Mold heating study proves pressurized water system

Among the many design problems toolmakers face is how to heat and cool a mold for optimum processing of thermoset composites. Toward that end, Westminster Solutions Inc. (Plainfield, Conn.), in partnership with Single Temperature Controls Inc. (Charlotte, N.C.), recently developed a resin transfer molding (RTM) temperature-control technology, using highly pressurized, heated water rather than a conventional system that uses hot oil. In the latter, oil is circulated through channels in the mold, as close to the mold face as possible, to provide evenly distributed heat during cure. Oil is used because processing temperatures can reach nearly 400°F/204°C, far above water’s normal boiling point. But oil-based systems have drawbacks: A large quantity of circulating oil is needed, which in turn requires a large amount of energy to preheat, and a heat exchange unit must be used to cool the oil at the end of the cure cycle. In addition, an oil system has a large equipment footprint with associated operating costs and maintenance requirements, says Mark Ypsilantis, Westminster’s VP of business development.

Known for its injection mold expertise, the company saw an unfilled need when it entered the composite tooling arena several years ago: “We felt there was room for … a better method for thermal management of high-temperature RTM molding,” Ypsilantis recalls. The Westminster/SINGLE closed-loop system pressurizes water to about 230 psi, which increases the water’s boiling point but maintains its liquid state so it can circulate at the target processing temperature. A year-long study by Westminster, which compared oil, electric and pressurized water systems in small- to medium-sized RTM molds made with a variety of tooling materials, demonstrated the benefits of pressurized water over oil and electric as a heat transfer medium. Specifically, water’s ability to absorb and release large amounts of heat due to its high thermal conductivity and specific heat allowed overall cycles to be reduced by as much as 35 percent, especially during the ramp down cycle of a cure, says Ypsilantis. Plus the system cost is reduced because less energy and equipment are needed and the mold design can be simplified.