Direct long-fiber composite parts are replacing machined aluminum
for hatch covers on the amphibious version
of the Quest Kodiak, a single-engine turboprop, backcountry utility aircraft
manufactured in Sand Point, ID.
"The aluminum parts were too heavy while these new ones weigh less than two pounds," says Dan Garrett, the Wipaire R&D engineer in charge of the project. "Also, they were subject to dents and bending, and they cost too much - approximately five times the cost of the composite parts." Corrosion resistance and ease of replacement were also issues in moving from aluminum to thermoplastic composite.
The 14 x16 inch hatch covers are made with a new technology called Pushtrusion D-GMT. Hot charges of metered, long fiber-reinforced polypropylene compound are transferred to a compression press and immediately pressed into finished parts, eliminating glass mat technology (GMT) sheet typically used in compression molding.
"The Pushtrusion process gives processors the flexibility to customize their specific formula by fiber loading, polymer type and color," says Steve Bowen, president of PlastiComp, the system developer. "They are not limited to standard GMT sheet offerings." Other thermoplastics that can be used in the system include polyamide, thermoplastic polyurethane and PBT-type polyester. Available fiber systems include E-glass, S-glass and carbon.
Garrett says he considered a thermoset sheet molding compound (SMC) with long fibers but was concerned with high material and development costs. Thermosets require curing cycles and are labor-intensive.
The hatch is molded in a precolored, black pigmented, UV-stabilized material and is then painted with a black non-skid coating on its exposed surface to match the upper, walked-upon deck surface of the float. Foot traffic dictated sufficient load bearing stiffness as a primary criterion in the design.
The exposed upper surface of the hatch cover is mostly smooth, with a channel for a sealing gasket. There are thicker sections at the perimeter for threaded fasteners as well as a perimeter wall. Ribs radiate to the wall from a central boss where a standard, snap-in-place inspection port is installed.
The new long glass-reinforced thermoplastic hatch covers
were a joint development. PlastiComp
provided technical guidance on materials and processes plus the actual part molding
while Wipaire designed the part
and made the mold in its in-house machine shop. The project was completed in
less than four months.
The floats add approximately $250,000 to the 10-seat aircraft's base cost of $1.1 million.
The hatch covers are the second application for the Pushtrusion process, which previously has been used to make covers and lids for water-treatment units.