EP1251404A2 - Cavities comprising elastomeric image carrier - Google Patents

Abstract

The invention relates to an image carrier, for example for use in electrophotography, which contains at least one elastomer with cavities. The image carrier can be used, among other things, in electrography, electrophotography (as a photoconductor) or magnetography and in color printing processes. <IMAGE>

Description

The present invention relates to an image carrier having at least one Contains elastomer with cavities. The image carrier can, among other things They are used in electrography, electrophotography or magnetography.

Digital printing processes have become increasingly popular in recent years Importance, especially electrophotography. Although most continue Devices only work in black and white, the proportion of colored devices always increases further. Colored devices have significantly higher requirements for the Image quality as black and white devices.

The central component in electrophotographic printing processes is the Photoconductor. With practically all today's electrophotographic color printers organic photoconductors are used. The thickness of the photoconductor is typically around 20 µm on an aluminum drum. Usually the photoconductor is neither elastic nor compressible. A detailed Description of the electrophotographic process can be found e.g. B. in "Electrophotography and Development Physics" by L. B. Schein, Springer Verlag, 1992, ISBN 0-387-55858-6.

DocuColor 2060 von Xerox: als Zwischenträger wird ein Endlosband über einen elektrostatischen Transfer eingesetzt.

NexPress 2100 von NexPress: Zwischenträger sind Trommeln (auch elektrostatischer Transfer).

E-Print 1000 von Indigo: Zwischenträger ist eine Art Drucktuch, weiches auf einen Zylinder gespannt ist. Der Transfer auf Papier erfolgt über Druck und Hitze.

In order to achieve good image quality, as is required in color printing, intermediate supports are often used in color electrophotographic printers. When using an intermediate carrier, the toner image is first transferred from the photoconductor to the intermediate carrier and from there to paper. These intermediate supports are partially elastic and / or compressible so that unevenness in the rough paper is compensated for. Such intermediate carriers are used, for example, in the following commercially available electrophotographic printers:

DocuColor 2060 from Xerox: an endless belt via an electrostatic transfer is used as the intermediate carrier.

NexPress 2100 from NexPress: intermediate carriers are drums (also electrostatic transfer).

E-Print 1000 from Indigo: The intermediate carrier is a kind of printing blanket that is stretched over a cylinder. The transfer to paper takes place under pressure and heat.

Such intermediate carriers improve the image quality, but also have considerable ones Disadvantage:

So they cause considerable additional components and wear parts Costs for each printed page and increases the complexity of the machine. In addition, color printers typically use the four process colors Black, cyan, magenta and yellow worked in sequence on paper (or the intermediate carrier) are applied. A high quality Image requires a register accuracy of 100 µm or better between the Color separations. An intermediate carrier is used to achieve this accuracy difficult.

In US 3,945,723 a technique is described in which a flexible Layer pressurized under a photoconductor sleeve over two end pieces so that the photoconductor sleeve is attached there.

US 3,994,726 describes a flexible photoconductor. The flexibility will needed to have a wide contact zone for a special form of liquid development to reach.

In US 5,828,931 and the corresponding DE 19646348-A1 Achieving a better picture quality without an intermediate carrier an elastic Layer proposed under a photoconductor. In particular, the hardness the elastic and the photoconductive layer specified, the photoconductor is harder than the elastic layer.

Compared to the previously known systems according to US 5,828,931 both good image quality (the elastic photoconductor adapts to the rough paper well) as well as a simplified machine structure and cost savings achieved by eliminating an intermediate beam.

With this solution, however, a problem arises for those systems in which the four process colors in succession in four printing stations on paper transferred: elastic materials show a so-called Deformation slip. This is an effect that is based on Figure 1 explains in more detail: roller 1 has an elastomeric cover 3 (for example rubber) and is employed on roller 2. Roller 2 and the core of roller 1 are made of non-elastic material compared to reference 3 (for example steel). The employment creates a contact zone 4, too Nip called, in which the elastomeric cover is deformed. Now becomes roller 1 driven so that roller 2 is moved by friction of roller 1 the hard roller 2 has a higher surface speed than that of Roller 1 with an elastomeric cover firmly. This effect is considered more positive Designated slip. It comes about essentially because that elastic materials such as rubber or polyurethane are not are compressible, but are pressed in the nip through a gap that is smaller than the reference thickness. Basically, the positive deformation slip The bigger the two rollers are pressed together. In addition, the thicker the slip, the smaller the positive deformation slip Reference is.

