Electrophotography – Image formation – Fixing
Reexamination Certificate
2001-06-15
2004-04-20
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000, C219S619000, C399S330000
Reexamination Certificate
active
06725009
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an image heating device permitting reduction of warm-up time and an image forming apparatus. More particularly, the present invention relates to an image heating device for use in image forming apparatus, such as electrophotographical apparatus or electrostatic recording apparatus, that is suitable as a fixing device for fixing unfixed images, and to an image forming apparatus.
BACKGROUND ART
Conventionally, as image heating devices, for which fixing devices are a typical example, contact-heating devices such as heat roller type devices and belt type devices, generally have been used.
In recent years, due to the demand for shorter warm-up time and reduced energy consumption, electromagnetic induction heating, by which rapid heating and high efficiency heating are likely to be attained, are attracting great attention (see JP 10(1998)-123861 A).
FIG. 23
shows a cross-sectional view of an image heating device utilizing the electromagnetic induction, which is disclosed in JP 10(1998)-123861 A. As shown in
FIG. 23
, a magnetization coil
114
is provided inside a heat-generating roller
112
. By this magnetization coil
114
and a core
117
, an alternating magnetic field is generated to induce an eddy current in the heat-generating roller
112
, thereby heating the heat-generating roller
112
. Then, an unfixed toner image
111
formed on recording paper
110
can be fixed after the recording paper
110
has passed through a nip portion formed between the heat-generating roller
112
and a pressure roller
113
. Further, an image heating device with a heat-generating roller that is made thin has been proposed, as disclosed in JP 10(1998)-74007 A.
FIG. 24
shows this device.
In
FIG. 24
, reference numeral
310
denotes a magnetization coil, which generates a high-frequency field when a high-frequency current is applied thereto from an inverter circuit, and reference numeral
311
denotes a metal sleeve, which generates heat through electromagnetic induction and is rotated. An external pressure member
313
rotates in arrow direction “a”. The metal sleeve
311
, which is held between the external pressure member
313
and an internal pressure member
312
, rotates following the external pressure member
313
.
A recording paper
314
carrying an unfixed toner image thereon is fed to the nip portion formed between the external pressure member
313
and the internal pressure member
312
in the arrow direction shown in the drawing. The unfixed toner image on the recording paper
314
is then fixed by the heat from the metal sleeve
311
and the pressure from both the pressure members
312
and
313
.
Further, to prevent electromagnetic induction heating from being performed while the metal sleeve
311
is at rest, a heating signal for the inverter circuit is set to be a logical product of an operation signal and a heating signal from a drive motor for rotating the external pressure member
313
.
In image heating devices utilizing such electromagnetic induction, a heat-generating member such as a heat-generating roller or the like is directly heated through electromagnetic induction. Such image heating devices thus can attain higher heat-exchanging efficiency as compared with those using a halogen lamp for heating, so that the surface of a fixing roller can be heated up to a fixing temperature rapidly with a smaller power.
However, the image heating device in which a normal metal heat-generating roller is simply heated through electromagnetic induction cannot attain remarkably reduced warm-up time as compared with conventional image heating devices using a halogen lamp for heating.
Further, if a heat generating roller is made thinner to decrease the thermal capacity for shortening warm-up time, it becomes difficult to control the temperature of the roller.
JP 8(1996)-137306 A has proposed an image heating device using a belt with a smaller thermal capacity for shortening warm-up time. In this image heating device, the belt formed of a conductive material is heated through electromagnetic induction and the belt itself thus can be heated rapidly. However, since the thermal capacity of the belt is too small, the heat generated by the belt is removed by a tension roller and an oil roller, which brings about a problem that it is difficult to raise the temperature of the entire system.
For shortening warm-up time, a rotating operation of the heat-generating roller is generally started after the heat-generating roller is heated up to a predetermined temperature. However, since the roller can be heated rapidly according to the electromagnetic induction heating, if the heat-generating roller at rest is heated in the image heating device with a small thermal capacity, an abrupt temperature rise may occur at a portion of the heat-generating roller. This may result in deterioration of the belt, an elastic material provided on the belt, and the like.
Especially in an image heating device performing heating with a heat-generating roller and a heat-resistant belt looped around the roller, the temperature of the heat-generating roller is made too high by the rapid heating, resulting in permanent set of the heat-resistant belt in accordance with the curvature of the roller. It is to be noted here that this problem seldom occurs in the case of a conductive belt and never occurs in an image heating device in which a straight portion of the belt is heated. This problem occurs only in an image heating device in which a heat generating roller is heated and the heat from the roller is conveyed by the belt formed of a resin.
From the viewpoint of saving energy, it is preferable that a heat-generating member in an image heating device is heated only when the device is used. Image heating devices of heat roller type generally include a heat-generating member in a nip portion. However, in image heating devices of the belt type, a heat-generating member is away from a nip portion, resulting in time lag between the temperature change in the heat-generating member and in the nip portion.
In addition, in the image heating devices in which the heat-generating member is away from the nip portion, the heat from the belt, which has been heated by the heat-generating member, is not only consumed for melting toner on recording paper but also for heating a pressure roller and a fixing roller. The pressure roller and the fixing roller are heated by removing the heat from the belt. Accordingly, the amount of the heat removed by these rollers depends on the amount of the belt that has been passed, i.e., the process speed. The heat removed by these rollers is not directly involved in the fixing operation. Therefore, it is necessary to minimize the amount of this wasted heat for performing the fixing operation quickly.
In an image heating device including a magnetization coil and a rotatable conductive heating element, if the device is configured so that the conductive heating element is heated through electromagnetic induction only when the element is rotating, the magnetization coil should magnetize the element after a rotating operation of the element is started. Otherwise, the temperature of the element is only partially made high, resulting in uneven temperature distribution. Although this configuration permits a relatively short warm-up time, it is necessary that the conductive heating element keeps residual heat during a standby period for immediately satisfying the user's demands for printing. However, in the image heating device with this configuration, a rotating operation of the conductive heating element has to be performed for heating the element, which brings about a problem that the element needs to be kept rotating even in the standby period. Besides, since the conductive heating element is heated rapidly, it is difficult to maintain the element at low temperatures.
In the belt-type image heating device, if a temperature sensor is provided on the surface of the belt, the sensor is liable to damage the surface of the belt, thereby reducing the life of the
Asakura Kenji
Imai Masaru
Tatematsu Hideki
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Ngo Hoang
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