Extrusion, Wood

Composite wood extrusion

Composite wood extrusion

Conenor invented conical extruder technology in the 1990s, and licensed it to produce extruded plastics products such as pipes and cable coatings. The company has now gone further, developing a follow-on technology – the CONEX® Wood Extruder – for producing composite extrusions of wood and other natural fibre-plastic composites.

Extrusions produced from wood and other natural fibre-plastic composites (WPC) are a rapidly growing business in North America, and are gaining increasing attention in Europe and elsewhere. Durable, maintenance- free, and moisture-resistant WPC products are replacing treated wood in applications such as decking and fencing. Not only do they offer technical advantages, they also offer a sustainable path for recycling plastics waste from both consumer and industrial sources.

Conventional WPC extrusion utilises precompounded pellets made from dried and sized wood fibres and selected plastics – either virgin and/or recycled – and incorporating the necessary processing additives for both compounding and extrusion. These pellets are extruded into WPC products using standard plastics industry single- and twinscrew extruders, dies, and other line equipment.

This chain of multiple processing steps, however, is inherently costly and unecological, as wood and plastic materials are handled separately, heated and cooled, and mixed in a long series of steps using a variety of equipment.

Layout of the patented Conex® Wood Extruder.

All in one

Conenor’s solution involves a novel process and an extruder in which everything is done on-line in a single step and by a single piece of equipment: the Conex® Wood Extruder.

The Conex unit grinds the wood, removes excess moisture, grinds the plastics, compounds the two together with any necessary additives, and extrudes WPC products. As an added plus, the fibre orientation resulting from the helical material flow pattern inside the extruder gives the resulting products extra strength.

The savings in manufacturing, and the freedom to use the lowest-cost raw materials, are obvious. All raw materials are handled and heated only once, mixed and processed directly, using short residence times, and involving no material degradation.

Extrusion takes place at just 10 ºC above the polymer melting temperature, and at about 60-bar die pressure. The process can also utilise composite plastic waste from the paper and textile industries, produce multilayer products, and accommodate exceptionally high wood contents of over 80%.

Extrusion of WPC board using the Conex unit.

Low-cost processing

The Conex Wood Extruder can utilise various wood-based raw materials, such as undried sawdust from sawmills, as well as mechanically crushed thermoplastic waste without compounding, such as polypropylene and polyethylene. A wood content of between 60% and 70% by weight is normally used. Depending on the quality of the plastics available and the application in question, higher or lower wood content can be used, and the user can develop customer- or product- specific compositions in-house. Switching can be made easily on-line, without even stopping the extruder.

Material handling is minimised, as everything is processed in a single step, which saves on energy and money, while the freedom to use the lowest-cost materials offers a low-cost basis for WPC end-products. Machinery investments are low as well, as there is no need for separate units such as compounders.

Ideally, a WPC product manufacturing facility could be located at an existing forest products site, where clean sawdust is readily available.

Value-added products

The Conex is ideal for producing hollow pro- files. The hollow core of the extruder means that, when the composite melt enters the die, a simple sizing die is sufficient to give the product its final form, leaving a cavity inside the extruded product. The hollow core of the unit can also be used for cooling purposes.

Multilayer products can be extruded without the need for complex multilayer dies or side extruders. This is beneficial in optimising products such as board, by using different materials and/or additives, such as on-line colouring throughout or on surface layers only.

Cross-linking polyethylene on-line with peroxide and/or silane makes WPC products more crack-, wear- and impact-resistant, as well as dimensionally stable at both elevated and low temperatures. Foaming the middle layer reduces product weight and gives balanced thermal insulation.

Embossing can be used directly on-line to give products a wood-like finish; while thermoforming techniques can be used to produce 3-D shapes from WPC board.

Using virgin polymers and selected wood materials, composites combining specific mechanical characteristics with acoustic and visual properties can be tailored for use in applications such as window and door frames, furniture, automotive and yachting products, and musical instruments.

Conenor