to form a nip portion; a temperature detection member that detects the temperature of the heater, The control unit a power supply process that starts supplying power to the heater when a print job is received; a paper feeding process in which, after a predetermined time has elapsed since the temperature detected by the temperature detecting member has reached a predetermined temperature, the paper feeding unit starts feeding paper; The control unit, in the paper feeding process, If the number of print sheets included in the print job is equal to or greater than a predetermined number, paper feeding by the paper feeding unit is started after a first time has elapsed since the detected temperature reached a first temperature; An image forming apparatus that, when the number of printed sheets included in the print job is less than a predetermined number, starts feeding paper using the paper feeding unit after a second time, which is shorter than the first time, has elapsed since the detected temperature reached a second temperature that is lower than the first temperature.

2. 2. The image forming apparatus according to claim 1, wherein the first temperature is a fixing temperature that is a target temperature for thermal fixing by the fixing unit.

3. 3. The image forming apparatus according to claim 1, wherein the second temperature is a lower limit of a temperature range in which fixing can be performed in the fixing unit when the sheet reaches the nip portion.

4. A paper feed unit that feeds sheets; a process unit that forms a toner image on the sheet fed by the paper feed unit; a fixing unit that heats and fixes the toner image formed on the sheet by the processing unit; a control unit, The fixing unit is a heater having a substrate and a resistive heating element; an endless belt including an inner circumferential surface in contact with the heater and rotating around the heater; a pressure member that sandwiches the belt between itself and the heater to form a nip portion; a temperature detection member that detects the temperature of the heater, The control unit a power supply process that starts supplying power to the heater when a print job is received; a paper feeding process in which, after a predetermined time has elapsed since the temperature detected by the temperature detecting member has reached a predetermined temperature, the paper feeding unit starts feeding paper; The control unit, in the paper feeding process, If the number of print sheets included in the print job is equal to or greater than a predetermined number, paper feeding by the paper feeding unit is started after a first time has elapsed since the detected temperature reached a first temperature; If the number of print sheets included in the print job is less than a predetermined number, paper feeding by the paper feeding unit is started after a second time, which is shorter than the first time, has elapsed since the detected temperature reached a second temperature that is equal to or lower than the first temperature; An image forming apparatus that, before starting paper feeding by the paper feeding unit, executes a process including a speed change process that makes the sheet conveying speed in the fixing unit faster when the number of printed sheets included in the print job is less than a predetermined number than the conveying speed when the number of printed sheets included in the print job is equal to or greater than a predetermined number.

5. The image forming apparatus according to claim 1 , wherein the control unit executes a process including a temperature change process that changes the first temperature in accordance with at least one of a type and a size of a sheet included in the print job.

Images