Image forming appartus and image fixing apparatus

Abstract: An image forming apparatus includes image forming means for forming an unfixed image on a recording material (P); a heater (1); a film (6) movable with the recording material (P) carrying the unfixed image, wherein said heater (1) heats the unfixed image through said film (6); and a control means for controlling a fixing condition in accordance with a thermal property of said film.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus comprising a heater for heating and fixing an unfixed developer image on a recording material and to an image fixing condition setting method. Particularly it relates to an apparatus wherein a film member is slid relative to a fixed heating member, and the recording material is close contacted to a surface of the film member opposite from the heating member side while moving through the heating position with the film member so that the heat energy is supplied to the recording material through the film member from the heating member. Heretofore, as for the heat-fixing device for the unfixed image and/or an image heating device for executing improvement of a surface property of an image, a heat roller system has been widely used wherein the recording material supporting the image is nipped and transported by a heating roller and a pressing roller. However, in such a heat roller system the heat capacity of the heating roller is large, and therefore, there is a problem that the time (so-called warm up time) required for heating the heat roller up to a predetermined fixing temperature is long. Under the circumstances U.S. Patent No. 5149941, U.S. Ser. No. 444802 published as US 5 262 834 A or the like has proposed a film heating fixing device using a thermal head of the low heat capacity and a film of a small thickness sliding relative to the thermal head, thus reducing the warm up time. According to the fixing device, the electric energy consumption is also very low due to the reduction of the warm up time. An example of such a fixing device is shown in Figure 8. When the recording material P carrying the unfixed toner image Ta enters the nip formed between the pressing roller 209 and the heating member 200, the toner image Ta is heated by the heating member 200, and the sheet with the fixed toner image Tb is discharged from the nip. The heating member includes, the support member 204 made from the resin material such as PPS which is excellent to the heat insulation property, the heat resistivity, the rigidity supporting the heat generating member 202 and/or the base plate 201, the temperature sensing element 203 detecting the temperature of the heat generating member 202 substantially detects the temperature of the base plate 201, the protection layer 208 comprising glass or the like is excellent to the durability protecting the heat generating member 202, "(" the thickness approx. 1mm the ceramic base plate 201 such as the alumina is excellent to the thermal conductivity and the resistance heat generating member 202 of the resistance heat capacity such as the tungsten and/or the nickel-chrome the heating member 200. The film 205 moving in contact with the unfixed toner image comprises a resin material such as polyimide having a thickness of approx. 20-70 µm and having high heat resistivity, parting property, durability. The film 205 is stretched around a heating member 200 and a follow roller 207, a driving roller 206. As described hereinbefore, the temperature sensing element 203 detects the heat transmitted toward the base plate 201 from the heat generating member 202. Electric power supply is controlled by unshown control means so that the detected temperature by the detection element 203 maintains the predetermined temperature. The heat generating member 202 heats the toner image through the protection layer 208 and the film 205. It is difficult to manufacture the very thin film with a predetermined thickness such as several tens µm, and among individual films, there is a variation of thickness of approx. ±60% at the maximum. It has been found that the variation of the thickness brings about a slight variation in the fixing property. The film is very thin, but is of the resin material in order to assure the heat resistivity, the parting property and the durability, and it is difficult to say that it has a good thermal conductivity, and in addition it also has a heat insulation property. It has been found that when the use is made with a film thicker than a predetermined thickness, the heat emitted from the heat generating member 202 and transmitted to the temperature sensing element 203 increases, the toner image is not sufficiently heated despite the fact that the control means controls the temperature to the predetermined temperature. On the contrary, when the use is made with a film thinner than the predetermined thickness, the heat insulation function is smaller than of the film of the predetermined thickness so that the heat transmission to the temperature sensing element 203 is smaller as compared with the case of the film of the predetermined thickness, and therefore excessive heating results. EP-A-0 436 955 discloses an image fixing apparatus including a heater maintained at a controlled temperature, a film contacted to the heater and movable together with and in sliding contact with a recording material carrying a visualized image, wherein the visualized image is heated by the heater through the film, a temperature detecting element for detecting a temperature of the film or a member contacted to the film, and a controller for changing a level of the controlled temperature on the basis of an output of the temperature detecting element before start of an image fixing operation of the image fixing apparatus. It is an object of the invention to provide an image forming apparatus, a corresponding fixing device and an image fixing condition setting method ensuring stable fixing properties. This object is solved as for the image forming apparatus by the features of claim 1, as for the fixing device by the features of claim 8 and as for the method by the features of claim 15. The invention is further developed by the features of the subclaims. