EP1658528B1

EP1658528B1 - Externally heated fuser member

Info

Publication number: EP1658528B1
Application number: EP04782072A
Authority: EP
Inventors: Knut Behnke; Hans-Otto Kraus; Frank-Michael Morgenweck; Domingo Rohde; Detlef Schulze-Hagenest; Lars Seimetz; Arun Chowdry; Cumar Sreekumar; Dinesh Tyagi; Bernd Geck
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 2003-08-28
Filing date: 2004-08-24
Publication date: 2008-05-28
Anticipated expiration: 2024-08-24
Also published as: EP1658528A1; DE602004014161D1; ATE397238T1; WO2005024526A1

Abstract

A fuser mechanism (1) and a fuser device (2) that is at least partially externally heated for fusing toner, on a printing medium (3), incorporating a microwave heating mechanism (9) for contact-free heating of the surface of the fuser device. In addition, a fuser device (2) that incorporates a surface layer that absorbs microwaves.

Description

FIELD OF THE INVENTION

The invention pertains to a fuser device, in particular, a fuser device that is at least partially externally heated, for fusing toner on a printing medium in a printing machine, preferably in an electrophotographic printing machine.

BACKGROUND OF THE INVENTION

In electrophotographic printing machines, as well as copiers, fuser mechanisms are used to fuse toner on printing media. For this purpose, fuser mechanisms are used which, for example, have as the fuser device a fuser roller and, as a pressure device, a pressure roller. In this approach, the fuser roller is heated and the fuser roller and the pressure roller are pressed one against the other so that the toner becomes fused onto the printing medium as a result of the heating and the pressure. Additionally, the pressure device can be heated such that it, too, becomes a fuser device. In such case, two fuser devices are then pressed one against the other and the toner thus becomes fused onto the printing medium.
It is proposed in patent specification U.S. Patent No. 6,445,902 that the fuser devices be heated from the outside by rotating, contact-making heater rollers whose diameters are significantly smaller than those of the fuser device.
It is becoming increasingly necessary, particularly for modem, electromagnetic printing processes, to regulate with all possible speed the surface temperatures of the fuser devices, and to adjust such temperatures for different printing media that are to be imaged one after the other. The benefit to be gained from the ability to do this is provided by an externally-heated fuser device because the different temperatures on the surface of the fuser device that are required for fusing different printing media can be more quickly adjusted by the external heating roller than would be the case with internal heating, whereby the entire volume of the fuser device must first assume the changed temperature before the change in temperature effectively reaches the surface.
In addition, the use of external heating makes it possible, for example, whenever the fuser devices are mechanically worn out, to change the fuser devices as necessary without having to replace temperature sensors, heating elements, or associated electronics. These disadvantages exist with conventional fuser devices that are internally heated.
The disadvantage of the process proposed in patent specification U.S. Patent No. 6,445, 902 is that the additional, contact-making heating rollers cause increased wear on the surface of the fuser device.
US Patent N° 4,025,751 discloses a fuser roll structure, for use in a xerographic reproducing apparatus, comprising a core member having a sleeve carried thereby and a heat source disposed internally or externally of the core member.
US 2002/0114642 Patent application discloses means for using frictional drives including conformable members in electrostatography, and more particularly to the use of frictional drives for transferring and fusing toner images in electrophotography.
US 2002/0098021 Patent application discloses a fusing system in an image forming system for fixing a toned image to a support member. An external heating source is associated with a fusing member so as to provide heat to the surface of the fusing member.

