EP0125581A1 - Heat fixing station - Google Patents

Abstract

A heat fixing station for fixing a wet-developed toner image, in which a curved heating plate, over which the copy bearer with the moist toner image is passed, has a temperature profile over the length of the plate with two different temperature ranges which may have a temperature difference of 35 DEG C to 160 DEG C. In order to implement this temperature profile, a different electric power is applied to each of the two heating elements of the heating device for heating the heating plate and a zone of reduced heat conductivity is provided in the heating plate in the region of a fixing gap formed by the heating plate and a contact pressure roller. <IMAGE>

Description

The invention relates to a thermal fixing station for fixing a liquid-developed toner image by means of heat in a copier with a curved heating plate, along which the copy sheet with the liquid-developed toner image is passed through a guide roller within the fixing station, and with a heating device for heating the heating plate.

The thermal fixing station is used in electrophotographic copiers, in which a charge image is generated on a photoconductor layer, which is developed with developer liquid through deposited toner pigments to form a toner image. The toner image is transferred to a copy carrier such as paper. The developer-moist copy carrier with the toner image is introduced for fixing in the thermal fixing device in which the toner image is to be fixed on the paper in a wipe-proof manner by the action of heat.

DE-OS 25 39 642 discloses a device for fixing a toner image by means of heat in an electrophotographic copier working with liquid developer. A heating device is arranged in this fixing device, along which the liquid-developed copy sheet with the toner image on its upper side is moved. The heating device has a curved heating surface, on which the copy sheet rests with its rear side and slides along in order to fix the toner image by means of heat. The heating The area is curved in one section. At a short distance from the heating surface there is a guide roller which is brought into contact with the surface of the sheet carrying the toner image in order to guide and introduce it to the curved surface area of the heating surface. The circumferential surface of the guide roller is knurled and arranged at a distance from the curved or curved surface area of the heating surface which is greater than the sheet thickness.

The liquid toners generally consist of dielectric liquids such as Isopar G, an isoparaffinic hydrocarbon that boils at about 164 ° C., in which charged pigments, preferably of carbon black, are dispersed. In addition, the liquid toners contain additives to control the charging of the pigments, binders and possibly easily melting waxes with melting temperatures of around 80 ° C. When the toner image is transferred to the copy carrier such as paper or film, the copy carrier is also moistened with dispersing liquid. This transferred dispersing liquid is evaporated in the fixing device by the action of heat and, in the case of toners containing wax, the aim is to melt the separated wax particles. For this purpose, the cemented copy carrier is passed in a known manner (DE-OS 25 39 642) over a heating plate which in practice has a temperature of 180 ° C to 210 ° C. The higher temperature range is preferably set for waxy toners. Fixing temperatures will be higher than the specified values not used, since in the event of unpredictable jams of the copy material, paper would yellow or even scale or films would melt.

During operation of the copying machine, despite the uniformly high heating of the known heating plate, it often happens that toner from the individual copy sheet sticks to the pressure or guide roller and is transferred from this to locations on the continuous copy sheet or subsequent copy sheets, i.e. the pinch roller shows an offset printing effect. As lengthy tests have shown, it is not possible to avoid this undesired toner transfer during fixing by changing the fixing gap between the heating plate and the pressure roller, neither with a thin fixing gap of, for example, 0.3 mm or with a thicker fixing gap of about 3 mm. This is particularly disadvantageous when using wax-containing toners which result in very high density and thus blackening of toner images and, above all, tend to print.

The object of the invention is to improve a fixing station of the type described at the outset in such a way that an offset effect on the copy sheets caused by the guide or pressure roller cooperating with the heating plate in the fixing station is prevented.

This object is achieved in that the heating plate has a temperature profile with two different temperature ranges over its plate length.

In an embodiment of the invention, the temperature T ₁ in the inlet zone of the heating plate, which extends from the beginning of the plate to the fixing gap between the heating plate and the pressure or guide roller, is lower than the temperature T ₂ in the outlet zone of the heating plate, which differs from the fixing gap extends to the end of the plate. The temperature T _{i of} the inlet zone is at least 35 ° C. lower than the temperature T _{2 of} the outlet zone.

In a further embodiment of the invention, the temperature difference between the temperature T _{2 of} the outlet zone and the temperature T _{i of} the inlet zone is up to 160 ° C.

The further advantageous embodiment of the invention results from the features of claims 5 to 14.

With the invention the advantage is achieved that by relatively simple measures, such as different performance of the heating elements of the heating device for the heating plate, the drilling of holes, an air gap, a heat-insulating layer or the like. or leadership role can be largely suppressed by toner transfer.