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic sectional view of a fixing device according to a first embodiment of the present invention. Figure 2 is a schematic sectional view of an image forming apparatus incorporating the fixing device according to an embodiment of the present invention. Figure 3 is a block diagram of control means in embodiment 1 of the present invention. Figure 4 is a schematic sectional view of the fixing device of another example of the present invention. Figure 5 is a sectional view of a fixing film according to an embodiment of the present invention. Figure 6 is a schematic sectional view of a fixing device of embodiment 2 of the present invention. Figure 7 is a schematic sectional view of a fixing device of embodiment 3 of the present invention. Figure 8 is a sectional view of a fixing device illustrating an operation of a fixing device using a film. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described referring to Figures 1- 7. Figure 2 shows an image forming apparatus using the present invention. As shown in Figure 2, the exemplified image forming apparatus of the present invention is an original carriage fixed type, an optical system moving type, a rotatable drum type and an image transfer type electrophotographic copying machine. In the device of Figure 2, an original 19 is placed in a predetermined manner on a fixed original carriage glass 20, and predetermined copying conditions are set, and thereafter a copy start key is pushed, then a photosensitive member drum 39 is rotated at a predetermined peripheral speed in the clockwise direction indicated by an arrow. A reflection shade and a first mirror 23 is moved at a the predetermined speed V to the right-hand side of the glass from the home position at the left-hand side of the glass along surface the bottom surface of the original carriage glass 20, and a second mirror 24, a third mirror 25 are moved at the speed of V/2 in the same direction mirrors 26, 27, 28 sixth - fourth fixed and the imaging lens 29 through-the rotating photosensitive drum 39 the surface of on (slit exposure) projected and imaged is the original reflected by the illumination scanning light, and right-hand side to left-hand side from the light source 21 and 22 by illuminated and scanned Is the original carriage glass 20 on placed the original 19 of the faced-down image surface, by this. The surface of the rotatable photosensitive drum 39 has been charged uniformly to a positive or negative predetermined potential by a primary charger 30 before the image exposure, and the above-described exposure is effected to the charged surface, by which an electrostatic latent image of the pattern corresponding to the original image is formed sequentially on the surface of the drum 39. The electrostatic latent image formed in the surface the photosensitive member drum 39 is visualized as the toner image by the developing roller 32 of the developing device 31. On the other hand, a recording material P is fed by sheet feeding rollers 51, and is introduced to the transfer portion between the transfer charger 34 and the drum 39 at a predetermined timing through a guide 33, and receives transfer corona. It is contacted to the drum 39 so that the toner visualized image is transferred sequentially from the surface of the drum 39 to the surface of the recording material. The recording material P passing the image transfer portion is subjected to the discharging to remove the back surface charge by discharging needles 35, and is separated sequentially from the surface of the drum 39. And, it is introduced to a fixing device 60 through an inlet guide 10 and a transportation portion 38, and is subjected to toner image fixing operation which will be described hereinafter, and then it is discharged from the apparatus. The surface of the drum 39 after the transfer operation is cleaned by a cleaning blade 37 of a cleaner 36 to remove contamination such as the residual toner, and is subjected to the image forming operation repeatedly. When the movable optical member 21-25 having moved through the forward passage as described hereinbefore reaches the predetermined forward passage end portion, it moves through the backward passage to return to the original home position and waits until the start of the next copy cycle . In the case that a plurality of sheets (for example, 100 sheets) is set before the copy start key is depressed, the above-described process is repeated at predetermined intervals by a micro computer (in the following "MPU") 18 as indicated in Figure 3, after the the completion of the back movement process of the optical system. Referring to