Patent Grant
7010255
1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to a fixing device included in an image forming apparatus for heating a toner image carried on a sheet or recording medium.
2. Description of the Background Art
Generally, an image forming apparatus includes a fixing device configured to fix a toner image transferred to a sheet or recording medium with heat and pressure. A heat roller type of fixing system belongs to a family of conventional fixing devices and includes a heat roller and a press roller facing each other. The heat roller and press roller convey a sheet carrying a toner image therebetween to thereby fix the toner image with heat and pressure.
In the heat roller type of fixing system, a heat source may be disposed in the heat roller, as taught in, e.g., Japanese Patent No. 3,153,732 (column “0015, FIG. 1). Alternatively, a heat source may be positioned outside of the heat roller for heating the surface of the heat roller, as proposed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-129313 (columns “0055” and “0056”, FIG. 2) and 10-133505 (column “0110”, FIG. 1).
Further, to promote rapid warm-up of the heat roller heated by the heat source, Japanese Patent Laid-Open Publication No. 11-84933 (column “0022”, FIG. 1), for example, discloses a fixing device configured to sense the temperature of the heat roller at positions preceding and following a sheet conveying zone and feed, when a pressure difference reaches a preselected value, power to the heat source. Also, Japanese Patent Laid-Open Publication No. 2002-72731 (column “0049”, FIG. 2), for example, proposes to provide the surface of a heating member with light absorbing capability. Further, Japanese Patent Laid-Open Publication No. 10-10919 (column “0033”, FIG. 1), for example, proposes to feed power to the heat source when the surface temperature of a heating member drops below a preselected value.
However, the conventional fixing devices cannot surely prevent the surface temperature of the heating member from dropping, particularly in the sheet conveying zone or nip, extending the warm-up time of the heating member.
It is an object of the present invention to provide a fixing device capable of obviating temperature drop to thereby reduce the warm-up time and maintain preselected fixing temperature at all times, and an image forming apparatus including the same.
A fixing device included in an image forming apparatus for fixing a toner image formed on a sheet of the present invention includes a rotatable member and a pressing member forming a nip, which corresponds to a conveying zone, therebetween. The rotatable member and pressing member convey the sheet via the nip for thereby fixing the toner image on the sheet. A heating member heats the surface of the rotatable member in contact with therewith. Part of the rotatable member and part of the heating member contacting each other respectively have a convex shape and a concave shape complementary to the convex shape.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:
Referring to
As shown in
Recording media applicable to the apparatus 20 include plain papers customary with, e.g., a copier, OHP (OverHead Projector) sheets, cards, postcards and other thick sheets belonging to the at least 90K, 100 g/cm2 class, and envelopes and other special sheets greater in thermal capacity than sheets.
Because the image forming devices 21Y through 21Bk are identical in configuration with each other except for the color of toner to use, the following description will concentrate on the image forming device 21Y by way of example. The image forming device 21Y includes a photoconductive drum 25Y, which is a specific form of an image carrier, rotatable in a direction A. Sequentially arranged around the drum 25Y in the direction A are a charger, a developing device, a cleaning device and other conventional process units. Scanning means with a polygonal mirror scans the drum 25Y with a light beam 29Y at a position between the charger and the developing device.
The drum 25Y may be replaced with a photoconductive belt, if desired. It is to be noted that in the image forming device 21Bk, a light beam 29Bk may be implemented as two beams, so that the image forming device 21Bk can form an image at higher speed than the other image forming devices 21Y, 21M and 21C.
The sheet cassettes 23 and 24 are respectively loaded with sheets of size A4 and sheets of size A3, which are elongate in the right-and-left direction as viewed in
Pickup rollers 41 and 42 are respectively associated with the sheet cassettes 23 and 24 for paying out the sheets. Roller pairs 43 convey the sheet thus paid out from the sheet cassette 23 or 24 toward the registration roller pair 30. An outlet roller pair 45 drives the sheet out of the casing 26 toward the stack tray 27 via an outlet 46.
To better understand the present invention, brief reference will be made to conventional fixing systems. Generally, a fixing roller must have its surface maintained at preselected temperature and is heated by a heat source for such a purpose.
