Thermal fixing system for recording media of a printer or copier device that are printed on one or both sides

In a thermal fixing system for fixing toner images on the front side of a recording medium in an electrographic printer or copier device, wherein the back side of the recording medium can already have a fixed toner image. The thermal fixing means contains a heat transfer fixing station that fixes the toner images on the recording medium, and contains a pre-heating saddle that precedes the heat transfer fixing station in a running direction of the recording medium. A sliding surface that accepts the recording medium over its back side is allocated to the pre-heating saddle. The sliding surface is constructed of a toner-repellant material at least in a contact region with the recording medium. The preheating saddle is designed as a low temperature saddle with the largest possible constructional length, so that a temperature difference between recording substrate and saddle surface is as small as possible. The preheating saddle has, in the recording substrate running direction, a plurality of heating zones. A control device controls the heating zones in such a manner that, along the preheating saddle, an approximately constant thermal energy flow occurs on the saddle surface to the recording substrate. For matching the preheating saddle to various recording substrate widths, the preheating saddle is subdivided, transversely to the recording substrate running direction, into individually drivable transverse heating zones.

BACKGROUND OF THE INVENTION 
The invention is directed to a thermal fixing system for fixing toner 
images on the front side of a web-shaped recording medium in an 
electrographic printer or copier device, whereby the back side of the 
recording medium can already have a fixed toner image. 
Thermal fixing devices that comprise a pre-heating saddle with a following 
fixing zone composed of a heated fixing drum and a pressure roller are 
employed in printer or copier devices for heat transfer fixing of toner 
images on a recording medium that is usually composed of paper. 
Such thermal fixing devices are disclosed, for example, by U.S. Pat. No. 
4,147,922 or Japan Abstract Vol. 13, No. 120, 24 March 1989, 
(Japan-A-63-292177). 
It is beneficial in electrographic printer devices that work in the highest 
speed range with, for example, a printing speed of more than 0.5 m/s, and 
that employ a heat transfer fixing station for fixing, to heat the paper 
web or the paper sheet to temperatures of approximately 100.degree. C. or 
more before the actual heat transfer fixing process in order to thus 
obtain a good joining of the toner image to the paper surface. 
When a paper web or a single sheet of paper that is already printed and 
fixed on one side, for example on the back side, is to be printed and 
fixed on the other side, then the first side which is already fixed must 
be conducted over the hot surface of the pre-heating saddle for heating 
the paper for the second fixing process. The following problems thereby 
arise in this second fixing process: 
a) Continuous printer operation: 
The print image that is already fixed and that runs over the hot surface of 
the pre-heating saddle is heated to such an extent that it assumes a 
condition ranging from tacky through fluid, and is partly smeared on the 
saddle surface. The more toner is transferred from the toner image onto 
the saddle surface the more toner collects on the saddle surface, until a 
visible destruction of the toner image on the paper occurs. 
b) Waiting or Standby Operation: 
While the printer is in the waiting or standby mode, the paper web having 
the already fixed print image lies on the hot saddle. The print image is 
heated to such an extent in the region of the surface of the pre-heating 
saddle that it assumes a tacky through fluid condition and sticks to the 
hot surface of the pre-heating saddle. When the paper web is started, the 
toner image is then torn from the surface of the paper web and remains 
sticking on the hot surface of the saddle. 
In the case of the known fixing devices, there is another problem. It has 
previously been assumed that it is necessary to preheat the paper very 
rapidly over a relatively short path, via the preheating saddle, and then 
to fix the toner image on the paper via the rollers. For this purpose, the 
heating elements are arranged in the preheating saddle in such a way that 
the greatest quantity of heat is emitted to the recording substrate in the 
region of the paper inlet of the preheating saddle and that the emitted 
quantity of heat is then reduced over the heating elements in the 
direction of the paper exit. Thus, the relatively hottest region of the 
saddle is the paper inlet. 
However, it has appeared that a rapid heating up of the paper over a short 
path leads to a high loading of the paper. This loading is expressed as a 
deformation, an embrittlement or an ageing of the paper and as a 
non-uniform loss of water from the paper during passage through the fixing 
station. Hence, post-processing of the paper by cutting or sorting is made 
more difficult or there occurs a non-uniform fixing of the toner images 
and thus an impairment of the quality of the print. 
In addition, a rapid heating up requires a high specific heating power 
using high-power heating elements and a complicated control system. 
Because of the high heating power it is therefore necessary to lift the 
recording substrate immediately from the saddle in the event of a printer 
stop, in order to prevent burning of the paper. His makes comprehensive 
control devices necessary, which impairs the paper handling as a whole. 
In modem electrophotograhic printing devices, furthermore, recording 
substrates of the most different widths are processed in the same machine. 
If the same amount of energy is fed to the saddle over the entire width, 
the saddle heats up severely in that region where there is no paper 
running, since in this region no energy is dissipated, apart from losses 
due to convection. 
A temperature distribution of this type has considerable disadvantages. The 
paper is heated up non-uniformly, which leads to fluctuations in the 
fixing quality and can also cause paper running problems. The maximum 
heating saddle temperature must be reduced, since there exists the risk of 
overheating of the heating elements and the lifetime of the heating 
elements is thereby shortened. The energy losses are relatively large and 
the inner region of the machine is heated up unnecessarily. 
In the case of thermofixing devices with a preheating saddle, the recording 
substrate is guided over a heated gliding surface of the saddle. Direct 
contact between paper and saddle is essential for a good thermal transfer 
between paper and saddle surface. In the case of high printing speeds and 
in the use of pre-folded papers or papers of non-uniform thickness, 
fluttering movements of the paper can occur in the region of the saddle. 
In consequence, the paper lifts partially off from the saddle, which 
impairs the thermal transfer. Also, paper contains a relatively high 
proportion of water, which is released during warming. The released steam 
can be deposited in the machine and can lead there to disturbances or to 
corrosion. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a thermal fixing means having a 
pre-heating saddle for fixing toner images on the front side of a 
recording medium in an electrographic printer or copier device, whereby 
the back side of the recording medium can already have a fixed toner 
image. 
