Dual Phase

Air entrapment during the mould filling in high pressure die casting is a major problem that often leads to high amount of scrap, due to unexpected and dangerous gas porosity, especially in high performance and structural parts.

The air flow in HPDC must be considered much more important than metal flow: many commercial software solutions (the so-called single phase software) are available to predict the metal flow pattern inside the die, but at present no one of them is able to model the air flow through the air vents in order to predict the actual air entrapment. Commercial simulation software are not actually considering air movement inside the die as they do not consider the air compressible fluid at all.

For them air is merely “the space not filled by metal” and the effect of air on fluid flow is not considered at all: no air vents or escape is modelled, no air compression and heating is considered, no air velocity at outlet is calculated.

Both theory and practice suggest that the flow of metal inside the die during the second phase injection cannot be considered a coherent flow with a compact fluid front but it’s composed by an high velocity stream of air and small metal droplets (or small bubbles inside a metal stream, as you prefer) that fills up the die. The size of these small droplets (or bubbles) is much smaller than the typical dimension of a practical mesh cell and the effect of actual air counterpressure on the flow is really meaningful.

In order to model the spraying phenomena Castle utilizes a dual phase (air+metal) immiscible VOF (Volume of Fluid) solver: with this kind of technology air can be compressed inside the die, can move, escape through vents and can be finely dispersed inside the metal flow simulating for the first time what really happens inside the steel die with an high degree of accuracy as never seen before.

Thermal implications of air heating during the filling of the die are taken into account: air is heated up by the hot die and by the incoming liquid metal front so that it expands and its physical properties change time to time.
Real velocities and operating pressures are taken into account during the three stages of injection, not only during first or second phase like many other models do. Final intensification pressure is then applied to the entrapped air to compress it just like in the real process happens during solidification. Process parameters like phase’s strokes and plunger speed are calculated with an high degree of accuracy for a given casting geometry in order to simulate real casting conditions.

Progressive clogging of vents and vacuum channels are considered as function of metal flow inside the cavity, taking into account position of air escapes too, not only their shape and section.