The internal heating system described above has a problem that the metallic core of the fixing roller J must be thick enough to serve as the rigid member of the roller J and is therefore apt to increase thermal capacity, resulting in low heat conduction efficiency to the surface of the roller J.
An external heating system solves the above problem by locating a heating section outside of the fixing roller and heating only the surface of the fixing roller.
The external heating system shown in
The external heating system shown in
Further, when an unmovable, external heating member is held in contact with a fixing roller, the heating member heats only part of the fixing roller. This brings about a problem that when the fixing roller is locally heated while in a halt, it is apt to catch fire. It is therefore necessary to dispose a heat source in the fixing roller, too, and selectively use this heat source and an external heat source to thereby protect the fixing roller from local heating. This, however, cannot be done without resorting sophisticated configuration and control.
The fixing device of the present invention prevents the heat-insulating effect from decreasing by obviating, e.g., the elastic deformation of the surface of a fixing roller, thereby solving the problems stated above.
Referring to
It is noteworthy that while the fixing roller 1A and press roller 1B both are members applied to a heat roller heating system, they do not accommodate a heat source therein, as illustrated.
The fixing roller 1A is made up of a metallic core formed of iron (Fe), a heat insulating layer formed on the core and implemented by foam silicone, liquid silicone or foam ceramic, and a parting layer surrounding the heat insulating layer and implemented by, e.g., a PFA tube. If desired, a heat conduction layer formed of foil of nickel or stainless steel (SUS) may intervene between the heat insulating layer and the parting layer.
The press roller 1B is made up of a metallic core also formed of iron, an elastic layer formed of foam silicone or liquid silicone by way of example, and a parting layer covering the elastic layer and implemented by, e.g., a PFA tube.
The external heating member 2 includes an endless belt 2B passed over a plurality of rollers 2A1 through 2A3 and movable while partly contacting the circumference of the fixing roller 1A. In this configuration, the fixing roller 1A constitutes a rotatable member having a convex circumference, so that the belt 2B has a concave contact surface complementary to the convex circumference of the roller 1A. To implement the concave contact surface, the belt 2B may be replaced with a roller provided with an elastic layer, if desired. Stated another way, the belt 2B included in the heating member 2 is made concave in place of the fixing roller or rotatable member 1A. It follows that even when an elastic layer including a foam portion is present in the fixing roller 1A, the foam portion is prevented from being crushed or losing the heat-insulating function.
In the illustrative embodiment, the rollers 2A1 and 2A2 should only rotate by following the rotation of the fixing roller 1A in the opposite direction to the fixing roller 1A. If desired, either one of the rollers 2A1 and 2A2 may be driven by a drive source, not shown, so as to move the belt 2B independently of the fixing roller 1A. Further, the belt 2B may be configured to slide on the fixing roller 1A without following the rotation of the fixing roller 1A.
Part of the belt 2B is pressed against the fixing roller 1A by the rollers 2A1 and 2A2 positioned at both sides in the direction in which the belt 2B extends along the surface of the roller 1A. The belt 2B is therefore capable of moving while uniformly contacting the surface of the fixing roller 1A. In this sense, the rollers 2A1 and 2A2, positioned side by side and causing the belt 2B to extend along the surface of the fixing roller 1A, play the role of members for pressing the belt 2A against the fixing roller 1A. Alternatively, part of the belt 2B between the rollers 2A1 and 2A2 may be pulled upward so as to be concave complementarily to the convex surface of the fixing roller 1A without contacting the roller 1A.
In the illustrative embodiment, the belt 2B is made up of a base implemented by 30 μm to 100 μm thick foil of nickel, stainless steel or similar metal. The inner surface of the base is painted black in order to enhance absorption efficiency by obviating the reflection of light. Further, a 20 μm to 50 μm thick surface layer formed of silicon rubber or fluorine-containing resin is formed on the surface of the belt 2B that contacts the fixing roller 1A, protecting the fixing roller 1A from damage.
