Patent Grant
7251447
1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a fixing apparatus mounted in a copying apparatus or a printer utilizing a recording technology as an electrophotographic recording method or an electrostatic recording method, and a conveying roller for use in such apparatus.
2. Related Background Art
A copying apparatus or a printer of an electrophotographic process is equipped with a fixing apparatus for heat fixing a toner image formed on a recording material. Such fixing apparatus is available in various types, such as a heat roller type in which a fixing roller heated by an internal halogen lamp and a pressure roller pinch and convey a recording material to execute a heat fixing thereof, an on-demand type (also called film heating type) in which a ceramic heater is contacted with an internal surface of a flexible sleeve (fixing film or fixing belt) formed by a heat-resistant resin or a metal, and a recording material is heated by a fixing nip portion formed by the ceramic heater and a pressure roller, and an electromagnetic induction heating type in which a rotary member itself in contact with a recording material generates heat. In any of these types, a fixing nip portion formed between a fixing roller (or a heater) and a pressure roller pinches and conveys a recording material thereby heat fixing a toner image thereon.
In case of employing an ordinary paper as the recording material, it is necessary to avoid formation of creases in the course of a fixing step. A principal cause of crease formation in the ordinary paper is a shrinkage of paper fibers by an excessive heat supply. As a countermeasure against such phenomenon, there is known a method of employing a fixing roller or a pressure roller (hereinafter such rollers being collectively called a conveying roller) having an inverted crown shape, in which diameter becomes larger from a central portion toward both end portions in the longitudinal direction. For example, a conveying roller, having a metal core of a uniform diameter over the longitudinal direction and an elastic layer provided around the metal core and having a thickness gradually increasing toward the end portions in the longitudinal direction, generates a force of stretching an ordinary paper in the course of the fixing step, thereby suppressing creasing in the ordinary paper.
However, in an ordinary fixing apparatus, in order to form the fixing nip portion, a pressure is applied for example by a spring between an end of the fixing heater (or heater) and an end of the pressure roller, and also between the other end of the fixing roller (or heater) and the other end of the pressure roller. As the nip portion is formed by applying a force in each of both ends of the apparatus, the roller itself shows a certain bending even though the roller has a metal core, so that the fixing nip at the central portion in the longitudinal direction of the roller becomes narrower than in both end portions, thus tending to result in a deficient pressure. Besides, in case of employing a conveying roller having an inverted crown profile as explained above, a further deficiency in pressure tends to be generated in the central portion in the longitudinal direction of the roller, thereby leading to a fixing failure or a toner offsetting.
On the other hand, the pressure roller not internally provided with a heat source tends to show a temperature change by an operation status of the printer. For example in a continuous printing operation of printing on plural recording materials in succession, a large amount of heat is taken away by the recording materials, so that the temperature of the pressure roller does not become very high (for example about 80-90° C.). On the other hand, in an intermittent printing operation in which a preceding recording material and a succeeding recording material have a long interval, a heat amount supplied from the fixing roller (or heater) supplied during such interval becomes larger so that the pressure roller tends to assume a high temperature (for example about 140-150° C.). A fixing property of the toner image is influenced by a heat amount and a pressure given thereto, and a fixing failure and a toner offsetting tend to be generated in a continuous printing operation in which the pressure roller has a low temperature whereby the heat amount given to the recording material and the toner image tends to become low. On the other hand, in an intermittent printing operation, even if the pressure applied to the central portion in the longitudinal direction of the roller is deficient, the fixing failure or the toner offsetting is not generated as in the continuous printing operation as the pressure roller has a high temperature.
Thus, a configuration of merely employing a pressure roller of an inverted crown shape can suppress creases on the recording material but cannot necessarily satisfy a fixing property, and it is very difficult to obtain an ability of suppressing creases and a satisfactory fixing property at the same time. It is therefore conceived to adopt a metal core of a tapered shape in which the diameter becomes gradually smaller from a central portion in the longitudinal direction toward both end portions and to provide an elastic layer in such a manner that the pressure roller has a straight or inverted crown profile at the room temperature (about 10-30° C.) (cf.
