Abstract:
The present invention is a novel sprue rod assembly for lost wax investment casting and its method of use. The sprue rod is reusable because it is made of material with a high melting point, such as metal, instead of wax. The sprue rod assembly also comprises a weight pull system, which causes the sprue rod to fall out of the investment mold when it is heated prior to dewaxing or early during dewaxing. The reusable sprue rod assembly reduces wasted wax, increases the efficiency of dewaxing and decreases flaws from the expansion of trapped wax during dewaxing. In addition, the sprue rods can be stacked to create the desired length.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention generally relates to investment casting, more specifically to lost wax investment casting using wax trees. 
     2. Description of the Background Art 
     Lost wax investment casting is a method of creating small molded metal designs. The casting process begins with a wax pattern made from a plastic master mold of the design. The wax pattern has one or more small wax rods, called gates, attached to it in strategically selected areas. The gates are welded onto a wax rod, called a sprue rod, typically at a 45 degree angle. The weld between the gate and the sprue rod is called the filet. The patterns can be welded radially, lengthwise or both. The sprue rod/wax pattern combination is called a wax tree. The bottom end of the sprue rod is inserted into a rubber base, which is then fitted onto the base of a metal flask. The resulting configuration consists of the wax tree being encapsulated by the flask without the patterns touching anything. The flask is filled with slurry, called investment, usually under vacuum pressure. Investment is similar to plaster, becoming hard in a matter of hours. Once the investment has hardened, the rubber base is removed from the flask, and the flask is placed into an oven or other heated space for the dewaxing cycle. During the wax removal, the wax melts and drips out, leaving a negative space where the wax tree used to be. The negative space is an exact duplicate of the wax tree, with the area where the sprue rod and gates were creating a path for molten metal to fill the patterns. The investment mold then cures in the heat, allowing it to withstand the temperatures of the molten metal. Finally, the molten metal is poured into the heated mold, creating a metal version of the wax tree. The investment is removed from the cooled metal, and the patterns can be removed from the tree. 
     Dozens of problems can arise during the lost wax investment casting process, resulting in casting defects ranging from minor flaws that can be reworked to irreparable defects that require recasting. One common problem results from wax melting within an enclosed space because the adjacent wax has not yet melted, blocking the wax&#39;s exit from the investment mold. Wax expands as it melts, causing problems for the investment mold. One of these problems is wax infiltrating pores in the investment mold. This wax can be extremely difficult to remove from the mold and can cause defects in the metal cast. 
     In addition to casting problems, the traditional wax sprue rod is completely lost in the dewaxing cycle. This is not only wasteful, but also represents a continuing expense that can be eliminated with a non-disposable reusable sprue rod. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a reusable sprue rod assembly for lost-wax investment casting that presents several advantages over the traditional method of lost wax investment casting. The reusable sprue rod of the invention is preferably made of metal or ceramic that will not melt during the dewaxing cycle. The invention further comprises a cap that keeps investment out of the sprue rod; a weight that fits on the end of the sprue rod to aid the removal of the sprue rod during dewaxing; a locking mechanism to lock the weight into place on the sprue rod; a rubber base that holds the sprue rod in place by firmly engaging the weight while the wax tree is welded together and during the investment process; and a puller base and spring-loaded latch that locks onto the locking mechanism in place of the rubber base during the dewaxing cycle. 
     An object of the invention is to replace the wax sprue rod with a reusable sprue rod that can be removed during or prior to the wax removal cycle, allowing more wax to be removed from the investment mold, reducing problems with wax expansion during heating, and resulting in a substantial reduction in wasted wax. This can be accomplished by heating the sprue rod without heating the investment mold, as during traditional dewaxing. The sprue rod can be heated either because it is a conductive material or because it comprises a heating means, such as a nichrome wire. 
