Injection molding opens up new applications of wood plastic composites

Wood Plastic Composites (WPCs) offer the combined properties of wood and plastic—excellent moisture barrier plus the ability to be screwed or nailed like wood, along with an organic look and feel. Wood-fiber/thermoplastic blends have already made a name for themselves in extruded decking and fencing boards. Wood plastic composites are now finding application in the field of injection molding as recent developments in the manufacture of WPC compounds have significantly improved the quality, consistency, and capability of this environmentally friendly material. In fact, the latest generation of WPCs can be run smoothly through traditional injection molding equipment with minimal adjustments to process settings and no physical hardware modifications. However, these materials do have some processing characteristics that distinguish them from familiar molding resins.

WPCs, part of the emerging family of materials that can be termed “thermoplastic biocomposites”, can be made with a variety of plastics, such as polyethylene, polypropylene and polystyrene. Besides wood, these biocomposites can utilize other natural fibers such as rice hulls, palm fiber waste or flax. As they contain up to 50% organic fibers, these materials offer injection molders a material option that is more environmentally friendly than typical petrochemically derived polymers. In addition to the “green” factors, thermoplastic biocomposites reduce a molder's exposure to increasing petroleum prices, reduce the energy costs associated with production, and produce an end-product with great structural rigidity, an aesthetically pleasing finish, and new, highly marketable performance capabilities.
One of the benefits of the current generation of WPCs is that they can be blended very efficiently with additional unfilled PP or other resin. Through blending, molders can achieve different performance characteristics: improve the shatter resistance of components such as car bumpers or increase the structural rigidity of neat resin. When molded using proper temperatures, speeds and a non-restricted flow path, WPC parts will exhibit uniform color and dispersion of wood fibers, minimal stress, smooth surfaces and show no evidence of gassing. The two most important principles to keep in mind for molding WPCs and other biocomposites are to avoid excessive heat and shear. Wood/PP biocomposites tend to be lower in cost and weight than unfilled resins or glass-filled resins. WPCs are competitive with calcium carbonate-filled or talc-filled PP in cost, performance and processing. But WPCs have the advantage of lower density, which reduces their effective cost and can be beneficial in transportation and other uses that put a premium on light weight. Applications can include automotive, construction, sports, toys, and other consumer goods.
Prospective applications for injection molding with WPCs are parts with thick walls and ones that would benefit from excellent rigidity and dimensional stability. However, they should not be subjected to excessive impact, since WPCs are less shatter-resistant than some traditional injection molding materials. While WPCs are best suited to parts with thick walls, processors can compensate for thinner walls by blending WPC with additional neat polymer.

Wood/PP flows very quickly at relatively low temperatures and pressures (generally similar to mineral-filled PP). As a result, injection molders can achieve significant energy savings. They can also achieve shorter cycle times and higher productivity due to reduced filling and cooling times. Typical temperatures for molding wood/PP composites are 171-188 degree C for the rear zone, 182-199 degree C for the middle zone, 193-210 degree C for the front zone, and 199-210 degree C for the nozzle tip.

Molding pressures of course depend on the design of the part as well as the runner system and gates. Injection molding with WPCs generally requires less pressure than molding with traditional materials.
While the material tends to flow very quickly, it is important to avoid excessively short fill times as the material is shear sensitive. Increased heat due to overly fast fill times typically manifests itself in a resin-rich streaking on the surface of the component. This streaking is remedied simply by slowing down the injection rate. Given the lower temperatures used for molding WPCs, hold times are often lower than with regular materials.

Typically, runner systems with a generous flow path and minimal obstruction will minimize shearing of the material. Likewise, injection gates should be as large because very small gates will cause excessive shear and possible discoloration, as well as a resin-rich appearance at the gate region. Ideally, parts should be gated directly into a thick section of the part. Gates should be located so as to avoid joining of flow fronts and weld lines at points that may be susceptible to stress in use. Breakage is more likely to occur at such weld lines.

Injection molded WPC components are rather natural in appearance, with a light-brown tone and a uniform grain. However, a high-gloss finish is achievable, and the material can be dyed in various colors with excellent color uniformity.