Abstract:
An apparatus and method for strip casting of metals on at least one endless belt. The apparatus employs a tapered molding section that is large at the point of molten metal entry and tapers to a smaller thickness where a pair of pinch rolls apply a compressive force that sets the final thickness of the cast strip.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for the continuous casting of metals, and particularly the casting of metal strip. The continuous casting of thin metal strip has been employed with increasing success. The conventional twin-belt caster is employed to cast in widths up to 80 inches, but typically 0.75 inch thick, requiring three in-line rolling mill stands to produce coils with strip 0.1 inches thick. 
     In heat sink thin strip casters, such as disclosed in U.S. Pat. Nos. 5,564,491, 5,515,908, and 6,044,896 and World Patents WO 09517274A1 and WO 9714520A, also using twin-belts, the thickness cast is typically 0.1 inch. The ability to cast wider than 20 inches with this technology, however, is unproven. In the prior art heat sink belt casters, the molten metal is fed to the curved portion of the belts on the entry pulleys, and solidification of the metal is complete by the nip of the belts on the entry pulleys. The gap between entry pulleys is adjusted to create sufficient force to cause some elongation of the strip. In adjusting the gap force, using the apparatus described in U.S. Pat. No. 6,044,896, the horizontal distance to the nozzle tip from the top pulley is adjusted at the same time as the vertical distance between top and bottom pulleys. The belts are cooled in the return loop where the belts are not in contact with molten or solid strip. The cast gauge, about 0.1 inch, is the same as that obtained with conventional twin-belt casters after three rolling passes. In the prior art heat sink casters, side dams are located before the nip of the entry pulleys by means of a combination of stationary mechanical and electromagnetic edge dams. One example of such edge dams is shown in World Patent , WO 98/36861. The solidification rate is semi-rapid, which is a metallurgical advantage for many products, but unsuitable for making can body stock requiring galling resistance. Typically, with prior art heat sink belt caster operations, after three rolling mill stands the strip thickness is down to 0.01 inch. 
     In conventional twin-belt strip casting equipment, two moving belts are provided which define between them a moving mold for the metal to be cast. Revolving mechanical side dam blocks fill the gap between the belts in the molding section, which necessitates that the belts be parallel in the molding section. Such parallel belts mandate that the thickness of the cast product will be nearly the same as the height of the tip delivering molten metal. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite the side in contact with the molten metal. As a result, the belt is subjected to extremely high thermal gradients, with solidifying metal in contact with the belt on one side and a water coolant in contact with the belt on the other side. The dynamically unstable thermal gradients cause distortion in the belt, and consequently neither the upper nor the lower belt is flat without adding various devices to prevent areas of segregation and porosity. The belts are more prone to distortion when the machine is wider. 
     Various improvements have been proposed in the prior art, including preheating of the belts as described in U.S. Pat. Nos. 3,937,270 and 4,002,197, continuously applied and removed parting layers as described in U.S. Pat. No. 3,795,269, moving endless side dams as described in U.S. Pat. No. 4,586,559 and improved belt cooling as described in U.S. Pat. Nos. 4,061,177, 4,061,178 and 4,193,440. These various improvements and others have helped the quality of the cast surface, but the cast thickness is too large to achieve important economies in the downstream rolling. Furthermore, good surface quality is more difficult to achieve as the width is increased. 
     Another continuous casting process that has been proposed in the prior art is that known as block casting. In that technique, a number of chilling blocks is mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form therebetween a casting cavity into which a molten metal such as an aluminum alloy is introduced. The liquid metal in contact with the chilling blocks is cooled and solidified by the heat capacity of the chilling blocks themselves. Block casting thus differs both in concept and in execution from continuous belt casting. Block casting depends on the heat transfer, which can be effected by the chilling blocks. Thus, heat is transferred from the molten metal to the chilling blocks in the casting section of the equipment and then extracted on the return loop. Block casters thus require precise dimensional control to prevent flash (i.e. transverse metal fins) caused by small gaps between the blocks. Such flash causes sliver defects when the strip is hot rolled. As a result, good surface quality is difficult to maintain. Examples of such block casting processes are set forth in U.S. Pat. Nos. 4,235,646 and 4,238,248. 
