Direct extrusion of WPCs offers material savings option

New machinery optimized for wood-polymer composite (WPC) profile extrusion helps processors extrude their own recipes.

Suppliers of wood/plastic compounds (WPCs) won’t like hearing it, but new extruder developments could mean the loss of this market for them in some instances.

German processing equipment maker Reifenhäuser (Troisdorf) has, over the last several years, been aggressively promoting profile extrusion of WPC (for instance at this event), a development, at least in Europe, which has been relatively slow in terms of market growth. The process typically involves first compounding wood or natural fiber flour with polyolefins, followed by a second step of final extrusion into siding, decking, fence posts, and other items meant to replace wood and/or plastics (mainly PVC) profiles.

A BiTrudex direct extrusion line for WPC processing includes a gearless single-screw extruder placed above a counter-rotating twin-screw extruder. Increasingly, says Dieter Thewes, head of the business unit Extrusion Center at Reifenhäuser, processors' interest is turning to direct WPC extrusion, with the to-be-processed elements mixed, melted, and extruded in a single step. One reason he cites is the ability to process up to 80% wood flour (by weight) at substantially higher (>40%) processing speeds, but with lower investment costs, compared to first compounding and then extruding the material. Lower overall energy use also is in processors’ favor since there is no need for a heating-cooling mixer; time savings are also possible.

For its part, Thewes’ firm offers a line consisting of its counter-rotating twin-screw Bitruder technology coupled with a gearless single-screw REItorque extruder for this process. The line includes a single atmospheric and two vacuum degassing units to siphon off unwanted moisture and volatiles. “This assembly (marketed as BiTrudex) enables processing fibers with a humidity (level) up to 12% at line speeds that can be up to three times higher than competitive direct extrusion equipment,” says Thewes. Soft wood fibers are generally used, but he says lab tests show hard wood flour is also suitable for direct extrusion.

Why a twin-screw counter-rotating extruder for this application? Thewes says, in this particular configuration, the twin-screw is able to achieve the necessary thorough mixing of the wood flour and polymer melt. It also builds up the pressure required to push the mixture through a die without the need of a melt pump.

The L/D of the Bitruder has been reduced from 36 to 30, which translates into a more compact design than a comparable WPC direct extrusion line, says Andreas Reimann, project manager, Extrusion Center. He reckons processing WPC on this configuration confronts processors with the same sort of abrasive wear of screws and barrels as when extruding glass fiber-filled compounds, so the manufacturer include a special armor coating, developed at its Reiloy screw/barrel manufacturing subsidiary, for WPC on all of the line’s plasticizing elements. He says depending on the WPC mixture, processors should be able to use such screws and barrels up to 25,000 hr before replacement, compared to non-treated equipment that might only last 6000 hrs. So far the company has sold three such direct extrusion WPC lines, and Thewes says interest from markets around the world is running high. —[email protected]

Riga, Latvia-based packaging thermoformer diversifies, adds WPC profiles to its mix, and realizes 20% faster line speeds than promised. Read about it here.