METHOD AND APPARATUS FOR COUNTER-GRAVITY CASTING OF METAL

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 means of 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: METHOD AND APPARATUS FOR COUNTER-GRAVITY CASTING OF METAL TECHNICAL FIELD The present invention relates to a method of casting metal products in the manner set forth in the preamble of claim 1. 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 of the kind referred to above, with which it is possible to fill the casting cavities concerned as quickly as possible whilst avoiding undue turbulence and shock, and this object is achieved by proceeding in the manner set forth in the characterizing clause of claim 1. 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. This apparatus is of the kind set forth in the preamble of claim 5, and according to the invention, it also comprises the features set forth in the characterizing clause of this claim 5. Advantageous embodiments of the method and the apparatus, the effects of which - beyond what is obvious - are explained in the following detailed part of the present description, are set forth in claims 2-4 and 6-12, respectively. 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 Figures 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, Figure 3 is a graph showing an example of a mould-filling profile shown in the form of pressure as a function of time, and Figures 4-6 show various examples of sensing arrangements that can be used in the apparatus shown in Figure 1 and/or Figure 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS The mould-filling station shown in Figure 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 Figure 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 Figure 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 Figure 2, the supply of molten metal is contained in a furnace 3, that need not necessarily be closed like the one shown in Figure 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 13a and a current coil 13b. Instead of the first pressure sensor 7 shown in Figure 1 , the embodiment shown in Figure 2 comprises - a current regulator 12 adapted to control the current flowing through the field coil 13a and current coil 13b 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 13a and 13b, 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'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. Figure 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. Ill: 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 signalled by it/them to the main control unit 22. Figure 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. Figure 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, signalling the arrival of the melt in the outlet of the mouthpiece 6. Figure 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 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. LIST OF PARTS 1 mould support 2 mould 3 furnace 4 gas-supply unit 5 filling tube 6 mouthpiece 7 first pressure sensor 8 second pressure sensor 9 melt-level sensor 10 lower filling sensor 11 upper filling sensor 12 current regulator 13 electromagnetic pump 13a field coil 13b current coil 15 current sensor 16 Venturi restriction 17 pressure gauge 18 pressure gauge 19 pressure gauge 20 thermocouple 21 opening 22 main control unit

Claims: CLAIMS 1. 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 said molten metal a pressure sufficient to lift it to said higher level, characterized in that said pressurizing device is controlled to vary said pressure in a manner ensuring that said casting cavity is filled according to a predetermined value of molten-metal level as a function of time by means of a closed-loop arrangement with at least one input derived from at least one of the following parameters: a) the pressure of said molten metal in a duct leading from said furnace to said casting cavity, b) the flow velocity of said molten metal in said duct, c) the level of said molten metal in said casting cavity, d) the power supplied to said pressurizing device, e) previously programmed control parameters, and f) the decrease in weight per unit time of said furnace. 2. Method according to claim 1 and for said furnace using a substantially closed chamber, and for said pressurizing device using a controllable gas-pressure unit adapted to create a variable gas pressure in said chamber, characterized in that said gas-pressure unit is controlled as set forth in claim 1 , using as input representing the power supplied to said pressurizing device either the power supplied to said gas-pressure unit or the pressure created by it in said chamber. 3. Method according to claim 1 and for said pressurizing device using an electromagnetic pump comprising means for creating an alternating or travelling electromagnetic field creating a pressure differential in said molten metal, characterized in that said electromagnetic pump is controlled by means of a closed-loop arrangement with at least one input as set forth in claim 1 , using as input representing the power supplied for said pressurizing device, the electrical current flowing through said electromagnetic pump. 4. Method according to claim 1 or 2 and for use with a casting cavity with an opening at the top, characterized by including in said parameters the absence or presence of molten metal in said opening in order to reduce the filling velocity to zero when said presence occurs. 5. Apparatus for carrying out the method according to any one or any of the claims 1-4 and comprising a) a furnace (3) for containing the molten metal to be cast, b) a pressurizing device (4,13) capable of creating in said molten metal a pressure sufficient to lift it to said higher level, c) a duct (5) leading from said furnace (3) to d) a casting cavity in a mould (2), and e) control means (22) for controlling the supply of power to said pressurizing device (4,13), characterized by f) a pressure sensor (8) adapted to sense the pressure in said duct (5) and to transmit a corresponding signal to said control means (22), and in that g) said control means (22) are adapted to control said supply of power to said pressurizing device (4,13) in a manner ensuring that said casting cavity is filled according to a predetermined value of molten-metal level as a function of time. 6. Apparatus according to claim 5 and comprising a pressurizing device in the form of an electromagnetic pump (13) comprising means for creating an alternating or travelling electromagnetic field creating a pressure differential in said molten metal, characterized in that said control means (22) are adapted to control the electrical current supplied to said pump. 7. Apparatus according to claim 5 or 6, characterized by a level sensor (9) adapted to sense the arrival of the surface of molten metal in said duct (5) and to transmit a corresponding signal to said control means (22). 8. Apparatus according to any one or any of the claims 5-7, characterized by a level sensor (10) adapted to sense the arrival of the surface of the molten metal at the entry into said mould (2) and to transmit a corresponding signal to said control means (22). 9. Apparatus according to any one or any of the claims 5-8 and comprising or for use with a mould (2) having an opening at the top, characterized by a sensor (11) sensitive to thermal radiation and adapted to sense such radiation from said opening but not or to a substantially lesser degree from said mould (2). 10. Apparatus according to claim 9, characterized in that said thermal-radiation sensor (11) is placed on a line extending obliquely from said opening, the latter extending at an angle with said line, such as vertically. 11. Apparatus according to claim 9 or 10, characterized in that said thermal radiation sensor (11) is a thermal image camera adapted to transmit image information to said control means (22). 12. Apparatus according to any one or any of the claims 6 and 7-11 as dependent on claim 6, characterized by a) a digital controller (22) adapted to process the signals received from one or more of said sensors (7-11) and, on the basis of signals thus received and/or a programme previously supplied to said controller, to supply a signal representative of a desired value of the electrical current supplied to said electromagnetic pump (13) to a first input on a current controller (12) controlling the current through said pump (13), b) a current sensor (15) adapted to measure the momentary RMS-value of the current through said pump (13) and to supply a corresponding signal to a second input on said current controller (12), whereas c) said current controller (12) is adapted to regulate the current through said pump (13) in a manner minimizing the difference between the signals on its said first and second inputs.

Inventors: ANDERSEN HENRIK KLARSKOV, JENSEN KAJ J, LARSEN HENNING, ANDERSEN, HENRIK, KLARSKOV, JENSEN, KAJ, J., LARSEN, HENNING
Applicants: ANDERSEN HENRIK KLARSKOV, GEORG FISCHER DISA AS, JENSEN KAJ J, LARSEN HENNING, ANDERSEN, HENRIK, KLARSKOV, GEORG FISCHER DISA A/S, JENSEN, KAJ, J., LARSEN, HENNING
Application year: 1998
Main classification: B22D18
IPC codes: B22D18, B22D33, B22D35, B22D37, B22D39