Figure 1, detailed description will be made as to the fixing device 60 mounted to the present embodiment apparatus. Figure 1 is an enlarged sectional view of the fixing device 60. In Figure 1, a heating member 1 is provided with a heat generating member 3 of linear shape having a low heat capacity, and a resistance material 3 such as silver palladium is coated with a width of 1.0mm on an alumina base plate 2 having a length of 350mm, a width of 10mm and a thickness of 1.0mm, for example, it is supplied with electric energy at longitudinal opposite ends of the fixing device. The electric power supply is controlled by control means (not shown) including a micro computer, so that a detected temperature of a temperature sensing element 5 such as a thermister in contact with the surface opposite from the base plate surface supporting the heat generating member 3, is kept at a predetermined target temperature. 3a, 3b shown in Figure 3 are the opposite ends of the resistance material 3. The fixing film 6 moves in the direction of the arrow in contact to the heating member 1 which is under the temperature control in such a manner The fixing film 6 is a heat resistive film having a thickness of approx. 20 µm, and is an endless film having a base layer comprising material such as polyimide, polyetherimide, PES or PFA and a parting layer, coated thereon, and comprising fluorine resin material such as PFA or PTFE added with an electroconductive material. The parting layer is contactable to the toner image. Generally the film total thickness is less than 100 µm to assure proper heat conduction, and more preferably, less than 40 µm. The fixing film 6 is stretched by the heating member 1, a follow roller 8 and a driving roller 7 so that it moves without creases in the direction of the arrow even if the driving roller 7 is rotated. The heating member 1 is pressed through the fixing film 6 by the total pressure of 6-12kgf, by the pressing roller having the rubber elastic layer having a good parting property such as the silicone rubber 9. The recording material P carrying the unfixed toner T is guided to the fixing portion by the inlet guide 10. In this embodiment, the film 6, the pressing roller 9, the follow roller 8, the driving roller 7 and the heating member 1 are in the form of a unit detachably mountable relative to the main assembly of the image forming apparatus, but only the film 6, the rollers 7, 8 and the heating member 1 may constitute the unit. In the embodiment of Figure 2, the fixing film 6 is in the form of an endless belt, but may be a non-endless film as shown in Figure 4. Figure 5 is a sectional view of the fixing film 6. The base layer 6a is slidable relative to the heating member 1, and the parting layer 6b is in contact with the recording material P. In this embodiment, use is made of a polyimide film as the base layer 6a, and a carbon black dispersed semiconductive PFA coating film as the parting layer 6b. The predetermined value of the thickness ta of the base layer 6a is 20 µm, but it is 20 µm±10 µm in view of the error during the manufacturing. The thickness tb of the parting layer 6b is 15 µm±10 µm relative to a predetermined value 15 µm. Therefore, relative to a predetermined value 35 µm of the total thickness, the actual manufactured film involves a variation in the range of 35 µm±20 µm. At least one of ta, tb and ta+tb (ta+tb=t) is measured in the production line before the assembly of the fixing device, and is stored in a film thickness register 41 (storing means) such as ROM mounted to the fixing device 60 which is mountable to the main assembly of the image forming apparatus as the unit. On the basis of the stored information the power supply of the control means for the main assembly of the image forming apparatus is controlled. At least one of the pieces of information ta, tb, ta+tb with respect to the fixing film thickness may be registered in the MPU18 per se of the image forming apparatus. Material for recording the fixing film thickness and mounted to the fixing device is not limited to ROM but may be a dip switch and/or a variable resistance. The fixing film thickness is measured actually using a micro-meter or the like, but in addition to the method, for example, while rotating the fixing film, after assembling the fixing device, the heat generating member is supplied with electric energy, and the rising of the fixing film surface temperature is measured, and the fixing film thickness may be predicted. Following table 1 gives the control temperature for the heating member in this embodiment. As will be understood, the control temperature is changed in accordance with the total thickness of the fixing film. More particularly, if the fixing film is thin the control temperature is low, and if the fixing film is thick the control temperature is high. In addition, in consideration of the temperature rise of the pressing roller and/or the film by the continuous copy operation, the target temperature is switched in response to the number of the continuous copy sheets. More particularly, the target temperature is lowered in accordance with the continuous copy operation. Total thick. t(µm) Control Temp. 1st 2-9th 10 et seqq 15≦t<20 190°C 180°C 170°C 20≦t<25 200°C 190°C 180°C 25≦t<30 210°C 190°C 180°C 30≦t<35 210°C 200°C 180°C 35≦t<40 220°C 200°C 180°C 40≦t≦55 230°C 210°C 190°C By the use of such a temperature control system, even if there is a variation