This slip of deformation has an adverse effect on color printing the method of US 5,828,931, such as Figure 2 on a printing process for two Colors exemplified: rollers 5a and 5b are photoconductors to US No. 5,828,931. Known facilities are shown here, but not known Photoconductor 5a generates a toner image of the first color and onto photoconductor 5b Image of the second color. Paper 7 runs in the direction of arrow 8 and is through Back pressure roller 6a first pressed against photoconductor 5a, so that the image of the first color is transferred to the paper. Then the paper pressed against counter-pressure roller 6b against photoconductor 5b and the second color will be transferred to the paper.

The exact positioning of the two color images is problematic here to each other, in particular due to the deformation slip and manufacturing tolerances in the production of photoconductors 5a and 5b. For a good Image quality must have register accuracy at 100 µm or better (i.e. more precisely). This must also cover the entire paper surface can be achieved. The deformation slip lies in typical circumstances at about 1%. For a sheet of length A4 (29.7 cm), this corresponds to one Length difference of 2970 µm, which is considerably more than the desired 100 microns. In the prior art, three techniques are used to achieve the To keep deformation slip low: With known contact pressure, the Deformation slip can be measured. To the extent that the deformation slip the image is lengthened, the image on the photoconductor is shortened applied so that the image transferred onto paper has the correct length. The second technique for reducing register errors is the same Employment for counter pressure rollers 6a and 6b, d. H. Back pressure roller 6a placed against the photoconductor 5a with the same force as the counter-pressure roller 6b against photoconductor 5b. The third possibility of correction is to make the reference as possible to be thick because the positive deformation slip is relatively small is held.

Despite these correction mechanisms, it is very difficult to get the one you want To achieve register accuracy. In practice, it is generally not possible to do exactly the same job for all (typically four) colors to reach. Even small differences in employment lead to considerable ones Register errors. Another major source of error is manufacturing accuracy the photoconductor according to US 5,828,931. The manufacture of a high-precision roller with elastomeric cover is complex and caused high costs. Furthermore thick (typically 1 cm or more) covers needed, which lead to large and heavy rollers. This is in the practice of considerable disadvantage, since the photoconductor is a wearing part, which must be changed relatively frequently (e.g. after 50,000 A4 pages). Accordingly, this component must be both inexpensive and lightweight be manageable for the exchange.

The invention is based on the problem of providing an image carrier which has no or very little deformation slip, so that Problem of registration accuracy with the associated disadvantages is solved. The image carrier according to the invention is also intended as a photoconductor be usable.

Surprisingly, the problem is solved by an image carrier, the contains at least one elastomer with cavities.

The elastomer is preferably a foamed polymer, in particular a foamed polyurethane, silicone, EPDM rubber and / or NBR. Foamed here means, for example, that the polymer is produced in such a way that a gas, for example nitrogen or CO ₂ , is released during the production process. Alternatively, the gas can also be introduced during the production of the polymer. In both cases, an elastomer is created that contains cavities and has a relatively low density.

In a further preferred embodiment, the cavities in the Elasomer created by expanding hollow spheres or not expanding Hollow spheres, which are then expanded, are introduced. Both Elastomers are in particular those mentioned above materials polyurethane, silicone, EPDM rubber and / or NBR. Such Systems based on polyurethane are described in German patent application DE 10111618.7. The thermoplastic hollow spheres are made preferably from an acrylate-vinylidene fluoride copolymer and contain a gas, for example butane.

The volume fraction of the cavities in the elastomer is preferably 5 to 95%, especially 20 to 80%. The ratio of the voids to the elastomer Material can be easily optimized so that the Deformation slip is minimized. If the elastomer with the cavities has other components, such as a photoconductive or magnetic material, then the volume fraction of the voids in the Material containing the elastomer and additives, preferably 5 to 95%, especially 20 to 80%.

In a preferred embodiment, the image carrier is a photoconductor. The photoconductor according to the invention is preferably used in one of the two following Embodiments provided:

First, it can have at least two layers, one of which has at least one layer of photoconductive properties and at least one layer contains an elastomer with cavities. On the other hand, he can Contain layer that contains an elastomer and cavities and at the same time has photoconductive properties.

