Fixing means for electrophotographic copier

A fixing means for an electrophotographic copier includes a first roller and a second roller which are compressed against each other and serve to fix toner images on an image transferring medium passed therebetween. A control means is programmed to operate the rollers such that the heating of the second roller is continued for a predetermined length of time or until the surface temperature of the second roller reaches a certain predetermined temperature level after the surface temperature of the first roller reaches another predetermined temperature level such that the second roller is prevented from cooling down suddenly after the fixing operation is started. In another aspect of the present invention, the size of the image transferring medium passed between the rollers is detected and the heaters for the rollers are controlled accordingly such that temperature variations on the roller surfaces can be reduced and hence the roller lifetimes can be improved.

This invention relates to a fixing means for an electrophotographic copier 
and more particularly to such a fixing means adapted to fix toner images 
by causing a medium with toner images to pass between a heat roller and a 
contact roller. 
Most electrophotographic copiers make use of the so-called thermal fixing 
method or heat roller fixing method to fix toner images formed on an image 
transferring medium (hereinafter simply referred to as a copy paper). 
According to this method, the on which toner images are formed is passed 
between a heat roller and a contact roller compressed against it, and the 
heat roller is heated prior to the fixing operation for a predetermined 
period of time until the surface temperature of the heat roller reaches or 
exceeds the melting point of the toner images or otherwise a level at 
which the desired fixing process can be properly carried out. The prior 
art method of determining this preheating time period, however, frequently 
proves to be unsatisfactory when a large number of copies must be made by 
a high-speed operation. This is because, although the surface temperature 
of the heat roller may have reached a predetermined level by the 
preheating process, heat is immediately transferred away therefrom and the 
temperature of the fixing means drops below the required level for a 
successful fixing operation. 
In FIG. 11 which illustrates how a situation described above may come 
about, curves T'.sub.H and T'.sub.C show how the temperatures of the heat 
roller and the contact roller may typically change with time in a prior 
art fixing means. According to a prior art preheating method represented 
by FIG. 11, power is switched on at time t'.sub.1 and the roller 
temperatures gradually increase. When the temperature of the heat roller 
reaches a predetermined level a'.sub.1, the so-called first preheating 
period ends and a second preheating period is started immediately. In FIG. 
11, this switch-over time is indicated by t'.sub.2 and the temperature of 
the contact roller at this time is indicated by b'.sub.1. During the 
second preheating period, heat flows from the heat roller to the contact 
roller at an increased rate. The end of the second preheating period is 
reached when the temperature of the heat roller reaches another 
predetermined level a'.sub.2.The end time of the second preheating period 
is indicated by t.sub.3 and the temperature of the contact roller at this 
time is shown by b'.sub.2. After the end of the second preheating period, 
the temperature of the contact roller drops rapidly (down to a level 
indicated by b'.sub.3 in FIG. 11). This increases the rate of heat 
transfer from the heat roller to the contact roller and a large amount of 
electric power must be supplied to the heat roller in order to maintain 
the fixing means at a sufficiently high temperature level for fixing 
operations. 
Another problem to be considered with a conventional electrophotographic 
copier of the type described above relates to situations where a copy 
paper of less than the maximum size is passed between the rollers because 
the heating means for a conventional fixing means is usually adapted to 
generate heat at a fixed rate such that fixing can be appropriately 
effected when a copy paper of the maximum size admissible by the rollers 
is passed through. Thus, if a copy paper of the maximum size is passed 
between the rollers, the surface temperatures of the rollers will go down 
uniformly since the width of the paper and that of the rollers coincide 
approximately. If a smaller copy paper is passed through, however, there 
results a large temperature difference on the roller surfaces as 
illustrated in FIG. 12 which represents a situation where the roller width 
W.sub.R is greater than the paper width W.sub.P so that the surface 
temperature T.sub.2 where the paper has passed is much lower than the 
surface temperature T.sub.1 where the paper has not passed. In the past, 
there was the tendency to set the temperature T.sub.1 higher than 
necessary so that a copy paper of even the maximum size can be properly 
fixed. This has the consequence of making the local temperature 
differences T.sub.1 -T.sub.2 significantly large when a copy paper of a 
smaller size is used. As a result, possibilities of local deformations 
(expansion, swelling, etc.) of the roller surfaces (for example, of 
silicone rubber), separation of rubber materials from the cores of the 
rollers, wrinkles resulting on the copy papers and uneven fixing have to 
be seriously considered. 
It is therefore an object of the present invention to provide a fixing 
means for an electrophotographic copier by the heat roller fixing method 
using a heat roller and a contact roller such that the contact roller can 
be sufficiently heated. 
It is another object of the present invention to provide a fixing means for 
an electrophotographic copier by the heat roller fixing method such that 
the useful lives of the rollers can be improved and high quality copying 
can be effected. 
