US 3529129 A
Description (OCR text may contain errors)
Sept. 15, 1970 J. D. REES REFLECTION TYPE FLASH FUSER 3 Sheets$heet 1 Filed Feb. 23, 1968 LINE SOURCE (c oss SECTION) INVENTOR. v JA 8 D. RE S BY 3? FIG. 3
Sept. 15, 1970 J. D. REES REFLECTION TYPE FLASH FUSER 3 Sheets-Sheet 3 Filed Feb. 23, 1968 TRIGGER cmcun DC POWER SUPPLY xm m a m I G Em W N M 1 m 6 A W E c fi l an II. Y W- m WA r 2 Hi1 llrfi A E R 2 4 6 I F wlt ll JAMES D. REES United States Patent Office 3,52%129 REFLECTION TYPE FLASH FUSER James D. Rees, Pittsford, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Feb. 23, 1968, Ser. No. 707,612 Int. Cl. G03g 13/20, 15/20; H05]: 3/00 US. Cl. 219-216 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to heating apparatus and, in particular, to apparatus to produce uniform irradiance over an extended area.
More specifically, this invention relates to a xerographic flash fusing apparatus for rapidly and efliciently producing uniform image fixing on a flat support material. In the xerographic process, a plate, generally comprising a conductive backing upon which is placed a photoconductive insulating surface, is uniformly charged and the photoconductive surface then exposed to a light image of an original to be reproduced. The photoconductive surface is caused to become conductive under the influence of the light image so as to selectively dissipate the electrostatic charge found thereon to produce what is known as a latent electrostatic image. The image is developed by means of a variety of pigmented resin materials specifically made for this purpose which are known in the xerographic art as toners. The toner material is electrostatically attracted to the latent image areas on the plate in proportion to the charge concentration found thereon. Areas of high charge concentration become areas of high toner density while correspondingly low charge image areas become proportionally less dense. The developed image is transferred to a final support material, typically paper, and fixed thereto to form a permanent record or copy of the original.
Many forms of image fixing techniques are known in the prior art, the most prevalent of which are vapor fixing, heat fixing, pressure fixing or a combination thereof. Each of these techniques, by itself or in combination, suffer from deficiencies which make their use impractical or diflicult for specific xerographic applications. In general, it has been diflicult to construct an entirely satisfactory heat fuser having a short warm up time, high efliciency, and ease of control. A further problem associated with heat fusers has been their tendency to burn or scorch the suppor material. Pressure fixing methods, whether hot or cold, have created problems with image offsetting, resolution degradation and producing consistently a good class of fix. On the other hand, vapor fixing, which typically employs a toxic solvent has proven commercially unfeasible because of the health hazard involved. Equipment to sufficiently isolate the fuser from the surrounding ambient air must by its very nature be complex and costly.
With the advent of new materials and new xerographic processing techniques, it is now feasible to construct automatic xerographic reproducing apparatus capable of producing copy at an extremely rapid rate. Radiant flash fusing is one practical method of image fixing that will lend itself readily to use in a high speed automatic 3,529,129 Patented Sept. 15, 1970 process. The main advantage of the flash fuser over the other known methods is that the energy, which is propagated in the form of electromagnetic waves, is instantaneously available and requires no intervening medium for its propagation. As can be seen, such apparatus does not require long warm up periods nor does the energy have to be transferred through a relatively slow conductive or convective heat transfer mechanism.
Although an extremely rapid transfer of energy between the source and the receiving body is afforded by the flash fusing process, a major problem with flash fusing, as applied to the xerographic fixing art, has been designing apparatus which can fully and efiiciently utilize a preponderance of the radiant energy emitted by the source during the relatively short flash period. The toner image typically constitutes a relatively small percentage of the total area of the copy receiving the radiant energy. Because of the properties of most copy materials,
as for example paper, most of the energy incident thereon is wasted by being transmitted through the copy or by being reflected away from the fusing area. Another disadvantage associated with the prior art flash fusing apparatus has heretofore been the non-uniformity of image fixing produced. This phenomena is primarily due to the fact that it is difiicult to produce highly uniform irradiance on a large receiving surface, as for example a sheet of paper, from a relatively small source such as a flash lamp.
It is therefore a primary object of this invention to improve heating apparatus.
Another object of this invention is to improve xerographic fixing apparatus.
A further object of this invention is to improve xerographic flash fusing apparatus.
A still further object of this invention is to improve apparatus for rapidly fixing a heat fusable image to a final support material.
A further object of this invention is to efliciently heat fuse xerographic images of varying densities with a pulse flash array in energy.
Yet another object of this invention is to provide method and apparatus for rapidly and uniformly heat fusing a xerographic image to a paper support material.
