EP0035947A1 - Process for applying a synthetic coating onto a body, device for carrying it out and body so obtained - Google Patents

Abstract

The invention relates to a process for applying a synthetic coating onto the interior cylindrical surface of a hollow metal body, to the device for carrying out this process and to the coated hollow body obtained by means of this process. It is characterised in that the device for carrying out the process comprises a centrifuging bench (16) provided with a heat source (17), together with means of which some (18) are for regulating the speed of centrifuging and the others (19) are for initiating and stopping the action of the source (17). Application to hollow bodies such as tubular elements intended to form, in isolation, guide pieces, such as for example jack bodies, or after being assembled together a pipeline for conveying gas, more or less viscous liquids or even solids. <IMAGE>

Description

The invention relates to a method of fixing a plastic coating on the inner cylindrical surface of a hollow metal body, to the device for carrying out this method and to the coated hollow body obtained using this process. It applies more particularly but not exclusively to hollow bodies such as tubular elements, intended to form, in isolation, guide parts, such as for example cylinder bodies, or, after assembly between them, a pipe for the transport of gases, more or less viscous liquids or even solids.

In its various applications, the body must have, at least internally, on the one hand, a perfectly smooth surface to promote sliding and, on the other hand, sufficient abrasion resistance to avoid an early replacement of the body. For this purpose, it is known to fix, inside the body of the plastic material, such as for example polyamide "11" better known under the brand "RILSAN" and this, in a layer at the same time most regular and as thick as possible, in order to offer both a sufficient geometric correction to avoid subsequent machining, as well as a large reserve of material, in anticipation of wear by abrasion.

Before fixing, these plastics generally come in a pulverulent form, with a very fine particle size and for example of twenty to two hundred microns, so that the mass of the solid particles flows practically like a fluid. For the internal coatings of tubes, after preparation of the surfaces or even after application of a layer of primary material, various fixing methods are currently used: According to a first method, the metal part to be coated is firstly heated in an oven and then immersed in the plastic powder contained in a tank, which powder is kept in suspension by air. On contact with hot metal surfaces, the powder melts and thus forms a deposit whose thickness, which varies from two hundred fifty to five hundred microns, depends on the temperature of the part and the duration of its retention in the powder.

In a second fixing method, the tube to be coated is heated by an induction coil after being inserted in a circuit in which a flow of powder-charged air is established. As above, on contact with the heated tube, the powder melts and forms a deposit with a thickness of one hundred and fifty to two hundred and fifty microns.

In a third method, the powder is suspended in water and this solution is applied with a paint gun which is carried by a rod around which the tube is rotated and this more or less quickly depending on the thickness of the desired film. Then, the tube is heated first slowly, to allow the elimination of water and then quickly to cause melting, before fixing by cooling a film of fifty to three hundred microns of plastic material.

If they are more than enough for certain uses, the thicknesses permitted by the three aforementioned processes are manifestly insufficient for many applications such as, for example, for pipelines, for the hydraulic transport of solids and whose desired service life is about twenty years.

Indeed, to resist the corresponding abrasion, the coating should have a thickness of at least six millimeters.

To approach this result, a process akin to rotary gravity molding has been developed.

According to this method, the tube to be coated, the axis of which is horizontal, rests on a support which causes it to rotate about its axis, and this at low speed; whole, for example four meters per minute.

In this still cold tube, pour the dose of powder just necessary to make the coating layer. Due to its natural fluidity, the powder collects in the lower cylindrical segment of the tube under which is provided a heating means such as a gas rail, or several for large diameter tubes.

After putting this heating into service, in contact with the heated tube, the powder dose melts over a certain thickness. By gravity, the plastic, both that thus melted and that still solid, tends to remain in the lower cylindrical segment, despite the rotation of the tube.

Therefore, on the inner wall thereof, this plastic material only slides except for a film which by adhesion remains attached to said wall.

It is this film which, when it solidifies, will constitute the coating which unfortunately hardly reaches several millimeters in thickness and which moreover the surface condition is irregular.

