Ceramic molding is to make ceramic raw materials into green bodies with specified shape, size and relative strength according to the actual production requirements. The molding process depends on the properties of the ceramic raw materials and the molding process. There are various methods for creating the shape of ceramic products, but in general, they can be divided into dry forming and wet forming.

Dry forming includes dry pressing, isostatic pressing, ultra-high pressure forming, powder electromagnetic forming and other methods. Wet forming can be roughly divided into two categories: plastic forming and colloidal casting. Plastic molding, also known as wet pressing, refers to a molding method in which plastic materials are pressed into rigid molds, including extrusion molding, injection molding, and film rolling (calendering). Colloidal casting molding is a molding method that makes a fluid slurry into a self-supporting shape. This method utilizes the fluidity of the slurry to dry and solidify the material to obtain a relatively shaped molded body. It mainly includes grouting molding, injection molding, tape casting, direct solidification molding, colloidal vibration injection molding, etc.

dry pressing

Dry pressing is to add a relative amount of organic additives (binders, lubricants, plasticizers, defoamers, water reducers, etc.) to the ceramic powder, and make it molded in the mold under the action of external pressure.

Advantages: It is easy to realize automation, so it is widely used in industrial production.

Disadvantages: During the molding process, the delamination of the green body is often caused by uneven radial and axial pressure distribution, and cracking and uneven density also often occur.

isostatic pressing

Isostatic pressing is a method of compressing powder by applying isotropic pressure. According to the different molding temperature, isostatic pressing is divided into hot isostatic pressing and cold isostatic pressing. Cold isostatic pressing is an isostatic pressing method for forming workpieces at room temperature. Hot isostatic pressing refers to the isostatic pressing method in which the workpiece is isostatically formed and sintered under high temperature and high pressure. Advantages: can press parts with concave, hollow, slender and other complex shapes; small friction loss, low molding pressure; pressure is transmitted from all aspects, uniform density distribution of green compact, high compact strength, convenient mold making, long life long and low cost.

Disadvantages: It is not easy to precisely control the size and shape of the green compact, the productivity is low, the investment is large, the operation is more complicated, the molding is operated under high pressure, and the container and other high-pressure components need to be protected.

Ultra high pressure forming

Ultra-high pressure molding is a rapidly developing molding method, which is mostly used in the molding of nano-ceramics. The particle size of nano-ceramics is greatly affected by the sintering temperature. The lower the sintering temperature, the smaller the particle size, and the easier it is to obtain nano-ceramics; and by increasing the molding pressure and increasing the initial density of the green body, the sintering temperature of the nano-ceramics can be reduced. Therefore, ultra-high pressure forming came into being. Ultra-high pressure molding significantly changes the sintering properties of the green body, so that nano-ceramics can be easily obtained.

Powder Electromagnetic Forming

Powder electromagnetic pressing is a new high-efficiency molding process that utilizes strong pulse electromagnetic force to act on powder to densify it. This method is usually used for the forming of metal materials and can obtain higher density. The densities of the samples formed by the powder electromagnetic pressing method all reach more than 95%, and they have good piezoelectric and dielectric properties.

Extrusion

Mix the powder, adhesive, lubricant, etc. with water evenly and fully knead, and then use the hydraulic press to push the piston to extrude the plasticized billet from the extrusion nozzle. As the inner shape of the extrusion nozzle is gradually reduced, the piston produces a great extrusion force on the mud mass, which makes the billet dense and shaped.

Advantages: The ceramic membrane tube obtained by extrusion molding has large porosity, density and compressive strength, and the pore size distribution is concentrated, the gas permeation flux is large, and it is an excellent ceramic membrane tube. It is suitable for making pipes or bars with round, oval, polygonal and other special-shaped fracture surfaces.

Disadvantages: The material has low strength and is easy to deform, and may produce surface pits and blistering, cracking and internal cracks and other defects.

injection molding

Injection molding, also known as hot die casting, is a technology that gives metal powder, ceramic powder and polymer similar fluidity by adding a relative amount of polymer and additive components and slightly heating, and filling the slurry into the metal mold under pressure. , and after cooling, the blank is obtained by de-blanking. It is listed as an important "national key technology" by developed countries such as the United States. Advantages: high utilization rate of raw materials, fast and automatic mass production; special-shaped parts with small volume, complex shape and high dimensional accuracy can be prepared; due to the flow punching die, the green body density is uniform, and the sintered product performance is excellent; the production cost is low.

Calendering

The powder, additives and water are uniformly mixed to make plastic material, and then the material is rolled by two oppositely rotating rolls, thereby becoming a forming method of plate-shaped china. The green body obtained by rolling film forming has high density and is suitable for forming sheet-like and plate-like objects.

grouting

The grouting method is to inject the prepared mud into the gypsum model. Because the gypsum model has the properties of air permeability and water absorption, after the mud contacts the model, the water in the mud will be gradually sucked into the model wall, and the fine particles in the mud will follow the model. When the thick mud layer reaches the expected thickness, the excess mud in the model can be poured out. After the moisture in the thick mud layer is continuously absorbed by the model to achieve independent molding, the green body can be taken out and dried for repair.

