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Ceramic hot die casting

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Although ceramic hot die casting is also a grouting method, it is different from the previous grouting process. It utilizes the heat flow characteristics of paraffin, cooperates with the blank, and is molded under pressure using a metal mold, and the green body can be maintained after condensation.

1. Preparation of wax paste
The purpose of this process is to add the prepared blank to a paraffin-based binder to form a wax board for molding.
The paraffin wax (according to the ratio) is weighed to a certain amount (generally 12.5% to 13.5%), and then heated and melted into a wax liquid, and the desired material is dried in an oven so that 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 will hinder the complete infiltration of the dressing and the paraffin, and the viscosity will increase, making it difficult to form. Further, when heated, moisture forms small bubbles dispersed in the slurry, so that the sintered product forms closed pores, and the performance is deteriorated.
When preparing the wax slurry, a small amount of a surfactant (generally 0.4% to 0.8%, such as beeswax) is added to the powder to reduce the paraffin content and improve the molding properties.

There are two specific mixing methods:
1. Heat the paraffin to melt it, then pour the powder into the person and stir while heating;
2. Heat the powder and pour it into the paraffin solution while stirring. The preparation of the wax slurry can be carried out in a rotary kiln, and then the slurry is poured into a container and solidified to form a wax plate for molding.

The performance indicators of the slurry are as follows:
(1) Stability
Stability means that the slurry retains its uniform non-layering properties under conditions of prolonged heating without agitation. Usually expressed by the stability indicator:
Vo "2
V, the u-stability index in the formula;
Vo - the volume of the slurry being tested (CTI13);
Vt - volume of wax liquid (CIT13) separated after heating.
The test conditions are: 100 mL of slurry, kept at 70 ° C for 24 h. Separated wax
The liquid should be no more than 0.2 mL. At this time u>500.

(2) Castability
Castability refers to the ability of the slurry to fill the mold cavity and maintain the desired shape. It is a comprehensive measure of slurry viscosity and solidification rate. Generally speaking, if the fineness of the powder is appropriate, the powder is dried, and the binder (paraffin and surfactant-containing agent) is added in an appropriate amount, the castability of the slurry is good.
To determine the castability of the wax paste, a mold can be used as shown in Figure 1-2-10. The cavity is a square pyramid with a bottom surface of 5 mm × 5 mm and a cone height of 200 mm. Castability is the mold height of the mold cavity of the oil bath with a certain slurry temperature, a certain mold temperature and a certain pressure. The slurry temperature is 70 ° C, the mold temperature is 25 ° C, and the pressure is 5 atmospheres). Its H should be in the range of 70 to 120 mm. Below 70 mm, the viscosity of the slurry is too large, and the castability is poor; above 120 mm, that is, the slurry is too thin, of course, the molding is also difficult.

(3) Shrinkage rate
Shrinkage refers to the volume shrinkage of a wax slurry when it is solidified by a molten liquid state and solidified into a solid state. The shrinkage is related to the expansion coefficient of the powder and paraffin, the particle size of the powder, the shape of the granule, the gradation, the mixing ratio of the binder in the wax slurry, and the molding temperature. The volume shrinkage of the slurry as it solidifies in the mold cavity is referred to as cold shrinkage, typically about 1%.

2. The working principle of hot die casting machine

At present, there are generally two types of hot die casting machines used in production, one is manual and the other is automatic, but the basic principle is the same.
The working principle is that the prepared wax board is placed in a hot die casting machine cylinder, heated to a certain temperature to melt, and driven by compressed air, the slurry in the cylinder is pressed into the cavity through the suction casting port, according to the product. After the shape and size are kept for a certain period of time, the pressure is removed, the slurry is cooled and formed in the cavity, and then the mold is released, the blank is taken out, and some can be processed, or turned, or punched.

3. High temperature wax removal
The green body formed by hot die casting is first subjected to wax removal treatment before firing. Otherwise, since the paraffin is melted, volatilized, and burned at a high temperature, the green body will lose its bond and dissolve, and the shape cannot be maintained.
The wax is used to bury the body in a loose, inert protective powder. This protective powder is also called an adsor-bent. It is stable at high temperatures and is not easily bonded to the body. Although the paraffin will melt and diffuse during the heating process, the adsorbent supports the green body. When the temperature continues to rise, the paraffin wax volatilizes and burns completely, and a certain sintering occurs between the powders in the green body. At this time, neither the reaction nor the adsorption occurs between the green body and the adsorbent, and the green body has a certain strength. Usually, the wax discharge temperature is about 900 to 1100 ° C, depending on the nature of the blank. If the temperature is too low, there is no certain sintering between the powders, there is no certain mechanical strength, the blank is loose, and the subsequent process cannot be carried out; if the temperature is too high, until it is completely sintered, serious adhesion will occur, which is difficult. Clean the surface of the blank.
After the wax is discharged, the adsorbent on the surface is cleaned and then sintered.

4. Advantages and disadvantages of hot die casting
The hot die casting process is suitable for the production of small and medium-sized products with complex shapes and high precision requirements.
The advantages of hot die casting are simple equipment, convenient operation, low labor intensity, high production efficiency, small mold wear and long service life, so it is often used in the production of special ceramics.

The disadvantage of hot die casting is that the process is complicated, the energy consumption is large (multiple firing is required), and the construction period is long. For a large and long product with a thin wall, it is not suitable because it is not easy to fill the cavity.

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