Abstract:
A casting apparatus for pouring molten metal into molds comprises a vessel having a molten metal-receiving chamber generally extending between a filling end and a pouring end. The vessel includes a nozzle disposed in a bottom surface of the chamber proximate to the pouring end. A stopper cooperates with the nozzle to control a downward gravity flow of molten metal through the nozzle. A first support pivotably supports the vessel to provide a horizontal tilt axis substantially coincident with the nozzle. A second support is connected to the vessel at a point away from the tilt axis and has a drive for controlling a pivot position of the vessel. A tilt angle controller detects a level of molten metal within the chamber and engages the drive to maintain the level at a predetermined level.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates in general to the pouring of molten metal into molds for manufacturing cast metal articles, and, more specifically, to stopper-controlled pouring of metal from a vessel wherein the flow rate of molten metal through a nozzle into a mold is accurately controlled. 
     One type of automated pouring device for filling casting molds with molten metal includes a stopper-controlled pouring vessel. One example of such a pouring vessel utilizing a coreless induction heater is shown in U.S. Pat. No. 5,282,608. There is an inlet for admitting molten metal into a main holding chamber within the vessel and a bottom nozzle outlet for discharging the metal into underlying casting molds. A mechanically operated stopper rod interacts with the nozzle to regulate the flow of molten metal through the nozzle. 
     In order to optimize the properties of the cast article, a variable flow rate into the mold is necessary. Initially during the pouring of a mold, a high rate of metal flow from the pouring vessel into the mold is desired. Metal is poured into a sprue cup formed in the top of the mold and drains from the sprue cup through passages into the mold cavity. The sprue cup must be quickly filled to provide a smooth and even flow of metal into the mold cavity. Once the level of molten metal in the sprue cup reaches the desired height, a slower rate is maintained that matches the flow of metal out of the sprue cup into the mold cavity. This rate is maintained until sufficient metal has been poured to fill the mold cavity. Preferably, the flow of metal is stopped in time to avoid overspill of metal outside the sprue cup after the mold cavity is filled. 
     In a conventional stopper-controlled pouring system, a variable rate of molten metal flow through the nozzle is obtained by controlling the stopper rod height over the nozzle. Specifically, the rate of flow is given by
 
 R=δAk √{square root over (2 gh )}
 
where R is the rate of flow in pounds/second, A is the area of the orifice between the stopper rod and the nozzle in square inches, δ is the molten metal density in pounds per cubic inch, g is the gravitational constant, h is the head height of the molten metal bath above the orifice, and k is a constant which is the product of a coefficient of velocity, a coefficient of turbulence, and a coefficient of viscosity.
 
     In prior art stopper-controlled pouring systems, the variable A is controlled in order to achieve a desired profile of the flow rate during mold filling. The above equation is solved for A and a controller uses a known target flow rate at any moment together with nominal constant values for δ and k in order to determine the appropriate stopper rod position corresponding to the solved value for area A. The value of A is approximate since during a particular pour, certain elements of the equation are in fact not constant. In particular, the height of the metal bath h changes as the metal in the chamber is consumed and the coefficients of velocity and turbulence may change as a result of the change in h. 
     It is possible to measure these changing values so that they can be updated dynamically within the controller during the pour and used to update the above equation. However, this adds complication and expense to the pouring system and may still yield unsatisfactory results. Area A and flow rate R are directly related so that a robust control is achieved. Flow rate R varies exponentially as a function of head height h, making control of flow rate R more difficult. 
     A target flow rate in a typical casting application may range from about 3 lbs/sec to about 30 lbs/sec, for example. A maximum depth of the metal bath may be about 24 inches. In order to accommodate the ability to pour at 30 lbs/sec when the bath height is depleted down to 4 inches, a relatively large nozzle diameter is required in order to achieve the necessary area A. When pouring at the slower rate of 3 lbs/sec when the height of the metal bath is 24 inches, the stopper height over the nozzle necessary to achieve the desired value for area A is very small due to the large nozzle diameter. Under these conditions, the change in flow rate is very sensitive to minute changes in the stopper position. Consequently, the flow rate is hard to control and becomes inconsistent from pour to pour because of the variable head height. A further problem is that, at small stopper heights, the metal flow through the nozzle begins to roostertail due to an increased velocity. 
