CHICAGO (July 14, 4:10 p.m. EDT) — Extruder manufacturers at NPE 2000 featured an array of systems for the fast-growing wood/plastic composite market.
Combining the workability and aesthetics of wood with plastics´ durability, the composites are gaining ground in decking, window profiles and other wood strongholds.
Composite decking alone could be a $350 million market in the United States by 2005, according to Louisiana-Pacific Corp. The Portland, Ore., wood products manufacturer is making sure it will ride this growth. It is starting up a big wood composites plant in Selma, Ala., and recently bought Hoff Cos. Inc., a composite decking producer in Meridian, Idaho.
Wood and thermoplastics composites are not exactly a new technology. American Leistritz Extruder Corp. was a pioneer when it sold a system to Ford Motor Co. more than 15 years ago to make composite sound-deadening door panels. That machine is still in operation, at Plastic Machinery Enterprises in Dundee, Mich., according to Charlie Martin, sales manager for Leistritz of Somerville, N.J.
But demand for the composites is growing after the construction industry accepted pure plastics in the form of vinyl siding and windows, and in non-appearance applications, plastic lumber. The composite is an ideal way to use wood byproducts that would otherwise to be landfilled or burned. It also can be a steady market for recycled plastic.
Getting moisture out of the wood is key to making the composite extrusion process work. Wood fiber can contain up to 12 percent moisture, which can create problems of high-pressure steam in the extruder.
Davis-Standard Corp. addresses the moisture problem upstream of the wood/plastic mixing process. Its Woodtruder displayed at NPE removes moisture in the first section of the primary extruder. Midway along the primary extruder, a side-injection extruder separately heats and melts the plastic before injecting it into the primary extruder where it encapsulates the wood.
ExtrusionTek Milacron uses its vertical integrated feed system, which acts as a crammer, to densify the wood fiber and plastic mix before feeding to the material enhancement delivery system. The latter drives off most of the moisture and delivers a compressed, solid mixture into the firm´s twin-screw conical extruder.
Krupp Werner & Pfleiderer Corp. uses a single-screw pump to feed wood fiber into the compounding extruder. Moisture is removed by mechanical energy and steam escapes through vents along the process section before the polymer and additives are fed in.
Leistritz employs multistage devolatilization to rid the mixture of water during compounding. Its twin-screw machines are well-suited to moisture removal.
Berstorff Corp. uses a low-shear mixing element that wets out the fiber and steam is vented in a special section. The firm´s vent stuffer prevents steam from mixing with solids, according to Martin Mack, vice president of research and development for the Florence, Ky., firm.
Farrel Corp. said its Continuous Mixer can remove moisture levels as high as 10 percent. The mixer has counter-rotating, non-intermeshing twin rotors that melt, mix and de-gas wood/plastic mixtures. A single-screw extruder or melt pump connected downstream pressurizes the melted mixture for downstream applications. The Ansonia, Conn., firm also is entering wood/plastic markets with its new Ultima compounder.
NFM/Welding Engineers mainly dries the wood fiber in the first barrel of a 12-barrel twin-screw extruder, said Ed Wykoff, director of compounding for the Massillon, Ohio, firm. With a 58-millimeter machine with two atmospheric and a vacuum vent, the system can bring moisture down from 6 percent to 0.05 percent in the finished product, he claimed.
Martin said Leistritz has sold systems that directly make wood/plastic profiles and ones that make pellets of the composite for subsequent extrusion into profiles. The simplest and perhaps more cost-effective setup is a two-stage one, he said.
There may be as many as eight feed streams into the twin-screw compounder. Controlling them all in tandem with the profile extrusion is complicated. It is easier to use the compounder to make pellets and then extrude pellets through a separate single-screw extruder.
If direct-to-profile extrusion is planned, a gear pump or single-screw front-end attachment is needed to maintain tight product dimensions, according to Martin. The pump or single-screw provides more stable front-end pressure for the profile die. At NPE Krupp Werner & Pfleiderer´s composite setup included a Merritt 6-inch, 8:1 single-screw hot-melt extruder.
ExtrusionTek sales manager Tom Brown said his Batavia, Ohio, firm´s conical twin-screw extruder provides exceptionally high output rates — up to 1,700 pounds per hour for its 92mm model — for direct extrusion of decking. The large diameter of the back of the extruder allows feeding in a lot of material, which gets increasingly compressed as it gets near the extruder exit.
Whichever setup one chooses, a composite producer has to ensure the wood fiber is well encapsulated and environmentally durable, according to consultant Brent English.
"Anybody can make a stiff and strong (profile), but making it durable is what´s important," English said from his English Engineering & Consulting office in Barneveld, Wis.
He said there have been a few isolated examples of wood composite decking failure, probably related to improper encapsulation or choice of a wrong stabilizer or pigment.
While the numerous wood/plastic composite producers strive to make good, standard profiles, one equipment producer reveals it is helping develop foamed profiles.
B&P Process Equipment and Systems LLC of Saginaw, Mich., is working with an undisclosed company in Europe to make thin profiles using nitrogen as the blowing agent injected into a twin-screw extruder. B&P sales director Harold Possler said the process can reduce the composite weight by 30 percent. The customer will have a few big plants up and running in a few years, he predicted from his Mühldorf, Germany, office in a post-NPE interview.