US2965868A

US2965868A - Resistance type heating elements

Info

Publication number: US2965868A
Application number: US797143A
Authority: US
Inventors: Rolf W Eichler
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1959-03-04
Filing date: 1959-03-04
Publication date: 1960-12-20
Anticipated expiration: 1977-12-20
Also published as: DE1810418U; GB948916A

Description

Dec. 20, 1960 R. W. ElCHLE-R RESISTANCE TYPE HEATING ELEMENTS ZSheets-Shet'l' Filed March 4. 1959 INVENTOR. I Rolf W. Eichler A TTOQNE V Dec. 20, 1960 R. w EICHLER RESISTANCE TYPE HEATING. ELEMENTS 2 Sheets-Sheet 2 Filed March 4, 1959 v QQQ mvsmox Rolf W. Eichler 0 u w mt g a NQ v MN i mm 2 xx mm 8x 4V] \v y .3 \r RP kl \W a r I v I} a WK. 5 G v 3 v 3 H 7 R a Q a 3 2 mm. mvm wk w wt 3 (KR v A T TORNE Y United States Patent ce 2,965,868 RESISTANCE TYPE HEATING ELEMENTS Rolf w. Eichler, Fairport, N.Y., assignor to Haloid Xerox This invention relates to improvements in resistance type heating elements and, particularly, to an improved apparatus for fixing xerographic powder images.

More specifically, the invention relates to an improved heating element of low thermal mass that can be supported in parallel, closely spaced relation to a work piece. Although the invention is considered to have general application for this purpose, it is particularly useful in the field of xerography and has an important application in the fusing of resinous powder images producedby electrophotography or xerography onto sheets of paper and the like to which the powder images have been transferred after they have been formed by deposition of powder on an electrostatic latent image. Therefore, for convenience of illustration, the invention is described with reference to its use as a heat fuser for xerographic powder images. However, it is to be understood that it may be employed with equal facility in other fields.

In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, issued October 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure dis charges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided developing material or toner which is brought intov surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferredto a support surface such as paper to which it may be fixed by any suitable means.

One of the methods is common use for developing the electrostatic latent image is described in Walkup Patent 2,618,551 and is known as cascade development, and is in, general use for line copy development. In this technique, the powder or toner is mixed with a granular carrier material, and this two-component developer is poured or cascaded over the plate surface. The function of the carrier material is to improve the flow characteristics of the powder and to produce, on the powder, by triboelectrification, the proper electrical charge so that the powder will be attracted to the image. More exactly, the function of the carrier material is to provide the mechanical control to the powder, or to carry the powder to an image surface and, simultaneously, to provide homogeneity of charge polarity.

In the Carlson patent it is noted that a variety of types of finely divided electroscopic powders may be employed for developing electrostatic latent images. However, as the art of xerography has progressed, it has been found preferable to develop line copy images with a powder or toner formed of any of a variety of pigmented thermoplastic resins that have been specifically developed for the 2,965,868 Patented Dec. 20, 1960 for producing dense images of high resolution and to have characteristics to permit convenient storage and handling. Such developing materials are compounded to permit them to be fixed to the surface of a transfer material either by heat fixing or vapor fixing techniques, in accordance with the particular application in which they are employed, that is, the individual particles of resin (toner) soften and coalesce when heated or plasticized by solvent, so that they become sticky and readily adhere to the surface of the transfer material.

One of the important applications of the process of xerography comprises its use in automatic copying machines for general office use wherein the powder images formed on a xerographic plate are transferred to cut sheet paper and then fixed thereon by heat fusing. In order to fuse resinous powder images formed of the powdered resins now commonly used, it isnecessary to heat the powder and the paper to which it is to be fused to a relatively high temperature, such as approximately 325 F. It is undesirable, however, to raise the temperature of the paper substantially higher than 375 F. because of the tendeney of paper to discolor at such elevated temperatures. v

To keep powder requirements at a minimum and in order to obtain rapid warming of the heating element of the heat fuser it is customary to use a conventional heatingelement in the form of a coiled resistance wire element. However, coiled wire heating elements of the length necessary to overlap large sheets or webs of paper tend to sag in the center clue to expansion when heated, thereby preventing the use of the coiled wire heating elements in uniform, closely spaced relation to the sheets of paper. This sagging of the center portion of a relatively long coiled wire heating element when heated brings the center portion of the heating element closer to the paper than the remainder of the heating element, thereby causing uneven heating of the paper and possible charring of the paper.

