Fixing Roller and Its Manufacturing Process

1. Field of the Invention:

The present invention relates to a fixing roller and said fixing roller manufacturing process for a heat roller fixing device used in electrophotographic copying machines, facsimiles, printers and like machines using an electrophotographic process.

2. Description of the Prior Arts:

Conventional electrophotographic copying machine, heat roller fixing devices are known, like the device disclosed in Tokkai Sho 59-111177.

Conventional fixing devices comprise a heating roller incorporating a heater which have a separation layer on the electrically conductive core member. A pressure roller makes pressure contact with said heating roller, and an insulating roller on an electrically conductive core member. In such devices, copy paper, having a charged toner image thereon, pass between both rollers with the toner-bearing surface facing the heating roller, whereby the toner image is fixed onto the copy paper.

With this type of heat roller fixing device having a heat-fixing roller, the heated roller surface has sufficiently good release properties to prevent thermal adhesion of the toner without necessitating the application of silicone oil or a like offset preventing agent, or with use of only a very small amount of such agent. Thus, the copy paper feels pleasant to the touch.

However, the separation surface layer of the heated roller can be triboelectrically charged by the insulating surface layer of the pressure roller or by contact with the copy paper and as such is liable to electrostatically attract the toner and permit offset. This type of triboelectrically-induced offset cannot be prevented by the application of a offset preventing agent.

While it is known, as disclosed in Tokkai Sho 57-150869, that electrical resistance is reduced when a large amount of carbon black or other conductive substance is incorporated in the primer layer to prevent such charging if a large amount of carbon black, or like substance, is incorporated in the primer layer, the adhesive property of said primer layer is reduced and its essential function is lost. A further limitation is the occurrence of cracking in the thin primer layer caused by a large content of carbon black or like substance.

Another limitation of conventional devices employing resin powder coatings which incorporate large amounts of conductive substances applied over the primer, is the impossibility of effectively preventing triboelectrically-induced offset.

The primary object of the present invention is to provide, without the aforesaid limitations, a fixing roller for a heat roller fixing device, and a manufacturing method, for said fixing roller wherein triboelectrically-induced offset, caused via electrostatic attraction of the charged toner to the outer peripheral surface of the heating roller, can be prevented.

The object of the present invention is accomplished by providing a copying machine with a fixing roller comprising at least an electrically conductive core member and a polytetrafluoro ethylene perfluoro alkoxy ethylene co-polymer resin (PFA resin) wherein a PFA dispersion layer is obtained by dispersing the conductive material in a dispersing agent.

FIG. 1 is a side elevation view showing the heat roller fixing device incorporating a fixing roller of the present invention;

FIG. 2 is an enlarged view showing the surface processing sequence for the fixing roller of the present invention; and

FIGS. 3 to 5 are graphs showing the offset ranks obtained via the prescribed grinding of the roller and a graph showing the weight relationships of a third layer of the roller in an embodiment of the present invention.

FIG. 1 shows a sectional view of a heat roller fixing device having a fixing roller according to the present invention, said fixing device comprising a heating roller 4 incorporating a heater 3 and having a grounded conductive core member 1 with a primer layer 2, and a resin separation layer 11 successively layered thereon. A pressure roller 7 makes pressing contact with the heating roller 4 and is comprised of an insulating layer 6 on a conductive core member 5. Copy paper P having a charged toner image T formed thereon is passed between both rollers, 4 and 7, with the toner-bearing surface facing the heating roller 4, whereby toner image T is fixed onto copy paper P. Copy paper P bearing the toner image T is separated from the roller via separating pawl 8 and proceeds to guide plate 9 from which it is discharged via paper discharge roller 10.

Examples of materials for the electrically conductive core member, used with the heating roller and pressure roller, are aluminum, aluminum alloy, iron alloys such as stainless steel, and other metals.

