The injection molding process consists in pouring molten metal into the compression (pressing) chamber of the casting machine and then pushing it through the gating system into the cavity of the metal casting mold, which is filled under pressure. The cavity is filled at a high injection rate of the metal melt.
At the moment of completion of the filling of the form, the movement of the melt instantly stops, causing a water hammer. The pressure, which instantly increases, presses the metal against the working surface of the mold and contributes to a clear formation of the shape of the casting. The result is precise and smooth castings.
In addition, due to hot chamber die casting, the intensity of heat transfer increases, which leads to accelerated hardening. Due to the water hammer, the surface layer (0.02 – 0.2 mm) of the casting is dense, without gas porosity, gas bubbles remain in the inner layers of the casting or the solution, reducing porosity and increasing strength.
Stages of the die casting operation
During hot chamber die casting, molten metal is poured into the pressing chamber, and then by a plunger (piston), it is fed under pressure into the cavity of the split mold, which consists of stationary and moving parts. The internal cavities of the castings are formed by rods.
Air and gases are removed through channels 0.05 – 0.15 mm deep and 15 mm wide, located in the plane of the mold part, or by evacuating the working cavity before pouring molten metal.
After holding under the pressure required for solidification of the casting, the mold is opened, the core is removed, and the casting is removed from the working cavity of the mold by a pusher. The holding time under pressure depends on the maximum wall thickness of the casting and is approximately 1 – 15 s. Before pouring, the mold is heated to 120 – 320 � C. After removing the casting, the working surface of the mold is blown with air and lubricated with special materials to prevent welding of the mold casting.
When choosing an alloy, the following requirements must be taken into account:
- the alloy must have sufficient strength at high temperatures so that the casting does not break when ejected;
- have minimal shrinkage;
- have high fluidity at low overheating and have a small crystallization temperature range.
These requirements are met by alloys based on zinc, aluminum, magnesium, and copper.
The dimensional accuracy of the castings depends on the accuracy of mold manufacturing. In the manufacture of casting molds with dimensions according to the 9th grade, the castings can have the 10th – 11th grade of accuracy. Castings, made in split molds, come out in the 11th – 13th grade.
The mechanical properties of the material of the parts differ significantly from the mechanical properties of the original alloys. Upon rapid cooling, a casting crust with a very fine-grained structure is formed in the castings, the thickness of which is about 0.5 – 1.0 mm. Therefore, thin-walled castings have a fine-grained structure, increased strength and hardness by 20 – 30%, while reducing plasticity by about 30% compared to other casting methods.
The main reason for rejects, it turns out during machining, is porosity. The sources of porosity are alloy shrinkage, gases released from the liquid metal, and the air is entrained by the flow into the mold cavity. To improve the quality of castings, it is necessary to provide for the manufacture of casting holes. The parameter of machining allowances should not exceed 0.5 mm.