within the thickness tolerance of the fixing film, the fixing defect and/or the offset does not occur. (COMPARISON EXAMPLE) The temperature control was executed with 210°C for the first, 190°C for 2-9th, 180°C for after the 10 th sheet, irrespective of the thickness of the fixing film. In the case that the fixing film is thick the fixing defect occurs at the first sheet, and in the case that the fixing film is thin the high temperature offset occurs after the 10 th sheet. In embodiment 1, in the case that the thickness of the film, of one layer of the two layers constituting the fixing film is decisive for the fixing property, the control temperature may be determined in accordance with the thickness of the particular layer not of the total thickness of the fixing film. As to the thermal property of the film stored in the storing means such as the dip switch and/or ROM, it is not limited to the thickness of the film but may be a thermal conductivity. For example, in another alternative, in the case that the thermal conductivity of the parting layer 6b depends on the content of the dispersed carbon, the electric resistance of the parting layer 6b is measured, and the value of the electric resistance is recorded in the storing means of the fixing device or in the MPU of the main assembly, and the control temperature may be adjusted in accordance with the value. EMBODIMENT 2 Referring to Figure 6, embodiment 2 of the present invention will be described. According to the present embodiment, even if the thickness of the fixing film decreases by the sliding with the recording material and/or the heating member, the heating operation can be always most preferable. In addition, in the case of the use over a long period, if the method of the present embodiment is incorporated, the control temperature can be gradually lowered, and therefore the long lifetime of the device can be also expected. EMBODIMENT 3 Referring to Figure 7, an embodiment 3 will be described. The major portion of the structure of the present embodiment is similar to embodiment 1, but the shaft 9a, of the pressing roller 9 is supported by an eccentric bearing 103. The bearing 103 is rotatable while the recording material is not contacted with the pressing roller 9 by an unshown stepping motor, and by a predetermined degree rotation of the stepping motor, the urging force of the pressing roller 9 to the heating member 1 is adjustable. In this embodiment, the above-described urging force is changed as given in the following table 2 in accordance with the thickness of the fixing film. Pressing force is lowered with the decrease of the thickness of the fixing film, and pressing force is raised with the increase of the thickness of the fixing film. In addition, the temperature control condition follows table 1. Total thick. t(µm) Pressing Force 1st 2-9th 10 et seqq t<20 8kg.f 8kg.f 6kg.f 20≦t<30 10kg.f 8kg.f 8kg.f 30≦t<40 10kg.f 10kg.f 8kg.f 40<t 12kg.f 12kg.f 10kg.f According to the present embodiment, the fixing property in the case that the fixing film is thick is further improved, and in addition, the pressing force can be reduced without damaging the fixing property, and therefore, the durability in the case that the fixing film is thin is improved. EMBODIMENT 4 Embodiment 4 of the present invention will be described. The major portion of the structure of the present embodiment is similar to embodiment 1, but in accordance with the thickness of the fixing film, the driving of the optical system motor driving circuit 44, the driving of the sheet feeding roller 51 and the driving of the high voltage voltage source 42 are delayed through a predetermined period of time (the wait time) by the MPU18. During this time, the heating member is energized by a heating member driving circuit 16 to effect preliminary heating . Also during this, the main motor is rotated by the main motor driving circuit 43, and the fixing film slides on the heating member and is heated thereby. The following table 3 gives the control temperature and the wait time relative to the thickness of the fixing film. Total thick. t(µm) Wait time (sec) Control temp. (°C) 1st 2-9th 10th et seqq 15≦t<20 0 190 180 170 20≦t<25 0 200 190 180 25≦t<30 0 210 190 180 30≦t<35 0 210 200 180 35≦t<40 5 210 200 180 40≦t≦55 10 210 210 190 The present embodiment is effective to reduce the maximum temperature of the heat generating member in the case that the thick fixing film is incorporated, and, as a result excessive rise of the heat generating member temperature in the non-sheet processing area upon the small size sheet processing can be prevented. On the other hand, in the case that the fixing film having the normal thickness is incorporated, the wait time for the fixing is not produced to avoid the inconvenience to the user. EMBODIMENT 5 Embodiment 5 of the present invention will be described. The major portion of the structure of the present embodiment is similar to embodiment 1, but when the thickness of the fixing film is small, the maximum electric power is reduced as compared with the case that it is thick. According to this embodiment, even if the heat capacity of the fixing device is small, the temperature ripple of the heat generating member can be suppressed. EMBODIMENT 6 Embodiment 6 of the present invention will be described. The major portion of the structure of the present embodiment is similar to embodiment 1, but the film transportation speed