The invention is explained below with reference to the drawings.

• Fig. 1 eine schematische Seitenansicht einer thermischen Fixierstation mit einer Heizeinrichtung aus zwei in der Unterseite der Heizplatte eingelassenen Heizstäben,
• Fig. 2 ein idealisiertes Temperaturprofil über die Länge der Heizplatte der erfindungsgemäßen Fixierstation,
• Fig. 3 eine weitere Ausführungsform der Fixierstation, bei der die Heizstäbe der Heizeinrichtung nur in der Auslaufzone der Heizplatte angeordnet sind,
• Fig. 4 eine andere Ausführungsform der Fixierstation, ähnlich der Fig. 3, mit Bohrungen in der Heizplatte nahe dem Fixierspalt,
• Fig. 5 eine Ausführungsform der Fixierstation ähnlich der Ausführungsform nach Fig. 3, mit einer wärmeisolierenden Folie in der Einlaufzone der Heizplatte, und
• Fig. 6 eine andere Ausführungsform der Fixierstation, ähnlich zu derjenigen nach Fig. 3, mit einer zusätzlichen Kühlzone, die an die Auslaufzone der Heizplatte anschließt.

Show it:

• 1 is a schematic side view of a thermal fixing station with a heating device consisting of two heating rods embedded in the underside of the heating plate,
• 2 shows an idealized temperature profile over the length of the heating plate of the fixing station according to the invention,
• 3 shows a further embodiment of the fixing station, in which the heating rods of the heating device are arranged only in the outlet zone of the heating plate,
• 4 shows another embodiment of the fixing station, similar to FIG. 3, with bores in the heating plate near the fixing gap,
• Fig. 5 shows an embodiment of the fixing station similar to the embodiment of FIG. 3, with a heat insulating film in the inlet zone of the heating plate, and
• Fig. 6 shows another embodiment of the fixing station, similar to that of Fig. 3, with an additional cooling zone which connects to the outlet zone of the heating plate.

An example of the mechanical structure of a fixing station 1 is shown in FIG. 1. A heating plate 4 made of cast aluminum has a curved course and contains two heating rods 5, 5 'as heating devices, one of which heating rod 5 in front of the fixing gap 2 and the other heating rod 5 'is arranged behind the fixing gap 2. The continuous copy sheet 3 carries the liquid-developed toner image on its upper side and is directed against the heating plate 4 by a surface-structured pressure roller 6. The heating rods 5,5 'of the heating device can also have a plate or other suitable shape.

Heater plates and heaters of the form shown in Fig. 1 are among others. used in the Infotec copiers of the KALLE branch of Hoechst AG. Similar arrangements of the heating devices result for modified forms of such heating plates.

In the embodiment according to FIG. 1, the heating plate 4 is expediently subdivided into an inlet zone AC and an outlet zone CB, wherein A denotes the beginning of the plate and B denotes the end of the plate of the heating plate 4. The point C lies in the concavely curved part of the heating plate 4 in the area of Fixing gap 2. The heating rod 5 in the inlet zone AC is operated with a lower electrical output than the heating rod 5 'in the outlet zone CB.

Fig. 2 shows the temperature profile along the heating plate 4 in an idealized form. The temperature T ₁ in the inlet zone AC of the heating plate 4 between the start of the plate and the fixing gap 2 is lower than the temperature T ₂ in the exit zone CB of the heating plate 4 between the fixing gap 2 and the end of the plate B. There are no reliable data on the operation of such a temperature profile Findings that allow an absolutely correct interpretation of the found effect. It can be concluded from the observations with a high degree of probability that when the copy sheet is guided over the entry zone AC of the heating plate, the copy sheet with the moist toner image is not yet heated up to the melting temperature of the waxes or other components added to the toner, so that these substances do not accumulate the pressure roller can stick. The temperature T ₁ can correspond to the ambient temperature, but it is advantageous if T _{1 is} greater than the ambient temperature so that the copy sheet after the fixing gap ₂ as quickly as possible as it enters the outlet zone CB of the heating plate 4 with the higher temperature T ₂ a temperature is heated at which the entrained dispersing liquid evaporates and the components such as waxes added to the liquid toner melt.

