Injection molded parts get unique properties from wood filler

Extruded profiles are currently the dominant end-use for wood-fibre plastic composites (WPC), but there is potential for injection molded WPC with carefully crafted performance characteristics, according to industry experts at a Society of Plastics Engineers technical conference in Toronto.
Industry consultant Michael Burgoyne, speaking at the SPE Injection Molding Minitec on Plastic/Wood Fibre Composites (held March 20), noted that "we somewhat glibly speak of wood-fibre plastic composites as if they were one material. Nothing could be farther from the truth."
Composites of wood fibre and plastic can be made from various polymers most commonly high-density polyethylene, polypropylene, and polyvinyl chloride in combination with wood particles from a broad variety of processes and species.
Although wood fibre has the advantages of being readily available and economical, there are certain challenges involved with injection molding wood-filled plastics.
Due to the thermal instability of wood, Burgoyne explained, only those thermoplastics that can be processed at less than 200 degrees C can be used as the matrix if the objective is to maximize the properties of the filler and obtain a consistent product.
However, Mohini Sain, a professor at the University of Toronto and an experienced researcher of wood-filled composites, feels that further research and development could elevate these materials beyond the 200 degrees C threshold.
"If we could avoid the volatiles (that are released as the wood fibres are heated), I think we could injection mold wood-filled engineering resins. I think future technology is headed that direction."
Sain noted that part design and process simulation can lead to wood-filled parts with custom-designed properties. For example, fibre orientation and the direction of flow can be planned to make some areas of a part more stiff than others.
What’s more, WPC parts are by no means confined to wood-replacement applications, says Bill Crostic of Onaga Composites. Onage is a compounder of woodfibre-plastic composites, and can offer part development assistance. Crostic advises molders to think of wood fibre as just another reinforcing filler, one which has more stiffening power than talc or calcium carbonate, and a lower specific gravity than either of those or glass fibre.