     Another technique, which has been proposed in continuous strip casting, is the single drum caster. In single drum casters, a supply of molten metal is delivered to the surface of a rotating drum, which is internally water cooled, and the molten metal is dragged onto the surface of the drum to form a thin strip of metal which is cooled on contact with the surface of the drum. The strip is frequently too thin for many applications, and the free surface has poor quality by reason of slow cooling and micro-shrinkage cracks. Various improvements in such drum casters have been proposed. For example, U.S. Pat. Nos. 4,793,400 and 4,945,974 suggest grooving of the drums to improve surface quality; U.S. Pat. No. 4,934,443 recommends a metal oxide on the drum surface to improve surface quality. Various other techniques are proposed in U.S. Pat. Nos. 4,979,557, 4,828,012, 4,940,077 and 4,955,429. 
     Another approach, which has been employed in the prior art, has been the use of twin drum casters, such as in U.S. Pat. Nos. 3,790,216, 4,054,173, 4,303,181, or 4,751,958. Such devices include a source of molten metal supplied to the space between a pair of counter-rotating, internally cooled drums. The twin drum casting approach differs from the other techniques described above in that the drums exert a compressive force on the solidified metal, and thus effect hot reduction of the alloy immediately after freezing. While twin drum casters have enjoyed the greatest extent of commercial utilization, they nonetheless suffer from serious disadvantages, not the least of which is an output typically ranging about 10% of that achieved in the prior art devices described above. Once again, the twin drum casting approach, while providing acceptable surface quality in the casting of high purity aluminum (e.g. foil), suffers from poor surface quality when used in the casting of aluminum with high alloy content and wide freezing range. Another problem encountered in the use of twin drum casters is centerline segregation of the alloy due to deformation during solidification. These machines have demonstrated the ability to make wide product, but the production rate is typically only 10% per unit of width of heat sink and conventional belt casters. 
     There is thus a need to provide an apparatus and method for continuously casting metallic strip at high speeds, thin thickness and wide widths as compared to methods currently employed. 
     It is accordingly an object of the present invention to provide an apparatus and method for continuously casting thin metallic strip (i.e. 0.1 inch thick) using conventional wide twin-belt casters that apply coolant to at least one belt in the molding section. 
     Another objective of the invention is to provide an apparatus and method for the continuous casting of thin metallic strip which permit the production of wide strip (i.e. up to 80 inches) on heat sink belt casters, while retaining the high speed and thin thickness, with no cooling applied in the molding section. 
     Another specific objective is to provide, in one machine, a range of solidification rates for different product requirements, including a slow rate for can body stock to provide galling resistance. 
     These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention. 
     SUMMARY OF THE INVENTION 
     The concepts of the present invention reside in a method and apparatus for continuous strip casting of metals utilizing a twin-belt strip casting approach in which the molding section between the belts is large at the point of molten metal entry and tapers to a smaller thickness part way through the length of the machine where a pair of pinch rolls sets the final thickness near the end of the molten metal sump. The pinch roll gap, pinch roll-separating force, and caster speed are regulated to provide the desired strip thickness. The pinch force serves to reduce cracking and to control the strip thickness profile across the width, which is critical for successful downstream rolling. 
     In the present invention, the molten metal is preferably applied to the belts after the nip of the entry pulleys. Because the belts converge toward one another in the molding section, conventional tip designs, which are thick, can be utilized for feeding molten metal into the machine, while making thin strip. Solidification takes place in the tapered molding section with the belts converging toward each other by means of a pair of pinch rolls located between the tip of the casting nozzle and the exit pulleys. The strip is solidified in the molding section, which extends from the molten metal entry point to the pinch rolls. There is a strip conveyance section extending from the pinch rolls to the exit pulleys. 
     In the preferred embodiment of the present invention, the heat sink capacity of the belts is used for solidifying the molten metal in the molding section, and the belts are cooled in the return loop where no solidification is occurring. In that way, the method and apparatus of the present invention minimize or avoid the erratic distortion effects caused by high non-uniform thermal gradients across twin-belt strip casters of the prior art. However, the tapering of the molding section does not preclude the use of applying cooling means on the opposite side of the belts in the molding section to make thicker product, if desired. 
     In the present invention, the containment of molten metal on the tapered edges, after the casting nozzle tip, can be accomplished by electromagnetic means. Alternatively, edge containment can be accomplished by mechanical edge dam blocks moving with the belts and sealing on the top of the bottom belts and the side edges of the top belts. 
