Fundamentals of vacuum die casting
Lowering the air pressure in the die casting mould – in the mould cavity and the shot sleeve – reduces the resistance that the metal flowing in needs to overcome. Faster and more even mould filling is expected, and air pockets in the casting are avoided.
Here the following technical particularities need to be considered:
Tightness of the mould
The less air is sucked in through the mould, the more easily the vacuum can be adjusted. Pressure differences between the mould cavity and the pressure after the air bleed valve (Figure 1) are the quality criterion for the tightness of the die casting mould.
Figure 2 shows a typical pressure curve, which normally occurs in the single-stage vacuum die casting process. The upper curve represents the attainable pressure in the mould cavity and therefore the primary result of the evacuation process. The lower curve represents the values measured after the air bleed valve. The attainable pressure curve first shows a steep pressure drop and then levels off at a certain value. Figure 2 shows the pressure curve in a relatively tight mould.