Abstract:
A metal distribution system for the simultaneous production of a plurality of logs or round billets from molten metal comprising: 1) a trough for the introduction of molten metal; 2) a plurality of side streams extending from the trough and each of the side streams including a plurality of opposing apertures each of the apertures including a thimble for the shaping of molten metal passing through the trough and the side streams and into the thimbles. A uniform flow of molten metal into the side streams and the individual apertures is provided by the controlled negative angular orientation of the most upstream opposing pair of apertures thereby providing relative uniformity of the temperature of molten metal reaching each of the plurality of apertures. A unique unitized thimble configuration and trough damming arrangement are also described.

Description:
FIELD OF THE INVENTION 
   The present invention relates to apparatus useful in the casting of molten metal and more particularly to such devices as are utilized in the casting of so-called “logs”, “billet” or “round ingots” from, for example, molten aluminum. 
   BACKGROUND OF THE INVENTION 
   In the casting of molten metals such as aluminum apparatus and processes have been developed for the simultaneous casting of a plurality of logs, billets or round ingots, hereinafter logs, so as to increase the efficiency and productivity of the casting processes. In such processes and apparatus, a casting table having a plurality of apertures or molds is mounted over a pit from which emerge an equally numbered plurality of hydraulically operated bottom blocks. Each of the bottom blocks is registered, i.e. aligned with, one of the molds. The casting table includes troughs or distribution channels for the dissemination of molten metal introduced thereto to each of the individual molds or apertures located in the casting table. As metal from the distribution channels or troughs in the casting table enters the individual molds, the plurality of bottom blocks is lowered in unison to allow for removal of metal that has solidified in the mold therefrom and to provide space for the introduction of additional incoming molten metal. Such a prior art casting table is shown in FIG.  1  and described in greater detail hereinafter. 
   While the metal distribution of the casting tables of the prior art as depicted in  FIG. 1  have proven highly useful and reliable over many years of service in a multitude of installations, they suffer a number of shortcomings. 
   As those skilled in the molten metal casting arts are well aware, it is critically important that molten metal reaching each of the molds or apertures at substantially the same time with minimal temperature loss to obtain a successful cast of the plurality logs being simultaneously cast. If metal reaching one or more apertures is too hot or hold time is too short and does not solidify as the base plate descends, a “bleedout” can result. In such a condition, molten metal can be brought into contact with water applied as a spray in the process to cool the solidifying metal. Such a conditions requires rapid plugging of the aperture or mold that is experiencing the “bleedout” with the result that that portion of the production is lost for the cast. Alternatively, if metal has resided in the mold for too long a period, it may be cooler than the balance of the molten metal and therefore solidify more quickly in the mold than metal entering other molds in the casting table resulting in a “freeze-in”, i.e. the solidified metal becomes caught in the mold. Freeze in can drop out during casting and also result in bleedout. Such a condition can result the aborting of the cast entirely and necessitating a freeing up of the metal caught in the mold and a restart of the cast. Such errors can cause significant productivity losses and place operators in significant danger from a safety standpoint. If metal enters the mold with too much velocity or too hot, penetrates between the mold and the head, solidified ingot head “flashing” may occur. Flashing is another condition that may result in molten metal coming into contact with cooling water applied to the ingot below the solidification point. Flashing also causes damage to molds or distortion or delays in the bottom block movement that can also result in casting defects, bleedouts or complete table freeze in. 
   In addition to the foregoing, as will be explained in greater detail below, the design of the prior art “dams”, i.e. barriers that control the flow of molten metal into the distribution troughs within the casting table, often required the presence of at least two operators on the casting table at the initiation of a casting drop to “lift” or remove the dams at the start of the cast. The presence of operators in the immediate vicinity of the molten metal casting operation is always a safety concern, and the ability to eliminate the exposure of operators to such a risk is critically important to casting facilities. 
   Finally, the mold portions of the prior art casting tables comprise multi-part elements that require assembly in the casting table costing valuable assembly or set-up time and which because of their design leave exposed joints between the individual elements of the assembly that are sometimes prone to leaking, particularly if not properly assembled. 
   OBJECTS OF THE INVENTION 
   It is therefore an object of the present invention to provide a multi-strand metal distribution system that provides more uniform molten metal distribution at the start of a cast, minimizes heat loss and controls the velocity and fill time differences of molten metal entering the molds. 
