Abstract:
A molten metal alloy, such as bismuth and tin, is injected into a die block to form a tightly packed rectangular array of shot pellets. The ratio of waste sprue to shot pellets is minimized and the shot pellet yield per casting is maximized by allowing the shot pellets in the rectangular shot array to touch other shot pellets in adjoining rows and columns through small interconnecting vias. The interconnecting vias allows the molten metal to flow between shot pellets, as well as from the sprue into the shot pellets. This allows the molten metal to bypass blockages that reduce the shot pellet count per casting. The flow of the molten metal through the die block is also improved by machining away a small amount of metal from the face of the die in order to form a flashing between the shot pellets.

Description:
BACKGROUND  
         [0001]    In recent years there has been an increase in the demand for a non-toxic replacement for the traditional lead shot pellets used in shotguns. The use of toxic lead shot by hunters leads to the poisoning of many natural habitats, particularly the lakes and ponds where ducks, geese and other waterfowl are found. Presently, the only approved alternatives to lead shot are steel shot and bismuth-tin alloy, which are not toxic like lead. Unfortunately, steel shot has significant drawbacks compared to lead shot. Steel shot is much less dense than lead shot and therefore slows down much more rapidly than lead shot when fired. This makes hunting with steel shot less accurate because hunters must “lead” a moving target by a greater amount when using steel shot. Furthermore, because steel shot is so much lighter than lead shot, it causes far less shock and injury to the hunter&#39;s quarry. Rather than dying quickly after impact, the wounded animal will frequently run or fly a great distance before bleeding to death. Additionally, steel shot is more damaging to shotgun barrels because steel is harder than lead.  
           [0002]    A more suitable replacement for lead is bismuth shot, in particular bismuth alloy shot. Bismuth may be alloyed with other metals, such as tin, to thereby produce bismuth alloy shot that is very nearly the same density as lead. Bismuth alloys are soft like lead and non-toxic. For these reasons, bismuth alloy shot does not damage shotgun barrels the way steel shot does and, because of its comparable density, has the accuracy and killing power of lead shot.  
           [0003]    U.S. Pat. No. 4,949,644 to Brown discloses a non-toxic wildlife shot pellet for use in shotgun shells that is formed from bismuth or bismuth alloys. U.S. Pat. No. 5,279,787 to Oltrogge discloses methods of manufacturing and compositions of non-toxic projectiles containing bismuth, wherein the projectiles are made of high melting point powders mixed with molten metals of a low melting point to thereby produce a sintered metal projectile. These two prior art references are hereby incorporated by reference.  
           [0004]    The principal drawback to using bismuth alloy shot is producing it economically in commercial quantities so as to be a viable replacement for lead shot. Unlike lead and steel, bismuth is highly crystalline and extremely nonductile. Bismuth also has a very low melting temperature and expands upon cooling. These metallurgical properties make it difficult or impossible to economically produce bismuth shot by conventional methods.  
           [0005]    Normal shot sizes for hunting and shooting loads range from 0.008 inches to 0.33 inches. (No. 9 shot to No. 4 Buckshot) Conventional methods of making smaller sizes of shot (i.e., No. 9 to No. 6) are the drop method and the Bleimeister method (sometimes called the “short drop” method).  
           [0006]    In the drop method, molten lead is poured through a screen at the top of a shot tower. The “screened” lead chills and solidifies into spheres as it falls through the air before landing, typically, in a tank of cold water.  
           [0007]    In the Bleimeister method, molten lead is pumped through a pair of arms having small orifices along their bottom surfaces. The arms are positioned about 4 inches above the surface of almost boiling water. Droplets of molten metal fall through the small orifices in the bottom of the arms into the water and then roll along the surface of inclined plane pine boards to form spherical beads of molten lead. The spherical beads of molten lead harden as they roll and then drop to the bottom of the water tank, cooling on the way down.  
           [0008]    Bismuth is highly crystalline in nature and resists forming a sphere. Bismuth also expands when cooled and has a high degree of heat retention, which prevents it from cooling quickly enough to be made using the drop method. These properties make it impossible to manufacture large bismuth alloy shot pellets using the drop method and the Bleimeister method. It is also not possible to manufacture lead or steel shot greater than size No. 5 (0.12″) using either of these methods.  