It is another object of the invention to provide a thermofixing device and 
a process for fixing, in which the recording substrate is exposed to as 
small a thermal loading as possible during passage through the fixing 
device. 
It is a further object of the invention to provide a thermofixing device 
which makes it possible, without fluctuations of the fixing quality, to 
fix recording substrates of the most different widths and in which warping 
and deformations of the fixed recording substrate are avoided. 
According to the invention, a thermal fixing system is provided for fixing 
toner images on a front side of a recording medium in an electrographic 
printer or copier device wherein a back side of the recording medium 
already has a fixed toner image. A heat transfer fixing station is 
provided for fixing the toner images on the recording medium. A 
pre-heating saddle precedes the heat transfer fixing station and a running 
direction of the recording medium has a sliding surface allocated thereto 
for accepting the recording medium over its back side. The sliding surface 
comprises a toner-repellant material at least in a contact region of the 
recording medium. 
The specification of front side and back side of a recording medium is a 
purely relative matter for describing the two sides of a recording medium. 
When the recording medium, which can be composed of single sheets or of 
continuous form paper, is conducted over a pre-heating saddle having a 
sliding surface that exhibits a repellant property for the tacky through 
fluid toner and has high abrasion resistance with respect to the paper web 
sliding thereon, then the thermal fixing means can be employed in printer 
or copier devices that work both in a simplex as well as in a duplex mode. 
Materials that are manufactured of fluorine compounds such as, for example, 
PTFE or, respectively, PFA compounds, have proven beneficial. The material 
can be vapor-deposited, sprayed, or glued on an appropriate acceptance 
surface of a pre-heating saddle. PTFE or, respectively, PFA compounds 
exhibit extremely good repellency with respect to the toner material and 
exhibit extremely good properties regarding abrasion, due to the paper 
web. 
In order to enhance the abrasion resistance, wear-reducing constituents 
such as graphite or glass fibers can be mixed to the PTFE or PFA to a more 
or less pronounced degree. 
Since such pre-heating saddles are usually utilized in electrographic 
printer devices of the higher performance category (between 2 and 10 
million DIN A4 pages per month), non-wearing operation over years is 
impossible. For this reason, it is meaningful when the saddle surface can 
be unproblematically and simply renewed as needed, without the expensive 
base structure of the heating saddle with heating elements having to be 
renewed. For this purpose, a toner-repellant layer can be vapor-deposited, 
sprayed, or glued onto thin metal plates, whereby these coated, individual 
plates are then interchangeably secured on the base structure of the 
pre-heating saddle. 
In an advantageous embodiment of the invention, the toner-repellant layer 
is executed as a film which has a thin, thermally conductive adhesive 
layer on one side. The adhesive layer is implemented such that the film 
can be easily pulled from the saddle in the hot condition of the saddle. A 
fast renewal of the saddle surface is thus rapidly possible, as needed on 
site by the customer. 
The toner-repellant layer can also be implemented as a thin film that is 
taken from a supply reel, is guided over the surface of the pre-heating 
saddle and is then again wound up. The film is thus moved extremely slowly 
relative to the running direction of the paper. 
In order to obtain a fold-free entry of the paper web into the fixing gap 
between fixing drum and pressure drum, it has already been proposed to 
design that end of the pre-heating saddle facing toward the fixing gap as 
a smoothing edge over which the recording medium is deflected to a great 
degree. However, extremely high wear of the toner coating on the recording 
medium occurs in the wrap region in the region of the smoothing edge. This 
wear can be prevented when rollers that may potentially be provided with a 
toner-repellant coating are provided in the wrap region. 
When a relatively high proportion of graphite or glass fibers is added to 
the toner-repellant material in order to achieve high wear resistance of 
the surface, then the repellency of the surface relative to the toner 
image may potentially be reduced. In order to prevent a transfer of the 
toner image onto the saddle surface in such cases during a long waiting or 
standby mode of the printer devices, it is beneficial to lift the 
recording medium off from the saddle surface. This can occur wherein an 
air pillow is produced between the paper web and the saddle surface or 
sliding surface with the assistance of a blower means in the standby 
condition of the printer device. Another possibility for lift-off is 
comprised in providing a suitable lift-up element designed, for example, 
as a tension wire that engages under the recording medium over its entire 
width. The pre-heating saddle and lift-off element are thereby moved 
relative to one another such that, in a lift-off status, the recording 
medium is guided over the lift-off element at a distance from the 
pre-heating saddle. 
As a rule, the paper web is automatically placed into the printer in 
electrographic continuous form printers of the new generation. Among other 
things, the paper web must thereby be guided over the pre-heating saddle. 
Coatings composed of fluorine compounds electrostatically charge at their 
surface when paper slides thereon. Due to the electrostatic forces, the 
paper web adheres so firmly to the pre-heating saddle that it may 
potentially no longer be capable of being transported. An advantageous 
admixture of electrostatically conductive substances such as graphite or 
the like can prevent the formation of electrostatic charges. It is 
beneficial, given glued layers of material, when the adhesive is likewise 
conductive in order to thus produce a conductive connection between 
toner-repellant material and grounded carrier. 
Also, according to the invention, if the saddle is configured as a low 
temperature saddle with as large a constructional length as possible, so 
that the temperature difference between recording substrate and saddle 
becomes as small as possible, and if, furthermore, the saddle is 
subdivided in the recording substrate running direction into heating zones 
which are individually controllable and uniformly heated, the heating 
zones can then be controlled in such a way that, along the saddle, an 
approximately constant thermal energy flow occurs from the saddle to the 
recording substrate. 
By means of this measure, the thermal loading for the recording substrate 
becomes very low. Nevertheless, the thermofixing device can also be used 
in printing devices of high and very high printing speed. 
Furthermore, the subdivision of the saddle, transversely to the recording 
substrate running direction, into heating zones which can be driven as a 
function of the width of the recording substrate is of advantage. 
In consequence, the heating behavior of the saddle can be matched directly 
to the width of the recording substrate running through, which guarantees 
a constant fixing quality, irrespective of the width of the recording 
substrate used. 
