Duplex apparatus having a roller fuser

A copier or printer uses the principle of single pass duplexing which produces simplex output at full machine speed and duplex output with a gap of at least one sheet between sheets. It has a duplex fuser that fuses both duplex images simultaneously. The duplex fuser has a first relatively hard roller which contacts the side of receiving sheets carrying simplex images and a soft heated roller which contacts the opposite sides of sheets. The soft roller is heated, in part, by a lamp in its center, and, in part, by heat from the first roller which is passed to the soft roller between duplex sheets. The soft roller therefore runs at a higher temperature for duplex than it does for simplex. The hard roller preferably has a thin elastomeric layer on a metal core and therefore transfers heat to its surface more efficiently and faster than the soft roller.

TECHNICAL FIELD 
This invention relates to electrostatographic apparatus of the type which 
can generate receiving sheets having toner images on one side (simplex) or 
toner images on both sides (duplex). More specifically, this invention 
relates to such apparatus having a roller fuser for fixing duplex images 
in a single pass. 
BACKGROUND ART 
U.S. Pat. No. 4,429,990, issued Feb. 7, 1984 to E. J. Tamary, shows an 
electrophotographic copier which forms a series of toner images. The 
copier has two transfer stations. In a simplex mode, one transfer station 
is used to transfer toner images to a single side of receiving sheets 
which are then fed to a fuser for fixing. In a duplex mode, receiving 
sheets are fed to a first transfer station for receiving toner images on 
one side, immediately turned over, fed to a second transfer station to 
receive a toner image on the other side and then fed to the fuser where 
both images are fused simultaneously. 
The machine shown in the Tamary patent has been successful commercially. It 
has the very important advantage of providing duplex output without 
passing the copy sheet through the fuser twice. It is termed a "single 
pass" duplexing apparatus. It is contrasted with "double pass" duplexing 
systems in which a series of sheets receive one image, are fused and 
placed in an intermediate tray. They are fed back to the image member to 
receive the second image and pass through the fuser again. One image 
receives twice the fusing of the other and the sheet is heated twice. 
Although the advantages of "single pass" duplexing are many, the task of 
fusing duplex images in one mode and simplex images in another mode while 
controlling the surface temperatures of the fusing members is challenging. 
The commercial duplex fuser utilizing the Tamary technology has a pair of 
fusing rollers which are each heated from within. The fusing roller which 
contacts the side of a receiving sheet carrying a simplex image (called 
the "simplex roller") has a thick elastomeric covering on a metal core. 
The other fusing roller (the "duplex roller") has a thin elastomeric 
covering also on a metal core. Both fusing rollers have temperature 
control devices which sense the surface temperature of the metal core 
outside the image area to control the power applied to internal heating 
lamps within the rollers. Because of the thickness of the elastomer on the 
roller contacting the simplex side of the receiving sheets, a relatively 
high set point must be used for the metal core of that roller. This is 
because the temperature of the exterior surface of the elastomer for any 
given roller with a constant heat source decreases as a function of the 
thickness of the elastomer. 
Although this apparatus produces excellent images, the power consumption of 
the fuser is substantial, even working with relatively low fusing 
temperature toners. Further, while the core temperature of the simplex 
roller is not too high using toners fusable at 340.degree. F. for which it 
is designed, to use higher fusing temperature toners (for example, 
380.degree. F.) the core temperature leaves little upper latitude with 
respect to a high shut off point for the fuser and the char point of 
paper. Even at lower fusing temperatures, throughput is limited by the 
limit on the core temperature. Additionally, the thick outer layer causes 
a droop in temperature at the beginning of a run requiring a toner that 
fuses over a wide range of temperatures. 
A number of references show simplex fusers which include heating lamps in 
both rollers. The heating lamp in the roller that does not contact the 
image is generally used to prevent that roller from lowering the 
temperature of the roller which does contact the image when the rollers 
are in contact between images. See, for example, U.S. Pat. No. 4,231,653, 
Nagahara et al, issued Nov. 4, 1980; U.S. Pat. No. 4,549,803, Ohno et al, 
Oct. 29, 1985; U.S. Pat. No. 4,595,274, Sakurai, Jun. 17, 1986; U.S. Pat. 
