Abstract:
When casting e.g. aluminium alloy in a mould ( 2 ), that may be one of a string of moulds extending at right angles to the plane of the drawing, the pressure used for filling the mould ( 2 ) against gravity by a pressurizing device ( 4 ) is controlled according to a predetermined pressure/time programme having previously been read into a control unit ( 22 ), e.g. a digital computer, at least one of a number of sensors sensing pressure ( 8 ), levels ( 9,10 ) and full level ( 11 ) being used to supply inputs to the control unit ( 22 ). Sensors are chosen according to need for each particular casting programme.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of casting metal products by transferring molten metal from a furnace into a casting cavity at a higher level by means of a pressurizing device capable of creating in the molten metal a pressure sufficient to lift it to said higher level. Similar methods have been disclosed in a number of publications, but none of them provides any practical guidance with regard to achieving a filling of the casting cavity as quickly as possible and at the same time avoiding undue turbulence and shock, such as when the surface of the molten metal hits the top wall of the casting cavity. 
     DISCLOSURE OF THE INVENTION 
     It is the object of the present invention to provide a method with which it is possible to fill the casting cavities concerned as quickly as possible whilst avoiding undue turbulence and shock. By, in this manner, controlling the action of the pressurizing device on the basis of one or more parameters relating to the flow of the molten metal into the casting cavity, it is possible to achieve a “mould-filling profile”, i.e. the level of molten metal in the mould as a function of time, corresponding to optimum filling conditions, e.g. first filling the major part of the casting cavity at a relatively high pressure, but not so high as to cause undue turbulence, and then reducing the pressure to achieve a gentle and shock-free filling of the top of the casting cavity. 
     The present invention also relates to an apparatus for carrying out the method according to the invention. 
     Advantageous embodiments of the method and the apparatus, as well as the effects thereof—beyond what is obvious—are explained in the following detailed part of the present description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following detailed part of the present description, the invention will be described in more detail with reference to the exemplary embodiment of an apparatus according to the invention shown diagrammatically in the drawings, in which 
     FIGS. 1 and 2 are overall views of two exemplary embodiments of a mould-filling station comprising an apparatus according to the invention comprising several sensing functions, 
     FIG. 3 is a graph showing an example of a mould-filling profile shown in the form of pressure as a function of time, and 
     FIGS. 4-6 show various examples of sensing arrangements that can be used in the apparatus shown in FIG.  1  and/or FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The mould-filling station shown in FIG. 1 comprises as its main operational components 
     a mould support  1 , in the exemplary embodiment shown being adapted to support a string of 
     moulds  2 , said string extending at a right angle to the plane of the drawing, 
     a supply of molten metal contained in a substantially closed furnace  3 , 
     a gas-supply unit  4  adapted to apply a suitably controlled gas pressure to the space inside the furnace  3  so as to cause molten metal to flow through 
     a filling tube  5  extending upwardly to 
     a mouthpiece  6  adapted for temporary connection to the mould  2  being in a position for filling on the support  1 . 
     In addition to the operational components listed above, the mould-filling station shown in FIG. 1 comprises various sensing and control components, viz. 
     a first pressure sensor  7  adapted to measure the pressure inside the furnace  3 , 
     a second pressure sensor  8  adapted to measure the pressure in the filling tube  5 , 
     a melt-level sensor  9  adapted for inductively sensing the presence or absence of melt in the filling tube  5  at a level lower than that of the mouthpiece  6 , 
     a lower filling sensor  10  adapted for sensing the presence or absence of melt in the mouthpiece  6  immediately upstream of its connection to the mould  2 , 
     an upper filling sensor  11  adapted to sense melt having reached a position in or close to an opening (not shown) in the top of the mould  2 , and 
     a main control unit  22  adapted to receive and process signals from the sensors  7 - 11 , and, on the basis of such processed signals, to send a control signal to the gas-supply unit  4 . 
     At this point it should be emphasized that the sensors  7 - 11  need not always all be in operation in each and every mould-filling process, the choice of which of them to use being based upon circumstances in each particular case. 
     In addition to the sensors described above or in place of some of them, the mould-filling station could also comprise the following sensors or sensing functions, none of which are shown in FIG.  1 : 
     level sensing based upon electrical capacity measurements in the mould  2 , the ascending melt constituting one electrode, the other electrode being a conductor embedded in the mould close to the casting cavity, 
     level sensing using electrodes in or facing the casting cavity in the mould  2  and being short-circuited upon the melt having ascended to a particular level, 
     electromagnetic flow sensing, 
     flow sensing of the Venturi type. 
     The use of any one or any of these additional functions will, of course, enter into the choice of sensors referred to previously. 
     The various sensing functions and their use in the present connection, i.e. controlling the flow of melt into the mould, will now be described. 
     The first pressure sensor  7  will measure the gas pressure in the furnace  3  and send a corresponding signal to the main control unit  22 , enabling the latter to compare the actual gas pressure in the furnace to the pressure specified in the programme to exist at any given moment. 
