MXPA96005328A - Ceramic roller for printing and coating machines with electrostat help - Google Patents

Abstract

The present invention refers to an electrostatically chargeable ridille in a coating or printing machine, the roller comprising: a cylindrical ridillo core of a fiber-reinforced mixed material, a semiconductor ceramic layer attached to the cylindrical roller core; The semiconductive ceramic layer is formed of at least one plasma-sprayed coating of ceramic material, the strength of which is controlled by spraying plasma of the ceramic material in the cylindrical roller core to form a semiconductor ceramic layer; The electrical resistance of the semiconductor ceramic layer is further controlled by the manner in which the ceramic material is sprayed by plas

Description

CERAMIC ROLLER FOR PRINTING AND COATING MACHINES WITH ELECTROSTATIC HELP FIELD OF THE INVENTION The invention relates to rollers for use in electrostatic-assisted printing and coating (ESA) machines.

BACKGROUND OF THE PREVIOUS TECHE With reference to Figure 1, in printing machines with ESA, the ink is transferred from an engraved printing cylinder 10 to the underside of a strip 1 1 of a non-conductive material aided by electrostatic attraction. A roller 12 placed on the web 1 1 provides the electrostatic attraction of the ink towards the web 11. In some applications, this roller 12 is referred to as a printing roller, since it forms a line of contact 14 with the printing cylinder 10 and comes into contact with the web 1 1. The printing roller usually has an elastic synthetic rubber outer cover of a durometer hardness of 60 to 95 Shore A, which abuts against the web 1 1 and the printing cylinder 10. The engraved printing cylinder 10 can be rotated through a reservoir 16 supporting the ink or the coating material, the thickness of which is controlled by the scraper blade 17.

In other applications, the ESA roller is separated from the web in a machine known as a gap coating machine, also known as a meniscal or bubble coating machine. In this machine, the web is in contact with the print roller but not with the engraved cylinder. The engraved cylinder doses the coating to the region of the contact line, where there is a small, but precise, air gap. Said machines can use lead-coated copper, a laser-etched ceramic cylinder, or a cylinder coated with smooth chrome with a scraper blade that controls the coating material on the cylinder. The voltage is applied to the roller with ESA through a slip ring arrangement or through a third roller in the machine, known as the voltage applicator roller 18, as seen in Figure 1. ESA rollers typically have at least two layers, an insulating base material 19 on metal core 20 to prevent spillage to the ground, and a semiconducting synthetic rubber material 21. If the core of the roller can be isolated from the ground, only a layer of semiconductor rubber 21 is needed. A typical ESA printing machine, which includes a print roller, is described and shown in Adamson, patent of E. U.A. No. 3,477, 369 and Hyllberg et al., Patent of E. U.A. Do not . 4,493,256, issued January 15, 1985. The technical problems with ESA rollers are mechanical wear, chemical deterioration, and heating at higher compression operation speeds. The heating can cause a greater aging of the rubber layer typically used as the outer layer in printing rollers. When ESA rubber coated rollers wear out, their diameter changes can affect printing operations. Finally, wear on the outer cover will require replacement or recovery of the roller. Recently, in Hyllberg, patent application of E. U.A. Series No. 07 / 973,447, filed on November 9, 1992, which was mentioned above, described ceramic materials for a charge donor roller for a copying machine. One difference is that the charge donor roller usually has a voltage applied directly to its core and does not need to be grounded. In comparison, a roller with ESA is preferably isolated from the rest of the machine and has an applied voltage from another roller. One advantage of ceramics for ESA applications is that ceramics can be formed in thinner layers than semiconducting rubber. A ceramic layer maintains its dimensions of operation over a substantial use. The present application with ESA is a coating operation, in which a coating must be applied in a thin, uniform layer of liquid coating on a band. A further problem in the art is the vibration of the roller with ESA at higher machine speeds. In Carlson, patent of E. U.A. No. 5,256,459, issued October 26, 1993, describes a fiber-reinforced mixed tube for roller applications. Said core exhibits good damping characteristics at high rotation speeds. However, it has not been known that said tube is combined in a roller with ceramic layers, due to the difficulties presented in the joining of the two materials. The present invention seeks to overcome the limitations of the prior art by providing methods and constructions of ceramic rolls with ESA for coating and printing applications.