For example, in the first embodiment, a photoconductive layer is applied to an elastomeric layer with a thickness of 1 mm and a relatively low resistivity (e.g. less than 10 ⁹ Ωm), which consists of polyurethane, which contains expanded hollow spheres. In the second embodiment, however, the elastomeric layer with the cavities itself has photoconductive properties.

The photoconductors according to the invention have compared to known photoconductors The following advantages: Not foamed or generally not compressible When used against a hard roller, elastomers show a positive result Deformation slip (i.e. the hard roller turns faster than the elastomer Roller), while compressible materials have a negative deformation slip (i.e. the hard roller rotates slower than the elastomer roller) demonstrate.

In contrast, when using the materials of the invention correct choice of the ratio of cavities to elastomer a deformation slip usually completely avoided. This makes a high one Register accuracy achieved, both largely independent of the Setting the counter pressure rollers for the different colors at electrostatic transfer from photoconductor to paper, as well as to a high degree is independent of manufacturing tolerances (e.g. concentricity). Through the high registration accuracy becomes an extraordinarily good image quality Receive transfer.

This goes hand in hand with a low consumption of elastomeric material (in Comparison, for example, with a photoconductor according to US 5,828,931). This will be at achieved the photoconductors of the invention in several ways. First material is saved by introducing cavities. Second, it is in the case of a material according to the invention which has no deformation slip does not need to make the cover particularly thick to the Minimize deformation slip, such as when using materials without cavities. Third, it is with materials with voids considerably easier to achieve a low shore hardness. This is beneficial since this creates a relatively wide nip even with slight pressure. On The relatively wide nip of typically several millimeters is used in the Electrophotography during electrostatic transfer from photoconductor to paper very advantageous. The low consumption of the elastomeric material has under other advantages are that lower costs arise and that a simplified replacement of the photoconductor is possible.

In one exemplary embodiment of the first embodiment described above, the elastomeric layer with the cavities has a thickness of 1 mm. The layer has a Shore A hardness of 20 and a specific resistance of 10 ⁸ Ωm. This value applies to a speed range up to approx. 0.5 m / s. At higher speeds, a lower specific resistance, inversely proportional to the speed, is required in the first approximation. The structure of the photoconductor is, in a first approximation, similar to that of conventional photoconductor drums, i.e. there is a barrier layer less than 1 µm thick on the elastomeric layer, then a generation layer for charge carriers about 1 µm thick (here charge carriers are generated by the incidence of light) and finally as the top one is an approx. 20 µm thick transport layer for load carriers. Optionally, a thin (e.g. approx. 3 µm thick) and hard wear protection layer can be applied to the top layer. Obviously, the number of layers can be reduced if one of the layers mentioned performs several functions.

For these 3 layers, or 4 layers when using a wear protection layer, a variety of materials are available today (see e.g. "Electrophotography and Development Physics" by L. B. Schein, Springer Verlag, 1992, ISBN 0-387-55858-6 or DE 19951522). For the invention Those materials that are flexible are preferred for photoconductors that they are the bends and loads that go through the nip arise without enduring damage. Accordingly, there are organic ones Photoconductor compared to harder and therefore less flexible inorganic Preferred photoconductors.

In the photoconductors according to the invention without deformation slip, it is of The advantage of optimizing the ratio of cavity to elastomeric material. If the proportion of cavity is too high, you get a negative deformation slip, while with a very small amount of void receives a positive deformation slip. The percentage of void that needed to avoid the deformation slip depends in particular depends on the type and hardness of the elastomer used. It varies in particular between 5 and 95%.

There are various basic bodies for the application of the elastomeric layer State of the art variants: a fixed drum, a rigid sleeve (e.g. Aluminum with a wall thickness of 1 mm), a flexible sleeve (e.g. stainless steel with a wall thickness of 50 µm) or a flexible tape (e.g. PET with a Thickness of 100 µm). For the adhesion of the elastomer layer to the substrate adhesion promoters can be used.

In a preferred embodiment of the invention, no substrate is used. The photoconductor is then manufactured as a sleeve in the form described above on, for example, a drum and then removed from it. This has the advantage of low cost, since a substrate can be saved. As already mentioned, the photoconductor is a wear component that has to be replaced relatively frequently (e.g. after 50,000 A4 pages), so that costs play an important role here. In order to ensure a good electrical connection to the receiving drum in the printing press in this embodiment (this is essential for the functioning of the photoconductor), two configurations are preferred: First, a comparatively low specific resistance of the elastomeric material (e.g. less than 10 ⁷ Ωm ) or secondly, a "highly conductive" coating inside the sleeve (example: an approx. 1 µm thick layer of graphite).