           [0009]    Large lead shot is conventionally made by a process that extrudes lead wire which is then flattened into an oval ribbon by a pair of rollers. The flattened oval ribbon is run through a pair of die wheels that punch out spheres of the appropriate size. Round steel shot is made by drawing a wire and snipping it into short segments using a header machine. The header machine then grinds the short chunks of steel wire into spheres.  
           [0010]    Because of the nonductility of bismuth and bismuth alloys, it is not commercially feasible to make bismuth alloys into wire. Therefore, neither the header machine process nor the ribbon tape process may be used to make bismuth shot because both processes require that the bismuth alloy first be extruded into a wire.  
           [0011]    The shortcomings of the previously discussed methods of manufacturing bismuth alloy shot leads to the conclusion that one method of manufacturing bismuth alloy shot that has a chance to produce shot economically for commercial use is to use high-speed die casting machinery capable of producing large quantities of shot in a short time frame. The manufacturing cost of bismuth shot is very critical due to the greater cost of raw bismuth compared to lead. The cost of bismuth is typically ten times that of conventional materials such as steel and lead. Thus, any die casting process must yield a high product to waste ratio.  
           [0012]    Analysis revealed that manufacturing costs of $5 a pound on top of raw material costs would probably result in shot shells that were too expensive to be commercially acceptable to purchasers. Given the purchase price (or lease cost) of commercially available high-speed die casters and the operating costs associated with the die casters, it was determined that a minimum production level of at least a hundred pounds per hour of bismuth alloy shot had to be attained. Unfortunately, there were no die-blocks commercially available for producing shot made from bismuth alloy. The initial die-blocks custom fabricated for the inventors using traditional die design techniques failed to produce an acceptable number of shot pellets per casting in order to attain one hundred pounds of shot pellets per hour.  
           [0013]    For example, number 4 shot requires approximately 1600 pellets per pound, at a rate of 100 pounds per hour, to be commercially viable. Traditional mold design techniques used to cast small parts cannot attain these numbers because they have an unacceptably high ratio of sprue and runner material to shot pellets. For example, in most cases, approximately 70% of the metal injected into the die block forms sprues and runners to which the shot pellets are attached. These sprues and runners are waste metal that is remelted in the melting pot and represent a cost of approximately $3.11 per pound of waste per casting, exclusive of any other manufacturing costs.  
           [0014]    Conventional high speed die casters and die blocks were also found to be unsuitable for casting bismuth alloy shot pellets because the heat retention of the bismuth increased the cycle time of the machines, minimizing production rates. Also, the expansion of bismuth when cooled was in part responsible for slowing down the flow of the material through the die block.  
           [0015]    There is therefore a need for a method of quickly and economically manufacturing shot made from bismuth and bismuth alloys in various sizes.  
         SUMMARY OF THE INVENTION  
         [0016]    The foregoing problems inherent in the prior art methods of manufacturing shot are solved by the present invention, which provides a method of die casting bismuth alloy shot in commercial quantities at an economical cost.  
           [0017]    The problems inherent in commercially available high speed die casters and die blocks are solved by using a die block that casts an array of tightly packed shot pellets using a minimum amount of sprue. The shot pellets in one embodiment form an array around the sprue, wherein each pellet in the pellet array touches at least two other pellets and often four other pellets. Thus, as the molten bismuth alloy flows out of the sprue into the pellet cavities within the die block, the molten bismuth alloy flows between shot pellets thereby bypassing blockages and flowing a greater distance away from the sprue before solidifying. This produces a higher yield of good pellets per cast and also allows the use of a larger die capable of producing more pellets per cast.  
           [0018]    A further improvement on conventional die blocks includes the machining away of approximately 0.0015 inches of material from both die faces so as to form a narrow gap, or “fill void,” across the entire die surface, thereby allowing a thin “flashing” to form between all pellets in the shot pellet array. The combination of the multiple contact points between shot pellets in the array and the fill void allows the molten bismuth alloy to flow from the common sprue to the outer limits of the die much more quickly, before the molten bismuth alloy is able to solidify. The fill void and multiple contact points between shot pellets significantly reduces the sprue needed to spread the molten bismuth alloy throughout the die, while at the same time ensuring that the die is completely filled during each die casting. A less than complete fill results in partially hollow shot pellets, broken shot pellets, or a reduced number of pellets per cast due to empty, or partially empty cavities in the array.  
