Direct Extrusion of Wood Fibreboard
For direct extrusion of wood fibres with PP, the individual components are metered directly into the feed throat of the extruder. The resulting fibreboard has a width of 1200 mm and can be processed like wood.
Together with Pro Poly Tec GmbH (PPT), Lichtenfels, Hans Weber Maschinenfabrik GmbH, Kronach, has developed a system for extrusion of thermoplastic-bonded fibreboard (TPF). The system achieves throughputs of up to 700kg/h. The greatest fibreboard width extruded to date with this material is 1200 mm.
The TPF material consists of 80% wood fibre with 17% PP and 3% additives. The individual material components for the direct extrusion process are fed into the feed throat of the extruder via volumetric metering. Mixing of the fibre in the form of wood chips with the plastic pellets and additives occurs directly prior to entry into the feed throat. In this way, the ratios in the mixture may be varied, in contrast to processing of precompounded material.
The fibres can be added as pellets or as loose fibre. Pelletized fibres are more readily metered and permit achievement of a constant bulk density, so that a stable output results. To process loose fibres, a stuffer is required to ensure problem-free feeding while maintaining the mixture ratio. The length of the wood fibres varies between 0.5 und 1.5 mm. Not only wood fibres, but also other suitable natural fibres such as sisal, hemp, bamboo, coconut fibre, flax and straw can serve as the fibrous material. In addition to polypropylene, polyethylene and polyvinyl chloride are suitable as matrix materials. Strength, moisture uptake and impact properties can be varied through appropriate modification.
The combination die is modular in design and permits fibreboard thicknesses of 5 or 13 mm through use of inserts. The system works without calibration. The fibreboard is dimensionally stable upon exiting the die, cools to ambient temperature on a roller table and is subsequently conditioned at about 90°C. The end product can be processed like wood, but is insensitive to moisture.