No. 4,618,240, Sakurai et al, Oct. 21, 1986; U.S. Pat. No. 4,019,024 
Namiki, Apr. 19, 1977; U.S. Pat. No. 3,945,726, Ito et al, Mar. 23, 1976 
and U.S. Pat. No. 3,268,351, VanDorn, Aug. 23, 1966. 
DISCLOSURE OF THE INVENTION 
The object of the invention is to improve the temperature control and/or 
power consumption of a duplex fuser in an apparatus for forming either 
simplex or duplex copies. Improved power consumption can be taken 
advantage of by 1) reducing power consumption, 2) increasing throughput, 
and/or 3) increasing the fusing temperature. 
This and other objects are accomplished by a duplex fuser for apparatus 
generally of the single pass duplex type described in the Tamary patent, 
which fuser has first and second independently heated fusing rollers. The 
first fusing roller (the simplex roller) contacts the side of the 
receiving sheets carrying simplex images while a second fusing roller 
(duplex roller) contacts the opposite side of the sheets, thus, contacting 
images only in duplex. The first fusing roller is a relatively hard roller 
while the second fusing roller has a thick elastomeric material on top of 
a metal core. Both rollers have outside surfaces resistant to toner 
offset. 
According to a preferred embodiment, the first roller has a thin 
elastomeric layer on a metal core which because of its thinness does not 
substantially inhibit the passing of heat from the core to a simplex image 
being fused. In simplex operation, the first roller, because of the 
thinness of this layer, is efficient in heat transfer, fixing simplex 
images at full machine speed with relatively small spaces between 
receiving sheets. Compared to a simplex roller having a thick elastomeric 
coating, its core runs cooler and the temperature droop at the beginning 
of a run is reduced. 
In duplex, sheets are received only half as fast as they are received in 
simplex. More significantly, and unlike "double pass" systems, all duplex 
sheets are separated from each other by a space at least equal to another 
sheet. The second roller has a thick elastomeric outer layer and is also 
heated from within. Because of the space between sheets in duplex, the 
second roller is also heated by contact with the first roller during such 
spaces. Because this particular apparatus provides its duplex output with 
regular one sheet spaces between sheets, the second roller can reliably 
obtain enough heat from the first roller in duplex to raise its 
temperature adequately to fuse toner images on the opposite side of the 
sheet. Thus, with this arrangement, a large amount of the heat for fusing 
both images is supplied by the first fusing roller, which fusing roller 
has at most a thin elastomeric covering and is therefore efficient in 
transferring heat.

BEST MODE OF CARRYING OUT THE INVENTION 
FIG. 1 shows an electrophotographic printer 1 which uses the principle of 
single pass duplexing. According to FIG. 1, an image member 2 is an 
endless belt having one or more electrophotosensitive layers 9 on a 
conductive backing 8. Image member 2 is entrained about a series of 
rollers 10, 11, 12, 13, 14 and 15 and is driven past a series of stations 
by a motor 16 connected to roller 10. Image member 2 is uniformly charged 
at a main charging station 18, imagewise exposed at an electronic exposure 
station 19 to create a series of electrostatic images in response to an 
electronic signal coming from an electronic source 21 which electronic 
source can be a computer, a scanner, a memory, or the like. The series of 
electrostatic images on image member 2 are toned at a toning station 20 to 
create a series of toner images defined by the electrostatic images. 
Printer 1 has a pair of transfer stations 35 and 36 for transferring toner 
images to receiving sheets fed from either of receiving sheet supplies 30 
or 31. If simplex output is desired, receiving sheets are fed from either 
of receiving sheet supplies 30 or 31 to second transfer station 36 where 
the receiving sheets arrive in timed relation with the toner images and 
are transferred by conventional corona transfer. The receiving sheets 
separate from image member 2 as image member 2 passes around small roller 
14 and are transported to duplex fuser 45 which include rollers 46 and 47. 
The simplex images are fused by application of heat and pressure by 
rollers 46 and 47 and transported through an inversion to arrive face up 
in an output hopper 48. Image member 2 is cleaned at cleaning station 50 
for reuse. 
