JP5340908B2 - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to achieve a paper feedable state in a short time, while maintaining the simple configuration of a fixing device, and prevent poor conveyance or the like, without causing excessive curling. <P>SOLUTION: A control part 30 which controls the fixing device 12 includes: a paper-feeding target temperature-setting part 33 configured to set as a paper-feeding target temperature, the target temperature of a heating element on feeding the paper to the fixing device; a measuring part 31 configured to measure a rate of temperature change in accordance with an output of a temperature detection element after starting the power supply to the heating element of the fixing device 12; a rise-up target temperature-setting part 35 configured to set a rise-up target temperature higher than the paper-feeding target temperature in accordance with the obtained rate of temperature change; and a heating element driving part 37 configured to, after increasing the temperature of the heating element to the rise-up target temperature on supplying the power to the heating element, reduce the temperature of the heating element to the paper-feeding target temperature. The rise-up target temperature-setting part 35 is configured to change the rise-up target temperature in accordance with the rate of temperature change. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an image forming apparatus including a fixing device that fixes a toner image formed on a recording material by passing the recording material between fixing nips in which a heating rotator and a pressure rotator are pressed against each other.

  2. Description of the Related Art Conventionally, many image forming apparatuses such as copying machines, printers, and facsimiles that employ an electrophotographic method use a thermal fixing method as a means for fixing a recording material carrying an unfixed image. The fixing device to be used is provided.

  A typical conventional fixing device includes a fixing film rotating along a film guide, a heating source disposed on the film guide as a heating means for heating the fixing film, and a heat-resistant elastic layer formed on an aluminum or iron core. Pressure roller.

  In such a fixing device, a pressure roller is pressed against a fixing film provided with a heating source by a spring or the like, and a pressure contact width (hereinafter referred to as a fixing nip width) formed by the pressure contact is not fixed. By passing a recording material carrying an image (hereinafter referred to as unfixed toner) through a sheet and heating and pressurizing, the unfixed toner is fixed on the recording material.

  When the recording material is heated, the moisture in the recording material evaporates, and the recording material shrinks due to the decrease in moisture. If a difference in evaporation occurs between the front and back surfaces of the recording material, a dimensional difference occurs between the front and back surfaces of the recording material, and the recording material curls. In order to reduce the curling of the recording material, it is preferable that the temperature difference between the fixing film side and the pressure roller side of the fixing device is made small, and the moisture evaporation amount on the front and back sides of the recording material is made uniform. However, in a fixing device that performs fixing by providing a heating element only on one side of the recording material, a temperature difference occurs between the fixing film side and the pressure roller side. Due to this temperature difference, a dimensional difference occurs between the front and back of the recording material, and the recording material curls. As a result, major problems such as conveyance failure and stacking failure have occurred.

  In order to solve such a problem, Patent Document 1 is set based on the detection temperature of the temperature detection element for the pressure roller so that the image forming operation is started when the temperature of the pressure roller reaches a predetermined temperature. In addition, an image forming apparatus is described in which the temperature difference between the fixing roller and the pressure roller is reduced, thereby suppressing the occurrence of curling and performing good fixing.

  Japanese Patent Application Laid-Open No. 2004-228561 waits at a temperature lower than the target fixing temperature and controls to reheat to the target fixing temperature after the image development is completed, or according to the detected temperature detected immediately before starting up the heater. The heater is controlled to change the standby temperature, or after being heated to the target fixing temperature, the fixing temperature is controlled to be lowered according to the standby time during image development, or the standby is performed while preheating is performed. Control to lower the target fixing temperature according to the length of time to be pressed (lowering the fixing temperature to an appropriate temperature for fixing the toner image according to the temperature rise of the pressure roller by preheating) and pressurizing with the fixing roller An image forming apparatus is described in which the temperature difference between the rollers is reduced, thereby suppressing the occurrence of curling and good fixing is performed.

Japanese Patent Laid-Open No. 07-114295 JP 2002-91230 A

  In the method described in Patent Document 1 described above, temperature detection means for the pressure roller is newly required. Since the installation space is required when the temperature detection means is a non-contact temperature detection element, the size of the image forming apparatus is increased. In the case of a contact temperature detecting element, there is a possibility that the pressure roller may be damaged due to long-term use, resulting in an image defect.