SUMMARY OF THE INVENTION

In view of the above, this invention is to introduce a fuser mechanism that can adjust the surface temperature of a fuser device more quickly than has been possible to date. One result should be reduced wear of the surface of the fuser device. In addition, a corresponding fuser device is introduced, which allows the temperature of its surface to be quickly changed.
As an additional objective of the invention, a process is proposed for fusing toner onto a printing medium by a fuser device that is at least partially externally heated, in a printing machine, preferably an electrophotographic printing machine, which will reduce the wear on the fuser device and make possible a quicker adjustment of the surface temperature of the fuser device. The objective of the invention is achieved by a fuser mechanism in accordance with appended claim 1.
Beneficially, a microwave heating mechanism located outside the fuser device does not subject the fuser device surface to further mechanical stress, the result of which is that wear is reduced and the maintenance interval, i.e., the time before the fuser device is next exchanged, is extended. Beneficially, a microwave heating mechanism located outside the fuser device does not subject the fuser device surface to further mechanical stress, the result of which is that wear is reduced and the maintenance interval, i.e., the time before the fuser device is next exchanged, is extended.
With better microwave absorption of the fuser device surface, the efficiency of the microwave mechanism can be enhanced more successfully. For this purpose, then, a material for the fuser device surface should be selected whose microwave absorption is optimized. Thus, the surface layer of the fuser device should be purposely selected such that it absorbs microwaves and, in particular, that it absorbs microwaves better than is the case with conventional materials used for the surface layer of a fuser roller.
The response time of the fuser device is reduced because the temperature of the fuser device surface is changed more quickly by a change in the applied microwave energy. Thus, fusing speed is increased, in particular, when different printing media have to be fused directly behind one another, which then requires that the fuser device be adjusted to different temperatures. In addition, wear on the fuser devices can be decreased.
An additional benefit to be gained from using microwave irradiation for heating fuser device surfaces is that the temperature of the fuser device surface is determined by the microwave energy that is applied. In this regard, one can use materials for the fuser device surface that, have a low heat retention capacity, so that beneficially, the surface will cool off quickly once the microwave irradiation is stopped. Then, if faulty sheets get into the area which is being heated by microwaves, the microwave heating mechanism can be quickly switched off and there will be no danger of the faulty sheets catching fire, because the cooling occurs so quickly. The heat retention capacity of the surface layer must, of course, be high enough for the heat level to be maintained until such time as the nip is reached between the fuser roller or the belt fuser and a pressure roller.
As a matter of routine, it should be impossible to apply the microwave energy unless the fuser device is rotating, because otherwise the surface layer of the fuser device would be destroyed very quickly. Additionally, it is also possible for the fuser device to be heated internally. The mechanism used for such internal heating can heat the fuser device surface to a low temperature so that the external microwave heating mechanism would then be used solely for bringing about temperature changes above this lower temperature.
Provision is made for the microwave heating mechanism to incorporate at least two microwave applicators. By precisely measured dimensions of the applicators, including the entrance panels, a standing microwave forms inside the applicators. The applicators incorporate evenly distributed exit openings that are offset from one another in pairs and are oriented toward the fuser device surface.
In this way, the fuser device surface can be evenly heated. The exact position, the intervening distances, and the lengths of the exit openings must be matched to the wavelength of the microwaves used. In this way, it is possible to assure that the openings are located in the applicator in the area where the electromagnetic field is at its maximum, and that they, therefore, make maximum irradiation possible. The intervening distances should be equal to one-half of the waveguide wave length; the length of the slot is one-half of the wavelength of a free microwave.
The applicators can be offset, such that the exit openings of one of the applicators, is located in areas in which the other applicator has no exit openings. The microwave radiation that is emitted from the openings or the areas that are heated by the radiation overlap one another so that a sufficiently even heat profile arises on the fuser device surface. Using two or more microwave applicators has the additional benefit that more microwave energy per area can be applied to the fuser device surface, whereby the temperature of the surface reaches a desired value more quickly, and the speed of fusing is increased.
In addition, provision is made for a decoupling element, a thin metal sheet, to decouple the microwaves emitted from the microwave applicators. This decoupling element should be made of metal. In this way, the most even heat profile possible can be achieved. Such an electricity-conducting metal sheet can shield the different microwave applicators from one another such that the microwaves emitted from these applicators are decoupled from one another.
In addition, provision is also made for shielding elements for reducing the microwave radiation that is randomly emitted into the environment of the fuser mechanism. These shielding elements can be shields made of metal sheets.
The shielding elements can also be, according to the invention, chokes that have dimensions that result at least in a reduction of the intensity of microwave radiation. These chokes can, according to the invention, also be made of metal sheet shielding. In addition, these shielding elements can supplementally, or instead, be materials that absorb microwaves particularly well. In particular, provision is made for combinations of sheet metal shielding, chokes, and materials that absorb microwaves well.
The use of these shielding elements results at least in the attenuation of the microwave radiation emitted into the surroundings. Waxes, or even foam materials with suitable characteristics, for example, can serve as the absorbent materials.
By a layer on the surface of the fuser device that has, according to the invention, a material whose microwave absorption characteristics are temperature dependent, in particular, in the sense that it absorbs less microwave radiation at higher temperatures, it is beneficially possible to avoid overheating the fuser device surface. Conceptually, it is even possible that the fuser device surface would have only one maximum temperature. If such a maximum temperature would then lie below the ignition temperature of the printing medium in use, ignition of the printing medium, for example, such as that which might otherwise be caused by a faulty run in the fuser mechanism, would be precluded.
