Toner image fixing apparatus capable of keeping constant fixing roller temperature

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for and a method of fixing a toner image to a recording medium by fusing and pressing the toner image to the recording medium in an image forming system such as a copying machine, a printer, a facsimile machine, etc.

2. Description of the Related Art

FIG. 14

of the accompanying drawings shows a conventional recent toner image fixing apparatus for use in electrophotographic machines. As shown in

FIG. 14

, the toner image fixing apparatus has a belt fixing system comprising a fixing roller R

1

, a heating and tensioning roller R

3

, an endless fixing belt B trained around the rollers R

1

, R

3

, and a pressing roller R

2

disposed below and pressed against the fixing roller R

1

with the fixing belt B interposed therebetween. When a recording medium D in the form of a sheet with an unfixed toner image carried thereon is fed into the toner image fixing apparatus by a sheet feeder, the recording medium D is reheated by the heating and tensioning roller R

3

, and then the toner image is fixed to the recording medium D by the fixing belt B in a nipping region between the rollers R

1

, R

2

. Since the recording medium D is preheated, the nipping region may be set to a relatively low temperature. The fixing belt B is of such a small heat capacity that when the recording medium D passes through the nipping region, the temperature of the fixing belt B is quickly lowered to increase the coherent ability of the toner which is separated from the fixing belt B at the outlet of the nipping region, for thereby allowing the toner to be easily separated from the fixing belt B. Even if the fixing belt B is free of oil or coated with a small amount of oil, a clear fixed toner image can be produced on the recording medium D without offsets. The toner image fixing apparatus shown in

FIG. 14

is thus capable of solving the problems of toner separation and oil coating, which have not been eliminated by other toner image fixing apparatus using only a heating roller.

The conventional toner image fixing apparatus shown in

FIG. 14

will be described in greater detail. The pressing roller R

2

is positioned directly beneath the fixing roller R

1

, and the heating and tensioning roller R

3

is disposed upstream of the fixing roller R

1

with respect the direction in which the recording medium D is fed into the toner image fixing apparatus along the fixing belt B that is trained around the rollers R

1

, R

3

.

The toner image fixing apparatus also has an oil coating roller R

4

disposed above an upper run of the fixing belt B. A guide plate G for supporting the recording medium D is disposed below a lower run of the fixing belt B, and a gap between the guide plate G and the lower run of the fixing belt B serves as a preheating passage P for preheating the recording medium D when the recording medium D travels below the heating and tensioning roller R

3

toward the nipping region.

The fixing belt B is tensioned to a desired tension level when the heating and tensioning roller R

3

is pushed away from the fixing roller R

1

by a pressing lever U. The fixing belt B is actuated by the fixing roller R

1

which is coupled to an actuator. Since the fixing belt B is appropriately tensioned, it can stably rotate around the rollers R

1

, R

3

without undesirable slippage and sagging.

A heater H is housed in the heating and tensioning roller R

3

. The heating and tensioning roller R

3

is associated with a thermistor S for measuring the temperature of the surface of the heating and tensioning roller R

3

. The fixing belt B on the heating and tensioning roller R

3

has a sheet-contact area which is contacted by the recording medium D that is fed from the sheet feeder and a non-sheet-contact area which is not contacted by the recording medium D that is fed from the sheet feeder. The thermistor S is kept out of contact with the sheet-contact area of the fixing belt B on the heating and tensioning roller R

3

, but held in contact with the non-sheet-contact area of the fixing belt B on the heating and tensioning roller R

3

.

During the fixing process, based on a signal from the thermistor S, a controller (not shown) connected to the thermistor S controls the amount of heat generated by the heating and tensioning roller R

3

so that the temperature of the surface of the heating and tensioning roller R

3

will be kept at a preset level.

The temperature of the fixing belt B on the fixing roller R

1

varies depending on the period of time in which the fixing belt B has rotated, and is not constant when the recording medium D passes through the nipping region. If the period of time in which the fixing belt B has rotated is short, then the temperature of the fixing belt B on the fixing roller R

1

is low. In order to increase the temperature of the fixing belt B on the fixing roller R

1

, it is necessary to increase a temperature setting for the heating and tensioning roller R

3

for thereby bringing the temperature of the fixing belt B on the fixing roller R

1

into a toner image fixing temperature range at all times.

If the toner image fixing apparatus shown in

FIG. 14

is used to produce successive full-color copies, since the period of time in which the fixing belt B has rotated increases, the temperature of the fixing belt B on the fixing roller R

1

also increases, and so does the temperature of the outlet of the nipping region.

FIG. 15

of the accompanying drawings shows temperature characteristics of the toner image fixing apparatus shown in FIG.

14

.

In

FIG. 15

, the horizontal axis represents the period of time in which the fixing belt B has rotated, and the vertical axis represents the temperature of the fixing belt B on the rollers R

1

, R

3

. First, a temperature characteristic of the toner image fixing apparatus at the time the amount of heat radiated by the heater H is controlled in order to equalize the temperature of the fixing belt B on the heating and tensioning roller R

3

to a preset temperature T

2

will be described below. A solid-line wavy curve W

1

represents the temperature of the fixing belt B on the heating and tensioning roller R

3

, and a solid-line curve C

1

represents the temperature of the fixing belt B on the fixing roller R

1

.

After a standby period, as the period of time in which the fixing belt B has rotated increases, the temperature of the fixing belt B on the fixing roller R

1

increases. When the temperature of the fixing belt B on the fixing roller R

1

exceeds an upper limit temperature T

1

of a toner image fixing temperature range, the possibility of hot sheet offsets, i.e., sheet offsets at high temperatures, or sheet jams increases. When the temperature of the fixing belt B on the fixing roller R

1

becomes lower than a lower limit temperature T

1

′ of the toner image fixing temperature range, the possibility of cold sheet offsets, i.e., sheet offsets at low temperatures, or unfixed toner regions increases. Therefore, the temperature of the fixing belt B on the fixing roller R

1

should be kept in the toner image fixing temperature range which lies between the upper limit temperature T

1

and the lower limit temperature T

1

′.

The above drawback, i.e., sheet offsets and sheet jams, can be avoided when the temperature of the fixing belt B on the heating and tensioning roller R

3

is set to a temperature T

2

′, lower than the preset temperature T

2

, such that the temperature of the fixing belt B on the fixing roller R

1

will be equal to or below the upper limit temperature T

1

at its maximum, as indicated by broken-line characteristics curves W

2

, C

2

. However, it will take a longer period of time for the temperature of the fixing belt B on the fixing roller R

1

to reach the lower limit temperature T

1

′ of the toner image fixing temperature range, with the result that a fixation readiness time, i.e., a period of time required for the toner image fixing apparatus to become ready for fixing toner images, increases from TS to TS′.

After the toner image on the recording medium D is fixed, the sheet feeder for feeding the recording medium D into the toner image fixing apparatus is deactivated, the operation of the fixing belt B is stopped, and the heater H is de-energized, whereupon the toner image fixing apparatus enters a standby mode. Once the toner image fixing apparatus enters the standby mode, the surface temperatures of the fixing belt B and the fixing roller R

1

fall gradually. If the standby mode continues for a long period of time, then the fixing belt B and the fixing roller R

1

become so cold that when a fixing process is started again, it will take a long period of time before the fixing roller R

1

is heated to the toner image fixing temperature range. As a result, the operator has to wait a long period of time before the toner image fixing apparatus is operational again.

To alleviate the above deficiency, there has been proposed a priority control process which employs an auxiliary thermistor (not shown) for measuring the temperature of the surface of the fixing roller R

1

. According to the proposed priority control process, as shown in

FIG. 16

of the accompanying drawings, until the surface temperature of the fixing roller R

1

rises nearly to the toner image fixing temperature range, the amount of heat radiated by the heater H is controlled on the basis of the surface temperature of the heating and tensioning roller R

3

as measured by the thermistor S. When the surface temperature of the fixing roller R

1

increases beyond the toner image fixing temperature range, the amount of heat radiated by the heater H is controlled on the basis of the surface temperature of the fixing roller R

1

as measured by the auxiliary thermistor. The priority control process is effective to prevent sheet offsets and sheet jams from occurring, and also to shorten the period of time required to heat the fixing roller R

1

to the toner image fixing temperature range after the standby mode.