As stated above, in the illustrative embodiment, the rotatable member and heating member contact each other in a convex and a concave shape, respectively. Therefore, when the rotatable member includes an elastic layer including a foam portion, the foam portion is prevented from being crushed or losing the expected heat insulating function. It follows that temperature at the nip between the two members is prevented from dropping and lowering fixing efficiency. In addition, because the position where a sheet begins to be nipped and conveyed is also included in the heating zone, a sheet entered this zone is immediately heated to preselected fixing temperature.
A halogen heater or similar heat source 3 that radiates heat is disposed in the loop of the belt 2B and controlled in synchronism with the movement, i.e., rotation of the fixing roller 1A. More specifically, power is fed to the heat source 3 substantially in unison with the rotation of the fixing roller 1A.
If a roller (belt) is continuously heated for 5 seconds or more while stopped, then the belt is heated to 300° C. or above and has its base or surface layer deformed or degenerated, as determined by experiments. Further, if the roller is continuously heated for 10 seconds or more while stopped, it catches fire, as also determined by experiments. While continuously heating the roller in a halt for 2 seconds or less does not matter from a safety standpoint, it makes the temperature of the belt irregular and therefore brings about irregular fixation. It is therefore important to feed power to a heat source substantially in unison with the rotation of the roller. While the illustrative embodiment sets a specific condition on power feeding or heating timing, it is not necessary that the roller (belt) be stopped when power feed is interrupted for cooling. The issue to be addressed is that the problems stated above do not arise. It is to be noted that the duration of rotation of the roller (belt) after the interruption of power feed is open to choice.
As shown in
In the specific procedure shown in
The control over power feed to the heat source 3 may be based on a speed ratio between the fixing roller 1A and the belt 2B instead of the rotation of the fixing roller 1A. More specifically, the amount of heat based on the speed ratio between the fixing roller 1A and the belt 2B is used to reflect how frequently the new surface of the belt 2B faces the fixing roller 1A; the amount of heat transferred to the fixing roller 1A varies with such frequency. For example, when the belt 2B moves at higher speed than the fixing roller 1A, the chance that the new surface of the belt 2B, transferred heat to the fixing roller 1A and again heated by the heat source 3, faces the fixing roller 1 A increases, so that the amount of heat transferred to the roller 1A increases. When the above relation between the speeds is inverted, the amount of heat absorbed by the belt 2B increases. For this reason, power fed to the heat source 3 and therefore the amount of heat may be controlled on the basis of the speed ratio, so that an amount of heat required of the heat roller 1A can be achieved.
As for the control over the temperature of belt 2B, simple ON/OFF control is apt to aggravate temperature ripple on the belt surface and therefore bring about irregular fixation. It is therefore preferable to delicately control a mean amount of heat for a unit period of time stepwise by using duty control, phase control or inverter control relating the ON/OFF timing of the heat source 3, thereby equalizing the surface temperature of the fixing roller 1A and that of the belt 2B. Further, the amount of heat may be controlled in accordance with both of the rotation speed of the fixing roller 1A and that of the belt 2B. Such control over the amount of heat is practicable even when the belt 2B, serving as the external heating member 2, is replaced with a convex surface contacting the fixing roller 1A.
As shown in
As stated above, the amount of heat is controlled in accordance with the temperature of the fixing roller sensed by the temperature sensor 4, i.e., a mean amount of heat generated by the heat source 3 for a unit period of time is increased or decreased in accordance with the above temperature. Therefore, the temperature of the fixing roller 1A, which varies with the rotation speed of the fixing roller 1A, can be delicately corrected, equalizing the surface temperature of the roller 1A. This obviates defective fixation ascribable to temperature drop. Further, the heat source 3 assigned to the belt or external heating member 2, which is movable in association with the movement of the fixing roller 1A in contact with the roller 1A, is controlled in synchronism with the movement of the fixing roller 1A. This prevents the fixing roller 1A from being excessively heated when, e.g., it is stopped due to an error. Particularly, the heating condition set in synchronism with the movement of the fixing roller 1A is matched not only to the ON/OFF of the heat source 3 but also to the speed ratio between the fixing roller 1A and the belt 2B. This makes it possible to optimize the transfer of heat from the belt 2B in accordance with the condition in which the fixing roller 1A moves, and therefore to match the rise of the temperature of the fixing roller 1A and the maintenance of the temperature to conditions necessary for fixation.
