Shot Cylinder Performance
In high-pressure die casting, a shot cylinder is used to rapidly push liquid metal into a die. Typically, the cylinder is oriented horizontally and is filled with molten metal through a hole in the top. The metal is allowed to settle for a short time before a piston-plunger pushes the metal into a runner system leading to the die.
A properly designed shot cylinder must push metal into the die fast enough to avoid early solidification but not so fast that air is entrained into the metal. The presence of partially solidified metal or air in the metal can lead to internal defects that reduce the strength and integrity of the cast part.
Key to the successful operation of a shot cylinder is determining the maximum speed the piston can be pushed without causing the surface of the metal in the cylinder to overturn (or surf), which is a source of air entrainment. It is also important to minimize the early formation of solid metal in the cylinder, as this is another mechanism for possible defects.
Estimating the amount of overturning and early solidification is not easy because both processes involve non-linear phenomena. Fortunately, users of FLOW-3D do not have to worry since its accurate free-surface modeling capability makes it the ideal tool for this type of analysis.
A simple, but typical, example will illustrate how effective parametric studies of a shot cylinder design can be. In this example, a horizontal shot cylinder having a diameter of 3.25 inches and a length of 36.0 inches is a little less than half filled with molten aluminum (383Al). Initially, the metal is assumed to be at rest and at a temperature of 682°C. The piston begins pushing the metal with a speed of about 6 in/s, which sends a surface wave traveling down the metal in the cylinder. After 0.67s of elapsed time the speed is increased to 31.5 in/s. At this speed the fluid is seen to overturn (surf), which causes some air to be entrained into the metal.
Next, the simulation is repeated with a slower second-stage piston speed of 23.0in/s. In this case, the majority of surfing is eliminated. This reduction in surfing is beneficial, as can be seen from the comparisons, in Figure 2, of three types of potential sources for defects: surface oxides, air entrainment, and the formation of solidified metal in the cylinder. For ease of visualization the comparisons use vertical cross sections through the center of the shot cylinder and having roughly the same piston displacement.
Air entrainment is at least three times greater in the initial case than in the second case. Most of the air entrainment occurs during the early large-scale overturning of metal and resides near the piston. Air entrained at the front of the surface wave is distributed more uniformly in the metal.
On the other hand, the solid fraction (amount of solid metal formed in the cylinder) resides mostly near the piston face and is similar in both cases. Likewise, the entrained surface oxide is similar in both cases. Both quantities, whose growth depends on time, are slightly greater in the second case because the time taken to reach the same piston displacement is greater.
The detailed simulations provided by FLOW-3D give high-pressure die casters a real shot at predicting the optimum performance of shot cylinders without the time, expense, and waste of materials associated with multiple test shots.