Image fixing roller and image fixing apparatus using same

An image fixing apparatus includes a couple of rotatable rollers press-contacted to each other, for fixing a toner image by passing a toner image carrying material carrying the toner image, through a nip formed between the rotatable rollers, at least one of the rollers including a surface coating of anodized aluminum having fine pores filled with perfluoroalkyl compound material.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image fixing roller and an image fixing 
apparatus using the same for fixing an unfixed toner image, usable with an 
electrophotographic or electrostatic recording apparatus or the like. 
A roller image fixing apparatus using a couple of rollers is widely used in 
the field of an image fixing apparatus for fixing an unfixed toner image, 
since it is advantageous from the standpoint of reducing the size of the 
apparatus and others. Among the roller fixing apparatuses, a heating 
roller type fixing apparatus is mainly used, since it is advantageous in 
the quality of the image and image fixing performance. 
In the heating roller type fixing apparatus, a toner image carrying 
material carrying thereon an unfixed toner image, such as a transfer 
sheet, is passed through a nip formed between a heated roller and a 
back-up or pressing roller press contacted to the heated roller to fix the 
toner image. 
As for the materials of the rollers, the heating (heated) roller directly 
contactable with the toner image is made of a hollow aluminum cylinder 
having a surface layer of several tens microns thickness of a high parting 
property material such as fluorine resin (PFA, PTFE, for example) or the 
like; the pressing roller is made of a hollow metal cylinder of aluminum 
or stainless steel having a heat resistive rubber of several mm thickness 
of silicone, fluorine rubber or the like. With this structure, the image 
fixing operation is carried out under such conditions that the surface 
temperature of the heating roller is raised up to 180-200.degree. C., that 
a nip width, which is a width of contact between the rollers in the 
circumferential direction, is 6 - ten and several mm and that the contact 
pressure is 5 - ten and several kg/cm.sup.2. This method of image fixing 
is sufficient in that the toner image and the carrying material are 
sufficiently heated, and the toner image is sufficiently fused and fixed 
on the carrying material. 
However, this conventional fixing apparatus involves a problem that the 
surface material of the heating roller, which is of fluorine resin 
material such as PFA or PTFE resin, is worn or peeled, due to contact of 
separation pawls thereto for the purpose of assuredly separating the toner 
image carrying material from the pressing roller, due to contact of a 
temperature sensor thereto and/or due to passage of the carrying material 
such as a sheet of paper. The problem is, in addition to the wearing and 
the deterioration in the image quality by the peeling, the requirement 
that the pressure of contact of the separating pawl to the roller has to 
be reduced to reduce the wearing, the reduction resulting in frequent 
occurrence of paper jam. 
Therefore, it is desired that the wearing is reduced without decreasing the 
contact pressure of the separating pawl. U.S. Pat. Ser. No. 4,043,747 
proposes that the positions of the contacts are gradually changed to 
reduce the wearing without producing the jam. 
However, this results in a complicated structure of the apparatus. 
Additionally, a certain level of the wearing due to the passage of the 
sheet can not be avoided. 
Japanese Laid-Open Pat. Application No. 23946/1977 proposes that a surface 
of a good thermal conductor is provided with pores having diameters of 
several tens - several thousands Angstroms, which pores are impregnated 
with off-set preventing material containing a fluorine resin to constitute 
an image fixing roller. However, good thermal conductor metals such as 
aluminum and copper generally have a lower hardness, and therefore, are 
easily damaged if separating pawls for preventing the sheet from wrapping 
around the roller are contacted thereto with a high pressure. This is not 
desirable from the standpoint of increasing the durability of the roller. 
In addition, due to the good thermal conductivity of the surface of the 
roller, a large amount of heat is transferred to the image carrying 
material having the toner image at the time of the image fixing operation, 
with the result that the toner is fused more than necessary, thus 
producing a toner off-set, that is, the toner is deposited onto the fixing 
roller. Further, since the heat is very quickly removed from the fixing 
roller, it is difficult to continuously produce the images of good 
quality. Furthermore, a large amount of heat application to the sheet 
produces an unnecessary curling of the sheet, which leads to occurrence of 
sheet jam and difficulty in handing the sheet when plural images are to be 
formed on the same sheet, which requires the sheet to be refed. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide 
an image fixing roller having a high surface hardness and a high 
durability. 
It is another object of the present invention to provide an image fixing 
roller which has a good surface hardness and a good parting property, 
simultaneously. 