In
The above relation L1≦L2 is selected to guarantee a period of time over which the fixing roller 1A and belt 2B are required to contact each other for replenishing an amount of heat lost by the nip width. More specifically, in the nipping and conveying zone having the length L1, the temperature of the fixing roller 1A is simply absorbed by the sheet S and lowered thereby. Therefore, by replenishing heat over a period of time longer than the period of time over which the temperature drops, it is possible to restore the temperature to the fixing temperature. Assume that such a period of time necessary for heat replenishment is implemented by a roller instead of by a belt. Then, assuming that a length corresponding to the contact length of the belt 2B is a circumferential length, the roller must be provided with an extremely large radius of curvature that would increase thermal capacity and would thereby slow down temperature elevation.
Furthermore, the belt 2B, passed over the rollers 2A1 and 2A2, is simply held in contact with the surface of the fixing roller 1A in a concave shape complementary to the convex shape of the roller 1A. The surface of the fixing roller 1A is therefore protected from local deformation; otherwise, the foam portion of the heat insulating layer included in the fixing roller 1A would be crushed to degrade heat insulation. In this manner, the illustrative embodiment allows the fixing roller 1A to be efficiently heated to the fixing temperature and stably maintained thereat.
Why the belt 2B is passed over three rollers 2A1 through 2A3 is that the belt 2B needs a roundabout way for implementing the contact length L2.
A modification of the illustrative embodiment will be described with reference to
As shown in
The modification described above promotes efficient heat transfer from the heat source 3 to the belt 2B for thereby preventing power consumption by the heat source 3 from increasing.
Reference will be made to
In
As stated above, the portion close to the conveyance start position is included in the heating zone of the external heating member 2, so that the heat radiation path to the conveyance start position is minimized. This successfully minimizes the temperature drop of the fixing roller 1A. Particularly, when sheet conveying speed is high, the contact length L2 of the belt or heat inlet 2B must be increased relative to the nip width or heat outlet L1 in order to guarantee the heat transfer time. In this sense, the belt 2B may be passed over the rollers 2A1 through 2A3 in the condition shown in
In the illustrative embodiment, the length L2 over which the belt 2B contacts the fixing roller 1A is selected to be equal to or greater than the nip width, or the length of conveying zone, L1 between the fixing roller 1A and the press roller 1B, as stated above. It is therefore possible to guarantee a period of time necessary for replenishing heat absorbed by a sheet, thereby surely restoring the preselected temperature. Further, the above contact length is achievable without increasing the size of the fixing device. This, coupled with the fact that the foam portion of the fixing roller 1A is prevented from being crushed and losing the heat insulating function, prevents the surface temperature from dropping and therefore reduces the warm-up time. Moreover, the heating zone includes even the conveyance start position and therefore reduces the heat radiating portion, so that a sheet reaching the conveyance start position can be quickly heated to the fixing temperature.
As stated above, in the illustrative embodiment, the member that presses the belt 2B serves as a heat source at the same time. This, coupled with the fact that direct heat transfer is substituted for radiation, makes it unnecessary to use a special member as a pressing member and reduces the space to be occupied by the heat source, compared to the case wherein radiant heat is used. It is therefore possible to reduce the apparatus cost and the period of time necessary for the fixing roller to reach the fixing time while, after the roller has reached the fixing time, minimizing temperature variation.
When the roller 2A2 is driven by the same drive source, not shown, as the roller 2A1, drive torque may be varied on the basis of a speed reduction ratio in a drive transmission mechanism. On the other had, when a particular drive source is assigned to each of the rollers 2A1 and 2A2, the output torque of the drive sources may be varied from each other.
In the illustrative embodiment, the downstream roller 2A1, driven by heavier drive torque than the upstream roller 2A2, applies tension to the belt 2B. Therefore, when the rollers 2A1 and 2A2 are positioned closer to the axis of the fixing roller 1A than the belt 2B, the belt 2B is pressed against the surface of the fixing roller 1A. Consequently, the contact of the belt 2B with the fixing roller 1A is insured by using the structure supporting the belt 2B, i.e., without resorting to any special pressing structure, so that heat transfer from the belt 2B to the roller 1A is enhanced.
