Electric heating – Heating devices – Combined with diverse-type art device
Reexamination Certificate
2001-04-16
2002-05-07
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with diverse-type art device
C219S543000, C399S335000, C338S308000
Reexamination Certificate
active
06384378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heating-type toner-fixing unit in which (a) a pressure roller drives a continuous heat-resistant film by pressing it against a ceramic heater positioned on a heating cylinder and (b) a toner image is fixed on a copying sheet fed between the pressure roller and the heat-resistant film. The present invention particularly relates to a ceramic heater for a toner-fixing unit in which the ceramic heater is positioned on a heating cylinder.
2. Related Background Arts
Toner-fixing units used in fax machines, copiers, printers, and other image-forming machines transfer a toner image formed on a photosensitive drum onto a sheet of paper or another copying sheet before fixing it on the surface of the sheet by concurrent heating and pressing.
These toner-fixing units comprise a heating roller and a plastic pressure roller. One of these units, for example, uses a cylindrical metal roller provided with a halogen lamp or another heat source as the heating roller so that the surface region of the metal roller is heated by the heat source to fix the toner.
In addition to this fixing system, another toner-fixing system has been offered and used in recent years which uses a ceramic heater (hereinafter also simply called “a heater”) without using a heating roller. The latter system has been disclosed in published Japanese patent applications Tokukaihei 1-263679, Tokukaihei 2-157878, and Tokukaishou 63-313182, for example. Specifically, these disclosed systems (a) mount a ceramic heater on a plastic support, (b) use a pressure roller to press a heat-resistant film against the ceramic heater so that the heat-resistant film can move at the same speed as the peripheral speed of the pressure roller to feed a sheet of paper, and (c) fix a toner image on the surface of the sheet. The heat-resistant film is composed of a material comprising heat-resistant plastic, metal, or both.
The latter fixing system, using a toner-fixing unit comprising a ceramic heater and a heat-resistant film, has a heater significantly smaller in thermal capacity than that of the former system, using a metal roller. Therefore, the latter system can reduce the power consumption and eliminate the preheating of the heater after the power is supplied, so it is advantageous in having an excellent quick-start property.
FIG. 1
schematically shows a modified latter fixing system in which a continuous heat-resistant film revolves around a heating cylinder. In
FIG. 1
, a ceramic heater
1
is positioned on a heating cylinder
2
. A continuous heat-resistant film
3
revolves on the periphery of the heating cylinder
2
. A pressure roller
4
has a rubber layer or another elastic layer formed on its periphery and its rotation revolves the heat-resistant film
3
at the same speed as its peripheral speed. The heating cylinder
2
and the pressure roller
4
are pressed against each other by springs (not shown in
FIG. 1
) provided at the fixed portions of both of their ends. This pressure deforms the elastic layer on the periphery of the pressure roller
4
, forming a nip portion
5
having a width of W. A toner image
6
a
is formed on a copying sheet
6
such as a sheet of paper. The copying sheet
6
is fed between the heat-resistant film
3
and the pressure roller
4
each rotating in the direction of the arrow. The toner image
6
a
on the copying sheet
6
is heated and pressed at the nip portion
5
to be fixed as an image
6
b.
The ceramic heater
1
attached to the heating cylinder
2
has a structure shown schematically in
FIG. 2
, for example.
FIG. 2
is a plan view viewed from the downside. In
FIG. 2
, a ceramic base material
11
supports (a) one or more heating elements
12
provided at the face (tile fixing face) where the ceramic heater
1
is positioned opposite to the pressure roller
4
via the heat-resistant film
3
and (b) current-feeding electrodes
13
for supplying electric power to the heating elements
12
. The ceramic base material
11
generally has a shape of a thin rectangular flat plate as a whole. Generally, the heating elements
12
are formed at the fixing-face side of the ceramic heater
1
, and an overcoat glass layer is formed on the heating elements as a protective layer. The glass layer not only ensures electrical insulation but also protects the heating elements
12
and other members against the sliding contact with the heat-resistant film
3
.
Alumina is generally used now as the ceramic base material
11
of the foregoing ceramic heater
1
. The ceramic heaters using alumina base materials in this image-fixing system have a fixing rate of 6 to 16 ppm. The unit “ppm” is the abbreviation of “papers per minute” and signifies the number of sheets of paper in A4 size (210×297 mm) fed in a minute. The foregoing fixing rate, however, no longer satisfies the market's requirement. The market now requires to increase the rate to 24 ppm or more.
In a ceramic heater in this image-fixing system, a voltage of 100 or 200 V is applied to the heating elements to produce a Joule heat of hundreds of watts or more. This heat raises the temperature to about 200° C. in 2 to 6 seconds. In this rapid heating process, ceramic heaters using alumina base materials pose a problem of fracture caused by heat shock. When the fixing rate is increased, the time for transferring the heat from the heater to a sheet of paper (a copying sheet) is shortened. This requires the increase in the amount of heat to be supplied from the heater to the copying sheet per unit time, because the toner fixing requires a certain amount of heat. As a result, the increase in the fixing rate tends to increase the heat shock applied to the heater, increasing the percentage of heater fracture.
In order to cope with this problem, ceramic heaters using aluminum nitride, which has excellent resistance to heat shock, as the base material have been disclosed in published Japanese patent applications Tokukaihei 9-80940 and Tokukaihei 9-197861. Tokukaihei 9-80940 improves the temperature responsivity of the heater by exploiting the fact that aluminum nitride has a higher thermal conductivity than alumina. Tokukaihei 9-197861 intends to improve the fixing quality, to increase the possibility of high-speed printing, and to reduce the power consumption by utilizing the highly heat-conductive quality of aluminum nitride.
As stated above, the use of aluminum nitride as the base material of the ceramic heater can solve the problem of heater fracture. However, further increase in the fixing rate poses another problem of a reduction in the durability of the heat-resistant film (hereinafter also simply called “film”) revolving around the heating cylinder provided with the ceramic heater.
As mentioned previously, the conventional ceramic heater has a shape of a rectangular flat plate as a whole. Consequently, the continuous heat-resistant film, which is tubular when viewed from the side, is pressed against the flat fixing face provided on the heating cylinder by the pressure roller and deformed to a flat shape at this position, the nip portion. This deformation accompanied by the high-speed revolution applies a heavy load to the film and may cause the film to fracture at an early stage. In particular, a fixing rate exceeding 24 ppm increases this tendency. This problem has been an obstacle for increasing the fixing rate in a system using a heat-resistant film and a ceramic heater.
SUMMARY OF THE INVENTION
Considering the above-described problems, an object of the present invention is to offer an improved ceramic heater for a fixing system using a heat-resistant film and a ceramic heater. The ceramic heater is characterized by the following features:
(a) it reduces the degree of deformation of the heat-resistant film to lighten the load applied to the film at the time of revolution;
(b) it thereby lengthens the lifetime of the film, i.e., the time that elapses before the film fractures;
(c) it suppresses the above-described problems of the heater; and
(d) it enable
Nakata Hirohiko
Natsuhara Masuhiro
Fuqua Shawntina
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Walberg Teresa
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