Image forming systems such as electronic copying machines, electronic printers, etc. which incorporate the above toner image fixing apparatus are required in recent years to operate at a higher speed to meet demands for a higher sheet feed speed, i.e., an increased number of sheets fed per unit time through the toner image fixing apparatus. To meet such requirements, the fixing belt B needs to run at a higher speed, which results in a reduction in the amount of heat that is transferred per unit time from the heating and tensioning roller R

3

to the fixing belt B.

As described above, the thermistor S is held in contact with the non-sheet-contact area of the fixing belt B on the heating and tensioning roller R

3

. When sheets, e.g., recording mediums D, are successively fed into the toner image fixing apparatus, since the non-sheet-contact area of the fixing belt B on the heating and tensioning roller R

3

is not contacted by the sheets, the heat in the non-sheet-contact area of the fixing belt B is not dissipated, but stored therein, so that the temperature as measured by the thermistor S increases to a level beyond a heater control switching point shown in FIG.

16

. When the heater control switching point is reached while successive sheets are being fed into the toner image fixing apparatus, the controlling of the amount of heat radiated by the heater H on the basis of the surface temperature of the fixing roller R

1

as measured by the auxiliary thermistor switches to the controlling of the amount of heat radiated by the heater H on the basis of the surface temperature of the heating and tensioning roller R

3

as measured by the thermistor S.

As a consequence, though the amount of heat radiated by the heater H is kept at a constant level based on the temperature measured by the thermistor S, the heat of the fixing roller R

1

is greatly absorbed by the sheets that are being fed successively at a high speed. Therefore, as shown in

FIG. 17

of the accompanying drawings, the surface temperature of the fixing roller R

1

gradually falls. According to the priority control process, since the surface temperature of the fixing roller R

1

gradually falls while sheets are being fed successively at a high speed, toner images may not be fixed to the sheets with good toner image fixability.

It has been proposed to incorporate another heater in the pressing roller R

2

to meet the requirements for the toner image fixing apparatus to operate at a higher speed.

When small-size sheets or recording mediums D are successively fed into the toner image fixing apparatus, those sheets are not brought into contact with a non-sheet-contact area of the heating and tensioning roller R

3

which is associated with the thermistor S. Therefore, the non-sheet-contact area of the heating and tensioning roller R

3

stores a large amount of heat, and hence its temperature rises excessively, as shown in FIG.

15

.

When the temperature non-sheet-contact area of the heating and tensioning roller R

3

increases excessively, the surface temperature of the fixing roller R

1

also increases excessively. The fixing roller R

1

thus tends to deteriorate soon, have a shortened service life, cause an increased energy loss, and pose safety problems.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a toner image fixing apparatus which is capable of fixing an unfixed toner image carried on a recording medium to the recording medium with good toner image fixability even when the recording medium is fed at an increased speed.

Another object of the present invention is to provide a toner image fixing apparatus which is capable of holding the surface temperature of a fixing roller substantially in a toner image fixing temperature range even when a recording medium with an unfixed toner image carried thereon is fed at an increased speed.

Still another object of the present invention is to provide an apparatus for and a method of fixing a toner image to a recording medium while preventing the surface temperature of a fixing roller from increasing excessively even when the recording medium is fed at an increased speed.

Yet another object of the present invention is to provide an apparatus for and a method of fixing a toner image to a recording medium while holding the surface temperature of a fixing roller substantially in a toner image fixing temperature range even when the recording medium is fed at an increased speed.

The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a sectional front elevational view of a toner image fixing apparatus according to an embodiment of the present invention;

FIG. 2

is a schematic view showing the manner in which a fixing roller and a pressing roller are held in rolling contact with each other;

FIG. 3

is a cross-sectional view of a heating roller with a first heat source disposed therein;

FIG. 4

is a schematic front elevational view of an actuating mechanism of the toner image fixing apparatus shown in

FIG. 1

;

FIG. 5

is a block diagram of a control system for controlling heat sources in the toner image fixing apparatus shown in

FIG. 1

;

FIG. 6

is a diagram showing angles employed in an experiment conducted to check an allowable range of positions of the heating roller with respect to the fixing roller;

FIG. 7

is a flowchart of the main routine of a control sequence carried out by a controller of the control system for controlling the heat sources;

FIG. 8

is a flowchart of the subroutine of a standby mode control process in the main routine shown in

FIG. 7

;

FIG. 9

is a flowchart of the subroutine of a sheet feed mode control process in the main routine shown in

FIG. 7

;

FIG. 10

is a diagram showing the manner in which the temperatures of a fixing belt on the rollers vary when the control sequence is carried out;

FIG. 11

is a flowchart of a standby mode control process according to a first modification;

FIG. 12

is a flowchart of a sheet feed mode control process according to the first modification;

FIG. 13

is a block diagram of a circuit arrangement for detecting a temperature failure according to a second modification for the toner image fixing apparatus;

FIG. 14

is a sectional front elevational view of a conventional toner image fixing apparatus;

FIG. 15

is a diagram showing the manner in which the temperatures of a fixing belt on rollers of the conventional toner image fixing apparatus shown in

FIG. 14

vary when a control process is carried out to keep the surface temperature of a heating roller at a constant level;

FIG. 16

is a diagram showing the manner in which the temperatures of the fixing belt on the rollers of the conventional toner image fixing apparatus shown in

FIG. 14

vary when a priority control process is carried out; and

FIG. 17

is a diagram showing the manner in which the temperatures of the rollers of the conventional toner image fixing apparatus shown in

FIG. 14

vary when the priority control process is carried out while successive sheets are fed into the toner image fixing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Structure of Toner Image Fixing Apparatus

10

As shown in

FIG. 1

, a toner image fixing apparatus

10

according to an embodiment of the present invention has a housing

12

to be fixed to a frame of an electronic image forming system (not shown) such as an electronic printer, for example. The housing

12

comprises a base plate

14

to be fixed directly to the frame, a pair of vertical side plates

16

erected from respective side edges of the base plate

14

, an upper cover

18

mounted on the side plates

16

to cover upper right regions of the side plates

16

, and a left cover

20

mounted on the side plates

16

to cover left side regions of the side plates

16

.

The upper cover

18

is fixedly mounted on the side plates

16

. A swing lever

22

is swingably supported on right portions of the side plates

16

by a first pivot shaft

24

positioned on a right end of the swing lever

22

, for swinging movement about the first pivot shaft

24

to provide an open space at a left end of the swing lever

22

. The left cover

20

is swingably supported on the side plates

16

by a second pivot shaft

26

positioned on a lower end of the left cover

20

, for swinging movement about the second pivot shaft

24

to provide an open space at an upper end of the left cover

20

.

The toner image fixing apparatus

10

has a roller assembly including a fixing roller

28

rotatably supported on the side plates

16

for rotation about a fixed axis, a pressing roller

30

positioned obliquely downwardly of the fixing roller

28

in rolling contact with the fixing roller

28

and rotatably supported on the side plates

16

for rotation about a fixed axis parallel to the fixed axis of the fixing roller

28

, and a heating roller

34

positioned obliquely upwardly of the fixing roller

28

and rotatably supported on the swing lever

22

for rotation about its own axis.

The toner image fixing apparatus

10

also has a first heat source

32

such as a halogen lamp or the like disposed in the heating roller

34

, a second heat source

33

such as a halogen lamp or the like disposed in the pressing roller

30

, and an endless fixing belt (heat transfer belt)

36

trained around the fixing roller

28

and the heating roller

34

.

