We said: the molding process of new industrial ceramic materials is divided into 9 categories. In the last issue, we have introduced the 2 categories of dry method. Today we will continue to talk about the wet molding as follows7 Category:
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1. Grouting molding The 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 capillary action of the porous gypsum mold. To eliminate 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 gypsum mold wall. . Slip injection molding is suitable for preparing large ceramic parts with complex shapes, but the quality of the green body, including shape, density, strength, etc., is high, labor-intensive and not suitable for automated operations.
2. 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. Inject the metal mold under the action of compressed air, keep the 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, and high production efficiency. High, 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.
3. Tape casting is to bond ceramic powder with a large amount of organicThe agent, plasticizer, dispersant, etc. are fully mixed to obtain a flowable viscous slurry. The slurry is added to the hopper of the casting machine, the thickness is controlled with a scraper, and 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. Strict control of process parameters is required, otherwise defects such as peeling, streaks, low film strength or difficulty in peeling may easily occur. 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.
4. Gel injection molding technology was developed by researchers at the Oak Ridge National Laboratory in the United States in the 1990s. A new colloidal rapid prototyping process first invented in the early 1990s. Its core is the use of organic monomer solutions that can polymerizeBecomes a strong, laterally linked polymer-solvent gel. The slurry formed by dissolving 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 polymer is present in the laterally linked polymer-solvent, the solvent in the gel part is 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, and can form complex shapes, accurate Ceramic parts with net size, and their green strength is as high as 20-30Mpa or more, which can be reprocessed. 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.
5. Direct solidification injection moldingShaping 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-solids content slurry, chemicals that can change the pH of the slurry or increase the electrolyte concentration are added to it, and the slurry is then injected into a non-porous 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 green body does not need to be degreased, the density of the green body is uniform, and the relative density is high (55%~70%), it 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.
6. Injection molding It was early 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).type. 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. The injection molding process began to be applied to the molding of ceramic parts in the late 1970s and early 1980s. This process achieved the formation of infertile materials by adding a large amount of organic matter. Plastic molding is a common plastic molding process for ceramics. 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 dimensions 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.
Seven. (Colloidal injection molding) In order to solve the problems of large amounts of organic matter added and difficulty in removal in the traditional injection molding process, Tsinghua University creatively proposed a new colloidal injection molding process for ceramics , independently developed a colloidal injection molding prototype and realized the injection molding of barren ceramic slurry. 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. accomplish. 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 the organic monomers after being injected into the mold to form an organic network skeleton, which evenly wraps the ceramic powder. Among them, not only the debinding time is greatly shortened, but also the possibility of debinding cracking is greatly reduced. There is a huge difference between injection molding and colloidal molding of ceramics. The main difference is that the former belongs to In the category of plastic molding, 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 process of colloidal injection molding of ceramics is different from both general colloidal molding and traditional injection molding. It combines the advantages of colloidal in-situ solidification molding The green body has good uniformity, low organic content, and has the advantages of high injection molding automation. It is colloidal moldingA qualitative sublimation of craftsmanship that will It has become the hope for the industrialization of high-tech ceramics.
