Electrophotography – Image formation – Fixing
Reexamination Certificate
2001-05-11
2002-08-20
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000
Reexamination Certificate
active
06438349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a fixing member of a fixing assembly used in image-forming apparatus employing an image-forming process such as electrophotography or electrostatic recording. More particularly, this invention relates to a fixing member used in a heat fixing assembly by means of which an unfixed toner image formed and carried on a recording material (such as a transfer material, printing paper, photosensitive paper or electrostatic recording paper) by a transfer system or a direct system in an image-forming processing section is treated by heat fixing to form a fixed image. It also relates to a fixing assembly having such a fixing member and an image-forming apparatus having the fixing assembly.
2. Related Background Art
Conventionally, in fixing assemblies provided in image-forming apparatus employing an image-forming process such as electrophotography or electrostatic recording, heat fixing assemblies are widely used in which a recording material (hereinafter also “transfer material”) holding an unfixed image thereon is passed through a nip formed between a fixing roller and a pressure roller which are rotated in pressure contact with each other, to fix the toner image to the recording material. An example of an image-forming apparatus having a conventional heat fixing assembly is shown in FIG.
5
.
In the apparatus shown in
FIG. 5
, a fixing roller
10
which is a fixing member having a heating element comprises a hollow mandrel made of aluminum and a heating element halogen lamp provided inside the mandrel. From the interior of the hollow mandrel, a recording material is heated at a temperature high enough to cause the toner (toner image) held thereon to melt by supplying electricity from a power source (not shown).
On the periphery of the hollow mandrel, a releasing layer comprised of a material such as a polytetrafluoroethylene copolymer (PTFE) or a perfluoroalkoxytetrafluoroethylene copolymer (PFA) is formed which has an excellent performance for releasability and resistance to heat. The releasing layer is formed on the hollow mandrel by covering its surface with a material formed in a tube, or by coating it with a material by electrostatic spraying, dip coating or the like.
Such a fixing roller, however, has had a problem that it may cause a phenomenon of electrostatic offset in which the toner held on the recording material is electrostatically transferred to the fixing roller, resulting in a low image quality level.
Because of triboelectric charging taking place between the recording material and the fixing roller or because of transfer electric charges accumulated on the recording material, an electric field through which the toner on the recording material is attracted to the fixing roller is produced, so that a part of the toner image is transferred onto the fixing roller. After the fixing roller is rotated once, the toner thus transferred is fixed to the recording material to become a ghost on the image. This is called “electrostatic offset”.
The electrostatic offset is roughly grouped into two types, whole-area offset and release offset. The whole-area offset is a phenomenon where the recording material and the fixing member such as the fixing roller give and take electric charges mutually through triboelectric charging to cause an offset electric field steadily and the offset appears over the whole image area continuously. Meanwhile, the release offset is a phenomenon where the recording material hops at its rear end to come into strong contact with the fixing roller when the rear end of the recording material goes through the fixing assembly, so that it leaves potential history linearly on the fixing roller in its longitudinal direction, which potential causes an offset, and on the image the offset appears linearly in the principal scanning direction. Thus, the both are distinguishable.
To prevent such electrostatic offsets, in conventional apparatus the potential of the fixing roller is controlled at a constant value. Stated specifically, where a negatively chargeable toner is used, the fixing roller is subjected to antistatic treatment so as not to be positively charged, or is set electrically conductive and grounded so as to be made to have a potential of 0 V.
In an experiment, surface potential of the surface layer of such a fixing roller was measured with a surface potentiometer during paper feeding, to find that the surface layer stood charged only at tens of V even during paper feeding, thus an antistatic effect was confirmed.
Meanwhile, as image quality and process speed of electrophotographic apparatus have been made higher in recent years, a phenomenon called “smeared image trailing edges” (reading “smeared image-trailing-edges”; also “bleeding images”) has come to occur conspicuously, in which a part of horizontal-line images diffuses toward its rear end side to become broken when the horizontal-line images are fixed. The cause of such smeared image trailing edges is presumed to be chiefly the pressure ascribable to steam generated from the interior of paper. The phenomenon of smeared image trailing edges is detailed here with reference to FIG.
6
.
As shown in
FIG. 6
, steam having spouted at a space between a fixing roller
10
and a recording material
21
flows in the direction of an arrow and is compressed between the fixing roller
10
, the recording material
21
and a toner image
41
to disorder the toner image
41
.
As one of countermeasures for the smeared image trailing edges, it is proposed to provide a means for applying a bias voltage to the fixing roller
10
.
FIG. 7
shows an example of construction provided with such a bias-applying means (or bias power source)
12
as an example of countermeasures for the smeared image trailing edges.
As shown in
FIG. 7
, a fixing roller
10
has a fixing roller mandrel
10
a
formed in a hollow roll, and a releasing layer
10
b
which covers the periphery of the fixing roller mandrel
10
a.
To the fixing roller mandrel
10
a,
a DC bias of about 500 to 1,000 V is applied from the bias power source
12
. Here, a resistance element (not shown) of several M &OHgr; to tens of M &OHgr; is provided as safety resistance across the bias power source
12
and the fixing roller mandrel
10
a.
By the aid of an electric field generated by this bias, the toner image on the recording material
21
is electrostatically strongly held on the recording material
21
when the recording material
21
rushes into a fixing nip
31
. In this state, the recording material
21
with the toner image is held between the fixing roller
10
and a pressure roller
11
and transported therethrough. Hence, the smeared image trailing edges can be prevented from occurring on the recording material
21
even when the flow of steam as stated above is produced.
In the construction where the bias-applying means is provided as a countermeasure for the smeared image trailing edges, it is important to produce an electric field across the toner image
41
and the back of the recording material
21
through the recording material
21
, which electric field attracts the toner to the part of the recording material
21
. Accordingly, in such construction, a transport roller (not shown) serving as a contact member which comes into contact with the recording material
21
is provided immediately behind the fixing nip
31
, and this roller is set electrically conductive and grounded so that the voltage applied to the fixing roller
11
causes an electric current through the recording material
21
to produce the electric field across the toner image
41
and the back of the recording material
21
.
Here, any too low resistance of the fixing roller
10
may cause an increase in the voltage allotted to the safety resistance to lower the voltage that contributes to the formation of the electric field across the fixing roller
10
and the back of the recording material
21
, so that the effect of preventing the smeared image trailing edges may lower.
In the controlling of resista
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
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