The fixing roller

28

comprises a resilient roller, and the pressing roller

30

comprises a roller harder than the fixing roller

28

. As shown in

FIG. 2

, the fixing roller

28

and the pressing roller

30

have respective centers O

1

, O

2

spaced from each other by a distance D which is slightly smaller than the sum (R

1

+R

2

) of their radii R

1

, R

2

. In a rolling contact region (nipping region) between the fixing roller

28

and the pressing roller

30

, the fixing roller

28

and the pressing roller

30

are held in rolling contact with each other under a predetermined pressure P

1

, so that the fixing roller

28

has an outer circumferential surface made partly concave by the pressing roller

30

held in rolling contact therewith, thus providing a sufficient nipping width in a direction across the axes of the fixing roller

28

and the pressing roller

30

.

The toner image fixing apparatus

10

also has an oil applying roller

38

for applying silicone oil to an outer circumferential surface of the fixing belt

36

and cleaning the outer surface of the fixing belt

36

, a first helical spring

40

for normally pressing the oil applying roller

38

against the fixing belt

36

perpendicularly thereto to tension the fixing belt

36

, and a second helical spring

42

for normally urging the heating roller

34

in a direction away from the fixing roller

28

to tension the fixing belt

36

in coaction with the first helical spring

40

.

The upper cover

18

has a right lower portion bent inwardly into the housing

12

. A guide plate

44

is positioned below and largely spaced from the bent right lower portion of the upper cover

18

. The guide plate

44

and the bent right lower portion of the upper cover

18

jointly define an inlet port

46

therebetween for introducing therethrough a sheet S with an unfixed toner image carried thereon (hereinafter referred to as an “unfixed toner sheet”) into the housing

12

in the direction (feed direction) indicated by the arrow in FIG. The guide plate

44

is inclined obliquely upwardly to the left such that the height of the guide plate

44

progressively increases into the housing

12

. The guide plate

44

has an inlet end, i.e., a right end, positioned in confronting relation to an outlet end of a sheet feeding endless belt EB that is positioned in the electronic printer adjacent to the right end of the inlet port

46

. The guide plate

44

has an outlet end, i.e., a left end, positioned in confronting relation to the rolling contact region (nipping region) between the fixing roller

28

and the pressing roller

30

.

When the unfixed toner sheet S is fed in the feed direction indicated by the arrow toward the toner image fixing apparatus

10

by the endless belt EB, the leading end of the unfixed toner sheet S contacts the guide plate

44

, and is then guided thereby to travel obliquely upwardly into the rolling contact region between the fixing roller

28

and the pressing roller

30

.

A sheet discharge passage

48

is defined above the left cover

20

for discharging a sheet with a toner image fixed thereto with heat and pressure by the fixing roller

28

and the pressing roller

30

in the rolling contact region. Such a sheet will hereinafter be referred to as a “fixed toner sheet”). The sheet discharge passage

48

is oriented such that it discharges the fixed toner sheet substantially upwardly along a vertical plane.

A lower discharge roller

50

is rotatably mounted on the left cover

20

between the sheet discharge passage

48

and the rolling contact region. The lower discharge roller

50

is actuated by an actuating mechanism

52

(described later on) to rotate at a speed greater than the pressing roller

30

, i.e., at a speed which is 5% greater than the speed at which the pressing roller

30

rotates. An upper discharge roller

54

is positioned obliquely upwardly of the lower discharge roller

50

and held in rolling contact with the lower discharge roller

50

under resilient forces from a leaf spring

56

. The upper discharge roller

54

is positioned with respect to the lower discharge roller

50

such that a line interconnecting the centers of the upper and lower discharge rollers

54

,

50

extends substantially perpendicularly across a sheet discharge passage along which the fixed toner sheet is delivered from the rolling contact region to the sheet discharge passage

48

.

In the toner image fixing apparatus

10

thus constructed, the unfixed toner sheet S fed onto the guide plate

44

by the endless belt EB has its lower surface, opposite to the unfixed toner image, borne by the guide plate

44

, and is guided by the guide plate

44

toward the rolling contact region (nipping region) between the fixing roller

28

and the pressing roller

30

, with the fixing belt

36

being trained around the fixing roller

28

. When the unfixed toner sheet S passes under pressure between the fixing roller

28

and the pressing roller

30

, the unfixed toner image is fixed to the sheet S with heat and pressure.

Fixing Roller

28

The fixing roller

28

comprises a core

28

A rotatably supported on the side plates

16

by bearings (not shown) and a roller sleeve

28

B fitted coaxially over the core

28

A. The fixing belt

36

is trained around the roller sleeve

28

B. The fixing roller

28

has an outside diameter of 38.0 mm in this embodiment. The core

28

A comprises a shaft of iron having a diameter of 25 mm, and the roller sleeve

28

B is made of a heat-resistant resilient material of silicone rubber having a wall thickness of 6.5 mm. Specifically, the roller sleeve

28

B is made of a heat-resistant resilient material of silicon rubber having a JIS Model A hardness of 15.

As shown in

FIG. 4

, the core

28

A has an end combined with a shaft which is coaxially coupled to a first driven gear

58

through a one-way clutch

60

(described later on). The first driven gear

58

is held in mesh with a transmission gear

62

of the actuating mechanism

52

. Drive forces produced by the actuating mechanism

52

are transmitted through the transmission gear

62

to the first driven gear

58

which is rotated clockwise to rotate the fixing roller

28

through the one-way clutch

60

.

Pressing Roller

30

As shown in

FIG. 1

, the pressing roller

30

comprises a core

30

A rotatably supported on the side plates

16

by bearings (not shown) and a roller sleeve

30

B fitted coaxially over the core

30

A. The pressing roller

30

has an outside diameter of 35 mm in this embodiment. The core

30

A comprises a shaft of iron having a diameter of 32 mm, and the roller sleeve

30

B is made of a heat-resistant resilient material of silicone rubber having a wall thickness of 1.5 mm. Specifically, the roller sleeve

30

B is made of silicone rubber having a JIS Model A hardness of 20, which is harder than the roller sleeve

28

B. The outer circumferential surface of the roller sleeve

30

B is covered with a tube of fluoroplastics having a wall thickness of 50 μm.

As shown in

FIG. 3

, the core

30

A has an end combined with a shaft which is coaxially coupled to a second driven gear

64

which is held in mesh with the first driven gear

58

. Drive forces are transmitted from the first driven gear

58

to the second driven gear

64

, which rotates the pressing roller

30

counterclockwise.

In this embodiment, the pressing roller

30

is used as a primary drive roller for feeding the unfixed toner sheet through the nipping region. The ratio of gear teeth of the first and second drive gears

58

,

64

is selected such that the peripheral speed of the fixing roller

28

as it is thermally expanded is not greater than the peripheral speed of the pressing roller

30

. Specifically, the speed at which the fixing roller

28

is rotated by the first driven gear

58

is slightly lower than the speed at which it is rotated in frictional engagement with the pressing roller

30

through the fixing belt

36

.

The pressing roller

30

is not positioned directly downwardly of the fixing roller

28

, but is displaced downstream in the feed direction of a position directly downward of the fixing roller

28

. Specifically, the pressing roller

30

is positioned with respect to the fixing roller

28

such that an acute angle is formed between a vertical line passing through the center of the fixing roller

28

and a line segment passing through the centers of the fixing roller

28

and the pressing roller

30

. The line segment passing through the centers of the fixing roller

28

and the pressing roller

30

extends perpendicularly to the feed direction across the rolling contact region.

One-Way Clutch

60

The one-way clutch

60

allows the fixing roller

28

to rotate clockwise relatively to the first driven gear

58

, but prevents the fixing roller

28

from rotating counterclockwise relatively to the first driven gear

58

, i.e., rotates the fixing roller

28

and the first driven gear

58

in unison with each other. Specifically, when the fixing roller

28

is cold, i.e., when the fixing roller

28

and the fixing belt

36

are driven by the pressing roller

30

while the fixing belt

36

is held in frictional engagement with the pressing roller

30

and the fixing roller

28

is held in frictional engagement with the fixing belt

36

, the peripheral speed of the fixing roller

28

upon clockwise rotation thereof is the same as the peripheral speed of the pressing roller

30

, and hence is slightly higher than the peripheral speed of the first driven gear

58

. The difference between the peripheral speeds of the fixing roller

28

and the first driven gear

58

is absorbed by the one-way clutch

60

.