DESCRIPTION OF THE INVENTION The invention relates to a mechanical roller with superior mechanical and electrical properties for electrostatic applications. The surface layer is a mixture of at least two materials, one of which is an electrical insulator, and the other is a semiconductor. In a specific embodiment, the roller with ESA comprises a cylindrical roller core, and a ceramic layer, which is attached to the cylindrical roller core. The ceramic layer is formed as a mixture of an insulating ceramic material and a semiconductor material, wherein the ratio of the mixture is selected to control the electrical resistance of the ceramic layer at an applied voltage differential. The ceramic materials are mixed in a selected ratio to produce an electrical resistance on the scale of a semiconductor. A specific insulating material can be either alumina or zirconia applied by plasma or thermal spraying, and a specific semiconductor ceramic material can be either a titanium dioxide or chromium oxide applied by plasma or thermal spraying. The invention can be modalized in a roller with a semiconductive ceramic layer, or with a semiconductive ceramic layer and an insulating ceramic layer, or with a conductive ceramic layer, and an insulating ceramic layer, and a relatively more conductive layer arranged between the two layers. The invention also relates to a method for making an electrostatically loadable roll that can be used in a machine for coating, printing or copying, the method includes the steps of applying a binding liner to a tubular core of fiber-reinforced material; plasma spray a mixture of an insulating ceramic material and a semiconducting ceramic material to form a ceramic layer, which is joined, by the bonding liner, to the roller core, the ceramic layer having a selected resistance to produce an electrostatic attraction in response to an applied voltage differential; and seal the ceramic layer with a sealing liner. The volume resistivity of the semiconductor layer of a roller with ESA coated with rubber is in the range of 107 to 108 ohm-cm, and its thickness is approximately 1.27 cm. In contrast, a semiconductor ceramic layer has a volume resistivity of approximately 5 x 109 ohm-cm, and may have a thickness of the order of 254 microns. The use of ceramic layers is also advantageous in reconditioning rollers, placing their semi-conductive rubber layers with ceramic layers. In this discussion, rubber includes both natural and synthetic rubbers. Other objects and advantages, in addition to those described above, will be apparent to those skilled in the art, from the description of the preferred embodiment that is presented below. In the description, reference is made to the accompanying drawings, which form a part thereof, and which illustrate examples of the invention. However, said examples are not detailed of the various embodiments of the invention, and, therefore, reference is made to the claims that follow after the description, to determine the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of the machine with ESA of the prior art; Figure 2 is a cross-sectional view of a coating machine incorporating a first embodiment of the ESA roller of the present invention; and Figure 3 is a cross-sectional view of a coating machine incorporating a second embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Referring to Figures 2 and 3, the invention is incorporated into an electrostatic assisted roller (ESA) 30, 31, and a method for making the same. Figure 2 shows said roller 30 in a coating machine with ESA, wherein an electrical voltage is applied to the roller 30 with ESA through a load bar 32, without contact, at a potential of 25,000 volts of DC. A strip 33 of material is fed vertically downwards and around the bottom of the roll 30 with ESA and then is fed vertically upwards. A coating liquid is transferred from a reservoir 47, by the coating cylinder 46, to a bottom side of the band 33, aided by electrostatic attraction provided by the roller 30 with ESA. The band 33 is formed of a non-conductive material. The thickness of the liquid on the coating cylinder is controlled by a