FIELD OF THE INVENTION 
     This invention relates to a plug and more particularly to a plug finding particular utility as a casting mold plug and to a method for sealing a casting mold. 
     BACKGROUND OF THE INVENTION 
     Relatively large dies are used to mold engine blocks, transmission housings, and other large die-cast parts out of aluminum. Such dies are made out of an exotic metal that is capable of retaining its solidity and shape at a temperature higher than the aluminum that is being cast in the die. The die has a series of passages that are used to conduct water or oil for heating and cooling the die. In the case of oil, the die is heated initially by the use of oil at a certain temperature being conducted through these passages and then later the same oil may be used to carry heat away from the die in an effort to maintain a consistent temperature of the die. In the case of water, the molten aluminum being injected into the die is relied upon to raise the die temperature to a working level. When the working temperature of the die is established, water flows through the die to carry away excess heat. During the manufacturing of the die, holes are drilled in the die to provide the necessary passages through which the oil or water will pass. Before the die can be used the holes must be plugged to retain the water or oil. 
     The passages in the die over a period of time become filled with debris so that it is desirable that the plugs that are used to seal the passages can be removed easily and conveniently and the passages cleaned out. Presently existing plugs for these passages have been found to fail over a period of time causing the water or oil to leak form the die. Because leaking from the die is not only wasteful and inefficient but can cause damage to the cast part, it is desirable that an improved means be provided for sealing the passages with plugs that do not leak after a period of use. 
     An alternative means of constructing the die would be to weld the plug in place but then the plug could not be removed in order to clean out the passages. The plugs must be easily removable. The plug openings are threaded NPT ports, that is, they are “National Pipe Thread” ports. The plugs are screwed into these ports and have a screw thread on them for mating with the threading in the ports. Over a period of time it is found that these plugs begin to leak and come out of the tapered thread openings. What is needed is plug that will seat, remain sealed, will not leak and will not come out over a period of time while the die is in use and yet is removable when it is desired to clean out the passages. The sealing plug must be suitable for use for both media and ambient temperatures of 350° F. 
     SUMMARY OF THE INVENTION 
     One embodiment of the plug of the invention might include an outwardly threaded first member having a bore therethrough. There is also provided a second member of resilient expandable material which has a cylindrical shape with a central opening. An outwardly threaded third member is received within the bore and extends through the central opening of the second member. Also there is provided an inwardly threaded disc threadedly received on the third member and having projections which extend therefrom into the second member. The second member is received between the first member and the disc whereby rotating the third member into the threads of the disc moves the disc toward the first member and squeezes and expands the second member. 
     Another embodiment of the invention involves a plug which includes a first piece which is externally threaded and has a tapped hole. There is also provided a second piece made out of resilient expandable material and having a tubular shape with a central opening. Finally a bolt having an enlarged head and a threaded shank extends through the central opening of the second piece and is threadedly received in the tapped hole. The bolt is rotatable relative to the first piece to move the head toward the first piece to compress and expand the second piece. 
     Still another embodiment of the invention involves a plug for a casting die which includes an outwardly threaded first member. There is also provided a resilient expandable second member mounted on said first member. Finally a threaded third member is mounted on said first member and connected to the second member for expanding the second member. 
     A further embodiment of the invention is a method for sealing the passage of a casting die which includes providing a first externally threaded member. A second resilient expandable member is mounted on the first member. The first and second members are inserted into the passage by threading the first member into threads in the passage and finally expanding the second member against the walls of the passage. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary view of a casting mold showing the plug of the present invention in a passage of the casting mold. 
     FIG. 2 is a side elevation of an alternative embodiment of the plug of this invention. 
     FIG. 3 is a top plan of the embodiment of FIG.  2 . 
     FIG. 4 is a side elevation of a screw forming a part of the embodiment of FIGS. 2-3. 
     FIG. 4A is an end elevation of the screw. 
     FIG. 5 is a side elevation of a part forming a portion of the embodiment of FIGS. 2-4. 
     FIG. 6 is an end elevation of the part of FIG.  5 . 
     FIG. 7 is a side elevation of a disc forming a portion of the FIG. 2-6 embodiment. 
     FIG. 8 is an end elevation of the FIG. 7 disc. 
     FIG. 9 is a side elevation of a part of the plug of FIG.  1 . 
     FIG. 10 is an end elevation of the FIG. 9 part. 
     FIG. 11 is a side elevation of a further part of the FIG. 1 embodiment. 
     FIG. 12 is an end elevation of the FIG. 11 part. 
     FIG. 13 is a side elevation of a further part of the FIG. 1, and 9-12 embodiments. 
     FIGS. 14 and 15 are opposite and elevations of the part of FIG.  13 . 