When the heating roller

34

is heated by the heater

32

and the fixing roller

28

is heated through the fixing belt

36

, the outside diameter of the fixing roller

28

is increased as it is thermally expanded, and the peripheral speed of the fixing roller

28

increases. Since the peripheral speed of the fixing roller

28

does not become higher than the peripheral speed of the pressing roller

30

, the increase in the peripheral speed of the fixing roller

28

is absorbed by the one-way clutch

60

.

The one-way clutch

60

offers the following advantages: If the one-way clutch

60

were not employed, when a sheet with a glossy and slippery surface, such as a coated sheet, is fed as an unfixed toner sheet into the rolling contact region, the fixing belt

36

would slip against the unfixed toner sheet, and drive forces would not be transmitted from the pressing roller

30

to the fixing belt

36

and the fixing roller

28

, which would not then be driven by the pressing roller

30

. Therefore, the unfixed toner sheet would be jammed in the rolling contact region, or even if the unfixed toner sheet passed through the rolling contact region, the unfixed toner image on the unfixed toner sheet would be abraded and disturbed by the fixing belt

36

kept at rest.

In this embodiment, however, since the one-way clutch

60

is connected between the fixing roller

28

and the first driven gear

58

, even if drive forces from the pressing roller

30

are not transmitted to the fixing belt

36

, the fixing roller

28

is rotated clockwise by the first driven gear

58

through the one-way clutch

60

when the peripheral speed of the fixing roller

28

starts being lower than the peripheral speed of the first driven gear

58

. Therefore, the unfixed toner sheet passes reliably through the rolling contact region for effective protection against a sheet jam in the nipping region and toner image disturbance on the sheet.

Heating Roller

34

In this embodiment, the heating roller

34

comprises a core in the form of an aluminum pipe having a diameter of 30 mm and a wall thickness of 3.5 mm. The core is coated with a polytetrafluoroethylene (PTFE) layer having a thickness of 20 μm. A circular collar

66

made of heat-resistant polyetheretherketone (PEEK) and having a diameter of 34 mm is press-fitted over each of opposite bearing ends of the core for preventing the fixing belt

36

from being tortured or displaced out of position.

As shown in

FIG. 3

, the first heat source

32

disposed in the heating roller

34

comprises an axially longer first halogen lamp

32

A for heating larger-size sheets and an axially shorter second halogen lamp

32

B for heating smaller-size sheets, the first and second halogen lamps

32

A,

32

B extending axially parallel to each other. The larger-size sheets may be A4-size sheets fed in landscape orientation, A3-size sheets fed in portrait orientation, B5-size sheets fed in landscape orientation, B4-size sheets fed in fed in portrait orientation, etc., and the smaller-size sheets may be B5-size sheets fed in portrait orientation, A4-size sheets fed in portrait orientation, postcard-size sheets fed in either landscape or portrait orientation, etc.

In this embodiment, the longer first halogen lamp

32

A is of such a length as to be able to cover the distance of 297 mm which represents the dimension of a shorter side of A3-size sheets, and the shorter second halogen lamp

32

B is of such a length as to be able to cover the distance of 210 mm which represents the dimension of a shorter side of A4-size sheets. Each of the first and second halogen lamps

32

A,

32

B has such a luminous intensity distribution that the luminous intensity is 30-50% greater at its opposite ends than at its center.

Fixing Belt

36

The fixing belt

36

preferably has a heat capacity of 0.002 cal/° C.-0.025 cal/° C. per cm

2

so as to be able to preheat the unfixed toner on the unfixed toner sheet S to a fixing temperature through heat radiation for thereby fixing the toner without applying excessive heat. In this embodiment, the fixing belt

36

comprises an endless belt base of polyimide having an inside diameter of 60 mm and a thickness of 100 μm and a heat-resistant resilient separating layer of silicone rubber that is coated to a thickness of 200 μm on an outer circumferential surface of the endless belt base of polyimide.

Alternatively, the fixing belt

36

may comprise an endless belt base of electroformed nickel having a thickness of 40 μm and a heat-resistant resilient separating layer of silicone rubber that is coated to a thickness of 200 μm on an outer circumferential surface of the endless belt base of electroformed nickel.

Oil Applying Roller

38

The oil applying roller

38

serves to apply a small amount of silicone oil to the outer circumferential surface of the fixing belt

36

for separating the sheet S easily from the fixing belt

36

. The oil applying roller

38

comprises a support shaft

38

A rotatably supported in a casing

68

for rotation about a fixed axis and a heat-resistant layer

38

B of paper fitted over the support shaft

38

A and impregnated with silicone coil. In this embodiment, the support shaft

38

A comprises a shaft of iron having a diameter of 8 mm, and the heat-resistant layer

38

B of paper is covered with a film

38

C of porous fluoroplastics having a thickness of 100 μm. The oil applying roller

38

has a diameter of 22 mm. The oil applying roller

38

thus constructed is capable of stably applying a small amount of silicone oil to the outer circumferential surface of the fixing belt

36

.

The outer circumferential surface of the oil applying roller

38

is smeared with dirt such as of toner particles that is transferred from the outer circumferential surface of the fixing belt

36

. A cleaning brush

39

is held in sliding contact with the outer circumferential surface of the oil applying roller

38

for removing such dirt off the outer circumferential surface of the oil applying roller

38

thereby to clean the oil applying roller

38

.

Tensioning Mechanism For the Fixing Belt

36

As described above, a mechanism for tensioning the fixing belt

36

has the first helical spring

40

for normally pressing the oil applying roller

38

against the fixing belt

36

perpendicularly thereto to tension the fixing belt

36

, and the second helical spring

42

for normally urging the heating roller

34

in a direction away from the fixing roller

28

to tension the fixing belt

36

in coaction with the first helical spring

40

.

The first helical spring

40

is attached to the left cover

20

for normally urging the casing

68

, on which the oil applying roller

38

is rotatably supported, toward the fixing belt

36

. The casing

68

is movably supported by a guide rib

70

on one of the side plates

16

for movement toward and away from the fixing belt

36

. When the left cover

20

is swung open to the left about the second pivot shaft

26

, the first helical spring

40

is disengaged from the casing

68

, releasing the oil applying roller

38

from the fixing belt

36

. When the left cover

20

is swung to the right about the second pivot shaft

26

, the first helical spring

40

pushes the casing

68

under a pressing force P

2

, causing the oil applying roller

38

to press the fixing belt

36

under a certain tension.

The second helical spring

42

is connected between the left end of the swing lever

22

and the side plate

16

for normally urging the swing lever

22

to turn clockwise about the first pivot shaft

24

, i.e., to push the heating roller

34

on the swing lever

22

under a pressing force P

3

in a direction away from the fixing roller

28

. In this manner, the fixing belt

36

is given a desired tension.

Therefore, the heating roller

34

is displaced away from the fixing roller

28

by the swing lever

22

under the bias of the second helical spring

42

, tensioning the fixing belt

36

trained around the heating roller

34

and the fixing roller

28

.

The fixing belt

36

thus tensioned by the first and second helical springs

40

,

42

is held in frictional engagement with the pressing roller

30

and driven thereby. When the fixing belt

36

is driven by the pressing roller

30

, the fixing roller

28

is stably driven thereby without slipping or sagging with respect to the fixing belt

36

.

