BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a fixing apparatus used with electrophotographic
systems such as a copying machine, electrophotographic printer and the like and with
image forming systems such as an electrostatic recording apparatus and the like, for
fixing or fusing a non-fixed toner image formed on a recording sheet material, and
more particularly, it relates to a fixing apparatus for fixing a non-fixed toner image
onto a recording sheet material by pinching and conveying the recording sheet material
bearing the non-fixed toner image thereon by means of a pair of rotary members.
Related Background Art
[0002] In the past, as fixing apparatuses used with electrophotographic systems or with
image forming systems such as an electrostatic recording apparatus, a fixing apparatus
of roller type comprising a fixing roller incorporating a heating source such as a
halogen heater therein and a pressure roller urged against the fixing roller to be
rotated therewith and having an outer elastic layer made of silicone rubber has generally
been utilized. Such a fixing apparatus of roller type has been commonly in practice,
since it is more excellent than any other fixing apparatuses in view of reliable conveyance
of a recording sheet and/or good safety against fire, which might result from the
overheating of the heater.
[0003] However, in the above mentioned conventional fixing apparatus of roller type, there
arose a problem that, since the non-fixed toner image directly contacted the outer
surface of the roller, a portion of the non-fixed toner was transferred to the roller
surface to cause a so-called offset phenomenon.
[0004] In general, the offset phenomena generated in the above-mentioned fixing apparatus
can be divided into two groups, i.e., thermal offset (offset due to temperature) and
electrostatic offset (offset due to electrostatics).
[0005] The former, i.e. thermal offset, includes a low temperature offset wherein the temperature
of the toner in the fixing apparatus is too low to be fixed onto the recording sheet,
thus causing the offset, and a high temperature offset wherein the toner temperature
in the apparatus is high enough to melt the toner, thus causing the offset. These
offset phenomena can be avoided by properly determining the temperature of the fixing
roller by means of a temperature controlling means such as a thermistor on the basis
of the feature of the developer (toner), conveying speed of the recording sheet and
roller pressure.
[0006] On the other hand, the latter, i.e. electrostatic offset, may be caused by the fact
that the charged toner on the recording sheet is transferred onto the fixing roller
electrostatically. Since this offset phenomenon largely depends upon the kind of the
recording sheet and circumferential condition, it is difficult to control the transfer
of the toner (from the recording sheet to the fixing roller). At present, rather than
performing the control for directly decreasing the electric field which may cause
the electrostatic offset, the disadvantage due to the electrostatic offset, such as
the jamming of the recording sheet which may be caused by the smudge of the recording
sheet and/or fixing roller due to the offset toner has been partly eliminated by removing
the offset toner transferred to the fixing roller by means of a cleaning means such
as a felt pad or cleaning blade pressed against the fixing roller. Therefore, the
electrostatic offset problem has not yet still been solved.
[0007] Further, as well as the provision of the cleaning means, there has been proposed
a means for positively preventing the offset of the toner by forming the roller surface
with toner-separable material, see e.g. US-A- 4 616 917, or by applying the separating
agent such as silicone oil to the roller surface. However, the electrostatic offset
problem has still not yet been solved.
[0008] Furthermore, in order to reduce the toner offset, it is also known eg. from JP-A-
59 34 573 to float the fixing roller without earthing electrically (referred to as
"floating condition" hereinafter). In the case where the fixing roller is electrically
held in the floating condition, when a certain amount of toner is transferred or offset
to the fixing roller, the fixing roller is charged to the same charge polarity as
that of the charged toner, whereby the toner on the recording paper is repulsed from
the fixing roller, thus reducing the toner offset.
[0009] However, if the fixing roller is electrically held in the floating condition, particularly
in the low moisture circumstances, the fixing roller is charged to have a value of
a few kV due to the friction with the recording paper, thus causing a discharging
phenomenon, which may generate electric noise leading to an erroneous operation of
the image forming system.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide a fixing apparatus which can avoid
an electrostatic offset of toner.
[0011] Another object of the present invention is to provide a fixing apparatus which can
prevent an excessive charging of rotary members such as a fixing roller to minimize
electric noise, thus preventing an erroneous operation of an image forming system.
[0012] Other objects and features of the present invention will be apparent from the following
description in connection with the accompanying drawings.
[0013] According to the invention this object is accomplished by a fixing apparatus comprising
a movable member used for pinching and conveying a supporting material bearing a non-fixed
toner image thereon, thereby fixing, in use, the non-fixed toner image on said supporting
material;
the fixing apparatus being characterized by further comprising a rectifier element
connected between said movable member and an earth, said rectifier element being so
connected that said movable member holds a charge to prevent, in use, a toner offset.
[0014] The advantages of the invention will become apparent and obvious to those skilled
in the pertinent art upon referring to the following description provided in connection
with the accompanying drawings, of which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0015]
Figs. 1 and 2 are schematic sectional views for explaining a fixing apparatus according
to a preferred embodiment of the present invention;
Fig. 3 is a sectional view of a fixing apparatus to which the present invention is
applicable;
Fig. 4 is a graph showing the relation between current and voltage of a diode;
Fig. 5 is a schematic sectional view for explaining an image forming system having
a transfer roller and incorporating the fixing apparatus of Fig. 1;
Figs. 6, 8 and 9 are schematic sectional views of main portions of an image forming
system incorporating a fixing apparatus according to a second embodiment of the present
invention;
Fig. 7 is a schematic sectional view of a main portion of an image forming system
incorporating a fixing apparatus comparative to the embodiment of Fig. 6;
Figs. 10 and 11 are schematic sectional views of a fixing apparatus according to a
third embodiment of the present invention;
Figs. 12 and 13 are schematic sectional views showing alterations of the fixing apparatus
of Figs. 10 and 11; and
Figs. 14 and 15 are schematic sectional views showing further alterations of the fixing
apparatus of Figs. 12 and 13.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0016] The present invention will now be explained in connection with embodiments thereof
with reference to the accompanying drawings.
