Nine precision molding processes of zirconia ceramics

Nine precision molding processes of zirconia ceramics
The molding process plays a linking role in the entire preparation process 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, the traditional hand-kneading method, wheel forming method, grouting method, etc. of traditional ceramics can no longer meet the needs of modern society for production and refinement, so a new molding process was born. ZrO2 fine ceramic materials are widely used in the following 9 types of molding processes (2 types of dry methods and 7 types of wet methods):

1. Dry molding

1.1 Dry pressing

Dry pressing uses pressure to press ceramic powder into a certain shape of the body. Its essence is that under the action of external force, the powder particles approach each other in the mold, and are firmly combined by internal friction to maintain a certain shape. The main defect in dry-pressed green bodies is spallation, which is due to the internal friction between the powders and the friction between the powders and the mold wall, resulting in pressure loss inside the body.

The advantages of dry pressing are that the size of the green body is accurate, the operation is simple, and it is convenient to realize mechanized operation; the content of moisture and binder in the green dry pressing is less, and the drying and firing shrinkage is small. It is mainly used to form products with simple shapes, and the aspect ratio is small. The increased production cost caused by mold wear is the disadvantage of dry pressing.

1.2 Isostatic pressing

Isostatic pressing is a special forming method developed on the basis of traditional dry pressing. It utilizes fluid transmission pressure to apply pressure evenly to the powder inside 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, so the difference in the density of the green body can be avoided.

Isostatic pressing is divided into wet bag isostatic pressing and dry bag isostatic pressing. Wet bag isostatic pressing can form products with complex shapes, but it can only work intermittently. Dry bag isostatic pressing can realize automatic continuous operation, but can only form products with simple shapes such as square, round, and tubular cross-sections. Isostatic pressing can obtain a uniform and dense green body, with small firing shrinkage and uniform shrinkage in all directions, but the equipment is complex and expensive, and the production efficiency is not high, and it is only suitable for the production of materials with special requirements.

2. Wet forming

2.1 Grouting
The grouting molding process is similar to tape casting, the difference is that the molding process includes physical dehydration process and chemical coagulation process. Physical dehydration removes the water in the slurry through the capillary action of the porous gypsum mold. The Ca2+ generated by the dissolution of the surface CaSO4 increases the ionic strength of the slurry, resulting in the flocculation of the slurry.
Under the action of physical dehydration and chemical coagulation, the ceramic powder particles are deposited on the gypsum mold wall. Grouting is suitable for the preparation of large-scale ceramic parts with complex shapes, but the quality of the green body, including shape, density, strength, etc., is poor, the labor intensity of workers is high, and it is not suitable for automated operations.

2.2 Hot die casting
Hot die casting is to mix ceramic powder with binder (paraffin) at a relatively high temperature (60~100℃) to obtain slurry for hot die casting. The slurry is injected into the metal mold under the action of compressed air, and the pressure is maintained. Cooling, demoulding to obtain a wax blank, the wax blank is dewaxed under the protection of an inert powder to obtain a green body, and the green body is sintered at high temperature to become porcelain.

The green body formed by hot die casting has precise dimensions, uniform internal structure, less mold wear and high production efficiency, and is suitable for various raw materials. The temperature of the wax slurry and the mold needs to be strictly controlled, otherwise it will cause under injection or deformation, so it is not suitable for manufacturing large parts, and the two-step firing process is complicated and energy consumption is high.

2.3 Tape casting
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 slurry to the hopper of the casting machine, and use a scraper to control the thickness. It flows out to the conveyor belt through the feeding nozzle, and the film blank is obtained after drying.

This process is suitable for the preparation of film materials. In order to obtain better flexibility, a large amount of organic matter is added, and the process parameters are required to be strictly controlled, otherwise it will easily cause defects such as peeling, streaks, low film strength or difficult peeling. The organic matter used is toxic and will cause environmental pollution, and a non-toxic or less toxic system should be used as much as possible to reduce environmental pollution.

2.4 Gel injection molding
Gel injection molding technology is a new colloidal rapid prototyping process first invented by researchers at Oak Ridge National Laboratory in the early 1990s. At its core is the use of organic monomer solutions that polymerize into high-strength, laterally linked polymer-solvent gels.

