Aluminum oxide ceramics are also known as precision ceramics, special ceramics, or high-tech ceramics. It is a ceramic that uses highly selected raw materials and is produced according to special manufacturing processes, with precise control of chemical composition and excellent performance. At present, alumina ceramics are mainly used in high-tech and cutting-edge industries, such as microelectronics, nuclear reactors, aerospace, magnetic fluid power generation, artificial bones, and artificial joints. In the manufacturing process of alumina ceramics, the following three requirements should be met. ① The selected raw materials should be high-purity, and the particles should be as fine as possible; ② Strictly control chemical composition. During the manufacturing process, it is necessary to prevent impurities from mixing and the volatilization of the components themselves, and strictly control the particle size, interface, porosity, etc. of the sintered parts to achieve stable quality and reproducibility; ③ Accurate shape and size. Alumina ceramic parts are generally not processed and are used directly, especially ceramic electronic devices that require high accuracy.
There are significant differences in composition and manufacturing process between alumina ceramics and ordinary ceramics. Ordinary ceramics are made through three processes: raw material preparation, billet forming, and kiln firing; Most alumina ceramics are manufactured using powder sintering method. In terms of forming technology, due to the extremely high hardness of ceramics, it is difficult to cut and process, especially for non aligned products with complex shapes, such as turbocharger rotors, bones, teeth, and other bioceramic products in automotive engines. After forming and sintering, they become finished products without further processing. In order to meet this requirement, people imitated the injection molding technology of the polymer material industry to produce plastic parts and processed alumina ceramic products, achieving satisfactory results.
Ceramic injection molding technology involves adding thermoplastic resin, thermosetting resin, plasticizer, and friction reducing agent to ceramic powder, making it a viscous elastic body. Then, the heated and mixed slurry is injected into the metal mold from the nozzle, cooled and solidified. Commonly used thermoplastic resins include polyethylene, polystyrene, and polypropylene, with an addition amount of 10-30%. This technology has greatly improved the accuracy and reliability of forming complex shaped products.
1、 Injection molding equipment
Injection molding machines are generally composed of plasticizing devices (or injection devices), clamping devices, oil pressure devices, and electronic and power control devices. According to the internal structure of the plasticizing device, it can be divided into plunger type and parallel propeller type. In recent years, it is generally believed that the latter has more advantages.
The injection molding machine is centered around electronic and power control devices, which drive the oil pressure device while allowing the plasticizing device and clamping device to work in sequence. The working procedure is as follows: ceramic raw materials are fed into the cylinder body through a funnel. While the raw materials are fed into the end of the cylinder body, they are melted and stirred, and then injected into the cavity of the metal mold through the nozzle at the end of the cylinder body to obtain the blank. Advanced control systems in foreign countries adopt screen display methods (such as using oscilloscopes, plasma, electroluminescence, and liquid crystal), as well as designing graphical control consoles or a combination of the two.
The mold material is generally made of alloy steel with excellent cleanliness, wear resistance, and corrosion resistance. The mold design should comply with the flow characteristics of the ceramic polymer system. In order to reduce the shrinkage of the formed body and prevent air from being drawn into the formed body, the mold should consider controlling the discharge outlet. On circular products, fusion lines are easily formed on the opposite side of the grid, so it is also important to pay attention to the position of the grid. In order to optimize injection molding conditions, it is necessary to rely on temperature management of molds, material barrels, etc., and meticulous management of pressure sensors inside the molds. In addition, there should be cooling slots on the mold that can cool and heat, and relying on temperature regulators to keep the mold temperature constant is very effective in improving the accuracy of the shape. Due to the extensive use of organic materials in raw materials, degreasing without carbon residue is also an important issue in order to prevent hot cracking of the blank.
2、 Process
The principle of ceramic injection molding is basically the same as that of plastic injection molding. It's just a large amount of ceramic powder mixed inside the plastic. In order to improve the injection molding conditions, it is necessary to select organic materials that match the raw materials used and select the amount of addition. In order to obtain a dense and uniform injection molded form, the concentration of ceramic powder should be higher. But too high will lead to poor formability. To improve the flowability of mixed billets, the viscosity of the dispersant polymer system should be reduced. As a pre-treatment, it is important to improve the dispersibility of ceramic powders. In order to improve the flowability of polymers, appropriate plasticizers and lubricants need to be added.
