Research progress of wood composite processing tools

1. introduction
In recent years, the modern wood processing industry has developed rapidly, and the processing of wood composite materials has been rapidly developed. With the enhancement of people's environmental awareness, the wood-based panel industry has gained more and more development space. The most important product is medium density fiberboard (MDF), which is currently the largest producer in Asia. In 2001, the actual production of laminate flooring has increased to 60 million m2, and such a large increase in demand puts new demands on cutting tools. At present, the tool material used in cutting wood composite materials such as wood-based panels is mainly cemented carbide, and the wear resistance, heat resistance and hardness of the cemented carbide are high. However, due to the high anisotropic structure of the wood itself, the friction coefficient with the tool is very large, and the wood composite itself contains hard spots that cause mechanical abrasion of the tool (some artificial board surfaces have hard to process hard). The coating), as well as the acidic medium that causes the chemical corrosion of the tool, will aggravate the tool wear and corrosion, which not only greatly shortens the service life, but also seriously reduces the quality of the product, which in turn affects the processing cost and production efficiency of the product. The existing cemented carbide tools can not meet the requirements in terms of wear resistance and corrosion resistance. In order to achieve good economic and social benefits, the market urgently needs high-performance, high-quality woodworking tools.

At present, China is not only poor in the theory of woodworking tools research, but also very backward in its production. Therefore, it is of great significance to understand and master the development status and research progress of woodworking tools at home and abroad.

2. Wear mechanism
2.1 Mechanical abrasion and wear
Due to the special structure of the wood composite material different from the metal material, the wear mechanism of the tool in the processing should be specially studied, which is necessary for the material selection of the tool and the selection of geometric parameters. . The wear of woodworking tools is mainly divided into two categories: mechanical abrasion and corrosion wear.

Mechanical abrasion wear is caused by the migration of tool materials by hard particles or protrusions in the wood composite. Cell walls, resins, minerals (such as quartz sand, etc.), knots, glues, etc., can all become hard spots for mechanical abrasion. It is generally believed that mechanical abrasion wear is the process by which the abrasive slips over the surface of the tool, causing scratches or micro-cutting, resulting in the formation of scratches or furrows on the surface of the tool. Determine whether the mechanical scratch is the main wear mechanism of the tool. In addition to considering the material properties of the tool, the chemical properties and moisture content of the workpiece should also be considered. If the workpiece has a high water content and a high acidity, the tool wear often has another reason that cannot be ignored - corrosion wear.

2.2 Corrosion wear
Carbide inserts tend to form volatile chlorides when cutting certain wood materials. The cobalt element in the cemented carbide tool will be deprived of electrons by the hydrogen ions in the organic weak acid to form metal ions, and then the polyphenol compound will react with the ions to form a loose chelate. It can be seen that the corrosion of the tool material by the organic weak acid and the polyphenol compound is gradually carried out, and finally the metal chelate is formed to cover the surface of the tool. For cemented carbide tools, cobalt is an indispensable binder phase. Once the cobalt is corroded, the resulting composition is quickly worn away by mechanical abrasion and wear, causing the tool material to be severely worn. In addition, electrochemical corrosion is also an important cause of tool wear. Therefore, the wear of the tool is essentially a combination of mechanical, thermal and electrochemical corrosion of the tool and the workpiece material, and is a process of continuously losing the metal material in front and behind the tool. For several typical tools, the wear mechanism of the diamond film is "wear deformation - crack - wear debris formation". High-speed steel blades are mainly abrasive wear, supplemented by corrosion and wear. The loss of binder (cobalt) is the root cause of the wear of cemented carbide inserts, so corrosion wear is the main wear mechanism of cemented carbide inserts.

3. Current status and development direction at home and abroad
In recent years, with the use of some new materials, woodworking tools have moved towards diversification. These new materials have advantages that are unmatched by traditional materials in some mechanical and physical properties.

In the woodworking industry, the earliest tool used was high-speed steel. With the use of hard materials such as cemented carbide and diamond, the development of woodworking tools has become increasingly mature. Coating treatment is undoubtedly the most researched and developed technology today. The United States, Germany, Poland and other countries have a lot of research in this area, mainly for coating adhesion, wear mechanism, coating material performance comparison and so on. Multi-layer coatings, multi-component composite coatings, as well as diamond coatings, CBN, and CN coatings will be the future direction of tool coatings. In addition, there are many applications for PCD tools. Germany's Leitz is one of the world's most famous companies producing woodworking tools. They have developed MCD single crystal diamond tools that are dozens of times more wear resistant than PCD tools.

At present, the research on wood processing tools is very important at home and abroad. With the high efficiency and high environmental awareness, the traditional industry of wood processing has been given a new development direction, that is, the research on the tool wear mechanism and the tool. There are many new prospects for the calculation and optimization of geometric parameters and surface treatment. Especially in the surface treatment of tools, many articles have been published abroad. Some experts in China have done a lot of research on the theory of woodworking tools, such as Nanjing. Professor Cao Pingxiang from the Forestry University has made great achievements in the research of woodworking cutter wear and diamond film cutters. Harbin Institute of Technology has also done a lot of research on tool wear and PCD woodworking tools.

4. Research dynamics
4.1 Coating Technology
Coating is a new technology suitable for woodworking tool processing. It is usually coated with a layer of wear-resistant TiC, TiN, TiCN3 and other materials on the surface of cemented carbide. The main coating methods currently available are chemical vapor deposition (CVD) and physical vapor deposition (PVD). The uncoated cemented carbide material has lower heat resistance and wear resistance than the hard phase itself, while the surface hardness and wear resistance of the surface coated cemented carbide tool almost completely reflect the coating material. performance. Since the coating generally has a higher hardness than the hard alloy (such as CVD coating TiC, the hardness reaches HV3800), and the impact resistance of the base material is maintained. In the anti-corrosion of woodworking processing, due to the presence of the coating, the phenomenon of "decobalting" of the hard alloy during processing is well controlled, so the coating treatment can improve the service life of the tool.

