1. Field of the Invention
The present invention relates to an apparatus and method to make composite insulators for electric overhead lines.
2. Description of
It is known that in electric overhead transmission lines, the electric conductors are engaged with respective supporting pylons after interposing appropriate insulators.
A so-called "composite" insulator has been widely used in recent years.
Insulators of this kind are described, for example, in the minutes of the "CONFERENCE INTERNATIONALE SUR LES GRANDES RESEAUX ELECTRIQUES" (CIGRE), 1978 15-10, 1980 22-10 and 22-11, 1986 15-12, 1988 15-07; composite insulators are also described in the review "L'ENERGIA ELECTRICA", No. 1, 1983, pages 36-46.
Generally, composite insulators are mainly comprised of a cylindrical central core of elongated form made of a composite material reinforced with fibre glass or the like, and an outer insulating coating of elastomeric material, and are provided on the outer surface thereof with a plurality of uniformly-distributed annular projections which extend in coaxial relation with the central core. In general, the central core has the function of withstanding the mechanical stresses transmitted from the conductor to the pylon, whereas the protecting coating substantially aims at protecting the central core from external agents and preventing possible discharges between the conductor and the pylon.
According to one of the known methods for the achievement of composite insulators, as disclosed in CIGRE 1980, 22-11, page 3, and also described in the French Patent Application No. 2 399 105, the central core is coated with a cylindrical protection sheath, to which the annular projections moulded separately and generally made of the same materials forming the sheath, are applied.
According to a further known method, composite insulators are made by the injection moulding technique.
This technique enables the outer coating together with the respective annular projections to be directly formed on the central core by a single moulding and cross-linking step.
This technique is described for example in the European Patent Application No. 0 120 787 and also in the already mentioned French Patent Application No. 2 399 105.
In greater detail, the central core is enclosed between two movable half-moulds defining a filling housing the shape of which conforms to the shape of the outer coating provided with the annular projections.
Subsequently, elastomeric material in a plastic state is injected into said housing, radially to the central core, in order to carry out the complete filling of the housing. Then, the elastomeric material forming the outer coating and the annular expansions is subjected to cross-linking.
Ultimately, the mould halves are moved apart from each other in order to enable the finished insulator to be withdrawn therefrom.
In the European Patent No. 0 120 787, the presence of supporting rods is provided which are held in contact with the central core during the injection step in order to avoid central core bending as a result of the thrust exerted by the elastomeric material. These rods are retracted at the end of the injection step and before the cross-linking of the polymeric coating.
In accordance with a further method used for the manufacture of said insulators, the outer coating is first made in two separate halves by injection moulding, each half being provided with a semi-cylindrical housing designed to receive the central core. This is achieved by interposing an intermediate plate between two mould halves each reproducing the outer shape of the respective half, said plate being provided, on opposite sides, with semicylindrical projections each reproducing the shape of one half of the central core. Afterwards both mould halves are filled with elastomeric material and before the material cross-linking takes place, the mould halves are opened and the intermediate plate is moved away.
At this point the mould halves are moved close to each other again, upon interposition of the central core therebetween, and the insulator halves formed at the inside of the mould halves are therefore joined together.
When the cross-linking has been completed, the two insulator halves will be firmly joined to each other and to the central core.