     The second pressure sensor  8  will measure the metallostatic pressure at the inlet to the filling tube  5 , this pressure giving an indication of the level reached by the free surface of the melt. If this level differs from that according to the mould-filling programme as previously stored in the main control unit  22 , this unit will signal to the gas-supply unit  4  to effect the requisite increase or decrease in the pressure inside the furnace  3 , thus causing a corresponding rise or fall in the level of the free surface of the melt. 
     The melt-level sensor  9  operates on the basis of the inductance of a coil surrounding the filling tube  5 , the value of this inductance depending on the presence or absence of melt in the tube  5  at that particular point. Thus, the signal from the sensor  9  is substantially a YES/NO signal that can be used, either as a “CLEAR” signal for the actual filling of the mould to begin, or as a corrective to modify the mould-filling programme according to whether the point in time, at which the signal changes from NO to YES, coincides with or is early or late in relation to the point in time, at which the programme “expects” the surface of the melt to arrive at this sensor. 
     The lower filling sensor  10  will, of course, signal the arrival of the free surface of the melt at the inlet to the mould  2 , while the upper filling sensor  11  will signal the arrival of said surface in the top of the mould, thus indicating that the latter has been filled. Like the signal from the melt-level sensor  9 , the signals from the filling sensors  10  and  11  are substantially YES/NO signals, useful mainly for any necessary corrections to the mould-filling programme in the manner indicated above. 
     In the embodiment shown in FIG. 2, the supply of molten metal is contained in a furnace  3 , that need not necessarily be closed like the one shown in FIG.  1 . The requisite pressure needed to transfer the molten metal from the furnace  3  to the mould  2  through the filling tube  5  is provided by an electromagnetic pump  13 , e.g. having a field coil  13   a  and a current coil  13   b.    
     Instead of the first pressure sensor  7  shown in FIG. 1, the embodiment shown in FIG. 2 comprises 
     a current regulator  12  adapted to control the current flowing through the field coil  13   a  and current coil  13   b  in the electromagnetic pump  13 , in the example shown using a double thyristor, the regulator  12  also receiving a comparison signal from 
     a current sensor  15  adapted to measure the current through the coils  13   a  and  13   b , producing said comparison signal on the basis of the value measured. 
     As will be understood, the current sensor  15  does not directly take part in the monitoring of the casting process as carried out by some or all of the sensors  8 - 11 , as it is a part of the closed loop controlling the current metered by the regulator  12 . This sensor may be adapted to produce an I 2  signal rather than an I signal, so as to represent the power input rather than the current input to the pump  13 , the former being more closely related to the pump&#39;s hydraulic power output. 
     Of the sensing functions not shown but described above 
     the capacitive level sensing could be used for continuous control of that part of the mould-filling process, during which the level of melt ascends through the casting cavity in the mould  2 , 
     the short-circuit level sensing could be used to provide a YES/NO signal useful for correcting the mould-filling programme, 
     the electromagnetic flow sensing as well as that of the Venturi type could be used for continuous control of the part of the mould-filling process referred to above. 
     The programme installed in the main control unit  22 —preferably a digital computer of the type used for controlling industrial processes—could be divided into five steps, cf. FIG.  3 : 
     I: Pre-filling pressure: mould being prepared for filling. 
     II: Filling pressure: programmed to fill the mould to a level slightly below the top as quickly as possible while avoiding turbulence and oscillations. 
     III: Holding pressure: increasing slowly to avoid melt impact to the top of the casting cavity. 
     IV: Closing pressure: held constant while the mould is being closed. 
     V: Relaxation pressure: adjusted for non-turbulent return flow of melt from the upper part of the filling tube to the furnace. 
     These pressures are preferably those measured by the pressure sensors  7  and/or  8  and signaled by it/them to the main control unit  22 . 
     FIG. 4 illustrates the possible use of a Venturi restriction  16  in the filling tube  5 . By using three pressure gauges  17 ,  18  and  19  placed upstream of, within and downstream of the restriction, it is possible to compensate for the flow resistance so as to achieve a more realistic value of the true Venturi drop, and thus the flow velocity in the filling tube  5 . 
     FIG. 5 illustrates the possible use of a thermocouple  20  to sense the temperature in the outlet of the mouthpiece  6 . This thermocouple  20  would then constitute part of the lower filling sensor  10 , signaling the arrival of the melt in the outlet of the mouthpiece  6 . 
     FIG. 6 illustrates how the upper filling sensor  11 , in this example in the form of an infra-red sensor or camera, monitors an opening  21  in the top of the mould  2 , so as to react when it “sees” the hot melt rising in this opening. By aiming a thermally sensitive sensor obliquely to the vertically extending opening as shown, the sensor senses radiation from the opening but not, or at least to a substantially lesser degree, from the mould cavity. By using a camera it is possible to achieve very accurate control of the termination of the filling process by comparing the image information transmitted by the camera to the main control unit  22  to image information having previously been read into the latter. 
     As indicated previously, the decision as to which sensing function or functions to include in the operation will depend on the conditions in each particular case, mainly the shape and size of the casting cavity in each mould  2 , as well as the characteristics of the melt.