Electrophotography – Image formation – Fixing
Reexamination Certificate
2000-12-04
2003-06-10
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000, C399S328000
Reexamination Certificate
active
06577840
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to Japanese patent application Nos. JPAP2000-078330 filed on Mar. 21, 2000 and JPAP 11-343340 filed on Dec. 2, 1999 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of effectively performing an image fixing process.
2. Description of the Related Arts
Conventionally, a fixing station for use in an image forming apparatus employs a heat roller mechanism in which a fixing roller having a heat source and a pressure roller for applying a pressure to the fixing roller are provided so as to form a fixing nip through which a recording sheet is conveyed and is subjected to a fixing process. In such a heat roller mechanism, from its structure, the melted toner is inevitably separated from the fixing roller before it is sufficiently cooled off. Accordingly, an offset phenomenon is prone to be caused in which the toner is erroneously deposited on the surface of the fixing roller.
In recent years, a belt-type fixing mechanism capable of allowing the toner to sufficiently cool off has been looked at and various proposals associated with the belt-type fixing mechanism have been made.
In a Published Unexamined Japanese Patent Application No. 6-318001 (1994), one example of a belt fixing mechanism is disclosed, in which a seamless fixing belt is extended and is rotated between a heat roller internally having a heat source such as a halogen heater and a fixing roller, and a pressure roller is arranged to push the fixing belt against the fixing roller so as to form a fixing nip between the pressure roller and the fixing belt. In this mechanism, the toner is melted by a heat of the fixing belt heated by the heat roller, and the processes of fixing and cooling are performed at the fixing nip located downstream from the heat roller. The feature of this example is that, in order to prevent the offset phenomenon by reducing a temperature of the fixing nip, a recording sheet is made close to the fixing belt and is guided to the fixing nip so as to be sufficiently heated before reaching the fixing nip.
In general, the fixing belt of the belt-type fixing mechanism has a far smaller heat capacity than the fixing roller of the roller type fixing mechanism and, therefore, the fixing belt can rapidly be cooled off during the time when it is moved to pass through the fixing nip, resulting in an accurate prevention of the offset phenomenon.
On the other hand, this mechanism has a drawback that a sufficient fixing heat capacity cannot be obtained because of the small heat capacity of the fixing belt.
A Published Unexamined Japanese Patent Application No. 9-160405 (1997) discloses a technique which attempts to solve the above-mentioned problem. In this technique, a pre-nip is additionally formed at an entrance of an ordinary fixing nip formed by a pressure applied to the fixing roller by the pressure roller. The pre-nip is formed by winding the fixing belt around the pressure roller with a supporting roller arranged inside the fixing belt. Accordingly, the entire nip length is extended and, thereby, the recording sheet can contact the fixing belt for a longer time period so that a sufficient heat will be transferred onto the recording sheet.
When the velocities at which the recording sheet is conveyed in the image forming apparatus and in the fixing station are different, in particular, when the velocity at the fixing station side is relatively slower, the recording sheet may be slacked and tends to touch various portions of the fixing station. As a result of this touching, the surface of the toner image which is not fixed may be rubbed and the toner image may be damaged. This is often called an image rubbing phenomenon.
In the technique described in the above-mentioned Published Unexamined Japanese Patent Application No. 9-160405 (1997), in which the length of the fixing nip is made longer than usual, the recording sheet has a risk of touching the fixing belt before entering the fixing nip. This mechanism is explained below with reference to FIG.
1
.
FIG. 1
shows a schematic representation of a belt-type fixing station which includes a fixing roller
100
, a heat roller
104
internally having a halogen heater
102
, and a fixing belt
106
extended between the fixing roller
100
and the heat roller
104
. The fixing station further includes a pressure roller
108
for applying a pressure to the fixing roller
100
via the fixing belt
106
and a guide member
110
for guiding an incoming recording sheet
112
having an unfixed toner image on the surface thereof to a nip portion which is formed at an area where the fixing belt
106
and the pressure roller
108
are in contact under pressure. This nip portion is referred to as a fixing-process area N. The fixing-process area N is composed of a fixing-process area N
1
and a fixing-process area N
2
. The area N
1
is formed upstream from the area N
2
, which is the fixing-process area commonly used.
In the fixing station shown in
FIG. 1
, an entrance of the fixing-process area N is inevitably formed narrower because of the formation of the fixing-process area N
1
. If the recording sheet
112
is slacked, the surface of the unfixed toner image contacts the fixing belt
106
.
In the type of fixing station illustrated in
FIG. 2
, when the recording sheet
112
is released at its trailing edge from transfer rollers (not shown) and becomes free during the time when the leading edge of the recording sheet
112
passes through the fixing-process area N, the recording sheet
112
is raised towards a tangent line A due to the stiffness of the recording sheet. This is referred to as a trailing edge rise phenomenon. With the trailing edge rise, the recording sheet
112
tends to contact the fixing belt
106
and, as a result, the image rubbing phenomenon is caused. Of course, a thicker recording sheet tends to cause more of a trailing edge rise than with an ordinary recording sheet.
Another example of the belt-type fixing station is described in a Published Unexamined Japanese Patent Application No. 9-90787 (1997), in which a seamless fixing belt is rotatably extended between a heat roller internally having a heat source and a fixing roller having an elastic layer, and a hard-structured pressure roller is arranged to push the fixing belt against the fixing roller so that a fixing nip is formed between the pressure roller and the fixing belt.
With this mechanism, the toner is melted by the heat of the fixing belt heated by the heat roller, and the processes of fixing and cooling are performed at the fixing nip located downstream from the heat roller.
Also, the elastic layer of the fixing roller is configured to have a heat-insulating function for protecting the fixing belt from losing unnecessary heat, as well as an elastically-deforming function for enlarging the fixing nip, having a thickness of at least 2 mm.
From the structure of the fixing station described in the above-mentioned Published Unexamined Japanese Patent Application No. 9-90787 (1997), it is understood that many of the belt-type fixing stations use a fixing roller having an elastic layer as well as a main driving roller for conveying a recording sheet. Also, it is understood that in many cases the position of the fixing roller is fixed in the fixing station because the driving force can easily be transmitted from an image forming apparatus to the fixing station.
However, when a fixing roller having a thick elastic layer is used as a main driving roller, a radius of the fixing roller measured from the center of the rotation axis to the fixing nip varies in an area between the leading and trailing edges due to deformation of the elastic layer and, therefore, it is difficult to reproduce the linear velocity of the fixing roller. Furthermore, the layer combining the elastic pr
Hachisuka Toshiharu
Yamada Masamichi
Ngo Hoang
Ricoh & Company, Ltd.
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