Shot in die casting is the core link of the die casting process, which refers to the process of injecting molten metal into the mold cavity at high speed and high pressure through the injection mechanism of the die casting machine.
Definition and core role of shot in die casting
Shot is a critical stage in the die casting process in which the molten metal is filled with a mold, the core of which is to apply a high pressure (usually 10-175 MPa) to the molten metal in the press chamber by the injection punch, so that it enters the mold cavity at an extremely fast speed (16-80 m/s).
According to the definition of GB/T 5611-2017 standard, the process of pushing the molten metal filling cavity in the pressure chamber of the injection punch is “injection”.
Injection effect
Overcome flow resistance
Through high pressure to break through the resistance of the mold pouring system.
To ensure that the molten metal quickly fills the complex cavity.
Promote tissue densification
Continuous pressure is applied during the pressurization stage to reduce porosity and shrinkage porosity inside the casting.
Improved surface quality
High-speed filling allows the molten metal to fully adhere to the surface of the mold before solidification, forming a high-precision profile.
Process Breakdown of Injection
The injection process is usually divided into multi-stage control to optimize the filling effect and reduce defects.
Slow Injection Stage
Objective: To smoothly push the molten metal to the inner gate to avoid gas involvement.
Speed: typically 0.1-0.5 m/s to ensure that the chamber is filled and the molten metal temperature is stable.
Fast Injection Stage
Objective: To inject molten metal into the cavity at high speed (more than 4-5 m/s).
Key function: shorten the filling time (0.01-0.2 seconds) and prevent the molten metal from solidifying in advance.
Pressurization stage
Purpose: To apply a higher pressure (e.g. to 200 MPa for a second booster) before the molten metal solidifies, to compensate for shrinkage and increase density.
Technical requirements
The pressurization start-up time should be less than 0.02 seconds.
Otherwise the molten metal at the inner gate may solidify and cause pressurization failure.
Key Process Parameters of Injection
Pressure Parameters
Injection Force
The total force exerted by the injection punch, which is related to the performance of the hydraulic system.
Injection specific pressure
The pressure per unit area (calculated as the injection force/pressure chamber cross-sectional area) directly affects the metal-hydraulic kinetic energy and filling capacity.
High specific pressures (e.g. above 100 MPa) can improve the tensile strength of castings.
But need to balance the mold capacity.
Speed parameters
Injection speed
Slow speed and fast speed, which need to be adjusted according to the wall thickness and complexity of the casting.
For example, thin-walled parts require higher speeds (more than 5 m/s) to avoid cold separation.
Inner Gate Velocity
The rate at which molten metal enters the cavity through the gate, affecting the flow pattern (ideally in the range of 30-60 m/s).
Time Parameters
Filling time
Usually controlled within 0.01-0.2 seconds, too long will cause the temperature of the molten metal to drop, too short may cause turbulent entrapment.
Temperature control
Molten metal temperature: affect the fluidity, such as aluminum alloy is generally controlled at 620-680°C.
Mold temperature: Preheat to 150-250°C to slow down the solidification rate.
Technological Evolution of Injection Systems
Hot Chamber Die Casting Machine
The injection chamber is immersed in molten metal, which is suitable for low melting point alloys (such as zinc, magnesium) and has a low injection speed.
Cold chamber die casting machine
The injection chamber is separated from the furnace and injected into the molten metal through the scoop.
which is suitable for high melting point alloys (such as aluminum) and can achieve higher pressure injection speeds.
Technological innovation
Three-stage injection system
From the early single-stage injection to the three stages of slow, fast and pressurization, which significantly improves the density of castings.
Real-time control system
Sensors monitor the injection curve and dynamically adjust the parameters to respond to process fluctuations.
Influencing Factors and Optimization of Injection Quality
Common Defects
Porosity: caused by high-speed filling of coiled air or insufficient pressurization.
Cold separation: caused by low injection speed or insufficient molten metal temperature.
Shrinkage porosity: caused by insufficient pressure or time lag during the pressurization phase.
Optimization strategy
Parameter matching
Determine the optimal combination of injection velocity and temperature through orthogonal tests (for example.
A study shows that pouring temperature has the greatest impact on the volume of coiled gas).
Mold design
optimize the cross-sectional area and position of the inner gate to reduce the flow resistance.
Equipment maintenance
Regularly check the wear of the injection punch and the pressure chamber to avoid pressure leakage.
Future Trend of Injection Process
Intelligent control
AI algorithm is introduced to predict filling behavior and realize adaptive parameter adjustment.
Ultra-high pressure technology
Develop equipment with higher specific pressure (such as more than 300 MPa) to improve the forming capacity of large thin-walled parts.
Green manufacturing
Reduce energy loss and metal spatter in the injection process, and improve material utilization.
