Xerographic fuser roller

Abstract

An arrangement for fusing dry xerographic toner to a paper sheet by passing the sheet between the rollers, at least one of which is heated. The heated roller is tapered along its length in concave configuration, so that the tendency of the paper to wrinkle is substantially eliminated.

Description

This invention relates to xerographic copying apparatus and processes, and more particularly to an improved apparatus for fusing xerographic toner to a support sheet.

In the practice of xerography, an electrostatic image of a desired pattern is formed on an insulating surface. This is usually accomplished by providing a photoconductive insulating material affixed to a conductive backing, uniformly electrostatically charging the photoconductive surface (typically by a corona charging technique), and subsequently exposing the charged photoconductive surface to an electromagnetic radiation pattern (usually a visible light pattern) of the image to be reproduced. The electromagnetic radiation pattern discharges the photoconductive surface in the areas where the surface is irradiated, thus forming an electrostatic charge pattern on the photoconductive surface corresponding to the pattern of the desired image.

In order to render the image defined by the electrostatic charge pattern visible and permanent, the photoconductive surface is contacted with thermoplastic microscopic particles which may be in the form of a fine powder, the particles having been provided with an electrostatic charge opposite in sign to the charge remaining on those portions of the photoconductor which have not been discharged (or which have only been partially discharged) by the incident electro-magnetic radiation. As a result, these microscopic particles (commonly known as "toner") adhere to the photoconductor only in those areas which retain an electrostatic charge, i.e. those areas which have not been irradiated.

The pattern of thermoplastic toner particles which corresponds to the pattern of the desired image is subsequently either (i) fused to the photoconductive surface to form a permanent image or (ii) transferred to another surface, which may comprise ordinary paper, and subsequently fused thereto.

A common fusing arrangement presently utilized employs a heated roller which is maintained at a temperature above the melting or softening point of the toner material. The supporting sheet having the desired pattern of toner particles thereon is brought into contact with the heated roller, which softens or melts the toner particles and presses them against the supporting sheet.

This heated roller fusing arrangement performs reasonably well when relatively small paper supporting sheets are used, such as the common 8 1/2 by 11 inch letter size or 8 1/2 by 14 inch legal size sheets. However, it has been found that wrinkling of the paper results when sheets of substantially greater length and width are passed through the fusing apparatus.

While the reason for this wrinkling effect has not been conclusively established, it is believed to be due primarily to lateral shrinkage of the paper due to evaporation of moisture as the paper passes through the fusing apparatus.

Accordingly, an object of the present invention is to provide an improved fusing apparatus of the heated roller type in which wrinkling of the supporting sheet is substantially reduced or eliminated.

As herein described, there is provided xerographic fusing apparatus, comprising a first generally concave cylindrical roller mounted for rotation about its longitudinal axis, and a second generally cylindrical roller mounted for rotation about its longitudinal axis. Means is provided for disposing the rollers in juxtaposition with their axes mutually parallel so that the peripheries of the rollers engage each other. Means is also provided for heating at least one of the rollers.

In the drawing:

FIG. 1 illustrates the wrinkling effect introduced by fusing apparatus according to the prior art;

FIGS. 2a and 2b illustrate cross-sectional and side views respectively of fusing apparatus according to a preferred embodiment of a present invention; and

FIGS. 3 and 4 illustrate the theoretical principle of operation of the present invention.

As shown in FIG. 1, a sheet of plain paper 10 contains a pattern of xerographic dry thermoplastic toner particles 11 thereon, the toner particles 11 being electrostatically adherent to the paper sheet 10 and arranged in a pattern corresponding to the image of a desired copy. The toner particles 11 have been disposed in the desired pattern by a suitable xerographic apparatus (not shown) which is well known in the art.

The fusing apparatus shown in FIG. 1 comprises an upper relatively rigid heated fuser roller 12 and a lower relatively compliant unheated pressure roller 13. The rollers 12 and 13 are disposed with their longitudinal axes mutually parallel and spaced apart a distance such that the periphery of the fuser roller 12 engages and slightly deforms the periphery of the pressure roller 13. An internal heating element within the fuser roller 12 maintains the surface temperature thereof at a value (typically on the order of 300.degree. to 400.degree. F.) above the melting or softening point of the toner particles 11.