Such pressure roller, having a large thickness in the elastic layer on both end portions even though having a profile of a straight shape or a little inverted crown shape at the room temperature state, assumes an appropriate inverted crown shape by a thermal expansion of the elastic layer at a high temperature state of the pressure roller (about 140-150° C.), and is therefore capable of suppressing creases that tend to be generated at a high temperature in the ordinary paper. Also showing a smaller inverted crown shape at a temperature of about 80-90° C. where a fixing failure is often generated, it can suppress a decrease in the pressure in the central portion in the longitudinal direction of the roller, whereby it is rendered possible to suppress, to a certain level, a fixing failure or a toner offsetting that is often generated in a temperature range of the pressure roller of 80-90° C.
However, as the metal core of such pressure roller is tapered from a center line in the longitudinal direction toward both ends, the elastic layer in its entire area only shows a monotonous increase in the thickness from the center line in the longitudinal direction toward the both ends. Stated differently, the metal core does not have an area of a uniform diameter in a central portion in the longitudinal direction, and also the elastic layer does not have an area of a uniform thickness in the central portion in the longitudinal direction. Consequently, the elastic layer, when thermally expanded, assumes a V-shaped cross sectional form with a bottom of a recess in the central portion (cf.
Also in a heat fixing apparatus of film heating type, a temperature detecting element for detecting the temperature of a heater is provided on a rear surface of the heater, corresponding to a central portion in the longitudinal direction of the pressure roller, and a current supply control to the heater is executed based on thus detected temperature. Therefore, in case a local concave shape is formed in the central portion of the pressure roller as explained above, the pressure roller takes away, in the central portion, a smaller heat amount from the heater through a fixing film, than in the both end portions, so that the heater temperature tends to become higher in the central portion. For this reason, the temperature detecting element tends to shows a higher detected temperature, thereby maintaining the heater at a temperature lower than an actually desired control temperature and leading to a fixing failure.
The present invention has been made in consideration of the foregoing situation, and an object of the present invention is to provide an image heating apparatus capable suppressing creases in a recording material and at the same time suppressing a failure in image heating, and a conveying roller adapted for use in such apparatus.
Another object of the present invention is to provide an image heating apparatus including a conveying roller for conveying a recording material, the conveying roller including a metal core and an elastic layer provided outside the metal core; wherein the metal core including a straight shape area, in a central portion of a longitudinal direction thereof, having a substantially uniform diameter along the longitudinal direction, and tapered shape areas, on both sides of the straight shape area, in which a diameter gradually decreases toward ends in the longitudinal direction; and wherein the elastic layer including a straight shape area, in a central portion of a longitudinal direction thereof, having a substantially uniform diameter along the longitudinal direction, and inverted crown shape areas, on both sides of the straight shape area, in which a diameter gradually increases toward ends in the longitudinal direction.
Still another object of the present invention is to provide a conveying roller including a metal core; and an elastic layer provided outside the metal core; wherein the metal core including a straight shape area, in a central portion of a longitudinal direction thereof, having a substantially uniform diameter along the longitudinal direction, and tapered shape areas, on both sides of the straight shape area, in which a diameter gradually decreases toward ends in the longitudinal direction; and wherein the elastic layer including a straight shape area, in a central portion of a longitudinal direction thereof, having a substantially uniform diameter along the longitudinal direction, and inverted crown shape areas, on both sides of the straight shape area, in which a diameter gradually increases toward ends in the longitudinal direction.
Still other objects of the present invention will become fully apparent from following detailed description which is to be taken in conjunction with the accompanying drawings.