[0017] First of all, a fixing apparatus to which the present invention is applicable will
be explained with reference to Fig. 3.
[0018] The fixing apparatus F comprises a pair of movable members, i.e., a fixing roller
1 and a pressure roller 2 pressed against the fixing roller 1 and rotated together
with the latter. Incidentally, in this embodiment, the fixing roller 1 contacts a
non-fixed toner image ta formed on a recording sheet P (image bearing sheet). Further,
in the illustrated embodiment, while the movable members 1,2 are embodied as rollers,
the movable members 1,2 are not limited to such rollers, but may comprise endless
movable members such as endless belts.
[0019] The upper fixing roller 1 comprises a hollow core made of aluminium, iron and the
like, and a cover layer covering an outer cylindrical surface of the core and made
of material having good toner-separating ability, such as PTFE, PFA or silicone. Within
the fixing roller 1, there is provided a heater h such as a halogen lamp by which
the fixing roller 1 is heated. The lower pressure roller 2 comprises a core made of
iron, stainless steel and the like. An outer cylindrical surface of the core is coated
by toner-separable elastomer such as silicone rubber, fluororubber and the like. The
fixing roller 1 and the pressure roller 2 are urged against each other by an appropriate
bias means such as a spring (not shown) and are rotated in directions shown by the
arrows. A temperature sensing element 3 such as a thermistor is arranged to contact
the surface of the fixing roller 1, and detects the surface temperature of the fixing
roller 1. The energization of the heater h is controlled by a temperature adjusting
circuit on the basis of the surface temperature detected by the temperature sensing
element 3, whereby the surface temperature of the fixing roller 1 is automatically
maintained in a predetermined fixing temperature.
[0020] The reference numeral 4 designates a separating pawl or claw for separating the recording
sheet P from the fixing roller 1. An edge of the free end of the separating pawl 4
is pressed against the surface of the fixing roller 1 at an appropriate pressure.
The reference numeral 5 designates a cleaner such as a felt urged against the surface
of the fixing roller 1, which cleaner 5 can sweep away the toner and/or paper powder
adhered to the surface of the fixing roller 1. The reference numerals 6 and 7 designate
an inlet guide and an outlet guide, respectively, which can guide the recording sheet
P and are attached to a frame of the fixing apparatus F.
[0021] A toner image formed on a surface of a photo-sensitive drum (image bearing member)
is transferred onto the recording sheet P by means of a transfer charger (transfer
charger means). Thereafter, the recording sheet P is conveyed or fed through the inlet
guide 6 to the fixing apparatus F, wherein the recording sheet P is introduced into
a nip between the fixing roller 1 and the pressure roller 2 which are pressed against
each other and are rotated altogether. While passing through the nip, the non-fixed
toner image ta on the recording sheet P is thermally fixed on the recording sheet
P as a permanent fixed image tb by the heat from the fixing roller 1 and the pressure
between the rollers 1 and 2.
[0022] The recording sheet P which has passed through the nip between the rollers 1, 2 and
on which the images has been fixed is separated, at its leading edge, from the fixing
roller 1 by means of the separating pawl 4, and then is fed to a sheet path (not shown)
through the outlet guide 7. In this way, the recording sheet P is ejected onto an
ejector tray.
[0023] Incidentally, in Fig. 3, offset toner transferred from the recording sheet P to the
fixing roller 1 and adhered to the surface of the fixing roller 1 due to the offset
phenomenon is shown as tc. The present invention aims to minimize an amount of such
offset toner tc.
[0024] Next, a first embodiment of the present invention will be explained with reference
to Figs. 1 and 2. Fig. 1 shows an example that the toner as the developer forming
the non-fixed toner image ta on the recording sheet P to be fed to the fixing apparatus
is charged negatively (minus charge), whereas, Fig. 2 shows an example that such toner
is charged positively (plus charge).
[0025] The fixing roller 1 comprises a hollow core 1b and a toner-separable coating layer
1a surrounding an outer surface of the hollow core 1b. The pressure roller 2 comprises
a central core 2b and an outer elastic layer 2a.
[0026] In Figs. 1 and 2, the hollow core 1b of the fixing roller 1 is earthed through a
diode 100 acting as a rectifier element, a connection direction of which is so selected
that the fixing roller 1 can hold the electric charge having the same polarity as
that of the toner, both in Fig. 1 and Fig. 2.
[0027] Now, Fig. 4 shows a characteristic curve of the current and voltage of the diode
100. As seen from Fig. 4, in an area A where the diode 100 is connected in a normal
direction, although there may flow a large current, the voltage does not increase.
To the contrary, in an area B where the diode 100 is connected in a reverse direction,
only small current flow causes the voltage to increase up to the voltage yielding
point C, and then, if a further current flows, the voltage does not increase so much.
[0028] In consideration of such characteristic of the diode 100, the diode 100 is connected
to the fixing roller 1 in the connection direction as shown in Fig. 1 and Fig. 2.
In this case, even when only little toner is offset or transferred to the fixing roller
1, since the diode 100 is connected to oppose to the charge polarity of the toner,
the potential of the fixing roller 1 is changed to have the same charge polarity as
that of the toner, thus creating a repulsion force between the fixing roller 1 and
the toner, whereby the offset of the toner can be reduced.
[0029] Further, in the low moisture circumstances, if the fixing roller 1 is charged by
other causes than the offset of the toner, for example, if the fixing roller 1 is
charged to have the charge polarity opposite to that of the toner, the fixing roller
1 becomes substantially in the earthed condition. Whereas, even if the fixing roller
1 is charged to have the same charge polarity as that of the toner, the voltage does
not increase more than the voltage yielding point C (Fig. 4). Accordingly, it does
not have to be feared that the fixing roller 1 is charged up excessively.
