EP1425632B1 - Electrophotographic printing device - Google Patents

Abstract

An electrophotographic printing device including a toner-developer unit, a lighting device, a developer drum, a photoconductor, a transfer unit and an earthed charging device. The substrate to be printed is placed on a transport device and moved along the transfer unit and the toner image of the transfer unit is transmitted to the substrate. A clear, sharp and shadow-free printed image is obtained by arranging the substrate on a non-earthed, electrically conductive layer which is insulated relative to the earthed transport device by an insulator extending along the charging device that is located above the substrate and the measurement of substrate that is to be printed and that is oriented in the direction of transport. The charging device can be charged at a potential, exciting voltage U<SUB>F</SUB>, of between 1 to 10 kV, more particularly 1.5 to 4 kV.

Description

The invention relates to an electrophotographic printing apparatus comprising a toner developing unit, an exposure apparatus, a developer drum, a photoconductor, a transfer unit and a grounded charging device, wherein the substrate to be printed on a transport device moves past the transfer zone of the transfer unit and the toner image of the transfer unit the substrate are transferred.

Such a printing device is known from DE 198 49 500 A1 known. The developer unit operates on a toner and is associated with a photoconductor drum. The photoconductor drum is activated by means of an exposure device on its surface, so that a toner application is possible thereon. The photoconductor drum is in contact with a transfer roller via a contact line. The transfer roller rolls on the surface of the substrate to be printed from and is thereby transferred with the aid of electrostatic charging of the substrate to the transfer unit facing the top of the substrate.

In this printing device, two transfers of the toner image take place. The first transfer process occurs during the transition from the photoconductor drum to the transfer roller, the second transfer of the toner to the substrate. The transfers do not completely transfer the toner. It is desirable to achieve the highest possible transition rate so that clear, contour-sharp print images are created. In this case, the uniform and sufficient formation of the charge image in the region of the surface of the substrate, i. the charge transfer from the charging device to the substrate is of crucial importance.

In particular, with thick substrates, insufficient charging occurs when it consists of a material with poor electrical conductivity.

It is an object of the invention to provide a printing device of the type mentioned, in which an effective and uniform toner transfer to the surface of a substrate regardless of the material thickness and the nature of the substrate takes place and non-homogeneous areas in the printed image (shadowing) can be avoided.

This object is achieved according to the invention in that between the grounded transport device and the substrate, an insulator and between the substrate and the insulator, an electrically conductive layer are arranged, which extends over the charging device located above the substrate and in Transport direction oriented dimension of the substrate to be printed extends.

To improve the toner transfer, the electrically conductive layer between the substrate and the insulator is charged to a potential (field voltage U _F ) of 1 to 10 kV, typ. Between 1.5 and 4 kV to ground. The electrically conductive layer is constructed insulated from the transport device.

Even with electrically non-conductive substrates, such as glass, glass ceramic or plastic plates, with the substrate mounted on the transport device and the insulator arranged between the substrate and the transport device, a uniform and sufficient charging of the surface of the substrate is achieved, if between the substrate and the insulator is still a continuous metal layer is arranged, which extends in the transport direction at least over the charging device and the aligned in the direction of transport dimension of the substrate. This may be due to the fact that a homogeneous field is generated, which is not affected by the transport device when it is set to a potential corresponding to the reference potential of the charge.

In this case, the charging device is preferably designed so that the charging device divides a partial charging device arranged in front of and behind the transfer zone, which are housed in grounded housings which are open towards the substrate.

In this embodiment of the printing device, the substrate to be printed is first fed to the arranged before the transfer unit sub-charging device and is thereby charged electrostatically on its surface before it is fed to the transfer zone. In the transfer zone the toner transfer takes place. As the substrate progresses, depending on the size of the substrate and the printed image, it may happen that toner transfer to the substrate has not yet been completed, but the substrate has already left the partial charging device arranged in front of the transfer zone. The arranged after the transfer zone sub-charger then prevents a charge drop by this recharges the substrate. In this way, a uniform and effective toner transfer across the entire transport path of the substrate is ensured by a homogeneous charging.

With segmented insulator, equipotential bonding between the individual segments can be achieved, resulting in better print results.

