Preparation of lost wax casting shells

Investment casting involves applying multiple layers of refractory paint to the surface of a wax pattern. After hardening and drying, the wax pattern is heated and melted to obtain a shell with a cavity corresponding to the shape of the wax pattern. A method of obtaining castings by pouring after baking, so it is also called lost wax casting. With the continuous improvement of production technology, new wax molding processes continue to emerge, and the variety of materials available for molding continues to increase. Now the demoulding method is no longer limited to melting, and the molding materials are not limited to wax materials. Plastic molds can also be used. Since the castings obtained by this method have higher dimensional accuracy and lower surface roughness values, it is also called investment casting.

At present, investment casting molds generally use shells made of multi-layer refractory materials. After the module is dip-coated with refractory paint, granular refractory materials are sprinkled on them and then dried and hardened. This process is repeated several times until the refractory material layer reaches the required thickness. This forms a multi-layer shell on the module, which is usually left for a period of time to fully dry and harden, and then demoulded to obtain a multi-layer shell. Some multi-layered shells require filling with sand, while others do not. It can be poured directly after baking and is called a high-strength shell.

The quality of the mold shell is directly related to the quality of the casting. According to the working conditions of the shell, the performance requirements of the shell mainly include

1) It has high normal temperature strength, suitable high temperature strength and low residual strength.

2) It has good air permeability (especially high temperature air permeability) and thermal conductivity.

3) The linear expansion coefficient is small, the thermal expansion is low and the expansion is uniform.

4) Excellent resistance to rapid cooling, rapid heating and thermochemical stability.

These properties of shells are closely related to the materials used in shell making and the shell making process. Shell materials include refractory materials, binders, solvents, hardeners, surfactants, etc. Among them, refractory materials and binders directly form the mold shell and are the main shell materials. The main refractory materials used in investment casting are silica sand, corundum and aluminosilicate refractory materials (such as refractory clay and aluminum).

Vanadium, etc.). In addition, zircon sand and magnesia are sometimes used.

Powdered refractory materials and binders are formulated into refractory coatings, and granular refractory materials are sprinkled on the refractory coatings during shell making. The main binding agents used in refractory coatings are ethyl silicate hydrolyzate, water glass and silica sol. The coating prepared with ethyl silicate has good coating performance, high shell strength, small thermal deformation, high dimensional accuracy and good surface quality of the resulting casting. It is mostly used to produce important alloy steel castings and other castings with high surface quality requirements. The SiO2 content of ethyl silicate produced in my country is generally 30% to 34% (mass fraction), so it is called ethyl silicate 32 (32 represents the average mass fraction of SiO2 in ethyl silicate). Ethyl silicate can only play a bonding role after hydrolysis.

Coated shells prepared with water glass are prone to deformation and cracking. Compared with ethyl silicate, the castings produced have lower dimensional accuracy and higher surface roughness. Water glass bonding agent is suitable for the production of small ordinary steel castings and non-ferrous alloy castings. The modulus of water glass used for investment casting is usually 3.0~3.4, and the density is 1.27~1.34gcm3.

Silica sol adhesive is an aqueous solution of silicic acid, also known as silica sol. Its price is 1/3~1/2 lower than that of ethyl silicate. The quality of castings produced using silica sol as a binding agent is higher than that of water glass. The binder has been greatly improved. Silica sol has good stability and can be stored for a long time. No special hardener is required when making the casing. The high-temperature strength of this shell is better than that of ethyl silicate shell, but the wettability of silica sol to investment is poor and the hardening time is longer. The main processes of shell making include module degreasing, coating and polishing, drying and hardening, demoulding, baking, etc.