GB2258844A

GB2258844A - Fixing device for electrophotographic imaging apparatus.

Info

Publication number: GB2258844A
Application number: GB9212999A
Authority: GB
Inventors: Yoshimi Saito
Original assignee: Asahi Kogaku Kogyo Co Ltd
Current assignee: Pentax Corp
Priority date: 1991-06-19
Filing date: 1992-06-19
Publication date: 1993-02-24
Also published as: GB9212999D0; JPH04371985A; DE4220203A1

Description

2 C2 -) 31 ') 4 4 1 FIXING DEVICE FOR ELECTROPHOTOGRAPHIC IMAGING

APPARATUS The present invention relates to a fixing device for an electrophotographic imaging apparatus.

Conventionally, there has been known an imaging apparatus employing a socalled electrophotographic imaging method, such as an electronic copier machine and a laser beam printer and the like. In such an electrophotographic imaging apparatus, first, an exposure operation is carried out by means of an exposing unit in such a manner that a uniformly charged circumferential surface of a photoconductive drum is exposed to light to form a latent image on the basis of image information. Then, by means of a developing unit, toner is attached onto the latent image so as to form toner images. The toner image is in turn transferred onto a recording sheet by a transferring unit. Subsequently, the transferred images are fixed by means of a fixing unit.

As the above-described fixing unit, there has been generally known a socalled heat roll fixing device, which uses a heated roller (i.e. heat roller) to apply heat, and pressure in order to fix the toner images onto the recording sheet.

2 The heat roll fixing device includes a heat roller having a cylindrical roller member, and a heater member such as a halogen lamp accommodated in the cylindrical roller member so as to heat the heat roller. The circumferential surface of the heat roller is controlled to fall within a predetermined temperature range. This heat roller and a press roller constitute a pair of fixing rollers.

The press roller is made of a material such as a silicon rubber or the like which has sufficient heatresistance and adequate elasticity. The press roller is press-contacted to the heat roller.

A recording sheet bearing an unfixed toner image is inserted between the nip formed by the pair of fixing rollers. Thus, the recording sheet bearing the unfixed toner image is heated and pressed by the fixing rollers so as to firmly fix the toner images onto the recording sheet.

This kind of heat roll-fixing device has advantages such as a relatively high heat efficiency, and a speedy fixing operation.

Fig. 5 is a schematic view showing a pair of fixing rollers, and a conventional driving circuit for 3 maintaining temperature of the heat roller at a predetermined value. A pair of fixing rollers 10 consists of a heat roller 11 and a press roller 12. The heat roller 11 accommodates a halogen lamp 11A as a heater member therein.

The halogen lamp 11A is driven by an AC power source 13 to generate heat. The driving circuit includes a thyristor 14 as a switching element. Furthermore, as a temperature detecting element, there is provided a thermistor 15 to contact a circumferential surface of the heat roller 11 so as to detect the temperature thereof. The thermistor 15 is an element whose electrical resistance varies in response to temperature change of the heat roller surface.

Temperature data thus obtained is transmitted from the thermistor 15 to a CPU (Central Processing Unit) 16. On the basis of the data from the thermistor 15, the CPU 16 judges whether the halogen lamp 11A should be driven, and controls to turn ONIOFF the thyristor 14 via a gate thereof. That is, the temperature is controlled to fall within a predetermined temperature range by controlling the current supply to the halogen lamp 11A based on the surface temperature of the heat roller 11 detected by the thermistor 15.

4 However, in an electrophotographic imaging apparatus using the abovedescribed fixing rollers, there are problems as follows.

If a printing operation is required shortly after an ordinary warming-up period has passed after an electrical power source has been turned on, the temperature of the heat roller is unexpectedly lowered. That is, the temperature of the heat roller is lowered when the fixing rollers are press-contacted with each other for the printing operation. As a result, at the time when the ordinary warmingup period has passed, the overall temperature of the fixing device including the press roller is not sufficiently increased. Thus, fixing of toner may fail.

Furthermore, even if the pair of fixing rollers are already pressed into contact with each other and the temperature of a contacting portion of the press roller is sufficiently increased, the temperature of the heat roller may be lowered upon rotation of the fixing rollers because the remaining surface of the press roller other than the contacting portion is not sufficiently heated. Thus, it may also result that the fixing of toner may fail.

If the operating temperature of the fixing device is set relatively high in order to overcome the above problems, further problems result. That is, electrical power consumption may be increased, and toner may be adhered on the heat roller due to the excessive heat thereof. Furthermore, it is feared that members constituting the fixing device may be deformed due to the excessive heat. Therefore, in such a case, it becomes necessary to install a special means such as a cooling fan to decrease the inside temperature of the fixing device.