The paper sheet 10, supported by a suitable web transport (not shown), is passed between the fuser roller 12 and pressure roller 13 in the direction indicated by the arrow 14. As the paper sheet passes between the rollers, the heated fuser roller 12 contacts the toner particle-covered surface of the paper sheet 10 and fuses the toner particles 11 thereto. In order to prevent toner particles from adhering to the rollers 12 and 13, the rollers are preferably covered with an outer layer of a suitable low surface-free-energy non-stick material such as polytetrafluoroethylene.

While relatively small paper sheets 10, having dimensions on the order of 8 1/2 inches wide (measured in a direction perpendicular to the arrow 14) by 11 or 14 inches long (i.e. measured in the direction of the arrow 14) pass between the rollers 12 and 13 with little or no distortion, it has been observed that extensive wrinkling of somewhat larger sheets, such as those 14 inches wide and 17 inches long, occurs when the sheets are passed between the rollers 12 and 13.

These wrinkles appear as long creases 15 oriented in the direction of paper travel, i.e. the direction of the arrow 14. The creases 15 begin as narrow folds in the paper and gradually widen as the length of the paper passed between the rollers 12 and 13 increases. Attempts to substantially and consistently reduce or eliminate the creases 15 by substituting various types of paper for the paper sheet 10 have proven unsuccessful.

On the other hand, when the fusing arrangement shown in FIG. 1 was replaced by the improved apparatus shown in FIG. 2, the creases 15 disappeared, even when paper sheets 14 inches wide and 17 inches long were passed through this fusing arrangement.

The major difference between the fusing apparatus shown in FIG. 2 and that shown in FIG. 1 is the substitution of a concave fuser roller 16 for the cylindrical fuser roller 12 shown in FIG. 1.

As seen in FIG. 2a, the improved fuser roller 16 comprises a hollow metal generally cylindrical core 17 formed of a suitable heat conductive material such as copper. The outer surface of the copper cylinder 17 has a sand-blasted finish with a nickel layer 18 plated thereon. Disposed on the nickel plated surface of the cylinder 17 is an outer coating or layer 19 comprising polytetrafluoroethylene.

Typically, the thicknesses of the copper core 17, nickel layer 18 and outer sheath 19 may be on the order of 0.250, 0.0005 and 0.002 inches, respectively.

In order to heat the fuser roller 16 to the desired xerographic toner fusing temperature, a radiant heating element 20 is situated on the longitudinal axis 21 of the fuser roller 16 within the hollow interior thereof. The heating element 20 preferably comprises a quartz infrared heating lamp.

The cross section of the fuser roller 16 is circular. However, the diameter of the fuser roller tapers along the length thereof, so that the side view of the fuser roller 17, as shown in FIG. 2b, presents a generally concave appearance, with the fuser roller 16 having a maximum diameter d.sub.1 at each end thereof and a minimum diameter d.sub.2 at the center thereof. Preferably, the cross section of the fuser roller 16 is tapered in a linear fashion between the center and the ends thereof.

Although tests were not conducted with various curved tapers, it is believed that the linear taper provides optimum performance. In addition, the linear taper roller is simpler and more economical to manufacture than a curved taper roller.

Typically, the fuser roller 16 may have a length L on the order of 15 inches, a maximum diameter d.sub.1 on the order of 2.3600 inches and a minimum diameter d.sub.2 on the order of 2.3525 inches, the difference between d.sub.1 and d.sub.2 being 0.0075 inches, corresponding to a linear taper of 0.001 inches per inch of length.

It is highly desirable that the taper of the fuser roller 16 be symmetrical with respect to the center thereof, since any asymmetry in the taper will result in a tendency for paper sheets passed through the fusing arrangement to drift to one side or the other.

Disposed in juxtaposition with the relatively rigid fuser roller 16 is a relatively compliant pressure roller 22, the longitudinal axis 23 thereof being spaced from the longitudinal axis 21 of the fuser roller 16 by a distance such that the peripheries of the rollers 16 and 22 are in contact along the entire lengths thereof. Since the fuser roller 16 is tapered in a generally concave manner, it is necessary for the pressure roller 22 to be deformed more at its outer ends than at the center thereof, in order to insure full line contact between the two rollers.

Preferably, the pressure roller 22 comprises a metal core 24, preferably steel, surrounded by a cylinder 25 comprising an elastomeric material such as silicone rubber, and an outer sheath or jacket 26 comprising polytetrafluoroethylene. The diameter of the steel core 24 may typically be on the order of 2.75 inches, with the diameter of the silicone rubber cylinder 25 and the thickness on the outer sheath 26 preferably being on the order of 3.50 and 0.020 inches, respectively.