           [0019]    After each shot pellet array is cast, the array is ejected from the mold and gathered into a container. The container is then dumped into a tumbling device containing tumbling media, such as ball bearings, in order to break up the shot pellet array, which resembles a sheet of pellets. The tumbling pulverizes the flashing between the shot pellets into a fine powder. The tumbling also grinds off the small “nipples” that are formed at the contact points between the shot pellets in the array. The tumbling process smoothens the properly formed pellets into nearly smooth spheres. Any imperfectly formed pellets or hollow pellets are pounded flat during tumbling.  
           [0020]    After tumbling, the shot is sifted through a sifting screen to eliminate all metal pieces that are not shot pellets, such as the sprues. Once sifted, the shot pellets are moved by a conveyor belt to the top of a shot classifier which rolls the shot pellets down a series of steps set at varying angles for varying shot sizes to thereby automatically discard all less-than-round shot pellets including the imperfectly formed and hollow pellets. The remaining shot is collected at the bottom of the classifier and is ready for commercial usage. The shot may be sold in bulk for reloading in the commercial marketplace or loaded into shells using conventional manufacturing methods.  
           [0021]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0022]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0023]    [0023]FIG. 1 is a front view of a die face used in the present invention.  
         [0024]    [0024]FIG. 2 is a side view of 2 die halves as used by the present invention.  
         [0025]    [0025]FIG. 3 is a plan view of a portion of the shot pellet array produced by the present invention.  
         [0026]    [0026]FIG. 4 is a plan view of a portion of a shot pellet array produced by the present invention wherein the rows of shot pellets are interlaced.  
         [0027]    [0027]FIG. 5 is an elevational cross-sectional view of a shot pellet array produced by the present invention, taken substantially along line  5 -- 5  of FIG. 3.  
         [0028]    [0028]FIG. 6 is an elevational cross-sectional view of a shot pellet array produced by the present invention, taken substantially line  6 -- 6  of FIG. 3.  
         [0029]    [0029]FIG. 7 is a depiction of a high speed die casting machine embodying the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0030]    The principles of the present invention and their advantages are best understood by referring to the illustrated embodiments depicted in FIGS.  1 - 7  of the drawings, in which like numbers designate like parts.  
         [0031]    [0031]FIG. 1 depicts a die face  100  contained on a die block used to produce an array of shot pellets in accordance with one embodiment of the present invention. The embodiment of the present invention shown in FIG. 1 comprises four quadrants  101 - 104  of hemispherical impressions  110  connected to a central sprue channel  105  that is disposed along die face  100 . Sprue channel  105  is connected to sprue hole  220 , through which molten bismuth alloy is injected. The molten bismuth alloy flows along sprue channel  105  and into quadrants  104 - 105  of hemispherical impressions  110  disposed along sprue channel  105 .  
         [0032]    [0032]FIG. 2 depicts a side view of both halves of die block  200  used to produce shot pellet arrays in accordance with the present invention. Stationary die block  205  contains sprue hole  220  and sprue channel  105   b . Stationary die block  205  also contains a number of hemispherical impressions  110   b , each of which form one-half of a shot pellet. Movable die block  210  is the opposing mirror-image half of die block  200  that mates with stationary die block  205 . Guideposts  215  are used to align movable die block  210  and stationary die block  205 . Movable die block  210  contains sprue channel  105   a  which mates with sprue channel  105   b  in stationary die block  205  to thereby form sprue channel  105 . Movable die block  210  also contains hemispherical impressions  11 O a  which mate with opposing hemispherical impressions  110   b  in stationary die block  205 , to thereby form the shot pellets in accordance with the present invention. In the embodiment of the present invention shown in FIG. 2, a small amount of metal has been milled from the face of stationary die block  205 , thereby forming flashing gap  225 . In some embodiments of the present invention, flashing gap  225  is formed by milling the surface of only one die face. In other embodiments of the present invention, flashing gap  225  may be formed by milling both die faces. In either embodiment, the flashing gap  225  that is formed between stationary die block  205  and movable die block  210  will typically be approximately 0.003 inches in thickness.  
         [0033]    Additionally, in some embodiments of the present invention, either sprue channel  105   a  or sprue channel  105   b  may be omitted, so long as the remaining sprue channel is connected to sprue hole  220 . Shot pellets may be produced by the present invention even if the sprue channel  105  is cut into only one die face and flashing gap  225  is also cut into only one die face, so long as the hemispherical impressions  110  in the die face are connected with the sprue channel  105 .  