In electric ovens, kilns, furnaces, and the like of the prior art, the heating wires are supported on suitable refractory or ceramic material support elements generally arranged along the side walls of the heating chambers. Supports of this type are unsuitable to support a coiled resistance element made of small diameter wire as desired for the fusing of powder images.

Another form of heating element in common use comprises a resistance wire wrapped in a helical groove in a cylindrical ceramic or quartz tube of relatively large diameter. This arrangement of a resistance wire within a support tube has certain disadvantages in that it has a relatively high thermal mass and a relatively high heat transmission lag although it may adequately support the resistance wire.

It is, therefore, the principal object of this invention to improve the construction of resistance type heating elements to attain a self-supporting apparatus having low thermal mass and low heat transmission lag.

Another object of the invention is to provide a novel self-supporting heating element which may be positioned in closely spaced parallel relation to paper without any danger of the heating element deforming when heated to thereby cause it to come into contact with the paper.

It is still another object of the invention to provide a novel heating element that will provide uniform heat.

These and other objects of the invention are attained by means of a heating element comprising the combination of a heat resistant monofilar support element, preferably a small diameter quartztube, encircled by a fine coil of high electrical resistance wire, such as, for exmeanness ample, Nichrome wire, to provide a heating element of low thermal mass which will both heat up and cool off rapidly.

- For a better understanding of the invention as well as other obiects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

Fig. 1 illustrates schematically a preferred embodiment of anelectrophotographic apparatus adapted for automatic operation, and incorporating a heat fuser having a heating element constructed in accordance with the invention;

Fig. 2 is a perspective view of a preferred embodiment of a heat fuser with parts partially broken away to show the internal structure of heat fuser including the internally supported heating coil in accordance with the in vention;

Fig. 3 is a detail sectional view taken along line 3-3 of Fig. 2;

Fig. 4 is a detail enlarged sectional view of the heating element of the heat fuser taken along line 4-4 of Fig. 3; and

Figs. 5 and 6 are views of modifications of the heating elements of the invention and their mounting in a heat fuser.

Although it forms no part of the subject invention, there is shown schematically in Fig. l a continuous xerographic apparatus for the purpose of illustrating a suitable environment for a heat fuser having mounted therein heating elements of the subject invention.

As shown in Fig. 1, the Xerographic apparatus comprises a xerographic plate including a photoconductive layer or light-receiving surface on a conductive backing and formed in the shape of a drum, generally designated by numeral 10, which is mounted on a shaft 11 journaled in' a frame (not shown) to rotate in the direction indicated by the arrow to cause the drum surface sequentially to pass a plurality of xerographic processing stations. Drum is rotated at a constant rate through the drive action of synchronous motor 12.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally, as

follows:

A charging station, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station, at which a light or radiation pattern of copy to be reproduced is projected onto the drum surface to dissipate the drum charge in the exposed areas thereof and thereby form a latent electrostatic image of the copy to be reproduced;

A developing station, at which a xerographic developing material including toner particles having an electrostatic charge opposite to that of the electrostatic latent image are cascaded over the drum surface, whereby the toner particles adhere to the electrostatic latent image to form a xerographic powder image in the configuration of the copy to be reproduced;

A transfer station, at which the xerographic powder the drum surface is exposed to a relatively bright light source to effect substantially complete discharge of any source and are substantially enclosed within ashielding member.

Next subsequent thereto in the path of motion of the xerographic drum is an exposure station. This exposure station may be one of a number of types of mechanisms or members such as desirably an optical projection system 14 or the like designed to project a line copy image onto the photoconductive surface of the xerographic drum from an original as is well known in the art.

Adjacent to the exposure station is a developing station in which there is positioned a developer housing 15 including a lower or sump portion for accumulating de veloping material 16. A bucket type conveyor 17 having a suitable driving means, such as motor 18, is used to carry the developing material to the upper part of the developer housing where it is cascaded down over a hopper chute 21 onto the xerographic drum.

As the developing material is cascaded over the xerographic drum, toner particles are pulled away from the carrier component of the developing material and depos ited on the drum to form powder images, while the partially denuded carrier particles pass off the drum into the developer housing sump. As toner powder images are formed, additional toner particles must be supplied to the developing material in proportion to the amount of toner deposited on the drum. For this purpose there is provided a container 22 for toner 23 to be added to the developing material as needed, the toner being added at a rate determined by control gate 24.

After development, the image thus formed is transferred to support surface web 25, which may be of paper or any other suitable material. Web 25 is continuously transported from supply spool 26 to take-up spool 27 by paper handling apparatus 28, which may be of a type disclosed in Crumrine et al. Patent 2,781,705. Paper handling mechanism 28 includes a synchronous motor 31 driving take-up spool 27 and-drive rolls 32, while guide rolls 33 and 34 serve to direct web 25 into contact against a powder image on the surface of drum 10. Electrostatic transfer unit 35, which may be of a type similar to unit 13, generates an electrostatic charge to electrostatically attract the powder image from the surface of drum 10 to web 25.

