JP2010002857A - Fixing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further shorten a warm-up period of a fixing device using an endless belt. <P>SOLUTION: During the warm-up period before starting a fixing process in a fixing nip section after the start of the rotation of the endless film, a quantity of heat generated by a heater is set so that the quantity is the largest in a position downstream of the middle of the fixing nip in the rotating direction of the endless film. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a fixing device mounted on a copying machine or a printer using electrophotographic technology. In particular, the present invention relates to a fixing device that includes an endless film and a heater that contacts an inner peripheral surface of the endless film, and heat-fixes an unfixed toner image on the recording material onto the recording material via the endless film.

  Since the fixing device using the endless film has a very low heat capacity due to its structure, it can shorten the time from the start of warm-up until the temperature at which fixing is possible. Therefore, it has excellent performance, such as reducing power consumption during standby waiting for a print instruction.

  Even in the fixing device having such excellent performance, in recent years, further performance improvement has been demanded. One example is to further shorten the time from the start of warm-up until the temperature at which fixing can be achieved. In addition, further improvement in fixing performance for fixing toner on paper is one of them.

In Patent Document 1, a heater that contacts an inner peripheral surface of an endless film has a plurality of heating resistors, and when a plurality of recording materials are continuously fixed, It is disclosed to secure the fixing performance by changing the heat generation distribution.
JP 2002-341682 A

  Japanese Patent Application Laid-Open No. 2005-228867 ensures the fixability during continuous printing by gradually shifting the heat generation peak of the heater in the recording material conveyance direction during continuous printing to the upstream side in the conveyance direction. However, it does not shorten the time to warm-up completion with less power consumption.

  The present invention for solving the above-mentioned problems includes an endless film, a heater having a substrate and a plurality of heating resistors provided on the substrate, and being in contact with an inner peripheral surface of the endless film, and the endless film And a backup member that forms a fixing nip portion together with the heater via, and heat-fixes the unfixed toner image on the recording material while nipping and conveying the recording material carrying the unfixed toner image at the fixing nip portion. In the fixing device, during the warm-up period before the endless film starts to rotate and the fixing process is started at the fixing nip portion, the heater generates heat at a position downstream from the center of the fixing nip portion in the rotation direction of the endless film. The amount is set to be the largest.

  According to the present invention, it is possible to shorten the time until the warm-up is completed with less power consumption.

Example 1
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

  FIG. 20 is a diagram showing a schematic configuration of a laser printer as an example of an image forming apparatus equipped with the fixing device of the present invention.

  In the figure, reference numeral 1 denotes a drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum). The photosensitive drum 1 is rotatably supported by the apparatus main body M, and is driven to rotate at a predetermined process speed in the direction of the arrow by a driving hand throw (not shown). Around the photosensitive drum 1, there are a charging device (charging roller) 2, an exposure device (scanning means) 3, a developing device 4, a transfer device (transfer means) 5, and a cleaning device 7 in this order along the rotation direction. It is arranged.

  In FIG. 2, the paper feed cassette 8 is disposed at the lower part of the apparatus main body M and stores a sheet-like recording material P. The recording material P is transported along the transport path R to a transfer device (transfer device) 5, a guide 35, and a fixing device (fixing device) 6 sequentially from the upstream side in the transport direction by a transport device (not shown).

  Next, an image forming operation in the laser printer will be described. When the image forming operation is started, first, the photosensitive drum 1 that is rotationally driven in the direction of the arrow by the driving unit is uniformly charged to a predetermined polarity and a predetermined potential by the charging roller 2. Then, the surface of the photosensitive drum 1 charged by the charging roller 2 is scanned by the light L based on the image information by the exposure device (scanning means) 3. As a result, the portion of the electric charge scanned by the light L is removed, and an electrostatic latent image is formed on the surface of the photosensitive drum 1.

  Next, the electrostatic latent image is developed by the developing device 4 that contains toner, and a toner image is formed on the photosensitive drum. The developing device 4 includes a developing roller 4a, and supplies a toner to the electrostatic latent image on the photosensitive drum 1 by applying a developing bias to the developing roller 4a.

  On the other hand, the recording material P stored in the paper feed cassette 8 in parallel with such a toner image forming operation is fed and transported by a transport means (not shown), and between the photosensitive drum 1 and the transfer roller 5. It is conveyed to. When the recording material P is conveyed to the transfer nip portion, the toner image on the photosensitive drum is transferred to a predetermined position on the recording material P by the transfer bias applied to the transfer roller 5.

