EP0079007B1 - Method of sealing a porous coating - Google Patents

Description

The invention relates to a method for surface treatment of a porous coating applied to a workpiece, in which the coating is impregnated with a plastic that seals the pores.

Such a method is known from DE-A-24 32 426.

In the known method, a steel tubular mast is first sandblasted on its outside and then a metallic base layer of pure aluminum is applied to the sandblasted surface by means of flame spraying. This layer is then provided with a covering layer which closes the pores from a synthetic resin.

The known method is intended to improve the surface quality with regard to chemical loads, in particular the influence of weather, and further treatment is therefore neither provided nor necessary. In particular, the surface treatment of the tubular mast is completed with the application of the cover layer made of a synthetic resin and the synthetic resin can consequently harden over a relatively long time.

It is also known from DE-A-19 34 654, in the manufacture of colored metal sheets from roll to roll, to provide the sheet first with an electrodeposited first layer and then with a final spread which can be polymerized by electron beams and then also polymerizes becomes.

In this non-generic manufacturing process, the focus is also on improving the surface quality with regard to chemical stress and there is no provision for post-processing of the sheet, because the sheet coated in the manner mentioned is rewound on rolls and only then delivered to the customers.

The generic method mentioned at the outset is not suitable for the surface treatment of workpieces which are to be produced in a cyclical industrial production and for which a high abrasion resistance of the surface is important, as is the case, for example, with pressure cylinders, piston rods and the like. The like. is the case. These machine parts require post-processing, in particular grinding, to achieve the required accuracy of fit. However, if one soaked the machine part with liquid plastic before grinding in accordance with the generic method mentioned at the outset, this plastic layer would be removed or torn off again immediately after grinding, since it is not yet hard. One could also think of avoiding these difficulties by first reworking and then soaking with plastic, but this possibility is ruled out because the pores of the hard material layer are smeared by grinding and therefore not soaking the surface is more possible.

The invention is therefore based on the object of developing a method of the type mentioned at the outset in such a way that machine parts with high abrasion resistance can also be produced, specifically within industrial production processes with a high cycle or flow rate.

This object is achieved in that the workpiece is a machine part with high abrasion resistance, that the workpiece is first provided with a coating of a hard material such as aluminum oxide, tungsten carbide, chromium in the plasma spray process, that a crosslinkable plastic or lacquer is used for the impregnation, that the Plastic is first hardened in the outward-facing area by irradiation with electrons, in particular crosslinked, and that the surface is further processed, in particular ground, immediately after crosslinking.

The invention thus completely solves the underlying task, because the successive steps of plasma spray coating, impregnation, curing and reworking are so coordinated in type and sequence that machine parts with extremely high abrasion resistance and accuracy of fit can ultimately be produced in industrial manufacturing processes can run at high cycle or flow speed. A completely closed layer of the hardened, in particular cross-linked, plastic or lacquer is created within the porous coating, which reliably prevents the access of acids or other aggressive substances to the surface of the metallic body if the plastic is appropriately selected. It is not absolutely necessary that the plastic or lacquer layer, which is simply referred to as a plastic layer in the following, is cross-linked in its full depth, but it is sufficient if the cross-linking takes place only in an outer region. Another advantage of the invention is that the crosslinking by means of electron radiation can take place very quickly, in the presence of suitable systems, for example in a few seconds or at most a few minutes, and that the plastic layer and thus the body carrying the coating are not noticeably heated as a result of the electron radiation . It is therefore also possible to treat bodies sensitive to temperature increases with the method according to the invention. If it is intended to subject the body to processing after crosslinking, for example grinding the coated surface, the crosslinking is carried out in such a way that the crosslinking in the applied plastic layer extends at least so deep that no areas of the plastic layer that have not yet been crosslinked are reached in the subsequent processing. Since at least the outer area of the plastic layer is cross-linked and therefore dry in an extremely short time, the coated bodies can be processed further immediately after the electron irradiation. Depending on the type of the coated object, it may then be sufficient if areas of the plastic which are not yet crosslinked in the depth are still completely crosslinked under the influence of a sufficiently high ambient temperature in the course of the following days or weeks.