The above and other objects of the present invention are achieved by 
providing a fixing means for an electrophotographic copier by the heat 
roller fixing method using a heat roller and a contact roller such that 
the contact roller is caused to rotate by the rotation of the heat roller 
after the heat roller has reached a predetermined temperature level 
adequate for fixing operations and that the temperature of the heat roller 
is transmitted to the contact roller by this rotary motion. According to 
another embodiment of the present invention, a control means is provided 
for a heating means based on the surface temperature of the contact roller 
such that the preheating process is completed after the surface 
temperature of the heat roller reaches a first predetermined level when 
the surface temperature of the contact roller reaches a second 
predetermined level. The surface temperature of the contact roller can 
thus be prevented from dropping suddenly to create a situation where 
proper fixing operations cannot be continued. In another aspect of the 
present invention, a means is provided for detecting the size of the copy 
paper being passed between the rollers and the heating means for the 
rollers is controlled on the basis of the result obtained by the detecting 
means.

The structure of a fixing means embodying the present invention is 
schematically shown in FIG. 1 wherein numerals 1 and 2 generally indicate 
a heat roller and a contact roller, respectively. These rollers 1 and 2 
may be adapted to become compressed against each other with a 
predetermined pressure therebetween only after the power source for the 
electrophotographic copier is switched on or they may be in this mutually 
engaged relationship independently of whether the power source is on or 
off. The heat roller 1 includes a thermister 3 disposed in contact with 
the surface thereof, a heat generating means 5 such as a halogen lamp 
disposed inside thereof, a felt piece 7 impregnated with silicone oil, a 
blade 8 and a teflon-coated roller 9. The contact roller includes a 
thermister 4 disposed in contact with the surface thereof, a heat 
generating means 6 such as a halogen lamp disposed inside thereof, a 
rubber roller 10 impregnated with silicone oil and a core piece 11. A copy 
paper 12 with unfixed toner images 13 is passed between the rollers 1 and 
2 and comes out with fixed toner images 14 to be delivered to an ejection 
tray (not shown) by a known paper-ejecting means (not shown). 
According to an embodiment of the present invention, the fixing means shown 
in FIG. 1 is operated in such a manner that the surface temperatures 
T.sub.H and T.sub.C of the heat roller 1 and the contact roller 2, 
respectively, will change with time as shown in FIG. 2. Reference being 
made to FIG. 2, power is switched on at time t.sub.1 and the time interval 
from t.sub.1 to t.sub.2 represents the first stage of the preheating 
period. Likewise, the time interval between t.sub.2 and t.sub.3 represents 
the second stage (of duration about one minute) and the time interval 
between t.sub.3 and t.sub.4 represents the third stage of the preheating 
period according to this embodiment of the present invention. As shown in 
FIG. 2, the surface temperatures of the rollers 1 and 2 are a.sub.1 and 
b.sub.1, respectively, at the end of the first stage (t.sub.2), T.sub.H 
=a.sub.2 and T.sub.C =b.sub.2 at the end of the second stage (t.sub.3), 
and T.sub.H is approximately equal to a.sub.2 and T.sub.C = b.sub.3 at the 
end of the third stage (t.sub.4). 
The rotary motions of the rollers 1 and 2 are controlled on the basis of 
the temperatures a.sub.1 and a.sub.2. In other words, the surface 
temperature of the heat roller 1 begins to rise when the power source is 
switched on and the first stage of preheating is ended when T.sub.H 
reaches a.sub.1. When the first stage comes to an end at t.sub.2, both the 
heat roller 1 and the contact roller 2 begin to rotate by operations of a 
main motor (not shown in FIG. 1). The rollers continue to rotate until the 
surface temperature T.sub.H of the heat roller 1 reaches a.sub.2 at which 
fixing operations can be properly effected, and the second stage of 
preheating ends. 
Thereafter, the heat generating means 5 inside the heat roller 1 is 
switched on and off intermittently, controlled by the thermister 3, to 
maintain the surface temperature of the heat roller 1 approximately equal 
to a.sub.2. When the main motor begins to operate, electric circuits of 
various high-voltage sections, etc. inside the copier also become active. 
In view of the total rate of power consumption by the copier, the heat 
generating means 6 inside the contact roller 2 is switched off at this 
moment. 