In order to make heating of the fuser device surface as even as possible, provision is beneficially made for the surface layer to be interspersed with carbon, whose density preferably should be a function of depth, for the purpose of improving the absorption characteristics of the surface layer. The result is that the surface becomes heated more quickly and the heat is also distributed better laterally. The density can also be varied such that the site of maximum absorption lies below the surface of the fuser device, so that the heat, temporally displaced, reaches the surface in that instant at which the surface makes contact with the printing medium. By this, a particularly effective heating of the fuser device surface is possible.
Provision is further made according to the invention for the rotating areas of the fuser device, with the exception of the surface layer, to include a non-absorbent material that is microwave-permeable. Such a material can, for example, be glass. In this way, overheating of the fuser device that might otherwise be caused by microwave radiation not absorbed by the surface layer can be optimally prevented, whereby then the material is subject to lower levels of stress.
If the rotating areas with the exception of the surface layer are microwave-permeable, the radiation can pass through to a reflector element and, from there, be reflected back to the surface of the fuser device. In this way, the remaining rotational areas do not become heated. Consequently, in a further beneficial embodiment of the fuser mechanism, according to the invention, provision is made for a focusing, stationary reflector element to be used for focused reflection of microwave radiation that is transmitted into the interior of the fuser device. Provision for a reflector element can be made, in particular, if the rotating areas of the fuser device are not, or are only partially, microwave permeable. Even in such a case, microwave radiation that at least partially passes through the fuser device can be reflected in a focused manner.
This reflector element must be suited to reflect microwaves. It must be concave and must be installed inside the fuser device such that it does not rotate, preferably on the center of rotation of the fuser device, so that reflection of microwave energy into the surrounding areas is minimized. Microwave radiation will be reflected back to the area of the fuser device surface, where it will contribute to the heating of that surface. In this way, both undesired reflection of microwave radiation into the surroundings is reduced and the efficiency of the fuser mechanism is increased.
If different sized printing media are used, then during the fusing process there can be areas of the fuser device surface that make no contact with the printing medium, but that are nevertheless heated by the microwave mechanism. Because the heat is not transferred from these areas onto the printing medium, each time the fuser device rotates, the temperature in these areas is undesirably raised. To prevent this outcome, provision is beneficially made for shields that can be laterally inserted into the space between the microwave heating mechanism and the fuser device surface. The insertion of these shields leads to a reduction in the effective heating width of the microwave heating mechanism. These shields can, for example, be made of metal and, as such, prevent in the shielded areas transmission of microwave radiation onto the fuser device surface, thereby inhibiting the heating thereof.
In addition, provision can be made for the fuser device to have a heat-insulating layer below the surface layer. This layer can prevent heat energy that is coupled into the surface layer of the fuser device from getting into the interior of the fuser device and heating, for example, the metal core located therein. This is particularly applicable when carbon is present whose density is a function of its depth. The insulating layer lies beneficially directly below the area of maximum absorption.
Provision is made according to the invention for the temperature of the fuser device surface to be sensed. For this purpose, provision is made for locating at least one temperature sensor for sensing the temperature of the fuser device surface in the vicinity of the fuser device. A temperature sensor can, for example, be located above the fuser device surface and/or inside the fuser device. The temperature sensors can then be located upstream of and/or downstream of the point at which the fuser device makes contact with the printing medium. If then, for example, the sensed temperature is too high, the fusing event can be interrupted.
Beneficially, the microwave energy will be adjusted as a function of the printing medium passing through the fuser mechanism and/or the sensed temperature of the fuser device surface. The different printing media can react differently to the temperature of the fuser device surface, and, in addition, it may be necessary to use different temperatures during fusing in order to produce on the printing medium the different gloss imprints that may be desired. An ideal adjustment of the microwave energy is, however, not possible until the temperature of the fuser device surface is known and the energy level is adjusted in accordance therewith. In this way, a better result with respect to the desired glossiness may be obtained during the fusing event and/or the printing medium can be protected.
It is possible at any given time that the desired temperature of the fuser device surface for fusing a printing medium already may have been exceeded. Reaction to such a situation is possible once the temperature has been sensed, and the surface could then be appropriately cooled. For this purpose, provision has been made according to the invention for a cooling mechanism to be located in the vicinity of the fuser device, preferably in the direction of rotation of the rotating fuser device, behind a nip formed by the fuser device and the pressure device. There is still enough space in this position for the cooling mechanism so that it can impact upon the largest possible area of the surface. The cooling mechanism can be a mechanism, which, for example, blows cooled air onto the surface. Beneficially, an undesired heating of the fuser device can also be prevented by use of this cooling mechanism.
In addition, provision has further been made for the cooling mechanism to be a component of the microwave heating mechanism. This makes it possible, then, for cooled air to be directed onto the fuser device surface via the exit openings of the applicators. This arrangement allows the space required for the cooling mechanism to be minimized or at least to be limited within the direct surroundings of the fuser device.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments from which additional characteristics according to the invention can be derived, but to which the invention is not limited, are represented in the drawings. The drawings show the following:

