Ceramic hot die casting
Ceramic hot die casting is a commonly used molding method for electronic ceramics, especially alumina ceramics. Usually, the prepared wax cake is placed in a hot press barrel to be heated and melted into a slurry. Under the action of compressed air, the slurry is applied. The material is injected into the cavity through the suction port, and the pressure is released after a certain time according to the shape and size of the product, and the slurry is cooled and formed in the cavity. The body after hot die casting can be processed, for example, turning, punching, etc., and then subjected to wax removal treatment to remove paraffin in the body and increase the strength of the body. After the wax is discharged, the adsorbent on the surface is cleaned and then sintered.
The ceramic hot die casting method is also grout molding in a certain sense, but the difference is that it mixes paraffin in the billet, utilizes the heat flow characteristics of paraffin, and uses metal mold to form under pressure. The process flow is shown in the figure as below.
Ceramic hot die casting
The ceramic hot die casting process is suitable for small and medium-sized products with complex shapes and high precision requirements, and has simple equipment, convenient operation, low labor intensity, high production efficiency, small mold wear and long service life, so it is often used in special ceramic production. However, ceramic hot die casting also has disadvantages. For example, the process is complicated, the energy consumption is large (multiple firing is required), and the construction period is long. For a thin, large and long product, it is not suitable because it is not easy to fill the cavity.
(1) Preparation of powder
Alumina ceramics are usually formed by hot die casting. However, the powder must be pretreated before molding. Due to the different crystal forms of AL203, the structure is different, the performance is different, and the most stable is α-AL203. Therefore, AL203 ceramics must be pre-fired with AL203 powder. The purpose of calcination is mainly to convert all of γ-AL203 into α-AL203 to reduce the firing shrinkage. In addition, the pre-firing can also eliminate Na2O in the AL203 raw material, improve the purity of the raw material, thereby ensuring the performance of the product.
From a practical point of view, the method of calcination is different, the additives are different, the atmosphere is different, the effect is different, and the quality of the calcination is also different. Therefore, calcination is one of the important links in the production of AL203 ceramics. Industrial alumina powder, usually added with an appropriate amount of additives, H3BO3, NH4F, aluminum fluoride, etc., the addition amount is generally 0.3% to 3%.
For the calcined Al203, the degree of conversion of γ-Al203 to α-Al203 can be examined by the following method.
Since the structure of α-Al203 does not adsorb dye, and γ-Al203 is a porous sphere structure, the adsorption capacity is strong, so the degree of conversion is judged by the amount of dye adsorbed. The dyes commonly used are halogen, methylene blue and the like. The unconverted Al203 was deeply dyed, and the fully transformed Al203 was lightly stained. This method is simple in practical use, but cannot be quantitatively determined.
2. Optical microscopy
This method is based on the fact that α-Al203 and γ-Al203 have different refractive indices to judge the conversion. Generally, iodomethane with a refractive index of 1.730 is used as the refractive index oil. Under a polarizing microscope, if the refractive index is greater than 1.730, the refractive index is greater than 1.730. It belongs to α-Al203. Conversely, less than 1.730 belongs to γ-Al203.
3. Density method
The density of α-Al203 and γ-Al203 is different, the density of α-Al203 is large, close to the theoretical density, and the density of γ-Al203 is small. Therefore, the amount of α-Al203 can be estimated from the density of Al203 after calcination, thereby judging whether the quality of the calcination is good or not.
In addition to the pre-burning of the powder, the fineness should be controlled. The finer the powder, the larger the specific surface area, and the more the amount of paraffin required under the same fluidity and injectability requirements, which is unfavorable for hot press molding. Product dimensional tolerance requirements are also disadvantageous. If the particles are too large, the wax slurry tends to be unstable in precipitation. The fineness of the powder is generally controlled to be no more than 0.5% in the 10,000-hole sieve, and all of them must pass through the sieve of 0.2 mm aperture. Tests have shown that if the large particle size can be further reduced to not exceed 60 μm and the fine particles of 1 to 2 μm are minimized, a wax paste and a product with good performance and stability can be obtained.