   It is another object of the present invention to provide a thimble assembly for the above-described multi-strand metal distribution systems that because of their design and construction provide simplified and more secure installation of the mold assemblies. 
   It is yet another object of the present invention to provide a metal distribution system that incorporates an improved dam release mechanism that obviates the need for the presence of operators on the casting table to release dams during start up of a cast. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided a metal distribution system for the simultaneous production of a plurality of logs or round billets from molten metal comprising: 1) a single main trough for the introduction of molten metal; 2) a plurality of side streams extending from the trough and each of the side streams including a plurality of opposing pairs of apertures each of the apertures including a mold for the shaping of molten metal passing through the trough and the side streams and into the molds. A controlled velocity and uniform flow of molten metal into the side streams and the individual apertures is provided by the controlled negative angular orientation of the entry angle of the most upstream of the opposing aperture pairs thereby providing relative uniformity of the temperature of molten metal reaching each of the plurality of apertures. A unique unitized thimble configuration and trough damming arrangement are also described. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a metal distribution system of the prior art. 
       FIG. 2  is a top view of one embodiment of the metal distribution system of the present invention. 
       FIG. 3  is a cross-sectional view of a mold of the prior art. 
       FIG. 4  is a cross-sectional view of one embodiment of a mold of the present invention. 
       FIG. 5  is a top plan view of a single secondary trough in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , in the prior art a metal distribution system  10  for the simultaneous production of multiple logs or round billets comprised an inlet  12  feeding a primary trough  14  that in turn fed secondary troughs  16   a ,  16   b  and  16   c . Located at approximately right angles to the major (long) axes  18   a ,  18   b  and  18   c  of secondary troughs  16   a ,  16   b  and  16   c  and on opposing sides thereof are pairs of opposed round apertures  20  (only some being specifically identified in  FIG. 1  for clarity) each of apertures  20  containing a mold as will be described below in connection with FIG.  3 . Insertion of manual dams  22  requires manual removal to begin the flow of metal into troughs  16   a ,  16   b  and  16   c . In the casting operation, molten metal was provide to primary trough  14 , passed therethrough to secondary troughs  16   a ,  16   b  and  16   c  and thence into apertures  20 . While, as previously mentioned such a structure has provided a highly useful arrangement, it did demonstrate several shortcomings. Among these were that all of apertures  20  did not fill at the same time, thus resulting in temperature and solidification differences inside the sump between the first and last to fill in molten metal entering, for example the aperture designated  20   a  and that designated  20   b  in FIG.  1 . Such a condition can and often did lead to the problems previously referred to as “bleedout” or “freeze-in”. Additionally, the casting practice commonly used with a metal distribution system of this type called for starting the flow of molten metal through inlet  12  and then sequentially and manually removing dams  22 . The need to manually operate the damming arrangement required the presence of operators, most generally 2 on the surface of casting table  10  to perform removal of the dams. This posed a significant safety hazard as the presence of personnel in the immediate area of the casting table is always a cause for safety concern. Thus, the design and availability of a casting table that eliminated such issues have been a long sought after objectives. 
   Referring now to  FIG. 2  that presents a top plan view of the metal distribution system  30  of the present invention, there is provided an inlet  32  feeding a single preferably centrally located primary trough  34  having a plurality of relatively short secondary troughs  36  each feeding a plurality of opposing apertures  38  (not all numbered in  FIG. 2  for clarity) that contain molds (not shown in FIG.  2 ). Dams  40  are provided at the entry of each of secondary troughs  36 . Dams  40  are controlled by a pneumatically or hydraulically operated dam control arm  42  that is remotely operated from an operators station (not shown). In operation, molten metal is flowed through inlet  32  into primary trough  34  where its flow is limited by the presence of dams  40 . Once primary trough  34  is filled to the appropriate level, dam control arm  43  is activated raising dams  40  allowing metal to flow simultaneously into all or selected secondary troughs  36  and thence into apertures/molds  38 . Thus, primary trough  34  and secondary troughs  36  are flowably connected. Because of the angular structure of entry angles  42  as described in greater detail below, molten metal of all relatively the same fill time and temperature rapidly fills apertures/molds  38  simultaneously thereby eliminating the problems of unequal temperature metal in the casting table at different locations, i.e. providing minimum fill time and accompanying minimum temperature loss with maximum velocity to avoid flashing. The incorporation of the remotely operated dams  40 , the need for the presence of operators on the casting table during the start up procedure is also eliminated. 
