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.
(2) Preparation of wax paste
The purpose of this process is to add the prepared powder to a paraffin-based binder to form a wax plate for molding. The main components in the wax paste are powder and thermoplastic binder. The latter is commonly used as paraffin wax. The melting point of paraffin wax is 55~60℃. The viscosity after melting is small, the source is rich and cheap, and it is suitable as a thermoplastic binder. The paraffin wax is weighed to a certain amount (generally 12.5% to 13.5%), and then heated and melted into a wax liquid, and the weighed powder is dried in an oven; the water content is not more than 0.2%. This is because when the water content in the powder is more than 1%, the moisture hinders the powder and the paraffin from completely infiltrating, and the viscosity is increased, making it difficult to form.
In addition, the surface of the powder particles is generally charged, polar and hydrophilic, while the paraffin is non-polar hydrophobic. Therefore, porcelain powder and paraffin wax are not easy to adsorb, and it is easy to precipitate after heating and slurry. Surfactant is often used as a medium for the contact between porcelain powder and paraffin in production. The surface active material is composed of a hydrophilic group (polar group) which is easily soluble in water or easily wetted by water, and a lipophilic group (non-polar group) which is easily soluble in oil. The porcelain powder and the paraffin are surface-active. Substances are indirectly adsorbed together as bridges. In addition, the surface active material adsorbs on the surface of the porcelain powder particles to form a thin layer of a single molecule, which reduces the surface energy at the interface between the powder particles and the paraffin wax, reduces the interaction between the molecules, and improves the fluidity of the wax slurry. Reduce the amount of wax in the wax slurry.
Commonly used surface active substances are oleic acid, stearic acid, beeswax and the like. The production practice proves that the surface active substance oleic acid is added after dry grinding to a certain fineness, and the effect is better after dry grinding for 3 hours. Thus, the surface of each porcelain powder particle is covered by the oleic acid film, which reduces the surface free energy of the porcelain powder particles, and the fluidity after the wax is good. The performance indicators of the slurry are as follows:
Stability refers to the ability of the slurry to maintain its uniform non-delamination under conditions of long-term heating without agitation. Usually expressed by the stability index U:
V0— the volume of the slurry being tested, cm3;
Vt— the volume of the wax liquid separated after heating, cm3;
The test was carried out with 100 mL of slurry and incubated at 70℃ for 24 h to obtain the upper layer of wax. The wax liquid required to be separated should be no more than 0.2mL, which means that the U should be greater than 500, the stability index of the wax liquid is qualified.
Injectability refers to the ability of the slurry to fill the cavity and maintain the desired shape. It is a comprehensive indicator for measuring the viscosity and solidification rate of the slurry. Generally speaking, if the fineness of the powder is appropriate, the powder is dried, and the amount of the binder added is appropriate, the slurry can be well injected. To determine the injectability of the wax paste, a mold can be used, as shown in Figure below. The cavity is a square pyramid with a bottom edge of 5mm x 5mm and a cone height of 200mm. The injectability is expressed by a certain degree of slurry temperature, a certain mold temperature, and a certain degree of pressure to inject the slurry into the cavity (the slurry temperature is 70℃, the mold temperature is 25℃, the pressure is about 0.5 MPa), and its H Should be 70~120mm. Below 70mm, that is, the viscosity of the slurry is too large, and the injectability is poor; when it is 120mm, the slurry is too thin, of course, the molding is also difficult.
3. Shrinkage rate
The shrinkage rate means that the wax slurry will shrink in volume when it is cooled and solidified by the molten liquid state. The shrinkage is related to the expansion coefficient of the powder and paraffin, the particle size of the powder, the shape of the grain, the gradation, the mixing ratio of the binder in the wax paste, and the molding temperature. The volume shrinkage of the slurry as it solidifies in the mold cavity is referred to as cold shrinkage and is typically about 1%.