Fixing device for electrophotography

A fixing device for electrophotography has a heating roller and a compression roller compressed against each other. Copy paper with toner image on its surface is passed inbetween to have the image fixed. The heating roller is hollow and contains a heater inside. To control the heat output of the heater, a temperature detector is positioned on the surface of the heating roller near the edge of the paper passing between the rollers where no toner image is formed. The wall thickness of the heating roller is reduced to about 0.8-1.5 mm for improved heat distribution characteristics.

BACKGROUND OF THE INVENTION 
This invention relates to a fixing device for fixing toner images in 
electrophotography and more particularly to such a fixing device 
characterized by the position on its heating roller where a temperature 
detector is disposed. 
The fixing device for an electrophotographic copying machine and the like 
typically comprises a heating roller which is made of metal and is heated 
to a temperature of about 200.degree. C. and a compression roller which is 
disposed opposite to the heating roller and, when in operation, is 
compressed against it and follows its motion. The fixing process is 
effected with these rollers compressed against each other and rotating 
together, and a sheet of copy paper with a toner image transferred on its 
surface passes between these rollers. As the paper passes between the 
rollers, the toner is melted by heat and the image becomes fixed by the 
pressure of the rollers. 
The heating roller is a hollow cylinder and holds a heater lamp at its 
center section. In addition, a temperature detector for the heater control 
such as a thermister is disposed in contact with the surface of the 
heating roller to detect its temperature. In order to maintain the heating 
roller at an appropriate temperature level for the fixing process, the 
power delivered to the heater lamp is controlled according to the output 
from this temperature detector. 
In FIGS. 3A and 3B, a previously designed fixing device is illustrated with 
a heating roller 10, a compression roller 20 compressed against it, and a 
heater lamp 3 at the center of the heating roller 10. FIG. 3C shows the 
temperature distribution on the surface of the heating roller 10 by the 
operation of the heater lamp 3. It shows that the surface temperature is 
slightly higher near the ends and flat near the center, and it is because 
the rollers are intentionally so designed in order to compensate for the 
temperature drop at these places caused by the heat loss through the 
bearings for the heating roller 10. Since the thermal resistance of the 
roller housing is relatively high, however, the temperature distribution 
on the surface of the heating roller 10 does not always become flat, 
depending on the number of sheets of paper which pass through per unit 
time. FIG. 3C represents such a situation where temperature is relatively 
higher at the end sections. In order to be able to detect an average 
fixing temperature even if there is some such distortion in the 
temperature distribution, the temperature detector is disposed on the 
heating roller at a point which may be expected to be nearly in the middle 
of the temperature distribution. Because of the contact with the 
temperature detector, however, the surface of the heating roller sometimes 
becomes scratched and this may adversely affect the fixed image. In order 
to eliminate such a problem, it may be considered preferable to put the 
temperature detector at an end section of the heating roller but a 
representative value of the surface temperature cannot be detected at such 
a position, and it becomes difficult to accurately control the temperature 
of the heating roller. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a fixing device with 
which the surface temperature of its heating roller can be accurately 
detected without adversely affecting the quality of the image fixed on the 
paper passing therethrough even if the heating roller becomes scratched 
where the temperature detector is in contact therewith. 
The above and other objects of the present invention are achieved by 
providing a fixing device of the type having two rollers, at least one of 
them being hollow and enclosing a heater at its center section, and the 
other roller being compressed against it and rotated with it, and passing 
therebetween a sheet of copy paper with a toner image. According to the 
present invention, the wall of the roller containing the heater is made 
thinner and a temperature detector for controlling the operation of the 
heater is placed in contact with the surface of this roller in the 
neighborhood of a side edge of the paper passing through these rollers 
where toner image is not formed. 
With the device so structured, even if the surface of the heating roller 
becomes scratched because a temperature detector is in contact, the fixed 
image is not affected because it is near the side section of the paper 
where toner image is not formed. Since the wall thickness of the heating 
roller is thinner, furthermore, the thermal resistance of the roller 
surface is reduced. As a result, the device has improved thermal response 
characteristics and the temperature distribution on the roller surface can 
be made flat over a wider range by properly designing the heat 
distribution characteristics of the heater such as a heater lamp. Thus, 
the full surface temperature of the heating roller can be detected even 
though the detector is set near an end section of the roller and the 
surface temperature of the heating roller can be maintained at a desired 
level.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1A and 1B show the structure of a fixing device according to an 
embodiment of the present invention and its characteristics. Numeral 10 
indicates a heating roller composed of a hollow aluminum housing 2 with 
reduced thickness about 0.8 to 1.5 mm and thermal conductivity about 0.53 
cal/cm.multidot.sec.multidot.deg and a heater lamp 3 disposed axially. Its 
axis is supported by ball bearings 4 and rotated by a driving means (not 
shown). Numeral 20 indicates a compression roller with a silicone rubber 
roller 7 around its central shaft 6. A tubular layer 8 of ethylene 
tetrafluoride resin is formed on its surface. This roller is also 
rotatably supported by a pair of ball bearings 5. 
FIG. 1C shows the distribution characteristics of the surface temperature 
of the heating roller 10. Since the roller's thermal response 
characteristics are improved by reducing the wall thickness t of the 
heating roller to the aforementioned range, the temperature drop caused by 
the heat loss at the end axis sections can be compensated by properly 
setting the heating characteristics of the heater lamp 3. As a result, a 
uniform temperature distribution can be attained over a wide range and, 
although the temperature detector is positioned in contact at a side 
section external to the image area (where a toner image is formed on copy 
paper which passes through the rollers), it can detect the average 
temperature which is representative of the overall surface temperature of 
the heating roller. With reference to FIG. 1C, the "void section" 
indicates the section at the edge of copy paper where no toner image is 
formed. Such a void section comes about, for example, in the case of a 
copying machine designed to form a film at the edge of its photosensitive 
drum such that copy paper can be forcibly removed from its surface. 
FIGS. 2A and 2B show how a temperature detector 1 is positioned in contact 
with the heating roller. The detector 1 is for controlling the heater and 
serves to detect the temperature of the section where it is attached. The 
detector 1 is about 10 mm angle, having a thermistor buried in the surface 
part of sponge-like foam silicone with a heat-resisting tape applied on 
its surface. The surface of the heating roller tends to develop scratches 
9 because of its contact with this heat-resisting tape but they do not 
affect the fixed image on the paper because the scratched areas correspond 
to where no toner image is formed on the copy paper which passes between 
the rollers. 
There are copiers which cause copy paper to naturally fall off from the 
photosensitive drum by means of an erase charger. With such copiers, there 
are no void sections, and toner images are formed to the edge of the 
paper. Even in such situations, a temperature detector may be positioned 
near the paper edge external to the range where the paper is passed, and 
the scratches and the like made on the roller surface can be prevented 
from affecting the fixed images. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and many modifications and variations are possible in light of 
the above teaching. Such modifications and variations which may be 
apparent to a person skilled in the art are intended to be included within 
the scope of this invention.