GB1580065A

GB1580065A - Electrical insulators

Info

Publication number: GB1580065A
Application number: GB3882177A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1976-09-22
Filing date: 1977-09-16
Publication date: 1980-11-26
Also published as: TR20336A; ATA679577A; DE2643197C2; CH632699A5; DE2643197B1; AT361559B; SE7710532L

Abstract

In order to achieve the object of providing an electrically insulating silicone-rubber moulding, in which no appreciable shrinkage of the moulding occurs in spite of the use of silicone rubber, and the moulding times, i.e. the residence times of the elements to be moulded in the moulds, are small, for moulding the mould is inclined through 20 DEG with respect to the vertical. Moulding is performed with a thermally crosslinkable silicone-rubber composition of predetermined viscosity at an injection moulding pressure of 30 to 100 bar. In this process, for crosslinking of the silicone rubber, the mould is heated by 50 to 180 DEG above the room temperature of the composition.

Description

(54) ELECTRICAL INSULATORS (71) We, SIEMENS AKTIENGESELLSCHAFT, a German Company, of Berlin and Munich, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of producing an electrically insulating silicone rubber moulding, having walls of non-uniform thickness, more particularly but not exclusively for use as a cable dividing box.

Plastics-insulated single-core mediumvoltage cables insulated with polyethylene or cross-linked polyethylene and used for 20 kV are increasingly replacing paper-insulated cables, and it is therefore necessary to develop a new fittings technique. Manual manufacture of individual dividing boxes is expensive and time-consuming; the aim is to replace it by mechanical manufacture at the factory.

There are various known methods of producing plastics mouldings.

According to German Offenlegungsschrift No. 24 32 057, a silicone rubber moulding is produced as follows. An initially fluid silicone rubber material containing a curing agent is poured through a filling aperture into a chamber or mould. After the silicone rubber has solidified, the aperture is closed by a plug.

Swiss Patent Specification No. 509 139 describes a process which is described as pressure solidification but is almost pressure-free, in which heated casting resin is poured into a mould heated to the curing temperature; to compensate for shrinkage, additional material is inserted under pressure until the curing process is complete.

German Offenlegungsschrift No.

2,017,506 likewise describes a method of manufacturing plastics mouldings of cast resin, but the cast resin material is introduced under pressure into the mould, the mould is at a higher temperature than the resin material, and the material remains at the pouring pressure until it solidifies in the mould.

The aforementioned methods, however, relate to cast resin, which has relatively low viscosity. Silicone rubber, a substance having considerably higher viscosity, must be used as the starting material in producing very resilient plastics mouldings, e.g. for use as prefabricated cable dividing boxes. The high viscosity makes it difficult to eliminate all bubbles or other cavities, as is necessary during moulding.

An object of the invention, therefore, is to provide a method of producing an electrically insulating moulding in which the moulding does not shrink appreciably even though silicone rubber is used, and in which the moulding time (i.e. the residence time in the mould) can be short (e.g. a few minutes).

According to the present invention, there is provided a method of producing a moulded electrical insulator having walls of nonuniform thickness from a silicone rubber starting material having a viscosity of from 5 to 90 Pascal seconds (ism~2) which material can be cross-linked by heat treatment to give a flexible polymeric material, wherein the starting material is forced under a pressure of from 30 to 100 bars and at ambient temperature into a mould while the mould cavity is held in an oblique orientation at an angle of from 10 to 300 to the vertical, and the mould is heated to a temperature sufficient to cause cross-linking of the starting material.

During the filling of the mould, the mould cavity is held at an angle of from 10 to 30 to the vertical; the preferred angle is one of about 20 to the vertical. It is not necessary for the inclined orientation of the mould cavity to be maintained during the heating step.

A pressure in the range of 30 to 100 bars is used to inject the starting material into the mould. The temperature to which the mould requires to be heated in order to cause crosslinking of the starting material is generally a temperature of 50-180"C above ambient temperature.

The heating of the mould can take place simultaneously with the filling of the mould; it is thus possible to supply the starting material at ambient temperature into a mould which is preheated to the desired temperature. Alternatively, heating of the mould can commence after the starting material has been injected thereinto.

In the method of the invention, vulcanisation begins at the mould walls (surface and core) and spreads gradually into the interior.

As a result, a flow channel, through which air can escape from the mould, remains open until the end of the moulding process, i.e.

until the mould has been completely filled.

Since semi-solid vulcanised layers initially form only on the mould walls and since the material, which is at a considerably lower temperature than the mould, can always flow in freely under pressure, no clots are formed, i.e. the relatively cool, flowing material takes longest to solidify in the injection channel, thus producing a moulding which is free from pores and cavities and has high mechanical and electrical quality. Since the mould is inclined to the vertical, it can efficiently produce even complicated mouldings having undercut portions and wide differences in cross-section, e.g. cable dividing boxes.

Another purpose of inclining the mould is to prevent the material overflowing during injection and thus enclosing air. Since cold material is supplied rapidly at high pressure, no appreciable shrinkage can occur during cross-linking.