FIG. 1 shows a schematic side view of an example of a fuser mechanism that incorporates an externally heated fuser device;
FIG. 2 shows an overhead view of an example of a fuser mechanism that incorporates an externally heated fuser device;
FIG. 3 shows the underside of the applicators used for the external heating;
FIG. 4 shows a schematic side view of a fuser mechanism;
FIG. 5 shows an overhead view of a fuser device that incorporates insertable metal shields;
FIG. 6 shows a cross-section through the fuser mechanism that incorporates metal shields;
FIG. 7 shows a cross-section through an examplary fuser mechanism that incorporates an alternative fuser device and alternative shielding elements;
FIG. 8 shows a cross-section through an examplary fuser mechanism; and
FIG. 9 shows a fuser mechanism that incorporates an pressure roller to serve as a fuser device.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the accompanying drawings, FIG. 1 shows schematically a side view of a fuser mechanism 1 not being part of the invention that incorporates an externally heated fuser device, which as shown here is a fuser roller 2. A printing medium 3, which here is, for example, a sheet of paper, is conveyed in the direction of the arrow 4 through a nip 5 that is located between the fuser roller 2 and pressure device, which here is a pressure roller 6, for the purpose of fusing toner that is not shown here. The fuser roller 2 and the pressure roller 6 rotate during the fusing process in the directions shown by the arrows 7 and 8. The fuser roller 2 rotates around its center of rotation 14.
The fuser roller 2 has a layer on its surface, which is not particularly emphasized in the drawing and which absorbs microwave radiation. Special embodiments of such a fuser roller 2 are shown in FIGS. 7 and 8. The fuser roller 2 receives the energy required for fusing from a microwave heating mechanism 9. This mechanism is located in the direction of rotation of the fuser roller 2 directly upstream of the nip 5 and heats the surface of the fuser roller 2 without making contact with that surface.
The microwave heating mechanism 9 incorporates two microwave applicators 10 and 11, hereinafter referred to simply as applicators 10 and 11. As shown in FIG. 3, the sides of the applicators 10 and 11 that face the fuser roller 2 are slotted so that microwave radiation 12 exits from applicators 10 and 11 and heats the surface of the fuser roller 2. Some of the microwave radiation that is not absorbed by the surface layers can be reflected back from the various other layers of the fuser roller 2, mainly from a metal core of the roller 2 that is not shown in the drawings.
In order to once again direct the reflected microwave radiation 12 toward the fuser device surface and thus to minimize reflection of radiation into the free space, provision is made for locating shielding elements including sheet metal shields 13 directly next to the applicators 10 and 11. These metal shields 13 are curved to match the curvature of the fuser roller 2 and are located on both sides of the applicators 10 and 11. The distance between the surface of the fuser roller and the external microwave emitter and the metal shields 13 is beneficially minimal, i.e., in the range of millimeters.
FIG. 2 shows a view of a fuser mechanism 1 not being part of the invention that incorporates an externally heated fuser roller 2. The same numbers that are used to identify elements in this drawing are used to identify the same elements in subsequent drawings. The applicators 10 and 11 run parallel to the center of rotation 14. Standing microwave fields are formed in them, which are then outputted via the slots 15, as shown in FIG. 3.
The microwaves are generated in microwave sources 18. The microwaves are coupled in the direction of the arrow 16 into the applicators 10 and 11 via intermediating guides 17 and panels 19 and 20. The dimensions of the applicators 10 and 11 and the panels 19 and 20 are selected such that a standing wave is formed. The invention is, however, not limited to applicators in which standing waves are formed; applicators in which traveling waves propagate can also be used and such waves are outputted in a similar fashion.
FIG. 3 shows the lower side of the applicators 10 and 11 that are used for external heating. The microwaves are coupled in the direction of the arrow 16 into the applicators 10 and 11 via the panels 19 and 20, where they then form into standing waves. The lower sides of the applicators 10 and 11 contain slots 15, through which the microwaves are outputted. The length l₀ of the slots 15, and their distance l₂ from one another are matched to the wavelengths of the microwaves being used. The length l₀ is one-half of the length of the microwave λ₀/2 in free space and the length l₂ is one half of the length of the wave guide λ_H/2 used for the microwaves in the applicators 10 and 11.
The geometries of the applicators 10 and 11 are selected such that the resonance conditions required for the microwave frequency being used are met. So that a maximum of microwave energy is outputted from applicators 10 and 11, each of the slots 15 is located at points of maximum field strength. Each first slot 15 of an applicator 10 or 11 is located at a distance l₁ from the corresponding panel 19 or 20, which corresponds to a quarter of the length of the wave guide λ_H / 4 used for the microwave in the applicator 10 or 11.