It is a further object of the present invention to provide an image fixing 
roller wherein the heat transfer to the image carrying material is 
reduced. 
It is a yet further object of the present invention to provide an image 
fixing apparatus wherein occurrences of toner off-set or the curling of 
the image carrying material are reduced. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of the preferred embodiments of the present invention taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be described in 
detail in conjunction with the accompanying drawings, wherein the same 
reference numerals or characters are assigned to the elements having the 
corresponding functions. 
Referring to FIG. 1, an image fixing apparatus according to an embodiment 
of the present invention is shown. The image fixing apparatus comprises a 
heating roller 1, a pressing roller 2, a separating pawls 5 and a parting 
agent applying means 6. 
The heating roller 1 is provided therein with a heating source 3 such as a 
halogen heater or the like for heating the heating roller 1. The surface 
temperature of the heating roller 1 is detected by a temperature sensor 4 
such as a thermister which is contacted to the outer periphery of the 
heating roller. In response to the output of the temperature sensor 4, a 
temperature control device not shown effects on-off control to the inside 
heating source 3 to provide a substantially constant surface temperature 
of the heating roller 1. 
On the other hand, the pressing roller 2 is a rubber roller having an 
aluminum or steel core metal covered with a silicone rubber layer (RTV or 
LTV) of a thickness of several mm. The pressing roller 2 is 
press-contacted by an unshown spring or the like to the heating roller 1 
to form a nip N. Through the nip N, an image carrying material 8 carrying 
an unfixed toner image formed by a known electrophotographic process or 
the like is passed, by which the toner image is fixed on the carrying 
material 8. 
Referring to FIGS. 2A and 2B, the heating roller 1 will be described in 
detail, which Figures are sectional views of a surface part of the fixing 
roller 1. Referring first to FIG. 2A, a core metal 42 made of aluminum or 
aluminum alloy is anodized at its surface by a known anodizing method to 
form anodized surface oxide layer (A1.sub.2 O.sub.3) 43, which will 
hereinafter be called "alumite" layer, having a thickness of several 
microns - several tens microns. Thereafter, a solid lubricant is filled 
into fine pores 45 existing in the alumite layer 43. The resultant product 
is shown in FIG. 2A. The fine pores 45 extend from the surface of the 
alumite layer substantially down to the aluminum core metal 42. At the 
interface between the aluminum core metal 42 and the alumite layer, there 
is an active layer (barrier layer 44). The diameter of the fine pores 45 
differs depending on the liquid used in the anodizing treatment. The 
diameter is preferably 100-500 Angstroms, and the number of the pores is 
preferably such that 5-40 %, further preferably, 10-40 % of the anodized 
area of the aluminum surface is occupied by the pores. The treating liquid 
is selected in consideration of those desirabilities. For example, when 
the anodization is performed in 4 % phosphoric acid both at 24.degree. C. 
by the voltage of 20 V, the diameter of the pores is 330 Angstroms, and 
the occupation of the pores is 16-18 %. When the alumite layer is produced 
by sulfuric acid treatment, which generally provides a hard coating, the 
diameter of the pores is 100-150 Angstroms, but the number of the pores 
can be increased by the voltage applied, and therefore, the occupation of 
10-20 % can be obtained. 
The anodized surface oxide layer produced in the manner described above is 
immersed in a mixture of water and anion or cation fluorine compound or in 
a mixture of water and isopropylalcohol or the like, and a DC or AC 
electric field is applied, placing the anodized aluminum roller as an 
electrode, by which the fluorine resin is accumulated in the pores 45 of 
the anodized coating from the bottom thereof. In this manner, the intended 
image fixing roller 1 can be provided. The fluorine resins usable for this 
purpose are perfluoroalkylphosphoric ester, perfluoroalkyl carboxylate, 
perfluoroalkyl ammonium salt and other perfluoro compound, which are 
generally called fluorine surface active agent. The perfluoroalkyl 
compound exhibits very good parting property when it is solidified. 
In order to assure the durability of the heating roll 1, the fine pores 45 
preferably has a thickness not less than 5 microns, and further preferably 
not less than 10 microns. Also, it is preferable that the fine pores in 
which the fluorine resin is substantially completely filled from the 
bottom to the top of the pores occupies 80 % or more of the total pores. 
However, if the depth of the fine pores 45 is not less than 50 microns, 
the hardness of the produced anodized coating decreases, and in addition, 
the number of fine pores not filled with the fluorine resin becomes large, 
and therefore, not practical. 
