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.
(3) Hot die casting
The main process of hot die casting is to place the prepared wax board in the hot press barrel, heat it to a certain temperature and melt it. Under the driving of compressed air, the slurry in the barrel is pressed through the suction port. Cavity cavity. After maintaining a certain time according to the shape and size of the product, the pressure is removed, the slurry is cooled and formed in the cavity, and then demolded, the blank is taken out, and some may be processed.
1. Influencing factors of hot press molding process
(1) Temperature of the wax slurry When the temperature of the wax slurry increases within a certain temperature range, the viscosity of the slurry is lowered, so that the particles of the green body can be arranged densely, and the shrinkage cavity in the blank can be reduced. When the temperature of the wax slurry is too high, the volume shrinkage of the green body is increased, and the surface is prone to pits. The temperature of the wax paste is related to the size, shape and thickness of the blank. Complex, large-sized, thin-walled blanks are injected with a slurry of higher temperature. The temperature of the wax is generally controlled at 65~85℃.
(2) Model temperature The model temperature determines the speed and quality of the solidification and solidification of the body. The model temperature is also related to the shape and thickness of the blank. When the shape is simple and the thick-walled blank is die-cast, the model temperature is lower; when the shape is complicated and the thin-walled blank is die-cast, the model temperature is higher. When using a model with many parts inserted, the model temperature is higher. However, increasing the temperature of the model reduces the density of the body and increases the internal pores. The general model temperature is 20~30℃. In production, two models are usually used for rotational casting, one model is die-casting, and the other model is cooled on ice.
(3) Pressure system The magnitude of pressure affects the rate of feed. The amount of pressure depends on the viscosity and fluidity of the wax paste. When using a wax slurry with a large viscosity and forming a thin or large blank, the pressure should be increased. Increasing the pressure reduces the shrinkage of the body during cooling, increases the density of the particle arrangement, and reduces shrinkage. The molding pressure commonly used in production is 0.3 to 0.5 MPa.
In addition to filling the wax slurry with the mold, it can also compensate for the volume shrinkage that occurs when the body is condensed, and the solid body is sufficiently solidified and hardened. The holding time is also related to the shape of the blank. The small blank is maintained at 0.3~0.4MPa for 5~15s, and the large blank is kept at 0.4~0.5MPa for 1min left. This can make the wax shrink when the blank shrinks. Supplement, reduce internal shrinkage and total shrinkage.
2. Hot die casting machine
At present, there are generally two types of hot presses used in production: one is manual and the other is automatic, but the principle is basically the same. The following figure shows the structural schematic diagram of the commonly used manual operation hot-press grouting machine, which includes the slurry barrel, the grouting tube, the oil bath box, the electric heating device, the pneumatic pressing mold device, the wax temperature automatic temperature control device, the work table and the machine. Shelf, compressed air source and pipeline, gas valve, mold, etc. When operating, place the mold above the grouting tube of the workbench. Press the foot pedal and compress the air into the upper part of the cylinder. Pass the diaphragm to force the head and press the mold against the table. At the same time, the compressed air also enters the upper part of the pulp bucket, forcing the slurry into the inner cavity of the mold through the grouting pipe. After the slurry is cooled and solidified, the foot pedal is released, the pressure and pressing force in the slurry tank are removed, then the mold is removed and the blank is taken out from the mold, and then the mold is returned to the workbench for the next injection.
1—Electric heating device 2—the grouting pipe 3—oil bath box 4—pulp bucket
5—point contact thermometer 6—Mold 7—compression head 8—cylinder 9—lift frame
10—lifting screw 11—machine board 12—The outlet electric heating device 13—Workbench
14—valve valve 15—foot pedal
The bottom of the machine board, as shown in the following figure, is equipped with a slurry tank and a grouting pipe. The electric heating wire is installed near the slurry discharge port to prevent the wax paste from clogging and blocking. The hand opening is used as the feeding cake and the slurry cleaning. Slightly larger than 10cm, there is a sealing cover on the top, and it should be tightly closed during work. The hot press grouting mold is generally made of alloy steel, and is usually formed into two mold halves which are opened and closed, and the cavity between the two mold halves is the shape of the product. For larger molds, the upper and lower molds are respectively provided with an interlayer of cooling air to accelerate the cooling and solidification process of the blank and improve productivity. The compression of the mold is pneumatic and reset by a spring. The indenter adjusts the height by means of a lifting screw to accommodate the needs of different height moulds. The temperature control of the wax paste is composed of a point contact thermometer, a relay, and a heating wire. The purpose of the oil bath is to make the heating even. The slurry temperature is generally 60 to 85℃.
The pressure of compressed air is 0.3~0.5MPa, which is related to the size of the product and the temperature of the slurry. In theory, the air entering the cylinder should take precedence over the air entering the drum, so that the mold starts to grout before the mold is pressed, causing the slurry to splash out. Therefore, some hot press grouting machines are designed with two gas valves, first stepping on the air valve leading to the cylinder, and the isostatic head presses the mold and then presses the gas valve leading to the tank to grout. In fact, since the compressed air to the cylinder has less waviness than the barrel, and the cross-sectional areas of the two are very different, the slurry splashing generally does not occur, so it is only necessary to use one gas valve and one tee joint.