A slurry of ceramic powder dissolved in a solution of organic monomers is cast in a mold, and the monomer mixture polymerizes to form a gelled part. Since the laterally linked polymer-solvent contains only 10%–20% (mass fraction) polymer, it is easy to remove the solvent from the gel part by a drying step. At the same time, due to the lateral connection of the polymers, the polymers cannot migrate with the solvent during the drying process.

This method can be used to manufacture single-phase and composite ceramic parts, which can form complex-shaped, quasi-net-sized ceramic parts, and its green strength is as high as 20-30Mpa or more, which can be reprocessed. The main problem of this method is that the shrinkage rate of the embryo body is relatively high during the densification process, which easily leads to the deformation of the embryo body; some organic monomers have oxygen inhibition, which causes the surface to peel and fall off; due to the temperature-induced organic monomer polymerization process, causing Temperature shaving leads to the existence of internal stress, which causes the blanks to be broken and so on.

2.5 Direct solidification injection molding
Direct solidification injection molding is a molding technology developed by ETH Zurich: solvent water, ceramic powder and organic additives are fully mixed to form electrostatically stable, low-viscosity, high-solid-content slurry, which can be changed by adding Slurry pH or chemicals that increase electrolyte concentration, then the slurry is injected into a non-porous mold.

Control the progress of chemical reactions during the process. The reaction before injection molding is carried out slowly, the viscosity of the slurry is kept low, and the reaction is accelerated after injection molding, the slurry solidifies, and the fluid slurry is transformed into a solid body. The obtained green body has good mechanical properties and the strength can reach 5kPa. The green body is demolded, dried and sintered to form a ceramic part of the desired shape.

Its advantages are that it does not need or only needs a small amount of organic additives (less than 1%), the green body does not need to be degreasing, the green body density is uniform, the relative density is high (55%~70%), and 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.

2.6 Injection molding
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 organics or high temperature curing of thermosetting organics. 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 large molding pressure, the obtained blanks have precise dimensions, high smoothness and compact structure; the use of special molding equipment greatly improves the production efficiency.

In the late 1970s and early 1980s, the injection molding process was applied to the molding of ceramic parts. This process realizes the plastic molding of barren materials by adding a large amount of organic matter, which is a common ceramic plastic molding process. In injection molding technology, in addition to using thermoplastic organics (such as polyethylene, polystyrene), thermosetting organics (such as epoxy resin, phenolic resin), or water-soluble polymers as the main binder, it is necessary to add certain Quantities of process aids such as plasticizers, lubricants and coupling agents 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 molding blank. However, the organic content in the green body of injection-molded ceramic parts is as high as 50vol%. It takes a long time, even several days to dozens of days, to eliminate these organic substances in the subsequent sintering process, and it is easy to cause quality defects.

2.7 Colloidal injection molding
In order to solve the problems of the large amount of organic matter added and the difficulty of eliminating the difficulties in the traditional injection molding process, Tsinghua University creatively proposed a new process for colloidal injection molding of ceramics, and independently developed a colloidal injection molding prototype to realize the injection of barren ceramic slurry. forming.

The basic idea is to combine colloidal molding with injection molding, using proprietary injection equipment and new curing technology provided by the colloidal in-situ solidification molding process. This new process uses less than 4wt.% of organic matter. A small amount of organic monomers or organic compounds in the water-based suspension is used to quickly induce the polymerization of organic monomers after injection into the mold to form an organic network skeleton, which evenly wraps the ceramic powder. Among them, not only the time of degumming is greatly shortened, but also the possibility of cracking of degumming is greatly reduced.

There is a huge difference between injection molding of ceramics and colloidal molding. The main difference is that the former belongs to the category of plastic molding, and the latter belongs to slurry molding, that is, the slurry has no plasticity and is a barren material. Because the slurry has no plasticity in colloidal molding, the traditional idea of ​​ceramic injection molding cannot be adopted. If colloidal molding is combined with injection molding, colloidal injection molding of ceramic materials is realized by using proprietary injection equipment and new curing technology provided by colloidal in-situ molding process.

The new process of colloidal injection molding of ceramics is different from general colloidal molding and traditional injection molding. The advantage of a high degree of molding automation is a qualitative sublimation of the colloidal molding process, which will become the hope for the industrialization of high-tech ceramics.