For the intended corrosion protection of the metallic surface of the coated body, it is not absolutely necessary for the plastic to penetrate all the way to the metallic surface.

However, since it is advantageous in many cases if the plastic layer penetrates as deeply as possible into the porous coating, it is provided according to one embodiment of the invention that the body is heated to approximately 80 ° C. before the liquid plastic is applied, and that the plastic subsequently applied to the heated body, in particular sprayed on. By heating to the relatively low temperature of, for example, 80 ° C, which can be withstood even by sensitive bodies without danger, the air contained in the cavities of the hard material coating is largely expelled and the liquid plastic penetrates into it Depth of the porous layer is facilitated. The plastic is applied while the body is still warm, so that the liquid plastic can penetrate into deeper areas of the porous layer when the air still contained in the pores cools. The amount of liquid plastic applied can be adapted to the particular requirements in a particularly simple manner by spraying.

According to one embodiment of the invention, it is advantageous to use a liquid plastic of low viscosity, preferably a plastic that is more liquid than water. This facilitates the penetration of the plastic into the cavities in the hard material coating.

In embodiments of the invention, the method is applied in such a way that only a single plastic coating is provided, which then generally takes up the entire thickness of the hard material coating. In other embodiments of the invention, however, it is provided that the porous coating is initially only soaked in its area adjacent to the metallic body, and that after curing by electron radiation the body is in turn soaked with a plastic which is also crosslinked by electron radiation. In the case of embodiments of the invention, the same plastic can be used here if, in special cases, for example, a desired crosslinking to a relatively great depth cannot take place in a single irradiation process because the plastic layer is too thick for this; however, different plastics are used in other embodiments of the invention. In particular, the outer plastic layer can be chosen so that a particularly low coefficient of friction of the body results; in this case the outer, fully crosslinked plastic layer can in particular be polytetrafluoroethylene.

In the case of flat workpieces, the penetration of the plastic can only be brought about into the area of the porous coating adjacent to the metallic body by applying only a limited amount of plastic required to fill the capillaries adjacent to the metallic body, which is then heated by appropriate heating as a result of gravity seeps down, the upper areas being cleared of the plastic again due to the limited amount of the liquid plastic and can be filled with another plastic after the introduced plastic has hardened. Where such a method is not possible, the entire coating can be filled with a specific plastic and the workpiece heated to a temperature at which the liquid plastic evaporates from the area near the surface, at which temperature the remaining plastic is already cured can. Another plastic can then be introduced into the area of the porous layer which has become free due to evaporation of the plastic and can be cured by electron radiation. In order to achieve the evaporation of the plastic on the surface, the heating can be carried out in such a way that this area of the layer located on the surface heats up more than the area of the porous layer adjacent to the metallic body.

An important advantage of the invention lies in the following: If a body coated with hard material, for example a cylinder of a printing press, has to be ground to achieve a sufficiently smooth surface, whole grains of the coating material are often torn out of the coating, so that a certain amount of Roughing depth cannot be fallen below. If, on the other hand, the body is provided with the sealant according to the invention, the crosslinked plastic layer holds the grains of the hard material coating firmly, especially if it extends to greater depths of the coating, so that individual grains which protrude too far outward are partially removed during the grinding process, such as this is desirable, but cannot be completely torn out of the coating.

The invention is described below using an exemplary embodiment. A metalli shear body, which in the exemplary embodiment is a printing cylinder of a printing press, and which consists of steel and has dimensions of 180 cm in length and 85 cm in diameter, is coated with a layer of tungsten carbide in the plasma spraying process. This coating has a thickness of approximately 100 µm. The plasma spraying process or, as it is also called, plasma spraying, and the devices required for this are known. A gas is fed to an arc plasma torch and dissociated and ionized in the arc depending on the type of gas. Shortly before the nozzle emerges, the pulverized spray material, ie the hard material, is fed to the plasma flowing out at high speed with the aid of a carrier gas. Argon is often used as the plasma gas.

The body provided with the hard material coating is then heated in such a way that the temperature of the body decreases somewhat from the inside to the outside. The heating causes most of the air contained in the pores of the surface layer to escape. The coated body is then sprayed with a liquid plastic, which is specified below. This plastic is drawn into the pores of the coating, at least in part as a result of capillary action, not least because the liquid plastic penetrates into the warmer areas of the coating, that is to say inward, with further liquefaction.

