EP0638855B1 - Thermal fixing device for image carriers printed on one or two sides in a printer or copying machine - Google Patents

Description

The invention relates to a heat fixing device for fixing toner images on the front side of a tape-shaped recording medium in an electrographic printing or copying device, in which the rear side of the recording medium can already have a fixed toner image.

To fix the subject of toner images on a recording medium, which usually consists of paper, heat-fixing devices are used in printing or copying devices, which have a preheating saddle with a downstream fixing zone comprising a heated fixing roller and a pressure roller.

Such heat fixing devices are such. B. from US-A-4 147 922 or Japan Abstract Vol.13, No.120, March 24, 1989 (Japan-A-63-292177) known.

In the literature reference RESEARCH DISCLOSURE, Vol.2244, No. 308, December 1989, Emsworth GB, page 977, a transport device for sheet-like recording media in copiers is described, which transports the recording media without contact. For this purpose, an air cushion is created below the record carrier. The contactless transport is intended to prevent the loose toner images from smearing before being fixed. A preheating saddle is not provided.

In electrographic printing devices that work in the top speed range with, for example, a printing speed of greater than 0.5 m / s and which Fixing using a thermal printing fixing station, it is favorable to heat the paper web or the paper sheet to temperatures of approximately 100 ° C. or higher before the actual thermal printing fixing process, in order to obtain a good connection of the toner image to the paper surface.

If a paper web or a single sheet of paper that is already printed and fixed on one side, for example the back, is to be printed and fixed on the other side, then the already fixed first side must be heated over the hot preheating saddle surface to heat the paper for the second fixing process be performed. The following problems arise during this second fixing process:

a) Continuous printing:

The already fixed print image, which runs over the hot preheating saddle surface, is heated so strongly that it assumes a pasty to liquid state and is partially smeared on the saddle surface. The more toner that accumulates on the saddle surface, the more toner is transferred from the toner image to the saddle surface until there is a visible destruction of the toner image on the paper.

b) Waiting or standby mode:

While the printer is in the waiting or standby mode, the paper web with the already fixed print image lies on the hot saddle. The print image in the area of the preheating saddle surface is heated so strongly that it assumes a pasty to liquid state and sticks to the hot preheating saddle surface. At the start of the paper web, the toner image is torn from the paper web surface and sticks to the hot saddle surface.

The aim of the invention is therefore to provide a heat fixing device with a preheating saddle for fixing toner images on the front side of a recording medium in an electrographic printing or copying device, in which the rear side of the recording medium may already have a fixed toner image.

This object is achieved according to the features of the first claim.

Advantageous embodiments of the invention are characterized in the subclaims.

The information on the front and back of a record carrier is purely relative to the description of the two sides of a record carrier.

If the record carrier, which can consist of single sheets or continuous paper, is guided over a preheating saddle with a sliding surface made of toner-repellent material, which shows a property that repels the pasty to liquid toner and has a high abrasion resistance compared to that has sliding paper web, so the heat fixing device can be used in printing or copying machines that work in both simplex and duplex mode.

Materials that are made from fluorine compounds, such as PTFE or PFA compounds, have proven to be favorable.
The material can be evaporated, sprayed or glued onto a corresponding receiving surface of a preheating saddle. PTFE or PFA compounds show a very good repellency against the toner material and very good properties against abrasion from the paper web.

In order to increase the abrasion resistance, components such as graphite or glass fibers that reduce wear can be mixed to a greater or lesser extent with the PTFE or PFA.

Since preheating saddles of this type are generally used in high-performance electrographic printing devices (between 2 million and 10 million A4 pages per month), wear-free operation over many years is impossible. For this reason, it is useful if the saddle surface can be replaced easily and easily if necessary, without the expensive basic structure of the heating saddle with heating elements having to be replaced. For this purpose, a toner-repellent layer can be evaporated, sprayed or glued onto thin metal plates, in which case these coated individual plates are then exchangeably attached to the basic structure of the preheating saddle.

In an advantageous embodiment of the invention, the toner-repellent layer is designed as a film which has a thin, thermally conductive adhesive layer on one side. The adhesive layer is designed so that when the saddle is hot, the film can be easily removed again. A quick renewal of the saddle surface is therefore possible very quickly on site at the customer's site.