The transport of the substrates can be carried out so that a table-like transport device is used, which is linearly guided past the transfer zone and covered by a one-piece or segmented insulating insulator as an insulator, and that the segments or the one-piece insulating plate on the substrate facing the top a conductive layer, eg a metal layer is (are) provided.

If functional elements are accommodated in the transport device, which come into contact with the substrate, such as suction openings, grooves, transport elements, sensors, cable bushings or other components, then provides a further embodiment that the table-like transport device functional elements carries, which are guided by the segments or the one-piece insulating plate and the conductive layer and are electrically conductively connected to the conductive layer, but are electrically isolated from the transport device.

In this way, inhomogeneities of the charging are avoided in the area of the functional elements, which can lead to disturbances of the toner transfer in the area of the functional elements.

The functional elements must always be flush with the conductive layer, e.g. is achieved by resilient support of the functional elements on the transport device and leads to the rich concern of the same on the underside of the substrate.

According to one embodiment, the transport of the substrates can also be carried out in such a way that the transport device has an endless conveyor belt which itself is formed as a metal strip or is provided with a metal layer on the outer side carrying the substrates, that the endless conveyor belt is guided over reversing rollers designed as an insulator, and in that the endless conveyor belt is movable between the reversing rollers on an insulating plate covering the transport frame.

The transport of the substrates can be carried out continuously without having to move the machine frame. The construction of a homogeneous and sufficient charging of the substrates remains ensured even in this embodiment of the transport device.

In order to carry out the charging also transversely to the transport direction in the same way, provides an embodiment that the charging device are designed as Flächencoronen extending over the entire, transversely to the transport direction extending width of the surface to be printed of the substrates and at least partially aligned over the transport direction Surface of the substrates, wherein it is also provided that the Flächencoronen have electrically non-conductive Coronendrahthalter which are stretched in grounded housings on which a plurality of juxtaposed, electrically conductive Coronendrähte are maintained, which is supplied with a uniform charge potential, the counter potential is grounded.

The printing device is further constructed so that the two partial charging devices have a distance which is smaller than the extent of the surface to be printed of the substrate in the transport direction.

The mentioned electrically conductive layer consists of a thin aluminum or copper foil. Also suitable are thin sheets or films made of steel and also made electrically conductive plastic films made of polyurethane, silicone and the like. The electrical conductivity of the layer must be large enough compared to the insulator. Advantageously, resistances are less than 1000 Ω / cm ² .

Suitable insulators are materials made of highly impact-resistant plastics, such as polyamide, polyimide, epoxy resins, kraft paper, bakelite.

The insulator may according to a further embodiment of abrasion-resistant and mechanically strong ceramic or silicate material, such as Al ₂ O ₃ or thin glass exist.

According to a preferred embodiment, it is provided that the metal layer consists of aluminum or copper foil, thin sheet metal, steel foil or electrically conductive made of plastic films of polyurethane, silicone and the like., Which have an electrical conductivity of less than 1000 Ω / cm ² .

Metal layer and insulator can also be combined into one unit and consist of a copper-clad epoxy resin.

According to a further embodiment, the conductive layer can also be made so that an elastic base with a conductive or metallized surface is applied to the insulator of the transport device, which leads to the uniform contact of the substrate underside. Also, a segmentation of the pad is possible if the segments are electrically conductively connected to each other. In order to achieve an effective transfer, the conductive surface of the pad is charged to a potential (field voltage U _F ) of 1 to 10 kV, in particular 3.5 to 5 kV to ground. The surface resistance of the elastic backing and the resistance of the functional elements embedded in the transport device, such as endless conveyors, should preferably be adapted to one another, since this leads to a homogeneous charging of the substrate.

In order to achieve a better insulation between the substrate to be charged and the transport device, a further embodiment of the printing device provides that the substrate to be printed is placed in a shape adapted to the substrate size. The mold is made of an electrically insulating material, the substrate underside facing surface of the mold is electrically conductive or with an electrically conductive layer or metal plate Mistake. The conductive layer or the metal plate is charged via sliding contacts, which are mounted directly in front of and behind the charging device located above the substrate, to the potential (field voltage U _F ) of 1 to 10 kV, in particular 1.5 to 4 kV with respect to ground.