     Previous attempts have been made in stopper-controlled pouring systems to maintain a constant head height in the molten metal bath. However, these attempts have been impractical and required complicated and expensive apparatus. For example, pressurized displacement of molten metal from a main chamber into a pouring subchamber has been used to provide a constant head height. In addition to the added expense, such a system required frequent maintenance resulting in down time and loss of productivity. 
     In order to increase productivity, it is desirable to pour metal into molds as the molds are carried in a conveyor line without stopping as is described in U.S. Pat. No. 5,056,584. As shown in that patent, a pouring vessel is suspended by a moving carriage in order to synchronize its movement with the moving molds. In a moving system, the pouring unit must have good mobility and should be contained completely above the height of the top of the moving molds on their conveyor system. The weight, complexity, and space requirements of prior art pouring systems having constant head height, however, have been unsuitable for these moving applications. 
     SUMMARY OF THE INVENTION 
     The present invention achieves important advantages of well controlled molten metal flow through a nozzle by providing a constant head height with a cost effective and easily maintained pouring system. In particular, the vessel is tilted to an appropriate position during pouring such that the constant head height results. 
     In one aspect of the invention, a casting apparatus for pouring molten metal into molds comprises a vessel having a molten metal-receiving chamber generally extending between a filling end and a pouring end. The vessel includes a nozzle disposed in a bottom surface of the chamber proximate to the pouring end. A stopper cooperates with the nozzle to control a downward gravity flow of molten metal through the nozzle. A first support pivotably supports the vessel to provide a horizontal tilt axis substantially coincident with the nozzle. A second support is connected to the vessel at a point away from the tilt axis and has a drive for controlling a pivot position of the vessel. A tilt angle controller detects a level of molten metal within the chamber and engages the drive to maintain the level at a predetermined level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross section showing a vessel of the present invention pouring molten metal into a mold. 
         FIG. 2  is a side cross section of the vessel of  FIG. 1  after increased tilting in order to maintain a predetermined level of molten metal in a substantially vertical column above the nozzle. 
         FIG. 3  is a top, plan view of the vessel of FIG.  1 . 
         FIG. 4  is a front cross section of the vessel along lines  4 — 4  of FIG.  3 . 
         FIG. 5  is a front, left, top perspective view of the vessel of FIG.  1 . 
         FIG. 6  is a top, plan view of the vessel of  FIG. 1  showing an alternative embodiment for suspending the vessel. 
         FIG. 7  is a front, plan view of the vessel of FIG.  6 . 
         FIG. 8  is a side view showing an alternative embodiment of a vessel of the present invention. 
         FIG. 9  is a side view of the vessel of  FIG. 8  raised into position for pigging off molten metal not to be poured into a mold. 
         FIG. 10  is a top, plan view of a factory floor layout for a casting system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a vessel generally indicated at  10  comprises vessel walls formed of refractory-lined steel plates and enclosing a molten metal chamber  11  extending between a filling end  12  and a pouring end  13 . A nozzle  14  is formed by an aperture in a bottom wall of vessel  10 . Nozzle  14  can be opened and closed by a stopper rod  15  contoured to provide a seal with nozzle  14  when pressed together. Molten metal in chamber  11  is heated by an induction heating coil  16 . Molten metal contained in vessel  10  has an upper surface  17  that rises and lowers as molten metal is supplied into vessel  10  and then poured out through nozzle  14 , for example. Vessel  10  is tiltable to bring molten metal forward from filling end  12  to pouring end  13  as the amount of molten metal in vessel  10  is depleted. Preferably, a bottom surface  18  of chamber  11  is sloped to provide a generally decreasing depth of chamber  11  from filling end  12  to pouring end  13  (with reference to a nominal position of vessel  10  wherein the top of vessel  10  is horizontal). In other words, there is an increasing depth of chamber  11  with increasing distance from pouring end  13  so that the tilting of vessel  10  can controllably shift molten metal between filling end  12  and pouring end  13 . While in the nominal, horizontal position, vessel  10  may have a molten metal capacity of about 1,500 to 8,000 pounds of iron, for example. 