Thereafter, image-bearing web 25 is transported beneath heat fuser 40 of the type described in detail hereinafter, whereby the developed and transferred xerographic powder image on the web 25 is permanently fixed thereto.

The next and final station in the device is a drum cleaning and discharge station where any powder remaining on the xerographic drum after transfer is removed by rotating brushes and the xerographic drum is flooded with light to cause dissipation of any residual electrical charge remaining on the xero-graphic drum. The residual powder image on the surface of drum 10 after transfer is removed by brushes 36 driven by motor 37 after which residual electrostatic charge is dissipated by illumination from lamp 38.

' Referring now to Figs. 2 to 4, inclusive, there is shown an embodiment of a heat fuser using a preferred embodiment of a heating element of the invention.

Structurally the heat fuser 40 consists of fuser casing 65 having a top wall 66, divergent side walls 67, part of which terminate in flanges 68 formed parallel with the top wall 66. To insulate electrical elements of the fuser from the casing and to support the heating elements of the fuser, supports 69 made of any suitable electrical insulating material, such as ceramic material, are rigidly connected to flanges 68 by bolts 71 and nuts 72, the supports 69 extending transversely across the fuser casing and spaced inwardly from the ends thereof. End plates 73 for the fuser casing are attached by screws 74 and nuts 75 to the supports 69, spacers 76 being used for alignment of the end plates 73 with the ends of the fuser casing.

millimeters, (.07874 inch).

' elementflti is formed of 28 gauge (.0126 inch) Nichrome wire, the coil portion-of the resistance element being approximately 11% inches long with a coil inside diameter of approximately Ms inch. A coiiof wire of this size and lengthisnbt self-supporting especially when heated to incandesccnce. The quartz tube supporting the coils of the resistance element has an inside diameter of one millimeter (.03937 inch) and an outside diameter of two This tube of quartz although having a low'thermal mass adequately supports the coils of the resistance element even when heated to incan- .desccnce and is not affected by thermal shock. -Making.

the outside diameter of the quartz tube considerably smaller than the inside diameter of-the coils of the resistance element limits the area of the coils that come into direct thermal contact with the quartztube, as seen in Fig. 4.

The heating elements 77, of which there are five shown, are supported by supports 69, the ends of the quartz tubes of the heatingnelements extending into apertures 81 formed in the supports. Due-to its high mechanical strength,'and low coefiicient of expansion, quartz tubing is a suitable support material forthe coils of the resistance element, whereas ceramics and other previously need support materials do not lend themselvesjtosuch an application because of the coil lengths and diameters used to obtain low thermal mass.

The ends of the resistance elements 77 are connected by screws 82 to conducting bars 83 secured to the sup-' ports '69 at opposite ends of the heat fuser by fasteners 84 passing through the conductor bars 83 and supports 69, the fasteners threadingly engaging nuts 75 to which may also be secured the terminal ends of a suitable elw trical power source (not shown). It is noted that the supports are'relieved opposite each screw-82 to prevent the screws from binding against the supports; and to insure electrical contact, the heads of fasteners 84 are welded to the conductor bars.

Since heat will be radiated from the heating elements '77 in all directions a reflector plate 85having polished surfaces is positioned above the heating elements to. re:

fleet heat back toward the heating elements and a sheet of transfer material passing under the heating elements.

To permit removal of the reflector plate for cleaning or replacement it is supported between the heating elements and the top wall of fuser casing 65 by the engagement of the T' buttons 86' secured to the reflector plate with a bracket 87 secured to the top wall 66 or the fuser casing 65 whereby the reflector plate may be readily removed to be eithercleaned or replaced.

Threaded bosses 88 fixed to the top-wall of the fuser casing, as by welding, are used for attaching the heat fuser by screws (not shown) to a suitable bracket or frame of the xerographic machine. In the embodiment of a heat fuser just described the quartz tubing of the heating element is supported at its ends in block-like supports 69. In Figs. and 6 two alternate methods of mounting the quartz tubing or rods are shown.

In Fig. 5 thereis disclosed an arrangement for a heat fuscr where supports 69a in the form of extruded metal U-shaped channels are secured by any suitable means, such as. welding, directly to a reflector plate BSasimilar to reflector plate 85 of Figs. 2 and 3, inclusive. The

quartz tubing or rod 79a of the heating element 77a is supported in apertures 81a in the supports 690, the ends of the resistance element 78a passing through an insulator tube 90 to be secured to a voltage supply source, as usual 5 in this class of device. The insulator tube 90 is also pret- -e'rably made-of quartz and is supported in apertures 89 is-the side plates of the support 6%.