  Next, the recording material P carrying the unfixed toner image is formed between the endless film 33 constituting the fixing device 6 along the conveyance guide 35 and the pressure roller 40 in contact with the endless film 33. It is conveyed to the fixing nip N. The unfixed toner image is heated and pressurized at the fixing nip portion, and the toner image is fixed on the surface of the recording material P. The recording material P after the toner image is fixed in this way is discharged onto a paper discharge tray provided on the upper surface of the apparatus main body M.

  On the other hand, the photosensitive drum 1 after the toner image transfer is cleaned by the cleaning blade 7a of the cleaning device 7 to prepare for the next image formation. By repeating the above operation, image formation can be performed one after another.

  Next, the configuration of the fixing device 6 according to the present embodiment is shown in FIG. FIG. 1A shows a plan view of the heater 31, and FIG. 1B shows a cross-sectional view of the fixing device.

  The heater 31 is a heater in which a heating resistor is formed on the ceramic substrate 31a. The ceramic substrate 31a is made of a low heat capacity material having excellent electrical insulation and thermal conductivity, such as alumina or aluminum nitride (A1N). A heating resistor 20 such as silver palladium (Ag / Pb) or Ta2N is formed on the ceramic substrate 31a by screen printing along the longitudinal direction of the substrate. Reference numerals 21a and 21b denote electrodes that are in contact with a connector (not shown). Further, the surface on which the heating resistor 20 is formed is covered with a thin glass protective layer (not shown). The heater 31 (the heating resistors 20a and 20b) is supplied with electric power from a commercial power source and generates heat.

  The temperature of the heater 31 is detected by a temperature detection element (hereinafter referred to as a thermistor) 34. The energization of the heater 31 is controlled by an energization control unit (not shown) according to the temperature detected by the temperature detection element 34. As a control method, a wave number control method for controlling the increase or decrease of the wave number of the current (sine wave) supplied from the commercial power source to the heater, or a phase control method for controlling the phase angle of the current (sine wave) supplied from the commercial power source to the heater There is. By these control methods, the temperature of the heater 31 is controlled to a predetermined target temperature.

  The endless film 33 is sandwiched between the heater 31 and the pressure roller 40 in the fixing nip portion N, receives power from the pressure roller 40, and rotates while being in close contact with the heater 31. The glass protective layer side of the heater 31 may be in contact with the endless film 33, or the ceramic substrate side may be in contact with the endless film 33.

  Further, since the endless film 33 rotates while rubbing against the internal heater 31 and the heater holder 32, it is necessary to keep the frictional resistance between the heater 31, the heater holder 32, and the endless film 33 small. For this reason, a small amount of lubricant such as heat resistant grease is interposed on the surfaces of the heater 31 and the heater holder 32. Thereby, the endless film 33 can be smoothly rotated.

  In order to efficiently apply the heat of the heater 31 to the recording material P, the thickness of the endless film 33 is preferably 100 μm or less. The endless film 33 has a configuration in which a heat-resistant resin such as polyimide, polyamideimide, or PEEK is used as a base layer, or a configuration in which a metal such as SUS (stainless steel), Al, or Ni is used as a base layer.

  In addition, as an endless film having sufficient strength and excellent durability for constituting a long-life fixing device, a thickness of 20 μm or more is required. Therefore, the thickness of the endless film 33 is optimally 20 μm or more and 100 μm or less. The endless film 33 has a three-layer structure including an endless film base layer, a primer layer, and a release layer, or a rubber layer provided between the base layer and the release layer. The endless film base layer side is the heater 31 side, and the release layer is the pressure roller 40 side. Further, the endless film base layer maintains the mechanical strength such as the tear strength of the entire endless film 33. On the outside of the base layer, a conductive primer layer in which a conductive filler is dispersed is formed with a thickness of about 2 to 6 μm. And in order to prevent the electrostatic offset by the charge-up of the endless film 33, it is comprised so that a film may be kept in a ground state or a fixed electric potential.

  Further, the release layer is made of a heat-resistant fluororesin having a good release property such as PFA, PTFE, FEP, or silicone resin having a thickness of 10 μm for the surface layer in order to prevent toner offset with respect to the endless film 33 and to ensure separation of the recording material P. To some extent, it is coated alone.

