Method for the Manufacture of Plastic Insulators With Screens for Indoor and Outdoor Use

It is known that insulators can be manufactured by the casting method with more or less filled plastic molding compounds on the basis of aromatic, cycloaliphatic or heterocyclic epoxy resins or also rubber systems such as silicone rubber (U.S. Pat. No. 3,484,983). For this mold casting, the compounds can be adjusted rigid or flexible. Frequently, plastic materials, particularly polytetrafluoroethylene (PTFE), are processed into insulators by the sintering method (British Pat. No. 1,066,209) and epoxy systems are frequently processed by the centrifugal casting method. For radiation-cross-linked silicone rubber, the assembly type of construction is used predominantly, with pre-fabricated individual screen elements being fabricated by the molding method.

Long rods are usually made by covering fiberglass-reinforced rods by the casting, sintering or assembly method. The rod is placed in a mold for this purpose and cast or sintered all around, or, alternatively, screen elements may be molded first and then drawn upon a core and joined or cemented to the latter by means of an adhesion agent.

These known methods have considerable disadvantages, especially for quantity production. Thus, there are limits due to the molds in the fabrication by the casting method. Furthermore, most of the available materials are not so flexible that they can follow, for example, in the case of suspension insulators, the elongation of the rod under tensile stress occurring in line operation. Thus, they cannot be applied to the fiberglass-reinforced core directly without the interposition of an elastic layer. It is a further disadvantage, for example, that for casting larger quantities, several molds are required. One most work in parallel operations, usually with expensive tools.

It is an object of the invention to manufacture plastic insulators with protective screens for indoor and outdoor use, especially support and suspension insulators, in quantity production and in continuous operation, using, if possible, only a single mold.

According to the invention, this problem is solved by the provision that molding compounds which are injection-moldable, electrically high-quality, hydrophobic, leakage current-resistant, erosion and temperature-resistant, and optionally highly elastic in the .+-. temperature range, and possessed of high notch impact resistance, are processed by injection-molding or transfer molding. The plastic molding compounds can be provided with any desired inorganic and/or organic fillers, where the filler content can be high, preferably 30 to 60% by weight, referred to the total weight. The molding compounds are processed in conventional plunger or worm-type injection molding machines. To manufacture the molded bodies, the thermoplastic, thermosetting-plastic or elastomeric molding compounds (with or without filler) are fed, according to the invention, to the injection unit and are plasticized while being heated. Subsequently, the plasticized compound is injected under pressure into a mold. The moldings obtained in this manner are free of voids and pores. The manufacturing cycles are very short for high production rates and are in the order of a few minutes. Through appropriate choice of the material, moldings with higher aging, oil, ozone and climatic resistance and also, specifically, with special property patterns, such as self-quenching or leakage-current resistance, can be manufactured.

It is a very special advantage of the procedure according to the invention that by injection molding partial sections onto the first-fabricated part, insulators of any desired length can be produced, independent of the tool.

According to the invention, the plastic molding compounds can be plasticized at temperatures of about 150.degree. to 280.degree. C. and injected under pressure into molds with temperatures of .ltoreq.100.degree. C. or, alternatively, plasticized at temperatures of .ltoreq.100.degree. C. and injected under pressure into hot molds, preferably into molds with a temperature of .gtoreq.140.degree. C.

For special applications, for example, when using thermoplastics and rubber systems for long rods, it has been found advantageous if the surface of the part to be injection-coated, such as the surface of GFK rods, is pre-treated, e.g., degreased or sand-blasted and/or if an adhesion agent or an adhesive layer is applied.

According to the method of the invention, single part as well as repeatedly injected-on insulators can be manufactured in plunger or worm-type injection molding machines. It is particularly suitable for manufacturing support and suspension insulators with screens, where entire units of one-piece insulators with screens or multiple-part units with screens can be produced; one-piece, for example, in the medium-voltage range and multiple-part for high-voltage series, for example, .gtoreq.110 kV. With this procedure, suspension insulators are produced to advantage. A degreased, sanded GFK rod provided with an adhesion agent is placed in a tempered mold and injection-molded all around.

Molding compounds suitable for carrying out the method according to the invention are injection-moldable thermosetting plastics, thermoplastics and elastomers, such as those based on aromatic, cycloaliphatic and heterocyclic epoxy resins, polyurethane (PUR) and polyester resins, polytetrafluoro ethylene (PTFE), polypropylene (PP) and rubber-modified polyolefins, such as ethylene-propylene monomer rubber (EPM) and ethylene-propylene-diene monomer rubber (EPDM) and their mixtures as well as mixtures with other molding compounds of duroplastic, thermoplastic and elastomeric nature.

The molding compounds may contain softeners, pigments, dyes, mold release agents, and the like. As stretchers and fillers may also be used, for example, calcite, rutile, quartz powder, kaolin, talc, aluminum oxide trihydrate, sulfates and their mixtures. Good properties were achieved with a high filler content, especially silanized quartz powder.

Especially well suited was found to be a flowable granulate of a mixture of 20 to 30% by weight polypropylene (PP), 50 to 60% by weight ethylene-propylene-diene monomer rubber (EPDM) and 20 to 30% by weight silanized quartz powder.

For the manufacture of support insulators, predominantly low-viscosity thermoplastic systems which can be processed at pressures of .ltoreq.50 bar have also proven themselves.

The subsequent fastening of fittings is performed in the usual manner as, for example, in long rods by wedging the rod ends into the conical interior of the fittings or by other practiced coupling methods such as prior placement of fastener bushings and cementing the insulator ends into forked caps.

To explain the invention in greater detail and to make clear the advance achieved thereby, reference is made to the drawing which illustrates the situation by means of embodiment examples.

In FIGS. 1 and 2, a support insulator is shown and in FIGS. 3 and 4, a suspension insulator in cross section. The left-hand side shows the respective insulator and the right-hand side the production design.

FIGS. 1 and 2 show support insulators such as are used indoors, for example, and which consist in the known manner of a base 10, the support insulator head 11 and the ribs 12 arranged therebetween. In a support insulator according to FIG. 1, the insulating material is made by a single injection at 1 and according to FIG. 2, by means of two injections at 7 and 8. The cementing or joining point is designated as the numeral 3.

In the suspension insulators shown in FIGS. 3 and 4, which are made by injection molding around GFK rods with the corresponding insulating material, the plastic is again designated with 4, the adhesion agent with 5 and the fiber-glass-reinforced core with 6. The suspension insulator as per FIG. 3 is made by a single injection at 14 and the one as per FIG. 4 by a first injection at 15 and a second injection at 16. The cementing line is designated with 17.