     FIG. 16 is a view similar to FIG. 1 of an alternative form of casting mold showing the FIG. 1 plug. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated devices, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring to the enclosed drawings and particularly to FIG. 2, plug  10  includes an outwardly threaded member  11 . The threads are tapered from a larger size at the end  12  of the member to a smaller size at the end  15  of the member. The member  11  has a recess  16  that has a hexagonal configuration so that the member  11  can be screwed into place by a wrench. The member  11  also has a bore  17  therethrough which receives a screw  25 . The screw  25  is also shown in FIGS. 4 and 4A and has an internal hex opening  30  in its head  31 . The screw  25  extends through a cylindrical silicone member  35  (detailed in FIGS. 5 and 6) and has threadedly mounted on its distal end a disc  40  (See FIGS. 7 and 8) having three projections  41  that extend upwardly and bite into the silicone  35 . When the screw  25  is rotated to screw into the disc  40  it forces the disc against the silicone  35  and squeezes it forcing it to expand outwardly so as to seal the passage within which the plug is mounted. 
     Another embodiment of the plug of the invention is shown in FIG. 1 mounted in the passage  50  of a die  51 . The plug  52  of FIG. 1 is constructed with three members or pieces. One piece  55  (FIG. 9) is a modified stainless steel, socket head, NPT plug that has a hole  56  drilled and tapped in the center of it. The external thread on the piece  55  is a standard “right-hand” thread. The tapped hole has a “left-hand” thread. A second piece  57  (FIGS. 1,  11  and  12 ) is a silicone material that has been custom molded into a tubular shape. This tubular shaped silicone piece  57  is placed over the threaded section  60  of a bolt  61  which is the third piece of the plug  52 . The bolt  61  is shown in FIGS. 13,  14  and  15 . Bolt  61  has a large diameter cylindrical head  62  and the shank  65  of the bolt is not threaded all the way to the head. The threaded shank has a socket head  64  or screwdriver slot  65  on the threaded end of the shank. Note FIG. 1 shows a socket head while FIGS. 13 and 14 show a slot). When assembled as shown in FIG. 1 the tubular silicone  57  is placed down over the threaded shank  65  of the bolt and the bolt is threaded into the stainless steel piece  55 . 
     In order to plug the passage  50  of the die  51 , the plug  52  is inserted into the passage with the silicone piece  51  being inserted first and then the piece  55  being screwed into the port of the die. When inserted into the die  51 , a large hex-head wrench is used to tighten the stainless steel piece  55  in the NPT port of the die. After the piece  55  is tightened, a smaller hex-head wrench or a screwdriver is used to turn the tapped shank of the bolt in the piece  55 . As the shank is turned the bolt head  62  moves closer to the piece  55  causing the silicone to compress on the shank and expand it&#39;s outside diameter against the walls of the passage  50  of the die, sealing the passage. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to he considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. For example, the plug can be made out of various materials and the elements, pieces and members making up the plug can have various shapes and sizes. Referring to the embodiment of FIG. 2, the dimension  68  might be 1.355 inches with all other dimensions in FIGS. 2-8 being scaled accordingly. As a further example the dimension  69  in FIG. 9 might be 0.43 inches with all other dimensions being scaled accordingly. 
     As another example within the scope of the invention the silicone member  35  (and the member  57 ) which might have to be molded members if made out of silicone could be replaced by a less expensive extruded member made out of, for example, a viton material. Both such materials and other such materials for this member should have the property of expanding to seal the passage when they are squeezed. Thus such materials have the property of maintaining or attempting to maintain the same volume when they are deformed. Other alternative forms of the invention within the scope of the invention can be conceived. For example the threading  60  on bolt  61  is preferably left-handed for the reason that the installer of the plug  52  can tighten the threads  60  by turning the bolt  61  in the same rotary direction as the piece  55  is turned to tighten the threads  70 . However, it is included within the invention to make the threads  60  right-handed as well. Various other alternative embodiments of the invention will be evident from a reading of the specification and a review of the drawings and are intended to be included by the following claims. 
     As still another example it is not always known what the depth of the threads are in the passage or hole in the die that is being plugged. FIG. 16 shows that the plug of the invention can be used to plug the passage even though the threads  75  in the mold passage  76  extend a greater distance than the threads  70 . In this application of the invention the piece  55  may be screwed into the passage  76  only to the point where it is generally flush with the die and then the bolt  61  is tightened to expand the piece  57 .
A plug for sealing and resealing the passages for heating and cooling liquid in a casting mold. The plug includes a stainless steel socket head NPT plug which is screwed into the passage to seal it. Inboard of the plug there is mounted a resilient expandable silicone tubular element that is expanded by squeezing it to further seal the passage.