  The heater holder 32 is formed of, for example, a heat-resistant plastic member (liquid crystal polymer, phenol resin, PPS, PEEK), and also serves as a guide for holding the heater 31 and guiding the rotation of the endless film 33. When the pressure roller 40 is rotated by a motor (not shown), the endless film receives a driving force from the pressure roller and rotates in the direction of the arrow in FIG.

  A pressure roller (backup member) 40 that forms the fixing nip portion N together with the heater 31 via the endless film 33 is formed by foaming heat-resistant rubber or silicone rubber such as silicone rubber or fluorine rubber on the outside of the core metal (not shown). A release layer (not shown) such as PFA, PTFE, or FEP may be formed on the elastic layer 42.

  In order to form the fixing nip N as shown in FIG. 1B, springs are applied to both ends in the longitudinal direction of the fixing device, and pressure is applied to the fixing nip N by the force of the springs. Yes.

  The unfixed toner image t is heated and fixed to the recording material P while the recording material P carrying the unfixed toner image t is nipped and conveyed at the fixing nip portion N formed as described above.

Next, FIG. 2 shows temperature control of the fixing device in the first embodiment. FIG. 2 shows the transition of the target temperature (control temperature) of the heater 31. The fixing device according to the present embodiment includes the following three operations.
(1) Start-up operation to warm the device to the target temperature (warm-up period)
(Heat storage operation in which the recording material P does not exist in the nip N)
(2) Fixing operation for fixing the toner image to the recording material P (fixing processing period)
(Heating operation in which the recording material P exists in the nip portion N)
(3) Falling operation for cooling the apparatus When a print instruction is input to the printer, first, (1) a startup operation (hereinafter referred to as pre-rotation) is performed. At this time, since the printer needs to start up various devices such as an image processing device, a drive motor, or a high voltage circuit, the power that can be input to the fixing device is limited. In this embodiment, 1000 W can be charged into the fixing device, a device with a process speed of 300 mm / sec and a throughput of 50 ppm is used, and the pre-rotation is 4 seconds. The endless film 33 is made of polyimide having an outer diameter of φ24 mm, the base layer is 50 μm, and the thermal conductivity of the endless film is about 0.1 W / mK. The pressure roller 40 had an outer diameter of 25 mm, the pressure was adjusted so that silicone rubber was used as the elastic layer 42 and the thickness was 4 mm and the width of the nip portion N was 8 mm.

  In the above conditions, when energizing only the upstream heating resistor 20a shown in FIG. 1 at a power ratio of 100% (that is, a state in which the sine wave supplied from the commercial power source is not limited) (setting A) FIG. 3 shows the temperature distribution in the fixing nip portion N when energizing only the downstream heating resistor 20b with a power ratio of 100% (setting B). The horizontal axis is the position in the nip N, and the vertical axis is the temperature in the nip. The measured values in FIG. 3 are obtained by measuring the temperature by passing a thermocouple through the fixing nip N instead of the recording material P at the end of the pre-rotation (just before the fixing operation 4 seconds after the start of energization).

  In setting A, the temperature of the thermocouple increases immediately after entering the nip portion N, and the temperature reaches a peak (about 130 ° C.) near the center of the conveyance direction, whereas in setting B, the thermocouple immediately before exiting from the nip portion N. Temperature reached a peak, and the ultimate temperature was about 150 ° C. Thus, it can be seen that the setting B has a higher peak temperature than the setting A, and the position of the peak is also closer to the nip exit than the setting A.

  Here, the temperature of the surface of the endless film 33 is shown in FIG. FIG. 4 (a) shows the temperatures of the endless film immediately after and immediately before the nip portion N in the upstream 100% heat generation (setting A), the downstream 100% heat generation (setting B), and the upstream 50% downstream 50% heat generation (setting C). Show. The temperature immediately after the nip portion N and the temperature immediately before the nip portion N are measured at the positions shown in FIG.

  As for the surface temperature of the endless film 33 immediately after the nip portion N, the one having 100% downstream heat generation (setting B) has the highest temperature (130 ° C.), and accordingly, the one having 100% downstream heat generation immediately before the nip portion N (setting B). Is the highest temperature (80 ° C.).