Actuating Mechanism

52

As shown in

FIG. 4

, the transmission gear

62

is held in mesh with an output gear GE that is connected through a gear train (not shown) to an actuator in the electronic printer when the toner image fixing apparatus

10

is installed in the electronic printer. The transmission gear

62

can be driven to rotate by the output gear GE. The actuating mechanism

52

also has, in addition to the transmission gear

62

, the first driven gear

58

held in mesh with the transmission gear

62

and coupled to the fixing roller

28

through the one-way clutch

60

, and the second driven gear

64

held in mesh with the first driven gear

58

and fixed coaxially to the pressing roller

30

.

The actuating mechanism

52

also has an idler gear

72

held in mesh with the transmission gear

62

. The idler gear

72

is also held in mesh with a third driven gear

74

fixed coaxially to the lower discharge roller

50

for rotating the lower discharge roller

50

at a speed equal to or higher than the rotational speed of the pressing roller

30

.

Other Structural Details

As shown in

FIG. 1

, the toner image fixing apparatus

10

has a peeler blade

76

for peeling the fixed toner sheet off the outer circumferential surface of the pressing roller

30

, and a sheet sensor

78

for detecting the leading end of the fixed toner sheet as it is fed to a rolling contact region between the upper and lower discharge rollers

54

,

50

.

Control System

The toner image fixing apparatus

10

further comprises a control system (see

FIG. 5

) which includes a controller

86

for controlling the actuating mechanism

52

, the first heat source

32

disposed in the heating roller

34

, and the second head source

33

disposed in the pressing roller

30

. To the controller

86

, there are electrically connected a first thermistor

80

for detecting the temperature (heating roller temperature) Th of a non-sheet-contact area (which is not contacted by the unfixed toner sheet S) of the fixing belt

36

on the heating roller

34

, a second thermistor

82

for detecting the temperature (fixing roller temperature) Tf of a sheet-contact area (which is contacted by the unfixed toner sheet S) of the fixing belt

36

on the fixing roller

28

, and a third thermistor

84

for detecting the temperature (pressing roller temperature) Tp of the outer circumferential surface of the pressing roller

30

. Based on the temperatures Th, Tf, Tp detected by the first, second, and third thermistors

80

,

82

,

84

, the controller

86

controls the heat generated by the first and second heat sources

32

,

33

.

The controller

86

also controls the first halogen lamp

32

A of the first heat source

32

through a first heater driver

88

A, the second halogen lamp

32

B of the first heat source

32

through a second heater driver

88

B, and a halogen lamp of the second head source

33

through a third heater driver

88

C according to a control sequence described later on.

Position of the Heating Roller

34

The heating roller

34

is positioned substantially upwardly of the fixing roller

28

. Therefore, the fixing belt

36

that is trained around the fixing roller

28

and the heating roller

34

is so spaced from the guide plate

44

that the unfixed toner sheet fed on the guide plate

44

will not be brought into contact with the fixing belt

36

. Stated otherwise, the fixing belt

36

is disposed in a position outside of a region where the unfixed toner sheet fed on the guide plate

44

possibly passes.

Because the heating roller

34

is positioned substantially upwardly of the fixing roller

28

, the unfixed toner sheet S carried on the upper surface of the guide plate

44

is reliably prevented from contacting the fixing belt

36

irrespective of how the unfixed toner sheet being fed may be curled. Consequently, the unfixed toner sheet S can be led to the rolling contact region between the fixing roller

28

and the pressing roller

30

without disturbing the unfixed toner image on the unfixed toner sheet S, so that the unfixed toner image on the unfixed toner sheet S can reliably be fixed to the unfixed toner sheet S in the rolling contact region.

Angle of the Heating Roller

34

The fact that the heating roller

34

is positioned substantially upwardly of the fixing roller

28

offers advantages inherent in the toner image fixing apparatus

10

. An experiment to determine an optimum angular range in which the heating roller

34

can be positioned substantially upwardly of the fixing roller

28

by changing the angle of the heating roller

34

as shown in

FIG. 5

will be described below.

In the experiment, a straight line passing through the centers of the fixing roller

28

and the pressing roller

30

was defined as a reference line B, and an angle θ was defined between the reference line B and a line segment L interconnecting the centers of the fixing roller

28

and the heating roller

34

. The angular position of the heating roller

34

with respect to the fixing roller

28

was changed to change the angle θ between 90° and 180°, and the frequency of rubbed states of toner images at the inlet of the rolling contact region between the fixing roller

28

and the pressing roller

30

and also the frequency of defects of toner images at the outlet of the rolling contact region between the fixing roller

28

and the pressing roller

30

were measured when the toner images were copied on one side and both sides of sheets.

The angle θ was defined as a positive angle when measured counterclockwise from the reference line B, and as a negative angle when measured clockwise from the reference line B. Therefore, the heating roller

34

positioned at the angle θ=+180° and the heating roller

34

positioned at the angle θ=−180° were in the same angular position, and the heating roller

34

positioned at the angle θ=+105° and the heating roller

34

positioned at the angle θ=−255° were in the same angular position. Defects of toner images at the outlet of the rolling contact region represent sheet offsets or sheet jams.

The experiment was conducted under the following conditions:

The nipping width in the rolling contact region was set to 8 mm, and the pressing roller

30

applied a pressure P

1

of 24 kgf to one side of the unfixed toner sheet S. The temperature of the fixing belt

36

trained around the fixing roller

28

was set to 160° C. The surface temperature of the pressing roller

30

was set to 140° C. The speed at which to feed the unfixed toner sheet S was set to 180 mm/sec. The pressing roller

30

was rotated in synchronism with the speed of 180 mm/sec. The toner used was an A color toner manufactured by Fuji Xerox. The sheet S used was plain paper having a weight of 64 g/m

2

.

The experiment was made for nine angles θ of 90°, 105°, 120°, 150°, 180°, −150°, −120°, −105°, −90°.

The results of the experiment are given in Table 1 shown below.

TABLE 1

Copied on

Copied on

one side

both sides

Angles

A

B

A

B

Evaluation

90°

3/5

0/5

5/5

0/5

Not acceptable

105°

0/5

0/5

1/5

0/5

Partly

acceptable

120°

0/5

0/5

0/5

0/5

Acceptable

150°

0/5

0/5

0/5

0/5

Acceptable

±180°

0/5

0/s

0/5

0/5

Acceptable

−150°

0/5

0/5

0/5

0/5

Acceptable

−120°

0/5

0/5

0/5

0/5

Acceptable

−105°

0/5

2/5

0/5

3/5

Not acceptable

−90°

0/5

5/5

0/5

5/5

Not acceptable

A: The frequency of rubbed states of toner images at the inlet of the rolling contact region.

B: The frequency of defects of toner images at the outlet of the rolling contact region.

As can be seen from Table 1, when the angle θ is greater than 105° and smaller than −105°, i.e., when the angle θ is in a range from 105° to 255° as measured only counterclockwise, toner images were neither rubbed at the inlet of the rolling contact region and nor defective at the outlet of the rolling contact region, indicating a good toner image fixing process. However, when the angle θ is equal to or smaller than 105°, toner images were either rubbed at the inlet of the rolling contact region and or defective at the outlet of the rolling contact region, indicating a poor toner image fixing process.

Heating Control by the Controller

86

A control process or sequence carried out by the controller

86

for controlling the heating of the first and second heat sources

32

,

33

will be described below with reference to the flowcharts of

FIGS. 7 through 9

.

The controller

86

comprises a CPU (Central Processing Unit) for controlling the control system shown in

FIG. 5

, a ROM (Read-Only Memory) for storing programs, a RAM (Random-Access Memory) for storing thresholds, settings, and other data, an interface for transmitting data between the controller

86

and a controller of the electronic printer which incorporates the toner image fixing apparatus

10

, and various I/O (Input/Output) ports. Unless a sheet feed command is supplied from the electronic printer, the controller

86

keeps the toner image fixing apparatus

10

in a standby mode, and executes a predetermined standby mode control sequence. When a sheet feed command is supplied from the electronic printer, the controller

86

operates the toner image fixing apparatus

10

in a sheet feed mode, and executes a predetermined sheet feed mode control sequence.