[0030] By the way, it was found that the less the electrostatic capacity of the diode 100,
the more the amount of the offset toner tc could be reduced. The reason may be that,
for example, when the same amount of toner is offset or transferred to the fixing
roller 1, the smaller the connection capacity between the fixing roller 1 and a chassis,
the greater the potential fluctuation of the fixing roller 1, and thus, the greater
the electric field for repulsion against the toner on the recording sheet P. From
the test results it was found that the electrostatic capacity of the diode 100 required
to prevent the offset of the toner was preferably 100 pF or less.
[0031] Now, the above-mentioned "electrostatic capacity" corresponds to an electrostatic
capacity when the reverse bias in an area D, i.e. the reverse bias having a value
below the voltage yielding point C in the reverse direction (reverse direction voltage
yielding point) is applied to the diode 100.
[0032] Further, it was found that a diode 100 having the reverse direction voltage yielding
point C (Fig. 4) of 50 V or more was desirable to prevent the toner offset. The reason
may be that, if the reverse direction yielding voltage of the diode 100 is small,
when the toner is offset, since the potential of the fixing roller 1 cannot reach
above the reverse direction yielding voltage, the electric field sufficient to repulse
the toner on the recording sheet P cannot be obtained.
[0033] Fig. 5 shows an example wherein the present invention is applied to an image forming
system including a transfer means comprising a transfer member such as a transfer
roller 50 (transfer movable member) for transferring the non-fixed toner image ta
formed on the image bearing member 10 onto the recording sheet P.
[0034] The transfer roller 50 acting as the transfer member comprises a metallic core 50b
made of iron, stainless steel and the like and an outer conductive elastomer layer
50a. The transfer roller 50 is subjected to a bias voltage from an appropriate power
source (not shown). Unlike to a transfer charger not having a transfer roller 50,
since the transfer roller 50 can apply the bias voltage to the recording sheet P by
directly contacting the latter, it has an advantage that the erroneous or poor transfer
does not occur even in the high moisture circumstances. However, it was found that
the conventional fixing apparatus including the fixing roller 1 had a disadvantage
that, in the high moisture circumstances, the transferring current flows to the fixing
roller 1 through the transfer paper (recording sheet) P which is wet to reduce its
resistance, whereby the surface of the fixing roller 1 is charged to have the same
polarity as that of the transfer roller 50, thus attracting the toner toward the fixing
roller 1 to offset the toner.
[0035] The embodiment shown in Fig. 5 aims to eliminate such disadvantage. This example
shows the case wherein the toner has the negative polarity.
[0036] For example, regarding the arrangement shown in Fig. 5 which has a maximum paper
passing width of 220 mm and a transferring bias of + 500 V, when comparing the amount
of the offset tone
tc generated in the apparatus wherein the fixing roller 1 is earthed through the diode
100 having the electrostatic capacity of 20 pF and the reverse direction yielding
voltage of 600 V, and the amount of the offset toner
tc generated in the apparatus wherein the fixing roller 1 is directly connected to the
earth without such diode 100, in the high moisture circumstances, it could be seen
that the amount of the offset toner tc generated in the apparatus having the diode
100 could be reduced to 1/100 of that generated in the apparatus having the fixing
roller 1 directly connected to the earth.
[0037] Next, a second embodiment of the present invention will be explained. This embodiment
is realized to reduce the electrostatic offset which may be caused by the fact that
the transferring charge applied to the back of the recording sheet P and having the
charge polarity opposite to that of the toner leaks through the pressure roller 2.
[0038] Fig. 6 is a sectional view showing a main portion of the image forming system incorporating
a fixing apparatus according to the second embodiment.
[0039] In Fig. 6, a fixing roller 1 comprises a hollow metallic core made of aluminium,
iron and the like, an outer surface of which is coated by good toner-separable material
or tube such as PTFE, PFA or silicone rubber, and a heating source such as a harogen
lamp arranged within the core. The outer surface of the fixing roller 1 is maintained
to a given temperature by means of an appropriate temperature controlling means (not
shown).
[0040] A pressure roller 2 pressed against the fixing roller 1 and driven by the rotation
of the fixing roller 1 comprises a central metallic core made of iron, stainless steel
and the like, and an outer elastic layer made of silicone rubber of fluororubber.
[0041] The core of the pressure roller 2 is connected to GND (earth) through a diode 101
acting as a rectifier element in such a manner that the electric charge having the
charge polarity opposite to that of the non-fixed toner image ta is held on the pressure
roller 2; whereas, the core of the fixing roller 1 is also connected to GND through
a diode 102 acting as a rectifier element in such a manner that the electric charge
having the same charge polarity as that of the non-fixed toner image ta is held on
the fixing roller 1. In this embodiment, the example that the toner having the negative
(minus) charge polarity is used is shown.
[0042] On the other hand, a latent image is formed on a photosensitive drum 10 acting as
an image bearing member, by the charge from a charger 11 and the illumination 11 of
light information according to image information. The latent image is developed by
the developer (toner) in a developing device 13 to form a toner image. Incidentally,
the reference numeral 14 designates a cleaner for cleaning the photosensitive drum
10.
[0043] The toner image formed on the surface of the photosensitive drum 10 is transferred
onto the recording sheet P by applying the charge having the charge polarity (in this
case, positive or plus polarity) opposite to that of the toner to the back of the
recording sheet P by means of a transfer charger means 15. The recording sheet P on
which the toner image has been transferred is introduced into the nip between the
fixing roller 1 and the pressure roller 2, where the toner image is thermally fixed
onto the recording sheet P.