  Therefore, there is a method of controlling the heating element by predicting the pressure roller temperature from the temperature of the heating element without actually measuring the temperature of the pressing roller (Patent Document 1).

  However, when the input voltage (peak value or effective value) to the heater fluctuates (changes), the heating time of the pressure roller varies depending on the amount of heat generated by the heater. As a result, there arises a problem that the pressure roller cannot be stably heated to a predetermined temperature. This is because when the input voltage is high, the heat generation amount of the heating element is large, and conversely, when the input voltage is low, the heat generation amount is small.

  When the input voltage is high, the amount of heat generated by the heating element is large, so that the temperature of the heating element rises to a predetermined temperature in a time shorter than the predetermined heat generation time, so that the pressure roller temperature does not rise to the target temperature. Therefore, the temperature difference between the fixing roller and the pressure roller becomes large, and the curling problem is not improved. Therefore, when waiting to raise the pressure roller temperature, the power is lowered after the target temperature of the fixing roller is reached, so that the rising slope of the pressure roller temperature becomes loose, resulting in a problem that the waiting time becomes longer. .

  Conversely, when the input voltage is low, the heat generation amount of the heat generator is small, so that the heat generator temperature reaches the predetermined temperature in a longer time than the predetermined heat generation time. As a result, the time required to rise to the target temperature becomes longer. Further, since it takes a long time to reach the target temperature of the fixing roller, the temperature of the pressure roller becomes higher than necessary.

  From the above, when the input voltage fluctuates, there is a problem that the pressure roller temperature at the time of paper feeding cannot be maintained at a desired temperature because the heating time of the pressure roller is different. Therefore, as a means for controlling the power, for example, there is a method using a power control circuit for controlling the power by directly determining the power from the current flowing through the heater. However, this method requires a dedicated power control circuit, which increases the cost.

  On the other hand, there is a method of controlling the temperature on the fixing film side by determining the electric power from the temperature detection change of the heating element temperature detecting means at the time of fixing driving (during fixing rotation). However, since this method performs power prediction control while driving the fixing device, it is necessary to increase the detection accuracy so that the temperature change during rotation of the pressure roller does not affect the temperature detection of the heater, and the input voltage can be stably In order to determine the above, it is necessary to monitor the temperature detection change of the heating element temperature detection means for a long time to determine. For this reason, it has been difficult to reduce the curl stably in a state where paper can be passed in a short time.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can pass paper in a short time while maintaining a simple configuration of a fixing device and can prevent conveyance failure without causing excessive curling. And

  An image forming apparatus according to the present invention includes a fixing device that fixes a toner image formed on a recording material by passing the recording material between fixing nips in which a heating rotator and a pressure rotator are pressed against each other, and the fixing device. Temperature detecting means for detecting the temperature of the heating element, and control means for controlling energization to the heating element of the fixing device. The control means includes a measuring means for measuring a temperature change rate according to an output of the temperature detection means after energization of the heating element of the fixing device, and a fixing device according to the obtained temperature change rate. Startup target temperature setting means for setting a startup target temperature that is higher than the paper passing target temperature, which is the target temperature of the heating element when passing paper, and the control means generates the heat when the heating element is energized. After raising the body temperature to the start-up target temperature, the body-passing target temperature is lowered, and the start-up target temperature setting means changes the start-up target temperature according to the temperature change rate. Features.

  When the heating element is energized, not only the temperature of the heating element is raised to a startup target temperature that is higher than the paper target temperature, but is also changed according to the startup target temperature temperature change rate (the higher the temperature change rate, the higher the By setting the target temperature to a large value), it is possible to satisfy both the fixing and curling requirements in a relatively short time.

  The startup target temperature setting means may change a relative temperature of the startup target temperature with respect to the sheet passing target temperature according to the temperature change rate.

