Structural, Aesthetic Injection Molded Wood Fiber Components
Robert Joyce had extruded enough wood fiber composite sheet products to know that to achieve structural integrity, calendaring was essential. “I learned that pressure was important,” says Joyce, principal of Innovative Plastics and Molding (Lambertville, Mich.). He also knew that one of the unachieved goals of the injection molding of wood fiber composites — particularly at high fiber loadings — was the generation of enough cavity pressure to produce a hollow part with a smooth outer surface that has the appearance of wood.
Joyce’s idea, for which he’s been granted a U.S. patent, was to provide the much-needed pressure not from the machine, but from gas injected at 3,000 psi to 5,000 psi (207 bar to 345 bar) into the mold immediately following the injection of the fiber-resin melt. He took this idea to Gain Technologies (Shelby Township, Mich.), a manufacturer of gas injection technologies for injection molding, which worked with him to develop and perfect the process.
One of the first parts Joyce produced was a car steering wheel, with 44 percent wood fiber in a polypropylene (PP) resin supplied by Basell Polyolefins (Elkton, Md.). Molded-in pigments — brown, with black streaking — simulate a wood-like appearance. The gas injection, combined with proper material balance, reportedly helps create a structural cellular wall (minimizing warpage and sinks), pushing the wood plastic composite to fill out the mold; it also forms an exterior integral skin that helps make the part weather- and mold-resistant. Gas injection also hollows out the steering wheel, reducing molded-in stress and minimizing part weight.
The resulting mechanical properties are similarly impressive, especially when glass fiber is used (6 percent) in the wood plastic composite: tensile strength is 5,700 psi/39.3 MPa, flexural modulus is 450,000 psi/MPa, unnotched izod is 6 ft-lb and material shrinkage is less than 0.05 percent, with a specific gravity of 0.7 to 0.8.
Joyce also is developing products for building and construction and furniture applications and expects to begin licensing the process soon, although he says he’ll also work with processor or OEM partners.
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