It is therefore an object of the present invention to provide an improved fixing device for an electrophotographic inaging apparatus which can prevent decrease of the temperature of a heat roller at the beginning of a printing operation even shortly after a warming-up period has elapsed, and can further maintain the temperature inside the imaging apparatus at a usual value, without being excessively increased.

According to the present invention, there is provided a fixing device for an electrophotographic imaging apparatus, said fixing device performing a fixing operation at a fixing temperature, said fixing device comprising:

first regulating means for regulating said fixing 6 temperature to fall within a first predetermined temperature range; second regulating means for regulating said fixing temperature to fall within a second predetermined temperature range, said second predetermined temperature range being set to be higher than said first predetermined temperature range; and control means for controlling said fixing temperature in such a fashion that the fixing temperature is maintained within said second predetermined temperature range for a predetermined period at the initial stage of operation of said electrophotographic imaging apparatus, while the fixing temperature is maintained in said first predetermined temperature range except at said initial stage.

Preferably, the fixing operation is allowed when said fixing temperature reaches a predetermined temperature within said second range.

Furthermore, the predetermined temperature may be the upper limit of daid first temperature range.

According to another aspect of the invention, there is provided a method of controlling the temperature of a 7 fixing device employed in an electrophotographic imaging apparatus at an initial stage of the operation of said imaging apparatus, said fixing device being operative at a fixing temperature, said method comprising the steps of:

controlling said fixing temperature to fall within a first predetermined range for a predetermined period at the initial stage of operation of said electrophotographic imaging apparatus; and controlling said fixing temperature to fall within a second predetermined range, said first predetermined range being higher than said second predetermined range.

An example of the present invention is described in detail below, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic block diagram which illustrates the temperature control of a fixing device embodying the present invention; Fig. 2 is a flow chart illustrating a printing operation of an electrophotographic printer employing th fixing device embodying the present invention; 8 Fig. 3 is a flow chart illustrating a temperature control interruption routine of the fixing device of the present invention; Fig. 4 is a graph showing the temperature of the heat roller of the electrophotographic printer embodying the present invention; Fig. 5 is a schematic view showing a pair of fixing rollers and an actuating circuit for maintaining the temperature of the heat roller; and Fig. 6 is a schematic crosssectional view showing an overall constitution of a laser beam printer employing the fixing device embodying the present invention.

Fig. 6 is a schematic cross-sectional view showing an overall construction of a laser beam printer employing a fixing device embodying the present invention.

The laser beam printer outputs a hard copy of images or printing information inputted from a computer or the like by printing them on a continuous recording paper 20 provided as a recording medium. The continuous recording paper 20 is a so-called fan fold paper which is conventionally used for line printers. The fan fold paper is provided with feeding holes at both side edges 9 thereof, and is provided with perforated lines at the boundaries of consecutive pages, so that it can be torn or folded along the perforated lines.

A photoconductive drum 21 is driven by a main motor (not shown) so as to rotate at a predetermined surface speed. Around the photoconductive drum 21, there are disposed, along the rotational direction of the photoconductive drum 21, a toner cleaning portion 22, a discharge unit 23, a charging unit 24, a scanning optical system 25, a developing unit 26, and a transfer unit 27.

The scanning optical system 25 is constituted in such a manner that a laser beam modulated and emitted by a semiconductor laser (not shown) is deflected by a polygon mirror 51, which is driven by a polygon motor 52 to rotate at a predetermined speed. The laser beam thus deflected is corrected by an f.P lens 53 and, in turn, is guided onto a surface of the photoconductive drum 21 by way of a reflection mirror 54 so as to form a scanning line.

There is further provided a recording paper feeding path between the ph6toconductive drum 21 and the transfer unit 27 in a left to right direction in the drawing.

There is further provided a tractor 9 to the right of the transfer unit 27 in the drawing.

The tractor 9 has a pair of endless belts 91 having a plurality of projections engaged with feed holes open at both edges of the fan fold paper 20. The pair of endless belts 91 are wound between a drive shaft 92 and driven shaft 93 in parallel with each other. The drive shaft 92 is connected to a main motor (not shown) via a field clutch and a gear train housed in a box (not shown).

In this embodiment, a feeding operation of the fan fold paper 20 is basically carried out by a fixing device 10. The endless belts 91 are driven in the same rotational direction as the fixing device 10 but slightly (approximately 1-2 %) slower than the speed at which the fixing device 10 feeds the fan fold paper 20.

That is, due to the rotational speed difference, the tractor 9 rotates with certain degree of resistance in accordance with the feeding operation of the fan fold paper 20 by the fixing device 10. The resistance gives tension to the fan fold paper 20 hung between the tractor 9 and the fixing device 10 so as to prevent a skew of the fan fold paper 20.