The method of the invention can produce very homogeneous, highly resilient silicone rubber mouldings having an excellent surface, for use e.g. as cable dividing boxes. The dividing boxes, which have somewhat smaller internal dimensions, can be drawn over the cable (using a lubricant) and then prestressed thereto, thus reliably preventing the formation of leakage paths.

Preferably, use is made of a silicone rubber substance containing terminal vinyl groups.

This substance greatly improves the resistance of the moulding to initial or subsequent tears. It also somewhat simplifies the manufacturing conditions.

It is particularly advantageous for the mould cavity surfaces to be wetted with a solution of a perfluorocarboxylic acid derivative before the moulding operation. These derivatives greatly reduce the surface tension of the moulded material. so that the material reaches all corners of the mould, even when the shape is complicated. The concentration of the perfluorocarboxylic acid derivative in the wetting solution is preferably 0.5 - 20 ppm. The perfluorocarboxylic acid derivative is preferably a mixture of the ammonium salts of perfluorinated C6 - C8 carboxylic acids, more particularly of perfluorocaprylic acid.

The method according to the invention will be illustrated with reference to two examples. The resulting mouldings are shown in section in Figs. 1 and 2 of the accompanying drawings.

Example 1 (Fig. 1) This Example relates to manufacture of a biconical moulding 10, about 280 mm long, as in Fig. 1. Moulding 10 has a wall thickness between 2 and 20 mm and is used as a dividing box for medium-voltage cables. A deflector 11 is embedded at its base. The operating conditions are as follows.

Amount of material poured in: 350 g catalysed silicone rubber material.

Viscosity of silicone rubber material at room temperature: approx. 20 - 40 Pa s.

Temperature of silicone rubber material: 20 - 25 C (room temperature).

Mould-filling time: approx. 45 sec.

Orientation of mould cavity: approx. 20 to the vertical.

Filling pressure: approx. 60 bars.

Vulcanisation time: approx. 2 minutes after pouring begins.

Mould temperature = 80 to 1000C.

Mould-release time: approx. 3 minutes after pouring begins.

At the top, the mould has a relatively small opening, so that at the end of the filling process the material at the opening finally solidifies and thus seals the mould; the pouring nozzle must be closed with a plug, to maintain the temperature prevailing during cross-linking.

Example 2 (Fig. 2) This relates to the manufacture of a biconical moulding 20, about 260 mm long, as in Fig.2. Moulding 20 has a wall thickness between 2 and 20 mm and is used as a dividing box for medium-voltage cables. In order to increase the length of the leakage paths, it has petticoats 22; these form undercut portions in the mould. A deflector 21 is embedded in its base. The operating conditions are as follows.

Amount of pouring material: 420 g catalysed silicone rubber material.

Viscosity of the catalysed silicone rubber material at room temperature: approx.

27 Pa s.

Temperature of the catalysed silicone rubber material: 20 - 25"C. (room temperature).

Mould temperature: 80 - 100"C.

Mould-filling time: approx. 70 seconds.

Orientation of mould cavity ; approx 20 to the vertical.

Filling pressure: 60 - 80 bars.

Vulcanisation time: approx. 6 minutes after pouring begins.

Mould release time: approx. 7 minutes after pouring begins.

WHAT WE CLAIM IS: 1. A method of producing a moulded electrical insulator having walls of nonuniform thickness from a silicone rubber starting material having a viscosity of from 5 to 90 Pascal seconds (ism~2) which material can be cross-linked by heat treatment to give a flexible polymeric material, wherein the starting material is forced under a pressure of from 30 to 100 bars and at ambient temperature into a mould while the mould cavity is held in an oblique orientation at an angle of from 10 to 300 to the vertical and the mould is heated to a temperature sufficient to cause cross-linking of the starting material.

2. A method according to claim 1, wherein the mould cavity is held at an angle of about 20 to the vertical during injection thereinto of the starting material.

3. A method according to claim 1 or 2, wherein the mould is heated to a temperature 50 to 1800C. above ambient temperature.

4. A method according to claim 1,2 or 3, wherein the silicone rubber material is a substance having terminal vinyl groups.

5. A method according to any preceding claim, wherein the mould cavity surfaces are wetted with a solution of a perfluorocarboxylic acid derivative before injection of the starting material into the mould cavity.

6. A method according to claim 5, wherein said solution contains from 0.5 to 20 parts per million of the perfluorocarboxylic acid derivative.

7. A method according to claim 5 or 6, wherein the perfluorocarboxylic acid derivative comprises a mixture of ammonium salts of perfluorinated carboxylic acids containing from 6 to 8 carbon atoms.

8. A method according to claim 7, wherein the perfluorocarboxylic acid derivative contains the ammonium salt of perfluorocaprylic acid.

9. A method as claimed in claim 1, and substantially as hereinbefore described.

10. An electrical insulator whenever made by a method according to any preced

Claims

ing claim.

11. An electrical insulator as claimed in claim 10, which is a cable dividing box.