     The belts utilized in the present invention can be provided with different coatings having different thermal resistances in order to provide rapid or slow solidification and short or long solidification lengths. Thus, by varying the coatings on the belts, the metallurgical structure can be varied depending on the needs of the product. For, example, slow solidification is desirable for making can body stock with good galling resistance. 
     The concepts of the present invention can be employed in the strip casting of most metals, including steel, copper, zinc and lead, but are particularly well suited to the casting of thin aluminum alloy strip, while overcoming the problems of the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the casting method and apparatus embodying the present invention. 
     FIG. 2 is a perspective view of one casting apparatus embodying the invention. 
     FIG. 3 is a cross-sectional view of the entry of molten metal to the apparatus and the pinch rolls illustrated in FIGS. 1 and 2. 
     FIG. 4 is a cross-sectional view of an electromagnetic edge dam that can be utilized in the apparatus shown in FIGS. 1 and 2. 
     FIG. 5 is a side view of an electromagnetic edge dam and the tapered molding section formed by two belts. 
     FIG. 6A is a cross section of an alternating segment electromagnetic edge dam. 
     FIG. 6B is a side view of an alternating segment electromagnetic edge dam. 
     FIG. 7 is an end cross-section view of moving side dam blocks that can be utilized in the apparatus shown in FIGS.  1  and  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus employed in the practice of the present invention is perhaps best illustrated in FIGS. 1,  2  and  3  of the drawings. As there shown, the apparatus includes a pair of endless belts  10  and  12  carried by a pair of upper pulleys  14  and  16  and a pair of corresponding lower pulleys  18  and  20  of FIG.  1 . Each pulley is mounted for rotation about an axis  21 ,  22 ,  24 , and  26  respectively of FIG.  2 . The pulleys are of a suitable heat resistant type, and either or both of the upper pulleys  14  and  16  is driven by a suitable motor means not illustrated in the drawing, for purposes of simplicity. The same is equally true for the lower pulleys  18  and  20 . Each of the endless belts  10  and  12  is preferably formed of a metal which has a surface that has a low reactivity or is non-reactive with the metal being cast. Quite a number of suitable metal alloys may be employed as well known by those skilled in the art. For example, steel and copper alloy belts can be employed in the apparatus. 
     The belts  10  and  12  define between them a molding zone which extends from the entry pulleys  14  and  18  to the nip of a pair of pinch rolls  15  and  17 . As illustrated in FIGS. 1 and 2, the pinch rolls  15  and  17  are located between the entry pulleys  14 ,  18  and the exit pulleys  16 ,  20 . The pinch rolls  15  and  17  are preferably movable so that the length of the molding zone may be adjusted from 5 inches to 120 inches, or more, according to the needs of a particular cast. These needs include consideration of speed, belt coatings, and product solidification rate. In the preferred practice of the invention, the gap between the pinch rolls  15  and  17 , less the thickness of the two belts, is dimensioned to correspond to the desired thickness of the metal being cast. Thus, the thickness of the metal strip being cast is determined by the dimensions of the nip between belts  10  and  12  passing over pinch rolls  15  and  17  along a line passing through the axis of pinch rolls  15  and  17  which is perpendicular to the belts  10  and  12 . As is described in the earlier issued U.S. Pat. No. 5,515,908, the thickness of the strip being cast is also limited by the heat capacity of the belts between which the molding takes place. 
     In accordance with the practice of this invention, there is provided means associated with the pinch rolls  15  and  17  to prevent displacement of the pinch rolls relative to each other. Any suitable apparatus to rigidly fix the relative positions of pinch rolls  15  and  17  may be used. FIGS. 1 and 2 illustrate a simple mechanism including pillow blocks  45  and  47  mounted on the axes  23  and  27  of the pinch rolls  15  and  17 , respectively, and secured to each other by means of a tension member  41 . The tension member may be either fixed or adjustable. Good results can be obtained by simply using a turnbuckle  41  as the tension member to prevent relative displacement of axes  23  and  27  relative to each other. As will be appreciated by those skilled in the art, various other and more sophisticated tension members may likewise be used. For example, use can be made of a hydraulic cylinder as the tension member to prevent relative displacement of the axes  23  and  27  relative to each other. The use of such a hydraulic cylinder has the further advantage that it is adjustable, and thus the tension can be conveniently changed depending on the application and the metal being cast. 