In the following there will be explained, with reference to the accompanying drawings, a first embodiment of a conveying roller of the present invention, and an image forming apparatus equipped with a heat-fixing apparatus (image heating apparatus) provided with such conveying roller. The present embodiment employs an image forming apparatus having a maximum conveyable width of the recording material (hereinafter called maximum sheet width) corresponding to an LTR size (216 mm in a width perpendicular to the conveying direction of the recording material) and of so-called center reference type in which a reference position for the conveying operation is provided at the center of a length in such perpendicular direction.
(1) Image Forming Apparatus M
At first, there will be explained, with reference to
The image forming apparatus shown in
Also in a lower part of the main body M of the apparatus, a sheet feed cassette 7 containing a sheet-shaped recording material P such as paper is provided, and, along a conveying path of the recording material P and in the order from an upstream side, there are provided in succession, a feed roller 15, conveying rollers 8, a top sensor 9, a conveying guide 10, a heat-fixing apparatus 11 according to the invention, a discharge sensor 29, conveying rollers 12, discharge rollers 13 and a discharge tray 14.
In the following, there will be explained a function of the image forming apparatus of the above-described configuration. The photosensitive drum 1, rotated in the direction R1 by the drive means (not shown) is uniformly charged at a predetermined potential of a predetermined polarity by the charging roller 2. The photosensitive drum 1 after charging is surfacially exposed to a laser light L, based on image information, by the exposure means 3 such as a laser optical system, whereby the charge in an exposed portion is dissipated to form an electrostatic latent image.
The electrostatic latent image is developed by the developing apparatus 4. The developing apparatus 4 is provided with a developing roller 4a, which is given a developing bias to deposit a toner onto the electrostatic latent image on the photosensitive drum 1, thereby developing a toner image (visible image). The toner image is transferred by the transfer roller 5 onto a recording material P such as paper.
The recording material P is contained in the feed cassette 7, then fed by the feeding roller 15, conveyed by the conveying rollers 8 and is conveyed through the top sensor 9 to a transfer nip portion between the photosensitive drum 1 and the transfer roller 5. In this operation, the recording material P is detected at a leading end thereof by the top sensor 9, and is thus synchronized with the toner image on the photosensitive drum 1. The transfer roller 5 is given a transfer bias thereby transferring the toner image from the photosensitive drum 1 onto a predetermined position of the recording material P.
The recording material P, bearing an unfixed toner image on the surface by the transfer operation, is conveyed along the conveying guide 10 to the heat fixing apparatus 11, in which the unfixed toner image is heated and pressurized thus being fixed to the surface of the recording material P. The heat fixing apparatus 11 will be explained later in more details.
The recording material P after the toner image fixation is conveyed by the conveying rollers 12 and discharged by the discharge rollers 13 onto the discharge tray 14 provided on an upper surface of the main body M of the apparatus.
On the other hand, on the photosensitive drum 1 after the transfer of the toner image, a residual toner not transferred onto the recording material P but remaining on the surface is eliminated by a cleaning blade 6a of the cleaning apparatus 6, and is used for a next image formation.
The image formation can be executed in succession by repeating the above-described operations.
(2) Heat-Fixing Apparatus (Image Heating Apparatus) 11
In the following, an example of the heat-fixing apparatus 11 of the invention will be explained with reference to
The fixing apparatus 11 is constituted, as principal components, of a ceramic heater (hereinafter called heater) 20 serving as a heating member for heating toner, a fixing film 25 serving as a flexible sleeve surrounding the heater 20, a pressure roller 26 serving as a conveying roller in contact with the fixing film 25, and temperature control means 27 for controlling the temperature of the heater 20.
The heater 20 and the pressure roller 26 are mutually pressed through the fixing film 25, thus constituting a fixing nip portion N. The pressure roller 26 is driven counterclockwise indicated by an arrow R26, thereby exerting a rotary force on the fixing film 25 by the contact frictional force of the pressure roller 26 and the fixing film 25 at the fixing nip portion N. Such rotary force causes the fixing film 25 to rotate clockwise as indicated by an arrow R25, in a sliding contact with a lower surface of the heater 20.