This embodiment of the present invention is characterized in that the main 
motor is programmed to resume its rotation for a specified length of time 
after the end of the second stage of preheating when the surface 
temperature of the heat roller 1 reaches a.sub.2. In FIG. 2, this 
specified length of time immediately after the conclusion of the second 
stage is identified as the third stage from t.sub.3 to t.sub.4. The 
purpose of keeping the main motor rotating for this additional time period 
is to significantly reduce the temperature drop on the surface of the 
contact roller 2 after the fixing means becomes ready to start the fixing 
operation. Experiments have shown that as the time interval from t.sub.3 
to t.sub.4 is changed from two minutes to three minutes and further to 
four minutes, the surface temperature b.sub.3 of the contact roller 2 at 
the end of the third stage changes relatively little in the range of 170 
to 180.degree. C. but the level b.sub.4 to which the surface temperature 
of the contact roller 2 drops after the main motor is switched off at the 
end of the third stage becomes 50.degree., 40.degree. and 35.degree. C., 
respectively. This is because heat is additionally supplied from the heat 
roller 1 to the contact roller 2 over an extended time during which the 
rollers 1 and 2 are kept rotating and this has the effect of reducing the 
difference between the surface temperatures of the contact roller 2 and 
the inside temperature. 
The effects of additional heating described above are demonstrated in FIG. 
3 wherein the fixing performance is shown as a function of the number of 
processed copy papers. The curve between two circles indicates a situation 
where the additional heating takes place for two minutes (from t.sub.3 to 
t.sub.4). The curve between two two minutes (from t.sub.3 to t.sub.4). The 
squares represents the case where the additional heating period is three 
minutes and the curve between two triangles represents the case where the 
additional heating period is four minutes. FIG. 3 shows that the fixing 
performance improves as the length of additional heating (the third stage) 
is made longer. 
FIG. 4 is a block diagram of a circuit for operating the fixing means of 
FIG. 1 according to the mode described above and illustrated in FIGS. 2 
and 3. Reference being made to FIG. 4, numeral 16 indicates a central 
processing unit CPU to which the measured temperature value by the 
thermister 3 in contact with the heat roller 1 of FIG. 1 is transmitted 
through an analog-to-digital converter A/D 25. The CPU 16 is adapted to 
control the motion of the aformentioned main motor 20, causing it to start 
rotating when the power switch 17 is set in the ON position. Numeral 21 
generally indicates the mechanisms for driving the rollers 1 and 2. The 
CPU 16 is further adapted to compare the temperature detected by the 
thermister 3 with the predefined temperature a.sub.2 stored in a memory 
means 18 and to start a timer 19 from the point in time when they become 
equal to each other. The main motor 20 is rotated until the timer 19 shows 
that a time interval corresponding to another predefined value stored in 
the memory means 18 has passed. As explained above, the aforementioned 
rollers 1 and 2 are caused to rotate by the motion of this main motor 20. 
In summary, there is added to a fixing means of a conventional type a 
heating means for heating the contact roller by the rotation of the heat 
roller after the surface temperature of the latter reaches a level 
sufficiently high for fixing operations. As a result, the heat flow from 
the heat roller to the contact roller is reduced after the condition for 
starting fixing operations has been established. This prevents the 
undesirable temperature drop in the fixing means and the fixing 
performance is improved especially for a copier operated at a high speed 
or when a large number of sheets are processed. Since the temperature of 
the fixing means can be maintained merely by rotating the heat roller, 
furthermore, the present invention also has the effect of reducing power 
consumption. 
Alternatively, the preheating process may be programmed in part on the 
basis of the surface temperature of the contact roller, reference being 
made again to the fixing means described in FIG. 1. Such a preheating 
program according to a second embodiment of the present invention is 
similar to the one described above and will be explained below with 
reference again to FIG. 2. According to the embodiment of the present 
invention, the end of the third stage of preheating is not determined by 
the predefined time interval represented by t.sub.4 -t.sub.3 but instead 
by the temperature level b.sub.3 to which the contact roller 2 must reach. 
To summarize, the first stage of preheating lasts until the surface 
temperature of the heat roller 1 as detected by the thermister 3 reaches a 
predetermined level a.sub.1 with the main motor remaining in the OFF 
condition and both heat generating means 5 and 6 being switched on; the 
second stage of preheating starts at t.sub.2 and lasts until the surface 
temperature of the heat roller 1 reaches a higher predetermined level 
a.sub.2 with the main motor and the heat generating means 5 for the heat 
roller 1 switched on but the heat generating means 6 for the contact 
roller 2 switched off as explained above in connection with the first 
embodiment of the present invention in view of the increased rate of total 
power consumption during this period caused by the operation of the main 
motor; and the third stage starts at t.sub.3 and lasts until the surface 
temperature of the contact roller 2 as detected by the thermister 4 
reaches a predetermined level b.sub.3 higher than the surface temperature 
b.sub.2 of the contact roller 2 at the end of the second stage of 
preheating. 
