Blending process of the hottest wood fiber and pla

2022-09-19
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Blending process of wood fiber and plastic composites

the composite of wood fiber and plastic has the advantage that the properties of these two different materials can be ideally combined (Fig. 1). Plastic extrudates filled with wood powder (sawdust) or wood fiber have been mainly used in the construction field, such as balcony covers5 Interior design: the interior is characterized by hand-made leather decoration and carbon fiber wings extending to the full height of the instrument panel, as well as profiles used for windows, doors and railings. The most important market is the United States, which produced about 200000 tons of wood plastic composites in 2000. The global annual growth rate is 25%. The main advantages of these composites are their wide range of design, waste free production and a wood like appearance

so far, the application of wood fiber/plastic composites in injection molding is not common, although they have a lot of advantages. The quality requirements related to residual moisture content and uniform mixing are generally higher than those in profile extrusion, especially when processing or producing thin-walled plastic parts in hot runner molds

Scandinavian wood fiber company of Sweden, in close cooperation with Germany's copellon company, launched a development project aimed at producing granular wood fiber/plastic composites, which has the high quality required for injection molding. Quality is mainly controlled by two factors:

◆ processing of wood fiber and types of wood

◆ process control in the production of composites in twin-screw mixer

mixing high-grade wood fiber materials

one condition for producing wood fiber/plastic composites with good mechanical properties is that the quality of wood fiber remains excellent. Wood fibers used in injection molding pellets must be classified as technical fibers by quality. The starting material for production consists of selected rough planed wood flowers. The sawdust that can be obtained in large quantities does not meet the quality requirements

planed wood flowers are ground into technical wood fibers in the woodworking factory, with the ratio of length to diameter as large as possible. At the end of the grinding process, the wood fibers pass through a sieve and are classified by fiber length with a specific length distribution. The length of wood fiber used for injection molding pellets is generally between 200 and 400mm, and can reach 3000mm in special cases

Figure 2

mixer for producing composites

when continuously processing wood fibers for the production of composites, there is an important limitation that needs to be considered. High precision ball screw: at a temperature above 160 ℃ and in the presence of oxygen, thermal oxidation will occur in the fibers, causing blackening and mechanical damage to the fibers. Because of this limitation, and according to the technical evaluation results of different mixing systems, the twin-screw mixer of copetron has proved to be particularly suitable. Its main advantages are:

◆ it can control the introduction of energy through the screw structure and process parameters, so as to ensure low processing temperature

◆ when there is no oxygen, the residence time used by wood fiber at the melting temperature of plastic is very short, only 10 to 20 seconds

◆ closely meshed co rotating twin screws have excellent mixing and dispersion effects

a series of system tests were carried out in the machine manufacturer's test workshop to determine the most ideal screw structure and the best steps suitable for the processing of high-grade and low-grade wood fiber/plastic composites. The test was carried out on a twin-screw mixer (ZSK megavolume), which has a screw with a diameter of 62mm and uses different screw structures (Title Figure). The screw thread of this mixer is particularly deep, with a diameter ratio of 1.8, so it has a larger free volume than ZSK megacompound. This brand is especially suitable for bulky wood fiber materials with extremely low density (0.15kg/dm3). These tests show that using the process described below, the device shown in Figure 2 not only produces high-quality injection molding composites, but also processes fibers particularly gently

the basic plastic, usually PP, but sometimes HDPE, together with the additives specified in the formula, such as adhesives, UV stabilizers, colorants, antibacterial agents or flame retardants, is transported to the first shell of the mixer through the gravimetry system and melted in the first processing area. Subsequently, wood fiber (the residual moisture content must be between 3% and 6% by weight, depending on the formula) is directly added to the melt through the gravity metering device and the two-way feeding device. This ensures a mild treatment of the fibers. In most cases, the gas carried by wood fiber can escape through the back exhaust system in the device

in the second processing stage, wood fibers are evenly dispersed in the melt, so that every fiber is completely wetted by the plastic melt. The subsequent vacuum exhaust ensures that residual moisture and all other volatile components are recovered. The next section accumulates pressure for the granulation process. Because there are special requirements for the granulation of wood fiber/plastic composites, a more advanced granulation system from copellon was developed after factory tests. The extremely uniform barrel shaped particles (Fig. 3) produced can be processed on the traditional injection molding machine without any problems, and produce clean plastic parts without streaks

depending on well-equipped pilot plants, the performance of compounds can be optimized, and the number of templates made under conditions similar to the actual conditions can be checked, so as to test their suitability for potential uses. This means that interested customers can familiarize themselves with the new materials and processing technologies used in production

according to the introduced concept, the machine has a mixer with a screw diameter of 76m to avoid damaging the experimental machine M. the first impact test standard gb229 ⑴ 96303 appeared by the machine manufacturer in 20 until 1963, and was delivered to Scandinavian wood fiber company at the end of 303 (Figure 4). This machine can produce composites with a weight content of%, using PP or HDPE as the base material. Its design output is 1000kg/hr. Some wood fiber/plastic composites produced by it were processed into injection molded parts by Scandinavian wood fiber company, and some were sold

(to be continued)

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