namely, the fixing speed is increased or decreased in response to the thickness of the fixing film. According to the present embodiment, even in the case of the fixing film of a different thickness, the equivalent proper fixing property is provided without changing the control temperature, and the toner offset can be prevented. In the above-described embodiment, the thermal property of the film is stored in the fixing unit, and the stored information is detected by the main assembly of the image forming apparatus, and the main assembly of the image forming apparatus automatically switches the fixing condition. However, the film may be selected for each the thermal property of the film in the film production process to permit manual setting of the fixing condition in accordance with the thermal property of the film. For example, the film is selected in accordance with the thermal property in the production line beforehand so that the serviceman can discriminate the difference of the film when the film per se is exchanged in the image forming apparatus. The serviceman may set manually the fixing condition of fixing device. As described in the foregoing, according to the present invention, by changing the fixing condition in response to the thickness of the fixing film, the always proper fixing property is provided even if the fixing film of a different thickness is incorporated, and the offset is not produced. In addition, the thickness of the usable fixing film and/or the tolerance of the material property can be large, and therefore the yield on the manufacturing of the fixing film is improved. The cost reduction of the device per se can be also expected. While the invention has been described with reference to the structures disclosed herein, this application is intended to cover such modifications or changes as may come within the scope of the appended claims.

Claims: An image forming apparatus comprising image forming means (39) for forming an unfixed image on a recording material (P) a heater (1), and a film (6) movable with said recording material carrying said unfixed image, wherein said heater heats said unfixed image through said film, characterized by a control means (18) for controlling a fixing condition in accordance with the thickness of said film. An apparatus according to Claim 1, wherein said film comprises a plurality of laminated layers (6a, 6b), and the thickness of said film is the thickness of at least one layer of said plurality of layers. An apparatus according to claim 1, further comprising a temperature detector (5) for detecting a temperature of said heater, wherein said control means controls electric power supply to said heater so that the temperature detected by said temperature detector is maintained at a control temperature, the control temperature being increased when the film thickness is increased. An apparatus according to Claim 1, further comprising a pressing roller (9) for urging the recording material toward said heater and pressing force switching means (103) for switching pressing force of said pressing roller relative to said heater, wherein said control means controls said pressing force switching means so that the pressing force of said pressing roller relative to said heater increases with the increase of the thickness of said film. An apparatus according to Claim 1, wherein said control means switches, in response to the thickness of said film, a preliminary heating period after start of electric power supply to said heater until start of heating of the unfixed image. An apparatus according to Claim 1, wherein a maximum amount of electric power supplied to said heater is controlled by said control means in response to the thickness of said film. An apparatus according to Claim 1, wherein said control means switches, in response to the thickness of said film, a fixing speed. A fixing device (60) for use in the image forming apparatus according to one of the preceding Claims, said fixing device comprising said heater (1), said film (6) movable with a recording material (P) carrying an unfixed image, wherein said heater heats the unfixed image through said film, storing means (41) for storing a thermal property of said film. A device according to Claim 8, wherein the thermal property of said film is a thickness of said film. A device according to Claim 8, wherein a thermal property of said film is a thermal conductivity of said film. A device according to Claim 8, wherein said storing means includes a ROM. A device according to Claim 8, wherein said storing means includes a variable resistor having a resistance switchable in accordance with the thermal property of said film. A device according to Claim 8, wherein said storing means includes a dip switch switchable in accordance with the thermal property of said film. A device according to Claim 8, wherein said fixing device is detachably mountable to a main assembly of said image forming apparatus. An image fixing condition setting method, comprising the steps of discriminating the thickness of a film for a fixing device, wherein said film is movable with a recording material carrying an unfixed image and setting a fixing condition of the fixing device in accordance with the thickness.

Inventors: KUSAKA KENSAKU, MARUTA HIDEKAZU, MASUDA KOJI, KUSAKA, KENSAKU, MARUTA, HIDEKAZU, MASUDA, KOJI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G03G15
IPC codes: G03G15, G05D23