     Sprue rods are traditionally made of paraffin, which melts and is lost during the wax removal process. The sprue rod of the present invention is made of a material with a high melting point that will not melt as the rest of the wax tree melts. The material may be conductive, non-conductive, a combination of conductive and non-conductive materials or microwavable materials. The sprue rod may be a tube or a solid rod, may taper internally or externally and may have different cross-sections. The inner surface or outer surface may contain grooves for heating elements or additional wax. The sprue rod may have equally spaced rows of holes drilled at a 45 degree angle for accepting gates. 
     Another object of the invention is to allow for the use of patterns that are made of materials with higher melting points than wax. For instance, patterns made of metals with low melting points, such as Indium, can be welded to the sprue rod. This is not possible with wax sprues. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front and side elevational view of the preferred embodiment of the reusable sprue rod and its cross section. 
         FIG. 2  is a perspective view of the assembled invention with rows of wax patterns welded onto the reusable sprue rod. 
         FIG. 3  is an exploded view of the sprue rod, weight means, locking means and rubber base. 
         FIG. 4  is a perspective view of the puller base and spring-loaded latch. 
         FIG. 5  is a cross-section of the puller base and spring loaded latch, showing the compression coil spring within the center hole of the base. 
         FIG. 6  is a perspective view of the puller base in position to lock into the locking means. 
         FIG. 7  depicts several alternative embodiments of the reusable sprue rod of the present invention. 
         FIG. 8  is a perspective view an alternative embodiment of the invention (Example 2), wherein the reusable sprue rod comprises internal or external grooves for nesting a heating means. 
         FIG. 9  is a perspective view of an alternative embodiment of the invention (Example 4), wherein a series of reusable sprue rods are stacked to extend the length of the sprue rod. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The reusable sprue rod assembly of the present invention comprises a reusable sprue rod, a sprue rod cap, a weight means, a locking means, a rubber base, a spring-loaded latch and a puller base. The reusable sprue rod ( 1 ), shown in  FIG. 1 , has a high melting point and can be manufactured from a conductive material, a non-conductive material or a microwave susceptible material. Conductive materials include aluminum, stainless steel, copper, brass or sterling silver. Non-conductive and insulating materials include composite plastic, porous engineered ceramics, refractory ceramics and high temperature resistant silicone rubber. The sprue rod could be made of non-conductive microwave-safe materials. A sprue rod of this type would then be combined with a heating means, such as nichrome wire or silicon carbide grains, which could be sprinkled or coated on the sprue rod to make it heat in a microwave. The sprue rod can also be made of combinations of materials. For instance, a sprue rod could be made of non-conducting material with grooves on its exterior surface coated with conducting material. 
     The reusable sprue rod can be straight or tapered. If it is tapered, the outer diameter of the sprue rod or the inner diameter or both could be tapered. The sprue rod can have many different cross-sections, including most regular polygonal shapes, some of which are shown in  FIG. 7 . A sprue rod could also be a solid rod, rather than a tube. The length and diameter of the sprue rod can also vary. A series of straight sprue rods can also be stacked to create different lengths of sprue rods. 
     The reusable sprue rod ( 1 ) is a hollow tube fabricated from a material with a melting point above the melting point of wax, typically above 200 degrees Fahrenheit. The sprue rod comprises a top end ( 27 ), a body ( 28 ), a bottom end ( 29 ), an interior surface, an exterior surface and a central axial opening ( 30 ). The top and bottom ends of the sprue rod are open. A metal or wax sprue rod end cap ( 6 ), shown in  FIG. 2 , fits into and seals the hole in the top end of the sprue rod. The sprue rod end cap is capable of preventing wax, investment and other liquids from seeping through the top end of the sprue rod when in place. 