In order to make possible a good contact between recording substrate and 
gliding surface of the saddle, irrespective of printing speed and paper 
used, in an advantageous embodiment of the invention openings can be 
arranged on the gliding surface, said openings being connected to a device 
producing a vacuum. By means of the vacuum, the recording substrate is 
sucked flat onto the gliding surface and, in the process, the steam 
released in the paper is simultaneously sucked away via the openings. 
Furthermore, if use is made for heating elements of heating cartridges 
which are arranged in passage openings of the heating saddle, said heating 
cartridges can easily be exchanged and the saddle itself can be 
cost-effectively produced from an extruded profile. 
A domed shaping of the gliding surface of the saddle ensures a force 
component, which pressed the recording substrate against the saddle 
surface, over the entire saddle length. This measure supports the contact 
of the recording substrate on the saddle surface, stabilizes the recording 
substrate guidance and thus leads to an improved thermal transfer. 
In a further advantageous embodiment of the thermofixing device, peripheral 
entry means, for example in the form of a keyboard, are provided on the 
machine, via which means, by means of the entry of operating parameters 
such as paper weight, fixing temperature, etc., the heating power of the 
fixing device is automatically matched to these parameters. 
Embodiments of the invention are shown in the drawings and shall be set 
forth in greater detail below by way of example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electrographic printer device for printing continuous form papers 
contains a thermal fixing means schematically shown in FIG. 1. The thermal 
fixing system is designed as a heat transfer fixing means. It contains a 
heating drum 11 heated via radiators 10 and contains a pressure roller 12 
that can be electromotively pivoted against and away from the heating drum 
11. The heating drum is composed of an aluminum cylinder having a 
heat-resistant coating arranged thereon. The pressure roller is likewise 
composed of an aluminum cylinder having a coating of silicone. The heating 
drum 11 is electromotively driven. The heating drum 11 has an oiling means 
13 allocated to it for applying mold lubricant onto the heating drum. A 
heated pre-heating saddle 15 with negative pressure brake 16 associated 
therewith precedes the rollers as viewed in the conveying direction of the 
recording medium. This pre-heating saddle 15 serves the purpose of 
pre-heating a recording medium 17 designed as a continuous form paper and 
supplies it to the actual fixing gap between the rollers 11 and 12 in its 
pre-heated condition. The recording medium 17 is conducted over the 
pre-heating saddle 15 in taut fashion because it is decelerated by the 
negative pressure brake 16 and is driven via the rollers. A loose toner 
image on the recording medium is pre-heated on the pre-heating saddle 15 
and is fixed between the rollers 11 and 12 by heat and pressure. 
A cooling device 18 following the rollers 11 and 12 in the paper running 
direction provides for a cooling of the entire paper. For this purpose, 
the cooling device 18 contains a cooling surface 19 provided with 
apertures across which the recording medium 17 moves. Cold air supplied 
via an air delivery channel 20 flows from the apertures and produces a 
cooling air cushion under the recording medium 17. At the same time, air 
is blown onto the tonered side of the recording medium via a profile lying 
opposite thereto. 
Given the described thermal fixing means, the pre-heating of the continuous 
form paper 17 occurs via a low-temperature pre-heating saddle 15 that is 
composed of two heated saddles connected following one another, namely of 
a stationary pre-heating saddle 21 and of a heating saddle 23 pivotable 
around a pivot point 22. Pre-heating saddle 21 providing a first heating 
zone of lower temperature and heating saddle 23 providing a second heating 
zone of higher temperature to thus form two separate heating zones as 
viewed in the paper running direction. The entire pre-heating path thereby 
has a length of approximately 500 through 700 mm. During the pre-heating, 
the paper 17 slides on sliding surfaces 24 of the pre-heating saddle 21 or 
heating saddle 23. 
In order to produce a good contact between the saddles and the paper and to 
thus keep the temperature difference small, the sliding surface or the 
saddles are designed arcuately and with an arc radius that amounts to 700 
mm in the illustrated example. Due to the arc of the sliding surfaces in 
combination with the traction by the rollers 11 and 12 and the 
deceleration by the negative pressure brake 16, a force component acts 
over the entire saddle length that presses the paper 17 against the 
sliding surfaces 24. Moreover, the stability of the paper running on the 
saddle is thereby enhanced. The saddles 21 and 23 comprise oblong 
depressions 25 transversely relative to the paper running direction which 
extend over the entire width of the saddles. They are connected to a 
channel 27 by lateral bores 26. The air channel proceeds under the saddles 
and is connected to a pneumatic means that produces an over-pressure and 
under-pressure, for example to a blower and to a pump. During the printing 
mode, the recording medium (paper) is suctioned against the sliding 
surfaces 24 of the saddles by under-pressure, and the water vapor being 
released due to the pre-heating is suctioned off. During standby mode, an 
air pillow is produced between the recording medium 17 and saddles or 
sliding surface 24 due to over-pressure. 
The heating of the saddles 21 and 23 occurs with electrical resistance 
elements in the form of interchangeably arranged heating cartridges that 
are arranged in bores 29. The pre-heating saddle is designed as a 
low-temperature saddle whose heating capacity is controlled via a 
microprocessor-controlled regulator arrangement. 
As shown in FIGS. 15A and 15B the thermal fixing means or station 58 is 
also suitable for fixing recording medium 17 that already have a fixed 
toner image 59' on their back side 17B. This toner image 59 can be printed 
and fixed on the back side 17B (or on the front side) of the recording 
medium in a first pass as shown in FIG. 15A. After this, a further toner 
image 60 is applied and fixed on the corresponding other side 17A in a 
further or second pass as shown in FIG. 15B. For this purpose, the sliding 
surface 24 according to the illustration of FIGS. 2 and 3 is composed of a 
toner-repellant plastic layer, of a fluorine compound, for example a PTFE 
or, respectively, a PFA compound, that is vapor-deposited, sprayed, or 
glued onto the worked surface of the pre-heating saddle 15. The compounds 
are described as follows: 
Polytetrafluorethylene (PTFE) having the structural formula: 
##STR1## 
Perfluoroalkoxy polymers (PFA) having the structural formula: 
##STR2## 
with R=C.sub.n F.sub.n+1, as a perfluoridated alkane side chain. 