Then an electric power is supplied to the heater 20 to elevate the temperature thereof thereby executing a temperature control at a predetermined temperature. In this state, the recording material P bearing an unfixed toner image T is introduced between the fixing film 25 and the pressure roller 26 in the fixing nip portion N. In the fixing nip portion N, a toner image bearing surface of the recording material P is contacted with an external surface of the fixing film 25, and is pinched and conveyed in the fixing nip portion N together with the fixing film 25. In such pinched conveying process, the heat of the heater 20 is given to the recording material P through the fixing film 25, whereby the unfixed toner image on the recording material P is heated and pressed to the recording material P and fused and fixed thereto. The recording material P after passing the fixing nip portion N is separated by a curvature from the fixing film 25.
(Heater 20)
The heater 20 is constituted by forming in succession, on a ceramic substrate, a heat generating member formed by printing a heat-generating paste and a glass coating layer for protecting and insulating the heat-generating member. The heater 20 generates a heat by supplying a power-controlled AC current to the heat-generating member on the heater 20. The ceramic substrate is formed for example of aluminum nitride or aluminum oxide. On a rear surface of the ceramic substrate, as shown in
The present embodiment employed, as an example, a heater 20 prepared by screen printing a silver-palladium alloy as the heat-generating member on a highly insulating substrate of aluminum nitride and applying a glass coating as an insulating protective layer.
(Temperature Control Means 27)
Temperature control means 27 includes a CPU 23 which controls a triac 24, based on a temperature detected by the temperature detecting element (thermistor) 21 mounted at the approximate center of the rear surface of the heater 20, thereby controlling a power supply to the heater 20.
(Fixing Film (Flexible Sleeve) 25)
The fixing film 25 is shaped as an endless belt and is loosely fitted on the heater 20 and the guide part of the heater holder 22. The fixing film 25 is pressed by the pressure roller 26 to the heater 20, whereby an internal periphery of the fixing film 25 is contacted with a lower surface of the heater 20. Thus the fixing film 25 is rotated in a direction R25 along with the conveying of the recording material P in a direction K by the rotation of the pressure roller 26 in a direction R26.
The fixing film 25 is restricted at both lateral ends thereof by a guide portion (not shown) of the guide part of the heater holder 22, so as not to be disengaged from the longitudinal direction of the heater 20. Also on the internal surface of the fixing film 25, grease is coated in order to reduce a sliding resistance with the heater 20 and the guide part of the heater guide 22.
The fixing film 25 of the present embodiment is a metal sleeve formed by coating, on a surface of a cylindrical base tube (base layer), a primer layer and a releasing layer. The cylindrical base tube (base layer) is formed by a heat-resistant metal or alloy of a high thermal conductivity such as SUS (stainless steel), Al, Ni, Cu or Zn. The fixing film may also be based, in place for such metal, on a heat-resistant resin such as polyimide. The releasing layer is formed by mixing a filler for resistance adjustment in a resin such as PTFE, PFA or FEP. The coating can be achieved, for example, by washing the surface of the base layer of the fixing film and by dip coating a primer layer serving as an adhesive and then a releasing layer.
The present embodiment employed, as an example, a base layer of SUS of a thickness of 40 μm, an electroconductive primer layer of a thickness of 5 μm and a releasing layer of a medium resistance of a thickness of 10 μm.
(Pressure Roller (Conveying Roller) 26)
The pressure roller 26 is formed by providing, on an external periphery of a metal core 26a, an elastic layer 26b of heat-resistant rubber such as silicone rubber or fluorinated rubber, or an elastic layer of foamed sponge. The external periphery of the elastic layer 26b contacts the external periphery of the fixing film 25. The heater 20 and the pressure roller 26 constitute, across the fixing film 25, the fixing nip portion N for pinching and conveying the recording material. In the fixing nip portion N, a width (nip width) a in the rotating direction of the pressure roller 26 is so selected as to advantageously heat and press the toner on the recording material P.
