Coming of age?

As the composites industry matures, manufacturers continue their search for technology that will maximize production and minimize cost. A growing number of resin infusion shops — including some aerospace manufacturers — are finding this combination in reusable vacuum bag (RVB) technology. Proponents say these stretchy membranes can replace disposable bagging films and even rigid countermolds, and they offer molders a wealth of advantages, including improved part-to-part quality and greater shop safety and cleanliness. No less important, they provide a way to cut down on consumables and reduce a shop’s waste stream. RVBs do, however, require capital expenditure and offer challenges that must be overcome for proper implementation, and they aren’t always the right way to go in every part process situation.

“As with any process tool, there’s a list of factors to consider,” says Rich Rydin of RVB system provider SR Composites LLC (Henderson, Nev.). “Each RVB system has pros and cons, and fabricators should consider the relative ease of building and using the RVB, its tolerance for a specific resin chemistry and processing temperature, and, of course, the aggregate cost per square foot.”RVBs are made with either synthetic or natural rubbers. The most prevalent, synthetics, include a broad range of silicones, but other elastomeric materials have been used in vacuum membranes, including (but not covered here) forms of polyurethane, ethylene/propylene, polysulfide, fluorosilicone, nitrile and chloroprene.Silicone is a polymerized siloxane, or polysiloxane, an inorganic/organic polymer. That is, it contains an inorganic silicon/oxygen backbone to which organic groups (methyl, ethyl or phenyl), derived from petroleum distillates, are attached. Formulated to exhibit rubber-like elongation, silicone rubber is a common RVB material. To be useful as RVB materials, silicones require in-situ polymerization via the addition of a catalyst — typically platinum, but also tin or water — and their cure can vary with environmental conditions.

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