PTFE or PFA compounds exhibit extremely good repellency with respect to the 
toner material and extremely good properties with respect to abrasion due 
to the paper web. In order to enhance the abrasion resistance, 
constituents such as graphite or glass fibers can be added to a greater or 
lesser extent to the toner-repellant layer. 
Since heating saddles are usually employed in electrographic printer 
devices of the upper performance category having a page capacity of 2 
through 10 million DIN A4 pages per month, these heating saddles are 
subject to relatively high wear. For this reason, it is beneficial when 
the saddle surface can be unproblematically and simply renewed as needed 
without the expensive basic structure of the heating saddle with heating 
element having to be replaced as well. 
In order to enable this replaceability, the toner-repellant layer 30 in an 
exemplary embodiment according to FIG. 4 is vapor-deposited, sprayed, or 
glued onto thin metal plates 31. The thin metal plates 31 can have a 
thickness of 1 through 5 mm and are interchangeably clamped or screwed on 
the basic structure 32 of the pre-heating saddle. 
The toner-repellant layer 30 can also be executed as a thin film which has 
a thin, highly thermally conductive adhesive layer on one side. The 
adhesive layer is implemented such that the film can be easily pulled from 
the saddle in the hot condition of the saddle. A fast renewal of the 
saddle surface is thus very rapidly possible as needed on site at the 
customer. 
In an exemplary embodiment shown in FIG. 5, the toner-repellant layer 30 is 
designed as a thin film 33. The film 33 extends over the entire width of 
the pre-heating saddle. It is wound on a reel 34 as a reserve supply, this 
reel 34 being attached under the pre-heating saddle in the saddle entry 
region. This film, proceeding from this supply reel 34, is stretched over 
the pre-heating saddle up to the outlet of the heating saddle in the paper 
running direction and is in turn wound up on a take-up reel 35 under the 
pre-heating saddle. With the assistance of a drive means coupled to the 
take-up reel 35, the film is moved extremely slowly in relationship to the 
speed of the recording medium and is wound onto the take-up reel 35. A 
film supply is located on the supply reel 34 of the admission side; this 
can be designed for the entire service life of the printer. The 
pre-heating saddle is designed maintenance-free in this way. 
In order to obtain a fold-free entry of the recording medium web 17 into 
the fixing gap between fixing drum 11 and pressure roller 12, it is 
beneficial to guide the paper web around the paper discharge saddle edge 
36 of the pre-heating saddle in a wrap. However, extreme wear of the toner 
image already fixed on the recording medium occurs in this wrap region. 
According to an embodiment shown in FIG. 6, this wear can be prevented in 
that one or more deflection rollers 37 in the form of smoothing rollers 
are arranged in the wrap region, these likewise being potentially provided 
with a toner-repellant coating 30A. The smoothing rollers 37 steer the 
recording medium 17 out of a running direction defined by the sliding 
surface 24 into an admission direction to the heat transfer fixing 
station, namely with a deflection angle that is dimensioned such that a 
smoothing effect is exerted on the recording medium 17. 
When a relatively high proportion of graphite or glass fibers is added to 
the toner-repellant layer 30 for achieving a high resistance to wear of 
the surface, then the repellency of the surface to toner can be 
potentially reduced. In order to prevent a transfer of the toner image 
onto the pre-heating saddle surface in such instances during a long 
waiting or standby status of the printer device, it is beneficial to lift 
the recording medium 17 off from the surface of the pre-heating saddle in 
the standby condition of the printer device. 
In the exemplary embodiment of FIG. 7, air is supplied to the bores 26 and 
to the slots 25 via the pneumatic channel 27 during the standby status for 
this purpose, so that an air pillow that holds the recording medium 17 at 
a distance from the sliding surface 24 arises between sliding surface 24 
and recording medium 17. Sticking of the recording medium to the surface 
of the pre-heating saddle is thus precluded. When a film 33 as shown in 
FIG. 5 is employed as a toner-repellant layer, then an air cushion can be 
similarly produced between film 33 and the pre-heating saddle. 
Another possibility for lifting the recording medium off from the 
pre-heating saddle in the standby mode of the printer device is shown in 
FIG. 8. A lift-off element, for example in the form of a tension wire 38, 
that engages under the recording medium 17 in the region of the 
pre-heating saddle, is stationarily arranged in mounts of the printer 
device, such that the tension wire 38 comes to lie in a recess 39 of the 
pre-heating saddle in a position D of the pre-heating saddle allocated to 
the printing mode. When the pre-heating saddle or the heating saddle is 
pivoted out of the printing position D into a waiting position W, the 
tension wire 38 remains stationary and the paper web 17 is thereby lifted 
off from the hot surface of the pre-heating saddle. 
Another possibility is a pivoting of the tension wire 38 or other paper 
deposit elements out of the surface of the pre-heating saddle and lowering 
them in turn into the surface when the printing mode is initiated. 
In electrographic printer devices of the newer generation, the recording 
medium 17 is automatically inserted into the printer device. Among other 
things, the paper web must thereby be conducted over the pre-heating 
saddle. Coatings of fluorine compounds such as PTFE or PFA 
electrostatically charge to an extreme degree on their surface when paper 
slides thereon. It can thus occur that the electrostatic forces produced 
in this way impede further conveying of the paper web 17. Such 
electrostatic charges can be prevented by mixing electrostatically 
conductive substances, for example graphite or similar materials, into the 
toner-repellant layer 30. When the toner-repellant layer 30 is composed of 
a layer glued onto the pre-heating saddle, it is necessary to likewise 
design the adhesive to be conductive in order to thus produce a conductive 
connection to the pre-heating saddle, which is beneficially grounded. 
CONTROLLED HEATING 
In the case of the thermofixing of a recording substrate with a toner image 
arranged thereupon, in a fixing gap, under pressure and heat, the toner 
image comprising polymeric material, for example polyester, is heated via 
a heated fixing roller until in the melting range and is thus bonded with 
the recording substrate. 