In order to obtain sufficient heat transfer from the fuser roller 16 to a paper sheet 39 which may be passed between the rollers 16 and 22 in the direction of the arrow 27 to properly fuse xerographic toner disposed on the upper surface thereof (i.e. the surface contacted by the heated fuser roller 16), the deformation of the pressure roller 22 should preferably be such that the length a of the contact region of the peripheries of the rollers 16 and 22 is on the order of 0.3 inches.

Positioned adjacent to the fuser roller 16 and pressure roller 22 is a paper transport 28, which is constructed in a manner well known in the art, and comprises a porous endless belt 29 which is moved about the rollers 30 in a counterclockwise direction, so that the upper surface of the belt 29 moves in the direction of the arrow 27. Paper sheets having toner particles on the upper surfaces thereof are retained in contact with the upper portion of the belt 29 by vacuum resulting from continuous removal of air from within the belt 29 by a suitable vacuum source (now shown). Movement of the belt 29 then conveys the paper sheets 39 toward the region of contact between the rollers 26 and 22, the sheets thereafter being moved through the fusing apparatus by frictional contact with the fuser and pressure rollers, which are mounted for rotation about their longitudinal axes 21 and 23 in the directions indicated by the arrows 31 and 32, respectively.

While the precise reason why the fusing arrangement shown in FIG. 2 is able to provide fusing of relatively wide and long paper sheets without wrinkling thereof has not been conclusively established, it is believed that the concave design of the fuser roller 16 results in subjecting the paper sheet to outward lateral stress, thus effectively tensioning the paper and permitting it to shrink uniformly in the lateral direction. The manner in which the concave design of the fuser roller 16 is believed to accomplish this result will be best understood by reference to FIGS. 3 and 4 of the drawing.

FIGS. 3a and 3b illustrate side and end views respectively of a convex roller 33. It is well known that when a web travels across such a convex roller, the web tends to center itself on the roller. That is, webs 34 and 35 respectively, moving in contact with the periphery of the roller in a direction into the drawing, are subjected to lateral forces which tend to move the webs 34 and 35 toward the center or point of greatest diameter of the roller 33. It is believed that this tendency is due to the fact that the peripheral speed of the roller increases toward the center thereof, thus causing the side of each web closest to the center to be drawn toward the center.

An experimental fuser roller having the convex configuration shown in FIG. 3 was fabricated and tested. It was found that the creases caused by this configuration were much worse than those caused by a precisely cylindrical fuser roller. This effect is believed to be due to the exertion of forces tending to push each area of the web toward the center thereof, thus causing a "bunching" of the web.

On the other hand, the concave web 36 shown in side and end views respectively in FIGS. 4a and 4b has a cross section such that the peripheral speed of the roller increases in a direction toward the outer ends thereof. This profile tends to cause individual small webs 37 and 38 thereon to drift toward the outer ends. Thus each region of a large single web disposed on the roller 36 is subjected to a lateral force away from the center of the roller, thus effectively subjecting the web to lateral tension and resisting any tendency toward formation of creases when the web shrinks laterally due to evaporation of moisture during the fusing process. Inspection of FIG. 4 makes it evident that it is important for the concave profile of the roller to be symmetrical about its center, since any asymmetry will result in unequal lateral outward forces, and cause the web to drift to one side or the other of the roller.

Claims

I claim:

1. Xerographic fusing apparatus, comprising:

a relatively rigid generally concave hollow cylindrical fuser roller mounted for rotation about the longitudinal axis thereof, the diameter of said roller being linearly and symmetrically tapered along said axis with respect to the center thereof, from a maximum value at each end to a minimum value at the center, said fuser roller comprising a metal cylinder having an outer layer comprising polytetrafluoroethylene disposed thereon;

a radiant heating element disposed along said longitudinal axis within the interior of said fuser roller for heating said fuser roller to a desired xerographic toner fusing temperature;

a cylindrical pressure roller comprising a metal core surrounded by a cylinder comprising elastomeric material, and an outer layer comprising polytetrafluoroethylene disposed thereon, said pressure roller being disposed in juxtaposition with said fuser roller, said pressure roller being mounted for rotation about the longitudinal axis thereof,

said longitudinal axes being mutually parallel and spaced apart a given distance such that the peripheries of said rollers engage each other along the entire length of the line of contact thereof,

said fuser roller having a taper from the center to each end thereof such that the diameter of said fuser roller increases on the order of 0.001 inch per inch of length thereof; and

transport means for feeding paper sheets having xerographic toner thereon between said rollers.