   Referring now to  FIGS. 4 and 5 , according to a specifically preferred embodiment of the present invention, aperture entry angles  42  located at the entry of apertures  38  those proximate primary trough  34 , i.e. those at the upstream end  37  of secondary troughs  36 , are negative and preferably range from about 15 to about 30 degrees and most preferably between about 20 and 25 degrees. The negative orientation of these angles and their particular pitch as specified herein provide for the rapid and uniform fill of apertures  38  downstream thereof toward extremities  44  with a minimum of metal fill time and velocity into apertures  38  thus preventing metal flash and inclusion causing turbulence and providing relative temperature uniformity in the molten metal. Stated differently, filling of secondary troughs  36 , because of the angular orientation of entry angles  42  results in secondary troughs  36  filling from the downstream ends  44  toward the upstream ends  37 . In operation, as molten metal enters secondary troughs  36  upon the raising of dams  40  molten metal immediately flows to the outermost extremities or downstream ends  44  of secondary troughs  36  whereupon it quickly fills apertures  38  further downstream of primary trough  34  and then commences to fill secondary troughs  36  “backwards” in the direction of primary trough  34  or the upstream ends of secondary troughs  36 . This action provides for the quick and controlled fill of all apertures  38  with a minimum of turbulence and with molten metal of relatively the same temperature to assure a uniform start to the cast with a minimum of the occurrence of “bleedthrough” or “freeze-in” and significant reductions in head and butt defects that reduce the need for head and butt crop and increase the productivity of the casting operation. Thus, relatively simultaneous fill time of all apertures  38  is achieved by the provision of negative entry angles  42  that are directed away from opposing apertures  38  closest to primary trough  34  thus insuring that the positions  38  furthest away from primary trough  34 , i.e, closest to extremities  44  or downstream, receive metal at approximately the same time as those closest to primary trough  34  or upstream. 
   Each of apertures  20  and  38  contains a “mold”. As shown in  FIG. 3 , (a cross-sectional view along the line  3 — 3  of  FIG. 1 ) in the prior art, mold  50  comprised a crossfeeder  52 , a thimble  54 , a blanket of back-up insulation  56 , a “paper” (mica or the like) or similar gasket  58 , a transition plate  60 , a mold body  64  and a graphite ring  62 . A water reservoir  66  that produced a water spray  68  through the emission of water through spray channel  70  provided cooling of the solidifying metal  72 . The letters L and L′ in  FIG. 3  indicates those areas where molten metal remains liquid as it moves through mold  50  before solidifying at  72 . The volume L′ is commonly referred to as the “sump”. 
   In the prior art, thimble  54 , crossfeeder  52 , back-up insulation  56  and transition plate  60  all represented individual components that were assembled “in situ” so to speak at the casting station or in a fabrication shop before the start-up of a cast. This clearly involved a significant amount of labor. Additionally, it was not uncommon for the vertical joint  74  between thimble  54  and crossfeeder  52  to leak resulting in a bleedthrough of molten metal into joint  76  at gasket  58  between crossfeeder  52  and blanket insulation  56  and casting table structure  78 . Such leakage was not only affected productivity, but could cause a safety issue under certain particularly severe leakage conditions. Additionally, the variability in assembly technique from operator to operator introduced a further element of uncertainty or variability into a casting operation that was already fraught with variables. Thus, a solution has been sought that would significantly reduce the labor intensity of the mold insertion/fabrication operation, reduce any variability in the assembly operation and reduce the potential for leakage at the previously described assembly joint(s). 
   Such a solution is shown in  FIG. 4  that is a cross-sectional representation along the line  4 — 4  of FIG.  2 . The improved metal handling system  80  of the present invention shown in  FIG. 4  also comprises a crossfeeder  82 , back-up insulation  84 , and a thimble  86 , but all fabricated as a monolith that simply drops into aperture  38  through horizontal engagement with mold table  88  at horizontal joint  90  and transition plate  78  that is part of mold  60  that further engages mold table bottom plate  62  supported on mold member  73 . The entire structure is retained in close and tight engagement through the action of a bolt down arrangement through steel upright  100  that includes a nut  102  or other suitable fastening arrangement to bring the entire structure together. A graphite lubricating ring  62  as used in the prior art is incorporated in much the same fashion and for the same purposes as in the prior art. Cooling water sprays and a water reservoir are also preferably incorporated into the mold assembly, as shown in FIG.  4 . The foregoing structure, has been found to: 1) reduce heat loss through the back-up insulation to a greater degree than the blanket back-up insulation used in the prior art; 2) results in fewer cracked logs at start up; 3) results in fewer cold start related defects such as bleedouts and freeze ins; and 4) quite obviously increases the ease of assembly, and greatly reduces the labor involved in the mold assembly operation. 