     Vessel  10  is suspended over a mold flask  20  that moves in a production line. At the top of mold flask  20 , a sprue cup  22  is aligned with nozzle  14  to receive a pour of molten metal. A mold cavity (not shown) receives the poured metal from sprue cup  22  via a plurality of passages (not shown) for distributing the molten metal. 
     As shown in  FIG. 2 , vessel  10  can be tilted forward (i.e., raised up at its filling end) in order to move more molten metal from the filling end into the pouring end so that a constant head height (designated by reference character H in FIGS.  1  and  2 ), is maintained in the vessel  10  between the upper surface  17  of the molten metal and a nozzle exit  25  of the nozzle  14 . Vessel  10  pivots in the direction of arrow  24  about a pivot axis that coincides the nozzle exit  25  and that is perpendicular to the plane of  FIG. 2 , whereby nozzle exit  25  maintains its position relative to the sprue cup of the mold underlying it. As more molten metal is refilled into vessel  10 , it is tilted back down in the opposite direction in order to maintain a predetermined head height at all times during pouring of molten metal into molds. 
     As shown in  FIGS. 3 and 4 , vessel  10  may include two nozzle/stopper assemblies in opposite fingers of a “U-shaped” vessel. The bottom surface of the chamber in each finger preferably has the same sloped profile from the filling end  12  to the pouring end  13 . The base of the “U” has a receiving trough  27  at one side and a pour-back trough  28  at the other side. Molten metal is charged into vessel  10  by pouring into receiving trough  27  from a launder system described with reference to FIG.  10 . Vessel  10  is emptied of molten metal by reverse tilting to pour off metal through pour-back trough  28 . Pour-back trough  28  is at a lower elevation than receiving trough  27  to ensure that pour back occurs only through pour-back trough  28 . In an alternative embodiment, vessel  10  may instead have a filling orifice located in an area above nozzle  14  or  26  since the filling launder does not then have to take into account the variations in the height of vessel  10  during tilting. 
       FIG. 5  shows a perspective view of vessel  10  including apparatus for supporting and controllably tilting the vessel. Substantially coincident with the pivot axis  30  through nozzles  14  and  26 , a pair of pivot bearings  31  and  32  are affixed to the outsides of vessel  10 . The bearings mate with a pair of trunnions  33  that are suspended from a movable carriage (not shown) for pivotably supporting the pouring end of vessel  10 . The filling end of vessel  10  is supported by a hoist  34  for controllably raising and lowering the filling end to achieve a pivot position that maintains the molten metal level at a predetermined level at the pouring end of vessel  10 . Hoist  34  may, for example, comprise a pair of cables  35  attached between a support plate  36  mounted to vessel  10  and a pair of reels  37  mounted to the movable carriage. A hydraulic or electric motor  38  coupled to reels  37  rotates to take up or pay out cables  35  under control of a tilt controller  40  as a closed loop feedback control. The level of molten metal can be determined by weighing the vessel contents using a load cell  41  upon which the hoist may be mounted. Knowing the weight and density of the molten metal, tilt controller  40  can determine the volume of molten metal. Since the geometry of the vessel chamber is known, tilt controller  40  can infer the level of the molten metal surface. Load cell  41  could alternatively be placed between hoist  34  and the moving carriage. 
