Lost wax casting is one of the casting methods known since antiquity. It is used for the manufacture of parts of high accuracy and complex configuration that cannot be performed by other casting methods (for example, turbine blades , etc.)
From fusible material: paraffin , stearin , etc. (in the simplest case, from wax ), by pressing it into a mold, an exact product model and a gating system are made. The most widely used was the P50C50 model composition, consisting of 50% stearin and 50% paraffin. For large products, salt formulations are used that are less prone to warping.
The model is then dipped in a liquid suspension of dusty refractory filler in a binder. As a binder, hydrolyzed ethyl silicate of the ETS 32 and ETS 40 grades is used, hydrolysis is carried out in a solution of acid, water and an organic solvent (alcohol, acetone). Currently, silica sols have been used in the PCM, which do not need hydrolysis in workshop conditions and are environmentally friendly. The following materials are used as refractory filler: electrocorundum, distensilimanite, quartz, chamotte, etc.
A suspension is applied to the model block (model and LPS) and sprinkled, from 6 to 10 layers are applied. With each subsequent layer, the grain fraction of the dusting is changed to form a dense surface of the shell shape. Drying each layer takes no less than half an hour; to speed up the process, special drying cabinets are used, into which ammonia gas is pumped. The model composition is smelted from the formed shell: in water, in the model composition, by burning, high-pressure steam.
After drying and refluxing, the block is calcined at a temperature of about 1000 ° C to remove gas-forming substances from the shell form.
After that, the shells enter the fill. Before pouring, the blocks are heated in furnaces to 1000 ° C. The heated block is installed in the furnace and the heated metal is poured into the shell. The filled block is cooled in a thermostat or in air. When the block is completely cooled, it is sent to a knockout. Hammer blows on the sprue bowl beats ceramic, then the LPS section. Thus we get a casting.
The advantages of this method: the ability to manufacture parts from alloys that are not amenable to mechanical processing; obtaining castings with an accuracy of sizes up to 11 - 13 quality and surface roughness Ra 2.5-1.25 microns, which in some cases eliminates cutting processing; the possibility of obtaining machine components that, with conventional casting methods, would have to be assembled from separate parts. Lost wax casting is used in a single (pilot), serial and mass production.
Due to the high consumption of metal and the high cost of the process, this method is used only for critical parts.
The investment casting process is based on the following basic principle:
- A copy or model of the final product is made of fusible material.
- This model is surrounded by a ceramic mass that hardens and forms a shape.
- Upon subsequent heating (calcination) of the mold, the casting model is melted and removed.
- Then, metal is poured into the cavity remaining at the site of the removed wax, which accurately reproduces the original casting model.