         [0034]    [0034]FIG. 3 is an enlarged view of a shot pellet array produced by one embodiment of the present invention. Shot pellet array  300  contains shot pellets  301 - 308  which are connected by vias  310 ,  320 ,  330 ,  340 ,  345 ,  350  and  355 . Shot pellets  301 - 308  are also interconnected by flashing  370 . As molten bismuth flows through die block  200 , it fills the cavities that form shot pellets  301 - 308  and flows between shot pellets  301 - 308  by means of vias  310 - 355  and flashing  370 .  
         [0035]    In another embodiment of the present invention, (not shown) vias  310 - 355  may be eliminated by packing the shot pellets  301 - 308  so tightly that the shot pellet  301 - 308  touch one another. This may be accomplished by positioning the center point of each hemispherical impression  110  in die face  100  sufficiently close to the center point of adjoining hemispherical impressions  110  so that the circumferences of the hemispherical impressions  110  overlap slightly. When the die blocks  205  and  210  are brought together, the overlaps in the circumferences of hemispherical impressions  110  will define holes between adjoining spherical cavities. Forming interconnections between the shot pellets  301 - 308  by this method allows the shot pellet  301 - 308  to be packed more tightly in die face  100 , thereby producing a slightly higher yield of shot pellets per casting and further reducing the “waste” material.  
         [0036]    However, this method also requires that the separation between the centers of the hemispherical impressions  110  in the die face  100  be very precisely located in order to accurately control the diameter of the hole connecting adjoining shot pellets  301 - 308 . If the hemispherical impressions  110  overlap by too much or by too little, the hole formed between the spherical cavities in the die block  200  may be too narrow to allow the molten bismuth to flow freely therebetween, or so wide that it is difficult to break the shot pellets apart. This method, if not tightly controlled, could also result in flat surfaces on the pellets.  
         [0037]    By using via channels  310 - 355  as shown in FIG. 3, it is easier to control the diameters of the interconnections between shot pellets  301 - 308 , although the yield of shot pellets per casting will be slightly lower.  
         [0038]    [0038]FIG. 4 depicts an alternative embodiment to the arrangement of shot pellet  301 - 308  shown in FIG. 3. In FIG. 4, shot pellets  401 - 408  are disposed in interlaced rows and columns of shot pellets. This allows a slightly tighter packing of shot pellets  401 - 408  than may be obtained using the rectangular grid of shot pellets  301 - 308  shown in FIG. 3. Of course, any grid configuration could be utilized, including oval, circular, or rectangular.  
         [0039]    The following explanation of FIGS. 5 and 6, which illustrate elevational cross-sectional views taken substantially along line  5 -- 5  and line  6 -- 6  through shot pellet array  300  as depicted in FIG. 3, also applies to the shot pellet array  400  depicted in FIG. 4. For the purpose of simplicity, however, FIGS. 5 and 6 will be explained with reference to the shot pellet array  300  shown in FIG. 3.  
         [0040]    [0040]FIG. 5 illustrates elevational cross-sectional view  500  taken along line  5 -- 5  through shot pellets  306 ,  307  and  308  in FIG. 3. Cross-sectional view  500  in FIG. 3 cuts through the center lines of via  345 , via  355  and shot pellet  307 . The two parallel dotted lines traversing the horizontal diameter of shot pellet  307  and the centers of via  345  and via  355  represent flashing  370 , which interconnects all pellets in the shot pellet array. When via channels are cut in both die faces in stationary die block  205  and movable die block  210 , via channels  345  and  355  will be centered on the horizontal diameters of the shot pellets  301 - 308 . If the via channels are cut between hemispherical impressions  110  in only one die face, then via channels  345  and  355  will be disposed on only one side of the horizontal diameters of the shot pellets  301 - 308 .  
         [0041]    Similarly, if flashing  370  is formed by milling flashing gap  225  in both die faces of stationary die block  205  and movable die block  210 , then flashing  370  will be centered around the horizontal diameters of shot pellets  301 - 308 . If, however, flashing gap  225  is milled in only one die face, then flashing  370  will be disposed on only one side of the horizontal diameters of shot pellets  301 - 308 . For purposes of further discussion, it will be assumed that the vias interconnecting the shot pellets in the shot pellet array were formed by cutting via channels in both die faces. It will also be assumed that flashing  370  was formed by milling flashing gaps in both die faces of die blocks  205  and  210 .  
