Stainless steels, because of their resistance to corrosion, are often used in highly aggressive conditions, which will cause most alternative materials to fail rapidly.
Casting of stainless steel is a method of manufacturing complex shapes of stainless steel components that are not easily or cost effectively fabricated or forged. Stainless steel casting involves heating the stainless steel material to above its melting point and introducing the material into a mould where it solidifies into the required shape.
In this article, we will focus on the Process of Stainless Steel Casting.
1. Mould Preparing
Mould for stainless steel casting is designed and produced according to the exact shape of a product. Normally, a pair of stainless steel cast mould is separated into upper and lower concave moulds, which is completed by a series of machining proceduces such as turning, planing, milling, ect. The cavity of casting mould will be consistent with the shape of stainless steel casting part.
Because the wax model is mainly produced from the pressing of industrial wax, so the mould material for stainless steel casting is selected by aluminum alloy, which features low melting point, low hardness, low price and light weight.
2. Wax Model & Tree Assembly
A large number of wax models are produced by injecting wax into aluminum alloy mould. Under normal conditions, a wax model can only produce a stainless steel casting part.
After taking out the wax models from mould, workers will need to check the dimensions of them, once there is any error, they need to be fixed or scrapped, otherwise they can not be used for further production. Once removing burrs, the wax model will be sticked to the sprue. Normally multi wax model will be assembled like a tree. So, this job is also called tree assembly.
The tree assembly will be first painted with industrial glue, then sprayed with a layer of fine sand, which is a kind of refractory sand with feature of high temperature resistance, usually used sand is silica sand. The sand particles are very small and fine, which ensures that the finished surface is as smooth as possible.
Naturally dry the wax model with fine sand sprayed under the room temperature, but cannot affect the dimentions of internal wax pattern. The time of natural drying will be depended by the complexity of the product itself. General drying time at the first time is around 5 to 8 hours. After first time sand spraying and natural air-dry, continue to paint industrial glue (silicon dissolving pulp), and spray the second layer of sand, the particle size used for the second sand layer is larger than the first layer of sand, also thicker. After spraying the second layer of sand, the wax model is naturally dried at the set constant temperature.
After the second sand spraying and natural air drying, go ahead for the third sand spraying, the fourth sand spraying, the fifth sand spraying… Requirements: Adjust the times of sand spraying according to the surface requirements, size, weight of stainless steel castings. In general, the sand spraying frequency is 3-7 times. The sand grain size of each sand spraying is different. Generally, the sand grains in the latter process are coarser than that of the preceding process, and the duration of air drying is different. The production cycle of a complete wax model is about
3~4 days.
3. Sand Shell Making and Dewaxing
Before baking process, evenly coat the shell with a layer of white industrial latex (silicon dissolving pulp), in order to adhesive and cure sand mold, and seal wax models. This step is used for the preparing for further baking process. At the same time, after the baking process, it can also improve the brittleness of the sand, so as to break the sand layer and take out of the stainless steel casting blank.
Heating the wax model in a special oven with metal seal (commonly used as a steam stove for burning kerosene). As the melting point of industrial wax is not high, the temperature is about 150℃. The wax model after heating will be melted and formed wax water, which will flow out along the sprue gate, this process is called dewaxing. The wax model after dewaxing is just an empty sand shell. The key of precision casting is to use this empty sand shell. (generally the wax can be repeatedly used for many times, but the wax must be filtered again, otherwise the reused wax will affect the surface quality of stainless steel casting blanks, such as: surface sand holes and hemp dot, meanwhile can also affect the shrinkage of stainless steel castings)
In order to make the sand shell more strong and solid, before pouring into stainless steel water, we must bake sand shell, usually roasted in the oven at high temperature (temperature at about 1000℃).
4. Pouring
Pour molten stainless steel into dewaxed sand shell, then liquid stainless steel water will fill fully the sand shell, including the middle part of sprue gate.
Note: The factory must detect the percentage of material, because the boiler that is used to melt stainless steel can be mixed with different chemical components.
After cooling and solidification of stainless steel liquid, break the outermost sand shell by means of mechanical tools or human power. The displayed solid stainless steel part is the original wax pattern shape, also is the final required blank. Then it will be cut, separated and then polished to become a single stainless steel casting blank.
5. Testing
Test the blanks. Once there is any surface defects such as sand hole, gas hole, the stainless steel blank can be welding repaired with argon arc. For serious defects, the scrapped product has to be cleaned and returned to the furnace for the casting production of other stainless steel components. Sometimes, the casting blanks will still need to be further machined for final finished products.
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