Low-pressure vacuum molding challenges spray-up rigidizing methods.

Producers of bathtubs, spas, and various transportation and construction products now will have a "clean" process for rigidizing thermoformed thermoplastic sheet. As an alternative to traditional FRP spray-up, a new low-pressure vacuum molding approach is offered by Merva Systems Inc., Burlington, Ontario, a joint venture between Laydon Industries of Burlington and Stauch GmbH of Germany.

The method involves low-pressure matched-die molding of thermoset FRP composite in a press operated entirely by vacuum. (These presses were formerly offered by Merva-Systems Corp. in Bridgeport, Conn. See PT, May '93, p. 25).

Brian P. Layfield, president of Merva Systems, says the low-pressure technology can allow FRP fabricators to better control their process while boosting daily production output and offering greater part-to-part accuracy and consistency. As compared with chopper-gun spray-up, the vacuum-molding process reportedly enables molders to more accurately control and document their process--an important consideration for processors that are seeking ISO-9000 certification or are subject to customer quality audits.

Thomas Stauch, president of Stauch GmbH, says the Merva system, on average, utilizes about 80% less electrical energy than a comparable hydraulic press. Merva officials say their system also saves material, owing to a scrap rate of about 10% compared with spray-up scrap rates of 18-25%. He says vacuum molding also provides inherent environmental advantages. A closed-mold process means less exposure to styrene fumes, and the press has a built-in filter to extract styrene when it evacuates the mold. Compared with hydraulic presses, the vacuum system generates no oil leaks, and eliminates the need to dispose of used hydraulic oil.

Platen length of Merva vacuum presses can range from 2 to 20 ft. All presses have 40-in. stroke and 160-in. height. Pressing force, supplied completely by vacuum, ranges from 3.4 to 28.3 tons. The machines are designed in Germany and built by Merva in Canada.

Company executives say virtually every press is custom built. Base prices range from $80,000 to $280,000. Safety options, computer controls, and special tooling capabilities are available. The company aims to deliver entire turnkey packages with molds and controls.

HOW IT WORKS

Merva officials recently provided a demonstration for tub and spa makers of a typical 25-min molding cycle for fabricating an acrylic-skinned bathtub. Sheets of continuous glass mat were hand laid in the bottom mold cavity, then covered with liquid polyester resin dispensed by a manually operated two-component metering system. Next, a thermoformed 125-mil-thick acrylic shell was placed in the mold cavity, covering the resin and glass mat. The male tool was lowered, fully closing the press. This demonstration used ambient-temperature curing, and the cycle was timed by Siemens computer controls on the press. Once cured, the part was loosened via compressed air and lifted out of the mold. In an actual production setting, operators could be preparing another part on a second machine while the initial part was curing.

Composite tooling for the Merva system can be made of polyester, epoxy, or vinyl ester, although nickel-shell tooling also can be employed. For faster cures, electric heater cloths can be added to the plastic tools, while the nickel-shell tools may incorporate water heating.

So far this year, at least two U.S. firms have begun to use the Merva technology. Airshield Corp., a manufacturer of automotive parts, recently initiated production with two Merva presses at its facility in Brownsville, Texas. Airshield will mold components for sleeper compartments of tractor-trailer truck cabs. The second firm using Merva presses is an unidentified California molder that supplies parts to the construction industry.