METHOD FOR COATING A STEEL PIPE BY EXTRUSION

Abstract: The invention relates to a method for coating a steel pipe (1) by extrusion, in which method the steel pipe (1) is pretreated and the pretreated steel pipe is led through an extrusion tool (3), wherein it is coated with an adhesive layer (4) and at least with a relatively thick thermoplastic layer (5), after which the coated pipe is cooled. The method is characterized in that the pretreated steel pipe is led into the extrusion tool (3) at a temperature below the solidification point of the extruded plastic material (4, 5) and that the coated steel pipe (1) is heated after cooling by induction heating (7) for achieving an effective adhesion between the steel pipe and the coating.

Description: Method for coating a steel pipe by extrusion The present invention relates to a method for coating a steel pipe by extrusion, in which method the steel pipe is pretreated and the pretreated steel pipe is led through an extrusion tool, wherein it is coated with an adhesive layer and at least with a relatively thick thermoplastic layer, after which the coated pipe is coo¬ led. According to known solutions, the coating of a steel pipe occurs normally in such a way that the steel pipe is mainly immediately after the pretreatment, comprising e.g. warming-up, blasting, heating and a potential epoxy coating, led into an extrusion tool at as high a tempera¬ ture as possible so that the activity on the epoxy layer does not decrease before the adhesive layer and the thermoplastic layer or layers are applied. It is namely generally believed that the adhesive capacity on an ap¬ plied epoxy layer considerably decreases after it has hardened. For achieving as good an adhesion as possible, attempts are therefore made to lead the epoxy-coated steel pipe as rapidly as possible into the extrusion tool. When steel pipes are coated without epoxy, the best adhesion is obtained, if the surface temperature of the steel pipe is above the melting point of the plastic material. The coated steel pipe is then immediately led into a cooling bath to make the layers applied during the extrusion solidify. Such a method is especially suitable when thin plastic layers of only a few millimeters are applied to the steel pipe, since the cooling and thereby the solidification of the extruded plastic then occurs relatively quickly, but if the extruded layers have a considerable thickness, e.g. centering problems quickly occur. On the basis of the insulating capacity of plas¬ tics in connection with thick plastic layers, the cooling of the plastic namely takes place relatively slowly, because of which the first supporting point of the pipe after the extrusion tool must be located at an extremely long distance therefrom, which may make e.g. the cente¬ ring of the steel pipe difficult. Since the cooling of coated steel pipes mainly takes place from the outer side, the applied plastic mass solidifies first on the outermost layer, from which the solidification slowly advances radially inward. The first supporting point of a coated pipe must therefore be located within such a distance from the extrusion tool, in which the cooling process has advanced through all plastic layers, whereby these obtain a sufficient bearing capacity. At every point along the pipe length, at which part of the plastic mass has a dough-like consistency, a risk arises in connection with a contact that the centering of the pipe is affected. Because of the long distance between the contact points, there is also a great risk that the applied plastic layers descend on the steel pipe, in which case the plastic layer on the upper side of the pipe becomes considerably thinner that the plastic layer on the lower side of the pipe. This is especially the case when a thick foamy plastic layer with a good insula¬ ting capacity is extruded on the steel pipe. The object of the present invention is to eliminate these problems especially when thick plastic layers are applied to steel pipes by extrusion during a single stage. This is achieved by means of a method characterized in that the pretreated steel pipe is led into the extrusion tool at a temperature below the solidification point of the extruded plastic material and that the coated steel pipe is heated after cooling by induction heating for ac¬ hieving an effective adhesion between the steel pipe and the coating. The plastic mass to be extruded on the steel pipe normal¬ ly has a temperature of ca. 200°C, whereas the temperatu- re of the pretreated steel pipe entering the extrusion tool is < 100°C. This results in that the cooling and solidifying of the extruded plastic layers are started immediately when they come into contact with the steel pipe. Accordingly, the cooling of the extruded layer occurs considerably more quickly, since the quantity of heat stored in the steel pipe itself is much lower and is therefore not led away through the extruded layers into the cooling bath at the same time as the solidification at least in the initial phase occurs in two sections, i.e. from the inner and outer side of the extruded layer- s. The total quantity of heat to be transported away from the coated pipe is thereby considerably lower than in connection with previously known processes. The extruded layers therefore quickly reach a sufficient bearing capacity for supporting the weight of the pipe. When the cooling is completed, the adhesion of the extruded layers to the steel pipe is not yet completely satisfactory, but this is facilitated according to the invention in an effective manner, i.e. the steel pipe is next heated by induction heating. During the induction heating, the electromagnetic waves enter directly through the extruded plastic layers into the steel pipe without heating of the plastic layers. Since the steel pipe is heated, heat is conducted to the adjacent plastic layers, which are thereby heated to a plastic state. The internal tensions on the upper layers are then released, which creates a shrink force on the extruded plastic coating in such a way that this very effectively presses against the outer shell of the steel pipe, which is clearly indicated e.g. by the fact that the adhesive layer extruded on the epoxy layer presses out at the ends of the pipe. The invention is next described in more detail with reference to the accompanying drawing, which shows a basic scheme of a production line according to this invention. The steel pipe 1 to be coated is pretreated on a pre- treatment station 2, on which the steel pipe 1 is first heated in such a way that possible moisture is removed, after which the outer shell of the steel pipe is blasted for obtaining a clean and new steel surface. According to one embodiment of the invention, the blasted steel pipe l can be led directly into an extrusion tool 3 at a tempe¬ rature, which is essentially below the melting point of plastic masses that are intended to be applied to the steel pipe in the extrusion tool 3. According to another embodiment, the blasted steel pipe 1 is heated to ca. 240°C, preferably by means of IR-radiation, and then led in the heated state through an epoxy coating unit of a pretreatment station 2, on which the epoxy powder is applied to the clean and heated steel surface. The epoxy coated steel pipe 1 is then allowed to cool so that its temperature is considerably below the melting point of plastic masses that are applied to the steel pipe 1 in the next extrusion stage, during which a so-called cross- head extruder 3 is preferably used. In the extrusion tool 3, an adhesive medium 4 as a relatively thin layer as well as at least a relatively thick layer of thermoplast 5 are applied to the steel pipe l. Since the temperature of the steel pipe 1 entering the extrusion tool 3 is below the solidification point of the adhesive medium 4 and the thermoplast 5, the solidification process starts immediately, which results in that all extruded layers solidify relatively quickly and require a considerably shorter cooling bath 6 after the extrusion than in con¬ nection with conventional methods. The first support point of the extruded pipe 1 can thus be located quite near the extrusion tool 3. In this way, the extruded layers quickly reach a solidified state, and a pipe with good centering is obtained, but the adhesion to the steel pipe l or the epoxy surface is relatively low. This is facilitated according to the invention in such a way that the cooled steel pipe 1 coated by means of extrusion is heated in an induction heating unit 7, which by means of high-frequency electromagnetic waves heats the steel pipe 1, from which heat is conducted to the inner section of the extruded layers. In connection with this heating, the innermost layer melts, whereby the entire coating around the steel pipe 1 shrinks and a very good adhesion between the steel pipe and the plastic layers is achieved. A suitable temperature for the steel pipe 1 entering the extrusion tool 3 has proven to be < 100 β C. The extruded thermoplastic layer can be a foamy insulating layer and/or a compact protective layer. When, especially, steel pipes with a thick foamy insulating layer of e.g. polypropylene are coated, the inventive method has proven especially suitable. Another suitable embodiment is a steel pipe coated with an adhesive layer, a relatively thick foam layer and a thinner compact protective cove¬ ring. To be able to reach the desired cooling of the steel pipe 1 before its entry into the extrusion tool 3, especially when an epoxy coating stage is included in the pretreat- ment stage, the process can either be interrupted after the pretreatment stage and steel pipe 1 can be transfer¬ red to a waiting storage, or the distance between the pretreatment station 2 and the extruder 3 must be adjus¬ ted long enough. In connection with the final induction heating, the steel pipe should preferably reach a surface temperature of ca. 160-180°C, which is approximately the same as the melting point of the adhesive layer.

