The molding process plays a connecting role in the entire ceramic material preparation process and is the key to ensuring the performance reliability and production repeatability of ceramic materials and components.
With the development of society, traditional ceramic hand-kneading methods, wheel forming methods, grouting methods, etc. can no longer meet the production and refinement needs of modern society, so A new molding process was born. The following 9 types of ZrO2 fine ceramic material molding processes are widely used (2 dry methods and 7 wet methods):
Dry press molding uses pressure to melt ceramic powder The essence of pressing a green body into a certain shape is that under the action of external force, the powder particles are close to each other in the mold and are firmly combined by internal friction to maintain a certain shape. The main defects in the dry pressing green body are layers. This is due to the internal friction between the powder and the friction between the powder and the mold wall, causing pressure loss inside the green body.
The advantages of dry pressing molding are accurate body size, simple operation, and convenient mechanization operation; the dry pressing green body contains less moisture and binder, and the shrinkage during drying and firing is small. It is mainly used to form products with simple shapes and a small aspect ratio. Mold wear and tearThe resulting increase in production costs is a shortcoming of dry press molding.
Isostatic pressing is a special molding method developed on the basis of traditional dry pressing. It uses fluid to transmit pressure and evenly apply pressure to the powder in the elastic mold from all directions. Due to the consistency of the internal pressure of the fluid, the powder bears the same pressure in all directions, thus avoiding density differences within the body.
Picture: Ceramic structural parts 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 operate intermittently. Dry bag isostatic pressing can realize automatic continuous operation, but it can only form products with simple shapes such as square, round, tubular cross-sections. Isostatic pressing can produce a uniform and dense green body with small firing shrinkage and uniform shrinkage in all directions. However, the equipment is complex and expensive, and the production efficiency is not high. It is only suitable for the production of materials with special requirements.
The grouting molding process is similar to tape casting. The difference is that the molding process includes a physical dehydration process and a chemical coagulation process. Physical dehydration passes through the capillaries of the porous gypsum mold. It removes the moisture in the slurry. The chemical coagulation process is because the Ca2+ generated by the dissolution of CaSO4 on the surface of the gypsum mold increases the ionic strength in the slurry, causing flocculation of the slurry.
Under the action of physical dehydration and chemical coagulation, ceramic powder particles are deposited on the plaster mold wall. Slurry injection molding is suitable for preparing complex-shaped molds. Large ceramic parts, but the body quality, including shape, density, strength, etc., is labor-intensive and not suitable for automated operations.
Hot die casting is to mix ceramic powder and binder (paraffin) at a relatively high temperature (60~100℃) to obtain a slurry for hot die casting. The slurry acts on the compressed air Inject into the metal mold, keep under pressure and cool, and demould to obtain a wax blank. The wax blank is dewaxed under the protection of inert powder to obtain a blank, which is then sintered at high temperature to become porcelain.
The green body formed by hot die casting has precise dimensions, uniform internal structure, small mold wear, high production efficiency, and is suitable for various raw materials. The temperatures of the wax slurry and the mold need to be strictly controlled, otherwise it will cause underfill or deformation, so it is not suitable for manufacturing large parts. At the same time, the two-step firing process is more complicated and consumes higher energy.
Tape casting is to fully mix ceramic powder with a large amount of organic binders, plasticizers, dispersants, etc. to obtain a flowable viscous slurry , add the slurryIn the hopper of the casting machine, a scraper is used to control the thickness, and the film flows out to the conveyor belt through the feeding nozzle. After drying, the film base is obtained.
This process is suitable for preparing thin film materials. In order to obtain better flexibility, a large amount of organic matter is added, which requires strict control of process parameters. Otherwise, it will easily cause peeling, stripes, low film strength or difficulty in peeling off. and other defects. The organic matter used is toxic and will cause environmental pollution. Non-toxic or less toxic systems should be used as much as possible to reduce environmental pollution.
Gel injection molding technology is a new colloidal rapid prototyping process first invented by researchers at the Oak Ridge National Laboratory in the early 1990s. . At its core is the use of organic monomer solutions that polymerize into strong, laterally linked polymer-solvent gels.
The slurry formed by dissolving the ceramic powder in a solution of organic monomers is poured into the mold, and the monomer mixture polymerizes to form a gelled part. Since only 10% to 20% (mass fraction) of the transversely connected polymer-solvent) polymer, therefore, the solvent in the gel component can be easily removed through a drying step. At the same time, due to the lateral connections of the polymer, the polymer cannot migrate with the solvent during the drying process.