In the embodiment disclosed in Fig. 6 the quartz tube. 7%, of the heating element 77 is supportw in apertures 10 81b in'depending thin-walled metal supports 69b secured to deflector plate 85!). In this modification the quartz tube 79b acts both as the support for-the resistance element 78b and as an insulator for. the ends of the resistance element since the ends of the resistance element are threaded through openings 91 formed through the wall of tube 79b at opposite ends thereof but inboard' of the supports 69b and the ends of the resistance element pass through the interior-of the tubing. Thus, the exposed wire of the resistance element, is insulated from the metal support by the walls of the quartz tubing.

By using a monofilar quartz-support of relatively small diameter in comparison to the inside diameter of the coiled resistance-wire, the heating element has low thermal mass and the resistance wire has substantially point contact with the monofilar quartz support thereby -permitting rapid heating and cooling of the resistance coil. These operating characteristics are important in a heat fuser, because the primary concern in this type of structure especially for use in a Xerogra'phic machine is the necessity for a low thermal mass heating element to. pet'- mit the inset to be brought to operating temperature quickly thereby eliminating a long stand-by period before the machine can be operated, and also toallow rapid cooling-ofthe element to prevent charring of a support material and possibly a fire in the event the machine is stopped as a result ofapower failure'with a support material, such as paper, stopped beneath the fuser. Using a quartz rod, a floating support can be provided for the tube by loosely inserting the ends of the rod into suitable apertures in end supports.

In each of the modifications disclosed there is used a very simply fabricated heating element which can be used for mounting in either an electrically conductive support or an electrically non-conductive support. 7 By this arrangement there is provided a heating element of low thermal mass that can readily be supported in parallel, closely spaced relation to a paper web or sheet. The quartz tube has a high melting point relalive-"to thetemperature generated by the resistance element so that it functions without noticeable sag even though the resistance wire and the tube is heated to incandescence. In addition, this arrangement permits the use of a relatively fine resistance wire whereby the heatingelement may be brought to its desired operating tem: perature in a sufiiciently short time to eliminate'the need for a supplemental rapid heating circuit. Although quartz tubing is preferred, it is obvious that any other material having similar physical characteristics of low mass, low thermal conductivity, high thermal shock resistance, and low coefficient of expansion may be used to support the resistance wires.

'While the present invention as to its objects and advantages, as described herein, has been carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. In a xerographic reproducing apparatus for forming fusible powder images on a support surface and including means for advancing such a support surface at a predetermined rate relative to a {using device located at a fusing station, the improvement in fusing device constructionincluding a pair of support brackets connectedspaced relation to the path of movement of such a nupport surface, a quartz tube suspended between the nupport brackets in substantially uniform spaced relation to thesurface of such a support surface, and a coil of resistance wire encircling the quartz tube to an extent sub stantially coterrninous with the width of such a support surface, said quartz tube forming a continuous support for said resistance wire coil. p

2. In a xerographic reproducing apparatus for forming fusible powder images on a support surface and including means for advancing such a support surface at a predetermined rate relative to a fusing device located at a fusing station, the improvement in fusing device construction comprising a pair of support brackets connected to the reproducing apparatus at the fusing station in spaced relation to the path of movement of such a support surface, a plurality of quartz tubes suspended between the support brackets in substantially uniform spaced relation to the surface of such a support surface and in substantially parallel relation to each other, and separate coilsof resistance wire encircling each of the quartz tubes to an extent substantially coterminous with the width of such a support surface, said quartz tubes forming continuous supports for said resistance wire coils.

3. In a xerographic reproducing apparatus for forming fusible powder images on a support surface and including means for advancing such a support surface at a predetermined rate relative to a fusing device located at a fusing station, the improvement infusing device construction comprising a pair of support brackets connected to the reproducing apparatus at the fusing station in spaced relation to the path of movement of such a support surface, a plurality of quartz tubes supported by the support brackets in substantially uniform spaced relation to the surface of iuch a support surface andin substantially parallel relation to each other, said quartz tubes extending through openings in the support brackets whereby to permit unimpeded expansion and contraction of the tubes, and separate coils of resistance wireencircling each of the quartz tubes to an extent sub-- stantially coterminous with the width of such a support I surface, said quartz tubes forming continuous supports for said resistance wire coils.

Bastian July 9, 1912 Mohn Nov. 10, 1953