Ready to Generate - Wacker Chemie AG

Ready to Generate

Gruppenbild mit Messwandlern (v. l.): Rainer Röder from Gardy, a consultancy; Martin Boss, vice president and technical head of Pfiffner; with Dr. Hans-Jörg Winter and Johan Dewitte from WACKER.

As the saying goes, “the devil is in the detail.” The viscosity of the liquid silicone rubber must be low enough to allow casting at atmospheric pressure without creating any bubbles in the mold. The rubber must also have an adequate processing life, yet still cure to a silicone elastomer as fast as possible in the heated mold at temperatures below 115 °C, as otherwise the GRP pipe would soften and buckle.

Conventional liquid silicone rubbers are unsuitable. “They are far too viscous, must be injected at high pressure and entail expensive injection molds,” explains Dr. Winter. In principle, a different rubber from the WACKER portfolio could be used, namely POWERSIL® 600, which cures at room temperature. It possesses the requisite low viscosity and cures to an elastomer which has excellent technical properties. “The only problem is that the mold needs to be cooled before each filling operation, as otherwise curing would start before it was completely filled,” adds the materials expert. This circumstance had the makers of the hollow-core insulators looking for an alternative.

The breakthrough for the pouring method came in the form of a two-component rubber launched by WACKER in 2000 under the name POWERSIL® XLR® 630. “This product embodies all the advantages of the two different rubber systems,” says Dr. Winter. The two-component, extra-low-viscosity liquid silicone rubber – XLR® stands for extra-liquid rubber – became the product of choice for low-pressure diecasting of the protective silicone and was instrumental in having the method adopted by the industry. Röder says that 80 percent of composite hollow-core insulators in Europe are now made by low-pressure diecasting. “Nearly all companies that have installed production lines for composite hollow-core insulators in recent years have opted for this process and use XLR® silicone from WACKER”, explains the sector expert.

Birthplace of the Method

Low-pressure casting was pioneered by TE Connectivity – formerly Cellpack AG – at its Wohlen site in Switzerland. The company is a member of the TE Connectivity Group, which is headquartered in Schaffhausen, Switzerland. “This site is where silicone shielding was first produced by low-pressure diecasting – it was here that the method was brought to production readiness and first used for industrial shielding in 1988,” comments Röde, a former managing director at the site. Thanks to its plant in Wohlen, TE Connectivity has become the world-leading maker of composite insulators, sold under the name Axicom. The thousands of composite hollow insulators for high-voltage equipment that are made yearly are all produced by low-pressure diecasting. For more than ten years, we have used only POWERSIL® XLR® 630 to sheathe the pipes. With this silicone, we get flawless, cost-effective production results,” says Dr. Robert Strobl, market director of high-voltage products at TE Connectivity.

Not only does the company apply the silicone shielding, it also winds the GRP pipes itself. Over the years, it has accumulated a vast store of expertise. This is hardly surprising, given that the Wohlen site has so far manufactured several hundred thousand composite hollow-core insulators for voltages ranging from 72 kilovolts to 1.1 megavolts. Dr. Strobl believes that collaboration with WACKER has been a key success factor: “The technical service engineers from WACKER were always on hand to resolve our technical queries and problems. What's more, the silicone has always been of the same consistently high quality over the years.”