In the foregoing embodiment, the heating roller having anodized coating or 
anodic oxidation coating is produced by a single electrolytic process. 
Referring to FIG. 2B, there is shown another embodiment, wherein in order 
to increase the pore percentage, the coating is dissolved chemically after 
the anodic oxidation coating is formed. Alternatively, the pores can be 
expanded by electrolytic process using phosphoric acid bath, by which the 
amount of the fluorine resin filled can be increased. Further, the area 
where the fluorine resin is exposed at the outer surface increases, 
thereby improving the parting property. The perfluoroalkyl compound is 
generally poor in ductility, and therefore it is difficult that the entire 
surface of the heating roller is covered with the perfluoroalkyl compound. 
In consideration of this, the expansion of the fine pores is advantageous. 
However, the expansion of the pores results in decrease in the hardness of 
the anodic oxidation coating. For this reason, it is practically desirable 
that the pores are not expanded beyond two-fold or 2.5-fold. 
As a further embodiment, the filler is not solely of the perfluoroalkyl 
compound, but fatty acid metallic salt or metallic sulfide exhibiting a 
water repelling property is added. Particularly, the former is excellent 
in the ductility which the perfluoroalkyl compound lacks, and therefore, 
is preferable to supplement the ductility. On the other hand, the fatty 
acid metallic salt has a low fusing temperature, and therefore, the 
mixture ratio with the perfluoroalkyl compound and the material thereof is 
property selected, depending on the temperature at which the heating 
roller is operated. More particularly, if it is constantly used above the 
temperature of 200.degree. C., the content, in the mixture, of the 
perfluoroalkyl compound is preferably not less than 50 %, and the fatty 
acid metallic salt is preferably magnesium oleate or the like having a 
relatively high heat durability. For the temperature approximately 
150.degree. C., it is possible to use sodium stearate or the like. Usable 
fatty acid metallic salt will be mentioned. Particularly, magnesium salt, 
potassium salt and sodium salt or the like shows good water solubility, 
and therefore, are preferable from the standpoint of production cost. For 
example, there are potassium stearate, sodium stearate, potassium 
palmitate, sodium palmitate, magnesium oleate or the like. 
Also, metallic sulfide, such as molybdenum disulfide or the like may be 
filled into the fine pores together with the perfluoroalkyl compound. In 
this case, the heat durability of the metallic sulfide is high, and the 
anticollosion property is high, and therefore, the product is durable to a 
higher temperature than the case where the fatty acid metallic salt is 
used. Similarly to the fatty acid metallic salt, the metallic sulfide is 
better in the ductility than the perfluoro compound, so that the poor 
ductility of the fluorine compound can be supplemented. As for a usable 
metallic sulfide, there are molybdenum disulfide, tungsten sulfide or the 
like. 
Also, it is possible to use a mixture of the above described metallic 
sulfide, fatty acid metallic salt and ionic perfluoroalkyl compound. 
The description will be made as to the method of filling the pores with the 
solid lubricant. In addition to the above described method wherein the 
electrolysis is used for filling the ionic perfluoroalkyl compound, there 
is a three-electrolytic process wherein a metal is deposited through the 
electrolytic process in the fine pores in the anodic oxidation coating, 
then an opposite electric field is applied to the metal to dissolve it 
into the electrolyte, and the ion perfluoroalkyl compound is filled into 
the pores as if the compound replace it. By this three-electrolytic 
process, when the metal is dissolved, the metal fatty acid metallic salt 
or metallic sulfide can be produced if water soluble fatty acid or sulfide 
is added in the electrolyte, so that it is simultaneously filled into 
pores together with the perfluoroalkyl compound. This method is 
particularly desirable in that the amount of the filler is larger than by 
the method wherein the above described three solid lubricants are directly 
applied or a method wherein they are dissolved, and the roller is immersed 
thereinto. 
Referring to FIG. 3, a further preferable embodiment of the present 
invention will be described. In this embodiment, the heating roller 1 
comprises a hollow core metal 42 made of aluminum, aluminum alloy, copper 
or the like, a heat resistance layer 46 made of material having a low 
thermal conductivity on the core metal, an aluminum layer 41 as the 
surface layer evaporated or plated on the heat resistance layer, wherein 
the surface is anodized. This is used for the heating roller. Because the 
aluminum layer 41 is anodized, it covers the entire circumferential and 
longitudinal end surfaces of the heat resistance layer 46, and the 
thickness thereof is 50-100 microns. 