The microwaves 12 that are outputted from the applicators 10 and 11 impinge upon the fuser roller 2 such that the surface thereof becomes heated. In order to heat the fuser roller surface as evenly as possible, the slots 15 of the two applicators 10 and 11 are mounted together in pairs. The distance l₃ between the middles of two neighboring slots 15, one of which is located in applicator 10 and the other of which is located in applicator 11, equals λ_H / 4.
As long as λ₀ > λ_H / 2 applies to the wavelengths of the microwave being used, the offset slots 15 of the applicators 10 and 11 will overlap laterally. The result is then generally even heating of the surface. If, however, overlapping of the slots 15 is not sufficient, or if λ₀≤λ_H / 2 applies such that the slots 15 of the applicators 10 and 11 do not overlap at all, even heating of the surface of the fuser roller 2 cannot be guaranteed. In such case, then, as the result of uneven gloss formation, streaks can form during the fusing event. Such streaking can, however, then be prevented by the use of three or more applicators. In this way it is always possible to make available applicators with sufficiently overlapping slots 15. The surface of the fuser roller 2 can then always be evenly heated and even fusing, and thus an image without gloss variations, can be achieved.
FIG. 4 shows a schematic side view of an additional fuser mechanism 1'. In this fuser mechanism 1', too, the applicators 10 and 11 of a microwave mechanism 9' are located directly in the vicinity of the nip 5. The distance from the nip 5 is essentially dictated by the overall dimensions of the metal shields 21, which are attached to the applicators 10 and 11 and are curved to conform to the curvature of the fuser roller 2. It is also possible in principle to position the applicators 10 and 11 at any other point opposite the fuser roller 2, for example, opposite the nip 5.
In the embodiment shown here, the fuser roller 2 incorporates an internal heat source 22 in addition to the external microwave heating mechanisms. With this heat source 22 the fuser roller 2 can be heated to a specific low temperature To and different fuser temperatures T_F can then be reached by using the microwave heating mechanism 9'. If, for example, the fuser temperature T_F of the fuser mechanism 1' is to be raised for the succeeding printing medium 3, an increase in the microwave energy coming from the microwave heating mechanism 9'can be used to raise the temperature of the surface of the fuser roller 2. If the temperature is to remain constant for two sequential fusing events, then the microwave heating mechanism 9' must balance out only the temperature loss suffered by the fuser roller surface during the fusing events and the subsequent rotation.
If the temperature is to be lowered for a subsequent fusing event, it is sometimes the case that the temperature lost from the surface during a rotation of the fuser roller 2 is not alone sufficient to attain this lower temperature. For this purpose, a cooling mechanism 23 is located in the direction of rotation downstream of the nip and above the surface of the fuser roller 2. Working in conjunction with a temperature sensor 38, which in this example is located upstream of the cooling mechanism 23, the cooling mechanism 23 can control the temperature of the surface of the fuser roller 2. For this purpose, the cooling mechanism 23 can also be located directly neighboring the microwave heating mechanism 9' or it can be connected with the applicators 10 and 11 in such a way that cooling air is fed into the applicators 10 and 11 and guided directly through the slots 15 onto the surface of the fuser roller 2. Other temperature sensors that are not shown here can be located, for example, in the immediate vicinity of the microwave heating mechanism 9'.
In this representation of the fuser mechanism 1', the metal shields 21 incorporate in addition, so-called chokes 24. The geometries of the chokes 24 are matched to the wave length of the microwaves being used. By these chokes, the microwaves are reflected back into themselves such that they extinguish themselves to the greatest extent possible. By the use of such chokes, the emission of microwaves into the surroundings of the microwave mechanism 1' can more easily be avoided.
The microwave energy that comes from the microwave heating mechanism 9' is to be adjusted as a function of the temperature sensed by the temperature sensor 38, the type of printing medium 3 that is being used, and the desired gloss. The length of the applicators 10 and 11 or the area of the applicators 10 and 11, which have slots 15, determines the extent of the area of the surface of the fuser roller 2 that is heated by the microwaves. The maximum fusible size F_max, of a printing medium, is thus, dictated by this area.
Even if a smaller sized printing medium 3 is used, the same width of the fuser roller 2 is heated by the microwave heating mechanism 9'. The areas of the heated fuser roller 2 which do not come into contact with the printing medium 3 will then not be cooled off to the same extent as the area that does come into contact with the printing medium and which, as a result of the contact, gives up energy. As the fuser roller continues to rotate, the areas that are not cooled continue to be heated to even higher temperatures. If such areas are then used for a subsequent fusing event, varying gloss formations on the printing medium will ensue. The elevated temperature can also adversely affect the fuser roller 2 itself. A danger of overheating the fuser roller 2 exists particularly when smaller sized printing media are used for an extended period of time.