For the temperatures T ₁ and T ₂ , only ranges can be specified within the scope of a general description. The temperatures required in each case depend, inter alia, on the shape and size of the heating plate 4. In the embodiment according to FIG. 1, an approximately 8 cm long heating plate has been used as an example, in which the lengths of the low-temperature range and the high-temperature range are approximately 1: 2. With a relatively shorter low-temperature range corresponding to the entry zone AC T ₁ can be chosen larger than for a longer low temperature range. Furthermore, the heat transfer from the heating plate to the copy carrier and the heat conduction through the copy carrier to the toner image on the surface determine the respectively optimal temperatures. It was found that the optimal temperature T _{2 is} up to 15 ° C higher for rough papers of 120 m thickness than for smooth papers of 80 m thickness. Another influencing factor is the process speed. The investigations of the thermal fixing station according to the invention were carried out at process speeds between 7 cm / sec and 19 cm / sec, without being limited in principle. The optimum temperatures are also determined by the composition of the liquid toner used. It was found that when changing from Isopar G as the dispersing liquid with a medium boiling range of 164 ° C to Isopar H with a medium boiling range of 180 ° C, the temperature T _{2 was} increased by approximately the temperature difference of the medium boiling ranges, ie by 16 ° C is. In addition, higher melting waxes require higher final temperatures T _{2 '.} In order to optimally adapt to these influencing variables, very high temperatures T ₂ up to approximately 250 ° C. would have to be specified in some cases. At such high temperatures, however, there is a risk that if the copy run is faulty, if there are jams in the fusing station, copies made of paper will scale or melt those made of foils. For practical reasons, the temperature T ₂ is therefore not higher than about 220 ° C., preferably in the range from 195 ° C. to 215 ° C. The temperature T ₁ is chosen to be greater than the ambient temperature and less than or equal to T ₂ . Good results are achieved with an otherwise strongly printing liquid toner in a fixing station with the dimensions given for FIG. 1 at a temperature T ₁ of 150 ° C. to 160 ° C. The pressure roller 6 has a temperature of about 80 ° C.

There are various possibilities for realizing the idealized temperature profile shown in FIG. 2 along the heating plate 4. It is important to note that the temperature T _{1 of} the inlet zone is at least 35 ° C lower than the temperature T _{2 of} the outlet zone. The temperature difference between the temperature T _{2 of} the outlet zone and the temperature T _{1 of} the inlet zone can be up to 160 ° C.

As has already been explained with reference to FIG. 1, a pronounced temperature profile is obtained through an asymmetrical distribution of the heating power of the heating rods 5 and 5 '.

In the embodiment of the fixing station shown in FIG. 3, both heating rods 5,5 ′ are arranged behind the fixing gap 2 in the outlet zone CB and the heating rod 5 closer to the fixing gap 2 is operated with a smaller electrical output than the heating rod located further away from the fixing gap 2 5 '.

The temperature jump in the temperature profile according to FIG. 2 is particularly pronounced if the heat conduction between the outlet zone CB and the inlet zone AC is reduced or partially interrupted. This can be achieved in that the heating plate has a zone of reduced heat conduction in the area of the fixing gap 2 compared to the heat conduction of the material of the heating plate 4.

For example, in the embodiment according to FIG. 4 there are bores 8 in the heating plate 4 in the zone of reduced heat conduction which are arranged in a row transversely to the longitudinal direction of the heating plate 4. Of course, these holes can also be provided in several rows in a row. If the cross-sectional area of the heating plate in the zone of reduced heat conduction is reduced by half through the bores, this results, for example, in a temperature T ₁ in the inlet zone of 150 ° C. to 160 ° C. of the heating plate 4, while the temperature T ₂ in the outlet zone approximately 210 to 215 ° C.

Another way of influencing heat Line between the inlet and the outlet zone is to let the heating plate 4 taper in the zone of reduced heat conduction in the area of the fixing gap 2. This configuration of the heating plate is not shown in the drawing. This also applies to a heat insulation layer embedded in the area of the fixing gap in the heating plate 4, which likewise leads to a reduction in the heat conduction between the inlet and the outlet zone of the heating plate.

Instead of holes 8, as shown in FIG. 4, an air gap can also be provided between the inlet and outlet zones of the heating plate.

It is advantageous not to interrupt or reduce the heat conduction exactly in the fixing gap 2, but offset in the direction of the plate end B, since the pressure roller 6 then remains relatively cool. The optimal displacement range corresponds approximately to the diameter of the pressure roller 6.