  According to the present invention, it is possible to provide an image forming apparatus that stably reduces curl and prevents poor conveyance even when the input voltage level fluctuates while maintaining a simple configuration.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view illustrating a configuration example of a fixing device illustrated in FIG. 1. It is a graph showing the relationship between the pressure roller temperature when a sheet having a water content of 10% or more is passed and the temperature control temperature related to curling and fixing properties. It is a graph showing the temperature transition of the main thermistor when energizing by changing the amplitude (level) of the input voltage applied to the heater in each case of fixing driving stop and fixing driving from a 25 degree environment. It is the figure which showed the result of having measured the relationship between the starting temperature of a heater, and the pressure roller temperature at the time of reaching | attaining that temperature for every input voltage level (an upper limit, a center, a lower limit). Changes in the main thermistor temperature (solid line) and pressure roller temperature (broken line) when the temperature on the fixing film side is lowered so that the temperature difference between the fixing film side and the pressure roller side is reduced to prevent curling. It is a represented graph. Main thermistor temperature (solid line) and pressure roller temperature (broken line) when raising the starting temperature higher than the target temperature adjustment temperature during paper feeding (paper feeding target temperature) and lowering to a temperature that can suppress curling during paper feeding It is a graph showing transition. 6 is a graph showing changes in the main thermistor temperature (solid line) and the pressure roller temperature (broken line) in a case where a stable sheet passing standby time and curling and fixing properties are compatible even when the input voltage fluctuates. 6 is a flowchart illustrating a procedure of a first processing example in which a startup target temperature is determined by detecting a state (level) of an input voltage from a predetermined environment in which it can be determined that the fixing device is cold. 12 is a flowchart illustrating a procedure of a second processing example in which a startup target temperature is determined by detecting a state (level) of an input voltage from a predetermined environment in which it can be determined that the fixing device is cold. 12 is a flowchart illustrating a procedure of a third processing example in which a state (level) of an input voltage is detected from a predetermined environment where it can be determined that the fixing device is cold and a startup target temperature is determined. 10 is a flowchart illustrating a procedure of a fourth processing example in which a startup target temperature is determined by detecting a state (level) of an input voltage from a predetermined environment in which it can be determined that the fixing device is cold. 10 is a flowchart illustrating a fifth processing example when it is determined that the fixing device is warmed from the temperature detected by the main thermistor.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus according to the present embodiment.

  As a flow of image formation, first, the image carrier 3 is charged to a predetermined constant potential from the charging high-voltage power source 1 by the charging roller 2, and a point on the image carrier 3 to form an image is exposed by the exposure device 4. The surface potential is lowered to a predetermined potential. Next, the toner in the developing container 5 is uniformly placed on the developing sleeve 6, and the difference between the lowered potential on the image carrier 3 and the potential applied to the developing sleeve 6, that is, the action of the electric field is utilized. A charged toner is adhered onto the image carrier 3 to form a toner image. The toner image attached to the image carrier 3 is transferred by a transfer roller 8 to a recording material such as paper fed by a paper feeding means and conveyed to a predetermined transfer area along a pre-transfer guide 7. The Thereafter, the recording material is conveyed along the fixing nip entrance guide 11, fixed by the fixing device 12, and discharged. The toner that adheres to the image carrier 3 and cannot be transferred is scraped off and collected in the waste toner storage space 10 by the cleaning blade 9.

  The fixing device 12 is controlled by the control unit 30. The control unit 30 receives the heater detection temperature from the fixing device 12 and controls the driving of the heater. The temperature change rate measuring unit (measuring unit) 31, the sheet passing target temperature setting unit 33, and the startup target temperature setting unit. 35 and a heater drive unit 37 are provided. The temperature change rate measuring unit 31 is a measuring unit that measures a change (increase rate) in the heater temperature in accordance with an output of a temperature detection element described later when the heater is energized. The paper passing target temperature setting unit 33 is a part for setting a target temperature when paper is passed to the fixing device 12. The start-up target temperature setting unit 35 is a part that sets a start-up target temperature that determines how far the heater is initially raised when the heater is energized. The heater driving unit 37 is a part that generates heat by applying an input voltage to the heater. The control unit 30 can be configured by a CPU, a memory (ROM) storing the control program, and the like.

  FIG. 2 is a schematic cross-sectional view illustrating a configuration example of the fixing device 12.

