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 Typhoon vacuum runners for high security in closure
















Vacuum for Die-Casting

In pressure die-casting, many important advantages are obtained by evacuating the gases that are found in the die and shot-sleeve just before and during injection.  Chief among these is a considerable reduction in porosity and hence a general improvement in quality.  Evacuating the die is especially important for components that will be welded or heat-treated, or where die filling is particularly difficult.

The evacuation of the gases must take place very rapidly before the shot and continue as long as possible during it.  It must cease reliably and absolutely when the shot sleeve plunger reaches a point a few millimetres before it stops.  Ideally, it should stop when the liquid metal has already penetrated at high speed the preliminary passages of the vacuum valve; at this instant there remain at most only a few milliseconds in which to close the vacuum exhaust channel, reliably and in such a way that it can resist the high injection pressure.

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The figure helps in understanding the different phases of the evacuation operation.  The main vacuum valve is placed close to the vacuum tank, which typically has a capacity of 250 litres with a pressure of less than 1 mbar. When the pouring slot of the shot sleeve has been hermetically sealed by the slow advance of the piston, the main vacuum valve is opened to initiate evacuation.  The liquid metal injection proceeds first at a moderate speed to fill the runners, and then very rapidly, generally one to three seconds after the metal has passed the gate(s).  Evacuation of the gases continues up to the instant where the fast-acting valve (solid line ellipse in the figure) closes under the pressure of the rapidly advancing liquid metal front, or alternatively on a signal given by the PLC controlling the casting machine.  (In the former case the valve is referred to as a mechanical, and in the latter case an actuated vacuum shut-off valve.)  If the cavity and the shot-sleeve are sufficiently vacuum-tight the pressure in the die is less than 100 mBar. 

Thus it can be seen that evacuation is very fast and precise, requiring very reliable control.  Depending on the size of the cast component the volume of gases removed ranges from 0.2 to more than 30 normal litres, with a flow velocity in the vacuum valve approaching the speed of sound in the atmosphere.  It is important that the fast-acting valve close at the very last moment of filling so as to ensure the evacuation of the last traces of gas pushed out by the advancing liquid metal.  In order to obtain this desirable result, the fast-acting valve is made to very close tolerances and is very robust; it ensures a closing time of typically one to three milliseconds.  For manufacturing high-integrity vacuum-cast components, VDS recommends evacuating the cavity to an absolute pressure of less than 50 mBar, i.e.  a residual of less than 5% of atmospheric pressure.



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Chill Vent inserts
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Vacuum Units
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Dual HS Pilot Valve


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