Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background of the Ihvention
The inuention relates to image fixing in electro-
photography, and is specifically concerned wïth a roller for
beat-fixing a toner image on a support sheet and method of
producing the same.
The most efficient way of fi2ing a toner image is
to heat the image by pressing the support sheet against a
heating roller to thereby fix the image by fusion or melt
adhesion. One disadvantage ,of such fixing is the so-called
"offset phenomenon". This phenomenon occurs when the toner
particles forming the image are rendered tacky by heating,
and portions of th'e toner image adhere to the periphery of
the heating roller~ and are thereby- transferred onto the
successive sheet or sheets which come in contact with the
roller. The cycle is repeated, and results in copies of
poor quality.
Several means have been devised to preclude or reduce
the effects of this offset phenomenon. For example, the
radially outw'~rd surface of the rollers used for heat fixing
of toner images has been made of materials which are very ~'
hard. Silicon oil has been applied to the radially outward
surface to provide a protective coating and to prolong the
service life of the roller. However, when there is no
silicon oil coating, or when there are discontinuities in
the silicon oil coating, the radially outward surface of
such rollers tends to deteriorate and their service life
tends to be shortened because of the heat to which thé
rollers are exposed and because of the friction bétween the
rollers and the toner image bearing sheets~
'Su'mmary of the InYention
The object of the invention îs to provide a device
for fixing electrophotographic toner images by means of
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an imaye fixing roller having a comparatively long service
life and tending to pre~ent the so-called offset phenomenon
in which a portîon of the toner image on one sheet adheres
to the roller and îs transferred onto a succeedîng sheet.
An image fixîng roller ~tilîzîng the invention
comprises a radially outer layer made of a material, consist-
ing of silicon rubber impregnated with silicon oil, which is
non-tacky, heat resistant and resîl;ent and is characterized
by rubber hardness of 20 to 70, tear strength of 10 to 15
kg/cm and tensile strength of below 70 kg/cm2. Ihe invention thus ingen-
iously utilizes the fact that a roller having a layer of such material sh~s
controlled surface deterioration when exposed to heat and friction, and
wears away at the surface so as to renew its fixing surface
as it is used for fixing toner images and to thus reduce the
possibility of toner particles from a previous toner image
transferred onto a successive sheet. The radially outer
surface of a roller constructed according to the invention
deteriorates when exposed to heat and to friction with the
support sheets bearing toner images, and wears off readily
so as to maintain a fresh fixing surface.
More particularly, there is provided: an image fixing
roller for electrophotography comprising a core, a roll layer
disposed around the core and made of silicon rubber which is
impregnated with silicon oil, said roll layer being adapted
to be heated for melting toner particles forming toner images
and the outer peripheral surface of the roll layer tending to
wear off when exposed to heat and frictî~on.
There i~ also provided: a method of producing an image
fixing roller for electrophotography comprising the steps of
setting a core in a mold, preparing a mixture of raw silicon
rubber having a viscosity- of above 1000 cs, vulcanizing agent
and silicon oil having a viscosity of beIow 1000 cs, înjecting
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the mixture into the mold and maintaining it at room
temperature until the mixture solid;fies, and removing a
thin layer of the outer surface of the molding.
Brief Descriptl~on of *he Drawing
Fig. 1 is a sectional view of an image fixing device
incorporating a roller constructed according to the invention;
Fig. 2 is a magnifi`cation of a portion of Fig~
Fig. 3 is a plan view of a fixing roller constructed --
according to the invention; and
Fig. 4 is a g~aph showing variations of rubber hardness
and tensile strength of silicon rubber against silicon oil
contained therein.
Detailed Description
Referring to Fig. 1, a support sheet 1 bearing a
toner image A on its upper side is moved in the direction
of the arrow B by suitable means (not shown). A fixing
roller 2 is disposed above the path of travel of the support
sheet 1, so that the radially outer peripheral surface of -
the roller 2 is in direct contact with the toner image A and
with the support sheet 1. The fixing roller 2 is supported
by a coaxial tubular shaft 3, which is made of a material
such as a metal and form the,core of the fixing roller 2. The
tubular shaft 3 is suitably rotatably supported such that
the roller 2 rotates in the direction of the arrow C. A
heat source 4 is disposed within the tubular shaft 3 to heat
the roller 2. The heat source 4 comprises a coil,heater which
is wound on a glass rod 5 disposed within the tubular shaft 3
and coaxial therewith. The tubular shaft 3 is surrounded -
by a roll layer 6 which is made of a material having a
special system which is prepared 1~n accordance with the
invention. The material preferably îs silicon rubber wh;ch
comprises network polymer and i-s lmpregnated with silicon oil,
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the oil being in the form of so-called free oil. Such
rubber is non-tacky, heat res;s-tant and resilient, has
rubber hardness of 20 to 7a ~Japanese Industrial Standards),
tear strength of 10 to 15 kg/cm and tensile strenyth of below
70 kg/cm .
A pressing roller 7 is di`sposed ïmmediately below the
fixing roller 2 and is constructed in an identical manner.
The corresponding portion of the two rollers are identified
by the same numerals, with the reference numerals of the
pressing roller 7 being primed. The pressing roller 7 is
suitably supported to rotate in the direction of the arrow D.
The rollers 2 and 7 are in pressing engagement with each other,
and the support sheet 1 bearing the toner image A moves
between the two rollers in the direction of the arrow B.