  The fixing device 12 is composed of a film unit 20 of a collar 30 and a pressure roller (pressure rotating body) 21 of a collar 25. The film unit (heating rotator) 20 includes a heating source 19, a fixing film 15, a film guide 13, a T stay 14, and a temperature detection element 18. The heating source 19 in the present embodiment is configured by a ceramic heater. The ceramic heater is composed of a heating element in which a heating paste is printed on a ceramic substrate, a glass coating layer for ensuring protection and insulation of the heating element, and the like. AC) current is supplied to generate heat. The fixing film 15 is made of polyimide and has a cylindrical shape with a thickness of about 70 μm, and efficiently transfers the heat from the heating source to the toner 17 on the recording material 16. The film guide 13 has a number of ribs in the longitudinal direction, thereby assisting the circumferential movement of the fixing film 15 while suppressing resistance to the fixing film 15. The T-stay 14 is made of a steel plate and applies a pressing force uniformly. In addition, the temperature detection element 18 on the back of the ceramic substrate is constituted by a thermistor in the present embodiment, and detects the temperature of the heater. Various target temperatures of the heater are determined based on the detection result. Based on this target temperature, the heater driving means is controlled to control the power to the heater, and the heater temperature is controlled to the target temperature. The pressure roller 21 is formed by covering a core metal made of aluminum of the flange 20 with silicon rubber. The pressure roller 21 is pressed against the heater with a predetermined pressure (nip pressure) with a fixing film sandwiched by a spring to form a fixing nip portion 22 having a width of 5 to 8 mm, which includes the film unit 20 and the pressure roller 21. Yes. The pressure roller 21 is rotationally driven by a pressure roller driving unit, and rotates the fixing film 15 and conveys the paper 16 introduced into the fixing nip portion 22 in close contact with the fixing film 15. As the sheet 16 is conveyed to the fixing nip portion 22 in this way, the unfixed toner image formed and supported on the recording material is fixed by the heat of the heater member and the fixing nip pressure. The cleaning roller 23 is made of aluminum of the collar 12 and is in contact with the pressure roller 21 and is driven to rotate.

  FIG. 3 shows the relationship between the pressure roller temperature when a sheet having a water content of 10% or more is passed, and the temperature control temperature related to curling and fixing properties. Regarding curling, it can be seen that when the temperature control temperature is high, it tends to deteriorate, and when the pressure roller temperature is high, it tends to improve. Regarding the fixability, it can be seen that when both the temperature control temperature and the pressure roller temperature are high, they tend to improve. In general, regarding the relationship between the temperature control temperature and the pressure roller temperature when the fixing device is started from the COLD state, the pressure roller temperature increases as the temperature control temperature increases. Considering these results, the range where both curl and fixability are good is only the curve showing the curl NG range of FIG. 3 and the hatched portion (right side of the intersection) sandwiched between the straight lines showing the fixability NG range. Become.

  FIG. 4 shows the temperature transition of the main thermistor when energized by changing the amplitude (level) of the input voltage applied to the heater in each of the fixing driving stop and fixing driving from the 25 degree environment. It is a graph. In this specification, “fixing driving” means not only simply driving the heater but also rotating the pressure roller 21. A solid line indicates when fixing driving is stopped, and a broken line indicates when fixing driving is performed. For example, it is assumed that the fixing drive is operated when the main thermistor detects a predetermined reference temperature (for example, thermistor 100 ° C.) during startup.

  When the input voltage is the upper limit of the swing width (hereinafter simply referred to as “when the input voltage is the upper limit”), the time from energization ON until the main thermistor detects the reference temperature is 0.5 seconds. When the input voltage is at the center of the swing width (hereinafter simply referred to as “when the input voltage is at the center”), the time from when the energization is turned on until the main thermistor detects the reference temperature is 0.75 seconds. When the input voltage is the lower limit of the amplitude (hereinafter, simply referred to as “when the input voltage is the lower limit”), the time from when the energization is turned on until the main thermistor detects the reference temperature is 1 second.

  Thus, it can be seen that the higher the input voltage, the shorter the time to reach the predetermined temperature. When comparing the thermistor transition at the time of fixing driving and when fixing driving is stopped, when the fixing is driven, the heat of the heater is taken away by the pressure roller, so that the rise of the main thermistor becomes dull and the time until the predetermined time becomes longer. From the above, it is possible to determine the input voltage in a short time by applying a voltage to the heater while fixing driving is stopped.