This undesirable heating of the areas of the fuser roller that are not needed for the fusing event can be prevented by shields 25. These shields 25 can be inserted from the side into the area between the fuser roller 2 and the microwave heating mechanism 9'. In this way, the effective heated width can be reduced. The shields 25 are inserted as a function of the size of the printing medium 3 that is to be fused. The smaller the printing medium that is to be fused, the greater are the outer areas of the fuser roller that are shielded. The metal shields 25 are curved so as to match the curvature of the surface of the fuser roller 2.
In contrast with that of the fuser mechanism shown in FIG. 1, the distance here between the microwave emitter and the fuser roller must be enlarged when such metal shields 25 are used. Otherwise the metal shields would influence the microwave radiation exiting from the applicator openings such that the entire geometry of the arrangement with respect to even heating of the fuser roller would no longer prevail.
While the distance of the microwave heating mechanism 9 in the fuser mechanism 1 shown in FIG. 1 without metal shields 25 is ideally located in the range of a few millimeters away from the fuser roller 2. When metal shields are used, this distance is about 2 cm or at least in the range of a few centimeters. This increased distance permits the easy use of the metal shields 25. In general, the microwave heating mechanisms 9 and 9' should be as close to the surface of the fuser roller 2 as possible, provided that the microwave field is not adversely affected.
To produce even impingement upon the surface of the fuser roller 2, the microwaves are guided onto the surface through a reverse-funnel shaped distance 26 after they have been outputted through the slots 15. The microwaves being emitted from the two distinct applicators 10 and 11 are decoupled thereby by a thin metal sheet 27 so that the field propagation of each of the applicators is not influenced by the other applicator.
FIG. 5 shows an overhead view of a fuser device with insertable metal shields 25. The metal shields 25 are located on the sides of the fuser roller 2 such that they can be inserted between the fuser roller 2 and the applicators 10 and 11. For this purpose, the metal shields 25 are movable along their axes and can be moved in the directions of the double arrow 28 toward the fuser roller 2 and away from it. This simple process allows the effective heating width of the microwave heating mechanism 9' to be varied.
FIG. 6 shows a section through the fuser mechanism 1' along with the insertable metal shields 25. FIG. 6 shows only one applicator 11 into which microwaves are fed in the direction of the arrow 16 such that a standing wave field is formed within it. The microwaves can exit from the applicator 11 through slots 15 that are not shown here and, having done so, can impinge upon the surface of the fuser roller 2.
Below the applicator 11 is a reverse funnel shaped distance 26. The distance 26 is, as described above, necessary to minimize the influence of the laterally insertable metal shields 25 on the field propagation in the applicators. The metal shields, which can be moved in the directions shown by the double arrow by a drive mechanism that is not shown here, are located between the roller 2 and the reverse funnel shaped distance 26. By inserting the metal shields 25 in the direction of the fuser roller 2 up to points a and a', the effective heating width of the microwave heating mechanism 9' can be adjusted to different printing media widths F. The minimal adjustable printing media width F_min and the maximal adjustable printing media width F_max delimit the widths of printing media 3 that can be fused by this fuser mechanism 1'.
FIG. 7 shows a cross section through the fuser mechanism 1 not being part of the invention that incorporates an alternative fuser device and alternative shielding elements. The fuser device, a fuser roller 2', incorporates a surface layer 29, which absorbs microwaves particularly well. For this purpose it is beneficially enriched with carbon. It is possible to incorporate the carbon in specific layers, i.e., just which layer of the roller material is to absorb most of the microwave radiation can be precisely specified. The density gradient of the carbon can be such that most of the microwave radiation will be absorbed at a distance below the surface that is selected such that the heat generated there reaches the surface of the fuser roller 2' just when this surface makes contact with the printing medium 3. In addition, the thermal conductivity of the surface layer 29 is positively influenced by the carbon enrichment such that non-homogeneities that might possibly arise in the heating are quickly balanced out, resulting in a very even heating of the surface of the fuser roller 2'.
Directly below the layer containing the carbon is an insulating layer 30. This insulating layer 30 prevents heat from the surface layer 29 from penetrating farther into the interior of the fuser roller 2' where it would not be available for the fusing process and would instead unfavorably heat the core of the fuser roller 2'.
In addition, an alternative or supplemental shielding element is shown in this special embodiment. While in FIG. 4 chokes, which prevent microwaves from exiting, are used as shielding elements, here, absorbent materials 31 are located in the area of, or beneficially on the ends of, the metal shields 21. These absorbent materials 31 are here, for example, made of carbon containing cellular plastic or wax, or the like, which can effectively absorb the microwave radiation not absorbed by the fuser roller 2'.