This method can be used to manufacture single-phase and composite ceramic parts. It can form ceramic parts with complex shapes and quasi-net dimensions. Its green strength is as high as 20~30Mpa and can be re-molded. processing. The main problems with this method are that the shrinkage rate of the embryo body is relatively high during the densification process, which can easily lead to deformation of the embryo body; some organic monomers have oxygen-inhibited polymerization, causing surface peeling and shedding; due to the temperature-induced polymerization process of organic monomers, Temperature gradient leads to the existence of internal stress, causing the green body to break apart, etc.
Direct solidification injection molding
Direct solidification injection moldingMolding is a molding technology developed by the Federal Institute of Technology in Zurich: solvent water, ceramic powder and The organic additives are thoroughly mixed to form an electrostatically stable, low viscosity, high solid content slurry., adding chemicals that change the pH of the slurry or increasing the electrolyte concentration, and then injecting the slurry into a nonporous mold.
Control the progress of chemical reactions during the process. The reaction proceeds slowly before injection molding and the slurry maintains low viscosity. After injection molding, the reaction proceeds faster and the slurry solidifies, turning the fluid slurry into a solid body. The obtained green body has good mechanical properties and the strength can reach 5kPa. After the green body is demoulded, dried and sintered, the ceramic parts of the required shape are formed.
Its advantages are that it requires no or only a small amount of organic additives (less than 1%), the body does not need to be degreased, the density of the body is uniform, and the relative density is high (55%~70%) , can form large-sized and complex-shaped ceramic parts. Its disadvantage is that the additives are expensive and gas is generally released during the reaction.
Injection molding has long been used in the molding of plastic products and the molding of metal molds. This process uses low-temperature curing of thermoplastic organic matter or high-temperature curing of thermosetting organic matter. The powder and organic carrier are mixed in a special mixing equipment, and then injected into the mold under high pressure (tens to hundreds of MPa). Due to the high molding pressure, the resulting green body has precise dimensions, high smoothness and dense structure; the use of specialized molding equipment greatly improves production efficiency.
In the late 1970s and early 1980s, the injection molding process began to be applied to the molding of ceramic parts. This process achieves the plastic molding of infertile materials by adding a large amount of organic matter. It is a ceramic A common type of plastic molding process. In injection molding technology, in addition to using thermoplastic organic matter (such as polyethylene, polystyrene), thermosetting organic matter (such as epoxy resin, phenolic resin), or water-soluble polymer as the main binder, a certain amount of Amounts of process aids such as plasticizers, lubricants and coupling agents are used to improve the fluidity of the ceramic injection suspension and ensure the quality of the injection molded body.
The injection molding process has the advantages of high degree of automation and precise size of the molded body. However, the green body of injection molded ceramic parts contains up to 50vol% organic matter. It takes a long time to remove these organic matter during the subsequent sintering process, even as long as several days to tens of days, and it is easy to cause quality defects.
Colloidal injection molding
In order to solve the problem of organic matter in the traditional injection molding process Due to problems such as large amounts of addition and difficulty in troubleshooting, Tsinghua University creatively proposed a new colloidal injection molding process for ceramics, independently developed a colloidal injection molding prototype, and realized injection molding of barren ceramic slurries. span>
The basic idea is to combine colloidal molding with injection molding, using proprietary injection equipment and the new solidification technology provided by the colloidal in-situ solidification molding process. This new process uses no more than 4wt.% of organic matter. It uses a small amount of organic monomers or organic compounds in a water-based suspension to quickly induce the polymerization of organic monomers after being injected into the mold to generate organic network bones.The frame evenly wraps the ceramic powder in it, which not only greatly shortens the debinding time, but also greatly reduces the possibility of debinding cracks.
There is a huge difference between injection molding and colloidal molding of ceramics. The main difference is that the former belongs to the category of plastic molding, while the latter belongs to slurry molding, that is, the slurry has no plasticity and is a barren material. . Colloidal molding cannot use traditional ceramic injection molding ideas because the slurry has no plasticity. If colloidal molding is combined with injection molding, the new solidification technology provided by proprietary injection equipment and colloidal in-situ molding process can be used to achieve colloidal injection molding of ceramic materials.
The new ceramic colloidal injection molding process is different from both general colloidal molding and traditional injection molding. It has the advantages of colloidal in-situ solidification and good uniformity of the green body. , with the characteristics of low organic content and the advantage of high injection molding automation, it is colloidal moldingA qualitative sublimation of craftsmanship will become the hope for the industrialization of high-tech ceramics.