The toner-repellent layer can also be designed as a thin film, which is drawn off from a supply reel and guided over the preheating saddle surface and then wound up again. The film is moved very slowly in relation to the paper running direction.

In order to obtain a wrinkle-free entry of the paper web into the fixing gap between the fixing and pressure roller, it has already been proposed to design the end of the preheating saddle facing the fixing gap as a smoothing edge over which the recording medium is strongly deflected. In the wrapping area in the area of the smoothing edge, however, there is very heavy wear on the toner coating on the recording medium. This wear can be prevented if rollers are provided in the wrapping area which are optionally provided with a toner-repellent coating.

If a relatively high proportion of graphite or glass fibers is added to the toner-repellent material in order to achieve a high wear resistance of the surface, the repellency of the surface against the toner image may be reduced. In order to prevent the toner image from being transferred to the saddle surface in such cases during a long waiting or standby operation of the printing device, it is advantageous to lift the recording medium off the saddle surface. This can be done by creating an air cushion between the paper web and the saddle surface or sliding surface in the waiting state of the printing device with the aid of a blower device. Another possibility for lifting is to provide a suitable lifting element, for example a tensioning wire, which engages behind the recording medium over its entire width. The preheating saddle and the lifting element are moved relative to one another in such a way that in a lifting state the recording medium is guided over the lifting element at a distance from the preheating saddle.

As a rule, with the new generation of electrographic continuous printers, the paper web is automatically inserted into the printer. The paper web must be guided over the preheating saddle, among other things. Coatings made of fluorine compounds become electrostatically charged on their surface when paper slides on them. Due to the electrostatic forces, the paper web adheres to the preheating saddle so strongly that it can possibly no longer be transported. An advantageous admixture of electrostatically conductive substances such as graphite or the like can prevent the formation of electrostatic charges. In the case of glued material layers, it is advantageous if the adhesive is also conductive, in order to establish a conductive connection between the toner-repellent material and the grounded carrier.

• Figur 1 eine schematische Schnittdarstellung einer Wärmefixiereinrichtung für eine elektrografische Druckeinrichtung,
• Figur 2 eine schematische Schnittdarstellung eines Vorwärmsattels mit einer Gleitfläche aus tonerabweisendem Material,
• Figur 3 eine schematische Schnittdarstellung eines Ausschnittes der Figur 2,
• Figur 4 eine schematische Darstellung eines Vorwärmsattels mit darauf angeordneten beschichteten Metallplatten als Gleitfläche,
• Figur 5 eine schematische Darstellung eines Vorwärmsattels mit zugehöriger Folientransporteinrichtung,
• Figur 6 eine schematische Schnittdarstellung eines Vorwärmsattels mit zugeordneter Glättungsrolle,
• Figur 7 eine schematische Schnittdarstellung eines Ausschnitts eines Vorwärmsattels mit einer pneumatischen Einrichtung zur bedarfsweisen Erzeugung eines Luftkissens zwischen dem Aufzeichnungsträger und dem Vorwärmsattel und
• Figur 8 eine schematische Schnittdarstellung eines Vorwärmsattels mit zugehöriger mechanischer Abhebevorrichtung für den Aufzeichnungsträger.

Embodiments of the invention are shown in the drawings and are described in more detail below, for example. Show it

• FIG. 1 shows a schematic sectional illustration of a heat fixing device for an electrographic printing device,
• FIG. 2 shows a schematic sectional illustration of a preheating saddle with a sliding surface made of toner-repellent material,
• FIG. 3 shows a schematic sectional illustration of a detail from FIG. 2,
• FIG. 4 shows a schematic illustration of a preheating saddle with coated metal plates arranged thereon as a sliding surface,
• FIG. 5 shows a schematic illustration of a preheating saddle with associated film transport device,
• FIG. 6 shows a schematic sectional illustration of a preheating saddle with an associated smoothing roller,
• FIG. 7 shows a schematic sectional illustration of a section of a preheating saddle with a pneumatic device for generating an air cushion between the recording medium and the preheating saddle as required
• Figure 8 is a schematic sectional view of a preheating saddle with associated mechanical lifting device for the record carrier.