The primer layer 2 of the heating roller is formed from a solution-type primer composed chiefly of fluorine resin, which is commercially available as an adhesive priming agent for the aforesaid iron alloys, aluminum alloys and like metals. Examples of useful primers are COOKWEAR A PRIMER WHITE 459-882 (proprietary name, product of Du Pont Co., Ltd., Japan) and MP902YL (proprietary name, product of Mitsui Phlorochemical Co., Japan), etc. Carbon black or metallic powder may be used as the electrically conductive material incorporated into this primer layer; the carbon black used may be furnace black, channel black or thermal black. Examples of such products commercially available are KETCHEN BLACK EC (proprietary name, product of Lion Yushi Co., Ltd., Japan), SPECIAL BLACK 4 (proprietary name, product of Degussa Co., Ltd., Japan), CARBON BLACK MA-100 and MA-8 (proprietary names, products of Mitsubishi Chemical Industries, Ltd., Japan), ACETYLENE BLACK (proprietary name, product of Denki Kagaku Kogyo Kabushiki Kaisha, Japan), etc.

The carbon black is incorporated in the primer, preferably at a rate of 6% or less by volume. Incorporation of an excessive amount of carbon black is undesirable as the strength of the primer layer is thus reduced.

The resin separation layer 11 is formed from a mixture of a fluorine resin with superior release properties and heat tolerance such as PFA resin, and electrically conductive particles, such as carbon black powder.

In more detail, the resin separation layer 11 comprises a PFA dispersion layer incorporating electrically conductive material, formed by dissolving said PFA resin in a dispersant and then incorporating a conductive material in the PFA dispersion layer with a resin powder separation layer laminated thereon. In the present embodiment, the PFA dispersion layer is formed from, for example, X500 (proprietary name, produced by Mitsui-Du Pont Phlorochemical Co.) comprising about 35% PFA resin particles of approximately 0.5 .mu.m diameter.

The electrically conductive material incorporated in the PFA dispersion layer is of the same type as that incorporated in the aforesaid primer layer, and in the case of carbon black the preferred amount is 9% or less by volume. Because more electrically conductive material can be incorporated in the PFA dispersion layer of the present invention than in conventional PFA powder layers, the electrical resistance of the outer peripheral surface of the roller can be reduced to 10.sup.6-10 .OMEGA.. This prevents triboelectrically-induced offset via electrostatic attraction of the toner.

Offset caused by thermal adhesion of the toner can be effectively prevented by the PFA dispersion layer due to the release properties and strength of said layer, which are superior to known conventional polytetrafluoro ethylene (PTFE resin) dispersion.

Additionally, the PFA dispersion layer obtained in the present invention can be applied in uniform coatings without irregularities, even in the case of small diameter rollers. Such uniformity is difficult to achieve using conventional methods.

The insulating layer of the pressure roller is formed from an insulating material such as natural rubber or synthetic rubber, or other conventional material having rubber-like elasticity.

The heating roller in the present embodiment may be obtained, for example, by the following process.

Using a piece of aluminum 25 mm in diameter, 230 mm in length and having a wall thickness of 1.3 mm as the electrically conductive member of the heating roller, was preheated for one hour at 70.degree. C. After coating the core member with a primer using an air spray gun at 2-3 kg/cm.sup.2 pressure and drying said primer for 10 min at 60.degree. C., the adhesive strength bonding the primer layer to the core member increases. Next, the PFA dispersion layer 2a incorporating carbon black was applied in the same manner using an air spray gun at 2-3 kg/cm.sup.2 pressure. After air drying for 30 min a second PFA dispersion layer was sprayed onto the core member and, without interruption, a coating of PFA powder 2b was applied with an electrostatic spray device as shown in FIG. 2(a). The overall coating was then dried at 100.degree. C. for 20 min. By means of the above drying process, and the hereinafter described baking process, pinhole generation induced by uneven evaporation of the moisture in the dispersion layer is prevented. Also, at this time the dispersion layer penetrates to the PFA powder (2b) as shown in FIG. 2(b).

Next, the core member was baked at 380.degree.-400.degree. C. for one hour, whereby the heating roller, formed from a PFA dispersion layer with a PFA powder overlaid thereon, was obtained, as shown in FIG. 2(c). The heating roller is subsequently ground in a final step to expose the PFA dispersion layer at portions of the outermost surface of the roller, e.g., the carbon black incorporated in the PFA dispersion layer is exposed at portions of the outermost peripheral surface of the roller.