     The bottom end ( 29 ) of the sprue rod, as shown in  FIG. 9 , engages a hollow end member ( 8 ) that is welded, glued or threaded into place on the open bottom end. In the preferred embodiment of the invention, the hollow end member is permanently affixed to the sprue rod. In alternative embodiments, the hollow end member can be pulled or threaded off of the sprue rod. The hollow end member comprises an inner diameter, an outer diameter, a crown, a center and a base. The inner diameter is constant from the base to the crown and matches the diameter of the sprue rod&#39;s central axial opening, essentially extending the central axial opening to the base of the end member. The outer diameter matching being such that the crown can fit snugly inside the bottom end of the sprue rod, the diameter widening to match the diameter of the diameter of the sprue rod&#39;s exterior surface below the crown, the diameter tapering from the center of the end member to the base. The end member further comprises two opposing L-shaped notches ( 7 ). The notches each comprise a vertical portion, a 90 degree turn, and a horizontal portion. Each notch begins with the vertical portion extending perpendicularly from the base of the end member through the center, the 90 degree turn and then the horizontal portion extending towards the other notch, parallel to the base of the end member. The horizontal portion is shorter than the vertical portion. The notches are capable of engaging the locking bar of the locking means, described below. The horizontal portion comprises a width and the vertical portion comprises a height. 
     In an alternative embodiment of the invention, the end member is a machine cut from the same piece of rod material as the reusable sprue rod, so the end member is an extension of the sprue rod that does not have to be welded or slip fit into place. This embodiment works best with a sprue rod that has a constant inner diameter, rather than a diameter that tapers. When the sprue rod has a tapering inner diameter, the end member is preferably welded into place. 
     The weight means ( 4 ), shown in  FIG. 3 , comprises a hemispherical weight, the weight comprising an apex, a base, a top opening at the apex of the weight, a bottom opening in the base of the weight and a channel extending from the top opening to the bottom opening. The diameter of the top opening is smaller than the diameter of the bottom opening. The bottom opening is sized so that the locking means ( 3 ) can fit through the bottom opening into the channel. The bottom opening can be reverse threaded, so that the locking means can be screwed into the weight means. Otherwise, the locking means can be welded into the bottom opening of the weight means to create a single piece. The weight means&#39; function is to weigh down the sprue rod, helping the sprue rod to automatically disengage from the investment mold during the dewaxing cycle, due to the additional pull of gravity, allowing the wax from the wax patterns and gates to drain more completely and easily. Another function of the weight means is to interface with the raised inner circle of the modified rubber base, holding the sprue rod in the rubber base. 
     The locking means ( 3 ), shown in  FIG. 3 , fits inside the bottom opening and channel of the weight means ( 4 ), but is too large in diameter to fit through the top opening of the weight means. The function of the locking means is to lock the weight means onto the sprue rod by engaging the notches in the end member of the sprue rod and to connect the puller base to the sprue rod by engaging the spring-loaded latch. The locking means is a hollow cylinder that comprises a top end, a body, a bottom end, an inner diameter and an outer diameter. The outer diameter is slightly smaller than the diameter of the bottom opening of the weight means and larger than the diameter of the top opening of the weight means, so that the locking means can be inserted into the weight means. The top end of the locking means comprises a flat outer surface, a center opening, and a locking bar ( 12 ) that spans the center opening. The center opening is slightly larger in diameter than the bottom end of the sprue rod, so that the sprue rod can be inserted into the opening if the notches in the end member are aligned with the locking bar. The center opening extends from the top end to the bottom end of the locking means. The locking bar comprises a rectangular prism, which comprises a length, height and width. The length is longer than the diameter of the center opening, but shorter than the diameter of the hollow cylinder of the locking means. The locking bar is slightly narrower in width than the horizontal portion of the L-shaped notch ( 7 ) in the end member ( 8 ) of the sprue rod and slightly shorter in height to the vertical portion of the same L-shaped notch. These dimensions allow the locking bar to be pushed up through the vertical portion of the notch until the locking bar meets the 90 degree turn and then rotated to slide the locking bar into the horizontal portion of the notch. The dimensions are close in size, ensuring a snug fit that requires some pressure to move the rod within the notch. The locking bar is fixedly attached to the flat surface of the top end of the locking means, extending across the widest part of center opening. The body comprises two opposing L-shaped notches ( 13 ), the notches comprising a vertical portion, a 90 degree turn, and a horizontal portion. Each notch begins with the vertical portion extending perpendicularly from the bottom end of the locking means towards the top end, the 90 degree turn and then the horizontal portion extend towards the other notch, parallel to the bottom end of the locking means. The horizontal portion is shorter than the vertical portion. The notches are capable of engaging the projections of the spring-loaded latch, described below. The horizontal portion comprises a width and the vertical portion comprises a height. The bottom end of the locking means comprises a flat outer surface bisected by the notches and a center hole. The locking means is locked onto the sprue rod by inserting the sprue rod through the weight means and into the center opening of the locking means when the locking bar and the sprue rod&#39;s notches are aligned. Either the sprue rod or the locking means is then rotated so that the locking bar slips into the horizontal portion of the sprue rod&#39;s notches. 