         [0042]    [0042]FIG. 6 shows elevational cross-sectional view  600  taken along line  6 -- 6  through shot pellet array  300 . Cross-sectional view  600  in FIG. 6 cuts through flashing  370  and vias  340  and  350 . Shown in the background of cross-sectional view  600  are shot pellets  302  and  303  and interconnecting vias  310 ,  320  and  330 .  
         [0043]    [0043]FIG. 7 depicts a high speed casting machine, such as a Horla DM250, that may be used to cast shot pellets in accordance with the present invention. Pure bismuth or bismuth and another metal are poured into hopper  710  in order to produce shot pellets of pure bismuth or bismuth alloy. The bismuth and other metal, such as tin, (if a bismuth alloy is desired) are heated in furnace  715  to produce molten bismuth alloy, which is injected through tube  720  into die blocks  205  and  210 . For purposes of further discussion of FIG. 7, it will be assumed that a bismuth alloy is being used.  
         [0044]    Prior to injection of the molten bismuth alloy, die blocks  205  and  210  are pressed together by ram  730  which is driven by motor  725 . In one embodiment of the present invention, the bismuth alloy is heated to a temperature of approximately 600.degree. Fahrenheit. Additionally, in one embodiment of the present invention, either stationary die block  205  or movable die block  210 , or both, may be heated to a temperature of approximately 125.degree. Fahrenheit in order to slow down the rate at which the bismuth alloy cools as it spreads through the die block. This helps to ensure that the molten bismuth alloy will spread to the farthest extremity of die face  100  before solidifying.  
         [0045]    Furnace  715  draws approximately 6 ounces of molten bismuth alloy into a plunger and injects the alloy at 600 p.s.i. through tube  720  into sprue hole  220  (not shown) in stationary die block  205 . The molten bismuth alloy then spreads through sprue channel  105  and into quadrants  101 - 104  of the shot pellet array. In some embodiments of the present invention, quadrants  102  and  103  are connected and quadrants  101  and  104  are connected to thereby form two halves on either side of sprue channel  105 , rather than quadrants.  
         [0046]    The interconnections between sprue channel  105  and the first row of shot pellets  110  connected to sprue channel  105  may be made slightly larger than the interconnections between individual shot pellets  110 . This will allow the molten bismuth alloy to flow more quickly out of sprue channel  105  and into the first row of shot pellets  110 .  
         [0047]    After the molten bismuth alloy has been injected into the mated die blocks  205  and  210 , the molten bismuth alloy is allowed to cool for a period of approximately 5 seconds. At that point, motor  725  withdraws ram  730 , thereby moving movable die block  210  away from stationary die block  205 . At the same time, ejector pins (not shown) in stationary die block  205  eject the shot pellet array from stationary die block  205 , causing it to fall into chute  740  and into container  745 .  
         [0048]    The entire process is controlled by control unit  750 , which may be used to vary the melting temperature in furnace  715 , the heating temperature of die blocks  205  and/or  210 , and the length of the time delay during which the molten bismuth alloy is allowed to cool in the die block before stationary die block  205  and movable die block  210  are separated.  
         [0049]    After a sufficient amount of shot pellet array has been gathered in container  745 , the shot pellet array is dumped into tumbling device  760 , which contains tumbling media (not shown), such as large ball bearings, which are used to break up the shot pellet array into individual shoe pellets.  
         [0050]    In other embodiments of the present invention, the shot pellet array may be moved directly from chute  740  to tumbling device  760  by an automated device, such as a conveyor belt. The shot pellet array is tumbled in tumbling device  760  for a sufficient period of time to break apart the individual shot pellets and to smooth off the flashing  760  between the individual shot pellets and to smooth away the small “nipples” formed when the vias connecting the shot pellets are broken.  
         [0051]    The time required to complete the tumbling process varies from 10 minutes to a half hour depending on the size of the shot that is being cast. After a sufficient time period to allow the tumbling device  760  to break up the shot pellet array, the contents of tumbling device  760  are poured through sifter  770  to separate the individual shot pellets from the tumbling media and the pieces of sprue. The shot pellets then fall into container  780 , which may contain a conveyor belt that moves the shot pellets to a classifier. As explained above, the classifier separates the spherical shot pellets from improperly formed shot pellets, such as broken shot pellets or hollow shot pellets that were flattened during the tumbling process.  
         [0052]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.