A modification of the illustrative embodiment will be described hereinafter. In the modification, the roller 2A1 closer to the conveyance start position than the roller 2A2 is formed of aluminum or similar good heat conductor and differs from the other roller 2A2 in that it is not provided with the cover layer R. The roller 2A1 can therefore maintain the temperature distribution in the axial direction uniform. More specifically, the sheet size to be dealt with is not constant in the axial direction of the fixing roller 1A. For example, when the sheet size occupies only a narrow range in the axial direction, the temperature distribution in the axial direction varies with the result that the supply and demand of heat on the belt 2B is disturbed. The roller 2A1 formed of a good heat conductor obviates the above problem. Particularly, when the roller 2A2 is formed of pure aluminum, temperature gradient in the axial direction is rapidly canceled, so that the temperature distribution in the axial direction is maintained uniform.
In the embodiment shown in
A cam 10 is held in contact with the end of the lever 7 remote from the shaft or fulcrum 8A and driven by a motor, not shown, to move the lever 8 such that the press roller 1B selectively moves into or out of contact with the fixing roller 1A. The cam 10 maintains the press roller 1B spaced from the fixing roller 1A via the lever 8 during the warm-up time of the fixing roller 1A, i.e., up to the time when the surface temperature of the fixing roller 1A reaches the fixing temperature. This timing corresponds to the start-up of the fixing device 1.
In the illustrative embodiment, it is also possible to control the contact pressure between the press roller 1B and the fixing roller 1A on the basis of the rotation phase of the cam 10. The control over the contact means control over the sheet nipping force. It follows the fixing roller 1A can desirably contact a toner image and efficiently transfer heat to the toner image.
Further, the press roller 1B is spaced from the fixing roller 1A up to the time when the surface temperature of the fixing roller 1A rises to the fixing temperature, as stated above. In this condition, heat transferred from the external heating member 2 to the fixing roller 1A is not absorbed by the press roller or another member 1B at all, insuring rapid warm-up of the fixing roller 1A.
A sixth embodiment of the present invention will be described with reference to
A receiving portion 1C should preferably be positioned between the upper portion of the belt 2B′ and the heat source 3 from the safety standpoint. When the upper portion of the belt 2B′ accidentally bends downward, as indicated by a phantom line in
Various modifications of the structural elements of the fixing device will be described hereinafter.
In the modification shown in
In the modification shown in
In the modification shown in
In the modification shown in
In the modifications of
In any one of the above modifications, the belt 2B or 2B′ of the external heating member 2 heats the fixing belt 1A′ in contact with the outer surface of the fixing belt 1A′, so that is suffices to set the temperature of only the outer surface of the belt 1A′. Such external heating successfully reduces the warm-up time, compared to internal heating that heats a member having a large thermal capacity. Moreover, because the belt itself includes a heat insulating layer, heat losses ascribable to heat radiation or heat conduction do not occur, so that the belt can be maintained at the fixing temperature.
Reference will be made to
Assume that a sheet, wrapped around the fixing roller 1A, is conveyed to the peeling means 12 by the roller 1A. Then, the peeling means lifts the leading edge of the sheet away from the surface of the fixing roller 1A for thereby peeling off the sheet. Otherwise, the sheet would, e.g., catch fire on reaching the external heating member 2.
If desired, the temperature sensor 4,
In addition to the peeling means, sensing means responsive to a sheet wrapped around the fixing roller 1A may be positioned between the outlet of the nip and the hating zone where the portion of the fixing roller 1A faces the belt 2B. In such a case, when the sensing means senses a sheet wrapped around the fixing roller 1A, the rotation of the roller 1A will be stopped while the peeling means will peel off the sheet. Such sensing means uses reflectance or similar optical factor. Further, when the sensing means senses the above sheet, the belt 2B may be released from the fixing roller 1A.
Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-160077 | May 2002 | JP | national |
| 2003-117777 | Apr 2003 | JP | national |
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| Number | Date | Country |
|---|---|---|
| 8-129313 | May 1996 | JP |
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| Number | Date | Country | |
|---|---|---|---|
| 20040022552 A1 | Feb 2004 | US |