These and other objects of the present invention are attained by a source of radiant energy capable of emitting energy wavelengths at which the support material is essentially non-absorbent and at which the images are highly absorptive, means to pulse said energy source for a predetermined period of time, and a reflective cavity arranged in respect to the source and the receiving surface whereby the image areas thereon are rapidly, efficiently, and uniformly fixed to the support material.
For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following description of the invention to be read in conjunction with the drawings, wherein:
FIG. 1 is an isometric view of the fuser housing suitable for use in fusing xerographic copy, the fuser having portions thereof broken away to show the internal construction of the apparatus;
FIG. 2 illustrates graphically the parameters important in the present fusing operation plotted against wavelength;
FIG. 3 is a curve showing the distribution in the relative irradiance produced by a line source on a relatively flat surface positioned parallel to said source;
FIG. 4 is a schematic representation of the electrical triggering circuit of the present invention;
FIG. 5 is a cross-sectional end view of the reflective cavity shown in FIG. 1;
FIG. 6 is a schematic representation of the reflective phenomena involved in the flash fuser of the present invention;
FIG. 7 is a sectional end view of a fuser in accordance with the present invention having a plurality of real energy sources.
The apparatus of the preferred embodiment of the present invention, as shown in FIG. I, basically comprises a rectangular shaped cavity, generally referred to as 20. The interior surfaces or walls of the cavity are specular reflectors of high reflectivity. An elongated generally tubular source of radiant energy 21 is supported in the front and rear walls of said cavity at a predetermined distance above the bottom surface 32 by means of brackets 19. Ingress and egress ports 22 and 23 are positioned in two opposing walls 24 and 25, respectively, and run parallel to the axial center line of the elongated lamp. The ports are positioned in the bottom portion of the side walls and permit the image bearing support material 30 to be transported through the cavity adjacent to and in close proximity with the bottom surface 31 of the cavity. The unfixed image bearing support material, which is shown in web configuration in this preferred embodiment, is fed from supply roll 32 over idler roll 33 and passes through the cavity housing by means of the above mentioned ingress and egress ports. The image is fused, as will be explained below, in the cavity. The fixed support material thereafter is guided by a second idler roll 35 to take up roll 36. The take up roll is driven by any suitable drive means, as for example motor means 37, at a predetermined rate.
In the present invention, the radiant energy source and the image bearing support material to be fixed thereto are placed within a reflective cavity or housing which is constructed to functionally approximate an integrating sphere. Basically, the theory of the optical integrating sphere is relatively straight forward and can be explained with a simple example. First, consider a point source of radiant energy which is irradiating an elemental surface at some given distance from the source. Let the irradiation incident upon the surface be of some magnitude (H If, without changing the intensity of the source or the distance that the elemental surface is positioned from the source, the source is now enclosed within the spherical reflector upon which the elemental surface falls, 21 resulting new irradiance (H) at the elemental surface is produced. This new irradiance is a function of the reflectivity of the inside surface of the sphere. If the reflectivity is a function of wavelength, the average reflectivity taken over the emissive bandwidth of the source can be used to find this new irradiance. Multiple reflections inside the sphere have now greatly increased the irradiance at the elemental surface and a gain factor, that is, the ratio of H to H also becomes a function of the average reflectivity of the sphere. The rectangular cavity 20, being an almost totally enclosed reflective cavity, conforms to the above principle.
Circuitry for achieving pulse generation in the preferred embodiment is shown in FIG. 4. A DC power supply 40 is connected across storage capacitor 41' and is grounded on one side 42. The storage capacitor typically has a capacitance of between 100 to 150 microfarads where applied voltages vary between 1800 and 5000 volts and hence electrical energy in the range of 160 to 1900 joules is stored for use when the flash lamp 43 is to be pulsed. The storage condenser is connected to the flash lamp through a variable inductor 44 which is in the range of 150 micro-henrys to 3 mill-henrys and determines the pulse duration produced by the flash lamp. Flash lamp 21 consists of an envelope containing xenon gas and a pair of electrodes at each end which are not electrically connected to each other. Surrounding the glass envelope of the flash lamp is a coil 45 which is connected to a high voltage pulse trigger 46. Approximately 20,000 to 30,000 volts is applied across coil 45 when the pulsing triggering circuitry is actuated. This high surge of current through the coil is such as to electrically couple the electrodes to the flash lamp causing a gas breakdown, which in turn, pulses the flash lamp resulting in a flash of suitable duration as determined by presetting variable inductor 44. In operation the lamp is periodically energized in timed relation to the movement of support material through the cavity.