Spraying takes place immediately after the workpiece has been heated to approximately 80 ° C., so that the air which has escaped from the pores of the coating does not penetrate again in the meantime.

After spraying, the workpiece is irradiated by electrons. A system suitable for such radiation is sold by Otto Dürr Anlagenbau GmbH, D-7000 Stuttgart under the type designation ESH 150. This system generates electrons with an energy of 150 keV, whereby the body to be irradiated can be in the free atmosphere and is only flushed with a suitable inert gas to keep out the oxygen in the air in the area in which the electrons strike the body . Nitrogen is an example of an inert gas. There are several embodiments of the system mentioned, which differ in the width of the electron beam generated. In such a system, the power of the electron beam available for irradiation is 7.5 kW.

It is not in itself necessary that the entire hard material layer is penetrated by the electron beam, rather it is sufficient if the layer penetrated by the electron beam is so deep that it is not completely removed during subsequent processing, so that the surface also after a subsequent processing of the workpiece is covered with hardened plastic. The plastic located below this hardened plastic layer then hardens over time, for example within about 2 to 10 weeks.

The sealing by the plastic cross-linked or hardened by means of electron bombardment is extremely effective. An unsealed, plasma sprayed layer is removed, for example, in a single operation by sandblasting with corundum. If a hardened plastic layer according to the invention is introduced into the hard material layer, about six such sandblasting processes are required to remove this combined layer.

The cross-linked plastic layer makes it possible for the first time to use workpieces with surfaces sprayed with a plasma spray in the chemical sector, where without the hardened plastic layer the hard material coating would be damaged by corrosion of the metal body supporting this layer. An impregnation of the hard material layer with an uncured plastic, however, could not prevent corrosion of the metal body with certainty.

If one wanted to crosslink the liquid plastic layer introduced into the hard material layer within a relatively short time by using elevated temperatures, temperatures between 130 ° C. and 250 ° C., depending on the type of plastic used, would have to be used for this.

It has surprisingly been found that, despite the shading to be expected from the individual particles of the hard material layer, the electron irradiation brings about an effective rapid hardening of the plastic layer. Curing down to a depth of 300 J.Lm can be achieved. In contrast, the hard material layers are only about 100 microns thick in many applications, so that the plastic layer can be fully cured in a single exposure.

At present, the use of a high-molecular plastic is considered favorable, which consists essentially of polyethylene glycol dimethacrylate or trimethylol propane trimethacrylate with the addition of aliphatic and / or aromatic hydrocarbons, especially amines, and which contains the usual additives of plasticizer, polymer Thickener, peroxide, toluidine, saccharin, fillers, pigments, stabilizers and the like, e.g. T. based on Si.

Claims (6)

1. A method of surface treatment of a porous coating applied to a workpiece, in which the coating gets soaked with a plastics which seals the pores, characterized in that the workpiece is a highly abrasion-resistant machine part, that the workpiece first of all gets provided by the plasma spray method with a coating of a hard substance such as aluminium oxide, tungsten carbide or chromium, that then for the soaking a crosslinkable plastics or varnish is employed, that the plastics at least in the region pointing outwards is hardened by electron radiation and in particular is cross-linked, and that subsequently the surface directly after the cross-linking gets further processed and in particular is ground.

2. A method as in Claim 1, characterized in that before the application of the liquid plastics or varnish the workpiece is heated to about 80 °C and that the plastics or varnish is applied to the heated workpiece.

3. A method as in Claim 1 or 2, characterized in that the plastics gets sprayed onto the coating.

4. A method as in one of the preceding Claims, characterized in that a plastics or varnish of low viscosity is employed.

5. A method as in one of the preceding Claims, characterized in that the amount of the liquid plastics or varnish is measured out in such a way that the porous coating gets soaked only in its lower regions, and that after the cross-linking of this plastics layer the workpiece gets soaked again with a liquid plastics or varnish which likewise gets cross-linked through electron radiation.

6. A method as in Claim 5, characterized in that different plastics or varnishes are employed for the different soaking processes.