scraper blade 45. Although the coating cylinder 46 is shown rotating in a clockwise direction in Figures 2 and 3, and can also rotate in a left-handed direction, with the scraper blade 45 placed on the right side as you can see in the drawings. As seen in Figure 2, a preferred embodiment of the roller with ESA has a tubular core 35. The tubular core 35 is formed of a fiber-reinforced mixed-type (FRP) material, such as that described in Carlson, U. U.A. No. 5,256,459, issued on October 26, 1993. An alloy bonding layer with a thickness of 36 of 76.2 to 127 microns and having a surface roughness of 762 to 1270 micrometers Ra, is formed on the entire outer surface of the tubular core 35. A material ceramic insulation 37 is then applied onto the tie layer 36. A semiconductive ceramic layer 38 with a thickness of 254 to 381 microns, is applied on the entire external surface of the insulating ceramic layer 37, except for the last 2.54 cm on each end of the roller. This keeps the coating away from the ends of the roller 30 and away from any mechanical support or electrical connections to the roller 30. Then, a sealing coating 39 is applied to penetrate the surface of the ceramic layer 38, and the roller 30 is cured. The roller 30 with ESA is made more particularly as follows: Step 1. A tubular FRP core 35 is formed using the methods described by Carlson, in the patent of E. U.A. Do not. ,256,459, cited above. For this application, the core 35 has a wall thickness of 1.27 cm or more, to limit the deflection to not more than 0.00254 cm. Step 2. Apply a tie layer 36 from 76.2 to 127 microns of a material such as Metco AE 7203, also known as Metco 900, which is 12% silicon, 25% plasticizer, and 63% aluminum. Step 3. Apply a layer of insulating ceramic 37 by plasma spray of a 95% alumina powder ceramic material, such as Metco 101 or Norton 1 10. This layer can have a thickness of 508 to 2540 microns, and in this example has a thickness of 1524 microns. Step 4. Apply a ceramic layer 38 with a thickness of 254 to 381 microns, with techniques and plasma spray equipment, using a mixture of alumina and titania, such as Metco 130 (87/13 alumina / titania) and Metco 131 (60/40 alumina / titania) in a mixture of 40/60 to 80/20. Metco products are available from Metco Corp., Westbury, NY. Alternatives for Metco 130 and 131 are Norton 106 and 108, respectively. This step is further carried out by spraying uniform, thin sublayers, to arrive at a desired thickness of the ceramic layer 38. The thinnest practical layer of the plasma material sprayed by plasma, for an electrical grade coating having a high integrity and uniformity , it is approximately 127 microns. As an option, a relatively more conductive layer 40 can be formed between the layer 37 and the layer 38. The resistance of this conductive layer must be at least 20 times smaller than the surface of the semiconductor layer 38, while the resistance of the insulating layer 37 must be at least 20 times greater than the semiconductor layer 38. A typical value The volume resistivity for the relatively more conductive layer 40 is 5 x 107 ohm-cm or less. The material could be 100% titania. It can also be used for this layer 40 nickel. The thickness of the layer could be of the order of 25.4 to 254 microns, and in this example it could be of 50.8 microns. As used herein, the term "insulator" material can mean a material with a volume resistivity of 1010 ohm-cm or greater. Alumina and zirconia are examples of oxide ceramics that are insulating materials. These typically have volume resistivities of 101 1 ohm-cm or greater. As used herein, the term material "semiconductor" can mean a material with a volume resistivity between 103 ohm-cm and 1010 ohm-cm. Titanium dioxide (T 2 O 2) and chromium oxide are examples of semiconducting or lower strength ceramics. These ceramics have volume resistivities typically of 108 ohm-cm or less. There are many other examples of materials in both categories that are commercially available. These relatively high and low strength materials can be mixed to obtain the proper balance of electrical properties for the load transfer roller application.