Claims: AMENDED CLAIMS [received by the International Bureau on 29 April 1991 (29.04.91); original claims 1-9 replaced by amended claims 1-7 (2 pages)] 1. A method for coating a steel pipe (1) by extrusion, in which method the steel pipe (1) is pretreated and the pretreated steel pipe is led at a temperature below the solidification point of those plastic materials (4, 5), which are to be extruded, through an extrusion tool (3), wherein it is coated with an adhesive layer (4) and at least one thermoplastic layer (5), after which the coated steel pipe is cooled, and the cooled steel pipe (1) is heated by induction heating (7) for achieving an effective adhesion between the steel pipe and the coating, c h a ¬ r a c t e r i z e d in that a relatively thick insulating layer of foamy thermoplast (5) is applied to the steel pipe (1) in the extrusion tool (3). 2. A method accordning to claim 1, c h a r a c t e r i ¬ z e d in that a compact protective layer is simultaneously applied to the foam layer. 3. A method according to claim 1 or 2, c h a r a c t e ¬ r i z e d in that the pretreatment (2) of the steel pipe (1) comprises warming-up and blasting. 4. A method according to claim 1 or 2, c h a r a c t e ¬ r i z e d in that the pretreatment (2) of the steel pipe (1) comprises warming-up, blasting, heating and epoxy coating. 5. A method according to claim 4, c h a r a c t e r i ¬ z e in that the steel pipe (1) is transferred to a waiting storage after the pretreatment for cooling it down to a desired temperature. 6. A method according to any of the preceding claims 1 to 5, c h a r a c t e r i z e d in that the distance between the pretreatment station (2) and the extrusion tool (3) is adjusted so that a sufficient cooling of the pretreated steel pipe (1) is reached. 7. A method according to claim 5 or 6, c h a r a c t e - r i z e d in that the steel pipe (1) in connection with the final heating reaches a temperature of ca. 160°C to 180°C, which is approximately the same as the melting point of the adhesive layer.

Inventors: ANTELL MATS TOM CHRISTIAN, LAANGNABBA RUNE JANE-ERIK, ANTELL, MATS, TOM, CHRISTIAN, LAANGNABBA, RUNE, JANE-ERIK
Applicants: MUOTEKNO AB OY, OY MUOTEKNO AB
Application year: 1990
Main classification: B29L23
IPC codes: B29C44, B29C47