A result which the invention aims to obtain is the fixing on the inner cylindrical surface of a hollow metallic body, of a coating of plastic material on the one hand, having both a regular surface condition and a perfect geometric correction , and on the other hand, which can reach a very large thickness.

• - une première phase dans laquelle la centrifugation s'opère à froid pour déjà bien répartir concentriquement la matière encore solide en une couche régulière,
• - une seconde phase pendant laquelle, outre la centrifugation par mise en service d'un moyen de chauffage, on opère une montée régulière de la température du corps jusqu'à fusion complète, à la place où elle se trouve en vertu de la répartition précitée, de toute la matière plastique quelle que soit l'épaisseur de la couche qu'elle forme,
• - une dernière phase pendant laquelle l'action de chauffage est interrompue, la centrifugation quant à elle se poursuivant jusqu'au refroidissement et donc le durcissement et la fixation de la matière plastique a la place où elle a été fondue dans la phase précédente.

To this end, it relates to a process of the type mentioned above, in particular characterized in that this body is subjected to centrifugation in three successive phases between which the centrifugation action continues and of which:

• - a first phase in which the centrifugation takes place cold so as to already distribute the still solid material concentrically in an even layer,
• a second phase during which, in addition to centrifugation by putting a heating means into service, a regular rise in the temperature of the body takes place until complete melting, in the place where it is found by virtue of the above-mentioned distribution , of all the plastic, whatever the thickness of the layer it forms,
• - a last phase during which the heating action is interrupted, centrifugation meanwhile continuing until cooling and therefore the hardening and fixing of the plastic in the place where it was melted in the previous phase.

• - figure 1 : la schéma bloc du procédé,
• - figure 2 : vus en coupe, le corps creux et le dispositif de mise en oeuvre de ce procédé.

It also relates to the device for implementing this process and the bodies thus obtained using it. The invention will be better understood with the aid of the description given below, by way of nonlimiting example with regard to the attached drawing, which represents:

• - Figure 1: the block diagram of the process,
• - Figure 2: seen in section, the hollow body and the device for implementing this method.

As in the known methods, on the one hand, the gross hollow body 1 passes, first of all, to a station 2 for preparing the surfaces ensuring the degreasing and the mechanical elimination of the surface oxides and on the other hand, this station 2 of surface preparation is optionally followed, and this depending on the nature of the plastic which must be fixed, by a station 3 for applying a so-called "primary" under layer intended to strengthen the bonding of this plastic.

Then, into the hollow body thus prepared 4 and not yet heated, at station 5 is introduced the dose of solid particles of plastic material 6, in powder or granules, just necessary for obtaining the desired coating and whose volume is previously calculated. This powder can then be distributed throughout the body by a squeegee.

According to an essential characteristic of the invention, this body 7 (then prepared and filled with powder) passes to a centrifugation station 8, in which there are three successive phases 9, 10, 11 between which the centrifugation action continues. In a first phase 9, the centrifugation takes place cold, to already distribute concentrically the material still solid in an even layer.

At the end of this first phase 9, the heating action is triggered 12. Then begins the second phase 10.

In this second centrifugation phase 10, a regular rise in the temperature of the body takes place until complete melting, in the place where it is by virtue of the aforementioned distribution, of all the plastic material regardless of the thickness of the layer which it forms.

Note that due to its distribution over the periphery of the tube, this thickness remains small compared to that which the material would have if it were concentrated in the lower cylindrical segment. Heat conduction is therefore ensured.

At this time, the heating action is stopped 13 and we then enter the third and last phase 11.

In this last phase 11 of centrifugation, the cooling is carried out and therefore the hardening and fixing of the plastic material in the place where it was melted in the previous phase after which the hollow body 14 is finished as regards its operations for fixing the coating 15.

• - il élimine les défauts tels que les soufflures,
• - il augmente la compacité du revêtement,
• - il conduit à un état de surface parfaitement lisse,
• - il corrige automatiquement tout éventuel défaut de concentri- cité du corps brut préparé,
• - il permet surtout de fixer un revêtement très épais et par exemple de l'ordre de onze millimètres.