     The circular rubber base ( 2 ), shown in  FIG. 3 , comprises a top side, a bottom side, an outer edge and a center. Rubber bases are common in lost wax casting, however, the top side of the rubber base of this invention is different from the typical rubber bases in the art to accommodate the reusable sprue rod and weight pull system. The bottom side of the rubber base is flat. The top side of the rubber base comprises a raised outer circle, a raised inner circle and a round center nub ( 16 ) in the center of the rubber base. The two circles are concentric. The outer raised circle ( 14 ) is located at the outer edge of the base&#39;s top side. Its circumference is such that it snugly fits over the outer circumference of an investment flask ( 23 ), which will be used during the investment casting process. The inner raised circle ( 15 ) is between the outer raised circle and the center nub. Its diameter is such that the base of the weight means ( 4 ) fits snugly within it, as shown in  FIG. 2 . The round center nub ( 16 ) has a diameter slightly shorter than the inner diameter of the locking means ( 3 ) and the central axial opening of the sprue rod. When the weight means with the locking means inside of it is pressed onto the rubber base, the base of the weight means ( 4 ) is engaged by the raised inner circle ( 15 ), and the center nub ( 16 ) of the rubber base engages the center opening of the locking means ( 3 ) and the central axial opening ( 30 ) of the sprue rod. The rubber base ( 2 ) is engaged to the locking means, weight means and sprue rod, allowing the sprue rod to be positioned vertically on the rubber base. This assembly can take place before or after the patterns ( 5 ) are welded onto the sprue rod, but must be in place for the investment process. The rubber base ( 2 ) is removed when the investment has hardened and is replaced by the puller base ( 10 ), which attaches to the locking means ( 3 ) via the spring-loaded latch. 
     The puller base ( 10 ), shown in  FIG. 4 , is circular and comprises a top side, a bottom side, an outer circumference, and a center hole ( 18 ). The center hole extends from the bottom side to the top side and acts as a slip-fit hole for the spring-loaded latch ( 11 ). The base further comprises a plurality of drain holes ( 17 ) around the center hole, the drain holes extending from the top side to the bottom side and being capable of allowing molten wax to flow through them. The bottom side of the base is flat. The outer circumference of the base is flush with the bottom side, but is raised above the top side, creating a raised rim that contains the molten wax that drips onto the top side during the dewaxing cycle, preventing leakage during dewaxing and promoting drainage of the molten wax through the drain holes. The center hole comprises a bottom diameter, a flat circular area ( 19 ), and a top diameter. The bottom diameter is larger than the top diameter. Examining the center hole from the bottom side to the top side, the large diameter at the bottom is necked down before reaching the top side, creating the flat circular area ( 19 ) parallel to the top side and the bottom side. 