Specifically, in the standby mode, the controller

86

controls the first heat source

32

to heat the heating roller

34

to a first temperature setting T

1

based on the heating roller temperature Th detected by the first thermistor

80

, and also controls the second heat source

33

to heat the pressing roller

30

to a second temperature setting T

2

based on the pressing roller temperature Tp detected by the third thermistor

84

. In the sheet feed mode, the controller

86

controls the first heat source

32

to heat the fixing roller

28

to a third temperature setting T

3

based on the fixing roller temperature Tf detected by the second thermistor

82

.

The controller

86

controls the amount of heat generated by the first halogen lamp

32

A of the first heat source

32

with the first heater driver

88

A, controls the amount of heat generated by the second halogen lamp

32

B of the first heat source

32

with the second heater driver

88

B, and controls the amount of heat generated by the halogen lamp of the second head source

33

with the third heater driver

88

C.

In the sheet feed mode, the controller

86

determines the size of a sheet being fed based on sheet information. If the controller

86

determines the size of a sheet being fed as a large size, then the controller

86

energizes only the first halogen lamp

32

A of the first heat source

32

with the first heater driver

88

A, and also energizes the halogen lamp of the second heat source

33

with the third heater driver

88

C in the same manner as with the standby mode. If the controller

86

determines the size of a sheet being fed as a small size, then the controller

86

energizes only the second halogen lamp

32

B of the first heat source

32

with the second heater driver

88

B.

The above control process or sequence carried out by the controller

86

will be described in more detail below with reference to

FIGS. 7 through 9

.

Main Routine of the Control Sequence of the Controller

86

As shown in

FIG. 7

, when the toner image fixing apparatus

10

is turned on, the controller

86

carries out a predetermined initializing process, and then executes a standby mode control process for controlling the heating of the first and second heat sources

32

,

33

in step S

10

. The subroutine of the standby mode control process in step S

10

will be described in more detail later on with reference to FIG.

8

.

The controller

86

executes the standby mode control process in step S

10

until a sheet feed command is supplied from the electronic printer in step S

12

. When a sheet feed command is supplied from the electronic printer, the controller

86

starts to operate various actuators of the actuating mechanism

52

and controls the actuators according to a predetermined actuator control process in step S

14

. The controller

86

also carries out a sheet feed mode control process for controlling the heating of the first and second heat sources

32

,

33

in step S

16

. The actuator control process in step S

14

will not be described below as it has no direct bearing on the present invention. The subroutine of the sheet feed mode control process in step S

16

will be described in more detail later on with reference to FIG.

9

.

The controller

86

executes the sheet feed mode control process in step S

16

insofar as there is a sheet feed command supplied from the electronic printer. When there is no longer a sheet feed command from the electronic printer in step S

18

, the controller

86

stop operating the various actuators of the actuating mechanism

52

in step S

20

. Then, control returns to step S

10

to execute the standby mode control process.

In this manner, the controller

86

basically controls the heating of the first and second heat sources

32

,

33

.

Subroutine of the Standby Mode Control Process

The subroutine of the standby mode control process in step S

10

shown in

FIG. 7

will be described below with reference to FIG.

8

.

When the standby mode control process begins, the controller

86

detects the heating roller temperature Th with the first thermistor

80

in step S

10

A, and decides whether the detected heating roller temperature Th is higher than the first temperature setting T

1

or not in step S

10

B. If the detected heating roller temperature Th is not higher than the first temperature setting T

1

, then since the heating roller temperature Th has not yet reached the first temperature setting T

1

as a target temperature, the controller

86

energizes only the first halogen lamp

32

A of the first heat source

32

in the heating roller

34

to generate heat therefrom in step S

10

C.

Conversely, if the detected heating roller temperature Th is higher than the first temperature setting T

1

in step S

10

B, then since the heating roller temperature Th has already exceeded the first temperature setting T

1

, the controller

86

de-energizes the first halogen lamp

32

A of the first heat source

32

in the heating roller

34

to stop generating heat therefrom in step S

10

D.

After having thus controlled the heating of the first heat source

32

in the heating roller

34

based on the heating roller temperature Th, the controller

86

detects the pressing roller temperature Tp with the third thermistor

84

in step S

10

E, and decides whether the detected pressing roller temperature Tp is higher than the second temperature setting T

2

or not in step S

10

F. If the detected pressing roller temperature Tp is not higher than the second temperature setting T

2

, then since the pressing roller temperature Tp has not yet reached the second temperature setting T

2

as a target temperature, the controller

86

energizes the halogen lamp of the second heat source

33

in the pressing roller

30

to generate heat therefrom in step S

10

G.

If the detected pressing roller temperature Tp is higher than the second temperature setting T

2

, then since the pressing roller temperature Tp has already reached the second temperature setting T

2

, the controller

86

de-energizes the halogen lamp of the second heat source

33

in the pressing roller

30

to stop generating heat therefrom in step S

10

H.

After having thus controlled the heating of the second heat source

33

in the pressing roller

30

based on the heating roller temperature Tp, controls returns from the standby mode control process shown in

FIG. 8

to the main routine shown in FIG.

7

.

Subroutine of the Sheet Feed Mode Control Process

The subroutine of the sheet feed mode control process in step S

16

shown in

FIG. 7

will be described below with reference to FIG.

9

.

When sheet feed mode control process begins, the controller

86

decides whether the size of an unfixed toner sheet fed from the electronic printer is a small size or not in step S

16

A. If the size of the unfixed toner sheet fed from the electronic printer is not a small size, i.e., if the size of the unfixed toner sheet fed from the electronic printer is a large size, then the controller

86

energizes only the first halogen lamp

32

A of the first heat source

32

in the heating roller

34

to generate heat therefrom in step S

16

B. Thereafter, the controller

86

controls the heating of the second heat source

32

in the pressing roller

30

in step S

16

C. Specifically, the controller

86

executes a subroutine for controlling the heating of the second heat source

32

, which is the same as the processing in steps S

10

E-S

101

H in the standby mode control process shown in

FIG. 8

, in step S

16

C.

If the size of the unfixed toner sheet fed from the electronic printer is a small size in step S

16

A, then the controller

86

energizes only the second halogen lamp

32

B of the first heat source

32

in the heating roller

34

to generate heat therefrom in step S

16

D. Thereafter, control goes to the subroutine in step S

16

C.

After having thus controlling the heating of the heating roller

34

and the pressing roller

30

depending on the size of the sheet being fed, the controller

86

detects the fixing roller temperature Tf with the second thermistor

82

in step S

16

F, and decides whether the detected fixing roller temperature Tf is higher than the third temperature setting T

3

or not in step S

16

G. If the detected fixing roller temperature Tf is not higher than the third temperature setting T

3

, then since the detected fixing roller temperature Tf has not yet reached the third temperature setting T

3

as a target temperature, the controller

36

energizes the first heat source

32

in the heating roller

34

to generate heat therefrom depending on the size of the sheet being fed in step S

16

H.

If the detected fixing roller temperature Tf is higher than the third temperature setting T

3

, then since the detected fixing roller temperature Tf has already exceeded the third temperature setting T

3

, the controller

36

de-energizes the first heat source

32

in the heating roller

34

to stop generating heat therefrom in step S

16

I.

After having thus controlling the heating of the first heat source

32

in the heating roller

34

based on the fixing roller temperature Tf, controls returns from the sheet feed mode control process shown in

FIG. 9

to the main routine shown in FIG.

7

.

As described above, according to the control sequence carried out by the controller

86

, when the actuating mechanism

52

starts operating, the standby mode control process in which the first heat source

32

disposed in the heating roller

34

is controlled to reach the first temperature setting T

1

based on the surface temperature Th of the heating roller

34

as measured by the first thermistor

80

changes to the sheet feed mode control process in which the first heat source

32

is controlled to reach the third temperature setting T

3

based on the surface temperature Tf of the fixing roller

28

as measured by the second thermistor

82

. The sheet feed mode control process continues insofar as a sheet feed command is supplied from the electronic printer.

