To keep disadvantages and defects – which constantly arise from an oxide skin forming on the melt – within limits, the gating system must guarantee low turbulence in the metal stream and also a smooth, controlled filling of the die cavity. With the transition from a liquid to a solid condition, volume contraction occurs; this can amount to up to 7 % of the volume. This shrinkage is controllable when the solid-liquid interface runs – controlled or directed – through the casting, mostly from the bottom to the top. This task, namely to effect a directed solidification, can be achieved with a good pouring system.
The castings are usually arranged “upright” in the die. The greatest mass can thus be placed in the bottom of the die.
Quality requirements can be, for example, high strength, high-pressure tightness or decorative anodizing quality.
One example of an “ideal” gating system which meets the highest casting requirements is the so-called “slit gate system”.
Here, the metal is conducted upwards continuously or discontinuously to the casting via a main runner. During mould filling, the melt is thus superimposed layer upon layer with the hotter metal always flowing over the already solidifying metal.
The standpipe ends in the top riser and supplies it with hot metal. This way, the solidification can be directed from below, possibly supported by cooling, towards the top running through the casting and safeguarding the continuous supply of hot metal. When there is a wide flare in the casting, the gating system has to be laid out on both sides. This symmetry ensures a division of the metal and also an even distribution of the heat in the die.
In low-pressure die casting, directing the solidification by means of the gating system is not possible. Nor is there any great possibility of classic feeding.
Directional solidification is only possible by controlling the thermal balance of the die during casting. This mostly requires the installation of an expensive cooling heating system.
Simulation calculations for die filling and solidification can be useful when laying out and designing the die and possibly the cooling. In actual production, the cooling and cycle time can be optimized by means of thermography (see section on “Solidification simulation and thermography”).