The description will be made as to the aluminum layer 41 functioning as a 
surface material having a surface alumite layer 43 and as to the heat 
resistance layer 46. The heat resistance layer 46 has a thermal 
conductivity of one tenth - one hundredth of the alumite layer 43, and the 
material and the thickness thereof are determined depending on the 
operating temperature, the image fixing speed and a width of the nip 
formed between the heating roller and the pressing roller. Usable material 
is, for example, a heat resistance plastic material such as epoxy resin, 
silicone resin, polyimide, polyamide or the like which can be applied onto 
the core metal by dipping, molding, spraying or the like which are known. 
The usable materials have the thermal conductivity of 0.5-5.0 W/cm.deg. 
For example, when the fixing speed is 400 mm/sec with the nip width of 9 
mm, and the toner to be fixed contain the resin containing as a major 
component polyester resin, the minimum fixing temperature is about 
130.degree. C., and therefore, the initial temperature of the fixing 
roller is set to 180.degree. C. When the heat resistance layer 46 is made 
of epoxy resin and has a thickness of 100-150 microns, and the thickness 
of the surface layer 40 containing the aluminum layer has a thickness of 
50 microns, it has been possible to continuously fix the images for not 
less than 250 sheets of A3 size (JIS). If, however, the thickness of the 
heat resistance layer 46 is not more than 50 microns, improper image 
fixing has been able to occur when about 100 A3 sheets are fixed. This is 
because the resistance to the heat conducting is not sufficient, and 
therefore, the heat is wasted into the image carrying member carrying the 
toner image. In this case, the heat resistance layer has a thickness not 
less than two-fold of that of the surface layer. However, if it is not 
less than twenty fold, the heat supply from the core metal 42 is retarded, 
and improper image fixing can result after only several tens sheets are 
fixed. Generally speaking, when the image fixing speed is increased or 
when the nip width is decreased, the amount of heat carried over to one 
sheet decreases, and therefore, it is preferable that the thickness of the 
heat resistance layer 46 is decreased or that the material exhibiting a 
higher heat conductivity is used. On the contrary, when the image fixing 
speed is decreased, or when the nip width is increased, it is preferable 
that the thickness of the heat resistance layer 46 is increased, or that 
the material having a lower heat conductivity is used. 
The material of the heat resistance layer is selected so as to satisfy: 
EQU L/v.varies.Ac/d 
where v is an image fixing speed; L is a nip width; c is a heat 
conductivity of the heat resistance layer; d is a thickness of the layer; 
and A is a constant determined for the surface temperature of the roller, 
and therefore, is a constant if the surface temperature is the same. 
In the foregoing descriptions, the heat resistance layer is made of a heat 
resistant (durable) resin, but it is possible to use a silicone rubber or 
the like added by silica, carbon or the like to increase the hardness 
thereof. As another material, there is a porous ceramic material which has 
a high heat resistance, when the porosity is properly controlled. 
In the case of the rubber material, the layer may be formed by molding, 
injection or the like. In the case where the ceramic material is used, the 
porosity is changed by changing an amount of a binder during production, 
and therefore, a proper amount of the binder is mixed into the ceramic 
material, and it is applied on the core metal, whereafter it is sintered. 
As described in the foregoing, according to the present invention, the 
surface of the heating roller is provided by anodizing an aluminum and by 
filling perfluoroalkyl compound into the fine pores of the anodized 
coating, whereby the hardness of the fixing roller surface is high, and 
therefore, the wear resistance is improved over the fluorine resin coated 
heating roller. The durability is not less than ten-folded. 
Since the surface of the fixing roller is hardened, it is possible to 
increase the pressure of contact with the peeling member such as a 
separating pawl, and therefore, the jam occurrence rate has been 
significantly reduced. 
Also, since the surface is of alumite having a high heat conductivity, the 
heat is efficiently transferred from the surface of the fixing roller to 
the toner image carrying material. Further, since the low thermal 
conductivity layer is provided between the surface material and the core 
metal, the heat is not extremely introduced. Thus, the energy loss is 
decreased at the time of the image fixing operation, and the toner offset 
to the fixing roller attributable to the extreme heating is reduced. The 
prevention of the excessive heat to the transfer material significantly 
reducing the occurrence of the curling of the image carrying material This 
further reduces the cause of the jam and improves the easiness of handing 
of the toner image carrying material. 
While the invention has been described with reference to the structures 
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 purposes of the improvements or the scope of the following 
claims.