[0044] Normally, in the image forming system including a transferring process, the non-fixed
toner image ta transferred from the image bearing member 10 to the recording sheet
P by means of the transfer charger means 15 is strongly held on the recording sheet
P by the charge of the toner and the transferring charge having the polarity opposite
to that of the toner and applied to the back of the recording sheet P.
[0045] From the test results, it was found that the electrostatic toner offset was caused
by the fact that the transferring charge having the charge polarity opposite to that
of the toner and applied to the back of the recording sheet P to electrostatically
hold the toner on the recording sheet P leaks in the fixing apparatus through the
pressure roller 2 contacting with the back of the recording sheet P.
[0046] Particularly, the electrostatic offset occurs noticeably in the case where a recording
sheet P such as an OHP transparent film of sandwich structure obtained by coating
middle-resistive material having a surface resistance of about 10⁹ - 10¹² Ω on both
surfaces of dielectric film which has been widely utilized is used, rather than a
plain paper.
[0047] In such an OHP film, when it is separated from the photosensitive drum 10, the reverse
charge - Q′ depending upon the transferring charge + Q applied in the transfer charging
process is applied to a recording surface (surface on which the toner image is transferred)
of the OHP film by the separating discharge. According to the test results, the reverse
charge Q′ was about 50 to 90% of the transferring charge Q. Accordingly, it is considered
that, if the transferring charge leaks through the pressure roller 2, since the separating
charge remains on the recording surface of the OHP film, the electrostatic force for
holding the toner is reduced quickly, thus causing the toner offset.
[0048] For example, as shown in Fig. 7, in the fixing apparatus wherein both of the cores
of the pressure roller 2 and the fixing roller 1 are directly connected to the earth,
tests were effected by using the OHP sheet (having the surface resistance of about
10¹² Ω) sold by 3M corp. in the United States to examine the relation between the
offset and the leak current during passing through the OHP sheet with respect to the
pressure rollers 2 having various resistances. The test results is shown in the following
Table 1.
[0049] The resistance of the pressure roller 2 was obtained in such a manner that a metallic
roller made of stainless steel was pressed against the fixing roller 1 to create the
nip of about 2 - 3 mm and the resistance between the core of the pressure roller 2
and the metallic roller was measured by a resistance meter. Further, the fixing roller
1 was formed from a cylindrical body made of aluminium and coated by an insulating
PFA tube having a thickness of about 30 µm therearound.
[0050] If the resistance of the pressure roller 2 becomes 10¹² Ω or less, the transferring
charge ⊕ on the back of the recording sheet P is beginning to flow as the leak current
I
P to the GND through the pressure roller 2. As the resistance of the pressure roller
2 decreases, the leak current increases.
[0051] At the same time, the charge ⊖ having the same polarity as that of the toner and
being applied to the recording surface of the OHP sheet loses its holding force, and,
thus, the charge ⊖ is beginning to leak onto the surface of the fixing roller 1, whereby
the current I
F flowing from the GND to the core of the fixing roller 1 can be measured. If the resistance
of the pressure roller 2 decreases below 10¹² Ω, the amount of the toner offset decreases
more and more. On the other hand, if the resistance of the pressure roller 2 is above
10¹³ Ω, the leak current I
P cannot be measured, but the toner offset does not cease.
[0052] Generally, in the fixing apparatus of roller type, since the pressure roller 2 is
urged against the fixing roller 1 at a pressure of few Kg to ten Kg or more (1 kg/cm²
= 9,806 65·10⁴ Pa), in some cases, the surface of the pressure roller 2 is charged
to have the same polarity as that of the toner by the frictional charging according
to the kind of the recording sheet P, which results in the electrostatic offset. This
phenomenon occurs noticeably when the toner is charged to the minus charge polarity.
[0053] Accordingly, in order to prevent the charge-up of the frictional charging between
the pressure roller 2 and the recording sheet P, it is desirable to use the pressure
roller 2 having the resistance of 10¹² Ω or less.
[0054] Further, when the pressure roller 2 is maintained in the floating condition without
connecting it to the earth, even if the resistance of the pressure roller 2 is below
10¹² Ω, the leak of the transferring charge can be reduced.
[0055] Due to the reduction of the transferring charge, even if the resistance of the pressure
roller 2 is below 10¹² Ω, the offset of the toner can be reduced when the pressure
roller 2 is in the floating condition.
[0056] In this way, although it is possible to reduce the electrostatic offset by using
the pressure roller 2 having the low resistance and by keeping the core of the pressure
roller 2 in the floating condition, if the continuous printing operation is effected
or in the low moisture circumstances, it is feared that the transferring charge leaks
to surrounding conductors by the charge-up of the pressure roller 2, thus causing
erroneous electrical operation of the image forming system.
[0057] Accordingly, in an embodiment shown in Fig. 6, the diode 102 acting as the rectifier
element is interposed between the fixing roller 1 and the earth in such a manner that
a charge having the same charge polarity as that of the toner is maintained, and the
diode 101 acting as the rectifier element is interposed between the pressure roller
2 and the earth in such a manner that a charge having the charge polarity opposite
to that of the toner is maintained.
[0058] With this arrangement, it is possible to prevent the transferring charge ⊕ on the
back of the recording sheet P from leaking to the GND by the rectifier function of
the diode 101, and to make the potential of the fixing roller 1 to the same charge
polarity as that of the toner by the diode 102 and by the charge ⊕ having the same
polarity as that of the toner and applied to the recording surface of the OHP sheet,
whereby the repulsion force for repulsing the toner on the OHP sheet acts on the surface
of the fixing roller 1. In addition, since when the diode 101 is connected in the
reverse direction the voltage does almost not increase above the yielding voltage,
by selecting any diode having a proper yielding voltage, it is possible to prevent
the erroneous electrical operation of the image forming system due to the discharge
in consequence of the charge-up of the pressure roller 2.