In a further preferred embodiment, the radius of curvature kept as large as possible for the photoconductor. This will reduce the burden of photoconductive materials kept as low as possible. For example for a drum can be achieved by a large outer diameter (Example: 200 mm). In the case of a band, this can be done by a sufficient length and a suitable tape guide can be achieved.

As already described above, in a preferred embodiment the Invention in the layer containing the elastomer and the cavities, too the photoconductor included. The elastomeric layer is then the transport layer for the load carriers. In this case, the structure is as follows possible: a barrier layer less than 1 µm thick is applied to the substrate applied. Then there is the approx. 1 mm thick elastomer layer, the also serves as a transport layer for load carriers. The follows Generation layer (approx. 1 µm thick). Because such layers are typically can only be mechanically loaded (heavy wear in contact with Paper), here is a wear protection layer as already described above used.

In general, the advantages of the image carrier according to the invention, as they are described above, also achieved in embodiments in which the Image carrier is not a photoconductor. The image carrier according to the invention is preferred also used for electrography and magnetography. In the Electrography (or also ionography, as described in "Electrophotography and Development Physics" by L. B. Schein, Springer Verlag, 1992, ISBN 0-387-55858-6) is used instead of a photoconductor an electrically insulating layer. The present invention can here with the same advantages as with a photoconductor. Alternatively, you can an insulating coating can also be dispensed with by using the elastomeric material uses a highly insulating material.

The same applies to magnetography (as described in "Electrophotography and Development Physics" by L. B. Schein, Springer Verlag, 1992, ISBN 0-387-55858-6). Instead of a photoconductive layer used a magnetic layer here. Alternatively, in the elastomer One or more magnetic substances (e.g. Magnetite). Except for the printing processes mentioned (electrophotography, Electrography, magnetography) is the image carrier according to the invention also for any other printing method applicable in which a toner image on a Image carrier created and from there without intermediate carrier on the substrate is transmitted, especially in color printing.

In a further variant, the transfer takes place thermally, i.e. the toner will heated so that it melts and sticks there when it comes into contact with the paper. This has two advantages: first, the steps of transfer and fixation in one Step summarized, which simplifies the process. Second, at This variant avoided some errors in electrostatic transfer may occur. For this variant, the elastomeric and photoconductive Layer be sufficiently temperature stable. Also, for the top one Layer used a material with low surface energy, so that the melted toner detaches well from the photoconductor for transfer to paper.

Characters:

Figure 1: Scheme of an arrangement for electrophotographic printing according to the prior art

Figure 2: Scheme of an arrangement for the electrophotographic printing process for two colors according to the prior art.

Claims (10)

Image carrier, characterized in that it contains at least one elastomer with cavities. Image carrier according to claim 1, characterized in that the volume fraction of the cavities in the elastomer is 5 to 95%, preferably 20 to 80%. Image carrier according to claim 1 and / or 2, characterized in that the elastomer is a foamed polymer, in particular a polyurethane, silicone, EPDM rubber and / or NBR. Image carrier according to claim 1 and / or 2, characterized in that the elastomer is a polyurethane which contains expanded thermoplastic hollow spheres. Image carrier according to claim 4, characterized in that the thermoplastic hollow spheres consist of an acrylate-vinylidene fluoride copolymer. Photoconductor for use in electrophotography according to at least one of claims 1 to 5, characterized in that it has at least two layers, at least one layer having photoconductive properties and at least one layer containing an elastomer with cavities. Photoconductor for use in electrophotography according to at least one of Claims 1 to 5, characterized in that it contains a layer which contains an elastomer with cavities and has photoconductive properties. Photoconductor according to claim 7, characterized in that the volume fraction of the cavities in the layer which contains an elastomer with cavities and has photoconductive properties is 9 to 95%, preferably 20 to 80%. Use of an image carrier according to at least one of claims 1 to 5 in an electrophotographic, electrographic or magnetographic Process and / or color printing process. Use of an image carrier or photoconductor according to one of the claims 1 to 9 in a process in which the transfer of the toner from the image carrier or photoconductor on the substrate is done thermally.

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