The entire interior surface of cavity 20 is constructed of or coated with a material which is highly reflective when taken over the emissive bandwidth of the flash lamp, a reflectivity in excess of 0.9 being preferred for greater efficiency. As will become apparent below, the efiiciency of the radiant flash fuser of the present invention is further enhanced if the spectral output of the energy source is such that the wavelength absorptivity of the toner is at a maximum and if simultaneously he wavelength absorptivity of the support material is at a minimum. For illustrative purposes, the design of the present invention will be explained in reference to fixing a toner image to a paper support material. However, it should be quite clear to one skilled in the art that the teachings of the present invention are not limited or restricted to the above mentioned materials. Represented graphically in FIG. 2 are the spectral absorption curves for xerographic toner and white bond paper which are superimposed over a typical spectral emission curve for xenon flash lamp 21. The xenon flash lamp has an emissive spectrum showing a strong continuum between 0.4 and 1.0 micron while the absorptivity of the white bond paper in this range for all practical purposes is non-existing. While the absorptivity of the paper about the missive output of the lamp is at a minimum, the toner essentially acts as a black body and will absorb well in excess of 90% of the energy incident thereon.
It should be understood by achieving the above spectral control in terms of absorptivity for a given bandwidth output of a flash lamp, the net effect is to heat the toner material and not the support material. As previously noted, the support material is transported through reflective cavity 20 in close proximity to the bottom surface thereof. The paper being highly reflective, in effect acts as a planar reflecting surface to the radiation emitted by the source. The radiation not reflected by the paper is transmitted through the paper and re-radiated back *by the highly reflective bottom surface 32 of the cavity so that the net effect is to return a preponderance of the energy not absorbed by the image back into the cavity. Most of the energy returned to the cavity is eventually reflected into the images where it can be utilized in the fixing process. As can be seen, what has herein been described is the principle of the reflecting integrating cavity applied to the xerographic fixing process to produce eflicient utilization of the energy emitted by a flash lamp. It should be further noted that this arrangement, because the spectral control afforded, eliminates the hazards of scorching or burning the support material.
To optimize image fixing in the present invention, the cavity must be highly reflective and also capable of producing uniform irradiance at the surface of the receiving body. The elongated tubular source of energy 21 (FIG. 1) is arranged such that the axial center line of the lamp is substantially perpendicular to the front and rear walls of the cavity, 27 and 28 respectively. Positioning the lamp in such a manner, that is, perpendicular to and between two highly reflective surfaces, in effect, optically produces a source of radiation which is infinitely long. With reference to FIG. 3, it can be shown that the irradiance produced by an infinitely long line source of radiant energy on a flat receiving surface can be determined. If the intensity of the source is J, and the source is positioned some distance Y from the receiving surface, the relative irradiance at the plane measured from a perpendicular from the plane passing through the source is:
and from this relationship it can be further shown that the relative irradiance varies along the flat plane in the X direction according to the relationship where H is the relative irradiance in the plane of the receiving surface at some distance X from the perpendicular previously defined and where Y is the distance the lamp is supported above the receiving surface. It has been determined that in a practical application with a source of finite length, the above relationship when applied aifords a reasonable approximation when the distance Y that the lamp is supported above the plane of the receiving surface is greater than ten times the bore diameter of the lamp. However, it should be clear to one skilled in the art that this invention is not limited to specific arrangement where the lamp is so supported ten diameters above the receiving surface. Although this arrangement does provide a convenient solution, smaller cavities can be designed using more complex equations available for other than line sources (see Walsh, John W., Photometry, Durwood Publications, 3rd Edition).
FIG. 5 is a sectional end view of the reflective cavity of the preferred embodiment shown in FIG. 1. The lamp is positioned midway between sidewalls .24 and and is supported above the surface of the support material 30. By positioning the lamp at a distance above the receiving surface (FIG. 1) at least ten times the bore diameter of the lamp, the irradiance produced by the source at the flat receiving surface closely approximates the theoretical irradiance produced by a line source. For greater efii ciency, top reflecting surface 31 is brought as close as practicable to the top of lamp 21 so that the height of the cavity (h) is minimized.