  This mechanism will be described with reference to the image diagram of FIG. In the upstream heat generation (setting A), heat is transferred from the heating resistor 20a to the endless film 33, and from the endless film 33 to the heater 31 and the pressure roller 40 side. Immediately after energization of the heater is started, heat is more easily transferred because the temperature difference between the heater and surrounding components is large. On the other hand, in the downstream heat generation (setting B), immediately after the heat is transferred from the heat generating resistor 20b to the endless film 33, the endless film 33 comes out of the nip portion N by rotation, so that it is insulated by the air layer. The endless film 33 passes through the nip portion N a plurality of times. However, since the temperature difference from the pressure roller 40 is small after the second time, heat does not easily escape and is stored in the endless film 33. Therefore, as shown in FIG. 4, in setting B, the surface temperature of the endless film immediately after the nip portion N increases, and the surface temperature of the endless film immediately before the nip portion N also increases.

  The heat transferred to the heater 31 due to upstream heat generation (setting A) permeates from the surface to the inside during one round, while the heat applied to the heater holder 32 downstream of the heater substrate and the pressure roller 40. .

  Next, the fixing operation (2) for the first sheet will be described. First, FIG. 6A shows the fixability of the first sheet. Here, the toner image on the recording material P is rubbed and thinned after fixing, and the density before and after the rub is measured to calculate the density reduction rate (%).

  In FIG. 6, the horizontal axis indicates the location in the conveyance direction of the recording material P, and the vertical axis indicates the density reduction rate. It can be said that the larger the number of the density decrease rate, the worse the fixing property. In this embodiment, whether the fixing property is good or bad is determined on the basis of 20%, that is, on the basis that the image density is about 80% by rubbing.

  In the graph showing the fixability of the first sheet in FIG. 6A, the density reduction rate for setting A is around 20%, whereas the density reduction rate for setting B is around 10%. is there. Setting B has particularly good fixability at the leading end in the conveyance direction of the recording material. In the case of setting C in which the two heating resistors 20a and 20b are uniformly heated, the density reduction rate is around 18%, and the fixing property is stable from the front end to the rear end of the recording material P.

  The reason why the fixing property of the setting B is good is that the amount of heat stored in the endless film 33 is gradually discharged from the front end of the recording material P in the nip portion N during the fixing operation.

  The fixability varies depending on the thermal conductivity and outer diameter of the endless film 33, but if the pre-rotation (heat storage operation) time is adjusted so that the density reduction rate of the first printed image is 20%, setting B Compared with the setting A, the pre-rotation time can be shortened by about 1 second. That is, it can be seen that if the warm-up is performed with the heat generation distribution as in setting B, the warm-up time can be shortened.

  Next, the fixing operation for the second and subsequent sheets will be described. In this embodiment, as shown in FIG. 2, when the pre-rotation (heat storage operation) is completed, the heater controls the temperature so as to maintain the target temperature of 180 ° C. and shifts to the fixing operation.

  During the fixing operation, the input power is reduced by temperature control that keeps the temperature of the thermistor constant, so that the power is tens of percent less than 1000 W (100%) during the previous rotation. Further, the power varies depending on the temperature and humidity of the usage environment and the thickness of the recording material P. FIG. 7 shows a monitor of the power consumed by the fixing device in this embodiment. FIG. 7 shows time on the horizontal axis and power consumption on the vertical axis. Although 1000 W is supplied at the time of the pre-rotation, the temperature control is started and the entire fixing device is warmed, so that the power consumption gradually decreases.

  The fixability of the third sheet at this time is shown in FIG. The electric power is adjusted by adjusting the arrangement of the thermistor, and 100% of the heat generated during the fixing operation of the third sheet is about 700 W of input power. Unlike the fixability of the first image shown in FIG. 6A, the fixability of the third image shown in FIG. 6B is better in the setting A than in the setting B. It has become. This is due to the following reason.

  In the temperature distribution shown in FIG. 3, in setting B, the temperature in the nip is 60 ° C. at a position about 3 mm from the inlet of the nip N, whereas in setting A, the temperature is about 0.5 mm from the inlet of the nip N. The temperature in the nip portion is 60 ° C. at the location.

  Next, the “temperature change in the elastic modulus of the toner” to be fixed is shown in FIG. The vertical axis represents the elastic modulus (G ′), and the horizontal axis represents the temperature. When the toner is heated, the elastic modulus is lowered and suddenly falls from around 60 ° C. Regarding the relationship between heating and pressurization in the fixing operation, the toner is more easily melted when the pressurization time after reaching 60 ° C. is longer. Therefore, the setting A is set according to the temperature distribution in the nip portion N shown in FIG. Fixability is further improved.