It is observed that plasma spray ceramics are not pure materials. Even the purest alumina, commercially available, has a purity of only 99.0% to 99.5%. Many grades of alumina contain various percentages by weight of other metal oxides. For example, white or gray alumina may contain titania (titanium dioxide) (T02) in amounts of less than 5% up to at least 40%. An increase in the percentage of titania in the mixture reduces the strength of the material. Although these materials are available as individual powders, these remain mixtures of various ceramics. The electrical properties of the final ceramic layer are the sum of the individual contributions to the resistance, capacitance, dielectric strength, etc. An individual powder may be available that could exactly meet the electrical requirements for the load transfer roller application. There is no doubt that it is not pure material. Preferred ceramics are Metco 130 (87/13 alumina / titania) and Metco 131 (60/40 alumina / titania) in a mixture of 40/60 to 80/20. Metco products are available from Metco Corp. Westbury, NY. The electrical properties of the coating are determined in large part by the ratio of alumina to titania in the finished coating. These two materials are easy to mix, since they can be purchased on the same particle size scale and have almost the same density. A typical value of titania, in the resulting mixture, is 20 to 24%.

For any ceramic layer containing titania (titanium dioxide), the resistance of the layer is also affected by the spray conditions. The titania can be partially reduced to a suboxide, by the presence of hydrogen or other reducing agents in the plasma flame. It is the suboxide (probably TiO instead of TiO2) which is the semiconductor in ceramic layer 38. Titanium dioxide is normally a dielectric material. The average chemical composition, typical of titanium dioxide, is 1.8 octane per molecule instead of 2.0 in a plasma sprayed coating. This level (and thus the coating properties) can be adjusted to a certain degree by increasing or decreasing the percentage of hydrogen in the plasma flame. The normal primary gas is nitrogen or argon, while the secondary gas is hydrogen or helium. The secondary gas increases the ionization potential of the mixture, thus increasing the power level at a given electrode current. For a typical Metco plasma gun, the hydrogen level is adjusted to maintain the electrode voltage in a 74-80 volt gun. The plasma spray parameters must be suitably adjusted to ensure that the mixture of materials in the finished ceramic layer 38 is the same. All mentioned powders do not require the same power levels, spray distance, and other parameters. In this way, the adjustment of the spray distance, for example, can increase the efficiency of depositing one powder on the other and change the mixture of the material in the finished coating. Ceramic coatings sprayed by plasma can be applied in one step (layer) of the plasma gun or in multiple passages. The normal method for most types of coating applications is to apply multiple thin ceramic coatings and develop them to the required thickness. Although the ceramic layer described above has a uniform ceramic composition, the ceramic sublayers in the resulting layer 38 do not have to have the same composition. The coating can be designed to have a different surface resistance than the average material volume. This can be done, 1) to change the shape of a load that is maintained on the roll surface without changing its overall properties, or 2) to compensate for the increased strength of a typical coating. The resistance of the semiconductor layer is selected to be on the scale of 100k ohms to 1 Megohm, to limit the current in the roller. In a roller with ESA, there is essentially no heating in the semiconductor layer and the roller operates almost at its ambient temperature. When a voltage application roller is used, such as element 18 in Figure 1, a relatively high voltage, 500-5000 VDC, is applied to produce a current of 3 milliamperes, for example, for a total power dissipation of 15 watts on the roller. When a 25 kilovolt load bar 32 is used, the maximum voltage on the roller surface is approximately 7000 VDC for a two-layer or three-layer roller. Step 5. As the roller 30 remains hot from the plasma or thermal spray of the semiconductive ceramic layer 38, a sealing coating 39 is applied to the ceramic layer 38 using an organic dielectric material, such as Carnauba wax or a Loctite 290 solder sealer. This sealer is left to soak for several hours at room temperature. If necessary, the sealant is cured (Loctite 290), with heat, ultraviolet light or sprinkler accelerators. When the roll 30 is cured, lower heat, 65.5 ° C, for example, must be used to avoid adverse effects on the ceramic. The level of porosity of the ceramic is generally less than 5% by weight (usually of the order of 2%). Once sealed, the level of porosity has a minimal effect on the coating properties for this application. Preferred types of materials are 100% solids and low viscosity. These include various types of waxes, low viscosity condensation cure silicone elastomers, and low viscosity epoxy materials, methacrylates, and other thermosetting resins. Liquid sealants, such as silicone oil, alone, or liquids in solids, such as silicone oil in silicone elastomer, can be used. These can produce additional benefits for the roller with ESA, to provide some measure of release (non-tacky properties). The sealant will generally be a high strength material, although the electrical properties of the sealant affect the overall properties of the sealed ceramic layers 38. For example, sealing with a Carnauba wax will result in a higher strength of the sealed ceramic layer 38 than the Loctite 290 weld sealer, since it is a better dielectric material. It is also poss to use a semiconductor sealer with a dielectric ceramic (without any semiconductor ceramic) to obtain the desired electrical properties. A low resistance sealant, such as a liquid or solid waxy type, of an antistatic agent may be used, provided that the combination of ceramics and sealant produce the appropriate electrical properties in the finished ceramic layer 16. Step 6. One final step is to grind and polish the ceramic layer 38, sealed, to the appropriate dimensions and surface finish (diamond, silicon carbide abrasives, etc.). After finishing, the ceramic layer 38 typically has a thickness of 254 to 381 microns with a surface finish of 50.8 to 177.8 micrometers Ra. In other embodiments, it can be thicker than 381 microns and vary in roughness from 25.4 to 254 micrometers Ra. The electrical and physical properties of the ceramic do not deteriorate over time, or due to exposure to oxygen, moisture, or chemicals, resulting in a long shelf life for the product. Figure 3 illustrates a second embodiment, wherein the roller 31 with ESA only has a ceramic layer 42 of semiconductor material formed on a conductive core 41, but not magnetic, which is connected to ground. The core 41 can be made non-conductive and the voltage applied to the semiconductive ceramic layer 42 through a voltage-applying roller. The web 44, the scraper blade 45, the coating roller 46 and the reservoir 47 of the coating material are similar for the embodiment illustrated in Figure 2. This has been a description of the examples of how the invention can be carried out. . Those skilled in the art will recognize that various details of the other detailed embodiments can be modified, and these embodiments will fall within the scope of the invention. Since the preferred embodiment of the invention is described with reference to a coating machine, the invention also has utility in printing machines and copying machines. Therefore, the public is appreciated of the scope of the invention and the modalities covered by it, in the following claims.