     The spring-loaded latch ( 11 ) is slip-fit into the center hole ( 18 ) of the puller base ( 10 ), as shown in  FIGS. 4 and 5 , and engages the notches in the locking means, locking the puller base onto the sprue rod, weight means and locking means. The latch comprises top end, a cylindrical central rod ( 21 ), and a bottom end ( 31 ). The top end of the latch comprises a circular center and two opposing rectangular projections ( 22 ), similar to the locking bar ( 12 ) of the locking means ( 3 ). The circular center&#39;s diameter is identical to the diameter of the central rod. The top end&#39;s circular center and projections create a flat surface, which is parallel to the latch&#39;s bottom end ( 31 ). The projections comprise a height and a width. The projections are sized to fit into the L-shaped notches ( 13 ) in the bottom end of the locking means, with their height being slightly smaller than the height of the horizontal portion of the notches and their width being slightly smaller than the width of the vertical portion of the notches. The projections and notches fit together the same way as the locking bar of the locking means and the notches of the sprue rod&#39;s end member. The top end of the latch is capable of fitting through the puller base&#39;s center hole, allowing the latch to be fully inserted into the puller base and allowing the projections to extend out the top side of the puller base. The diameter of the central rod is smaller than the top diameter of the puller base, allowing the central rod to slip inside through the center hole of the puller base. The central rod comprises a compression coil spring ( 20 ), shown in  FIG. 5 . The spring comprises a strip of flexible metal coiled around the central rod and resting on the bottom end of the latch. The bottom end of the latch ( 31 ) comprises a round disc with a diameter larger than that of the central rod, the diameter being slightly smaller than the bottom diameter of the puller base and larger than the top diameter of the puller base. The bottom end comprises an inner surface and an outer surface. The compression coil spring ( 20 ) rests on the inner surface of the bottom end. The outer surface is capable of being used as a knob to engage and disengage the latch, and hence the puller base, to and from the locking means by applying pressure with the thumb or fingers. When the spring loaded latch is inserted into the puller base, the inner surface of the bottom end faces the flat circular area ( 19 ) created by the necked down area inside the puller base&#39;s center hole and the compression coil spring ( 20 ) comes in contact with the flat circular area ( 19 ). When pressure is applied to the bottom end of the latch ( 31 ), the spring compresses between the bottom end of the latch and the flat circular area. When such pressure is applied, the top end of the latch and its projections protrude from the top of the puller base. The top end of the latch can then be inserted into the locking means if the projections and the locking means&#39; notches are aligned. The combined puller base/latch can be locked onto the locking means by applying pressure to the outer surface of the bottom end of the latch ( 31 ), aligning the projections ( 22 ) with the notches ( 13 ) in the locking means, inserting the top end of the latch into the center opening of the locking means, and turning the bottom end of the latch so that the projections are locked into the horizontal portion of the locking means&#39; notches. When locked together, the compression spring of the latch is under pressure, which assists the sprue rod assembly in coming free from the investment mold during dewaxing or when the sprue rod assembly is heated. 
     Various diameters of rubber bases and puller bases can be produced to coincide with different sizes of flasks. The size of the flask used will vary based on how far the wax patterns protrude from the sprue rod. Making certain that there is sufficient clearance between the sides of the flask and the wax patterns ensures that the investment mold will be thick enough to resist cracking or breaking when molten metal is poured into it. The rubber base does not have to be circular, if another shape is advantageous is a certain situation. 
     In addition, the weight means does not have to be dome-shaped. It could be manufactured in any number of shapes, so long as it still performs its function. However, the shape of the raised inner circle of the rubber base would also need change to accommodate the shape of the base of the weight means. 
     The method of use of the apparatus of the present invention is as follows. First, the reusable sprue rod ( 1 ) is dipped into a reservoir of molten wax to lightly coat its entire length with wax. Next, the sprue rod ( 1 ), weight means ( 4 ), locking means ( 3 ) and rubber base ( 2 ) must be assembled. In the preferred embodiment, the weight means and locking means are welded together. In other embodiments, the locking means must be placed or screwed into the weight means by inserting the top end of the locking means into the bottom opening of the weight means. The notches ( 7 ) on the bottom end of the sprue rod are aligned with the locking bar ( 12 ) of the locking means. The bottom end of the sprue rod is inserted through the top opening in the weight means ( 4 ) and into the center opening in the top end of the locking means ( 3 ). The notches ( 7 ) on the bottom end of the sprue rod engage the locking bar ( 12 ) of the locking means. The sprue rod ( 1 ) is then rotated, so that the locking bar ( 12 ) slides into the horizontal portion of the sprue rod&#39;s notches, locking the sprue rod to the weight means and locking means. The rubber base ( 2 ) is then attached to the assembly by lining up the inner raised circle ( 15 ) of the rubber base and the base of the weight means ( 4 ). This also aligns the center nub ( 16 ) of the rubber base with the center opening in the bottom end of the locking means ( 3 ). The rubber base is firmly pressed onto the assembly, so the inner raised circle ( 15 ) firmly encapsulates the base of the weight means ( 4 ), as pictured in  FIG. 3 . 