[0059] The following Table 2 shows the leak current while passing through the OHP sheet,
offset, potential of the core of the fixing roller 1, and potential of the core of
the pressure roller 2, when diodes 101, 102 having the yielding voltage of 1 kV are
used in this embodiment.
[0060] The fixing roller 1 having a diameter of 20 mm and made of an aluminium tube coated
by a PFA tube having a thickness of 30 µm was used. The pressure roller 2 having a
diameter of 16 mm and having various resistances according to the amount of the resistance
controlling material was urged against the fixing roller 1 at a total pressure of
about 7 Kg (1 kg/cm² = 9,806 65·10⁴ Pa). The transferring current of 1.5 µA was applied
to the OHP sheet moving at a speed of 24 mm/sec.
[0061] In the pressure roller 2 having the resistance of 10¹⁰ - 10¹² Ω, it was found that
the potential of the core of the pressure roller 2 was maintained at about + 800 V
and the potential of the core of the fixing roller 1 was maintained at about - 50
V and the currents flowing from both rollers to the GND were few below 1 nA, and the
electrostatic offset regarding the OHP sheet did not occur.
[0062] Further, in order to positively prevent the excessive charge-up of each roller, it
is desirable to use a diode having a voltage yielding point C of 3 kV or less, preferably
2 kV or less.
[0063] Fig. 8 is a sectional view of a fixing apparatus according to an alteration of the
aforementioned embodiment. In this example, in order to prevent the electrostatic
attraction between the recording sheets P thereby improving the stacking ability of
the recording sheets P, a charge removing brush 16 contacting the back of the recording
sheet P and connected to the GND is arranged at an outlet of the fixing apparatus.
The pressure roller 2 was the resistance of 10¹⁰ - 10¹² Ω, as in the previous embodiment.
In this embodiment, when the back of the recording sheet P contacts with the charge
removing brush 16, the current of 100 - 200 nA flows through the charge removing brush
16 in a direction from the OHP sheet to the GND, thus removing the charge.
[0064] By removing the charge from the back of the recording sheet P by means of the charge
removing brush 16, the holding force given by the electrostatic force of the charge
having the same polarity as that of the toner remaining on the recording surface of
the OHP sheet decreases quickly. In this case, if the core of the fixing roller 1
was directly connected to the earth, the current of 10 - 20 nA flowed in a direction
from the GND to the core of the fixing roller 1, and, thus, the toner offset occured.
However, in the illustrated embodiment, since the diode 102 is interposed between
the fixing roller 1 and the GND, such current does not flow, but the potential of
the core of the fixing roller 1 1 is increased up to about - 600 V, whereby the non-fixed
toner image ta having a charge ⊖ on the OHP sheet is subjected to the electrostatic
force directing toward the back of the OHP sheet, thus preventing the occurrence of
the toner offset.
[0065] By the way, in the aforementioned embodiment shown in Figs. 6 and 8, when the electrostatic
offset is caused only by the leak of the transferring charge applied to the back of
the recording sheet P, it is not necessary to connect the rectifier element 102 to
the fixing roller 1, but, it should be noted that, as shown in Fig. 9, the diode 101
acting as the rectifier element may be connected to the pressure roller 2 alone in
such a manner that the charge having the same polarity as that of the transferring
charge is maintained, thus preventing the leak of the transferring charge. Further,
according to Fig. 9, even when the charge ⊖ having the polarity opposite to that of
the transferring charge is generated on the pressure roller 2 due to the friction
between the pressure roller 2 and the recording sheet P, it is possible to leak the
charge ⊖ to the GND because of the connection direction of the diode 101, thereby
preventing the reduction (due to the friction charge) of the holding force for holding
the toner onto the recording sheet P in the contacting area between the recording
sheet P and the pressure roller 2.
[0066] Incidentally, in the aforementioned embodiment shown in Figs. 6 and 8, wherein examples
in which the charge polarity of the toner is the negative or minus polarity and in
which the transferring charge has the positive or plus polarity, respectively, were
given, when the toner has the positive polarity and the transferring charge has the
negative polarity, the diodes may be connected in reverse directions opposite to those
in the aforementioned embodiment. Also in this case, the same technical effect can
be obtained.
[0067] Next, a third embodiment of the present invention will be explained. This embodiment
is realized to minimize the electrostatic offset which may be caused when the roller
surface is charged by the friction charge between the roller and the recording sheet
P.
[0068] In Fig. 10, a fixing roller 1 comprises a hollow metallic core 1b made of aluminium,
iron and the like, an outer surface of which is coated by a toner separable and heat-durable
resin layer 1a made of PTFE or PFA. A pressure roller 2 urged against the fixing roller
1 comprises a central metallic core 2b made of iron, stainless steel and the like,
and an outer toner-separable elastic layer 2a made of silicone rubber, fluororubber
and the like. The core 1b of the fixing roller 1 is electrically connected to the
core 2b of the pressure roller 2, and these cores are connected to the earth through
a rectifier element 103 in a normal connection direction.
[0069] With this arrangement, it is assumed that when the recording paper (recording sheet
P) passes through the nip between these rollers the pressure roller 2 is charged to
have the minus charge by the strong friction between the pressure roller 2 and the
recording paper P. In this case, the surface of the fixing roller 1 will be charged
to have weak plus charge. Since the rectifier element 103 is connected as shown in
Fig. 10, the minus charge stored in the pressure roller 2 cannot escape to the ground
(GND), and, thus, is accumulated in the core 2b of the pressure roller 2, with the
result that the potential of the core 1b of the fixing roller 1 is changed to the
minus charge. Consequently, the potential of the outer surface 1a of the fixing roller
1 is also changed to the minus charge, whereby the repulsion force is generated between
the fixing roller 1 and the toner having the minus charge, thus preventing the toner
offset.
