Special ceramic materials such as silicon nitride have excellent properties such as high strength, high wear resistance, low density (light weight), heat resistance and corrosion resistance, and are suitable for manufacturing turbine feeder impellers, rocker burners, and auxiliary combustion. Ceramic parts for automobiles such as rooms. These components require complex shapes, high precision dimensions and high reliability. Intrinsic defects (cracks, pores, foreign matter, etc.) and surface defects are not allowed.
One of the forming techniques that can meet these quality requirements is the ceramic injection molding process. The ceramic injection molding technology is derived from the injection molding of polymer materials, and is formed by the properties of high molecular weight polymer melting at a high temperature and solidification at a low temperature, and the polymer is removed after molding. It is much simpler than traditional ceramic processing technology, and can produce high-precision ceramic parts of various complicated shapes, and is easy to scale and automate production.
At present, CiM (ceramic colloidal injection molding method and device) technology invented by Professor Yang Jinlong of the Department of Materials Science and Engineering of Tsinghua University is at the leading level in this field in China.
Ceramic injection molding technology has many advantages. It can be used to prepare ceramic components with complex shapes. It not only has high dimensional accuracy, good surface conditions, but also eliminates post-processing operations, reduces production costs, shortens production cycle, and has automation. High and suitable for large-scale production. The process generally includes the following steps: selection of ceramic powder, selection of binder, uniform mixing of ceramic powder and binder, injection molding, degreasing, sintering. Degreasing is the key.
The original ceramic molding injection technology is to mix a large amount of polymer resin and ceramic powder to obtain a mixture, and then put it into an injection machine at a certain temperature to inject into a mold, and quickly condense and then release the mold to form a green body. The technology is suitable for preparing large-scale production of wet blanks with high strength, high dimensional precision, low mechanical processing and uniform green body. It has obvious advantages for the preparation of products with complex shapes and thin thickness. However, since a large amount of polymer binder is contained, the degreasing of the ceramic body becomes an insurmountable problem, and the blank is easily deformed, and the pores are easily formed.
The binder fills the powder into the desired shape, which has an important effect on the overall process. The ideal binder should have the following characteristics:
1) The viscosity of the pure binder at the molding temperature is below 1 Pa·s, and does not separate from the powder when flowing, and has sufficient strength and hardness after cooling;
2) is an inert substance and does not react with the powder;
3) Decomposition only above the molding and mixing temperatures, the decomposed product is non-toxic, non-corrosive and has less residual ash;
4) The coefficient of expansion is low, and the residual stress caused by thermal expansion or crystallization is low;
5) It meets environmental protection requirements, and is inexpensive, safe, non-hygroscopic, non-volatile, and has a long storage life.
Most of the binders currently in use can be classified into three categories: wax-based or oil-based binders, water-based binders, and solid polymer solutions. The wax-based binder usually contains 3-4 components, and the polymer controls the flow viscosity, the strength of the raw product (the body before sintering), and the characteristics of degreasing. The short molecular chains are well formed and minimize the orientation in the shaped elements. The wax or oil is the primary filler and is removed at the beginning of the degreasing. Surfactants are used to improve the compatibility of the powder with the binder. Plasticizers are used to adjust the flow characteristics of the polymer. Water-based binders contain water soluble polymers, gels or water glass. Such binders are typically formed at low pressure to avoid separation of the powder from the binder and to reduce mold wear and residual stress. This makes it possible to manufacture thicker components since the water is easy to remove. Solidification or gelation of the binder solution gives the raw product strength. Before sintering, water evaporates or sublimates from the product to minimize distortion. A new binder formulation of a solid polymer solution using polystyrene has been employed to avoid deformation. The primary filler is removed by solution impregnation. Since the skeleton structure of polystyrene cannot be weakened, deformation of the raw material is avoided. The primary filler is a small organic molecule that has both a benzene ring and a polar group. The benzene ring makes it soluble in polystyrene when mixed, and the polar group makes it soluble in solvents such as water or alcohol when it is degreased.
Common binders are polypropylene (PP), random polypropylene (APP), polyethylene (PE), ethylene-vinyl acetate copolymer (EVA), polystyrene (PS), acrylic resin, and the like. Among them, PE has excellent formability; EVA has good compatibility with other resins, and has good fluidity and formability; APP has good compatibility with other resins, rich in fluidity and degreasing property; and PS has good fluidity. The additives include wax stone wax, microcrystalline paraffin wax, denatured paraffin wax, natural paraffin wax, stearic acid, compounding agent and the like. The fluidity of the shaped material can be evaluated using a high flow point meter and a melt indexer. When the degreasing has a large content of the binder, the degreasing property tends to be lowered, and the wax of the auxiliary agent is many, and the degreasing property is good. If the organic material cannot be completely scattered in a specific temperature region, it will affect the sintering of the ceramic. Therefore, it is necessary to consider the thermal decomposition characteristics and select it.
