Electrophotography – Image formation – Fixing
Reexamination Certificate
2000-11-16
2002-09-17
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000
Reexamination Certificate
active
06453145
ABSTRACT:
This application is based on an application No. H11-325790 filed in Japan, the content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flash-based image fixing apparatus that is used in an electrophotographic image forming apparatus such as a laser printer. In particular, the invention relates to an improvement in the supply of electricity to a flash lamp.
2. Related Art
Electrophotographic image forming apparatuses, such as laser printers, develop a latent image using toner, transfer the toner image onto a sheet of paper, and then fix the toner image using a fixing apparatus. Heated rollers are often used as this fixing apparatus, but take a long time to warm up. For this reason, increasing attention is being given to flash-based fixing apparatuses where a flash lamp is instantaneously illuminated to emit light (in particular infrared light) that supplies energy to melt the toner. Such toner includes a color agent, a binder, and the like.
FIG. 1
is a block diagram showing the circuit construction of a conventional flash-based image fixing apparatus.
As shown in
FIG. 1
, the flash-based fixing apparatus includes a flash lamp
200
, a flash power supply unit
300
, and a flash power supply control unit
500
. The flash lamp
200
is filled with xenon gas. The flash power supply unit
300
supplies power to the flash lamp
200
. The flash power supply control unit
500
controls the flash power supply unit
300
. The flash power supply unit
300
includes an AC-DC convertor
310
, a diode
320
, a power-supplying film capacitor
330
, a trigger circuit
340
, and a choke coil
350
. Of these, the diode
320
prevents reverse currents, the power-supplying film capacitor
330
has a capacitance of around 200 &mgr;F, and the choke coil
350
suppresses the discharge current.
In accordance with a charging indication given by the flash power supply control unit
500
, the AC-DC convertor
310
converts an AC voltage, supplied by the commercial power supply
800
via the power supply switch
900
, into a DC voltage of around 2000 V. This DC voltage is supplied to the film capacitor
330
via the diode
320
.
When the voltage across the terminals of the film capacitor
330
reaches a charging stop voltage, such as 2000 V, the flash power supply control unit
500
instructs the AC-DC convertor
310
to stop the charging of the power-supplying film capacitor
330
.
At the point where charging is stopped, an amount of electrostatic energy ((0.0002*2000
2
)/2=400J) that is required to melt toner on a piece of paper across a predetermined width has accumulated in the film capacitor
330
. Two thousand volts are applied across the main electrodes of the flash lamp
200
via the choke coil
350
as the charging stop voltage of the film capacitor
330
. However, the flash lamp
200
does not illuminate as long as a trigger signal is not inputted.
This charging stop voltage is set at a value that exceeds the minimum voltage (such as 1200 to 1500 V) required to initiate discharge (i.e., illumination) within the flash lamp
200
.
At a predetermined time following this, the flash power supply control unit
500
instructs the trigger circuit
340
to start the illumination of the flash lamp
200
. The trigger circuit
340
includes a capacitor (not illustrated) for generating a trigger signal. On being instructed by the flash power supply control unit
500
to illuminate the flash lamp
200
, the trigger circuit
340
discharges this capacitor and outputs the resulting voltage to the trigger electrode of the flash lamp
200
as the trigger signal. Once the trigger signal has been inputted into the trigger electrode, the electrostatic energy that has accumulated in the film capacitor
330
is released so that a discharge current flows though the main electrodes of the flash lamp
200
. As a result, the flash lamp
200
momentarily illuminates due to arc discharge, for a period of between several hundred microseconds to one millisecond.
FIG. 2
shows the transition in the discharge current that flows through the flash lamp during the illumination period.
As shown in
FIG. 2
, the discharge current that flows through the flash lamp
200
falls from an initial peak of
300
A to virtually
0
A during the illumination period of around one millisecond. In more detail, during the initial stage (shown as &dgr;
1
) in the illumination period, the discharge current hits a peak of
300
A at roughly the same time as the trigger signal is applied and thereafter drops to
130
A. In a short second stage (shown as &dgr;
2
), the discharge current stabilizes at around
130
A. In the third and final stage (shown as &dgr;
3
), the discharge current falls from
130
A to
0
A.
As described above, the flash lamp
200
is repeatedly illuminated to supply energy that melts the toner on the paper, thereby fixing the toner image to the paper.
As one example, When an image is formed using black toner, the coloring agent, such as carbon black, that is located near the surface of the toner particles absorbs the light energy produced by the flash lamp
200
and converts it into heat energy. This heat energy is transmitted from the surface of the toner particles to the center. As a result, the entire toner particles melt, which fixes the toner to the paper. Here, it is preferable of the illumination of the flash lamp
200
to be prolonged so that the heat energy can be properly transmitted into the centers of the toner particles.
In a conventional flash-based fixing apparatus, the discharge current that flows through the flash lamp
200
is suppressed by the choke coil
350
to extend the illumination period to around one millisecond. However, further extension of the illumination period is difficult due to the occurrence of ringing in the circuit. Also, in the first stage of the illumination (shown as &dgr;
1
in FIG.
2
), there is a sudden drop in discharge current from
300
A to
130
A, during which time the majority (up to around 83%) of the static charge that has accumulated in the film capacitor
330
is used up.
To extend the illumination period in a conventional flash-based fixing apparatus, it is only possible to increase the amount of static energy supplied to the flash lamp
200
. This raises the peak of the current value in the initial stage of the illumination period, and makes the illumination energy produced in this initial stage extremely high. This destroys the balance between the speed at which the coloring agent is heated and the transmission of the heat to the center of the particles, resulting in sublimation of the toner particles.
Conversely, if the amount of static energy supplied to the flash lamp
200
is reduced, the peak in the discharge current will be lowered, which reduces the sublimation of toner. However, if the illumination period is shortened, less heat energy is produced, so that the toner cannot be properly fixed. With a conventional flash-based fixing apparatus, therefore, improving the adhesion of the toner to the paper results in a great loss of toner through sublimation. In addition to this loss, sublimation also generates noise and unpleasant smells, and so is a first problem for conventional apparatuses. When fixing an image made with color (i.e., non-black) toner, the coloring agent present in the toner is poor at absorbing infrared radiation, so that toners are produced so as to include an infrared absorbing agent, such as a cyanine compound that has an absorbance peak for radiation with a wavelength of 800 to 1100 nm. When the proportion of such infrared absorbing agent reaches 3 to 5% by weight, however, there is an unavoidable rise in the cost of the toner.
Also, to fix the toner using an illumination period of around one millisecond, is necessary to raise the charging stop voltage of the capacitor in the flash-based fixing-apparatus to increase the illumination energy several fold. As mentioned above, however, raising the illumination energy of the flash lamp
200
also raises the disch
Grainger Quana M.
Minolta Co. , Ltd.
Morrison & Foerster / LLP
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