An electrographic printing device for printing on continuous paper contains a heat fixing device, shown schematically in FIG. 1, as described in US Pat. No. 4,147,922. The heat fixing device is designed as a thermal pressure fixing device. It contains a heating roller 11, which is heated by radiators 10, and a pressure roller 12, which can be pivoted and pivoted onto the heating roller 11 by an electric motor. The heating roller consists of an aluminum cylinder with a heat-resistant coating arranged on it, the pressure roller also consists of an aluminum cylinder with a coating of silicone. The heating roller 11 is driven by an electric motor. The heating roller 11 is assigned an oiling device 13 for applying separating oil to the heating roller. A heated preheating caliper 15 with associated vacuum brake 16 is arranged upstream of the rollers in the recording medium transport direction. Braked by the vacuum brake 16 and driven by the rollers, the recording medium 17 is guided tightly over the preheating caliper 15. A loose toner image on the recording medium is preheated on the preheating saddle 15 and fixed between the rollers 11 and 12 by heat and pressure.

A cooling device 18 arranged downstream of the rollers 11 and 12 in the paper running direction ensures cooling of the entire paper. For this purpose, the cooling device 18 contains a cooling surface 19 provided with openings, which is swept by the recording medium 17, cold air supplied via an air supply channel 20 flowing out of the openings and producing a cooling air cushion under the recording medium 17. At the same time, air is blown onto the concrete side of the recording medium via an opposite profile.

In the heat-fixing device described, the continuous paper 17 is preheated via a low-temperature preheating saddle 15, which consists of two heated saddles connected in series, namely a stationary preheating saddle 21 and a heating saddle 23 which can be pivoted about a pivot point 22, seen in the paper running direction two separate heating zones. The entire preheating section has a length of approximately 500 to 700 mm. During preheating, the paper 17 slides on sliding surfaces 24 of the preheating saddle 21 or heating saddle 22.

In order to make good contact between the saddles and the paper and thus to keep the temperature difference small, the sliding surfaces or the saddles are curved with a curvature radius which in the example shown is 700 mm. Due to the curvature of the sliding surfaces in connection with the train through the rollers 11 and 12 and the braking by the vacuum brake 16, a force component acts over the entire length of the saddle, which Presses paper 17 against the sliding surfaces 24. It also promotes the stability of the paper run on the saddle. The saddles 21 and 23 have transversely to the paper running direction elongated recesses 25 which extend over the entire width of the saddles. They are connected to a channel 27 by side bores 26. The air duct runs underneath the saddles and is connected to a pneumatic device that generates positive and negative pressure, for example a blower and a pump. In printing operation, the recording medium (paper) is sucked into the sliding surfaces 24 of the saddles by negative pressure and the water vapor released by the preheating is sucked out. In standby mode, an air cushion is generated between recording medium 17 and saddle or sliding surface 24 by overpressure.

The saddles 21 and 23 are heated by electrical resistance elements in the form of exchangeably arranged heating cartridges which are arranged in bores 29. The preheating saddle is designed as a low-temperature saddle, the heating power of which is regulated by a microprocessor-controlled control arrangement.

The heat fixing device is also suitable for fixing recording media which already have a fixed toner image on their back. This toner image can be printed and fixed in a first pass on the back or the front of the recording medium, after which a further pass is applied and fixed on the corresponding other side in a further pass. For this purpose, as shown in FIGS. 2 and 3, the sliding surface 24 consists of a toner-repellent plastic layer, of a fluorine compound, for example a PTFE or PFA compound, which is evaporated, sprayed or glued onto the machined surface of the preheating saddle 15.
The connections are described as follows:

Polytetrafluoroethylene (PTFE) with the structural formula:

mit R=C_nF_n+1 als perfluorierte Alkan-SeitenkettePerfluoroalkoxy polymers (PFA) with the structural formula:

with R = C _n F _{n + 1} as perfluorinated alkane side chain

PTFE or PFA compounds show a very good repellency against the toner material and very good properties against abrasion from the paper web. In order to increase the abrasion resistance, components such as graphite or glass fiber can be mixed to a greater or lesser extent with the toner-repellent layer.

Since heating saddles are generally used in high-performance electrographic printing systems with a page yield of 2 to 10 million A4 pages per month, these heating saddles are subject to relatively high wear. For this reason, it is favorable if the saddle surface can easily be replaced if necessary without the expensive basic structure of the heating saddle having to be renewed with heating elements.

In order to make this interchangeability possible, the toner-repellent layer 30 is evaporated, sprayed or glued onto thin metal plates 31 in an exemplary embodiment according to FIG. The thin metal plates 31 can have a thickness of 1 to 5 mm and they are interchangeably clamped or screwed onto the basic structure 32 of the preheating saddle.