Fig. 1

eine Druckvorrichtung mit linear bewegbarer Transportvorrichtung,

Fig. 2

schematisch die Potentialverteilung bei der elektrostatischen Aufladung eines Substrates,

Fig. 3

eine linear bewegbare Transportvorrichtung mit Funktionselementen, die mit dem Substrat in Kontakt stehen,

Fig. 4

eine als Endlosförderband ausgebildete Transportvorrichtung,

Fig. 5

schematisch das zusätzliche Potential zur elektrostatischen Aufladung des Substrates und der leitfähigen Schicht und

Fig. 6

isolierte Substrat Unterlagsplatte zur elektrostatischen Aufladung über Schleifkontakte.

Die Fig. 1 zeigt in Seitenansicht und teilweise im Schnitt eine elektrofotographische Druckvorrichtung für plattenförmige Substrate 30. Das Substrat 30 wird mit einer tischartigen Transportvorrichtung 25 linear an einer Transferzone 24 einer Transfereinheit vorbeibewegt. Dabei liegt eine Zwischenlage aus einem Isolator 17 oder Segmenten 17.1 bis 17.n derselben zwischen der Unterseite des Substrates 30 und der Auflageebene der Transportvorrichtung. Die Aufladung des Substrates 30 erfolgt über eine in Transportrichtung vor der Transfereinheit angeordnete Teil-Ladeeinrichtung 16 und eine nach der Transferein-heit angeordnete Teil-Ladeeinrichtung 18, die in Gehäusen auf elektrisch nichtleitenden Coronendrahthaltern eine Anzahl von elektrisch leitenden Coronendrähte gespannt festhalten. Die Teil-Ladeeinrichtungen 16 und 18 sind als Flächencoronen ausgebildet und erstrecken sich quer über die gesamte Breite zumindest der zu bedruckenden Substrate 30.The invention will be explained in more detail with reference to embodiments shown in the drawings. Show it:

Fig. 1

a printing device with a linearly movable transport device,

Fig. 2

schematically the potential distribution in the electrostatic charging of a substrate,

Fig. 3

a linearly movable transport device with functional elements that are in contact with the substrate,

Fig. 4

a transport device designed as an endless conveyor belt,

Fig. 5

schematically the additional potential for electrostatic charging of the substrate and the conductive layer and

Fig. 6

Insulated substrate Base plate for electrostatic charging via sliding contacts.

The Fig. 1 shows in side view and partially in section an electrophotographic printing device for plate-shaped substrates 30. The substrate 30 is moved with a table-like transport device 25 linearly past a transfer zone 24 of a transfer unit. In this case, there is an intermediate layer of an insulator 17 or segments 17.1 to 17.n thereof between the underside of the substrate 30 and the support plane of the transport device. The charging of the substrate 30 via a arranged in the transport direction in front of the transfer unit part-charging device 16 and arranged after the transfer unit-sub-charging device 18, the hold in a housing on electrically non-conductive Coronendrahalttern a number of electrically conductive Coronendrähte. The partial charging devices 16 and 18 are formed as Flächencoronen and extend across the entire width of at least the substrates to be printed 30th

The underside of the substrates 30 facing the top of the insulator plate 17 or the segments 17.1 to 17.n are provided with a metal layer 31.

As the scheme Fig. 2 can be seen, the transport device 25 is grounded, that is placed on the counter potential of the charge voltage Uc. The corona wires of the partial charging devices 16 and 18 are uniformly set to the potential of the charging voltage Uc. The metal layer 31 of the insulator 17 or the segments 17.1 to 17.n remains potential-free or is charged to further improve the toner transfer to a voltage (U _F ) of 1 to 10 kV, in particular from 3.5 to 5 kV to ground.

In the area of the transfer zone, the transfer unit is in contact with the substrate 30 for the toner transfer, wherein the transport speed of the substrate 30 is matched or coupled to the rotational speed of the transfer unit such that no slip occurs between the two.

Again Fig. 1 In addition, functional elements 34 can be integrated in the transport device 25, which are in contact with the underside of the substrates 30 to be printed through the insulator 17.