In this arrangement, the recording substrate is pressed against the fixing 
roller via one or more nip rollers. The boundary surface between the toner 
particles and the surface of the recording substrate is decisive for the 
fixing. In this region, the melting temperature of the toner must be 
reached carefully and without excessive heating, so that the toner ends 
with the recording substrate or sticks to the latter. If, during fixing in 
the fixing zone (fixing gap), the recording substrate has an essentially 
lower temperature than the toner, heat is withdrawn from the boundary 
surface via the recording substrate, which can lead to faulty fixing. For 
this reason, the recording substrate with the toner image arranged thereon 
is preheated before feeding into the fixing gap. In this case, it is 
favorable if the recording substrate is preheated to a temperature which 
already lies in the melting range of the toner material. In this range, 
which lies between 90.degree.-125.degree. in the case of a polymeric 
toner, the toner is already slightly sticky at the boundary surface with 
the recording substrate, which facilitates the actual fixing in the fixing 
gap. In the case of printing and copying machines which operate with 
endless paper, the recording substrate is commonly preheated via a 
preheating saddle, over which the recording substrate glides with its 
non-toner-laden side and thus picks up heat. In this case, the problem 
arises that the heat is picked up on the side facing away from the toner, 
so that heating of the boundary surface with the toner thus takes place 
only after heating of the actual recording substrate. As a function of the 
thickness of the recording substrate material and of its structure and of 
the printing speed, this requires a rapid supply of heating power via the 
preheating saddle. The processes in thermofixing are extensively described 
in U.S. Pat. No. 3,938,992, whose publication is a constituent of this 
application. 
For preheating the recording substrate to a temperature in the melting 
range of the toner material a heating power which is essentially dependent 
on the temperature difference between entry and exit temperature and the 
thermal capacity of the recording substrate must be supplied to the 
recording substrate in the preheating zone. 
Now, it has appeared that in the supply, of the heating power, which is too 
rapid and non-uniform, lasting deformations occur due to a temperature 
shock occurring in the recording substrate, said deformations being in the 
form of waves or bulges which influence the fixing process as a whole and 
in particular the post-processing of the printed recording substrate in a 
negative way. For this reason, it is favorable to heat the recording 
substrate as slowly as possible and as uniformly as possible in the 
preheating zone. The coefficient of temperature rise was established as an 
essential criterion for the speed with which the recording substrate can 
be heated without lasting deformations. The coefficient of temperature 
rise, measured in degrees Kelvin per second, denotes a limiting value for 
a permissible temperature rise per second during heating of the recording 
substrate. It is a material-dependent value, which can be determined by 
experiments. In this case, material samples are thermally loaded 
dynamically as a function of time and examined for any lasting 
deformations and warping. In the case of paper as recording substrate 
material, it was established that the coefficient of temperature rise is 
dependent on the basis weight (grammage, weight per unit area). The 
heavier the paper is, the smaller is the coefficient. This means that 
heavy papers must be heated up more slowly than thin light papers in order 
to avoid warping. However, if different paper grades are processed in a 
printing or copying machine, the geometry and the type of the preheating 
of the thermofixing device must be designed in accordance with this 
heaviest paper grade. The coefficient of temperature rise of the paper is 
120 K./sec. at 160 g/m.sup.2 basis weight; 155 K./sec. at 70 g/m.sup.2 
basis weight. 
The temperature coefficient is thus an essential parameter in the 
dimensioning of the length of the preheating zone or of the preheating 
saddle used for the heating. If the necessary heating power to be supplied 
has been determined as a function of the melting temperature which is to 
occur and of the heaviest recording substrate material to be used and of 
other parameters, such as printing speed, the necessary heating zone 
length or gliding surface length on the preheating saddle can be 
determined whilst keeping the other boundary conditions, such as constant 
specific power distribution (watts per cm) or uniform thermal energy flow 
(watts per area) along the saddle, at a minimum temperature difference 
between saddle surface and recording substrate. For this purpose, by way 
of example, proceeding from a calculated saddle length in a physical 
experimental construction, by means of infrared measuring devices 
operating without contact, the surface temperature of the recording 
substrate at the entry onto the saddle surface and on leaving the saddle 
surface is measured in the case of the heaviest recording substrate with 
the highest permissible printing speed and the temperature rise per second 
is determined therefrom. By means of comparing with the previously 
determined coefficient of temperature rise of the recording substrate 
material, an optimization is possible, the constructional length having to 
be dimensioned at least in such a way that the temperature rise lies below 
the coefficient of temperature rise. However, it should be pointed out 
that the coefficient of temperature rise is a statistical limiting value 
which, if exceeded, leads to the occurrence of a lasting quantitative 
material structure change, which makes itself noticeable in a disturbing 
manner. 
Thus, if the minimum saddle length and the saddle construction have been 
optimized for the worst case, the saddle length can be kept for other 
lighter papers. However, it is occasionally necessary, in accordance with 
the reduced heating power necessary for recording the thinner recording 
substrate, to match said heating power correspondingly. In order that this 
process is carried out automatically, an entry keyboard for the entry of 
operating parameters, such as basis weight of the paper, printing speed, 
etc., can be provided on the machine. A computer-controlled device 
arranged in the machine, for example within the framework of the machine 
control system, then automatically determines the necessary heating power 
and sets it on the heating elements of the heating zone. 
In the case of a preheating saddle as is shown in FIG. 1, which is composed 
of a preheating saddle and a heating saddle, the following relationship 
resulted for the calculation of the total heating saddle power. 
EQU P.sub.max =P.sub.Pap +P.sub.H20 +P.sub.H20steam +P.sub.convect 
EQU P.sub.max =G.sub.Pap.max .times.v.sub.Pap .times.b.sub.Pap.max 
.times.c.sub.Pap .times.T.sub.Pap +(G.sub.H20max /A*).times.v.sub.Pap 
.times.b.sub.Pap.max .times.c.sub.H20 .times.T.sub.H20 .times.+(q.sub.H20 
.times.G.sub.H20steam)/t.sub.2000 sheets +P.sub.convect. 
EQU P.sub.max =5895 W+1343 W+1608 W+300 W 
EQU P.sub.max =9146 W 
Description of the parameters and their values: 
These values are true for the most unfavorable conditions (heaviest paper, 
widest paper, maximum proportion of water) 
Temperature of the per preheating T.sub.pap. =100.degree. C.-25.degree. 
C.=75K. 
