When it comes to HV components made from polymeric materials, the possibility of inclusions occurring during production takes on great significance. For example, air bubbles trapped inside molded or cast insulators, become a source of internal partial discharge activity that will severely impact performance.
INMR travels to the German town of Bocholt to visit a manufacturer that has aimed to eliminate all risk of air inclusions by adapting vacuum mixing technology developed for epoxy resins and polyurethanes to the manufacture of insulators made from liquid silicone rubber.
Photo courtesy of Hübers.
This image is not a demonstration of good hydrophobicity but rather an x-ray revealing presence of numerous air inclusions trapped within sheds during molding.
Hübers Verfahrenstechnik( Hübers) is a supplier of a range of automatic pressure gelation( APG) clamping, metering and mixing machines traditionally used to produce items made from epoxy resin or polyurethane( PU). Apart from supplying the automotive sector, electrical insulation components are one of the main areas of application. In fact, the firm has its roots 75 years ago in impregnating windings of motors with resin under vacuum in order to extend their service life.
Hübers grew by developing machinery that allows electrical equipment manufacturers to mix and dose resin with other ingredients such as fillers to produce different types of insulators. However, with the
Hübers saw the opportunity to extend its experience with vacuum technology for epoxy and other resins to the liquid silicone rubber( LSR) now used to manufacture hollow core insulators. growing use of liquid silicone rubber( LSR) insulation across a spectrum of high voltage applications, management saw the opportunity to extend their mixing and dosing technology to this material as well.
Project Sales Engineer, Thomas Schedding, states that eliminating all risk of air inclusions is probably the major advantage of this production technology. However he notes that there are other benefits as well. For example, pre-treatment of the material under vacuum can help reduce the pressure needed during the molding process. This, he notes, offers the benefit of no mold parting line on the finished product as well as reduced flashing between sheds. Flashing is wastage and also requires labour to remove after molding.
Says Schedding,” we are the only ones in this industry who de-gas the LSR before dosing, mixing and molding. This results in a chain of downstream production advantages. There is also the possibility to use less costly aluminum molds instead of ones made from surface-hardened steel and also no longer any need for water to pre-heat the mold, resulting in energy savings.”
Photos: INMR ©
Hübers clamping machines, such as this R & D prototype for liquid silicone rubber, were originally developed for epoxy resin. They are typically horizontal for easier mold cleaning after each shot.
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