Polish researcher P Beer and other research pointed out that the friction coefficient of tool coating and logs is still relatively high, which puts higher requirements on the bonding force of the coating. A well-bonded coating can greatly increase the life of the tool and prevent premature wear of the tool. However, coated tools that have been used to a certain extent can severely reduce the quality of the machined surface of the wood board due to the rough tool surface. Therefore, low friction coefficient, smooth wear and longer service life have become the research direction of coatings. German researcher I. Endler et al. found that good adhesion between the coating and the body material, low surface roughness and small cutting edge radius can achieve good cutting results. When the coating thickness is less than 5 μm, the cutting edge radius can be increased little or not, which is important for woodworking tools that require a sharp edge. Despite the excellent performance conditions, the coated modified tools are still not very well used in woodworking. The difficulty lies in the combination of mechanical processability, physical properties of wood materials and frictional organization. parameter. This makes it difficult to select tool materials and determine tool geometry.

In coating technology, research on ion implantation technology has also made important progress. Studies have shown that ion implantation increases the hardness of the tool because the injected atoms enter the dislocation or the solid solution is solid solution strengthened. Ion implantation can also form a dense oxide film and change the electrochemical properties of the surface to improve corrosion resistance, which will be helpful for the improvement of woodworking tools.

(1) Nitriding technology
Carbon steel is widely used in woodworking processing because of its high performance and price performance. It is a traditional woodworking tool. With the gradual improvement of the processing quality requirements, the improvement and modification of carbon steel woodworking tools is also the direction of many researchers. Among them, carburizing and nitriding techniques have a good effect and are well received. Polish researcher J. Rudnicki et al. concluded that nitriding of woodworking tools has concluded that for carbon steels with lower yield temperatures, ion nitriding at temperatures below 400 °C can achieve good results, albeit with increased tooling and wood workpieces. The coefficient of friction, but nitriding technology can significantly improve tool strength and increase tool life by more than one time.

(2) Diamond film treatment
The diamond film has excellent characteristics, such as the highest hardness (100GPa), the highest thermal conductivity [>12W/(cm·k)], stable chemical reaction, and resistance to non-oxidizing acids at various temperatures. The diamond film has good performance conditions for processing wood composite materials, and its wear resistance and corrosion resistance can be fully exerted. At present, synthetic diamond mainly uses CVD method. A TiC/TiN layer is often used in the middle layer to prevent diffusion of cobalt and to carry the diamond film. MS Raghuveer et al. pointed out that when the diamond film is prepared by CVD, the outward transfer of cobalt from the substrate by the grain boundary is much more obvious than that of the diamond grains, but it can be combined with TiC/TiN by hydrogen ion chemical etching. The method of diffusing the resist layer makes it significantly lower. The sharpening method achieves a larger tool edge radius and maintains the sharpness of the cutting edge during CVD deposition, which can play an important role in improving the bonding force between the diamond and the tool base.

Diamond film coated tools have been used in wood processing to a limited extent, and there are still some problems to be solved. The main reason is that the bonding force between the diamond film and the body is always insufficient, and in the wood processing, a sharp edge and a small cutting edge, including the cutting angle, are required, which is an difficult process problem for the diamond film. Despite this, the diamond film has a good application prospect in wood processing due to its excellent properties.

4.2 Polycrystalline Diamond PCD and PDC

PCD (Polycrystalline Diamond) is a high performance synthetic superhard material. It has high hardness, good wear resistance and thermal conductivity, and has been increasingly used in the processing of wood composites in recent years. In 1979, the German company Blue Pack first produced PCD woodworking tools. When processing PB (particle board), its life is 125 times that of conventional cemented carbide tools. It can be seen that PCD has very good performance.

The research of Polycrystalline Diamond Compact (PDC) tools was developed on the basis of PCD tool research. It is to sinter the diamond micropowder on the cemented carbide body at ultra high temperature and high pressure, so that the PCD and the cemented carbide are combined into one whole, because it has the hardness and wear resistance of the diamond and the toughness of the hard alloy. And the advantages of solderability, so its appearance has an epoch-making significance.

At present, PDC tool composite sheet materials are developing towards large size, grain refinement, quality optimization and performance uniformity, and the application market is also growing year by year. The future development of China's wood processing industry is focused on further improving the quality of finished materials, the pass rate and the yield rate, and developing coating processing. Therefore, the accelerated adoption of PCD and PDC tools in the woodworking industry will be a development trend. First of all, due to the high hardness, good wear resistance and service life of 20 to 400 times that of similar carbide tools, the replacement tool time is from once every few hours to once every few months. Reduced the number of tool changes and sharpening hours, increasing equipment utilization and wood product productivity. Secondly, the amount of retraction of the tool tip is small, which can increase the cutting speed. Increasing the amount of feed per feed also increases productivity and results in lower roughness and higher precision. Although the tool cost is 10 to 40 times that of similar carbide tools, if the lifetime cost analysis is considered, the final product processing cost can be reduced by 20% to 69%.

5. Conclusion
At a recent international woodworking machinery seminar, the famous British woodworking machinery expert Geoff. Stainton predicted that China will become the world's largest woodworking machinery production base. China is a big country in the production of hard alloys. Cemented carbide is the main material for the production of woodworking tools. So how to use our advantages to mature as soon as possible in the market of woodworking tools, and to narrow the gap with foreign countries will be of great significance.

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