Speed of the paper web V.sub.pap. =0.86 m/s 
Maximum specific paper weight G.sub.pap.max =0.16 kg/m.sup.2 
Maximum paper width b.sub.Pap.max =0.457 m 
Specific heat of paper c.sub.Pap. =1250 J/(kgxK) 
Maximum H.sub.2 O proportion per 2000 sheets G.sub.H20max= 3.2 kg 
Heating temperature of the H.sub.2 O T.sub.H20 =70K 
Evaporated H.sub.2 O proportion per 2000 sheets G.sub.H20steam =0.5 kg 
Heat of evaporation of H.sub.2 O q.sub.H20 =2281.times.10.sup.3 J/kg 
Specific heat of H.sub.2 O c.sub.H20 =4180 J/(kgxK) 
Running time for 2000 12-inch sheets t.sub.2000 sheets =(609.6 m)/(0.86 
m/s)=709s 
Area of a 2000.times.12-inch-sheet long paper web A*=274 m.sup.2 
The power is distributed uniformly over the length of heating and 
preheating saddle. That means that, at a length of the heating saddle of 
300 mm and a length of the preheating saddle of 240 mm, there results a 
specific power distribution in the paper running direction of 169 W/cm. 
As previously pointed out with respect to FIG. 1, the heating of the 
saddles 21 and 23 is carried out by means of electrical resistance 
elements in the form of heating cartridges 28 (See FIGS. 3 and 9) which 
are arranged so that they can be exchanged. To accommodate the heating 
cartridges 28, the saddles 21 and 23 have continuous holes 29. These holes 
enable the exchange of each individual heating cartridge 28 in the event 
of a defect. Moreover, the saddles 21 and 23 can thus be cost-effectively 
produced from extruded aluminum profile. 
By means of the arrangement of the cartridges in the saddles, each saddle 
21 and 23, respectively, is subdivided into three heating zones 39/1, 39/2 
and 39/3, transversely to the paper running direction (FIG. 9). Here 
transverse heating zones 39/1 to 39/3 are used for matching the saddles to 
various recording substrate widths. The first heating zone 39/1 is limited 
on one side by the fixed paper running edge 40/1. This heating zone 39/1 
is as wide as the minimum recording substrate width. The remaining region 
of the saddles, up to the maximum recording substrate width, is subdivided 
into the equally wide heating zones 39/2 and 39/3. Each of the transverse 
heating zones 39/1 to 39/3 has a temperature sensor 41/1 to 41/3 for 
controlling the heating zones. Said temperature sensor is located in each 
case transversely to the paper running direction approximately in the 
center of the respective heating zones. Seen in the paper running 
direction, the sensor positions are selected such that control is possible 
to the same temperature both in the standby condition of the printing 
device (standby) and in the printing operation itself. In this way, the 
temperature control is simplified. The control temperature and the 
position of the sensors 41/1 to 41/3 are selected in such a way that the 
paper temperature at the end of the saddle during the start phase is just 
as high as during a longer printing phase. In this arrangement, the region 
from the center as far as the last third of the saddles has proved to be a 
favorable sensor position. 
The heating zones 39/1 to 39/3 are produced by means of the arrangement of 
the heating cartridges 28 in the holes 29. 
This is as follows: 
One cartridge in each case for the two outer heating zones 39/1 and 39/3 is 
pushed from both sides into the first hole, of a saddle, in the paper 
running direction. A heating cartridge 28 for the central zone 39/2 is 
pushed into the second hole. The third hole is equipped in the same way as 
the first, and so on. In this way, six heating cartridges 28 are located 
in each heating zone 39/1 to 39/3. 
As shown in FIGS. 4 and 5, the heating cartridges 28 of the heating zones 
39/1 to 39/3 are operated on phases R, S, T and N of a three-phase power 
supply. As a function of the type of the three-phase power supply (USA, 
Europe), the heating cartridges are connected in pairs in series (FIG. 12) 
(European three-phase power supply) or in parallel (FIG. 11) (three-phase 
power supply USA). 
There are thus three pairs of heating cartridges located in each heating 
zone 39/1 to 39/3. In order to achieve a uniform loading of all three 
phases, the connection is carried out of a first heating cartridge pair to 
the phases R, S; of a second heating cartridge pair to the phases S, T; 
and the connection of a third heating cartridge pair to the phases R, T. 
However, the possible wiring, of the individual heating cartridges 28, 
specified in FIGS. 11 and 12 can be varied as desired as a function of the 
operating power supply used. 
The surface temperature of the saddles and thus the temperature of the 
recording substrate is controlled with the aid of a control arrangement, 
as is shown in FIG. 10. 
The control arrangement contains an actuator 42, for example in the form of 
individual relays for coupling the heating cartridges 28 to a power supply 
unit 43. Connected downstream of the actuator is the control path 44 with 
the heating cartridges 28. The actual temperature is registered via the 
temperature sensors 41/1 to 41/3 and converted by the sensors into an 
electrical drive signal and amplified in a subsequent amplifier 46. A 
control arrangement 47 compares the actual temperature with a 
predeterminable desired temperature TS and controls to the desired 
temperature TS as a function of the control deviation. 
The microprocessor-controlled control arrangement 47 contains an 
analog-digital converter 48 with associated program-controlled two-state 
controller 49. Furthermore, it has a central unit CPU, which is connected 
to corresponding areas of memory SP1 and SP2. In addition, the 
microprocessor-controlled control arrangement 47 is coupled to the 
controller 50 of the printing device, which is commonly constructed with 
an operating panel 51 on the machine. The entire control arrangement can 
be a component of the machine control system of the machine. An additional 
low-voltage power supply unit 52, which is coupled to the actual power 
supply unit 43, ensures the power supply of the machine control system and 
thus of the microprocessor-controlled control arrangement 47. 
As already explained at the beginning, in the use of recording substrates 
of different material structure, in particular different basis weight, the 
heating power which is fed to the preheating saddle must be 
correspondingly matched. This is similarly true for the matching of the 
saddle exit temperature to the recording substrate to be printed. In order 
to be able to adjust this heating power or other parameters on the 
preheating saddle, such as for example the exit temperature, the machine 
contains an operating panel 51 for the entry of various operating 
parameters, such as basis weight of the recording substrate, desired exit 
temperature at the preheating saddle, etc. The operating panel is 
connected to a computer-controlled arrangement which can be a part of the 
control arrangement 47 and which contains a central unit CPU, which is 
connected to corresponding memories SP1 and SP2. 