By means of this construction the surface charge on the outer peripheral surface of fthe roller is grounded through the electrically conductive core member 1, said surface charge being transmitted to the conductive core member via the carbon black incorporated in the PFA dispersion layer, and the carbon black incorporated in the primer layer. Thus, triboelectrically-induced offset due to the electrostatic attraction of the toner can be effectively prevented.

Thermal adhesion of the toner can also be prevented due to the superior release properties and strength of the PFA dispersion layer of the present invention.

In the present embodiment, the fixing roller related to the invention was used in a heating roller, but it is also applicable for use in a pressure roller.

Although the present embodiment provides a PFA dispersion layer with a PFA powder layer applied thereon, the PFA powder layer may be omitted. However, provision of a PFA powder layer enhances the release properties of the roller and its effective in preventing offset.

When a PFA powder layer is provided, the surface of said layer is ground to expose portions of the underlying PFA dispersion layer, e.g., portions of the carbon black incorporated in the PFA dispersion layer are exposed through the surface of the PFA powder layer. Thus, triboelectrically-induced offset due to electrostatic attraction of the toner can be prevented.

When an excessive amount of the PFA powder layer is ground, the release properties are diminished, however, because the present invention provides a PFA powder layer applied on a PFA dispersion layer, only a small amount need be ground.

Although the present embodiment provides a core member with a primer layer applied thereon, this primer layer may be omitted. However, provision of a primer layer on the core member is preferred as it enhances the adhesion of the PFA dispersion layer to the core member.

A first layer (primer layer); and second layer (PFA dispersion layer) composed of the hereinafter listed material compositions were successively laminated on an electrically conductive core member, whereby a heating roller was obtained.

A roller was fabricated according to the aforementioned processes and material compositions with film thicknesses (coating weights) as shown in Table 1, said roller was then tested for the occurrence of offset and the amount of triboelectric charging of the roller.

The reslts are shown in Table 2.

The results shown in Table 2 were evaluated as follows: The absence of offset is indicated by 0, offset with an ambiguous pattern is indicated by .DELTA., offset with a definite pattern is indicated by X, and offset with a clear pattern and character is indicated by XX.

An offset chart made by Kodak Co., Ltd. was used as a test pattern.

It is understood from the results shown in Table 2 that a heating roller manufactured by the aforesaid process has minimal triboelectric charging and surface resistance.

A heating roller was obtained in the manner described in Example 1 except that on the second layer was applied a third layer having composition listed below.

A roller was fabricated according to the aforementioned process with film thicknesses (coating weights) as shown in Table 3, the resulting rollers were then subjected to the same tests as in Example 1, the results of which are listed in Tables 4 to 6 below.

A roller was fabricated with the coating material layer composition as indicated hereinafter and tested in the same manner as described in Example 1, the results of which are shown in Table 7.

The evaluation standards regarding offset are identical with those of Example 1. Since, from the results of Examples 2 and 3, a closer relationship was inferred between the film thickness of the third layer and the occurrence of offset caused by the heating roller, than existed with the volumes of carbon black in the first and second layers, the rollers exhibiting poor offset qualities in Examples 2 and 3 were thinned by grinding their surfaces in 0.2 g increments and then re-tested, the results of which are shown in the graphs in FIGS. 3, 4 and 5. The film thickness compositions in the graphs is as indicated below.

FIG. 3 . . . Relates to the roller in Example 2.

FIG. 4 . . . Relates to the identical roller in Example 2.

FIG. 5 . . . Relates to the roller in Example 3.

It is understood from the above results that when the coating weight of the third layer is 0.4 g or less, offset does not occur.

A roller was fabricated using identical coating weight compositions as in Example 1 but with a reduced percentage of carbon black incorporated in the second layer than was used in the aforementioned Examples 1 through 3, the roller then was tested and the results obtained are shown in Table 8.

The material composition of each layer is listed below.

The results shown in Table 8 verify that offset does not occur when the third layer coating weight is 0.4 g or less.

Using the material compositions described hereinafter, a conventional heating roller was fabricated and was then tested with the results as shown in Tables 9 and 10.

As can be seen from the results shown in Tables 9 and 10, when the PFA dispersion layer was not provided and the postgrinding coating weight of the third layer was 0.2 to 0.1 g or less, offset was not completely eliminated.