  And as shown in FIG.6 (b), with the setting A, the density | concentration fall rate of the 3rd sheet | seat will be about 10%.

  Next, specific control of the first embodiment is shown in FIG. Similar to FIGS. 2 and 7, the horizontal axis represents time, and the vertical axis represents the change in the heat generation ratio between the upstream side heating resistor and the downstream side heating resistor. At the time of pre-rotation (heat storage operation), the heat generation on the downstream side is set to 100% (setting B), and heat is positively stored in the endless film 33 to suppress heat transfer to the heater substrate 31a and the pressure roller 40. At the end of the pre-rotation (heat storage operation), the heat generation is switched to 100% upstream (setting A) (fixing operation), the peak temperature in the nip portion is moved upstream, and the toner is heated in the upstream portion of the nip portion N. give. Therefore, in the fixing operation, fixing efficiency is improved, so that heat fixing can be performed even with low power.

  The fixability of Example 1 is shown in FIG. The horizontal axis shows the number of sheets, and the vertical axis shows the average density reduction rate. As described above, the density reduction rate can be kept around 10% by switching the heat generation distribution between the pre-rotation (heat storage operation) period and the fixing operation period.

  As described above, during the warm-up period before the endless film starts rotating and fixing processing is started at the fixing nip, the heater generates the largest amount of heat at a position downstream from the center of the fixing nip in the endless film rotation direction. By setting so as to be, it is possible to shorten the time to warm-up completion with less power consumption.

  Also, during the period for fixing the unfixed toner image on the recording material, the position where the heat generation amount of the heater is the largest is set at a position upstream of the position during the warm-up period in the endless film rotation direction. The property can be sufficiently secured.

(Example 2)
Next, the control of the second embodiment will be described with reference to FIG. Since the mechanical configuration of the fixing device is the same as that of the first embodiment, the description is omitted.

  In the second embodiment, a rapid change is prevented when the heat generation ratio is shifted. In particular, when the pre-rotation (heat storage operation) with a large input power is shifted to the fixing operation, the heat generation ratio is gradually changed. At this time, the shift is made so that the upstream and downstream are always 100%.

  Further, in the second exemplary embodiment, when a plurality of recording materials carrying an unfixed toner image are continuously fixed, the fixing nip portion is between the preceding recording material and the succeeding recording material (paper). The heat storage operation for the endless film (setting B shown in Example 1) is also performed between the sheets so that the amount of heat generated by the heater is maximized at a position downstream from the center of the fixing nip. The term “between sheets” here refers to a state where the recording material P is not present in the nip portion N between the first sheet and the second sheet, for example. In this embodiment, the interval between sheets is set to about 75 mm (250 msec), and during that time, 100% heat generation (setting B) is set downstream. Between the sheets, heat transfer from the heating resistor 20 to the heater substrate 31a and the pressure roller 40 is suppressed, and heat is stored in the endless film 33. It is desirable that the period of the setting B between the sheets is at least one turn of the endless film. However, since there is heat transfer in the circumferential direction of the endless film 33 and penetration and diffusion into the inside, heat storage is performed even if less than one turn. There is an effect.

  Further, in the second embodiment, 50% heat generation (setting C) is performed at the time of operations other than the pre-rotation period, the heat storage operation period between sheets, and the fixing operation period. Thereby, since the temperature distribution change in the nip portion N becomes gentle, stress due to thermal stress on the heater 31 and the peripheral members can be relieved. Further, during the falling operation, power is not supplied depending on the control temperature, but heat is generated evenly in the upstream and downstream even when heating is performed after the rotation is stopped, for example.

  Further, even when the heating resistor is formed so as to protrude beyond the nip portion N, the thermal stress due to the thermal imbalance in the recording material conveyance direction of the heater 31 can be suppressed to a small value. Can make problems less likely.

  Further, in the second embodiment, the fixability is improved by the control between papers. A comparison with Example 1 is shown in FIG. The number of sheets is plotted on the horizontal axis and the density reduction rate is plotted on the vertical axis. According to the second embodiment, the fixability on the second and third sheets is improved. For this reason, it is effective when printing a plurality of sheets, and when the fixing property is uniform (density reduction rate is about 20%), the process speed can be increased to 350 mm / sec even with the same configuration.