     Molten metal to be cast is supplied to the molding zone through suitable metal supply means  28  such as a tundish. The inside of the tundish  28  corresponds in width to the width of the product to be cast, and can have a width up to the width of the narrower of the belts  10  and  12 . The tundish  28  includes a metal supply delivery casting nozzle  30  to deliver a horizontal stream of molten metal to the molding zone between the belts  10  and  12 . Such tundishes are conventional in strip casting. Thus, the nozzle  30 , as is best shown in FIG. 3 of the drawings, defines, along with the belts  10  and  12  immediately adjacent to nozzle  30 , the molding zone into which the horizontal stream of molten metal flows. Thus, the stream of molten metal flowing substantially horizontally from the nozzle fills the molding zone between each belt  10  and  12  past the nip of the pulleys  14  and  18 . It begins to solidify and is substantially solidified prior to the point at which the cast strip reaches the nip of pinch rolls  15  and  17 . Supplying the horizontally flowing stream of molten metal to the molding zone where it is in contact with a tapered molding section of the belts  10  and  12  passing from the nozzle tip  42  to pinch rolls  15  and  17  serves to allow a larger gap at the entry pulleys  14  and  18  than the gap between the pinch rolls  15  and  17 . The gap  48  between entry pulleys  14  and  18  remains fixed to maintain a good fit with nozzle  42  while the pinch roll gap  49  is adjusted. The belt linear speed, pinch roll gap, and gap separating force are regulated so that the last point to freeze  51  is substantially at the belt nip between pinch rolls  15  and  17 . The center of the strip may have a “mush” zone that is partially solidified that is capable of supporting a gap force. 
     The belts  10  and  12  also define between them a strip conveyance zone which extends from the pinch rolls  15  and  17  to the exit pulleys  16  and  20 . The belts  10  and  12  in the conveyance zone may be parallel to each other, or alternatively may be diverging so that the gap between the exit pulleys  16  and  20  is larger than the gap between the pinch rolls  15  and  17 . 
     In accordance with the preferred embodiment of the invention, the casting apparatus of the invention includes a pair of cooling means  32  and  34  positioned opposite that portion of the endless belt in contact with the metal being cast in the molding zone between belts  10  and  12 . The cooling means  32  and  34  thus serve to cool the belts  10  and  12  just after they pass over pulleys  16  and  20 , respectively, and before they come into contact with the molten metal. In the most preferred embodiment as illustrated in FIGS. 1 and 2, the coolers  32  and  34  are positioned as shown on the return run of belts  10  and  12 , respectively. In that embodiment, the cooling means  32  and  34  can be conventional cooling means such as fluid cooling nozzles positioned to spray a cooling fluid directly on the inside and/or outside of belts  10  and  12  to cool the belts through their thickness. Alternatively, the cooling means can be located in the molding section, the conveyance section, or on the exit pulleys depending on the thickness and speed of operation. For example, thicker cast strip, 0.2 inch to 0.8 inch, might require cooling in the molding section, while retaining the pinch roll concept. It is sometimes desirable to employ scratch brushes  36  and  38  which frictionally engage the endless belts  10  and  12 , respectively, as they pass over pulleys  14  and  18  to clean any metal or other forms of debris from the surface of the endless belts  10  and  12  before they receive molten metal from the tundish  28 . 
     Thus, in the practice of this invention, molten metal flows horizontally from the tundish through the casting nozzle  30  into the casting or molding zone defined between the belts  10  and  12  where the belts  10  and  12  are heated by heat transfer from the cast strip to the belts  10  and  12 . The cast metal strip remains between and conveyed by the casting belts  10  and  12  after the pinch rolls  15  and  17  until each of them is turned past the centerline of exit pulleys  16  and  20 . Thereafter, in the return loop, the cooling means  32  and  34  cool the belts  10  and  12 , respectively, and remove therefrom substantially all of the heat transferred to the belts in the molding zone. After the belts are cleaned by the scratch brushes  36  and  38  while passing over pulleys  14  and  18 , they approach each other to once again define a molding zone. 
