Injection molding method for advanced ceramics

1. Process principle of injection molding

The basic principle of the injection molding process is to add a small amount of organic monomers to a low viscosity, high solid content powder solvent suspension system, and then use catalysts and initiators to polymerize and crosslink the organic monomers in the suspension to form a three-dimensional network structure, allowing the liquid slurry to solidify in situ and form a green body with a composite structure of powder and polymer substances. Then, it is demolded, dried, organic matter removed, and sintered. Finally, obtain the required ceramic components.

2. The process and application of injection molding

(1) The basic components of the injection molding process are ceramic powder, organic monomer, polymerization catalyst, dispersant, and solvent. The injection molding process can be divided into water-based and non-water-based injection molding processes according to the different solvents used. If the solvent is water, it becomes water-based injection molding; If the solvent is an organic solvent, it is called non aqueous injection molding. The organic solvents in non aqueous injection molding have low vapor pressure and relatively low viscosity at crosslinking temperature. Compared with non aqueous injection molding, water-based injection molding has many advantages: its injection process is closer to traditional ceramic molding process; The green body is easy to dry; The viscosity of cosolvent gel is reduced; Avoiding environmental pollution caused by organic solvents.

① Preparation of slurry

Low viscosity slurry is beneficial for both mixing and grouting. Therefore, optimizing the solid content is very important to maintain the fluidity of the slurry. It can obtain a slurry with good fluidity through a low viscosity premixed solution. Choosing a suitable dispersant can increase the fluidity of the mud. In order to achieve good dispersion effect, an appropriate amount of dispersant needs to be added. Choosing appropriate dispersants and controlling the pH value of the slurry to ensure its flowability are prerequisites for injection molding.

② Gel

Since the gel reaction is a Exothermic reaction, the start of polymerization of the mixture depends on the temperature of the solution. The gel process is controlled by the residence time. The capture time refers to the time from the addition of an initiator or catalyst to the start of polymerization, which is the time that grouting can take place.

③ Drying

Drying is a crucial step in the production process of ceramic materials in order to reduce the deformation of the body. For thin billets, the main change is humidity, which can be dried at room temperature; Unlike injection molding and pure cementation, injection molding has no constant stage during drying; Changing humidity and temperature can significantly reduce drying time.

④ Rubber discharge

The glue discharge process can be completed simultaneously during sintering or separately before sintering at low temperatures. Ultimately, ceramic components of near net size can be obtained.

(2) With the continuous progress of modern technology, the application of injection molding has also led to rapid development of ceramic preparation technology, especially the emergence of new forming methods that have laid the technical foundation for the development of high-performance ceramics. The injection molding technology has a series of excellent characteristics such as simple process, low production cost, good uniformity of the produced body, high density, and close to net molding, which has taken high-performance ceramic molding technology to a new level. The application of injection molding process is also more extensive than other molding processes, and this process is mainly applied in the following aspects:

① Preparation of Nanoceramics

At present, one of the research hotspots in the field of ceramic materials is the preparation of high-performance ceramic materials using nanoscale powder systems.

② Preparation of Multicomponent Composite Ceramics

The biggest problem and difficulty in preparing composite materials using injection molding technology is how to prepare a slurry with high solid volume fraction, good flowability, and uniform dispersion.

③ Application in the field of powder metallurgy

The injection molding process technology was originally used as a near net size molding technology for ceramic materials, which is suitable for preparing large-sized and complex shaped parts.

④ Preparation of porous gradient ceramics by foam solidification method

With the continuous expansion of the application range of porous ceramic materials, significant progress has been made in the research of preparation techniques for porous ceramic materials.

3. Process characteristics of injection molding

The injection molding technology not only combines the advantages of other molding methods, but also overcomes the shortcomings of other molding methods. Its main characteristics are as follows:

(1) The formed body has high strength and can be mechanically processed into complex shaped components. Due to the action of organic polymers, the strength of the green body can reach 20-40MPan. Parts with this strength can be processed by various mechanical processes, resulting in more complex shapes, more precise dimensions, and smoother surfaces. Therefore, this process reduces the production cost of the parts.

(2) The overall uniformity of the body is good, which can reduce the sintering temperature and greatly improve the reliability of ceramic parts. Due to the unchanged composition and volume before and after liquid-solid conversion during the molding process, and the particles solidify in situ. So as long as the molding process is fully filled, all parts of the preform have the same density, which can form large volume parts and ensure the uniformity of the final product, improving the reliability of engineering ceramics.

(3) Compared with the injection molding process, the organic content is relatively low and there is no difficulty in molding. The organic matter in the slurry generally only accounts for 10% -20% of the liquid phase medium, which is equivalent to 3% -5% of the weight of the ceramic dry material. Therefore, its extrusion process is relatively easy and can be completed synchronously with the sintering process, avoiding the time-consuming and energy consuming extrusion process of the injection molding process, saving energy and reducing costs.

(4) Can achieve near net size molding. Due to the small volume change of the slurry during the liquid-solid conversion process, the size of the solidified green body basically depends on the model. Moreover, due to the high solid content of the slurry, the formed green body has a high density, small drying shrinkage and firing shrinkage, and small deformation, which can achieve near net size molding.

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