Stored in the memories SP1 to SP2 there are allocation tables or 
characteristics, via which, in accordance with entry of the corresponding 
parameters via the operating panel 51, the corresponding electrical values 
to be controlled and to be regulated of the preheating saddle are 
allocated. These values are then fed to the control arrangement 47 as 
desired value. In the exemplary embodiment shown, the desired temperature 
TS is entered via the operating panel 51, the temperature at which the 
paper leaves the saddle arrangement (preheating saddle 15) or the entry 
temperature of the paper into the fixing zone between the rollers 11 and 
12 being designated as desired temperature. The statement of the operating 
parameters was only by way of example. In the case of a change of the 
printing speed or in the case of a change of the paper width, a matching 
of the heating power is likewise necessary. This takes place automatically 
by means of corresponding switching-in of the transverse heating zones 
39/1, 39/2 and 39/3 designed to be individually drivable and arranged on 
the saddle 15 transversely to the recording substrate running direction, 
or by registering of the set printing speed, the variation of which indeed 
has an effect as a whole on many units of the machine. In the normal case, 
in electrophotographic printing devices which operate with endless paper, 
operations are carried out at a constant recording substrate advance speed 
(printing speed). 
The functioning of the control device is explained using the diagram of 
FIG. 13. The abscissa X of the diagram in this case designates the 
position in millimeters, proceeding from paper entry on the saddle 
surface, the ordinate Y designates the temperature in degrees Celsius. In 
this case, the temperature variation on the paper or recording substrate 
is represented in the curve P1. The curves VD and VS here designate the 
temperature variation on the saddle surface of the preheating saddle 21 in 
printing operation VD and in standby operation VS. The curves HD and HS 
the temperature variation in printing operation HD and standby operation 
HS on the heating saddle surface. The positions of the sensors of the 
preheating saddle and of the heating saddle are designated by SV and SH in 
the curves. In this context it should be noted that the diagram represents 
the temperature variation within the heating zone 39/1 both of the 
preheating saddle and of the heating saddle, specifically when only this 
heating zone 39/1 is active, that is to say a recording substrate of 
minimum width sweeps over the saddle. If recording substrates of other 
widths are used, a similar temperature variation is true in the case of 
additional activation of the heating zones 39/2 and 39/3. 
The saddle temperature of the preheating saddle 15 is controlled by means 
of the control arrangement, specifically by means of controlling the 
heating zones, namely the heating saddle 23 and the preheating saddle 21. 
In so doing, the aim of the control is a constant desired saddle 
temperature, the exit temperature of the paper after leaving the saddle 
being able to be entered as saddle temperature, via the operating panel 
51. The microprocessor-controlled control arrangement 47 then converts 
this desired saddle temperature into corresponding desired temperatures on 
the preheating saddle 21 and on the heating saddle 23 and controls these 
together. The level of the desired temperature to be set depends on the 
type and the material construction of the recording substrate used and on 
the printing speed, that is to say the paper advance of the machine. In 
the case of normal paper and a printing speed corresponding to a paper 
advance speed of approximately 0.89 m/sec, the paper at the saddle inlet 
has a temperature of 20.degree. and is intended to be heated to a paper 
exit temperature of approximately 100.degree.. The heating cartridges 28 
are now arranged along the heating zones 21 and 23 of the saddle 15 in 
such a manner and are controlled in such a manner that the thermal energy 
flow per surface from the saddle to the paper is constant along the 
saddle. Furthermore, the length of the saddle is fundamentally determined 
such that the temperature difference .DELTA. T between saddle surface 
(gliding surface) and paper becomes constant and as small as possible. The 
length of the saddle is limited, however, by the maximum constructional 
length available and can vary from machine to machine. However, as large a 
length as possible is the aim, so that most careful heating-up of the 
paper is achieved. 
In this case, one problem is the dynamic behavior of the temperature 
variation at the transition from the standby or start phase to printing 
operation. In the start phase, that is to say without paper or with paper 
deposited in the standby condition, thermal dissipation from the saddle 
takes place simply by means of convection. Nevertheless, it must be 
ensured that the paper is not excessively heated in the start or standby 
phase. This is ensured by means of the saddle construction described and 
by means of the control. 
In this arrangement, both in standby operation and in printing operation, 
the temperature of the saddle is kept constant, the preheating saddle 
having a temperature of approximately 80.degree. and the heating saddle a 
temperature of approximately 130.degree.. The result is thus the 
temperature variation which can be seen in FIG. 13. In standby operation, 
the preheating saddle has the temperature of 80.degree. over its entire 
surface, corresponding to the curve VS, and the heating saddle has the 
temperature of 130.degree. over its entire surface, corresponding to the 
curve HS. After initiation of printing operation, the temperature 
variation tilts around the sensor positions SV and SH, so that the 
steady-state temperature variation represented by the curves VD and HD is 
set in printing operation. In this steady-state condition, the temperature 
difference .DELTA. T between saddle surface and paper is approximately 
constant along the saddle surface. 
A still more exact setting of the constant temperature difference is 
possible, if the number of controlled heating zones is increased. However, 
this leads to an additional expenditure. As shown, the condition can also 
be approximately achieved using one saddle which has two heating zones, 
namely preheating saddle and heating saddle. In detail, the control 
sequence is as follows: 
After laying the paper in the printing device and threading through the 
fixing station, the desired temperature TS is entered via the operating 
panel 51, corresponding to the paper used. The microprocessor-controlled 
control arrangement 47 connects the heating cartridges 28 to the phases of 
the three-phase power supply of the power supply unit 43 via the actuator 
42. After the desired temperature is reached, the operational readiness of 
the fixing station is communicated to the controller 50 of the machine. 