The particle size of ceramic raw materials is generally 1 μ m. Add an adhesive (also known as an additive), thoroughly mix and stir. The process flow of injection molding is shown in the following figure.
During the stirring process, the ceramic powder is moistened and coated with adhesive, forming a uniform composite before injection molding can proceed. And it requires cooling, drying, and crushing to obtain particles suitable for feeding into the injection molding machine funnel.
There are four technical issues that should be noted and mastered throughout the entire process.
1. Liquidity of raw materials
The ceramic particles used in injection molding are generally composed of 80-90% (important ratio, the same below) powder and 10-20% binder. The binder is removed during the degreasing process, so the minimum amount of addition is appropriate. However, it should be noted that insufficient addition will affect the forming effect. In addition, the fluidity of ceramic particles deteriorates as the particle size decreases and the shape deviates from the spherical shape. Therefore, the liquidity should be tested using the simplest possible method.
2. Defects generated by forming conditions
If the forming conditions are not correct, various defects can occur. The most crucial one is the fusion welding line, which is prone to defects if the formed form has through holes or blind holes. Therefore, it is necessary to pay attention to the design of the mold, especially the type, position, size, and number of openings. At the same time, attention should be paid to the balance between the injection temperature and speed of injection molding.
In addition, in order to avoid defects such as surface roughness, cracks, elongated marks, and deformation. In difficult forming situations, adaptive controllers can be installed in the injection molding machine for fine control.
3. Defatting
This process, also known as removing the adhesive, usually heats up at a rate of 3-5 ℃/b for about 5-10 days. However, when carried out in a protective atmosphere with a pressure of 0.5MPa, the degreasing can be completed within 40 hours.
4. Sintering
The thermal and other parameters can be determined based on the type of ceramic. The linear shrinkage rate during sintering is about 15-20%, and work with complex shapes or thick walls is prone to cracking during sintering, and attention should be paid to preventing it.
3、 Additives
This is one of the most important issues in injection molding technology. The additive properties required for different forming methods also vary. If injection molding requires degreasing, fluidity, glue solubility, strength, and shrinkage; Extrusion requires plasticity, glue solubility, strength, and lubrication; Cold isostatic pressing (rubber molding) requires lubrication, granulation, and strength; Mechanical pressing requires lubrication, granulation, strength, demolding, etc. At the same time, different adhesives should also be used for different products.
The requirements for additives in injection molding technology are as follows.
1. Adhesive solubility: It is advisable to use as few additives as possible for various forming methods, and it is possible to use gelatinous (adhesive solubility) additives, which is beneficial for reducing production cycles and product costs. Especially beneficial for removing the adhesive in the future.
2. Flowability: Injection molding requires flowability under high pressure. The injection effect of using resin based binders is good because when using resin based binders, attention should be paid to maintaining viscosity and injecting slowly.
3. Expansion and shrinkage: Due to the use of a large amount of binder in injection molding, the shrinkage of the blank is large, affecting dimensional accuracy and geometric shape, and easily forming pores. Therefore, substances with low expansion and contraction such as paraffin can be selected as additives. This type of substance has a smaller expansion contraction effect than non crystalline substances such as rosin.
Injection molding technology, as early as the 1950s, Japan used A12O3 ceramic as the ignition plug for internal combustion engines. In the future, it will be used for production in small and complex parts one after another. In recent years, people have been actively engaged in the development and research of various heat-resistant and wear-resistant parts. For example, the swirl chamber head of diesel engines has been vitrified and manufactured using injection molding technology.
According to reports, recently, the Institute of Production Technology at the University of Tokyo in Japan successfully tested the "frozen injection molding method" using only water. This is achieved by utilizing the fluidity and freezing properties of water to solidify and demould ceramics. Fill the pre cooled mold with blanks at around 0-5 ℃. At the same time, the blanks freeze from the inner wall. When the internal freezing reaches the demolding strength, the blanks can be removed from the mold. As there is no need to use a large amount of organic additives as mentioned earlier, there is no need for a long degreasing process, which can greatly improve the performance; Shortening production time creates favorable conditions for reducing costs and increasing production.
Keywords: alumina ceramic injection molding, alumina ceramic powder injection, CIM powder injection, Shenzhen ceramic powder injection, zirconia ceramic powder injection, thermal insulation ceramics, heat-resistant ceramics, wear-resistant ceramics, ceramic processing