The forming process plays a pivotal role in the entire preparation of ceramic materials and is the key to ensuring the performance reliability and production repeatability of ceramic materials and components.

With the development of society, traditional ceramic techniques such as manual molding, wheel forming, and injection molding can no longer meet the demands of modern society for increased production and refinement. Thus, new forming processes have emerged. The main 9 widely used forming processes for ZrO2 fine ceramic materials are as follows (2 dry methods and 7 wet methods) :

1. Dry molding

Dry pressing molding

Dry pressing molding uses pressure to press ceramic powder into a green body of a certain shape. In essence, under the action of external force, powder particles approach each other within the mold and are firmly combined by internal friction, maintaining a certain shape. The main defect in dry-pressed green bodies is layer cracking, which is caused by the internal friction between the powder materials and the friction between the powder materials and the mold wall, resulting in pressure loss inside the green body.

The advantages of dry pressing molding are accurate blank size, simple operation and easy realization of mechanized operation. The moisture and binder content in dry-pressed green billets is relatively low, and the shrinkage during drying and firing is small. It is mainly used to form products with simple shapes and has a small length-to-diameter ratio. The increase in production costs caused by mold wear is a drawback of dry pressing molding.

Isostatic pressing forming

Isostatic pressing is a special forming method developed on the basis of traditional dry pressing. It uses fluid to transmit pressure and uniformly applies pressure to the powder in the elastic mold from all directions. Due to the consistency of the internal pressure of the fluid, the powder is subjected to the same pressure in all directions, thus avoiding the difference in density within the green body.

Ceramic structural components formed by isostatic pressing

Isostatic pressing can be divided into wet bag isostatic pressing and dry bag isostatic pressing. Wet bag isostatic pressing can form products with relatively complex shapes, but it can only be done intermittently. Dry bag isostatic pressing can achieve automated continuous operation, but it can only form products with simple cross-sections such as square, circular, and tubular. Isostatic pressing can produce uniform and dense green bodies with small firing shrinkage and uniform shrinkage in all directions. However, the equipment is relatively complex and expensive, and the production efficiency is not high. It is only suitable for producing materials with special requirements.

Ii. Wet Molding

Grouting molding

The grouting molding process is similar to the casting molding process, but the difference lies in that its molding process includes a physical dewatering process and a chemical coagulation process. Physical dewatering is achieved by the capillary action of the porous gypsum mold to remove water from the slurry. The chemical coagulation process is due to the dissolution of CaSO4 on the surface of the gypsum mold to generate Ca2+, which increases the ionic strength in the slurry, causing the slurry to flocculate.

Under the action of physical dehydration and chemical coagulation, ceramic powder particles are deposited and formed on the gypsum mold wall. Grouting molding is suitable for preparing large ceramic components with complex shapes, but the quality of the green bodies, including shape, density and strength, is relatively poor. The labor intensity of workers is high and it is not suitable for automated operations.

Hot die casting forming

Hot die casting is a process in which ceramic powder is mixed with a binder (paraffin) at a relatively high temperature (60 to 100℃) to obtain the slurry for hot die casting. The slurry is injected into a metal mold under the action of compressed air, held under pressure and cooled, and then demolded to obtain a wax body. The wax body is dewaxed under the protection of inert powder to obtain a plain body, which is then fired at high temperature to form porcelain.

The green billets formed by hot die casting have precise dimensions, uniform internal structure, less mold wear, high production efficiency and are suitable for various raw materials. The temperature of the wax paste and the mold needs to be strictly controlled; otherwise, underfilling or deformation may occur. Therefore, it is not suitable for manufacturing large components. Meanwhile, the two-step firing process is relatively complex and has high energy consumption.

Casting molding

Cast film forming involves thoroughly mixing ceramic powder with a large amount of organic binders, plasticizers, dispersants, etc., to obtain a viscous slurry that can flow. The slurry is then added to the hopper of the cast film machine, with the thickness controlled by a scraper. It flows out through the feeding nozzle onto the conveyor belt and is dried to obtain a film blank.

This process is suitable for preparing thin film materials. To achieve better flexibility, a large amount of organic substances are added. It is necessary to strictly control the process parameters; otherwise, defects such as peeling, streaks, low film strength or difficulty in peeling may occur. The organic substances used are toxic and can cause environmental pollution. Therefore, non-toxic or minimally toxic systems should be adopted as much as possible to reduce environmental pollution.

Rubber injection molding

Gel injection molding technology was first invented by researchers at the Oak Ridge National Laboratory in the United States in the early 1990s as a new colloidal rapid prototyping process. The core is to use an organic monomer solution, which can polymerize into a high-strength, laterally connected polymer-solvent gel.

The slurry formed by dissolving ceramic powder in the solution of organic monomers is poured into the mold, and the monomer mixture polymerizes to form a cementitious component. Since there is only 10% to 20%(by mass) of the polymer in the laterally connected polymer-solvent, it is easy to remove the solvent in the gel component through the drying step. Meanwhile, due to the lateral connection of the polymer, during the drying process, the polymer cannot migrate with the solvent.