The fixing device 10 comprises, as shown in detail 11 in Fig 5, a heat roller 11 in which a halogen lamp 11A is accommodated as a heating element so that the heat roller 11 can be heated up to a predetermined temperature, and a press roller 12 which is disposed to be press-contacted with the heat roller 11. The heat roller 11 is connected through a clutch and a gear train housed in a box (not shown) to a main motor (not shown) so that the heat roller 11 can be rotated synchronously with the photoconductive drum 21 at the same surface speed.

In the laser beam printer constituted as above, the surface of the photoconductive drum 21 is main scanned (exposed) by the laser beam emitted from the scanning optical system 25, while the photoconductive drum 21 is rotated (auxiliary scanned). The latent images formed on the surface of the photoconductive drum 21 are developed by the developing unit 26 to form a toner image. The toner image is transferred onto the fan fold paper 20, which is fed by the fixing device 10, by means of the transfer unit 27. Then, by means of the fixing device 10, the toner image is fixed onto the fan fold paper 20.

Fig. 1 is a schematic block diagram illustrating temperature control of a fixing device embodying the present invention.

Temperature data detected by a thermistor 15 (i.e. a 12 temperature detecting means) is converted from analogue signals to digital signals by an A/D converter of a CPU 16, and inputted into a CPU 16. Then, on the basis of the converted temperature data, the CPU 16 controls a driving circuit 2 to turn ON or OFF the halogen lamp 11A. A timer is connected to the CPU 16 for measuring a period of high temperature, which will be described later.

Fig. 2 is a flow chart illustrating a printing operation of an electrophotographic printer employing the fixing device embodying the present invention.

In step S1, the CPU 16 is initialized. That is, memory test, initialization of variables, etc. are executed in step Si. A heater ready flag and high temperature setting flag, which will be described later, are set to respective initial values 0 and 1 in step S1. In step S2, a self-test of the printer mechanism is executed. If the self-test of the printer mechanism has finished without disorder, a warming-up of the heater is initiated.

Then, in step S3, it is checked whether the warmingup of the heater is'finished. If the judgement in step S3 is YES, it is checked, in step S4, whether a print requirement is generated. If the judgement in step 54 is YES, the print operation is executed in step SS.

13 Fig. 3 is a flow chart illustrating a temperature control interruption routine of the fixing device of the present invention.

It should be noted that this interruption routine is executed once every predetermined period, e.g., 200 msec, as an interruption procedure. The interruption of this routine is allowed when warming-up of the heater is started.

In step S10, it is checked whether the heater ready flag is 11011 or 11111. When this interruption routine is executed for the first time after the power has been turned on,' the heater ready flag is set to an initial value of 11011 as aforementioned, and therefore, step S20 follows step S10. At this stage, if the surface temperature of the heat roller is not greater than 200 OC, the heater is turned ON, i.e., the halogen lamp 11A is turned on in step S24. Thereafter, the process returns to the main routine shown in Fig. 2.

Until the surface temperature of the heat roller exceeds 200 OC, the same procedure of step S10, S20, and S24 is executed when this interruption routine is executed.

14 If the surface temperature of the heat roller exceeds 200 'Cl the heater ready flag is set to 1 in step S21, and in step S22, a 60-second timer 3 is started. Then, in step S23, the heater is turned OFF, i.e., the halogen lamp 11A is turned OFF, and the process returns to the main routine shown in Fig. 2.

After the heater ready flag has been set to 1 in step S21, since the initial value of the high-temperature setting flag is 11111 as aforementioned, the heater, i.e., the halogen lamp 11A, is controlled so that the surface temperature thereof is maintained in a high- temperature range between 190 OC and 200 OC, inclusive, until the expiry of the timer 3 (in steps S11 through S16).

if it is judged that 60 seconds has elapsed, the hightemperature setting flag is changed from 11111 to 11011 in step S17.

After the hightemperature setting flag is set to 11011, when the temperature control interruption routine is executed, steps S10, S11, and S30 are subsequently executed. Then, the surface temperature of the heat roller is maintained in another temperature range between 180 OC and 190 OC, inclusive.

In accordance with the above-described control, shortly after the electrical power has been turned ON, the surface temperature of the heat roller is increased to 2000C. Then, the surface temperature is temporarily maintained, or regulated within a high- temperature range of 1900C to 2000C for 60 seconds. Thereafter, the temperature is regulated to fall within a temperature range of 1800C to 1900C.

Fig. 4 is a graph showing the temperature of the heat roller in accordance with the temperature control of the electrophotographic printer of the present invention.