According to the illustrated embodiment, while sheets are being fed into the toner image fixing apparatus

10

in the sheet feed mode, the rollers whose temperatures are to be measured do not change depending on the temperature measured by the first thermistor

80

, but the first heat source

32

is controlled always on the basis of the fixing roller temperature. As a result, even when sheets are fed at a high speed and pass through the nipping region highly frequently, depriving the fixing roller

38

of a large amount of heat, the first heat source

32

is controlled to transfer heat from the heating roller

34

through the fixing belt

36

to the fixing roller

28

to make up for the lost heat. Therefore, as shown in

FIG. 10

, the fixing roller

28

is kept substantially constant in the toner image fixing

5

temperature range at all times. Consequently, even when sheets are fed at a high speed into the toner image fixing apparatus

10

, unfixed toner images on the sheets can well be fixed to the sheets with good toner image fixability.

In this embodiment, since the second heat source

33

is disposed in the pressing roller

30

which is one of the rollers positioned across the nipping region, it can supply a sufficient amount of heat to heat the unfixed toner sheet S. As a consequently, even if the speed at which the fixing belt

36

is increased, the nipping region is supplied with a sufficient amount of heat. The toner image fixing apparatus

10

is thus capable of meeting requirements for higher speeds at which to feed sheets into the toner image fixing apparatus

10

.

With the second heat source

33

disposed in the pressing roller

30

, the size of a sheet being fed into the toner image fixing apparatus

10

in the sheet feed mode is determined, and if the sheet is of a small size, then the second heat source

33

is de-energized to prevent the pressing roller

30

from being heated. Accordingly, the non-sheet-contact area of the fixing belt

36

on the heating roller

36

which is associated with the first thermistor

30

is effectively prevented from increasing its temperature. Even though the temperature of the first heat source

32

in the heating roller

34

is controlled on the basis of the surface temperature of the fixing roller

28

as detected by the second thermistor

82

throughout the sheet feed mode, the surface temperature of the heating roller

34

is prevented from increasing excessively, but the heating roller

34

is heated well with safety.

In the illustrated embodiment, as described above, the first heat source

32

disposed in the heating roller

34

comprises the first halogen lamp

32

A for heating larger-size sheets and the second halogen lamp

32

B for heating smaller-size sheets. In the standby mode and the sheet feed mode in which larger-size sheets are fed, only the front halogen lamp

32

A is energized to heat the heating roller

34

. In the sheet feed mode in which smaller-size sheets are fed, only the second halogen lamp

32

B is energized to heat the heating roller

34

. As a result, the surface temperature of the heating roller

34

is prevented more reliably from increasing excessively for allowing toner images to be fixed to the sheets more stably.

Since the controller

86

effects the standby mode control process, even when the standby mode continues for a long period of time, a fixation readiness time, i.e., the period of time required for the toner image fixing apparatus

10

to become ready for fixing toner images, subsequent to the standby mode can be shortened, so that the operator does not need to wait long before a fixing process begins.

In the above embodiment, the heating roller

34

is positioned substantially upwardly of the fixing roller

28

, i.e., the heating roller

34

is angularly positioned with respect to the fixing roller

28

such that the angle formed between the line segment L interconnecting the center of the heating roller

34

and the center of the fixing roller

28

and the reference line B interconnecting the center of the fixing roller

28

and the center of the pressing roller

30

lies in a range from about 105° to about 255°. Therefore, the fixing belt

36

that is trained around the fixing roller

28

and the heating roller

34

is so spaced from the guide plate

44

that the unfixed toner sheet fed on the guide plate

44

will not be brought into contact with the fixing belt

36

. Stated otherwise, the fixing belt

36

is disposed in a position outside of a region where the unfixed toner sheet fed on the guide plate

44

possibly passes.

Consequently, no matter how the unfixed toner sheet being fed is curled due to jumping or sagging on account of the speed difference between the toner image fixing apparatus

10

and a preceding toner image transferring apparatus, the unfixed toner image on the upper surface of the unfixed toner sheet is reliably prevented from touching the fixing belt

36

, and can be led, without being disturbed, into the rolling contact region between the fixing roller

28

and the pressing roller

30

, so that the toner image can reliably be fixed to the sheet by the fixing roller

28

.

The fixing roller

28

comprises a resilient roller, and the pressing roller

30

comprises a roller harder than the fixing roller

28

. Therefore, even if the fixing roller

28

and the pressing roller

30

are small in diameter, they provide a sufficiently large nipping width in a direction across their axes. As a consequence, the toner image fixing apparatus

10

may be relatively small in size, and sheets can be fed through the toner image fixing apparatus

10

at high speed. The toner image fixing apparatus

10

is thus suitable for use in color printers.

As described above, inasmuch as the fixing roller

28

positioned above the pressing roller

30

comprises a resilient roller and the pressing roller

30

comprises a roller harder than the fixing roller

28

, the fixing roller

28

provides an upwardly concave surface in the nipping region, unlike the conventional structure shown in FIG.

14

. The upwardly concave nipping region provided by the fixing roller

28

produces forces tending to separate a sheet carrying a fixed toner image from the fixing belt

36

. Even though the toner is carried on the surface of the sheet held in contact with the fixing belt

36

, because the sheet can easily be separated from the fixing belt

36

due to the upwardly concave nipping region, the amount of oil applied to the fixing belt

36

by the oil applying roller

38

for preventing sheet offsets and jams may be relatively small. Actually, the upwardly concave nipping region provided by the fixing roller

28

is effective to avoid sheet offsets and jams between the fixing roller

28

and the pressing roller

30

even without the application of oil to the fixing belt

36

by the oil applying roller

38

.

Furthermore, the fixing belt

36

is made of a material having a small heat capacity, trained around the heating roller

34

at a large contact angle, and held in intimate contact with the heating roller

34

. As a result, even when sheets are passed at a high speed, i.e., even when a large number of sheets are passed in a unit time, through the nipping region, the temperature necessary to fix toner images to the sheets can reliably be maintained in the rolling contact region between the fixing roller

28

and the pressing roller

30

.

In the embodiment, the resilient fixing roller

28

does not house any heater, but the heating roller

34

spaced from the fixing roller

28

houses the heat source

32

therein. Thus, it is possible to sufficiently increase the thickness of the roller sleeve

28

B that is made of a heat-resistant resilient material of silicone rubber. Consequently, the nipping width in the rolling contract region can be sufficiently large while at the same time the fixing roller

28

may be relatively small in diameter.

In addition, the one-way clutch

60

disposed between the first driven gear

58

and the fixing roller

28

allows the pressing roller

30

, rather than the fixing roller

28

, as a primary drive roller for establishing a speed at which the unfixed toner sheet is fed through the nipping region. Therefore, even when the fixing roller

28

is heated in the fixing process and thermally expanded to increase its diameter, since the speed at which the unfixed toner sheet is fed through the nipping region is not established by the fixing roller

28

, it is not varied by the thermal expansion of the fixing roller

28

, but is maintained at a constant level. Consequently, the fixing belt

36

is maintained at a constant linear velocity to prevent toner images from being displaced or rubbed.

Modifications

The toner image fixing apparatus

10

has been described as being used in an electronic printer. However, the principles of the present invention are not limited to such an application, but are also applicable to other electronic image forming systems including an electronic facsimile machine, an electrophotographic copying system, etc.

In the above embodiment, the unfixed toner sheet is introduced laterally into the toner image fixing apparatus

10

. However, the unfixed toner sheet may be introduced vertically, e.g., upwardly, into the toner image fixing apparatus

10

. In such a modification, the pressing roller

30

is disposed laterally of the fixing roller

28

, and the heating roller

34

is disposed on one side of the fixing roller

28

which is opposite to the pressing roller

30

.