This method of fixing the plastic material has many advantages compared to a simple rotary and gravity molding:

• - it eliminates faults such as blowing,
• - it increases the compactness of the coating,
• - it leads to a perfectly smooth surface state,
• - it automatically corrects any possible concentricity of the prepared gross body,
• - it especially allows to fix a very thick coating and for example of the order of eleven millimeters.

d représente la densité de la matière plastique choisie et l'accélération due à la pesanteur.The centrifugation speed "N", expressed in revolutions per minute, is determined according to the size, expressed in meters, of the interior "r" and exterior "R" radii of the coating and according to the pressure "P", expressed in bars , which we want to print with plastic grains by said centrifugation, from the known relationship:

d represents the density of the plastic chosen and the acceleration due to gravity.

The heating temperature is obviously determined as a function of the characteristics of the plastic to be fix to be certain of reaching the melting point but also not to exceed the temperature from which the molten plastic material begins to agitate.

The device for implementing this process comprises a centrifuge bench 16, provided with a heat source 17 as well as means, some 18 for adjusting the centrifugation speed, the others 19 for switching on and off. heating or even intensity adjustment.

For example, this device will include a table 20 with which are associated drums or a series of rollers 21, 22 on which the body 7 rests, then formed from the raw body 1 having simply received the dose of powder 6.

The body 7 is held on these supports 21, 22 by at least one drum or a series of pressure rollers 23, which can be eclipsed for mounting and dismounting the bodies 7 to be coated.

At least one of these supports 21, 22 and / or pressure means 23 is rotated and, by adhesion to the body 7, communicates to the latter its own rotational movement. For this purpose, the table 20 is associated with a chair 24 supporting a motor 25 grouped with a variable speed drive 26 whose output shaft 27 is linked in rotation to the shaft 28, of the drum or of the series of rollers 22 , via a transmission 29 of any known type.

The heating can also be of any known type. In an exemplary embodiment, it comprises, running under the body between its supports 21, 22, a gas ramp 30 or more 30, 31 depending on the diameter of the body to be coated.

The assembly is housed under a removable casing 32 preferably lined internally with an insulating material 33 such as asbestos to both retain the heat given off by the heating and protect the environment against any projection.

In the chamber 34, delimited by the casing 32 and the table 20, a temperature control probe 35 can be immersed, as well as means for forced cooling of the body during the last phase.

Claims (3)

1. Method for fixing a coating of hot-melt plastic material on the duly prepared inner cylindrical surface, of a hollow metallic body, this process being CHARACTERIZED in that it is in the cold body and whatever it is at a temperature significantly lower than that at which the plastic material melts, this material is introduced in the form of solid particles in an amount just necessary to obtain the coating of the desired thickness, the volume and therefore the weight are previously determinable then in that this body is subjected to centrifugation (8) in three successive phases (9, 10, 11) between which the centrifugation action continues and of which: - a phase (9) of distribution of the material (6) still solid in a regular layer, in which phase the centrifugation also takes place cold to allow the material, still therefore in solid particles, the possibility of flowing in order to already distribute well concentrically, - A melting phase (10), during which, by commissioning ( ₁ 2) of a heating means, there is a regular rise in the temperature of the body (1) to a temperature just sufficient to obtain the fusion, complete and in the place where it is by virtue of the aforementioned distribution, of all the plastic material, regardless of the thickness of the layer that it forms, - A cooling phase (11) during which the heating action is interrupted (13) while the centrifugation continues until cooling and therefore hardening and fixing of the plastic in the place where it was melted in the previous phase. 2. Device for implementing the method according to claim 1, characterized in that it comprises a centrifuge bench (16) provided with a heat source (17), as well as means one (18) of adjustment the centrifugation speed the other (19) triggering of the action of the heat source (17), after being cold, the powdered material has been distributed and then stopping this action after melting said material. 3. Hollow body, characterized in that, on its internal cylindrical surface of the hot-melt plastic material is fixed by the method according to claim 1.

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