The toner-repellent layer 30 can also be designed as a thin film, which has a thin, good heat-conducting adhesive layer on one side. The adhesive layer is designed so that when the saddle is hot, the film can be easily removed again. A quick renewal of the saddle surface is therefore possible very quickly on site at the customer's site.

In an exemplary embodiment shown in FIG. 5, the toner-repellent layer 30 is designed as a thin film 33. The film 33 extends over the entire width of the preheating saddle. It is wound up as a supply on a spool 34, which is attached in the saddle inlet area below the preheating saddle. Starting from this supply reel 34, this film is stretched over the preheating saddle in the paper running direction to the heating saddle outlet and wound up again on a take-up reel 35 below the preheating saddle. The film is moved extremely slowly in relation to the recording medium speed with the aid of a drive device coupled to the take-up reel 35 and wound onto the take-up reel 35. On the inlet side supply spool 34 there is a film supply which can be designed for the entire printer service life. In this way, the preheating saddle is designed to be maintenance-free.

In order to obtain a wrinkle-free entry of the recording medium web 17 into the fixing nip between the fixing roller 11 and the pressure roller 12, it is advantageous to guide the paper web in a loop around the paper outlet saddle edge 36 of the preheating saddle. However, extreme wear of the already fixed toner image on the recording medium occurs in this wrapping area. According to an embodiment shown in FIG. 6, this wear can be prevented by arranging one or more deflection rollers 37 in the form of smoothing rollers in the wrapping area, which rollers can also be provided with a toner-repellent coating. The smoothing rollers 37 deflect the recording medium 17 from a running direction determined by the sliding surface 24 into an inlet direction to the thermal printing fixing station, with a deflection angle which is dimensioned such that a smoothing effect is exerted on the recording medium 17.

If a relatively high proportion of graphite or glass fibers is admixed to the toner-repellent layer 30 in order to achieve a high wear resistance of the surface, the repellency of the surface against toner can possibly be reduced. In order to prevent the toner image from being transferred to the preheating saddle surface in such cases during a long waiting or standby state of the printing device, it is advantageous to lift the recording medium 17 off the preheating saddle surface while the printing device is in the waiting state.

For this purpose, air is supplied in the waiting state in an exemplary embodiment according to FIG. 7 via the pneumatic channel 27, the bores 26 and the slots 25, so that between the sliding surface 24 and the recording medium 17 an air cushion is created which keeps the record carrier 17 at a distance from the sliding surface 24. This prevents the recording medium from sticking to the preheating saddle surface. If a film 33 is used as the toner-repellent layer, an air cushion can be produced in a similar manner between the film 33 and the preheating saddle.

A further possibility for lifting the recording medium from the preheating saddle in the waiting mode of the printing device is shown in FIG. A lifting element engaging behind the recording medium 17 in the region of the preheating saddle, for example in the form of a tensioning wire 38, is arranged in a fixed manner in holders of the printing device in such a way that the tensioning wire 38 comes to rest in a recess 39 in the preheating saddle in a position D of the preheating saddle assigned to the printing operation. When the preheating saddle or the heating saddle is pivoted from the printing position D into a waiting position W, the tensioning wire 38 remains stationary and the paper web 17 is thereby lifted off the hot preheating saddle surface.
Another possibility is to pivot the tensioning wire 38 or other paper depositing elements out of the surface of the preheating saddle and to sink it back into the surface when printing starts.

In the latest generation of electrographic printing devices, the recording medium 17 is automatically inserted into the printing device. Here, the paper web must be guided over the preheating saddle, among other things. Coatings made of fluorine compounds such as PTFE or PFA become extremely electrostatically charged on their surface when paper slides on them. It can thus occur that the electrostatic forces generated in this way hinder further transport of the paper web 17. Such electrostatic charges can be prevented by adding electrostatically conductive substances, for example graphite or similar materials, to the toner-repellent layer 30. If the toner-repellent layer 30 consists of a layer glued to the preheating saddle, it is necessary to also make the adhesive conductive, in order to establish a conductive connection to the preheating saddle, which is expediently grounded.