     In yet another embodiment, the molten metal surface can be directly measured using a laser sensor  42  mounted above the molten metal batch near a stopper rod  29 . For example, laser sensor  42  can be mounted to a side wall of vessel  10  or to a vessel cover  45  in the vicinity of a stopper rod aperture  46 . Laser sensor  42  optically determines the head height of molten metal and provides a corresponding signal to tilt controller  40 . Laser sensor  42  can be comprised of a laser distance sensor of the type commercially available from SICK AG, of Waldkirch, Germany, for example. 
     Another embodiment for suspending vessel  10  is shown in  FIGS. 6 and 7 . A trunnion rod  50  extends between bearings  51  and  52  mounted on the inward facing outer walls of the vessel fingers. A pair of support arms  53  and  54  extend from opposite sides at the filling end of vessel  10  for attachment to a hoist mechanism. A moving carriage  55  includes support beams  56  and  57  for securing trunnion rod  50 . 
     In the further embodiment shown in  FIG. 8 , a vessel  60  has a stopper mechanism  61  mounted thereon. Vessel  60  including the coreless induction heater may be comprised of the Horizontal Coreless Auto Pour (HCAP) system available from Hayes-Lemmerz International-Equipment and Engineering, Inc., in AuGres, Mich. Stopper mechanism  61  can, for example, be comprised of the commercially available Seaton model 676EC stopper unit. A frame  62  is attached to vessel  60  and has a load cell  63  attached thereto. Vessel  60  is connected to the hoist via load cell  63 . Even if a laser sensor is employed to measure head height, a load cell may still be desirable to estimate the weight of molten metal is vessel  60  in order to control refilling of metal. 
     Vessel  60  is preferably incorporated into a movable pouring system such as the Mobl-Pour automatic pouring system available from Hayes-Lemmerz International-Equipment and Engineering, Inc. Vessel  60  can be moved parallel to a mold line direction in synchronization with a moving mold to position the stopper nozzle(s) over the sprue cup(s) of the mold. It can be moved parallel to the mold line direction for alignment with the sprue cups and to move off of the line for cleaning of the stopper nozzles or other maintenance and for pigging or dumping the contents of vessel  60 . Thus, the tilting motion of vessel  60  permits tilting to the position shown in  FIG. 9  wherein molten metal is back-poured into a bull ladle  65  contained in a pit  66  in floor  67 . The range of tilting motion can be defined in relation to chamber surfaces comprising a sloped back wall  70  and a sloped bottom surface  71 . During pigging, vessel  60  pivots to the point where wall  70  has rotated just past horizontal so that all molten metal flows out into bull ladle  65  (e.g., for return to a main furnace). During pouring of molds, vessel  60  needs to pivot no farther than a point where bottom surface  71  has rotated just past horizontal in order to supply all molten metal available to the pouring end of vessel  60 . However, tilting to this extreme during pouring will not typically occur because the main chamber of vessel  60  will be frequently refilled so that the constant head height can be maintained. 
       FIG. 10  shows a top view of a portion of a factory layout for the tilt pouring system of the present invention. Molten metal is replenished into vessel  60  by an articulated launder system  75  which transfers the molten metal from a main furnace  76  to vessel  60  via a launder trough  77 . A gantry system  80  provides a rail system for the moving carriage carrying the pouring vessel and its support structures in order to follow molds on conveyor line  81 . A service platform  82  contains support and control equipment including an electrical control panel  83 , an inductor power unit  84 , a hydraulic power unit  85 , a water cooling system  86 , and a pneumatic panel  87 . 
     In view of the foregoing description, the present invention has provided a noncomplex, inexpensive solution to providing a constant head height of molten metal in a movable pouring system. Nozzle design and selection is greatly facilitated since a wide range of head heights does not need to be addressed. A more laminar flow can also be achieved because the nozzle can be better customized to the constant head height, and the roostertail problem is avoided. Furthermore, a shorter stopper rod can be used, which allows better stability of the vessel when molten metal is pouring in from the launder.