In the embodiment shown in Fig. 5, the desired temperature profile is obtained in that the two heating rods 5,5 'are arranged behind the fixing gap 2 in the outlet zone CB and the heating plate in the inlet zone AC is shielded by a temperature-resistant film 7. The film 7 reduces the heat transfer from the heating plate 4 to the copy sheet 3 in the desired manner. Another option is to rip the copy sheet over ribs, not shown to lead on the heating plate 4 in the inlet zone AC, in which case the air layer below the copy sheet between the ribs reduces the heat transfer from the heating plate to the copy sheet accordingly.

In the embodiment of the invention according to FIG. 6, an element 11 made of a material with high thermal conductivity connects to the plate end B of the heating plate 4. On the underside of the element 11 there are cooling fins 10, against which an air flow 9 is directed. The element 11 extends to a certain extent the outlet zone CB of the heating plate 4. In the area of the element 11, a temperature T ₃ is then set which is close to the ambient temperature.

The desired temperature profile across the heating plate 4 effectively prevents offset pressure from the pressure roller. The copies leave the hot plate 4 at the hot rear plate end B and are still so warm when they are dispensed that the melted toner images, especially with a high wax content, have not yet solidified in a smudge-proof manner. When stacked, the back is occasionally soiled from the copy below by small amounts of toner. This contamination of the back or printing by the output rollers, which are not shown in the drawings, can be largely prevented by cooling the copy while it is being output in a fixing station, as shown in FIG. 6.

Claims (14)

1. Thermal fixing station for fixing a liquid-developed toner image by means of heat in a copier, with a curved heating plate, along which the copy sheet with the liquid-developed toner image is passed through a guide roller within the fixing station, and with a heating device for heating the heating plate, characterized in that the heating plate (4) has a temperature profile with two different temperature ranges over its plate length. 2. Thermal fixing station according to claim 1, characterized in that the temperature T ₁ in the inlet zone AC of the heating plate (4) from the plate start A to the fixing gap (2) between the heating plate (4) and the pressure or guide roller ( 6) is lower than the temperature T ₂ in the outlet zone CB of the heating plate (4), which extends from the fixing gap (2) to the plate end B. 3. Thermal fixing station according to claim 2, characterized in that the temperature T _{1 of} the inlet zone is at least 35 ° C lower than the temperature T _{2 of} the outlet zone. 4. Thermal fixing station according to claim 2, characterized in that the temperature difference between the temperature T _{2 of} the outlet zone and the temperature T _{1 of} the inlet zone is up to 160 ° C. 5. Thermal fixing station according to claims 1 to 4, characterized in that the heating device consisting of two heating elements (5,5 '), of which one heating element (5) in front of the fixing gap (2) and the other heating element (5') behind the fixing gap (2) is arranged so that the temperature is regulated such that the heating element (5) in the inlet zone AC is operated with less electrical power than the heating element (5 ') in the outlet zone CB. 6. Thermal fixing station according to claims 1 to 4, characterized in that both heating elements (5,5 ') are arranged behind the fixing gap (2) in the outlet zone CB and the heating element (5) which is closer to the fixing gap (2) electrical power is smaller than the electrical power supplied to the further away heating element (5 '). 7. Thermal fixing station according to claims 1 to 4, characterized in that the heating plate (4) in the region of the fixing gap (2) has a zone of reduced heat conduction compared to the material of the heating plate 'te (4). 8. Thermal fixing station according to claim 7, characterized in that the zone of reduced heat conduction is offset relative to the fixing gap (2) in the direction of the plate end B of the heating plate (4). 9. Thermal fixing station according to claims 7 and 8, characterized in that in the zone of reduced heat conduction holes (8) in the heating plate (4) are present, which are arranged in a row transverse to the longitudinal direction of the heating plate (4). 10. Thermal fixing station according to claims 7 and 8, characterized in that the heating plate (4) tapers in the zone of reduced heat conduction. 11. Thermal fixing station according to claims 7 and 8, characterized in that the zone of reduced heat conduction has a heat insulation layer embedded in the heating plate (4). 12. Thermal fixing station according to claims 7 and 8, characterized in that the zone of reduced heat conduction is formed by an air gap between the inlet and the outlet zone of the heating plate (4). 13. Thermal fixing station according to claims 1 to 4, characterized in that both heating elements (5,5 ') behind the fixing gap (2) in the outlet zone CB are arranged and that the heating plate (4) in the inlet zone AC is shielded by a temperature-resistant film (7). 14. Thermal Fixing station according to claim 6, characterized in that on the plate end B of the heating plate (4) connecting an element (11) made of a material having a high thermal conductivity and that the _element (11) is equipped with cooling ribs (10), against which an air flow (9) is directed.

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