     The most preferred supply of molten metal from the tundish through the casting nozzle  30  is shown in greater detail in FIG. 3 of the drawings. As is shown in that figure, the casting nozzle  30  is formed of an upper wall  40  and a lower wall  42  defining a central opening  44  therebetween whose width may extend substantially over the width of the belts  10  and  12  as they pass around pulleys  14  and  18 , respectively. The distal ends of the walls  40  and  42  of the casting nozzle  30  are in substantial proximity of the surface of the casting belts  10  and  12 , respectively, and define with the belts  10  and  12  a casting cavity or molding zone  46  into which the molten metal flows through the central opening  44 . As the molten metal in the casting cavity  46  flows between the belts  10  and  12 , it transfers its heat to the belts  10  and  12 , simultaneously cooling the molten metal to form a solid strip  50  maintained between casting belts  10  and  12 . As in prior art belt casters, the molten metal contacts the casting belts  10  and  12  after the nip  48  of the entry pulleys  14  and  18  in the linear section. In the molding zone the gap between the belts  10  and  12  is tapered from the gap between entry pulleys  14  and  18  to the gap between pinch rolls  15  and  17 . Hence, in the present invention solidification is substantially complete near the nip  49  of the pinch rolls  15  and  17 . The space between the belts  10  and  12  at the time that they first come into contact with the molten metal just after the nip  48  of the entry pulleys  14  and  18 , is substantially larger then the gap  49  between the belts  10  and  12  at the pinch rolls  15  and  17 . In this way molten metal  44  can be delivered to the molding zone  46  through a nozzle  30  which is much thicker than the thickness of the strip  50 . This is an important distinction of this invention that enables thinner strip to be cast than prior art conventional twin-belt casters. In addition, because the space between the belts  10  and  12  where they first come in contact with the molten metal is much larger than the nip  49  of the pinch rolls  15  and  17 , any distortion in the belts in this region has little effect on the metal being cast. The high thermal gradient largely dissipates before the belts  10  and  12  reach the pinch roll nip  49 , and thus any distortions that do occur diminish as the belts approach the pinch roll nip. 
     The importance of freezing or solidification before the nip  49  also arises from the fact that as shown in FIG. 3 of the drawings, the metal solidifying between the tapered surfaces in the molding zone prior to the nip has a dimension or thickness greater than the corresponding dimension or thickness of the nip itself. That insures that when the solidified cast metal is advanced to the nip  49 , it has a larger dimension than that of the nip, thereby insuring that the nip  49  exerts a compressive force on the cast metal strip to thereby cause elongation to improve not only surface characteristics but also to reduce the tendency of the strip to crack. It should be noted that the central core of the strip might be semi-solid and able to support some separating force. In addition, the compressive force exerted on the cast metal strip between the pinch rolls insures good thermal contact between the cast metal strip and the belts and establishes a good thickness profile needed for subsequent rolling. 
     The amount of compressive force is not critical to the practice of the invention. By adjusting the gap between the pinch rolls  15  and  17  and/or adjusting the machine speed, the amount of compressive force that is applied to the cast strip can be controlled. The compressive force should be sufficiently high to insure good thermal contact between the cast metal strip and the belt as well as sufficiently high so as to cause elongation. The elongation is preferably sufficient to insure that the cast metal strip, as it is exits from the nip  49  is in a state of compression as distinguished from tension. Maintaining the cast strip under compressive force serves to minimize cracking that would otherwise occur if the cast strip were maintained under tension. In general, it is desirable that the percent elongation be relatively low, generally below 10 percent, and most preferably below 5 percent. 
     The thickness of the strip that can be cast is, as those skilled in the art will appreciate, related to the thickness of the belts  10  and  12 , the return temperature of the casting belts and the exit temperature of the strip and belts. In addition, the thickness of the strip depends also on the metal being cast. In general, aluminum strip having a thickness of 0.100 inches using steel belts having a thickness of 0.08 inches can provide a return temperature of 300 degree F. and an exit temperature of 800 degree F. The interrelationship of the exit temperature with belt and strip thickness is described in detail in application Ser. No. 07/902,997, now abandoned. For example, for casting aluminum strip for a thickness of 0.100 using a steel belt having a thickness of 0.06 inches, the exit temperature is 900 degree F. when the return temperature is 300 degree F. and the exit temperature is 960 degree F. when the return temperature is 400 degree F. 
