Wood extrusion

from Wikipedia, the free encyclopedia
Extruded WPC profile

The wood extrusion is a plastic technical process ( extrusion ), with which using wood and plastics in a continuous process Wood-plastic composites are produced. For this purpose, the compound materials used are first mixed, melted and homogenized in an extruder . Furthermore, the pressure required to flow through the spray nozzle is built up in the extruder . After exiting the nozzle, the material solidifies in the desired shape and can be processed further.

Compounds

Compound mixtures made of wood and different thermoplastics can be extruded. Polyethylene and polypropylene are currently used in particular ; For specific areas of application, polyvinyl chloride , but also polystyrene , continue to be used as plastics . It is important that the polymers used have a relatively low melting range so that there is no thermal damage to the wood during extrusion. Depending on the application, indoors or outdoors, a proportion of wood between 25 and 85% is usually chosen today. Wood is increasingly the load-bearing material with a small amount of plastic as a binding agent.

Wood is used in very different forms in wood extrusion: from wood flour to wood fibers with different geometries to pellets . Pellets are easy to feed and dose, but because of the inconsistent degree of compaction in the extruder, they are “broken down” to different degrees, so that the end product quality fluctuates. Wood flour can also be technologically disadvantageous for certain applications, despite the fact that it can also be metered well, since it has hardly any matrix-reinforcing properties. Different types of wood (e.g. spruce, pine, beech) can be used for wood extrusion.

Almost all additives known in the plastics industry such as UV stabilizers, antioxidants , lubricants , impact modifiers, flame retardants or heat stabilizers are used as additives in wood extrusion, depending on the application . The so-called coupling agents , with which the wood fiber-plastic matrix adhesion should be improved, represent a special case . These usually consist of a polar group and a long, non-polar chain residue (e.g. maleic anhydride grafted onto polypropylene).

Extruder types

Four different types of extruders are used for wood extrusion: the planetary roller extruder , the conical twin screw extruder, the parallel twin screw extruder and the single screw extruder. The size of the extruded wood particles is mainly determined by the extruder (e.g. cylinder size, screw geometry). In a given extruder, the particle size in the final product will not exceed a certain critical level; if longer fibers and chips are used, they are ground in the extruder.

The temperature sensitivity of the wood component poses a major challenge when using standard extrusion and injection molding machines. For example, the process parts of the planetary roller extruder have an exchange area that is at least five times larger than a conventional twin screw extruder, which allows the material to be mixed gently. On the one hand, this also contributes to the most precise temperature control compared to other compounding systems. On the other hand, the system offers a large processing window for different thermoplastic materials and allows an almost problem-free material change. It is a modular system with up to eight modules, depending on the output and application.

The conical twin-screw extruder with its large diameter in the feed area and the continuous compression by the conical screws is preferred for highly filled systems because it allows very high material densities to be achieved in the extruder. The relatively short design guarantees a short dwell time of the melt in the extruder and thus reduces the thermal load on the wood additives. In the last few years these tools have been adapted to the specifics of wood extrusion.

Single-screw extruders are mainly used where finished granulates are used as raw materials. It is true that the mixing effect is significantly worse compared to the twin screw extruders; However, this can be compensated for by the significantly lower costs under certain framework conditions.

Procedural steps

With the pretreatment and post-treatment of the product, wood extrusion comprises the following process steps:

  1. Possibly pre-drying of the wood fibers
  2. Compression and plasticizing in the extruder
  3. Relaxation and degassing (either atmospherically or with the help of vacuum )
  4. Further plasticizing and mixing
  5. Shaping through the extrusion nozzle at 100 to 300 bar
  6. Calibration, cooling, cutting to length
  7. Surface treatment

With direct extrusion, steps 2 to 5 take place in the extruder. As an alternative to this, the compounded material is first granulated and only reheated and extruded for shaping in a further step. This enables the production steps compounding and shaping to be separated in terms of time and space, but heating them twice is a disadvantage for the temperature-sensitive wood fibers.

Alternatives

Wood extrusion is the predominant process in the production of WPC products. However, only linear profiles can be produced with this method. Natural fiber injection molding offers extended shaping possibilities .

literature

Web links