Also as shown in
In the following, shapes of the metal core and the elastic layer of the pressure roller will be explained.
As shown in
Then there will be explained a shape of the elastic layer of the pressure roller, by explaining a shape of a metal mold 26d for forming the elastic layer of the pressure roller and a process for molding the elastic layer around the metal core 26a.
An internal surface of the metal mold 26d, as shown in
The metal core 26a is mounted in the metal mold 26d as shown in
The pressure roller 26 thus constructed, in which a rubber material such as silicone rubber constituting the elastic layer 26b has a property of expanding and contracting depending on the temperature, tends to show different external profiles at a room temperature (23 ±5°C.) and at a high temperature state (100°C. or higher) when the temperature of the pressure roller is elevated by a continuous printing operation.
In the following there will be explained a shape of the elastic layer at the room temperature state, namely when the pressure roller is cooled.
The external profile of the pressure roller in the longitudinal direction thereof, namely the surface shape of the elastic layer 26b in a state where it is contracted, is represented by a solid line in
The elastic layer at the room temperature state has a concave shape for following reason.
As will be understandable from
The elastic layer of a larger thickness shows a larger contraction when the temperature is lowered. Therefore, in comparison with the elastic layer formed on the straight shape area Sa of the metal core 26a, the elastic layer of a larger thickness formed on the tapered area shows a larger contraction when the temperature is lowered. Consequently, when the temperature of the pressure roller is lowered, within the elastic layer corresponding to the straight shape area Sd of the metal mold 26d, the diameter of the elastic layer excluding the straight shape area Sa of the metal mold becomes gradually smaller toward the end portion of the pressure roller (first phenomenon), whereby an area of a decrease in diameter is generated.
On the other hand, in the crown portions of the metal mold from the end portions of the straight shape area Sd of the metal mold 26d to the ends Pr, Pl of the pressure roller (more exactly to the ends of passing area of the recording material of the maximum size), the internal surface of the metal mold 26d is so formed as to secure a predetermined inverse crown amount on the both end portions (positions Pr, Pl) of the elastic layer 26b with respect to the straight shape area in the central portion, even when the pressure roller 26 is at a room temperature state, namely when the elastic layer 26b is contracted. In order to obtain such shape of the elastic layer at the room temperature state, the diameter of the metal core 26a decreases linearly (in a manner of a first-order function) toward the end portion, while the internal diameter of the metal mold 26d increases in a curve of a manner of a second-order function. As a result, the internal diameter of the metal mold 26d increases more than an increase in the contraction resulting from an increase in the thickness of the elastic layer 26b toward the end of the pressure roller. Thus, the diameter of the elastic layer in areas from the ends of the straight shape area Sd of the metal mold 26d to the ends Pr, Pl of the pressure roller increases gradually toward the ends of the pressure roller, regardless of the temperature thereof (second phenomenon).
Because of the first and second phenomena mentioned above, on the surface of the pressure roller 26 at the room temperature state, namely when it is cooled, concave portions are formed with a minimum diameter at points B (Br, Bl).
On the other hand, the pressure roller 26 at the high temperature state is at a temperature approximately equal to the hardening temperature as indicated by a broken line in
Also, as the pressure roller 26 has a straight portion in the central portion C in the longitudinal direction regardless whether the rubber material of the elastic layer 26b has a thermal expansion or not, the thermistor 21 can always be positioned on the rear surface (opposite to the nip forming surface) of the heater corresponding to the straight portion of the pressure roller 26.
As explained in the foregoing, the pressure roller of the present embodiment has different external profiles at a high temperature state and a low temperature state, and such difference is summarized in
Also the elastic layer 26b corresponding to the straight shape area Sa of the metal core has a substantially uniform thickness, while the elastic layer 26b corresponding to the tapered area of the metal core 26a increases gradually toward the ends in the longitudinal direction.