The carbon assures, in general, a better absorption of the microwave radiation by the fuser roller 2'. Use of carbon allows more microwave energy to be converted to heat. The carbon density gradient, and particularly the insulating layer 30, can assure optimal thermal conduction to the surface of the fuser roller 2'of the heat generated by absorption of microwaves, so that most of the heat is available at the surface at the moment when the fuser roller 2' makes contact with the printing medium 3.
A cross-section through another fuser mechanism that incorporates an additional alternative embodiment of a fuser device is shown in FIG. 8. A fuser roller 2" that is shown here incorporates a surface layer 32 that contains, for example, carbon for better absorption. Below this layer, the remainder of the rotatable area 33 of the fuser roller 2" is made of glass which is microwave permeable so that no absorption takes place therein.
The core area 34 of the fuser roller 2" is essentially hollow and contains a stationary reflecting element 35. This reflecting element 35 reflects microwave radiation 36 that has been transmitted through the surface layer 32 back to the surface layer 32 of the fuser roller 2". This reflected microwave radiation 37 can then be absorbed there and thereby contribute to the heating effort. Loss of energy derived from transmitted microwave radiation 36 can thus be prevented, or at least reduced. In addition, by suitable curving of the reflecting element 35, reflected microwave radiation 37 can be prevented from exiting from the area of the fuser mechanism 1; at least it is reflected into the area of the metal shields 21.
FIG. 9 shows a fuser mechanism that incorporates a pressure roller to serve as a fuser device.
Both the pressure roller 6' shown here, and the fuser roller 2, are heated by a microwave heating mechanism (9a and 9 respectively), and can consequently, for example, in a duplex process, support the fusing process. The structure of the microwave heating mechanism 9a is identical to the structure of the microwave heating mechanism 9. Otherwise, too, what has been previously described applies to this kind of pressure roller 6'. For the sake of graphic simplicity, the elements shown in this figure that do not differ from elements shown in previous drawings are not identified. The construction is here essentially the same for both the pressure roller 6' and the fuser roller 2.
Both fuser mechanisms 1 and 1' are capable of adjusting the fusing temperature to suit the different printing media 3. The fuser roller 2 can be preheated to a prescribed lower temperature To by of an internal heat source 22. As a practical matter, this temperature is the lowest temperature that can be used for fusing T₀ = T_Fmin. Then, by the microwave heating mechanism 9 and the cooling mechanism 23; the surface temperature of the fuser roller 2, can be adjusted to suit a subsequent printing medium quickly and without making contact. In making this adjustment, data about (1) the printing medium 3, (2) the temperature that has been sensed by the temperature sensor 38, and (3) the desired glossiness, can be taken into account.
In addition, sensors can be on hand for detecting faulty sheets. If such a faulty sheet is detected, all that needs to be done to prevent dangerous overheating of the printing medium 3 is to switch off the microwave heating mechanism 9.
By the reflecting element 35 in the interior of the fuser roller 2 and a density gradient of a carbon enrichment contained in the surface layer 29 of the fuser roller 2', the available microwaves can be maximally utilized. Enrichment with carbon also results in a quicker, homogeneous distribution of heat on the surface of the fuser roller 2'. An insulating layer 30 also contributes to a better utilization of the microwave radiation.
Exiting of the microwaves, from the area of the fuser mechanism 1, can also be prevented by the metal shields 13 and 21. Other elements suited for this purpose are chokes 24 and the absorbent materials 31. The curvature of the reflecting element 35 also helps prevent the escape of microwaves. These distinctive elements can be variously combined with one another.
The movable metal shields 25 can prevent undesirable heating of areas of the fuser roller 2' that are not used for the fusing event. The effective heating width of the microwave heating mechanism 9' can be optimally adjusted to the size F of the printing medium 3.
By using the microwave heating mechanism 9 and, additionally, by the improved utilization of the microwave radiation, as well as by the supplemental preheating of the fuser roller 2, the time required to adjust to a desired fusing temperature T_F can be reduced to a minimum.
The escape of microwaves can also be limited to a minimum by the structural measures already described.
The danger imposed by faulty sheets when conventional, contact-making, external heating elements are used can be significantly reduced by the use of microwave heating mechanisms 9.
With the use of several or more applicators 10 and 11 the time required to achieve T_F can be even further reduced because more energy can be converted into heat. For this purpose, an increase in the microwave output is also possible up to a certain point. Even more uniform heating, can also be achieved by this process.
The microwave heating mechanism 9, does not cause mechanical wear of the fuser roller 2, because, no contact is made between the mechanism and the roller.

