G SERIES VACUUM OVER PRESSURE CASTING MACHINES
The operator has simply to charge the crucible, place the casting flask in position and press one button! All machines lead the operator logically step-by-step through the casting sequence, irrespective of the type and quantity of the alloy. Frequency and power output are automatically and continuously adjusted and, just ahead of attaining the preset casting temperature, the computerized control starts modulation of the heating current by delivering low frequency impulses, thus inductively stirring the melt. Then, when all parameters are achieved and the temperature “spread” is no more that 4°C either side of the set point, pouring starts automatically followed by strong pressurization of the molten metal by inert gas. The complete cycle can be performed in minutes and crucible lasts for hundreds of casting it properly used.
EXPLANATORY NOTE OF THE “OVER PRESSURE” GAS IN MOLTEN METAL
In our system both the flask and crucible are located in a common chamber which is first fully evacuated by vacuum, before melting, followed by helium gas supply to create an inert atmosphere. Helium gas presents excellent gas fluidity which provides a cleaning effect of the molten metal and flask and guarantees a smooth filling of metal even with the most intricate filigree patterns. FIG. 1 The casting temperature is attained and low frequency pulses are given to vibrate and mix the molten metal by keeping it homogeneous. At this stage the flask is pushed against a long lasting special metal sealing blade system, or SBS, which, like a knife , cuts the investment by sealing the vacuum underneath the flask. This “long lasting” system avoids the use of silicon gaskets or flasks with flange with an important reduction in casting costs. FIG. 2. The metal, once completely poured into the flask, is pressurized by argon up to 4 bars. This pressure being applied to the molten metal, forces it into the flask while the vacuum pulls it. This results in a very smooth surface due to the effect of the vacuum and a dense well compacted casting due to the over- pressure which benefits both thin and thick sections. FIG. 3
The drawing below depicts the internal layout of most our competitor’s machines where the crucible and flasks are located in separate chambers divided by an aluminium base-plate. In this type of system, the crucible sits in the upper section and the flask is sealed beneath with a silicon gasket around its top edge. FIG. 1 The effect of this “useless” pressure is drastically reduced by “drag” of the crucible holder etc.; in fact, application of gas “over-pressure” by this method shows no significant advantage over a simple gravity/vacuum traditional and obsolete system. FIG.2 The metal cannot be effectively pressurized up to 4 bars because gas pressure applied above the crucible can only act on the metal, once the mould is occupied, by jetting through the crucible feed hole to the underside. Furthermore, at this high pressure this weak structures will not withstand and the cover will blow out. A further disadvantage of these machines is that there is usually a gap of 4-5 cm between the crucible feed hole and the flask entry, which frequently leads to splashing and gas absorption problems (our “G” series have the crucible almost attached to the flask). FIG.3.