Also when the pressure roller 26 is in a cooled state (room temperature state), the surface of the pressure roller 26 corresponding to the tapered area of the metalc core 26a has a recessed portion (concave portion), and such concave portion disappears when the elastic layer 26b is thermally expanded (high temperature state).
Also in the thermally expanded state of the elastic layer 26, the longitudinal length of the straight shape area of the elastic layer 26b is longer than the longitudinal direction of the straight shape area Sb when the pressure roller 26 is cooled.
The present embodiment employs, as an example, an aluminum metal core 26a having a length of 216 mm between Pr and Pl, a maximum diameter φDc of 23 mm in the central portion C, a length Sa of 50 mm of the straight portion, and a minimum diameter φDe of 21 mm in both ends (Pr, Pl). The elastic layer 26b is formed with silicone rubber. The metal mold 26d has a length Sd of 100 mm of the central straight portion, and a crown amount (difference between the internal diameter of the central portion and the internal diameter at the point P) of 120 μm. The pressure roller 26 in the room temperature state has an average diameter of 30 mmφ, a length of the straight portion Sb of 40 mm somewhat shorter than the straight portion Sa of the metal core, a difference in external diameter (ΔP−C) constituting the inverted crown amount of 60 μm, and a difference in external diameter between the concave point B and the central portion C (ΔB−C) of 25 μm. Also at the high temperature state, the pressure roller 26 has an average diameter of 30.4 mmφ, a length of the straight portion SB of 80 mm somewhat shorter than the straight portion Sd of the metal mold, and the external diameter difference constituting the inverted crown amount of 120 μm, wherein the concave shape in the point B vanishes.
(3) Comparative Experiment with Prior Example
A pressure roller 26 of the aforementioned embodiment (
(A) Evaluation of Toner Accumulation on the Surface of the Pressure Roller 26
A character pattern was printed continuously on 50,000 sheets, utilizing a paper Continental LX (manufactured by IGEPA Ltd.) containing calcium carbonate as a filler, in an environment of 15° C./10% RH, and a toner stain on the pressure roller and on the printed paper, at 5,000th and 50,000th sheets was evaluated.
(B) Evaluation of Paper Creases
A grid pattern of a pitch of 10 mm was continuously printed on 500 sheets, utilizing a thin paper Office Planner (manufactured by Canon Inc.), in an environment of 32.5° C./80% RH, and generation of creases in paper was confirmed.
(C) Evaluation of Fixing Property
A character pattern was printed continuously on 500 sheets, utilizing a rough surface paper Fox River Bond #24 (manufactured by Fox River Paper Co.), in an environment of 15° C./10% RH, and a fixing property was evaluated by a rubbing test and the like.
These results are shown in Table 1. In the table, (+) indicates a satisfactory level, (±) indicates a practically acceptable level, and (−) indicates an unacceptable level.
Based on these results it is confirmed that the pressure roller 26 of the present embodiment can achieve, in comparison with the prior examples 1 and 2, a prevention of toner accumulation on the pressure roller in a continuous printing operation, a prevention of creases in the paper and a stable fixing property by an appropriate temperature control, even at an elevated printing speed.
The pressure roller of the prior example 1 shows a low suppressing ability for the paper creases as it cannot form an inverted crown shape in a low temperature state of the pressure roller.
Also the pressure roller of the prior example 2, having a large local concave portion in the central portion in the longitudinal direction at a high temperature state, shows a lower pressure in such central concave portion than in other portions within the fixing nip, thus receiving a smaller heat amount from the heater in such central concave portion. Therefore, the heater temperature becomes higher in the central portion where the thermistor is positioned in comparison with other portions, so that a sufficient heat amount is not supplied even when the temperature control is executed with an appropriate temperature. Consequently, the fixing property is deteriorated in the central portion, and a toner accumulation takes place as the temperature of the pressure roller becomes lower in the central portion, in comparison with other portions.