[0070] While the example that the toner having the minus charge is used was explained, as
to the toner having the plus charge, as shown in Fig. 11, only the connection direction
of the rectifier element 103 may be reversed. By connecting the rectifier element
103 in the reverse direction, the minus charge stored in the pressure roller 2 escapes
to the ground; whereas, the outer surface 1a of the fixing roller 1 is charged to
the plus charge. Accordingly, the repulsion force is generated between the toner and
the fixing roller 1, thus preventing the toner offset.
[0071] Now, the test results effected by using the fixing apparatus according to the illustrated
embodiment will be explained.
[0072] The test conditions were as follows.
[0073] The fixing roller 1 comprising the core 1b having an outer diameter of 20 mm and
a wall thickness of 2 mm and coated by the resin tube layer 1a made of PFA and having
a thickness of 30 microns, and the pressure roller 2 comprising the core 2b having
an outer diameter of 10 mm and the outer elastic layer 2a made of LTV silicone rubber
and having a wall thickness of 3 mm were used. Coating lengths (longitudinal lengths)
of the PFA resin layer and silicone rubber layer were 226 mm and 222 mm, respectively.
The cores of these two rollers were electrically connected to each other and were
also connected to the earth through a diode DSA-1A4 manufactured by HITACHI SEISAKUSHO
Co., Ltd. (Japan), as shown in Fig. 10.
[0074] Further, the rollers were urged against each other to form a nip having a width (length
in a circumferential direction) of 2 mm and the OHP sheet was passed through the nip.
The toner having the minus charge was used.
[0075] As a result, the surface of the pressure roller 2 was frictionally charged to - 2000
V, and this charge was supplied to the core 1b of the fixing roller 1 to charge the
core 1b to have a value of - 750 V, with the result that the surface of the fixing
roller 1 was charged to - 250 V to generate the repulsion force between the toner
and the fixing roller 1, whereby the toner offset was prevented.
[0076] Incidentally, it is preferable that the rectifier element 103 which was used here
has the yielding voltage of 50 V to 500 V, because if the yielding voltage of the
rectifier element 103 is below 50 V it is impossible to apply the adequate charge
to the fixing roller 1 and to maintain such charge in the fixing roller 1, whereas
if the yielding voltage is above 500 V the discharge will be generated in the low
moisture circumstances, thus creating the electric noise which leads to the erroneous
operation of the image forming system.
[0077] Next, alterations or deformations of the apparatuses shown in Figs. 10 and 11 will
be explained with reference to Figs. 12 and 13. Incidentally, the same constructural
elements as those in Figs. 10, 11 will be designated by the same reference numerals
as those used in Figs. 10, 11, and the detailed description thereof will be omitted.
[0078] Fig. 12 shows a fixing apparatus used with the toner having the minus charge polarity.
In comparison with Fig. 10, in this embodiment, since the fixing roller 1 is connected
to the pressure roller 2 through a rectifier element 104 so that the fixing roller
1 is connected in the normal direction with respect to the pressure roller 2, the
core 1b of the fixing roller 1 is charged to the minus charge more strongly.
[0079] The potentials of the cores of the fixing roller 1 and the pressure roller 2 were
compared between the apparatuses shown in Figs. 10 and 12 using the same roller pair
as shown in the aforementioned test. As a result, in the apparatus shown in Fig. 10,
the potentials of the cores 1b, 2b of two rollers were the same and had a value of
- 750 V. On the other hand, in the apparatus shown in Fig. 12, the potential of the
core lb of the fixing roller 1 was - 950 V, whereas the potential of the core 2b of
the pressure roller 2 was - 650 V. In this way, by lowering the potential of the core
1b of the fixing roller 1b lower than that of the pressure roller 2, the surface 1a
of the fixing roller 1 was more strongly charged negatively to have a value of - 4000
V, thereby increasing the repulsion force against the toner to improve the offset
preventing effect.
[0080] Fig. 13 shows a fixing apparatus used with the toner having the plus charge polarity.
In this example, the rectifier element 104 is connected so that the plus charge can
be easily stored in the core 1b of the fixing roller 1. Also in the embodiment shown
in Fig. 13, the same technical effect as mentioned above was obtained.
[0081] Next, a further alterations of the apparatuses of Figs. 12, 13 will be explained
with reference to Figs. 14 and 15. Incidentally, the same constructural elements as
those in the aforementioned embodiments will be designated by the same reference numerals
as those used in the aforementioned embodiments, and the detailed description thereof
will be omitted.
[0082] In the embodiments shown in Figs. 10 to 13, while the material of the surface 1a
of the fixing roller 1 was fluororesin such as PFA and the material of the surface
2a of the pressure roller 2 was silicone rubber, in the embodiments shown in Figs.
14 and 15, these surface materials are reversed. That is to say, the material of the
surface of the fixing roller 1 is constituted by a thin elastic layer 1c made of silicone
rubber, and the material of the surface of the pressure roller 2 is constituted by
a fluororesin layer 2c such as PFA. In particular, as an example, the pressure roller
2 comprises a core 2b and a silicone rubber layer 2a, and further includes an outer
PFA tube 2c surrounding the silicone rubber layer 2a.
[0083] By constituting the surface of the fixing roller 1 with the elastic material in this
way, the fixing ability is increased, and the overpressure to the image is prevented,
thus obtaining a good image.
[0084] Fig. 14 shows the embodiment with a toner having the minus charge polarity. In this
embodiment, since the surface of the fixing roller 1 is covered by the silicone rubber,
the surface is apt to be charged to the minus charge polarity. Accordingly, the rectifier
element 104 is connected in the normal connection direction so that the potential
of the core 1b charged by the charge generated on the surface of the fixing roller
1 is held to the more minus charged condition.