FIG. 5 is a block diagram of a control system for operating the fixing 
means of FIG. 1 according to the second embodiment of the present 
invention described above. Reference being made to FIG. 5 wherein 
components corresponding to those defined in connection with FIG. 4 are 
indicated by the same numbers, a central processing unit CPU 16 is 
connected not only to the thermister 3 for the heat roller 1 through an 
analog-to-digital converter 25 and the heat generating means 5 inside the 
hat roller 1 but also to the thermister 4 for the contact roller 2 through 
another analog-to-digital converter 26 and the heat generating means 6 
inside the contact roller 2. Numeral 18 again indicates a memory means and 
three values a.sub.1, a.sub.2 and b.sub.3 are stored therein. As explained 
above, the values a.sub.1 and a.sub.2 are for making comparisons with the 
surface temperature of the heat roller 1 and the value b.sub.3 is for 
comparing with the surface temperature of the contact roller 2. The heat 
generating means 5 and 6 and the main motor 20 are switched on and off in 
accordance with the results of such comparisons as explained in detail 
above. 
The effects of the third preheating stage according to the second 
embodiment of the present invention explained above are demonstrated in 
FIG. 6. The curve between two circles in FIG. 6 represents the 
relationship between the fixing performance and the number of processed 
copy papers according to this embodiment of the present invention. The 
curve between two triangles represents the relationship when a prior art 
method of operation is used. 
The present invention is addressed also to the problem of temperature 
variations along the width of the rollers when a paper of less than the 
maximum admissible width is passed between them. As shown in FIG. 7 which 
is a circuit diagram of a circuit for controlling the voltage to be 
applied to the heat generating means 5 or 6 of FIG. 1 according to the 
present invention, each of the heat generating means 5 and 6 is connected 
to an AC power source 30 through a diode 31. A lead switch 32 which opens 
and closes in response to a signal P is connected in parallel with the 
diode 31. 
The signal P for opening and closing the lead switch 32 is indicative of 
the size of a copy paper, or it is a signal which is obtained on the basis 
of the detected size of a copy paper. For example, if it is determined 
that a paper of the maximum size is being passed through, the lead switch 
32 is closed by a detection signal indicative of this determination and if 
it is determined that a smaller paper is being passed through, the lead 
switch 32 is opened by a different detection signal indicative of this 
finding. It may be preferable to use for the size detection the 
size-indication signal at the time of copying operation. 
Thus, when a copy paper 12 of the maximum size carrying unfixed toner 
images 13 is passed between the rollers 1 and 2 as shown in FIG. 1, the 
signal P of paper size detection causes the lead switch 32 to close so 
that a voltage with a waveform obtained by full-wave rectification is 
applied to the heat generating means 5 and 6 and the toner images 13 are 
fixed with the amount of heat suited for processing a paper of the maximum 
size. When a copy paper 12 is of a smaller size, the lead switch 32 is 
opened by the signal P for paper size detection and a voltage of a 
waveform obtained by half-wave rectification shown by FIG. 8 is applied to 
the heat generating means 5 and 6 to heat the rollers 1 and 2. The amount 
of heat generated under this condition is naturally less although it is 
still sufficiently large for fixing a paper of a smaller size. 
Accordingly, the surface temperatures of the rollers 1 and 2 become lower 
than when a paper of the maximum size is processed. With reference to FIG. 
12, this has the effect of reducing the temperature T.sub.1 where the 
paper has not passed and hence the difference T.sub.1 -T.sub.2. 
FIG. 9 is a circuit diagram of another voltage control circuit for 
controlling the amount of heat generated by the heat generating means 5 
and 6. In this circuit, a triac circuit 34 is inserted between the AC 
power source 30 and the heat generating means 5 (or 6) such that the triac 
circuit 34 is controlled by the output of a gate circuit 36 which 
introduces the signal P of paper size detection. Since the phase angle of 
the voltage can be varied freely with a circuit thus structured, the 
amount of heat generated by the heat generating means 5 and 6 can be 
controlled more finely according to the size of the copy paper being 
processed and the temperature difference T.sub.1 -T.sub.2 on the rollers 1 
and 2 can be reduced most effectively. 
In summary, the present invention teaches to control the amount of heat 
generated inside the rollers for a fixing means according to the detected 
size of the copy paper being processed. This has the desired effect of 
improving the lifetimes of the rollers and the quality of copies produced 
by the fixing means employing them. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. For example, the end of the third preheating 
stage need not be defined in either of the manners described above. 
Reference being made again to FIG. 2, the time to end the preheating 
process may be chosen at t.sub.5 corresponding to the point where the 
extension of the line representing T.sub.C between t.sub.1 and t.sub.2 
crosses the extension of the same curve representing T.sub.C between 
t.sub.3 and t.sub.4. As another example, the first and second stages may 
be combined into a single stage. In this situation, both heat generating 
means 5 and 6 are switched on and the main motor 20 remains stationary 
until the surface temperature of the heat roller 1 reaches the level 
a.sub.2. Such modifications and variations which may be obvious to a 
person skilled in the art are intended to be within the scope of the 
present invention.