In contrast, the pressure roller 26 of the present embodiment has a straight portion Sb in the longitudinally central portion regardless of the presence/absence of thermal expansion of the elastic layer 26b by a temperature change, and also has a straight portion (Sa) with a maximum diameter of the metal core thereunder. Consequently, within the fixing nip, a high pressure is maintained in the central straight portion and an appropriate heat supply is realized from the heater 20 in the central straight portion. Thus the temperature of the heater becomes uniform in the central straight portion where the thermistor 21 is positioned, and a desired heat amount can be supplied from the heater 20 by an appropriate temperature control.
It is thus rendered possible to obtain a desired fixing property. Also the central straight portion of the pressure roller can have a desired uniform temperature, thereby avoiding a toner accumulation onto the pressure roller 26. Furthermore, in comparison with the central portion, the end portions corresponding to the maximum passable sheet size have a larger external diameter to form so-called inverted crown shape, thereby preventing creases in the paper.
Furthermore, as the metal core 26a has a straight portion (Sa) in the central portion and is tapered toward both ends, the pressure roller 26 is given so-called inverted crown shape, in which the thicker end portions of the elastic layer 26b show a larger diameter than in the central portion, by the thermal expansion of the elastic layer 26b for example in a continuous printing operation. Therefore, a tensile force toward the both ends is applied to the recording material P pinched and conveyed in the fixing nip portion N, thereby preventing generation of creases in the paper.
Also the pressure roller 26 is produced by setting the metal core 26a in a metal mold 26d, having a straight portion formed longer than the straight portion in the central portion of the metal core 26a, in such a manner that the straight portion of the metal mold 26d exceeds the straight portion of the metal core 26a on both lateral sides, and executing a molding operation in such state. In this manner it is rendered possible to form a straight portion securely in the central portion in the longitudinal direction of the pressure roller 26.
In the following there will be explained, with reference to the accompanying drawings, a second embodiment of a pressurizing rotary member, a fixing apparatus and an image forming apparatus. In the following, components equivalent to those in the first embodiment are represented by corresponding symbols and will not be explained in duplication.
The pressurizing rotary member, the fixing apparatus and the image forming apparatus of the present embodiment employ, a pressure roller 36 shown in
The pressure roller 36 shown in
The present embodiment for example employs, as in the first embodiment, an aluminum metal core 26a having a maximum diameter φDc of 23 mm in the central portion C, a length Sa of 50 mm of the straight portion, and a minimum diameter φDe of 21 mm in both ends. The elastic layer 26b is formed with silicone rubber. The metal mold 26d has a length Sd of 100 mm of the central straight portion, and a crown amount of 120 μm. The releasing layer 36c is formed by a seamless tube of PFA of a thickness of 50 μm. The releasing layer 36c can be provided on the elastic layer 26b for example by setting the releasing layer in the metal mold 26d in advance at the molding operation for the elastic layer 26b, or fitting the releasing layer 36c on the elastic layer 26b after the hardening step, but such examples are not restrictive.
As shown in
The present embodiment, in comparison with the first embodiment, shows somewhat different changes in the external diameters by the thermal expansion or contraction of the elastic layer 26b, because of the presence of the releasing layer 36c. The presence of the releasing layer as in the present embodiment tends to reduce the changes in the external diameters. However, the concave shape in the point B disappears at the high temperature state to provide effects similar to those in the first embodiment, and an improvement in the releasing property is additionally obtained by the addition of the releasing layer 36c, whereby the margin against the toner accumulation can be further increased.
The pressure roller 36 of the above-described present embodiment (
(A) Evaluation of Toner Accumulation on the Surface of the Pressure Roller 26
A character pattern was printed continuously, utilizing a paper Continental LX (manufactured by IGEPA Ltd.) containing calcium carbonate as a filler, in an environment of 15° C./10% RH, and a toner stain on the pressure roller and on the printed paper, at 50,000th and 300,000th sheets was evaluated.