  FIG. 5 is a diagram showing the results of actual measurement of the relationship between the heater start-up temperature and the pressure roller temperature when the temperature is reached for each input voltage level (upper limit, center, lower limit).

  When the paper passing target temperature, which is the target temperature adjustment temperature during paper passing, is set to 170 ° C., the pressure roller temperature at the beginning of paper passing is usually 70 ° C. or higher, so that it becomes 65 ° C. at the end of paper passing. Temperature. When the input voltage is at the upper limit, the time to reach the target startup temperature is short, so it is difficult to raise the pressure roller temperature. Conversely, when the input voltage is at the lower limit, the target startup temperature is reached. It can be seen that the pressure roller temperature is likely to rise because of the long time. When the input voltage is in the middle, the relationship is between the upper limit and the lower limit.

  6 to 8 show the temperature control temperature (main thermistor temperature: solid line) and the pressure roller temperature related to curling and fixing when a sheet having a moisture content of about 10% is passed stably even when the input voltage fluctuates. It is a graph showing the rising transition of the heater temperature for satisfying the relationship (broken line). The horizontal axis represents time (t seconds), the left vertical axis represents the temperature control temperature (main thermistor temperature) (° C.), and the right vertical axis represents the pressure roller temperature (° C.). A range indicated by a horizontal arrow in the figure indicates a time range in which the sheet is conveyed into the fixing device.

  FIG. 6 shows the main thermistor temperature (solid line) and the pressure roller temperature (broken line) when the temperature on the fixing film side is lowered so that the temperature difference between the fixing film side and the pressure roller side becomes small in order to prevent curling. ). For each solid line and broken line, the difference in temperature transition when the input voltage fluctuates is shown for each concentration (upper limit voltage: low concentration, center voltage: medium concentration, lower limit voltage: high concentration). Thus, when the temperature on the fixing film side is simply lowered, the start-up time from when the temperature starts to the target temperature is shortened, and the time until the paper arrives at the fixing device is shortened. However, since the start-up time is shortened, the pressure roller temperature does not rise to a temperature necessary for fixing, and the fixing property is deteriorated. This tendency is prominent when the input voltage is the upper limit.

  FIG. 7 shows a main thermistor in which the start-up temperature is made higher than the target temperature adjustment temperature during paper feeding (paper feeding target temperature) in order to achieve both curling and fixing properties, and the temperature is lowered to a temperature at which curling can be suppressed during paper feeding. It shows changes in temperature (solid line) and pressure roller temperature (broken line). By setting the start-up temperature higher than the medium temperature, the pressure roller temperature is raised to the temperature required for fixing, and during paper passing, the target temperature is lowered to a temperature at which curling can be suppressed. Compatibility and curl are possible. However, when the input voltage is the upper limit, since the time to reach the target rise temperature is short, the pressure roller temperature does not rise to the target temperature. Therefore, it is necessary to wait at the target startup temperature in order to increase the pressure roller temperature. When waiting at a predetermined start-up temperature, the power is lowered after reaching the target start-up temperature, so that the inclination of the pressure roller temperature rise becomes gradual, and the paper passing standby time becomes longer than when the input voltage is in the center. Conversely, when the input voltage is the lower limit, the amount of heat generated by the heater is small, so it takes longer to rise to the target startup temperature by raising the input voltage to the target startup temperature in a longer time than the central heating time. The paper passing standby time is longer than when the input voltage is in the center. In addition, the temperature of the pressure roller becomes higher than necessary.

  FIG. 8 shows changes in the main thermistor temperature (solid line) and the pressure roller temperature (broken line) when the stable sheet passing time, curl, and fixing property are both achieved even when the input voltage, which is the object of the present invention, fluctuates. It is a representation. (In this example, the sheet passing target temperature is set to 170 ° C. in accordance with the relationship between the pressure roller temperature and the temperature control temperature shown in FIG. 3.) By making the start-up temperature higher than the sheet intermediate temperature, the fixing property is improved. By raising the temperature to the required pressure roller temperature and lowering it to a temperature at which curling can be suppressed during paper feeding, both fixability and curling can be achieved. In addition, when the rising temperature is constant, when the input voltage fluctuates, the pressure roller temperature and the sheet passing standby time are affected. Therefore, the rising temperature is switched according to the input voltage.