     Next, the assembled sprue rod assembly can be clamped or otherwise attached to a jig, where the wax patterns ( 5 ) are welded onto the sprue rod. Alternatively, the wax (or other) patterns can be welded to the sprue rod after placing the sprue rod assembly on a flat working surface. Once the wax patterns have been welded onto the sprue rod, as pictured in  FIG. 3 , a metal investment flask ( 23 ) is placed over the wax tree and inserted into the rubber base ( 2 ), interfacing with the outer circle ( 14 ) of the rubber base to hold the flask firmly in place. Investment is poured into the flask and allowed to harden into an investment mold. 
     Once the investment mold ( 9 ) is completely hardened, the rubber base ( 2 ) is pulled off of the assembly and is replaced by the puller base ( 10 ) and spring-loaded latch ( 11 ). In order to lock the puller base onto the sprue rod assembly, the spring loaded latch must be inserted, top end first, into the center hole in the bottom side of the puller base and the puller base must be lined up with the base of the flask, as shown in  FIG. 6 . Pressure must be applied by a finger or other pushing means against the bottom end spring-loaded latch ( 11 ), so that the top end of the latch protrudes from the top side of the puller base. Next, the projections ( 22 ) on the top end of the latch are lined up with the notches ( 13 ) of the locking means. The top end of the latch is inserted into the bottom end of the locking means ( 3 ) by applying additional pressure to the latch&#39;s bottom end, so that the projections ( 22 ) of the latch enter the notches ( 13 ) of the locking means and the compression spring is compressed. The latch&#39;s bottom end is then turned while maintaining pressure on the latch, locking the puller base onto the sprue rod assembly by engaging the projections with the horizontal portion of the notch. This new assembly is then suspended base down in an oven or other heated or steam-filled space for the dewaxing cycle. While the metal sprue rod is heating, the wax can escape from the investment mold by running down the metal sprue rod and dripping out the drain holes ( 17 ) in the puller base ( 10 ). The reusable sprue rod will quickly become hot enough to disengage from the investment mold ( 9 ) because of the pressure from the spring-loaded latch ( 22 ) combined with the weight of the weight means ( 4 ) and the sprue rod ( 1 ). The sprue rod assembly can then be removed from the heated space. This leaves a central void in the investment mold where the remaining wax can escape as it melts. The rest of the investment casting process (the rest of dewaxing, heat-treating the investment mold, casting with molten metal or other material, and removing the cast pieces from the tree) is followed in the traditional manner. 
     In an alternative method, prior to the dewaxing cycle, the sprue rod assembly is heated and removed. This leaves a central void where the sprue rod was, allowing wax to easily escape the investment mold as it melts. The difference between this alternative method and the preferred method is that the sprue rod is removed earlier, eliminating problems with wax being unable to escape from the outset of dewaxing. 
     The specific apparatus and method described herein are representative of preferred embodiments and are exemplary and not intended as limitations on the scope of the invention. Other objects, aspects, and embodiments will occur to those skilled in the art upon consideration of this specification, and are encompassed within the spirit of the invention as defined by the scope of the claims. 
     Preferred Embodiment of the Invention 
     In the preferred embodiment of the invention, the reusable sprue rod is made of a conductive metal. The exterior surface of the sprue rod tapers slightly, with the bottom end having a larger diameter than at the top end, as shown in  FIG. 1 . The interior surface is not tapered. 