[0085] In this embodiment, the potential of the surface of the fixing roller 1 becomes -
2 kV to generate a strong repulsion force against the toner having the minus charge,
thus preventing the toner offset. Incidentally, in this case, the surface of the pressure
roller 2 is charged to + 250 V, and the rectifier element 103 is connected in such
a manner that the plus charge in the core 2b cannot be easily escape to the ground.
In this way, since an attracting force for attracting the toner toward the pressure
roller 2 from the back of the recording sheet P, the offset preventing effect is still
improved.
[0086] Fig. 15 shows the embodiment with a toner having the plus charge polarity. In this
example, the offset is prevented by designing that the plus charge is apt to be stored
in the core 1b of the fixing roller 1 and the minus charge is apt to be stored in
the core 2b of the pressure roller 2.
[0087] As mentioned above, according to this embodiment, since the fixing roller 1 and the
pressure roller 2 are connected to each other and are also connected to the earth
through the rectifier element so that the charge having the same polarity as that
of the toner can be maintained, even if the surface of the fixing roller 1 is charged
to the charge polarity opposite to that of the toner or developer, the offset can
be prevented, and, since the rollers are connected to the earth through the rectifier
element, the electric noise due to the discharge can be avoided, thus preventing the
erroneous operation of the image forming system.
[0088] As mentioned above, according to the present invention, by connecting the rectifier
element to the fixing movable member such as a roller, belt, endless sheet and the
like in consideration of the connection direction thereof properly, the electrostatic
toner offset can be prevented. Further, the electric noise generated by the excessive
charging of the fixing movable member can also be avoided, thus preventing the image
forming system from operating erroneously due to the electric noise.
1. A fixing apparatus, comprising:
a movable member (1, 1a, 1b, 2, 2a, 2b) used for pinching and conveying a supporting
material (P) bearing a non-fixed toner image (ta) thereon, thereby fixing, in use,
the non-fixed toner image (ta) on said supporting material (P);
said fixing apparatus being characterized by further comprising a rectifier element (100; 101, 102; 103, 104) connected between
said movable member (1, 1a, 1b, 2, 2a, 2b) and an earth, said rectifier element (100;
101, 102; 103, 104) being so connected that said movable member (1, 1a, 1b, 2, 2a,
2b) holds a charge to prevent, in use, a toner offset.
2. A fixing apparatus according to claim 1, characterized in that said movable member (1, 1a, 1b, 2, 2a, 2b) is contacted, in use, with the non-fixed
toner image (ta), and wherein said rectifier element (100; 101, 102; 103, 104) is
so connected that said movable member (1, 1a, 1b, 2, 2a, 2b) holds a charge having
the same polarity as the toner.
3. A fixing apparatus according to claim 1, characterized in that said movable member (1, 1a, 1b, 2, 2a, 2b) is contacted, in use, with a surface of
the supporting material (P) opposite to a surface on which the non-fixed image (ta)
is supported, and wherein said rectifier element (100; 101, 102; 103, 104) is so connected
that said movable member (1, 1a, 1b, 2, 2a, 2b) holds a charge having the same polarity
as the toner.
4. A fixing apparatus according to claim 1, characterized in that said fixing apparatus is used with an image forming system having an image bearing
member (10) for bearing the non-fixed image (ta), and transfer charger means (15)
for transferring the toner image formed on said image bearing member (10) onto the
supporting material (P); and wherein said movable member (1, 1a, 1b, 2, 2a, 2b) is
contacted with a surface of the supporting material (P) charged by said transfer charger
means (15), and said rectifier element (100; 101, 102; 103, 104) is so constructed
that said movable member (1, 1a, 1b, 2, 2a, 2b) holds a charge having the same polarity
as that obtained by said transfer charger means (15).
5. A fixing apparatus according to any of claims 1 to 4, characterized in that said movable member (1, 1a, 1b, 2, 2a, 2b) is heated by heating means (h).
6. A fixing apparatus according to any of claims 1 to 5, characterized in that said rectifier means (100; 101, 102; 103, 104) is a diode.
7. A fixing apparatus according to claim 3, characterized in that said movable member (1, 1a, 1b, 2, 2a, 2b) has a resistance of 10¹²Ω or less.
8. A fixing apparatus according to any of claims 1 to 7, characterized by further comprising charge removing means (16) contacting, in use, a back surface
of the supporting material (P) after the fixing of the toner image thereon and arranged
down stream of a nip portion formed between a pair of rotary members for fixing with
respect to a feeding direction of said supporting material.
1. Fixiergerät mit einem bewegbaren Element (1, 1a, 1b, 2, 2a, 2b), das für ein Klemmen
und Fördern eines Trägermaterials (P) verwendet wird, welches ein nicht-fixiertes
Tonerbild (ta) trägt, wodurch im Betrieb das nicht-fixierte Tonerbild (ta) auf dem
Trägermaterial (P) fixiert wird, dadurch gekennzeichnet, daß das Gerät ferner ein Gleichrichterelement (100; 101, 102; 103, 104) aufweist, das
zwischen dem bewegbaren Element (1, 1a, 1b, 2, 2a, 2b) und einer Erde angeschlossen
ist und das so verbunden ist, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) eine
Ladung aufweist, um im Betrieb eine Tonerversetzung zu verhindern.
2. Fixiergerät nach Anspruch 1, dadurch gekennzeichnet, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) im Betrieb mit dem nicht-fixierten Tonerbild
(ta) in Berührung steht und das Gleichrichterelement (100; 101, 102; 103, 104) so
angeschlossen ist, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) eine Ladung aufweist,
welche die gleiche Polarität wie der Toner hat.
3. Fixiergerät nach Anspruch 1, dadurch gekennzeichnet, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) im Betrieb mit der Oberfläche des Trägermaterials
(P) gegenüber zu einer Oberfläche in Berührung steht, auf der das nicht-fixierte Bild
(ta) gelagert ist, und das Gleichrichterelement (100; 101, 102; 103, 104) so angeschlossen
ist, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) eine Ladung aufweist, welche
die gleiche Polarität wie der Toner hat.