The lamp 21, positioned between two parallel reflecting surfaces, acts in relation to the support material as an infinitely long line source of radiant energy and therefore the relative irradiance at the surface of the support material will vary in accordance with the previously discussed relationship. However. the lamp is completely enclosed within a highly eflicient reflective cavity capable of producing an infinite number of mirror images of this real source. FIG. 6 shows schematically the real source of energy 21 and a number of mirror images of the real source produced by these planar reflecting surfaces. As can be seen, the total relative irradiance produced at the plane of the support material within the cavity is a summation of the irradiance produced by all the sources, real and apparent. That is, the total relative irradiance at the plane of the receiving surface measured some distance x from the perpendicular passing through the source is now where all the relative irradiances are evaluated at the point x and corrected for the reflectance of the cavity. By applying the above mentioned relationship for relative irradiance to the real and apparent sources, the total irradiance at all points x in the flat receiving plane within the cavity can be found. It is apparent that for some predetermined width W (FIG. 5) of the cavity there exists an arrangement at which the summed value of H is essentially constant or uniform across the cavity. It is possible by varying the parameters of the lamp height (D) and cavity height (H), to produce a summed value of H which is essentially constant or uniform across the cavity in the plane of the receiving surface. It should also be clear that the same results can be obtained by holding (D) constant and varying the width (W) of the cavity.
By positioning the lamp as described above in a cavity of predetermined width, uniform irradiance at the plane of the support material is produced during each flash period. As previously noted the interior of the cavity and the support material absorb very little energy whereas the toner images are highly absorptive. Obviously, a large amount of the emitted energy is absorbed by the toner after a varying number of reflections in the cavity.
Although the present invention has been disclosed in relation to the xerographic fixing process it is in no way so limited. It should be obvious that by properly selecting a lamp having a spectral output matched to the absorptive properties of the receiving material and making the interior surfaces of the cavity reflective to this output, it is possible to rapidly, efiiciently and uniformly heat the receiving material in accordance with the teachings of the present invention. Furthermore, it has been determined experimentally that the teachings of the present invention are not limited to a specular reflecting cavity and the teachings are equally applicable to designing a reflective cavity having difluse reflecting surfaces.
While reference has been made throughout to such elements as a xenon flash tube as the preferred embodiment, it is obvious that any other suitable lamp, flash or otherwise, may be used. Any suitable power supply and pulser may be used and other equivalent electric circuits utilized to produce a pulse of suitable intensity and duration. In similar manner, the teachings of the present invention are not contemplated to be limited to apparatus utilizing a single lamp but are equally applicable to a multi-lamp arrangement as shown in FIG. 7. Altering the number of lamps employed, of course, suggests alterations in the geometry of the reflecting cavity. Toners other than the typical electroscopic toner compositions referred to above may also be utilized. Specific modifications of support materials and toners consisting of different compositions and commonly employed within the terms may also be employed with suitable alterations in the tailoring of the pulsed shape and reflective characteristics of the walls in accordance with the principles set forth in this invention.
While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the scope of the following claims.
What is claimed is:
1. Apparatus for fusing heat fixable powder images to a final support material upon which the powder images are loosely adhering, said apparatus including an elongated lamp having a pre-determined diameter, said lamp being capable of emitting radiant energy concentrated at wavelengths at which said support material is relatively nonabsorptive and at which the powder images are highly absorptive.
a substantially enclosed housing having an interior made up of planar reflecting surfaces, said surfaces cooperating to form a box-like cavity being highly reflective at the wavelengths at which said lamp emits energy,
support means to mount said lamp perpendicularly between two parallel end walls of said cavity in close proximity to the top surface thereof at a distance above the bottom surface of said cavity at least equal to ten times the lamp diameter so that said lamp optically approximates an infinitely long line source of radiation to an object in the plane of said bottom surface,
said lamp being positioned in spaced relation with the interior side wall surfaces of said cavity so that the real source of radiation and the apparent sources of radiation reflected by said side wall combined to produce substantially uniform radiation in the plane of said bottom surface,
means positioned exterior said cavity to advance an image bearing support material along a path of travel in juxtaposition to bottom surface of said cavity, and
means to activate said lamp for a period wherein images carried on the support are irradiated by direct and reflected radiation sufficient to fix the powder image uniformly to the support surface.
2. The apparatus of claim 1 wherein the interior surfaces of said cavity are specular reflecting surfaces.
3. The apparatus of claim 1 wherein the interior surfaces of said cavity are difluse reflecting surfaces.
4. The apparatus of claim 1 wherein said elongated lamp comprises a xenon flash lamp having an emissive spectrum showing a strong continuum between 0.4 and 1.0 micron.
5. The apparatus of claim 4 further including the means to periodically energize said source of radiation in timed relation to the movement of said image bearing support material.
6. The apparatus of claim 5 having a plurality of elongated lamps placed in parallel relation of distance above said bottom surface at least equal to ten times the diameter of one of said lamps.
Hoso et al 2l9-388 Silberman 219-388 Smith et a1. 219-354X Bungay 219-216 X Carlson 219-216 X Young et a1 250-652 X Verderver 219-216 Hull et al. 219-388 X 15 JOSEPH V. TRUHE, Primary Examiner P. W. GOWDEY, Assistant Examiner U.S. Cl. X.R.