Advantages: low process cost, simple process, easy operation and control.

Disadvantages: The molding shape is rough, the grouting time is long, and the density and strength of the green body are not high.

Centrifugal grouting

Centrifugal grouting is developed on the basis of traditional grouting. By adjusting the pH value and other process parameters, the powder is uniformly dispersed in the liquid, and deposited and formed under the action of the centrifugal force of high-speed rotation. Centrifugal grouting combines wet chemical powder preparation with stress-free densification technology. On the one hand, it can prevent powder agglomeration and other defects; It can be used for the preparation of multi-layer and gradient composite functional materials.

Electrophoretic deposition molding

Electrophoretic deposition molding is to use a DC electric field to promote the migration of charged particles, and then deposit them on electrodes with opposite polarities.

Features: Simple operation, flexibility and high reliability, so it is suitable for the molding method of multilayer ceramic capacitors, sensors, and gradient functional ceramics, but it is sensitive to changes in process parameters.

Gel injection molding

Injection molding is to add vinyl organic monomers in the suspension medium, and then use catalysts and initiators to cross-link the organic monomers through free radical reaction, and the green body is cured in situ. Gel injection molding technology is a new colloidal rapid prototyping process first invented by researchers at Oak Ridge National Laboratory in the early 1990s.

Advantages: The green body has high strength and is easy to machine.

Disadvantages: The shrinkage rate of the green body is relatively large during the densification process, which causes the green body to bend and deform, and the organic monomers used are toxic, and the reaction atmosphere is difficult to control.

tape casting

Tape casting refers to a molding method in which solvent, dispersant, binder, plasticizer and other components are added to ceramic powder to obtain a uniformly dispersed and stable slurry, and a film of the required thickness is obtained on a casting machine.

Advantages: simple equipment, continuous operation, high production efficiency, high automation level, stable process, uniform green body performance, etc.

direct solidification

Direct solidification molding is a new concept of net size in-situ ceramic molding technology invented in the 1990s by Prof. Gauckler's research group at the Swiss Federal Institute of Technology in Zurich, which combines biological enzyme technology, colloidal chemistry and ceramic technology.

Advantages: no or only a small amount of organic additives (less than 1wt%) is needed, the green body does not need to be degreasing, the green body density is uniform, the relative density is high, and the ceramic parts of large size and complex shape can be formed.

Disadvantages: The green body strength is often not high enough.

Colloidal Vibration Injection Molding

Colloidal vibration injection molding is a colloidal molding technology invented in 1993 by Professor F.F. Lange of the University of California, Santa Barbara. The prepared dilute suspension containing high ionic strength (20% to 30% (vol)) is filtered or centrifuged to obtain a high solid content blank, and then poured under the action of vibration to achieve in-situ curing.

Advantages: Continuous production can be achieved, and ceramic parts of complex shapes can be formed.

Disadvantages: The green body has low strength, and the green body is easy to crack and deform when demoulding.

temperature induced flocculation

Temperature-induced flocculation molding is a net-size colloidal molding method invented by Sweden L. Bergstrom in 1994. DCC and colloidal vibration injection molding utilize the electrostatic stability of colloids and this method utilizes the space (steric hindrance) of colloids. Stable characteristics.

Advantages: Unqualified green bodies can be reused as raw materials after demoulding, and can be used to shape almost all ceramic powder systems.

Solid moldless molding

The concept of modern solid moldless molding technology appeared around the end of the 1970s. In the early 1990s, the University of Texas proposed the idea of free-form manufacturing and applied it to the field of ceramics. Solid moldless molding technology breaks through the limitations of traditional molding ideas and is a "growth" based molding method. In the field of ceramics, the solid moldless molding process can be divided into: laser selective sintering molding, three-dimensional printing molding, fused deposition molding, layered manufacturing molding, stereolithography molding, etc.

Features: high flexibility, high technology integration, rapidity, free-form manufacturing, etc. The main problems of this technology are: high equipment price, software development, material development, forming accuracy and quality and so on.

Generally speaking, dry forming technologies such as steel mold pressing and isostatic pressing have developed earlier, with high technological maturity and high degree of automation. They are the main methods of forming special ceramics at present. Applications in high-performance fine ceramic molding. The colloidal molding method represented by injection molding and tape casting is a new technology in wet molding, which not only has the advantages of low cost, but also has a high degree of automation and can realize large-scale production, which represents the special ceramic molding method. Direction of development. In the future, the future development of special ceramic molding technology will focus on the following aspects:

(1) Further develop the application of various dieless forming technologies that have been proposed in the preparation of different ceramic materials;

(2) The design of structural layers with complex properties and the three-dimensional changes of interspersed, interwoven, connected structures and components within the layers;

(3) Structural design and manufacture of large special-shaped parts;

(4) Manufacture and practical application of ceramic microstructure;

(5) Further develop new technologies that are non-polluting and harmonized with the environment.