Prototyping propels propeller designs

Waukegan, IL--Prototypers at Outboard Marine Corp. (OMC) excel in producing parts quickly and cost-effectively. The prototyped parts range from a myriad of propeller designs to 6-lb engine housings to 5-ft-long boat hull sections. Selecting the right material for each project makes the job go faster and smoother.

"We're constantly searching for materials that are easy to work with and will help us quickly produce high-quality prototypes for concept verification, fit and function, testing, and engineering evaluation," says Richard Brnot, OMC prototype team coordinator. He found such a material in Ren Shape(reg) polyethylene modeling, board, and epoxy laminating systems supplied by Ciba Specialty Chemicals Performance Polymers (East Lansing, MI).

Propeller design provides an example, especially since the new propellers must match varying engine horsepower and pitch requirements. In the past, to alter propeller designs, each of four blades was hand-built from wood or clay. These patterns formed a negative polyurethane mold in which multiple wax pieces could be cast and then glued to complete a foundry pattern for investment casting. This task normally took about six weeks.

Today, OMC technicians machine the master shape of a single blade from Ciba's Ren Shape 5168 board that serves as a fixture as well as a mold. A standard wax hub is then set in place, the fixture/mold moved in position to produce the first blade, and wax injected into the mold and directly onto the hub. The hot-wax bonds to the existing hub as it cools.

After about 30 min, the wax hardens, permitting the fixture to be repositioned to form the next blade. With this process, prototypers can move from concept to prototype wax propeller in one week. Investment-cast, stainless-steel parts are available in another week, for a total-part completion time of two weeks.

"The Ren Shape board has a relatively high density that allows wax blades to release easily from the fixture," Brnot explains. "Machining the master pattern for blades also increases design accuracy, compared with masters built by hand."