If duplex output is desired, receiving sheets are fed from receiving sheet 
supply 31 to first transfer station 35 where a first side of the receiving 
sheet receives a first toner image. The receiving sheet is separated from 
image member 2 and turned over by a turnover roller 37 and immediately fed 
back to second transfer station 36 to receive a second toner image on its 
opposite side. The receiving sheet again separates from image member 2 as 
image member 2 passes around small roller 14 and is transported to fuser 
45 where both images are simultaneously fused to opposite sides of the 
sheet and then deposited with its first side up in output tray 48. 
Fuser 45 is shown in more detail FIG. 2. According to FIG. 2, first fusing 
roller 47 contacts the image side of a receiving sheet 100 carrying a 
simplex image and is commonly called the "simplex roller." Simplex roller 
47 has a metal core 101 and is preferably covered by a thin elastomeric 
covering 102 of a material which defines the outside surface of simplex 
roller 47 which surface is resistant to offset of toner. For example, the 
elastomeric material can be a conventional silicone rubber presently used 
in fusers. 
The simplex fusing roller 47 is heated by a short filament quartz lamp 103 
which preferably does not stretch to the ends of the roller 47. Lamp 103 
is preferably relatively high power. For example, for fusing legal-sized 
sheets having a cross-track dimension of 14 inches and standard sized 
sheets having a cross-track dimension of 11 inches, lamp 103 can be 
powered by 1850 watts across a 141/4 inch filament. 
Second fusing roller 46, commonly called the "duplex roller," has a metal 
core 111 similar to core 101 in first roller 47. It is covered with a 
relatively thick elastomeric coating and is heated by a somewhat less 
powerful but longer lamp 113. For example, lamp 113 can be powered by 1250 
watts across a 16 inch filament. 
Elastomeric layer 102 is sufficiently thin, for example, 20 mils, to make 
simplex roller 47 relatively hard compared to duplex roller 46 whose 
elastomeric layer 112 is thicker, for example, 100 mils. Rollers 46 and 47 
thus form a nip which is curved into the duplex roller, approximately 
conforming to the cylindrical (uncompressed) outer periphery of first 
roller 47. The temperature of core 101 is monitored by a temperature 
sensor 104 and the temperature of core 111 is monitored by a temperature 
sensor 114 whose outputs are fed to a fusing control 130 which in turn 
controls the power supplied to lamps 103 and 113. 
During simplex fusing, printer 1 produces simplex output at full machine 
speed. That is, receiving sheets have images transferred to them and enter 
the fuser at a rate approximating that of the movement of image member 2 
with a small, for example, less than 1 inch, space between sheets. Core 
101 for simplex roller 47 has a simplex set point at a temperature sensed 
by sensor 104 that is high enough to fuse images at this rate taking into 
consideration the thickness of thin layer 102. Duplex roller 46 is heated 
by heat lamp 113 to supply some heat to the process, which reduces the 
amount of heat lost by the simplex roller 47 between the receiving sheets. 
However, the duplex roller 46 does not have a set point that would by 
itself be consistently high enough to fuse images on the back side of 
receiving sheet 100. 
In single pass duplex operation, sheets are received from image member 2 at 
a consistent, every-other-frame, rate. That is, consecutive receiving 
sheets are separated by a gap equal, at least, to the in-track dimension 
of a receiving sheet. During this time, heat from the simplex roller 47 
transfers to the duplex roller 46, thereby raising the exterior 
temperature of duplex roller 46 above that attributable to the heat from 
core 111. The heat from simplex roller 47 raises the temperature of the 
surface of duplex roller 46 adequately to allow roller 46 (with the heat 
received from its heat source 113) to fuse images carried on the back of a 
duplex receiving sheet while the simplex roller fuses images on the front 
side of the receiving sheet. 
This system has the advantage of supplying much of the heat for both 
simplex and duplex fusing from the simplex roller 47. Simplex roller 47 
has the thin elastomeric cover 102 and therefore readily transfers heat to 
the nip. 
