Precision Cast Parts

Precision casting, also known as investment casting, its basic principle is to use fusible materials (such as wax materials) to make soluble models. Then, several layers of refractory materials are coated on the models to form a shell. The models are melted and flowed out by heating. After that, the shell is put into a roasting furnace for high-temperature roasting. When it becomes firm, liquid metal is poured into the shell. After the metal cools and solidifies, the shell is broken, and precision castings can be obtained.

Firstly, patterns need to be made. For example, wax patterns can be used. The manufacturing precision of wax patterns has a great impact on the precision of the final castings. Usually, special molds are used, and wax materials are injected into the mold cavities by injection or other means to form wax patterns in the required shapes. This process is similar to making small plastic toys, except that wax materials are used here and higher requirements are placed on dimensional precision.

After the wax patterns are made, they also need to be trimmed and inspected to ensure that the surfaces of the wax patterns are smooth, free of defects, and the dimensions meet the design requirements. Because any flaws on the wax patterns may be magnified in the subsequent casting process, resulting in quality problems of the castings.

Shell-making is one of the key steps in precision casting. The wax patterns are soaked in coatings. The coatings are generally composed of refractory materials (such as quartz powder, zircon powder, etc.) and binders (such as water glass, silica sol, etc.). After soaking, a uniform coating layer is formed on the surface of the wax patterns, and then a layer of refractory sand (such as mullite sand) is sprinkled as a reinforcement layer.

This process needs to be repeated several times. Generally, about 6 – 10 layers of shells will be made so that the shells have sufficient strength to withstand the subsequent pouring process. The drying and hardening of each layer are very important. For example, water glass shells need to undergo chemical hardening treatment after being coated, while silica sol shells mainly rely on natural drying.

Wax removal is the process of melting and flowing out the wax patterns in the shells. Methods such as hot water wax removal and steam wax removal can be adopted. In hot water wax removal, the shells with wax patterns are put into hot water, and the wax patterns are melted by heat and float on the water surface and then removed.

The shells after wax removal need to be roasted. The roasting temperature is usually around 800 – 1000°C. The specific temperature depends on the shell materials and the casting materials. The purpose of roasting is to further improve the strength and air permeability of the shells, and at the same time remove impurities such as residual wax materials, moisture and organic substances in the shells, creating good conditions for pouring the molten metal.

Pouring is to inject the molten metal (such as stainless steel, alloy steel, aluminum alloy, etc.) into the roasted shells. Parameters such as pouring temperature and pouring speed need to be strictly controlled to ensure that the molten metal can smoothly fill the cavities of the shells and no defects such as pores and cold shuts will occur.

After the castings solidify, the castings need to be cleaned. Firstly, the shells need to be removed. Methods such as mechanical crushing and chemical shell dissolution can be used. Then, surface treatments such as cutting the risers and gates, grinding and sandblasting are carried out on the castings to obtain precision castings that meet the requirements of dimensional precision and surface quality.