     In the preferred embodiment, the weight means ( 4 ) and locking means ( 3 ) of the weight pull system are welded together into a single unit. The end member is also welded into the bottom end of the sprue rod. 
     Alternative Embodiments of the Invention 
     Example 1 
     In an alternative embodiment, the reusable sprue rod comprises a plurality of holes drilled at a 45 degree angle in rows and columns along its length. The holes are all the same size, with a diameter corresponding to a common gate diameter. The gates could then be directly inserted into the holes, assuring the perfect uniform angle for all of the wax pattern welds. A filet must still be created around the gate/hole interface after insertion of the wax pattern. This embodiment can be manufactured with different sizes and configurations of holes, as well as the holes being drilled at varying angles. 
     Example 2 
     The reusable sprue rod can comprise grooves in the exterior surface ( 24 ) or the interior surface ( 25 ). The grooves can be of various depths. Such grooves can comprise heating elements, preferably nichrome heating wires or strips extending through the length of the groove. These heating elements can be fixed, as shown in  FIG. 8 , or movable. The sprue rod can be a non-conductive metal if it has external grooves ( 24 ) and heating wires ( 26 ). 
     In addition, external grooves can be filled with melted wax prior to assembling the sprue rod assembly. The wax-filled grooves form reservoirs of wax that can be heated to facilitate welding of wax patterns and filet creation. If the area where a wax pattern is going to be welded is heated to locally melt the wax in the groove, the gate of the wax pattern can be inserted into the heated groove and then pulled back slightly and held in place until the wax solidifies, producing a nice, smooth filet that will minimize turbulence and inclusions when the molten metal is poured into the investment mold. 
     A smooth filet can reduce inclusions, which are caused by sharp edges of investment that break off when molten metal is poured into the investment mold. A smooth filet also reduces turbulence in the molten metal as it is poured into the investment mold. Turbulence can cause gas bubbles in the metal cast, resulting in porosity in the cast pieces. 
     Example 3 
     In another alternative embodiment of the invention, the sprue rod comprises a heating element that runs along the length of the sprue rod&#39;s interior. The heating element is sized to fit within the sprue rod. After the investment mold has hardened, the heating element can be connected to a power source, facilitating the removal of the sprue rod assembly as the wax surrounding the sprue rod softens and melts. Early removal of the sprue rod provides a great advantage in dewaxing the investment mold, as it is easier for the wax to exit the mold through the large cavity left by the sprue rod. 
     Example 4 
     Another alternative embodiment of the invention is a stackable sprue rod assembly. Two or more sprue rods are stacked vertically to create a single stacked sprue rod of the desired length, as depicted in  FIG. 9 . The rest of the sprue rod assembly and method of use remain the same as in the preferred embodiment. In this alternative embodiment, there is a top sprue rod and a bottom sprue rod, and there may be center sprue rods, depending on the desired length. 
     The top sprue rod can have an end cap ( 6 ) at its top end that press fits in place. The top sprue rod does not have a hollow end member at its bottom end. The bottom sprue rod does not have an end cap, but does have a hollow end member ( 8 ) threaded or press-fit onto the bottom end of the sprue rod. Any center sprue rods do not engage end caps or end members. The bottom end of each sprue rod fits snugly into the top end of another sprue rod, meaning that the interior diameter of the top end is slightly larger than the exterior diameter of the bottom end. The bottom end of each sprue rod is capable of being press-fit into the top end of any other sprue rod. 
     In an alternative embodiment of this embodiment, the sprue rods have any threading and reverse threading, so that they can be screwed together, rather than being press-fit together. Every sprue rod&#39;s interior surface at the top end of the sprue rod is reverse threaded. Each sprue rod&#39;s bottom end is threaded on the exterior surface. The threading on the bottom end of one sprue rod is capable of being threaded into the top end of another sprue rod.