This approach permits running at higher throughput and/or higher roller 
surface temperatures, without excessive roller core temperatures. At the 
same time, heat is more effectively utilized and therefore the fuser heats 
up the environment less than prior duplex fusers. 
Although both rollers 47 and 46 have elastomeric layers in the preferred 
embodiment shown in FIG. 2, roller 47 could have only a thin layer of 
offset preventing material such as polytetrafluoroethylene directly on 
core 101. To assure comparable appearance of both images in duplex, roller 
46 should then have a coating of the same offset preventing material on 
elastomeric layer 112. 
In the prior art duplex fuser presently in use, which has a 100 mil and a 
20 mil elastomer coatings on simplex and duplex rollers, respectively, 
core set points on the simplex and duplex rollers when in standby are 
approximately 345.degree. F. and 330.degree. F., respectively. For 11 inch 
simplex receivers these set points are increased to simplex run set points 
of approximately 415.degree. F. and 340.degree. F., respectively. The 
surface temperature of the simplex roller can droop to as low as 
305.degree. F. at startup using these parameters, with a power consumption 
as high as 2500 watts. This device is designed for use with toners having 
a desired fusing temperature of 340.degree. F. 
In a fuser constructed as shown in FIG. 2 standby set points of 340.degree. 
F. and 366.degree. F. are used for simplex and duplex rollers, 
respectively. These are increased to 395.degree. F. and 415.degree. F., 
respectively, during a simplex run. This provides a steady state surface 
temperature on the simplex roller of approximately 380.degree. F. with 
little droop and maximum overshoot to about 395.degree. F. Using these set 
points, the duplex roller surface temperature is maintained between 
340.degree. F. and 350.degree. F. during simplex operation. 
In duplex, the duplex roller set point is allowed to remain at 415.degree. 
F. while the simplex roller set point is reduced to 375.degree. F. This 
maintains the surface temperature of each roller at between 380.degree. F. 
and 390.degree. F. for duplex fusing, again with negligible droop. 
The temperatures in each mode are maintained with an average power 
consumption less than 2500 watts. This structure is designed for use with 
a toner having a preferred fusing temperature of about 380.degree. F. 
Thus, the FIG. 2 fuser provides more even fusing with less droop and danger 
of overheating than the prior art despite increasing the fusing 
temperature from 340.degree. F. to 380.degree. F. This structure therefore 
allows use of a higher fusing temperature toner in the FIG. 1 apparatus. 
The surface temperatures are measured in the middle of the image while the 
core set points are dependent upon sensors in the margins outside of the 
images which tend to be cooler than the middle of the core. This explains 
the simplex roller core set point in duplex being lower than either roller 
surface temperature. 
The specific examples of set points set out above are for 11 inch receiver 
sheets. Higher set point for 13 or 14 inch receiving sheets are required 
to provide essentially the same fusing temperature at each surface. This 
adjustment between letter and legal or other size sheets is a feature 
presently known in the art. 
Note that apparatus 1 is shown as a printer using an LED printhead 19 
connected to a source 21. The fuser according to FIG. 2 could also be used 
with an optical copier similar to that shown in U.S. Pat. No. 4,429,990, 
referred to above. 
If the source 21 is a computer which is generating data at a rate that, in 
some instances, due to its complexity does not keep up with the speed of 
the printer 1, a logic and control 200 of printer 1 may add one or more 
skip frames to its processing cycle. That is, until the data stream being 
fed by source 21 into printhead 19 is complete, one or more frames may not 
be imaged. If this happens continually in either simplex or duplex 
operation, the fuser 45 may run without sheets passing through it, but 
with a higher "run" set point. This will cause overheating of the fuser 
with known problems, such as "hot offset", charring of paper and 
overheating shutdown by safety sensors (not shown). 
To solve this problem, logic and control 200 for the apparatus can be 
programmed to adjust the set point(s) in fuser control 130 in a downward 
direction in response to the occurrence of skip frames. 