These functional elements 34 may be suction openings, grooves, transport elements, sensors, cable feedthroughs and other components which preferably terminate with the upper side of the metal layer 31 and, where necessary, are held with springs 32 under spring tension on the underside of the substrate 30, such as Fig. 3 shows. In this case, the functional elements 34 may be connected by potential equalization lines 33 to the reference potential of the charge voltage Uc and the metal layer 31, but they are kept electrically isolated in the transport direction 25, as the small air gap can be seen. Such transport devices 25 can successively pass through the transfer zone and be each covered with one or more substrates to be printed 30.

Based on Fig. 1 only briefly the parts of an electrophotographic printing device are presented, which are known per se and in their operation.

In a developer unit 10, a toner, such as a ceramic, a thermoplastic or duromeric plastic toner is stored. The developer unit 10 is associated with a developer drum 15 which supplies the toner to a photoconductor 20. The photoconductor 20 is cylindrical and is in a contact zone 21 in linear contact with the transfer unit 22. About the photoconductor 20, an exposure device 11 is arranged, which exposes a photosensitive layer on the circumference of the photoconductor 20. This creates a latent electrostatic charge image. Due to the charge image For example, toner particles are transferred from the developer drum 15 to the photoconductor layer 20 via electrostatic processes. These toner particles are passed on to the transfer unit 22 in the region of the contact zone 21. A downstream in the direction of rotation of the photoconductor 20 cleaning device 14 removes still adhering toner residues from the photoconductor 20. After the cleaning device 14 is followed by a quenching light 13, which discharges the photosensitive layer of the photoconductor 20. Thereafter, the photosensitive layer of the photoconductor 20 is brought back to a uniform charge structure with a charger 1 2 so that they can again provided by the exposure device 11 with an electrostatic charge image who can.

The transfer unit rolls on the substrate 30 to be printed. At this time, the toner on the transfer unit in the transfer zone is transferred onto the substrate 30. Since the partial charge devices 16 and 18 cause a full-surface charge of the substrate 30 with opposite potential to the charge on the photoconductor 20, a clear toner transfer takes place with high efficiency.

As the Fig. 1 can be seen, the distance in the transport direction between the sub-charge devices 16 and 18 is smaller than the dimension of the substrate 30 in this direction to ensure that the substrate 30 remains charged during the entire passage of the transfer zone.

The Fig. 4 shows a transport device 25 which is grounded and between two reversing rollers has an endless conveyor belt, which is itself electrically conductive and forms the conductive layer 31. The guide rollers form an insulator 17.3, which may also be formed by guide rollers with an insulating peripheral layer, eg PTFE layer. The base of the guide rollers may also consist of insulating material. The additional voltage is supplied, for example via additional sliding contacts 37.

The endless conveyor belt may be a close-meshed metal belt, which facilitates a fixation of the substrates 30 by means of suction.

Fig. 5 shows similar to the Fig. 2 a grounded transport device 25 with an insulator 17 arranged thereon. The electrically conductive layer 31 between the substrate 30 and the insulator 17 is charged to 1 to 10 kV, preferably 1.5 to 4 kV, via a field voltage U _F. The charging devices 16 and 18 and the transfer zone 24 above the substrate 30 are as in FIG Fig. 2 trained and arranged.

As Fig. 6 shows, the substrate 30 can also be absorbed by an insulating mold 35.1 with edges 35.2. The mold may be disposed on an electrically conductive layer 31, which is separated from the earthed conveyor 25 via an insulator 17, but is transported therewith. The inclusion of the mold 35.1 carries an electrically conductive surface 36, which is supplied via sliding contacts 37, the field voltage U _F.

Claims (15)