Reference list

10th

= Heater, radiant heater

11

= Heating roller, fixing roller

12th

= Pressure roller

13

= Oiling device

15

= Preheating saddle

16

= Vacuum brake

17th

= Record carrier

18th

= Cooling device

19th

= Cooling surface

20th

= Air supply duct

21

= Preheating saddle

22

= Pivot point

23

= Heating saddle

24th

= Sliding surface

25th

= elongated recess, slot, opening

26

= Hole

27

= Pressure channel, pneumatic channel

28

= Heating cartridge, heating element

29

= Hole for heating cartridge

30th

= toner-repellent layer

31

= Metal plates

32

= Basic structure of preheating saddle

33

= toner-repellent layer in the form of a film

34

= Supply spool

35

= Take-up spool

36

= Paper outlet saddle edge of the preheating saddle

37

= Pulleys

38

= Lifting element, tension wire

39

= Recess

D

= Print position

W

= Waiting position (standby mode)

Claims (17)

1. Thermal fixing system for fixing toner images on the front side of a recording medium (17) in an electrographic printer or copier device, in which the rear side of the recording medium (17) may already exhibit a fixed toner image, having
   a heat transfer fixing station (12, 11) fixing the toner images on the recording medium (17), and
   a preheating saddle (15), preceding the heat transfer fixing station (11, 12) in the direction of travel of the recording medium, having an associated sliding surface (24) receiving the recording medium (17) via its rear-side, the sliding surface (24) being constructed, at least in the region of contact with the recording medium (17), from a toner-repellent material (30).
2. Thermal fixing system according to Claim 1, having a toner-repellent material (30) comprising a fluorine compound.
3. Thermal fixing system according to Claim 2, having a PTFE or PFA compound as toner-repellent material (30).
4. Thermal fixing system according to one of Claims 1 to 3, having a toner-repellent material (30) exhibiting wear-reducing particles such as graphite and/or glass fibres.
5. Thermal fixing system according to one of Claims 1 to 4, having a toner-repellent material (30) which is designed to be electrostatically conductive.
6. Thermal fixing system according to one of Claims 1 to 5, having a sliding surface (24) which is exchangeably securable on the preheating saddle (15).
7. Thermal fixing system according to one of Claims 1 to 6, having a film as sliding surface (24), which exhibits, for securing on the preheating saddle (15), a thermally conductive adhesive layer which is as thin as possible.
8. Thermal fixing system according to one of Claims 1 to 6, having a film (33) as sliding surface (24), which is guided, proceeding from a supply region (34), via the preheating saddle (15) to a delivery region (35).
9. Thermal fixing system according to Claim 8, having a drive device moving the film (33) over the preheating saddle (15).
10. Thermal fixing system according to one of Claims 1 to 9, having a device which, if required, separates the recording medium (17) from the preheating saddle (15).
11. Thermal fixing system according to Claim 10, having a pneumatic device which, via air exit openings associated with the preheating saddle (15), generates an air cushion separating the recording medium (17) from the preheating saddle (15).
12. Thermal fixing system according to Claim 10, having a lift-off element (38) engaging behind the recording medium (17) over its width in the region of the preheating saddle (15), the preheating saddle (15) and the lift-off element (38) being designed to be movable relative to one another in such a manner that in a lift-off condition the recording medium (17) is guided via the lift-off element (38) at a spacing from the preheating saddle (15).
13. Thermal fixing system according to Claim 12, having a cable-type element (38) as lift-off element.
14. Thermal fixing system according to one of Claims 1 to 13, having a preheating saddle (15) which exhibits, at an end (36) facing the heat transfer fixing station, at least one smoothing roller (37), over which the recording medium (17) is deflected from a direction of travel determined by the sliding surface (24) into an admission direction to the heat transfer fixing station (11, 12), the angle of deflection being dimensioned in such a manner that a smoothing effect is exerted on the recording medium (17).
15. Thermal fixing system according to Claim 14, having a smoothing roller (37) exhibiting a toner-repellent layer (30).
16. Thermal fixing system according to one of Claims 1 to 15, having a preheating saddle (15) having a first heating zone of low temperature, which first heating zone is designed as a pre-heat-applying saddle (21), and a second heating zone of higher temperature, which second heating zone is located downstream in the direction of travel of the recording medium and is designed as a heat-applying saddle (23).
17. Process for producing a sliding surface (24) according to one of Claims 1 to 6 by applying a toner-repellent material layer (30) to a carrier surface associated with the preheating saddle (15).

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