Last Updated on June 16, 2021 by You Ling
Ceramic Injection Molding
Alumina is currently one of the most widely used special ceramic materials in industry. It is not only cheap and easy to obtain, but also has high mechanical properties, high electrical insulation and low dielectric loss. Therefore, it is used in aerospace, machinery, It is widely used in the fields of electronics, electric power, automobile, chemical industry, medicine and so on.
However, the high hardness and brittleness of alumina ceramics make it difficult to be machined. In other words, it is quite difficult to form and control the precision of alumina special-shaped parts, especially for micro-structures (millimeter level). Most molding methods are almost powerless.
The emergence and development of ceramic injection molding (Ceramic Injection Molding, CIM) technology has opened up a new path for alumina ceramic molding. The products prepared by injection molding not only have precise and controllable dimensions, but also can realize mass production without or requiring a small amount of mechanical processing, which greatly reduces the production cost of ceramics.
1. Process flow
CIM mainly includes four stages of feed preparation, injection molding, debinding and sintering. The basic process is to add a binder to the alumina ceramic powder and mix it evenly to form a viscoplastic feed. Under heating, an injection molding machine is used to inject the feed into the mold cavity for condensation. After the debinding process removes the binder, it can be used for sintering.
Generally speaking, the principle and process of alumina ceramic injection molding are not complicated, so it is very popular in the manufacturing industry. Especially in the fields of engines, gas turbines and rocket engines that require parts with complex shapes, high precision, high temperature resistance, and high hardness, the application of this aspect started very early. For example, in 1931, the German SiemensHalske company used alumina ceramics Used in spark plug materials; in 1942, Japan applied alumina ceramics to the spark plugs of military aviation aircraft.
2. Process points
However, in order to obtain high-quality alumina parts through the injection molding process, there are many places that need to be paid attention to. For example, the process of injection and demolding is restricted by many factors. Therefore, it is the field to control the changes of various key parameters in the injection process. The key research content.
1. Powder selection and optimization
At present, the research on alumina ceramic materials is concentrated in two categories, one is concentrated on α-Al2O3, γ-Al2O3 and the combination of the two in different proportions. Research has found that the mixing ratio of Al2O3 powders with different structures causes injection molding bonding. There is a big difference in the ratio of agent, feed fluidity, product shrinkage and grain boundary diffusion. The other type is composite materials prepared by doping other ceramic or metal materials based on alumina. The composite ceramic materials doped with Cr3C2, SiC, Ti, WC, ZrO2, Al, MgO in alumina can enhance the dislocation and Inhibiting the growth of grains is conducive to grain refinement and improving the mechanical properties of the product.
Another research area for powder selection is the influence of powder average particle size, bulk density, powder shape, specific surface area, and particle gap on the flow performance of the feed and the quality of the product. As the main raw material of injection molding, the most important requirement for powder is fine particle size and regular shape. The powder with a small particle size has good fluidity during injection molding, a large filling amount, and a low sintering shrinkage rate.
2. Binder
①Binder selection: The selection of binder system is the core link in alumina ceramic injection molding. It not only directly affects the injection performance of the feed, but also determines the degreasing method and affects the final performance of the product. The binder system has two basic functions in the injection molding process: first, to ensure that the feed has good fluidity; second, to ensure the strength of the green body.
②The influence of surfactant: According to the rheological theory, the surfactant controls the properties of the ceramic feed to a large extent, and the stability of the feed will increase as the absorption layer on the surface of the ceramic particles increases. In the injection molding process of alumina ceramics, the amount of surfactant will vary with the other components of the binder, and its content is mostly between 1% and 5%. Theoretically, there is an optimal value of surfactant for any component of the binder. If the content is too small, the adsorption layer on the particle surface is not thick enough, which will affect the flow properties of the feed; if the content is too much, it will bond when heated. The volatilization of the agent components will cause voids in the molded body and reduce its compactness.
③Research on mixing and rheological properties: Mixing is a key link in the preparation of feeds. The mixing method, time and order of discharging will all affect the rheological properties of feeds and the quality of later products. Wen-ChengJ studied the relationship between the mixing sequence and the performance of the alumina ceramic product and showed that the first addition of polypropylene can better preheat the alumina powder, improve the mixing power, and facilitate the molding of the product.
3. Injection process
Improper control of the injection molding process will cause many defects in the product, and these defects can not be found until debinding and sintering. Therefore, controlling and optimizing the injection process parameters is very important to improve the yield and material utilization. When Liu Ye et al. used finite element method to simulate the injection process, they found that the injection speed and mold temperature are the key process parameters of alumina ceramic micro gear injection molding, but the injection temperature has less influence on the injection process.
4. Degreasing process
The degreasing process is the longest time-consuming process in the injection molding process, and it is also the most critical process for quality control. Improper degreasing can cause numerous product defects. A lot of research work has shown that the degreasing process and the addition of the binder are inseparable, and the degreasing method used is also different for different binders.