In the above embodiment, the temperatures of the fixing belt

36

on the heating roller

34

and the fixing roller

28

are detected and used for the control of the heating of the heat sources

32

,

33

. However, the temperatures of the heating roller

34

and the fixing roller

28

may directly be detected and used for the control of the heating of the heat sources

32

,

33

.

In the above embodiment, the heating or energization of the second heat source

33

disposed in the pressing roller

30

is controlled on the basis of the surface temperature Tp of the pressing roller

30

which is detected by the third thermistor

84

, as shown in FIG.

8

. According to a first modification, the heating or energization of the second heat source

33

may be controlled on the basis of the surface temperature Th of the heating roller

34

which is detected by the first thermistor

80

or the surface temperature Tf of the fixing roller

28

which is detected by the second thermistor

82

, rather than the surface temperature Tp of the pressing roller

30

which is detected by the third thermistor

84

.

A standby mode control process and a sheet feed mode control process according to such a first modification will be described below with reference to

FIGS. 11 and 12

. Those steps shown in

FIGS. 11 and 12

which are identical to those shown in

FIGS. 8 and 9

will be denoted by identical reference characters, and will not be described in detail below.

In the standby mode control process according to the first modification, as shown in

FIG. 11

, if the heating roller temperature Th detected by the first thermistor

80

in step S

10

A is lower than the first temperature setting T

1

, then the controller

86

energizes the second heat source

33

disposed in the pressing roller

30

to generate heat therefrom in step S

10

G. If the heating roller temperature Th is higher than the first temperature setting T

1

, then the controller

86

stops energizing the second heat source

33

to prevent the second heat source

33

from generating heat.

In the standby mode control process according to the first modification, therefore, the processing in steps S

10

E, S

10

F shown in

FIG. 8

is dispensed with, and hence the third thermistor

84

for detecting the surface temperature of the pressing roller

30

is dispensed with. As a result, the standby mode control process is simplified, and the number of parts used is reduced and the cost of the toner image fixing apparatus is lowered because the third thermistor

84

is dispensed with. Since the third thermistor

84

which is held in contact with the outer circumferential surface of the pressing roller

30

is dispensed with, the outer circumferential surface of the pressing roller

30

is prevented from being damaged by a thermistor, and hence the pressing roller

30

will have a longer service life.

In the sheet feed mode control process according to the first modification, as shown in

FIG. 12

, if the fixing roller temperature Tf detected by the second thermistor

82

in step S

16

G is lower than the third temperatures setting T

3

, then the controller

86

energizes the first heat source

32

disposed in the heating roller

34

to generate heat therefrom in step S

16

H and then energizes the second heat source

33

disposed in the pressing roller

30

to generate heat therefrom in step S

16

J. If the fixing roller temperature Tf is higher than the third temperatures setting T

3

, then the controller

86

stops energizing the first heat source

32

disposed in the heating roller

34

to prevent the first heat source

32

from generating in step S

16

I and then stops energizing the second heat source

33

disposed in the pressing roller

30

to prevent the second heat source

33

from generating in step S

16

K.

Inasmuch as the heating of the second heat source

33

is controlled on the basis of the surface temperature Tf of the fixing roller

28

, the temperature of the nipping region can be controlled more reliably for improved toner image fixability.

In the first modification, the first thermistor

80

for detecting the surface temperature Th of the heating roller

34

is used to control the heating of the first and second heat sources

32

,

33

in the standby mode control process, and the first thermistor

80

is not used, but the second thermistor

82

for detecting the surface temperature Tf of the fixing roller

28

is used, to control the heating of the first and second heat sources

32

,

33

in the sheet feed mode control process. Therefore, according to a second modification shown in

FIG. 13

, the first thermistor

80

is used as a sensor for detecting a temperature failure in the sheet feed mode control process.

More specifically,

FIG. 13

shows a circuit arrangement according to the second modification. Those parts shown in

FIG. 13

which are identical to those shown in

FIGS. 1

,

3

, and

5

are denoted by identical reference characters. As shown in

FIG. 13

, an emergency shutoff switch

90

is connected in series to the first heat source

32

. The second thermistor

82

for detecting the surface temperature Tf of the fixing roller

28

is connected to the controller

86

through a fixing roller rotation control unit

94

A. The first thermistor

80

for detecting the surface temperature Th of the heating roller

34

is connected to the controller

86

through a heating roller standby control unit

94

B. The fixing roller rotation control unit

94

A and the heating roller standby control unit

94

B are selectively connected to the controller

86

by a first selector switch

92

.

The first thermistor

80

is connected to a heating roller rotation failure control unit

94

C through a second selector switch

96

. The heating roller standby control unit

94

B and the heating roller rotation failure control unit

94

C are selectively connected to the first thermistor

80

by the second selector switch

96

. If the heating roller rotation failure control unit

94

C detects a rotation failure of the heating roller

34

while the heating roller

34

is rotating, then the heating roller rotation failure control unit

94

C causes a relay

98

to turn off the emergency shutoff switch

90

.

In the standby mode, the first and second selector switches

92

,

96

have their movable contacts shifted to the broken-line position. According to the standby mode control process, the controller

86

controls the heating of the first and second heat sources

32

,

33

based on the temperature detected by the first thermistor

80

. In the sheet feed mode, the movable contacts of the first and second selector switches

92

,

96

are shifted to the solid-line position. According to the sheet feed mode control process, the controller

86

controls the heating of the first and second heat sources

32

,

33

based on the temperature detected by the second thermistor

82

.

In the sheet feed mode, the first thermistor

80

is connected to the heating roller rotation failure control unit

94

C through the second selector switch

96

. Therefore, the heating roller rotation failure control unit

94

C can detect a temperature failure of the heating roller

34

based on the temperature detected by the first thermistor

80

. For example, if the surface temperature Th of the heating roller

34

exceeds an allowable safety range, then the heating roller rotation failure control unit

94

C applies a control signal to the relay

98

to cause the relay

98

to turn off the emergency shutoff switch

90

for thereby cutting off the supply of an electric current to the first heat source

33

in the heating roller

34

. The heating roller

34

is thus prevented from being overheated for safety.

A circuit arrangement of the second heat source

33

is omitted from illustration in FIG.

13

.

In the first modification, the first and second heat sources

32

,

33

are energized for the same period of time. However, the period of time for which the first heat source

33

is energized may be made longer than the period of time for which the first heat source

32

is energized, using a timer, a latch, etc. This is because in general the heating capacity of the second heat source

33

disposed in the pressing roller

30

is smaller than the heating capacity of the first heat source

32

disposed in the heating roller

34

.

According to the present invention as described above, the toner image fixing apparatus can fix toner images to unfixed toner sheets with good toner image fixability even when the unfixed toner sheets are fed at an increased speed into the toner image fixing apparatus.

Furthermore, the toner image fixing apparatus is capable of holding the surface temperature of the fixing roller substantially in a toner image fixing temperature range even when a sheet with an unfixed toner image carried thereon is fed at an increased speed.

The toner image fixing apparatus can fix a toner image to an unfixed toner sheet while preventing the surface temperature of the fixing roller from increasing excessively even when the unfixed toner sheet is fed at an increased speed.

The toner image fixing apparatus can fix a toner image to an unfixed toner sheet while holding the surface temperature of the fixing roller substantially in a toner image fixing temperature range even when the unfixed toner sheet is fed at an increased speed.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Number	Date	Country
10-165849	Jun 1998	JP
10-165850	Jun 1998	JP
10-165851	Jun 1998	JP

Number	Name	Date
5041718	D'Hondt et al.	Aug 1991
5075732	Menjo	Dec 1991
5671462	Toyohara et al.	Sep 1997

Number	Date	Country
5-27620	Feb 1993	JP
5-80666	Apr 1993	JP
7-319321	Dec 1995	JP

Toner image fixing apparatus capable of keeping constant fixing roller temperature

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (3)

US Referenced Citations (3)

Foreign Referenced Citations (3)