In operation, when the support sheet 1 bearing the
toner image A on its upper side moves between the image
fixing roller 2 and the pressing roller 7, the toner image A
is brought into direct contact with the radially outer
peripheral surface of the roller 2, wh.ich is heated to a
suitable temperature by means of the heat source 4. The toner
particles forming the image A are fused, and the toner image
A is fixed by melt adhesion to the support sheet 1. The
rollers 2 and 7 press against each other with suitable force
such that the pressure facilitates the fixing of the toner
image A.
As the roller 2 is heated by means of the heat source
4, and as i.t engages the support sheet 1 frictionally by
vi`.rtue of the deformation of the surface portions of roll
layer 6 as apparently shown in Fi.g~ 2, the radially outer
surface of the roll l~yer 6 tends to deteriorate and to
wear away. Thl.s deterioration and ~earIng away is considered
to be essentially effected by tearïng of the network polymer
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forming the silicon rubber, and helps prevent or at least
reduce the offset phenomenon, b.ecause toner particles which
may adhere to the surface of the fixing roller 2 are worn
away together with the surface to w-hi`ch they have adhered
and thus a new surface on the fïxing roller 2 together with
silicon oil is presented for the next support sheet. It
should be understood that the surface of the fixing roller
does not wear away in the course of fix;ng while silicon oil
is present at the surface ~ut starts wearing away only
after the silicon oil that was present at the surface has
been used up, and stops wearing away when a new surface having
a sufficient concentration of silicon oil is presented. :
Experiments have been carried out by using a fixing
roller constructed according to the invention and having a
radially outer layer 6 made of silicon rubber having rubber
hardness of 40. The results have shown that the radially
outer surface of the roller keeps wearing away at the rate
of 0.05 to 0.2 mm/10,000 sheets of B4 size (JIS) and the
roller keeps contacting succes:sive toner image bearing sheets
with a new surface.
Fig. 3 shows a fixing roller which is constructed
taking into account of uneven consumption or tearing away
of the layer surface thereof which would otherwise occur
when two or more sizes of support sheets are used. me
fixing roller 12 is adapted to use for supp`ort sheets of B4
and A4 sizes ~JI:S~ and comprises core shaft 13 and silicon
rubber layer 16 in simï.lar manner to that of Fig. 1. The
layer 16 h.as a slightly larger diameter portion, the axial
length of wh.i.ch corresponds to the width of A4 size CJIS~,
than th.at of the remaining end portions by, to say, 1 mm~
This construction helps prevent or at least reduce the
uneven consumption of the layer surface by means of preliminary
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compensating it.
In the embodiment shown and described above, the heat
source 4 is built in the tubular shaft 3, but it is to be
understood that it ;s not essential to provide a fixing
roller with a built-in heater, and that the radially outer
layer of the image fi`xing roller 2 may be heated by other
means, such as from outside the fl~xing roller 2. It is also
to be understood that the temperature of the roll layer 6,
and particularly- the temperature of its radially outer
surface, may be increased to a temperature higher than that
necessary for fixing, so that the radially outer surface of
the layer 6 becomes even more fragile and wears away even
more readily, and that it is possible in this manner to
control the rate at which the rad;ally outer surface of the
layer 6 wears away.
Proceeding to show a method of making the image fixing
roller described above, there is provided first a discussion
of some problems in forming a silicon rubber layer on the
surface of a backing (core) of metal or the like.
One of such problem arises when spraying the backing
surface wi.th a mixture of raw silicon rubber used as a base,
vulcanizing agent, filler and solvent, such as toluene, and
then heating the mixture on the backing surface at about
150C in order to remove the solvent. The silicon rubber
thus formed, however, is of no use for the present invention
because of its undesirably high rubber hardness, tear
strength. and tensile strength above 30, Cabove 15 kg/cm,
and 5Q to 70 kg~cm2, respectively-~.
Another problem arises when a mixture similar to the
above but without solvent is molded by injection molding
and then maintaïned at room temperature for 3 to 24 hours.
The resultant layer is also of no use because its properties
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are similar to those of the silicon rubber discussed
immediately above. In addition, it should be noted that
case-hardening is found at the outer surface of the layer,
resulting from a well-known interface reaction.
The present invention pro~ides the use of silicon
oil having a low viscosity, e.g., below 1000 cs, preferably
below 20Q cs for mitigating crosslinking of raw silicon
rubber or silicon oil of above 1000 cs, preferably above
2000 cs so that the resultant silicon rubber presents such
properties as required. Fig. 4 shows variations of these
properties against silicon oil content. The mixture thus
containing the silicon oil of low viscosity may be molded
by injection molding and then maintained at room temperature
for several hours, the number of hours dependent on the ~-
amount of catalyst that is used. me shell-formed on the
periphery of the silicon rubber layer by the case-hardening
discussed above may be removed by means of grinding. Proceed-
ing to molding, the core surface is preferably coated with
a primer, such as silicon varnish, silane co~pling or
silicon rubber in view of the poor adhesiveness between
silicon rubber and the core material which may be metal or
the like.
The following show usable materials for the base and
catalyst, usable range of low viscosity silicon oil content
and properties of silicon rubber thus obtained.
Materials
Base: dimethyl-poly-siloxane,
methylvi`nyl-polysiloxane,
phenyl-poly-siloxane, and
phIoro-polysiloxane
Catalyst: dibutyl-tin-dilaurate,
dibutyl-tîn-diacetate,
C
.
: : : . ., ' .. , ',
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tin octanate, and
lead octanate
Silicon oil content:
4 to 40 w%, preferably 20 to 30 w%
Properties: (preferred)
Rubber hardness: 15 to 50 ~JIS2 35 + 5
Tear strength: 10 to 15 kg/cm below 15 kg/cm
Tensile strength: 20 to 50 kg/cm about 25 kg/cm
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