     One of the advantages of the method and apparatus of the present invention is that there is now, for heat sink twin-belt casting, an option to employ a thermal barrier coating on the belts to reduce heat flow and thermal stress, as is typically employed in the prior art conventional twin-belt casting. The absence of fluid cooling on the back side of the belt while the belt is in contact with hot metal in the molding zone significantly reduces thermal gradients and eliminates problems of film boiling occurring when the critical heat flux is exceeded. The method and apparatus of the present invention also minimizes cold framing, a condition where cold belt sections exist in three locations: (1) before metal entry and (2) on each of the two sides of mold zone of the belt. Those conditions can cause severe belt distortion. In addition, there may be molding conditions that require the use of parting agents to prevent sticking of the cast metal strip to either of the belts. These agents typically add thermal resistance, which therefore requires a longer molding zone than that provided by prior art heat sink casters, such as disclosed in U.S. Pat. No. 5,564,491, where solidification begins and ends on the curve of the entry pulleys. In contrast, the longer molding zone of the present invention, which extends from the nozzle tip  44  to the nip of the pinch rollers, allows the use of such parting agents. The longer molding zone and lower heat flux values results in less belt distortion, which in turn enables casting in wider widths (i.e. up to 80 inches) while keeping the strip thin (i.e. a thickness of 0.1 inches). 
     For some applications, it can be desirable to employ one or more belts having longitudinal grooves on the surface of the belt in contact with the metal being cast. Such grooves have been used in single drum casters as described in U.S. Pat. No. 4,934,443 and WO 09714520A. As will be appreciated by those skilled in the art, coolant can be applied to the belts in one or more of these locations: molding zone opposite the molten metal; conveyance zone opposite solidified strip; grooves in the exit pulleys; and in the return leg between the exit and entry pulleys. In a preferred embodiment of the invention, the bottom pinch roll is set so that there is very little wrap of the bottom belt on that pinch roll and most of the gap adjustment is by movement of the top pinch roll; additionally, there is no cooling applied in the molding section on the top or bottom belts or on the top belt in the conveyance section but cooling is applied on the bottom belt in the conveyance section and the return loop of the top belt. The purpose of the forgoing arrangement is the promotion of late release of the strip from the bottom belt, by minimizing the bending of the strip at the pinch roll and thermal contraction of the bottom belt as the strip is contracting in the conveyance section. The late thermal release cools the strip to a lower temperature where it is stronger and less brittle. 
     Containment of molten metal at the sides of the strip in the tapered molding section is a vital feature of this invention. In one embodiment, illustrated in FIGS. 4-6, electromagnetic edge dams are utilized to contain the molten metal  30  between the solidifying metal  65  adjacent the belts  10  and  12  and prevent the molten metal from running out the edges of the belts. The electromagnetic edge dam comprises a core  62  upon which is mounted a coil  64  which produces an electromagnetic field. The edges of belts  10  and  12  run through the core  62  and the field generated by the coil  64  contains the molten metal along the edges of the belts. Electromagnetic edge dams are described in further detail in World Patent WO 98/36861 which is hereby incorporated by reference. However, because the apparatus of the present invention employs a molding zone that is longer than that provided by prior art casting equipment, electromagnetic edge dams that extend substantially the entire length of the molding zone must be utilized in the present invention. 
     One way of extending the length of the electromagnetic edge dams is to use alternating upper and lower electromagnetic containment segments  68  and  70 , respectively, as illustrated in FIG.  6 B. Each segment butts an adjacent segment and the location of the coils  64  alternates between adjacent segments to allow room for each segment to have its own coil. 
     Another mechanism for containing the molten metal is to use moving edge dam blocks. Moving edge dam blocks are described, for example, in U.S. Pat. No. 3,795,269 which is hereby incorporated by reference in its entirety. Such edge dam blocks must be modified, however, to accommodate the tapered molding zone of the present invention. 
     Referring to FIG. 7, the top belt  10  is narrower than the bottom belt  12  so that the edge dam block  72  rides on the top of the bottom belt  12  and seals on the sides of the top belt  10 . An optional second set of edge dam blocks  74  can ride on the top belt  10  to further prevent the molten metal from running over the edges of the belts.