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.
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.
(5) Major defects
The defects of the hot die casting process mainly include the following:
Under the note, the mold is not filled with wax slurry, and the pressed product is not finished. The reasons for the formation of the owe are as follows:
(1) The viscosity of the wax slurry is large, the fluidity is poor, the viscosity and fluidity of the wax slurry and the water content of the raw material, the amount of paraffin and surface active material added, the particle size of the raw material, the stirring mode of the wax slurry and the stirring time, and the wax paste during molding. The temperature is related to the temperature. Before mixing the wax, the raw materials must be sufficiently dried to reduce the moisture as much as possible, otherwise the fluidity of the wax slurry will be deteriorated. If the amount of paraffin and surface active material is insufficient, the fluidity of the wax slurry is poor. Of course, the amount of the above-mentioned additive may not be too much, otherwise the quality of the product may be affected. If the particle size of the raw material is too small, the specific surface area thereof is large, and as a result, the wax paste has a large viscosity and poor fluidity. Therefore, care should be taken to control the particle size of the raw materials during material selection and ball milling. When the wax slurry is prepared by hand stirring, the air in the wax slurry is not easily excluded, the viscosity is large, and the fluidity is poor. If mechanical agitation is used, the mixing is uniform, the air is better excluded, the viscosity of the wax slurry is small, and the fluidity is good. If the mixing time is too short when the wax is mixed, the paraffin and the surface active material and the raw material are not uniformly mixed, and the air cannot be completely excluded, and the viscosity of the wax slurry is large and the fluidity is poor. Therefore, there must be sufficient mixing time when mixing the wax. In addition, the low temperature of the wax slurry during molding is also a cause of high viscosity and poor fluidity of the wax paste.
(2) If the temperature of the grouting port is too high or too low, the temperature of the grouting port is too high, then the shrinkage of the blank after cooling is too large, which will cause under-filling; if the temperature of the grouting port is too low, the viscosity of the wax slurry will increase. The fluidity is deteriorated, so that the slurry cannot fill the entire mold and cause a low injection.
(3) Insufficient pressure and grouting time The size of the pressure determines the filling speed of the slurry in the mold, and also determines the compensation ability of the slurry when it is cooled and shrunk in the mold. If the pressure is too small, the filling speed of the slurry in the mold is slow, which may result in insufficient slurry in the injection mold and cause a low injection. Of course, the pressure should not be too large, otherwise the filling of the slurry will be too fast, and eddy currents may be generated, thereby bringing the air into the grouting, so that the pores appear in the blank. Therefore, it is necessary to select a suitable molding pressure. The duration of the pressurization, in addition to filling the slurry with the mold, also compensates for the volume shrinkage that occurs when the body is condensed, and sufficiently solidifies the body. If the grouting time is not enough, the slurry may not be filled with the whole model, or the volume shrinkage that occurs when the billet is condensed may not be fully compensated and the underfill phenomenon occurs. Therefore, sufficient grouting time must be ensured during molding.
(4) The gas in the mold is not completely discharged. The gas in the mold is not completely discharged, which may cause the slurry to not fill the entire mold and cause underfilling. Therefore, it is necessary to try to completely remove the gas in the mold during molding.
The main reasons for the occurrence of pits are: the slurry and the mold temperature are too high, so that the shrinkage increases when the body cools, which causes pits on the surface of the blank; the demoulding is too early, and the mold is demolded when the body is not completely solidified. The pits on the surface of the blank are formed. In addition, the die feed port is too small or the position is unreasonable, which affects the injection of the slurry, so that the volume shrinkage that occurs when the blank is condensed is not fully compensated to cause pits. The improvement measures are to master the slurry and mold temperature and demoulding time, and choose the appropriate size and position of the slurry inlet.
The performance of the slurry is not good, the viscosity is large, the fluidity is poor, or the temperature of the slurry and the mold is too low to affect the fluidity of the slurry, which will make the slurry not fill the mold, and the surface of the body will wrinkle after cooling. In addition, the air in the mold is not cleaned during molding, which may cause wrinkles.
The main reasons for the bubble generation are: uneven mixing or insufficient stirring time when mixing the wax, the air in the slurry is not cleaned out, and bubbles appear in the blank; the fluidity of the slurry is too large or the pressure is too large, so that the slurry is The filling is too fast to generate eddy currents, so that air is brought into the slurry to cause bubbles in the body; and the mold design is unreasonable, and the air in the mold is excluded to eliminate bubbles. The improvement measures are: when the wax is mixed, the air in the slurry is fully excluded, the amount of the paraffin wax and the surface active material is controlled, the excessive fluidity caused by excessive addition is prevented, the appropriate molding pressure is selected, and the mold is designed reasonably.
5. Deformation and cracking
The mold temperature is too high or the demoulding is too early, that is, the mold is deformed when the green body is not completely solidified. If the mold is cold, the mold is released too late, or the mold grouting port has no slope, cracking will occur. In addition, if the mold temperature is too low and the billet cooling rate is too fast, the mold core will prevent the body from shrinking and cracking. The solution is to master the mold temperature and demoulding time, and the mold grouting port should have a certain slope.