After printing operation is initiated, heat is withdrawn from the saddle 
via the paper as a function of the paper temperature, the paper basis 
weight, the printing speed, the paper thickness, the surface finish of the 
paper and the width of the paper. This disturbance variable influence is 
symbolically represented in the control loop of FIG. 10 as disturbance 
variable SG. The actual temperature resulting after subtracting the 
disturbance variable is registered via the temperature sensors 41/1 to 
41/3 and fed in the form of electrical signals to the 
microprocessor-controlled control arrangement 47. The latter activates the 
actuator 42 in a corresponding manner until the prescribed desired 
temperature is reached and the temperature profile which can be seen in 
FIG. 13 occurs. 
As described at the beginning in conjunction with FIG. 1, the heating 
saddle 23 of the preheating saddle 15 is arranged in the machine so as to 
be pivotable. For this purpose--as can be seen in FIG. 14--the heating 
saddle is supported at its input and in a pivotable and detachable manner 
via a bearing 22 in the machine frame. The heating saddle has, 
approximately at its center, a cam roller 53 which is rotatably supported 
on the heating saddle and cooperates with an eccentric snail cam 54 
supported movably in the machine frame. The eccentric snail cam 54 is 
driven via a cam shaft 55, which is connected to a stepping motor, not 
shown here. By means of rotating the eccentric snail cam 54, the heating 
saddle 23 rotates about the point of rotation 22. Hence, it can be 
positioned in different positions as a function of the operational 
conditions of the machine, namely into an operating position (position A; 
shown in FIG. 14 with continuous lines) assigned to the fixing operation, 
with nip roller 12 pivoted in, and into a standby position (position B; 
shown in FIG. 14 with interrupted lines) assigned to the standby 
operation, with nip roller 12 pivoted out. In the standby position, the 
recording substrate 17 is pivoted away from the hot fixing roller 11. 
Furthermore, however, it is in contact with the heated preheating saddle 
15. 
In the preheating of the recording substrate 17, be it now of paper or 
paper-like material or, for example, of plastic, there exists the problem 
that, as a result of the gassing out of the recording substrate material 
or as a result of other effects such as loss of water, etc., the recording 
substrate will shrink, which leads to some reduction in width. Hence, in 
the transition into the unheated paper running region, small waves or 
warping occur. 
This effect is to be observed in particular in standby operation, in which, 
in the case of a continuously heated heating saddle, the immobile 
recording substrate is exposed for a very long time to the heat from the 
heating saddle. If then, in the event of a renewed initiation of printing 
operation, the saddle is brought into the operating position by pivoting 
in and the preheated recording substrate is fed in the fixing gap between 
fixing roller and nip roller, the warping produced during the passage 
thought the fixing gap is ironed into the recording substrate by means of 
pressure and heat, which disturbs the printed image appreciably. 
In order to prevent this, the heating saddle 23 has, at its end assigned to 
the fixing gap, a smoothing edge 56, which is designed as a relatively 
sharp-edged rounding of the gliding surface 24. If, on leaving the heating 
saddle 23, the recording substrate web wraps around this heating saddle 
edge (smoothing edge 56) arranged on the preheating saddle exit region, by 
as large an angle 57 as possible, this warping of the recording substrate 
is smoothed out over the wrapped-around saddle edge 56 before the entry 
into the fixing gap. 
The heating saddle edge or smoothing edge 56 should in this case be 
positioned as close as possible to the fixing gap. A deflection angle of 
at least 7 degrees of angle or larger has proved to be advantageous, the 
smoothing effect also occurring to a limited extent already at 5.degree. 
or 6.degree. deflection angle. Designated by deflection angle 57 is the 
angle by which the running direction of the recording substrate 17 changes 
on leaving the gliding surface 24 of the heating saddle 23. In the 
exemplary embodiment of FIG. 14, with a domed gliding surface 24 of the 
heating saddle 23, this is the angle between the gliding surface direction 
(tangential) in the region of the smoothing edge 56 and the feed direction 
of the recording substrate to the fixing gap between smoothing edge 56 and 
fixing gap. 
So that the smoothing edge 56 does not press into the recording substrate 
17 in the standby position (standby operation), the heating saddle 23 is 
pivoted out in standby operation to such an extent that the recording 
substrate 17 does not rest on the smoothing edge 48 or does not wrap 
around the latter. 
In the exemplary embodiment, shown in FIG. 1, of the thermofixing device, 
the preheating saddle 15 consists of a fixed preheating saddle 21 and a 
heating saddle 23 which is arranged so as to be pivotable. Such a 
subdivision is also sensible because only a low saddle mass thus has to be 
pivoted over the heating saddle 23. In addition, the subdivision opens up 
the possibility of composing the preheating saddle 15 of heating zone 
modules, for example of a fixed heating zone module "preheating saddle" 
and a pivotable module "heating saddle" or else, by way of example, of a 
module forming the heating saddle and a plurality of modules forming the 
preheating saddle, which then form the preheating saddle 15 in 
combination. In this way, preheating saddles for various machine variants 
having, for example, a different printing speed can be constructed in a 
simple manner. If, for example, the printing speed and thus the recording 
substrate running speed of a machine variant are reduced, the preheating 
saddle length needed also reduces. If necessary, the "preheating saddle" 
module can thus be dispensed with completely and only a pivotable heating 
saddle module is necessary as preheating saddle. On the other hand, in the 
case of an increase of the printing speed, the preheating saddle length 
can be extended by the addition of further heating zone modules. 
A crosslinked toner has emerged as a toner material which is particularly 
suitable for fixing on paper via the described thermofixing device. By 
means of the careful heating up in the fixing, the advantageous fixing 
properties, already present per se, of the crosslinked toner can be 
further improved. For example, there can be used as crosslinked toner a 
toner which has at least 25 percent by weight of toner particles made of a 
polymer comprising a polyester or a polymer having styrene groups or a 
polymer comprising styrene groups, which is crosslinked covalently or 
ionically to such an extent that the melting range of the toner particles 
is increased by at least 10% in comparison with corresponding toner 
particles having a non-cross-linked polymer. 
Although various minor changes and modifications might be proposed by those 
skilled in the art, it will be understood that I wish to include within 
the claims of the patent warranted hereon all such changes and 
modifications which reasonably come within my contribution to the art.