A METHOD FOR JOINING STRUCTURAL MEMBERS WITH THE USE OF FIXING MEANS OF SYNTHETIC THERMOPLASTIC RESIN, A METHOD FOR THEIR MANUFACTURE, AND A METHOD OF USE

Abstract: The joining of structural members of plastics material or of a structural member of plastics material to metal members, particularly component parts of a motor vehicle body, is effected with the use of fixing means of thermoplastic material incorporated in the respective structural members and having a concentration of conductive-magnetic fillers in their coupling surfaces, and by the fusion of the surface zones by inductive heating; the fixing means are made in a mould by subjecting the molten thermoplastic material to which the conductive-magnetic fillers have previously been added to a magnetic field generated within the mould and having an intensity and direction adapted to cause the migration of the fillers towards the surface of the moulded members.

Description: The present invention relates to the assembly of structural members of plastics material either together or with metal components, particularly component parts of motor vehicle bodies. More particularly, the invention concerns a method for joining structural members of the type mentioned above, provided with fixing means of thermoplastic material having respective surfaces intended to cooperate with each other with form coupling, in which the final joining is achieved by fusion of the thermoplastic material constituting the respective coupling surfaces. The methods of assembly by fusion proposed until now have been shown to be difficult to carry out and in any case uneconomic. Methods have also been proposed for joining structural members of plastics material with the use of adhesives, which harden more or less quickly, or screw members. These methods are not sufficiently reliable, however, and in any case cannot be adapted to equipment currently in use for the industrial assembly of conventional structural members by robotised welding. FR-A-1 493 562 describes the welding of tubings by means of a sleeve which includes, in contact with the external surface of the tubings, bands of thermoplastic material including metal powder which allow to carry out heating by induction. The metallic powder is incorporated in a film of material to be placed in a groove between two elements to be connected together. Such film does not have per se the function of joining two pieces. Such a function is achieved only as a result of the melting of the plastic material. According to a different embodiment suggested by FR-A-1 493 562, the metal powder can be provided in the material of at least one of the tubings to be assembled and preferably concentrated at the time of moulding nearer the surface to be welded, e.g. by means of a magnet. This second embodiment discloses the provision of metal powder in the material of an element to be connected to a tube wherein the metal powder is concentrated at the time of moulding near the surface to be connected to the tube by means of a magnet. Also in this case there is no disclosure of fixing means having coupling parts of thermoplastic material. The primary object of the present invention is to provide a method which, by overcoming the disadvantages of the solutions proposed up to now, enables structural members to be assembled quickly and economically, thus rendering the operation industrial and automated. The subject of the present invention is a method of the above described kind comprising the steps of providing fixing means consisting of resilient joint fasteners having a concentration of metal fillers of ferromagnetic material, in at least the surface portions of said coupling parts, said fasteners having been obtained by moulding in a mould for thermoplastic material, with the use of a thermoplastic polymer composition including said metal fillers and by concentrating said fillers in the surface portions of said coupling parts by subjecting the thermoplastic material while molten within the mould to a magnetic field having an intensity and direction arranged to cause the migration of the fillers to the surface portions of the coupling parts, incorporating said fasteners In the structural members intended to be connected together, assembling the structural members with the coupling parts of said fasteners in position of mutual coupling, and bringing the thermoplastic material of the coupling parts of said fasteners in said surface portions to the melting point by inductive heating of said portions. The fixing means are incorporated in the structural members intended to be connected together and are made by moulding, preferably injection moulding, with the use of a thermoplastic polymeric composition including the conductive fillers. The thermoplastic material constituting the fixing means may conveniently be selected from the group consisting of acetal, polyamide and polyester resins. In the method according to the invention the fixing means, include fillers of ferromagnetic material and their concentration in the surface portions of the fixing means is obtained by subjecting the molten thermoplastic material including the ferromagnetic fillers, within the mould and during injection, to an orientable magnetic field having an intensity and direction such as to cause to the ferromagnetic fillers to migrate into the surface portions of the coupling surfaces of the fixing means themselves. The fillers used may be in the form of fibres, powders or particles and are preferably constituted by powdered iron derivatives. It is naturally desirable for the electrically-conductive ferromagnetic fillers to be concentrated in the surface portions of the coupling surfaces of the fixing means, so as to obtain surface portions with high conductivities which may easily be melted by inductive heating. The concentration of the conductive fillers adjacent the surface prevents the complete fusion of the fixing means. The above-described method for producing the fixing means does not harm the basic properties of the plastics matrix. Synthetic-thermoplastic-resin-based fixing means are obtained, for use in the method according to the invention, which have having a concentration of conductive ferromagnetic fillers, in the surface portions of their coupling surfaces such as to render the mass of material in the surface portions electrically conductive. The fixing means are of the resilient-joint (fastener) type. In order to assemble structural members provided with the fixing means, the latter are positioned in coupling or fastening relationship and are exposed to high-frequency radiation, preferably in the radio frequency range, to cause the fusion of the contact zones and thus cause their permanent interlocking after their subsequent solidification. Robotised equipment of the type currently in use for welding metal parts may be used for the radio-frequency irradiation. The mechanical strength of the joints achieved may be checked by non-destructive automatic diagnostic systems. The method of the invention may be used for the connection of structural members of plastics material or for connecting a member of plastics material to metal components. In the latter case, the fixing means may be incorporated as inserts in the metal component.

Claims: A method for the assembly of structural members of plastic material, constituting component parts of a motor vehicle body by means of resilient joint fasteners having coupling parts of thermoplastic material comprising the steps of: providing resilient joint fasteners having a concentration of metal fillers of ferromagnetic material, in at least the surface portions of said coupling parts, said fasteners having been obtained by moulding in a mould for thermoplastic material, with the use of a thermoplastic polymer composition including said metal fillers and by concentrating said fillers in the surface portions of said coupling parts by subjecting the thermoplastic material while molten within the mould to a magnetic field having an intensity and direction arranged to cause the migration of the fillers to the surface portions of the coupling parts, incorporating said fasteners in the structural members intended to be connected together, assembling the structural members with the coupling parts of said fasteners in position of mutual coupling, and bringing the thermoplastic material of the coupling parts of said fasteners in said surface portions to the melting point by inductive heating of said portions. A method according to claim 1, wherein the fillers are in the form of fibres, powders or particles. A method according to claim 1, wherein the fillers are powdered iron derivatives. A method according to claim 1, wherein the thermoplastic material constituting the fixing means is selected from the group of acetal, polyamide and polyester resins.

Inventors: DA RE, MARIO
Applicants: FIAT AUTO S.P.A.
Application year: 1987
Main classification: B29C65
IPC codes: B29C65, B29C70