  Here, FIG. 13 shows the first set-out time (FSOT: time required from the start of energization to the heater to the output of the first recording paper) and the throughput (TP) in the conventional example, the first example, and the second example. A comparison table is shown. It can be seen that the spec of Example 2 is that FSOT is shortened by 1 second and TP is 8 ppm faster than the conventional example in which uniform heat generation (50% each) is achieved.

  In the second embodiment and the first embodiment described above, the number of heating resistors on the heater substrate is two. However, if independent control is possible, a plurality of heating resistors as shown in FIG. Needless to say, even if configured, it is effective.

  As described above, a plurality of heating resistors are formed on the heater substrate on the upstream and downstream sides, and by selectively selecting a resistance layer to be energized and heated according to the situation, fixing power can be suppressed, FSOT shortening and throughput can be reduced. A fixing device that can be improved can be provided.

  As described above, when a plurality of recording materials carrying an unfixed toner image are continuously fixed, the fixing nip portion is centered even during a period in which the fixing nip portion is between the preceding recording material and the succeeding recording material. If setting is made so that the amount of heat generated by the heater is maximized at a downstream position, not only the time to warm-up can be shortened with low power consumption, but also the fixability during continuous printing is improved.

(Example 3)
Depending on the heating resistor configuration of the heater 31, the heat generation ratio due to temperature can be changed. In the third embodiment, the short direction of the heater 31 is divided so that one is upstream, and the other is downstream, and the upstream / downstream heat generation ratio is optimized. That is, due to the resistance temperature coefficient (TCR) of the heating resistor, the downstream heat generation amount increases during the pre-rotation heat storage operation (the heating part is from room temperature to about 180 ° C.), and during the fixing operation (the heating part is about 200 ° C.). Then, the heater 31 which performed the resistance value setting and connection of the heating resistor so that upstream calorific value may increase is proposed.

  For example, the heating resistor 20 shown in FIG. 15 includes a PTC (Positive Temperature Coefficient) resistance layer that increases in resistance as the temperature increases, and 20a and 20b have resistance values (Ω) and resistance temperature coefficients (ppm / deg). ) Is different. Each resistance value and temperature characteristic are shown in FIG. The heater of FIG. 15 has a plurality of types of heating resistors having different resistance temperature coefficients.

  The horizontal axis shows the temperature of the heating resistor, and the vertical axis shows the resistance value. TCR (ppm / deg) is the rate of change in resistance per 1 ° C increase. If positive (plus), the resistance rises. If negative (minus), As resistance decreases, the change in resistance increases as the number increases. In the third embodiment, the upstream heating resistor 20a has a resistance value of about 20Ω at a normal temperature (about 25 ° C.) and a resistance temperature coefficient of about 1000 ppm / deg. The downstream heating resistor 20b has a resistance value of about 22Ω at a normal temperature (about 25 ° C.) and a resistance temperature coefficient of about 100 ppm / deg. That is, the resistance value of the heating resistor is set to be reversed upstream and downstream due to a temperature change during the previous rotation.

Since the heating resistor is 20a and 20b as shown in FIG. 15 are connected in series (current constant), the heat generation amount becomes RI ^2. Therefore, the heat generation ratio changes from the downstream large to the upstream large as the resistance values of the upstream and downstream heat generating resistors are reversed.

  The heat generation distribution at this time is shown in FIG. The horizontal axis indicates the position in the nip N, and the vertical axis indicates the temperature. The time distribution of the temperature distribution in the nip N is shown. As the temperature of the heating resistor increases due to energization, the heat generation ratio changes and the temperature peak shifts upstream. Due to this change, heat is stored in the endless film 33 during the pre-rotation heat storage operation, and shifts to upstream heat generation with good fixing efficiency during the fixing operation.

  As described above, according to the third embodiment, it is possible to shorten the FSOT and improve the throughput without requiring a power supply circuit or control.

In addition to the configuration (1) of the third embodiment, FIG. 18 shows a resistance value, a resistance temperature coefficient, and a connection method as a configuration example of the heating resistor. For example, the heating resistors of the configuration (2) are connected in parallel, the downstream resistance value is smaller than the upstream, and the downstream resistance temperature coefficient is positive (PTC) and larger than the upstream. The parallel connection power is V ² / R, and the smaller the resistance value, the greater the amount of heat generated. Therefore, also in the configuration (2), the heat generation ratio near the downstream is obtained during the pre-rotation heat storage operation, and the heat generation ratio near the upstream is obtained during the fixing operation.