Claims (5)

1 .- An electrostatically loadable roller useful for coating, printing or copying, the roller comprising: a cylindrical roller core of a fiber reinforced mixed material; a semiconductor ceramic layer bonded to the cylindrical roller core; wherein the semiconductive ceramic layer is formed of at least one plasma sprayed coating of ceramic material, the strength of which is significantly reduced by the plasma spray of the ceramic material onto the roll roller core to form a layer conductive ceramic, semiconductor; and wherein the electrical resistance of the semiconductive ceramic layer is further controlled by the manner in which the ceramic material is sprayed by plasma.

2. The roller of claim 1, wherein the core is insulated, and wherein the core is formed of a conductive core and an insulating ceramic layer arranged to cover an outer cylindrical surface of the core.

3. The roller of claim 2, further comprising an alloy bonding layer with a thickness of 0.00762 to 0.0127 cm between the insulating ceramic layer and the core.

4. The roller of claim 2, wherein the insulating ceramic material is alumina or zirconia; and further characterized in that the semiconductor material is a combination of titanium dioxide and aluminum oxide.

5. The roller of claim 1, wherein the ceramic layer has a thickness in the range of 0.0254 to 0.0381 cm, inclusive. 6 - The roller of claim 1, wherein the semiconductor layer is formed to exclude at least 2.54 cm on each side of the roller to keep the coating material away from the ends of the roller. 7. The roller of claim 1, further comprising a relatively more conductive layer disposed between the semiconductor layer and the core, wherein said relatively more conductive layer is at least 20 times smaller in strength than the semiconductor surface layer. 8. A method for making an electrostatically loadable roller that can be used in a machine for coating, printing, or copying, the method comprising: applying a binding liner to a tubular core of fiber-reinforced material; plasma spray a mixture of an insulating ceramic material and a semiconducting ceramic material to form a ceramic layer, which is joined by the binding liner to the core of the roller, the ceramic layer having a selected resistance to produce an attractive electrostatic in response to an applied voltage differential; and seal the ceramic layer with a seal coating. 9. The method of claim 8, further characterized in that the step of spraying by plasma is performed in a number of repetitions to apply successive sublayers, which form the ceramic layer.