1. Electrophotographic printing apparatus for printing glass and ceramic products having a toner/developer unit (10), an exposing apparatus (11), a developer drum (15), a photoconductor (20), a transfer unit (22) and an earthed charging device (16, 18), in which electrophotographic printing apparatus the substrate (30) to be printed, lying on a transport apparatus, is moved past the transfer zone (24) of the transfer unit (22) and the toner image of the transfer unit (22) is transferred onto the substrate (30), characterized in that, during the printing operation, the substrate (30) is arranged on an unearthed, electrically conductive layer (31) which is insulated via an insulator (17, 17.1 ... 17.n, 17.3) with respect to the earthed transport apparatus (25) and extends over the charging device (16, 17) which is situated above the substrate (30) and over the substrate (30) to be printed which is oriented in the transport direction.
2. Electrophotographic printing apparatus according to Claim 1, characterized in that the charging device (16, 18) divides a part charging device (16) which is arranged in front of the transfer zone in the transport direction and a part charging device (18) which is arranged behind the transfer zone in the transport direction, which part charging devices (16 and 18) are accommodated in earthed housings which are open towards the substrate (30).
3. Electrophotographic printing apparatus according to Claim 1 or 2, characterized in that a table-like transport apparatus (25) is used which can be guided linearly past the transfer zone and is covered by means of an insulating plate as insulator (17, 17.1 ... 17.n), which insulating plate is made from a single piece or is divided into segments, and in that the segments or the single-piece insulating plate (17) are/is provided with an electrically conductive layer (31) on the upper side which faces the substrate (30).
4. Electrophotographic printing apparatus according to Claim 1 or 2, characterized in that the table-like transport apparatus (25) carries functional elements (34) which are guided by the segments (17.1 ... 17.n) or the single-piece insulating plate (17) and by the electrically conductive layer (31) and are connected to the functional elements (34) in an electrically conductive manner, but are insulated electrically from the transport apparatus (25).
5. Electrophotographic printing apparatus according to Claim 1 or 2, characterized in that the transport apparatus (25) has an endless conveyor belt which itself is composed of electrically conductive material or is provided with an electrically conductive layer (31) on the outer side which carries the substrates (30), in that the endless conveyor belt is guided over reversing rollers which are configured as an insulator (17.3), and in that the endless conveyor belt (25) can be moved between the reversing rollers on an insulating plate (17.1) which covers the transport frame.
6. Electrophotographic printing apparatus according to one of Claims 1 to 5, characterized in that the charging devices (16, 18) are configured as flat coronas which extend over the entire width of the surface area to be printed of the substrates (30), which width extends transversely with respect to the transport direction, and at least partially over the surface area of the substrates (30) which is oriented in the transport direction.
7. Electrophotographic printing apparatus according to Claim 6, characterized in that the flat coronas have electrically non-conductive corona wire holders (16.1; 18.1) which are tensioned in earthed housings (16.3; 16.4 and 18.3; 18.4, respectively), on which a plurality of electrically conductive corona wires (16.2; 18.2) are held which are arranged next to one another and to which a uniform charge potential (Uc) is fed, the opposite potential of which is earthed.
8. Electrophotographic printing apparatus according to Claim 2, characterized in that the two part charging devices (16, 18) are at a spacing which is smaller than the extent of the surface area to be printed of the substrate (30) in the transport direction of the latter.
9. Electrophotographic printing apparatus according to one of Claims 1 to 8, characterized in that the insulator (17, 17.1 to 17.n; 17.3) is composed of a highly insulating impact-resistant plastic, such as polyamide, polyimide, epoxy resins, hard paper or Bakelite.
10. Electrophotographic printing apparatus according to one of Claims 1 to 9, characterized in that the insulator (17, 17.1 to 17.n; 17.3) is composed of abrasion-resistant and mechanically loadable, ceramic or siliceous material such as Al₂O₃ or thin glass.
11. Electrophotographic printing apparatus according to one of Claims 1 to 10, characterized in that the electrically conductive layer (31) is composed of aluminium or copper foil, thin metal sheet, steel foil or plastic films made to be electrically conductive from polyurethane, silicone and the like which have an electrical conductivity of preferably less than 1000 Ω/cm².
12. Electrophotographic printing apparatus according to one of Claims 1 to 10, characterized in that an epoxy resin plate which is coated with copper is used as insulator (17) and electrically conductive layer (31).
13. Electrophotographic printing apparatus according to one of Claims 1 to 12, characterized in that the conductive layer (31) can be charged between the substrate (30) and the insulator (17) to a potential (field voltage U_F) of from 1 to 10 kV, in particular from 1.5 to 4 kV.
14. Electrophotographic printing apparatus according to one of Claims 1 to 13, characterized in that the electrically conductive layer (31) is configured as an elastic endless belt made from conductive material or with a metallized surface.
15. Electrophotographic printing apparatus according to one of Claims 1 to 14, characterized in that the substrate (30) can be received by an insulating form (35.1) which is provided with edges (35.2) and the receptacle of which carries a conductive layer (36) which can be charged to the field voltage (U_F) via brushes (37).

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