  For example, when the input voltage is the upper limit, the time to reach the target rise temperature is shortened, so that the input voltage is higher than the rise temperature (for example, 185 ° C. from FIG. The start-up temperature when the input voltage is the upper limit (for example, 200 ° C. from FIG. 5) is set so that the voltage becomes the target temperature at the same time as the center start-up time. From FIG. 3, for example, the pressure roller temperature is set to 65 ° C. or more at a paper passing target temperature of 170 ° C., that is, the pressure roller temperature at the beginning of paper passing is set to 70 ° C. so that it becomes 65 ° C. at the end of paper passing. On the other hand, when the input voltage is the lower limit, the time to reach the target rise temperature becomes longer, so it is lower than the center rise target temperature (for example, 185 ° C. from FIG. 5) and the same time as the rise time at the center of the input voltage. Then, the rising temperature at the lower limit of the input voltage (for example, 175 ° C. from FIG. 5) is set so that the pressure roller temperature becomes the target temperature. At this time, in order to suppress curling, the startup temperature is always set higher than the sheet passing target temperature (for example, 170 ° C.).

  In this way, by detecting the input voltage level at the start of energization of the fixing device and separating the startup target temperature according to the detected input voltage level, excessive curling is achieved while satisfying the fixing property. Paper can be conveyed without generating it. Usually, it is assumed that the level of the input voltage does not change greatly at least during the period from the start to the end of energization.

(First processing example)
FIG. 9 shows a procedure of a first processing example in which the start-up target temperature is determined by detecting the state (level) of the input voltage from a predetermined environment (for example, 30 ° C.) where it can be determined that the fixing device is cold. It is a flowchart.

  First, a sheet passing target temperature is determined (for example, 170 ° C.) based on a temperature detected by the main thermistor (for example, 30 ° C.) (S 10), and energization of the heating source (heater) is started (S 11). Next, a process S100 for measuring the temperature detected by the main thermistor and measuring the temperature rise rate of the heating element is executed. In this example, the elapsed time Δt until the detected temperature of the main thermistor rises by a predetermined reference temperature change amount is measured (S12).

  If the elapsed time Δt is equal to or less than a predetermined first threshold time (0.6 seconds in this example) (S13, Yes), it is determined that the input voltage is the upper limit, and the startup target temperature is set to the upper limit startup target. The temperature is set to 200 ° C. in this example (S14). If the elapsed time Δt is equal to or longer than the second threshold time (0.9 seconds in this example) (S15, Yes), it is determined that the input voltage is the lower limit, and the startup target temperature is set to the lower startup target temperature (this In the example, it is set to 175 ° C. (S16). In cases other than the above, it is determined that the input voltage is in the center, and the startup target temperature is set to the central startup target temperature (185 ° C. in this example) (S17). After determining the startup target temperature in this way, fixing driving is started.

(Second processing example)
FIG. 10 is a flowchart illustrating a procedure of a second processing example in which the state of the input voltage is detected from a predetermined environment (for example, 30 ° C.) where it can be determined that the fixing device is cold and the startup target temperature is determined. .

  As in the first processing example, first, a sheet passing target temperature is determined (for example, 170 ° C.) based on the temperature detected by the main thermistor (for example, 30 ° C.) (S20), and energization is started for the heating source (heater) ( S21). Next, a process S100 for measuring the temperature detected by the main thermistor and measuring the temperature rise rate of the heating element is executed. In this example, when the reference time Δt3 (0.45 seconds in this example) has elapsed since the start of energization, the temperature change ΔT from the start of energization is measured (S22).