4. Fixiergerät nach Anspruch 1, dadurch gekennzeichnet, daß das Fixiergerät mit einem Bildherstellungssystem verwendet wird, das ein Bildträgerelement
(10) zum Tragen des nicht-fixierten Bildes (ta) und ein Übertragungsbeschickermittel
(15) für die Übertragung des auf dem Bildträgerelement (10) gebildeten Tonerbildes
auf das Trägermaterial (P) aufweist, daß das bewegbare Element (1, 1a, 1b, 2, 2a,
2b) mit einer Fläche des Trägermaterials (P) in Berührung steht, welches durch das
Übertragungsbeschickermittel (15) geladen ist, und daß das Gleichrichterelement (100;
101, 102; 103, 104) so aufgebaut ist, daß das bewegbare Element (1, 1a, 1b, 2, 2a,
2b) eine Ladung aufweist, welche die gleiche Polarität wie diejenige Ladung aufweist,
die durch das Übertragungsbeschickermittel (15) erzielt wird.
5. Fixiergerät nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) durch ein Heizmittel (h) erhitzt wird.
6. Fixiergerät nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, daß das Gleichrichtermittel (100; 101, 102; 103, 104) eine Diode ist.
7. Fixiergerät nach Anspruch 3, dadurch gekennzeichnet, daß das bewegbare Element (1, 1a, 1b, 2, 2a, 2b) einen Widerstand von 10¹² Ω oder weniger
aufweist.
8. Fixiergerät nach einem der Ansprüche 1 bis 7, gekennzeichnet durch ein Ladungsentfernungsmittel (16), das im Betrieb nach der Fixierung des Tonerbildes
mit einer Rückenfläche des Trägermaterials (P) in Berührung steht und das bezüglich
einer Zuführungsrichtung des Trägermaterials stromab von einem Spaltenabschnitt angeordnet
ist, der zwischen einem Paar von Drehelementen für die Fixierung ausgebildet ist.
1. Appareil de fixage, comportant :
un élément mobile (1, 1a, 1b, 2, 2a, 2b) utilisé pour pincer et transporter un
milieu de support (P) portant une image (ta) en toner non fixée, afin de fixer, pendant
l'utilisation, l'image (ta) en toner non fixée sur ledit milieu de support (P) ;
ledit appareil de fixage étant caractérisé en ce qu'il comporte en outre un élément
redresseur (100 ; 101, 102 ; 103, 104) connecté entre ledit élément mobile (1, 1a,
1b, 2, 2a, 2b) et la terre, ledit élément redresseur (100 ; 101, 102 ; 103, 104) étant
connecté de manière que ledit élément mobile (1, 1a, 1b, 2, 2a, 2b) maintienne une
charge pour empêcher, pendant l'utilisation, un maculage par le toner.
2. Appareil de fixage selon la revendication 1, caractérisé en ce que ledit élément mobile
(1, 1a, 1b, 2, 2a, 2b) est mis en contact, pendant l'utilisation, avec l'image (ta)
en toner non fixée, et dans lequel ledit élément redresseur (100 ; 101, 102 ; 103,
104) est connecté de manière que ledit élément mobile (1, 1a, 1b, 2, 2a, 2b) maintienne
une charge ayant la même polarité que le toner.
3. Appareil de fixage selon la revendication 1, caractérisé en ce que ledit élément mobile
(1, 1a, 1b, 2, 2a, 2b) est mis en contact, pendant l'utilisation, avec une surface
du milieu de support (P) opposée à une surface sur laquelle l'image non fixée (ta)
est supportée, et dans lequel ledit élément redresseur (100 ; 101, 102 ; 103, 104)
est connecté de manière que ledit élément mobile (1, 1a, 1b, 2, 2a, 2b) maintienne
une charge ayant la même polarité que le toner.
4. Appareil de fixage selon la revendication 1, caractérisé en ce qu'il est utilisé avec
un système de formation d'images comportant un élément porteur d'image (10) destiné
à porter l'image non fixée (ta), et un moyen (15) de charge de report destiné à reporter
l'image en toner formée sur ledit élément porteur d'image (10) sur le milieu de support
(P) ; dans lequel ledit élément mobile (1, 1a, 1b, 2, 2a, 2b) est mis en contact avec
une surface du milieu de support (P) chargée par ledit moyen (15) de charge de report,
et ledit élément redresseur (100 ; 101, 102 ; 103, 104) est réalisé de manière que
ledit élément mobile (1, 1a, 1b, 2, 2a, 2b) maintienne une charge ayant la même polarité
que celle obtenue par ledit moyen (15) de charge de report.
5. Appareil de fixage selon l'une quelconque des revendications 1 à 4, caractérisé en
ce que ledit élément mobile (1, 1a, 1b, 2, 2a, 2b) est chauffé par un moyen chauffant
(h).
6. Appareil de fixage selon l'une quelconque des revendications 1 à 5, caractérisé en
ce que ledit moyen redresseur (100 ; 101, 102 ; 103, 104) est une diode.
7. Appareil de fixage selon la revendication 3, caractérisé en ce que ledit élément mobile
(1, 1a, 1b, 2, 2a, 2b) possède une résistance de 10¹² Ω ou moins.
8. Appareil de fixage selon l'une quelconque des revendications 1 à 7, caractérisé en
ce qu'il comporte en outre un moyen (16) d'élimination de charge en contact, pendant
l'utilisation, avec une surface arrière du milieu de support (P) après le fixage de
l'image en toner sur celui-ci, et agencé en aval d'une partie de serrage formée entre
deux éléments rotatifs pour réaliser un fixage par rapport à une direction d'avance
dudit milieu de support.