Next, in the configuration (3), a negative NTC (Negative Temperature Coefficient) resistance layer whose resistance decreases with increasing temperature is used. FIG. 19A shows a change in resistance value with respect to the temperature of the heating resistor used here. The resistance value on the downstream side is large at room temperature and reverses at a predetermined temperature to increase the upstream resistance value. Configuration (3) is the power of the RI ² for serial connection, continue to ratio change from the downstream heating before during rotation upstream heat generation during fixing operation. Furthermore, by using a heating resistor having different resistance temperature characteristics (positive and negative) as shown in FIG. 19B, it is possible to further increase the heat generation ratio change.

  Similarly, in the configuration (4) as well, downstream heat generation occurs during the pre-rotation and upstream heat generation occurs during the fixing operation, and the heat generation ratio with respect to the temperature can be set by changing the resistance value, the resistance temperature coefficient, and the connection method. There are other heating resistor arrangements as shown in (5), (6), (7), and (8), but even in this case, heating resistors can be arranged upstream and downstream, and optimally set according to the above conditions. it can.

  In the above example, two heating resistors have been described, but the same effect can be obtained by using a plurality of heating resistors as in (9) and (10). In this case, the resistance value and the resistance temperature coefficient may be set in accordance with the arrangement of the heating resistors.

  Further, as shown in (11) and (12), the heat generating resistors (resistive layers) and the conductive portions may be alternately arranged to provide the upstream and downstream heat generation ratios.

  As described above, in the third embodiment, it is possible to provide a fixing device capable of high-speed fixing at low cost without increasing the size of the device.

Heating member and fixing device according to the present invention Temperature control diagram related to the fixing device of this embodiment Comparison of temperature distribution in nip N between setting A and setting B Comparison of temperature on endless film surface in setting A, setting B, and setting C and figure showing temperature measurement position Image of heat transfer in nip N Comparison of fixability by difference in heat generation ratio The figure which showed the time change of the electric power of a present Example Change in temperature of toner elastic modulus Heat generation ratio control diagram of the first embodiment Comparison of fixability between conventional example and Example 1 Heat generation ratio control diagram of the second embodiment Comparison of fixability between Examples 1 and 2 Comparison of specifications between the conventional example and Examples 1 and 2 The figure which showed the example of another heater structure concerning this invention The figure which showed the heater in connection with this Example 3 The figure which showed the resistance temperature characteristic of the heat generating resistor in connection with the present Example 3 Temperature distribution diagram in the nip N of the third embodiment The figure which showed the other heater structural example in connection with this Example 3. The figure which showed the resistance temperature characteristic of the heat generating resistor in connection with the present Example 3 Schematic sectional view of an image forming apparatus according to the present embodiment

Explanation of symbols

6 Fixing device 20 Heating resistance layer (heating resistor)
31 Heating member (heater)
32 Support (heater holder)
33 Cylindrical member (endless film)
34 Temperature detection means (thermistor)
40 Pressure roller P Recording material N Nip part t Toner

Claims (4)

A heater having an endless film, a substrate, and a plurality of heating resistors provided on the substrate, which contacts an inner peripheral surface of the endless film, and a fixing nip portion are formed together with the heater via the endless film A fixing member that heats and fixes an unfixed toner image on a recording material while sandwiching and transporting a recording material carrying an unfixed toner image at the fixing nip portion.
During the warm-up period before the endless film starts rotating and fixing processing is started at the fixing nip portion, the heater generates the largest amount of heat at a position downstream from the center of the fixing nip portion in the endless film rotating direction. A fixing device that is set to be   During the period for fixing the unfixed toner image on the recording material, the position where the amount of heat generated by the heater is the largest is set at a position upstream of the endless film rotation direction with respect to the position during the warm-up period. The fixing device according to claim 1.   When continuously fixing a plurality of recording materials carrying an unfixed toner image, the fixing nip portion is downstream from the center of the fixing nip portion even during a period in which the fixing nip portion is between the preceding recording material and the succeeding recording material. The fixing device according to claim 1, wherein the heater is set so that the amount of heat generated by the heater is maximized at a side position.   The fixing device according to claim 1, wherein the heater includes a plurality of types of the heating resistors having different resistance temperature coefficients.

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