  If the temperature change amount ΔT from the start of energization to the time after the predetermined elapsed time Δt3 is equal to or greater than the predetermined threshold T1 (50 ° C. in this example) (S23, Yes), the input voltage is determined to be the upper limit, and the startup target The temperature is set to the upper limit startup target temperature (200 ° C. in this example) (S24). If the temperature change amount ΔT is equal to or less than a predetermined threshold T2 (<T1, 35 ° C. in this example) (S25, Yes), it is determined that the input voltage is the lower limit, and the startup target temperature is set to the lower limit startup target temperature ( In this example, it is set to 175 ° C. (S26). In cases other than the above, it is determined that the input voltage is in the center, and the startup target temperature is set to the central startup target temperature (185 ° C. in this example) (S27). After determining the startup target temperature in this way, fixing driving is started.

(Third processing example)
FIG. 11 is a flowchart illustrating a procedure of a third processing example in which the state of the input voltage is detected from a predetermined environment (for example, 30 ° C.) where it can be determined that the fixing device is cold and the startup target temperature is determined.

  As in the first processing example, first, a sheet passing target temperature is determined (for example, 170 ° C.) based on the temperature detected by the main thermistor (for example, 30 ° C.) (S30), and energization is started for the heating source (heater) ( S31). Next, a process S100 for measuring the temperature detected by the main thermistor and measuring the temperature rise rate of the heating element is executed. In this example, a temperature change amount ΔT is measured when a predetermined reference time Δt4 (0.25 seconds in this example) has elapsed since the detection of a predetermined reference temperature (for example, main thermistor 60 ° C.) during startup. (S32).

  If the temperature change ΔT obtained by this measurement is equal to or greater than a predetermined threshold T4 (40 ° C. in this example) (S33, Yes), it is determined that the input voltage is the upper limit, and the target startup temperature is raised to the upper limit. The target temperature is set (200 ° C. in this example) (S34). If the temperature change amount ΔT is not more than a predetermined threshold T5 (<T4, 25 ° C. in this example) (S35, Yes), it is determined that the input voltage is the lower limit, and the startup target temperature is set to the lower limit startup target temperature. (In this example, 175 ° C.) is set (S36). In cases other than the above, it is determined that the input voltage is in the center, and the startup target temperature is set to the central startup target temperature (185 ° C. in this example) (S37). After determining the startup target temperature in this way, fixing driving is started.

(Fourth processing example)
FIG. 12 is a flowchart showing a procedure of a fourth processing example in which the state of the input voltage is detected from a predetermined environment (for example, 30 ° C.) to determine the startup target temperature.

  As in the first processing example, first, the sheet passing target temperature is determined (for example, 170 ° C.) based on the temperature detected by the main thermistor (for example, 30 ° C.) (S40), and energization is started for the heating source (heater) ( S41). Next, a process S100 for measuring the temperature detected by the main thermistor and measuring the temperature rise rate of the heating element is executed. In this example, after energization is started, an elapsed time Δt from when the main thermistor detects the first reference temperature (60 ° C. in this example) until the second reference temperature (100 ° C. in this example) is detected. Is measured (S42). If this elapsed time Δt is equal to or less than a predetermined threshold t4 (0.25 seconds in this example) (Yes in S43), it is determined that the input voltage is the upper limit, and the startup target temperature is set to the upper limit startup target temperature (this In the example, it is set to 200 ° C.) (S44). If the elapsed time Δt is equal to or greater than a predetermined threshold value t5 (> t4, 0.5 seconds in this example) (S45, Yes), the input voltage is determined to be the lower limit, and the startup target temperature is set to the lower limit startup target temperature. (In this example, 175 ° C.) is set (S46). In cases other than the above, it is determined that the input voltage is in the center, and the startup target temperature is set to the central startup target temperature (185 ° C. in this example) (S47). After determining the startup target temperature in this way, fixing driving is started.

(Fifth processing example)
In the first to fourth processing examples, it is assumed that energization is started in an environment where it can be determined that the fixing device is cold. On the other hand, FIG. 13 shows a processing example when it is determined that the fixing device is warmed from the temperature detected by the main thermistor.

  First, the sheet passing target temperature is determined (for example, 166 ° C.) based on the detected temperature of the main thermistor (S50). This temperature is lower than the temperature when the fixing device is not warmed. Next, energization of the heating source is started (S51).