   What clearly differentiates refractory module  80  of the present invention is that it comprises a module that combines in a single integral unit, a hot face refractory for crossfeeder  82  and thimble  86 , with a peripheral, low density, cold face refractory, back-up insulation  84  thereby eliminating the need to separately insulate behind crossfeeder  82  and thimble  86  or to assemble the individual elements at the casting station or at some remote location. It also eliminates the need for a separate vertical joint ( 74  in  FIG. 3 ) since thimble  86  is cast into the refractory module  80  providing the formation of a horizontal seal  90  (rather than a vertical seal) directly with the transition plate  78 . 
   The aim of the crossfeeder is mainly to distribute molten aluminum to the mold while minimizing turbulence and heat losses. The refractory material should be inert vis-à-vis molten aluminum, easy to clean and show a low heat storage. Prior art cross-feeders are made of light density refractories that have to be well preheated to avoid cold start-up. Depending on the material and design, maintenance can be quite extensive. The main mode of failure in such devices is crack propagation with time that renders the crossfeeder unusable. Typical life is difficult to determine because it depends on many variables such as: casting technology, design, casting parameters, maintenance, etc. 
   According to the present invention, two different refractory materials are used to extend the useful life of the crossfeeder and to enhance the aluminum casting process itself. 
   The material directly in contact with the aluminum  87  is a dense and hard refractory material showing excellent non-wetting characteristics to molten aluminum. It is provided in the form a thin skin, preferably between 6 and 10 mm thick. This material is a fiberglass fabric reinforced wollastonite that exhibits outstanding mechanical and non-wetting properties and is suitable for the fabrication of complex shapes. According to a highly preferred embodiment of the present invention the non-wetting properties of this material are further improved by coating its surface with a thin layer of boron nitride (not shown). Thin skin  87  is then backed up with a layer  84  of a highly insulating refractory material, preferably, Wollite, a mineral foam based wollastonite material. The skin  87  is used as the mold external surface and the Wollite insulation  84  is cast around this externally. The two materials constituting thin skin  87  and insulating refractory  84 , have very similar thermal expansion coefficients, which avoids delamination and cracking during the heat up and casting cycles. This material combination exhibits a number of desirable characteristics/advantages. Among these are: mechanical strength; crack propagation minimization because of structure; repairability; reduced heat transfer and therefore more consistent molten metal temperature; significantly reduced cross-feeder weight and casting table weight significantly reduced heat storage and table preheating schedule; and reduced steel shell temperature due to increased insulation factors thereby minimizing steel expansion, joint maintenance and crack propagation. 
   Thus, in the casting insert  80  of the present invention, cylindrical crossfeeder  82  and cylindrical thimble  86  present a continuous, joint free and uninterrupted cylindrical interior surface  87  surrounded by an integral peripheral layer of back-up insulation  84 . 
   While the elements of the monolithic assembly of the present invention can be fabricated from a wide variety of compatible materials, according to a highly preferred embodiment of the invention, crossfeeder  82  is formed from an SH or RFM Insural material available from Pyrotek, Inc. East 9503 Montgomery Ave, Spokane, Wash. RFM Insural is a moldable light density refractory composite material comprised of fiberglass fabric reinforced wollastonite. Back-up insulation  84  comprises Wollite an insulating castable also available from Pyrotek, Inc. Wollite is a solid lightweight mineral foam that is stable during its preparation and during curing and drying. It is a phosphate bonded foam insulation that can be made in densities ranging from 320 to 880 kg/m 3  and is mainly composed of wollastonite, a calcium silicate. Crossfeeder  82 , thimble  86  and backup insulation  84  can also be cast as a single unit. This is made possible by the compatibility of the various materials of fabrication. 
   There have thus been described: a novel metal distribution system incorporating; an automated and remotely operable dam removal system; and a monolithic mold insert assembly that each individually demonstrate significant operating advantages and which when combined into a single operating system provide a significantly improved log or round ingot casting system that is economically desirable and simultaneously provides noteworthy safety improvements. 
   As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in any ways without departing from the spirit and scope thereof. Any and all such modifications are intended to be included within the scope of the appended claims.