Casting classification of investment casting

Investment castingThe casting molds can be divided into two types: solid type and multi-layered shell. At present, multi-type shell is commonly used. will

After the module is dipped and coated with refractory coating, sprinkle the material-like refractory material, and then dry and harden, and repeat this process many times, so that the refractory coating layer reaches the required thickness, so that a multilayer shell is formed on the module. Usually, it is parked for a period of time to fully harden, and then the module is melted to obtain a multi-layer shell. Some multi-layer shells need to be packed with sand; some do not need to be cast directly after roasting. When the melt mold is lost, the mold shell will be subjected to the pressure of the molten mold material that is increasing in volume; during firing and injection, the various parts of the mold shell will have mutual restraint and uneven expansion and contraction. Therefore, the metal may still be High temperature chemical reaction with shell material. Therefore, there are certain performance requirements for the shell, such as small expansion rate and shrinkage rate; high mechanical strength, thermal shock resistance, and chemical stability under fire and high temperature; the shell should also have a certain degree of air permeability , So that the gas in the shell can escape smoothly during pouring. These are all related to the refractory materials, binders and processes used in manufacturing the shell.

1. Materials used to make the shell

There are two types of materials used to make the shell. One is used to directly form the shell, such as refractory materials, adhesives, etc.; the other is to obtain high-quality shells, simplify operations, and improve process usage. Materials such as flux, hardener, surfactant, etc. 1) Refractory

At present, the refractory materials used in investment casting are mainly quartz and corundum, as well as aluminum silicate refractory materials, such as refractory clay, bauxite, and scorched gemstones. Sometimes zircon, magnesia (MgO), etc. are also used. 2) Binder

The most common binder used in investment casting is colloidal silicate solution (silicate sol for short), such as ethyl silicate hydrolysate, water glass and silica sol. They are mainly composed of silicic acid (H2SiO3) and solvents, and sometimes stabilizers, such as NaOH in silica sol. Ethyl silicate hydrolysate is the earliest and most common binder used in the casting of silicate sol molds obtained after the hydrolysis of ethyl silicate; the water glass shell is easy to deform and crack, and the dimensional accuracy of castings cast with it The surface finish is poor. However, in our country, when producing carbon steel castings with higher precision requirements and non-ferrous alloy castings with lower melting points, water glass is still widely used in production; silica sol has good stability, can be stored for a long time, and is not required for molding shells. Special hardener, but silica sol has a slightly poorer wetting of the investment mold. The hardening process of the shell is a drying process and takes a long time.