  Thereafter, the level of the input voltage is determined by measuring the temperature change rate (temperature increase rate) by any of the methods described in the first to fourth processing examples (S52 (S100)). If it is determined that the input voltage is the upper limit (S53, Yes), the startup target temperature is set to 190 ° C. When it is determined that the input voltage is the lower limit, the startup target temperature is set to 170 ° C. When it is determined that the input voltage is in the center, the startup target temperature is set to 180 ° C. After determining the startup target temperature, fixing driving is started.

  As described above, when it is determined that the fixing device is warm at the start of energization, the start target temperature is set to the upper limit and the lower limit in association with setting the lower sheet passing target temperature (166 ° C.) than when the fixing device is not warm. Set lower at each central input voltage level. The relationship between the sheet passing target temperature and the rising target temperature of each input voltage level can be determined in advance in a data table (not shown) or the like.

  According to the present exemplary embodiment, after the energization of the fixing device is started, the input voltage is detected and the startup target temperature is determined so that the sheet can be conveyed without causing excessive curl while satisfying the fixing property. I can do it.

  The preferred embodiments of the present invention have been described above, but various modifications and changes other than those mentioned above can be made. For example, the shape, material, and specific numerical values of the parts listed above are merely examples, and the present invention is not limited to them.

DESCRIPTION OF SYMBOLS 1 Charging high voltage power supply 2 Charging roller 3 Image carrier 4 Exposure device 5 Developing container 6 Developing sleeve 7 Pre-transfer guide 8 Transfer roller 9 Cleaning blade 10 Waste toner storage space 11 Fixing nip entrance guide 12 Fixing device 13 Film guide 14 T stay 15 Fixing film 16 Recording material (paper)
DESCRIPTION OF SYMBOLS 18 Temperature detection element 19 Heat source 20 Film unit 21 Pressure roller 22 Fixing nip part 23 Cleaning roller 25 Target temperature setting part 30 Control part (control means)
31 Temperature change rate measuring unit 33 Paper passing target temperature setting unit 35 Target temperature setting unit 37 Heater driving unit 60 Main thermistor

Claims (7)

A fixing device for fixing the toner image formed on the recording material by passing the recording material between fixing nips where the heating rotator and the pressure rotator are in pressure contact with each other;
Temperature detecting means for detecting the temperature of the heating element of the fixing device;
Control means for controlling energization to the heating element of the fixing device,
The control means includes
Measuring means for measuring a temperature change rate according to an output of the temperature detecting means after starting energization to the heating element of the fixing device;
A startup target temperature setting means for setting a startup target temperature that is higher than a paper passing target temperature that is a target temperature of the heating element when passing through the fixing device in accordance with the obtained temperature change rate;
The control means raises the temperature of the heating element to the startup target temperature when the heating element is energized, and then lowers the temperature to the sheet passing target temperature.
The image forming apparatus characterized in that the start-up target temperature setting means changes the start-up target temperature according to the temperature change rate.   The image forming apparatus according to claim 1, wherein the startup target temperature is set to be larger as the temperature change rate is larger.   The image forming apparatus according to claim 1, wherein the startup target temperature setting unit changes a relative temperature of the startup target temperature with respect to the sheet passing target temperature according to the temperature change rate.   The measuring means measures an elapsed time from the start of energization until a temperature rise of the reference temperature change amount is detected by the temperature detection means, and obtains a temperature change rate from the reference temperature change amount and the elapsed time. The image forming apparatus according to claim 3.   The image according to any one of claims 1 to 3, wherein the measuring unit measures a temperature change amount at a reference time lapse time by the temperature detection unit from the start of energization, and obtains a temperature change rate from the reference time and the temperature change amount. Forming equipment.   The measuring means measures the amount of temperature change from the detection time of a predetermined reference temperature during the start-up by the temperature detection means after the start of energization to the lapse of a predetermined reference time, and the reference time and the temperature change amount The image forming apparatus according to claim 1, wherein a temperature change rate is calculated from the image forming apparatus.   The measuring means calculates an elapsed time from a detection time of a first reference temperature, which is set in advance by the temperature detection means, to a detection time of a second reference temperature higher than the first reference temperature after the start of energization. The image forming apparatus according to claim 1, wherein the temperature change rate is measured and a temperature change rate is obtained from a temperature difference between the first and second reference temperatures and the elapsed time.