(B) Evaluation of Paper Creases
A grid pattern of a pitch of 10 mm was continuously printed on 500 sheets, utilizing a thin paper Office Planner (manufactured by Canon Inc.), in an environment of 32.5° C./80% RH, and generation of creases in paper was confirmed.
(C) Evaluation of Fixing Property
A character pattern was printed continuously on 500 sheets, utilizing a rough surface paper Fox River Bond #24 (manufactured by Fox River Paper Co.), in an environment of 15° C./10% RH, and a fixing property was evaluated by a rubbing test and the like.
These results are shown in Table 2. In the table, (+) indicates a satisfactory level, (±) indicates a practically acceptable level, and (−) indicates an unacceptable level.
Based on these results it is confirmed that the pressure roller 36 of the present embodiment can achieve, in comparison with that of Example 1, a prevention of toner accumulation on the pressure roller in a continuous printing operation, a prevention of creases in the paper and a stable fixing property by an appropriate temperature control, even at a further elevated printing speed and a longer service life.
In the pressure roller 26 of the prior example 1, the temperature of the pressure roller is not so elevated by a further increased printing speed so that the prevention of the toner accumulation by the roller shape only reaches a limit, and the roller surface is roughened by an abrasion by the paper surface in the prolonged paper-passing durability test, thereby resulting in a toner accumulation.
On the other hand, the pressure roller 36 of the present embodiment has, in addition to the effects of the shape explained in the first embodiment, an improved releasing property of the roller surface itself by the addition of the releasing layer 36c, thereby reducing the amount of the toner transferred from the fixing film onto the pressure roller, and the roughening of the roller surface in the paper-passing durability test is negligibly small, whereby the toner accumulation on the pressure roller can be prevented.
The first and second embodiments have been explained by an example of the shapes of the metal core and the metal mold for the pressure roller. Therefore, the present invention is not particularly restricted in shape, as long as the external profile of the pressure roller in the longitudinal direction thereof can be provided with a straight portion in a central portion and with a concave shape between each end of such central straight portion and each end of the paper of the maximum passable size.
The heat fixing apparatus of the above-explained film heating method is of a type driven by a pressurizing rotary member, but it may also be of a type in which a driving roller is provided on an internal periphery of the endless fixing film and driving such film under a tension. It can also be of a type in which the film is constructed as a rolled web which is driven in a running motion.
The heat fixing apparatus of the present invention is not limited to a film heating type but can also be, for example, a heat roller type in which an image bearing recording material is pinched and conveyed by a nip between a heating member and a pressurizing rotary member thereby heating the image on the recording material.
Also the heater is not limited to a ceramic heater but can be an electromagnetic inductive heat-generating member such as an iron plate. For example there can be employed a configuration of positioning an electromagnetic inductive heat-generating member such as an iron plate, as the heater in the fixing nip portion, and applying thereto a high frequency magnetic field generated by a magnetic coil and a magnetic core as AC magnetic flux generating means thereby generating a heat. Also there may be employed a configuration in which the film itself as the moving member is constructed by an electromagnetic inductive heat-generating member and heat is generated by AC magnetic flux generating means.
Furthermore, the heat fixing apparatus of the present invention is applicable not only to a fixing apparatus for heat fixing an unfixed image permanently on a recording material, but also to a heating apparatus for temporarily fixing an unfixed image on a recording material, or a heating apparatus for re-heating an image-bearing recording material thereby improving a surface property such as gloss of the image.
Furthermore, the image forming method of the image forming apparatus is not limited to an electrophotographic method, but can also be an electrostatic recording method or a magnetic recording method, and can also be a transfer process or a direct process.
The present invention is not limited to the aforementioned embodiments but includes any and all modifications within the technical concept.
This application claims priority from Japanese Patent Application No. 2004-099651 filed Mar. 30, 2004 and Japanese Patent Application No. 2005-087831 filed Mar. 25, 2005 which are hereby incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-099651 | Mar 2004 | JP | national |
| 2005-087831 | Mar 2005 | JP | national |
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