The principle of the hot chamber die casting process is based on the forced filling of the working cavity of a metal mold with a melt and the formation of casting under the action of the pressure of a pressing piston moving in a pressing chamber filled with melt. Let’s see what are the main advantages of hot chamber die casting.
Unlike a chill mold, the working surfaces of the mold in contact with the casting do not have a refractory coating. This leads to the need for short-term filling of the mold with melt and the action of excess pressure on the crystallizing casting, which is hundreds of times higher than the gravitational pressure.
Features of the formation of castings and their quality
In hot chamber die casting, the main parameters of casting quality – dimensional accuracy, surface roughness, mechanical properties, density, and tightness – are determined by the following features of its formation:
Short-term filling of the mold cavity with melt. The rate of inflow of the melt into the mold for different castings and alloys ranges from 0.3 to 140 m / s, the duration of its filling is 0.02 – 0.3 s, and the final pressure on the melt can reach 500 MPa. This makes it possible, despite the high cooling rate of the melt in the mold, to produce very complex body castings with a wall thickness of less than 1 mm from alloys with low and even close to zero fluidity (such property is possessed, for example, by alloys in a solid-liquid state). The high kinetic energy of the moving melt and the pressure transferred to it at the moment of completion of the mold filling, contribute to the production of castings with low surface roughness.
Gas tightness of the mold material. Ventilation of the working space is carried out using special ventilation ducts. At high rates of melt entry into the mold cavity, air, as well as gaseous decomposition products of the lubricant formed during its interaction with the melt, do not have time to completely leave the mold during its filling with the melt. They prevent the filling of the mold and get into the melt, leading to the formation of non-lithium, non-fusion, cavities, and gas-air porosity in the castings. Gas-air porosity leads to a decrease in the density of castings, a decrease in their tightness and plastic properties. Air, gases, decomposition products of the lubricant, which are in the pores of the casting under high pressure, complicate its heat treatment: when heated, the strength of the casting decreases, and the pressure of gases in the pores increases, which causes warping of the casting, bubbles appear on its surface.
High intensity of thermal interaction between the casting material and the mold due to its high thermal conductivity and heat capacity, the low thermal resistance of the lubricant layer and its decomposition products, significant pressure of the melt and casting on the mold walls, which improves the contact between them. This contributes to obtaining a fine-grained structure, especially in the surface layers of the casting, increasing its strength and high productivity of the process.
Transfer, at the end of filling the mold with metal, the pressure developed by the press piston in the pressing chamber to the melt in the mold cavity. This improves the feeding of the casting, helps to reduce the shrinkage porosity, and the compression of gas-air inclusions. As a result, the density, tightness, and mechanical properties of the casting increase. However, the effectiveness of the pre-compaction action is limited since this pressure on the melt in the mold only acts as long as the feeder is not solidified.