The broken line shows a characteristic of a conventional control method. In the conventional method, after the power source is turned on, the temperature of the heater is always controlled to fall within the same temperature range for a print operation. Therefore, if the print operation is started immediately after the temperature has reached its temperature range, the temperature of the heat roller may be undesirably lowered since accumulated heat is abruptly transferred to the press roller etc, which have a lower temperature at this stage. Thus, the previously described problems would be caused.

The solid line in Fig. 4 shows a characteristic according to the temperature control of the 16 electrophotographic printer of the present invention. In this case, the temperature of the heater is controlled to fall within the hightemperature range in which the surface temperature of the heat roller is maintained higher than the surface temperature for a normal print operation during the predetermined period immediately after the power source is turned ON.

Thus, if the print operation is started while the surface temperature of the heat roller is in the hightemperature setting period, the temperature of the heat roller is not excessively lowered. Thus, the fixing operation can be well performed without failure.

As is explained in the foregoing description, with the fixing device embodying the present invention, numerous problems encountered in the conventional art can be overcome.

For example, if the printing operation is required shortly after a warmingup period has ended in an initial stage of a printing operation after a power source is turned on, the temperature of the heat roller can be prevented from being excessively lowered. This is because the temperature of the heat roller is controlled to be increased to a higher temperature than the normal operational temperature at this initial stage.

17 Though the temperature of the heat roller is usually lowered when the press roller contacts the heat roller for the printing operation since heat of the heat roller is transferred to other members of the fixing device, which have not been sufficiently heated, it is possible to cancel such a temperature decrease by increasing the temperature of the heat roller itself. Thus, the fixing of toner can be maintained well without deterioration.

Though the pair of fixing rollers are already pressed into contact with each other, and the temperature of the contacting portion of the press roller is sufficiently increased, the temperature of the heat roller is lowered upon rotation of the press roller since the remaining surface of the press roller other than the contacted portion has not been sufficiently heated. Even in such a case, the heat roller can also cancel this kind of temperature decrease by controlling the surface temperature of the heat roller to be in the hightemperature range at the time when the fixing rollers start rotating.

If the operating temperature of the fixing device is always set to an excessively high temperature, it is feared that toner will be adhered on the heat roller due to the excessive heat. It is also feared that members 18 constituting the fixing device may be deformed d excessive heat.

ie to the The present invention can prevent this kind of overheating phenomenon by controlling the surface temperature of the heat roller to be maintained in the high-temperature range only for a predetermined period. Thereafter the temperature is controlled to fall within a lower, normal temperature range. Thus, printing images of lower quality or deformation of the fixing device can be safely prevented without installing any extra devices such as a cooling fan for decreasing the temperature inside the fixing device.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them. All changes that fall within the bounds of the claims, or equivalent of such bounds are therefore intended to be embraced by the claims.

19

Claims

1. A fixing device for an electrophotographic imaging apparatus, said fixing device performing a fixing operation at a fixing temperature, said fixing device comprising:

first regulating means for regulating said fixing temperature to fall within a first predetermined temperature range; second regulating means for regulating said fixing temperature to fall within a second predetermined temperature range, said second predetermined temperature range being set to be higher than said first predetermined temperature range; and control means for controlling said fixing temperature in such a fashion that the fixing temperature is maintained within said second predetermined temperature range for a predetermined period at the initial stage of operation of said electrophotographic imaging apparatus, while the fixing temperature is maintained in said first predetermined temperature range except at said initial stage.

2. A fixing device according to claim 1, wherein said 20. fixing device comprises a heat roller, and a heater member to be driven to heat said heat roller which is provided inside said heat roller.

3. A fixing device according to claim 2, wherein said heater member comprises a halogen lamp.

4. A fixing device according to any one of the preceding claims, wherein the fixing operation is allowed when said fixing temperature reaches a predetermined temperature within said second range.

5. A fixing device according to claim 4, wherein said predetermined temperature is the upper limit of said first temperature range.

6. A fixing device according to any one of the preceding claims, wherein said predetermined period comprises substantially 60 seconds.

7. A method of controlling the temperature of a fixing device employed in an electrophotographic imaging apparatus at an initial stage of the operation of said imaging apparatus, said fixing device being operative at a fixing temperature, said method comprising the steps of:

21 controlling said fixing temperature to fall within first predetermined range for a predetermined period at the initial stage of operation of said electrophotographic imaging apparatus; and controlling said fixing temperature to fall within a second predetermined range, said first predetermined range being higher than said second predetermined range.

8. A fixing device for an electrophotographic imaging apparatus, said fixing device performing a fixing operation at a fixing temperature, said fixing device substantially as herein described with reference to the accompanying drawings.

9. A method of controlling the temperature of a fixing device employed in an electrophotographic imaging apparatus at an initial stage of the operation of said imaging apparatus, said fixing device being operative at a fixing temperature, said method substantially as herein described with reference to the accompanying drawings.