This approach involves decreasing the core temperature set points from 
simplex values (used for maximum heat loss conditions) to standby values 
(used for minimum heat loss conditions) as a function of the number of 
skip frames per fused receiver. A fuser roller consisting of a elastomeric 
coating on a metal core can be modeled as a hollow cylinder, which is the 
coating, with the inner surface held at a uniform temperature equal to the 
temperature of the core. According to well known thermal equations, the 
flow of heat per unit length of the cylinder is equal to: 
EQU 2.pi.K(v.sub.1 -v.sub.2)/ln(b/a) 
where K is the thermal conductivity of the elastomer, v.sub.1 and v.sub.2 
are the temperatures of the inner and outer surfaces of the cylinder and a 
and b are the radii of the inner and outer surfaces of the cylinder. From 
this it can be seen, that, if the outer surface at b is to be kept at 
v.sub.2 and the heat flow out of the system is reduced (for example, due 
to skip frames), then the difference (v.sub.1 -v.sub.2) must be reduced 
proportionally to reduce the flow of heat to the outer surface to keep 
v.sub.2 from increasing. It is also apparent that the heat flow from a 
roller with a thick elastomeric coating is less than that of a thinly 
coated roller. Therefore, one method of controlling roller surface 
temperature is to first decrease the thinly coated simplex roller set 
point incrementally toward standby. Once the standby position is reached 
for the simplex roller, the duplex roller is reduced incrementally to 
standby. For example, for one skip frame the simplex roller is set to a 
value necessary to maintain constant net heat flow and roller surface 
temperatures at the aim fusing temperature. For two skip frames it is 
decreased slightly more, and so on, until the standby values for both 
rollers is reached. 
However, in actual practice, this much sophistication does not appear to be 
necessary. The printer shown in FIG. 1 may have five or six image frames. 
Logic and control 200 receives three inputs relevant to fuser control, the 
appearance of a frame indicator at a sensing point relevant to exposure, 
an indication as to whether exposure is to be made for that frame, and an 
indication as to whether printing will be in duplex or not (simplex). In 
response to the frame indication signal, if the frame is to be exposed and 
printing is in simplex, the fuser is set at it simplex run set points. In 
response to a frame indication signal and an exposure indication signal 
with printing set at duplex, the temperature set points are positioned for 
duplex. If a frame indication signal is received and no exposure signal is 
received, the logic and control immediately sets the fuser set points down 
"one skip frame increment", unless both rollers' set points are already at 
standby, in which case no further adjustment is made. 
Note that the fuser is immediately adjusted even though the frame to be 
skipped is four or five frames away from the fuser (the distance between 
the exposure station and the fuser). Four frames in a high speed printer 
may be equal to two or three seconds of time. Actual heat adjustment in 
this time is not fast enough to make a serious difference. However, if a 
series of skip frames occurs, definite overheating can result when sheets 
stop arriving at the fuser which this algorithm will adjust for. 
A specific example of an approach to incremental setting of the fuser shown 
in FIG. 2 in response to skip frames is to adjust the simplex roller set 
point to its duplex value on occurrence of the first skip frame, then 
adjust the simplex roller to the standby value at the second skip frame. 
Then the duplex roller is adjusted to the standby value for the third and 
subsequent frames. A less precise approach would be to adjust the simplex 
roller to the standby at the first skip frame and the duplex roller to 
standby at the second skip frame. Both of these approaches substantially 
eliminate hot offset in a condition of a substantial and upredictable skip 
frames. When the raster image processor catches up, an image is exposed, 
and the fuser returns to its run set points. 
Note that this algorithm for handling skip frames is not limited to single 
color printers. In printers that run often with a single color but 
occasionally combine images from consecutive frames onto a single side of 
a single sheet will also generate a condition similar to skip frames. That 
is, the flow of paper through the fuser will stop for a while. This 
invention can be used for such conditions. Note that the absence of paper 
in the fuser that causes the increase in temperature in a color copier or 
printer is due to superposing multiple frames on a single side of a sheet, 
rather than skipping an exposure frame. Thus, it may be preferable to key 
off the feeding of sheets rather than exposure as indicative of "skip 
frames", since exposure of images would not be a good indication of the 
paper passage through the fuser in this instance. 
The invention has been described in detail with particular reference to a 
preferred embodiment thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention as described hereinabove and as defined in the appended claims.