This method can be used to manufacture single-phase and composite ceramic components. It can form ceramic components with complex shapes and precise dimensions, and its green body strength can reach over 20 to 30Ma, allowing for further processing. The main problem with this method is that the shrinkage rate of the embryo is relatively high during the densification process, which is prone to cause deformation of the embryo. Some organic monomers have oxygen polymerization inhibition, resulting in surface peeling and shedding. Due to the temperature-induced polymerization process of organic monomers, temperature scaling occurs, resulting in the existence of internal stress and causing cracking and damage of the green body, etc.

Direct solidification injection molding

Direct solidification injection molding is a forming technology developed by the Swiss Federal Institute of Technology in Zurich: solvent water, ceramic powder and organic additives are thoroughly mixed to form a statically stable, low-viscosity and high-solid-content slurry. Chemical substances that can change the H value of the slurry or increase the concentration of the electrolyte are added to it, and then the slurry is injected into a hole-free mold.

Control the progress of chemical reactions during the technological process. The reaction proceeds slowly before injection molding, keeping the slurry of low viscosity. After injection molding, the reaction accelerates, the slurry solidifies, and the flowing slurry is transformed into a solid green body. The obtained green body has excellent mechanical properties, and its strength can reach 5ka. After the green body is demolded, dried and sintered, ceramic components of the required shape are formed.

Its advantages include no or only a small amount of organic additives (less than 1%), no need for degreasing of the body, uniform body density, high relative density (55% to 70%), and the ability to form large-sized and complex-shaped ceramic components. Its drawback is that the additives are expensive and gas is usually released during the reaction process.

Injection molding

Injection molding has long been applied to the molding of plastic products and metal molds. This process utilizes the low-temperature curing of thermoplastic organic substances or the high-temperature curing of thermosetting organic substances. The powder and organic carriers are mixed in a dedicated mixing device and then injected into the mold under high pressure (tens to hundreds of Ma) for shaping. Due to the high forming pressure, the obtained green body has precise dimensions, high smoothness and dense structure. The adoption of specialized forming equipment has greatly enhanced the production efficiency.

Injection molding technology was first applied to the molding of ceramic parts in the late 1970s and early 1980s. This process achieves plastic molding of non-plastic materials by adding a large amount of organic matter and is a common plastic molding process in ceramics. In injection molding technology, in addition to using thermoplastic organic compounds (such as polyethylene, polystyrene), thermosetting organic compounds (such as epoxy resin, phenolic resin), or water-soluble polymers as the main binders, a certain amount of plasticizers, lubricants, and coupling agents and other process AIDS must also be added to improve the fluidity of ceramic injection suspensions. And ensure the quality of the injection molding green body.

Injection molding process has the advantages of high degree of automation and precise size of the formed blank. However, the organic matter content in the green bodies of injection molded ceramic components can be as high as 50vol %. It takes a long time, even several days to dozens of days, to remove these organic substances during the subsequent sintering process, and it is prone to cause quality defects.

Gel injection molding

To address the problems of large amounts of organic substances added and difficult removal in traditional injection molding processes, Tsinghua University has creatively proposed a new colloidal injection molding process for ceramics, independently developed a colloidal injection molding prototype, and achieved the injection molding of refractory ceramic slurry.

The basic idea is to combine gel molding with injection molding, and achieve it by using the proprietary injection equipment and the new curing technology provided by the gel in-situ solidification molding process. This new process uses no more than 4wt-% organic matter. It rapidly induces the polymerization of organic monomers or organic compounds in a small amount of water-based suspension after injection into the mold to form an organic network framework, which uniformly encapsulates the ceramic powder. This not only significantly shortens the glue discharge time but also greatly reduces the possibility of glue discharge cracking.

There is a significant difference between injection molding and gel molding of ceramics. The main distinction lies in that the former falls within the category of plastic molding, while the latter belongs to slurry molding, meaning the slurry has no plasticity and is a non-plastic material. Due to the lack of plasticity of the slurry, colloidal molding cannot adopt the traditional idea of ceramic injection molding. If colloidal molding is combined with injection molding, that is, by using the proprietary injection equipment and the new curing technology provided by the colloidal in-situ molding process, the colloidal injection molding of ceramic materials can be achieved.

The new colloidal injection molding process for ceramics is distinct from both the common colloidal molding and the traditional injection molding. It will not only feature the uniformity of the colloidal in-situ solidification molding body and low organic matter content, but also have the advantage of high injection molding automation. It represents a qualitative elevation of the colloidal molding process and will become the hope for the industrialization of high-tech ceramics.

Ceramics have high hardness and relatively high brittleness. During processing, situations such as chipping and cracking are prone to occur. Usually, ceramic parts are processed using dedicated ceramic engraving machines, which can effectively reduce costs and achieve high-precision processing.