Claims

A fuser device that is at least partially externally heated, that is used for fusing toner on a printing medium (4) in an electrophotographic printing machine, for use in a fuser mechanism (1') characterized by a microwave mechanism (9') incorporating at least two microwave applicators (10 and 11) that have evenly distributed exit openings that are offset from one another and a decoupling element of a thin metal sheet (27) that is used to decouple said microwaves emitted from said microwave applicators (10 and 11), for externally, contact free heating of said fuser device, and a surface layer for said fuser device that absorbs microwaves.
The fuser device according to Claim 1, wherein focusing stationary reflecting element (21), used for focused reflection, onto said fuser device surface of microwave radiation that has been transmitted into said interior of said fuser device.
The fuser device according to Claim 1, wherein shields (25) can be inserted into space between said microwave mechanism (9') and said surface of said fuser device for said purpose of variably adjusting said effective heating width of said microwave heating mechanism (9').
The fuser device according to Claim 1, wherein a cooling mechanism (23) located in said vicinity of said fuser device, downstream of said nip (5) that is formed by said fuser device and a pressure device (6).
The fuser device according to Claim 4, wherein said cooling mechanism (23) being a component of said microwave heating mechanism (9').
The fuser device according to Claim 1, wherein said surface layer being made of a material whose microwave absorption characteristics vary with said temperature, in particular, such that less microwave radiation is absorbed as said temperature rises.
The fuser device according to Claim 1, wherein said surface layer containing carbon that has a density profile that is a function of depth, for said purpose of improving said absorption characteristics of said surface layer.
The fuser device according to Claim 1, wherein rotating areas of said fuser device, with exception of said surface layer, are made of microwave permeable material.
The fuser device according to Claim 1, wherein said fuser device incorporating a heat insulating layer (30) below said surface layer.