Abstract:
The mold that is intended to be used integrally with casting material that is contemplated by this invention is formed with a surface that exerts pressure on and retains set casting material in the mold by providing a pin in the mold that has an upper region that is larger in a dimension perpendicular to the base of the mold than the base of the pin.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the art of mold manufacturing and specifically to the manufacturing of molds that are to be used integrally with the casting material with which they are filled. 
   2. Description of the Prior Art 
   The manufacture of molds for the purpose of casting is a well known technology. Most commonly, a mold is used to form a casting material, such as metal, concrete, or plastic into a specified shape. The casting material is introduced into the mold in a fluid form and is allowed to set, thereby assuming the shape of the mold. 
   In most instances, a mold is a tool for forming casting material into a predetermined shape that is separated from the set casting material and either discarded or used again. For this reason, molds commonly are concave. This shape facilitates separation of the mold from the casting material. 
   In certain instances, however, the mold and the casting material are intended to be used integrally. In this instance, a concave mold may have shortcomings. The mold and the casting material may separate unintentionally, frustrating the performance of the integrated mold and casting material. 
   It is therefore desirable, in instances where the mold and the casting material are to be used integrally, to have a mold with at least one surface which exerts pressure on the set casting material such that the set casting material remains biased into the mold. 
   It is also often desirable to have the mold made of the least expensive, substantially rigid material available. Commonly, the material that is best suited for this purpose is plastic. Plastic is usually formed by injection molding, which is also a well known art. Injection molding requires a mold into which fluid hot thermoplastic is injected. After a suitable cooling period, the plastic is then removed from the exterior mold. Such injection molding is used to produce plastic structures that are concave on one surface and convex on the opposite surface. This is so because injection molding generally requires an injectible mold formed of two movable plates. The first of these plates has a concave portion, the second of these plates has a slightly smaller convex portion that fits approximately into the concave portion of the first plate. The two portions are separated by a gap that defines the injectible mold. The first and second plates are concave and convex, respectively, because the resulting plastic must separate easily from the plates. No current technology exists that provides for an injection molded plastic mold created from a single piece that has a surface that would retain set casting material within the mold. 
   SUMMARY OF THE INVENTION 
   The mold that is intended to be used integrally with casting material that is contemplated by this invention could be of any general shape. Such a mold, if it is produced by injection molding, must be concave so that the mold may be extracted from the injectible mold by which it is produced. The mold of this invention is formed with a surface that exerts pressure on set casting material to remain in the mold by providing a pin in the mold that has an upper region that is larger in a dimension perpendicular to the base of the mold than the base of the pin. 
   It is therefore an object of this invention to provide a mold having a structure within it that retains set casting material within the mold. It is a further objection of this invention to provide such a structure through injection molding. 
   It is yet another object of this invention to provide a mold which contains such a structure that is frustoconical in shape. 
   These and other objects of the invention will be apparent to those skilled in the art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial perspective view of the mold. 
       FIG. 2 . is a cross sectional view of the mold. 
       FIG. 3  is a top view of the mold. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The mold  10  of this invention may be formed by injection molding. It may be of any general shape, but, for purposes of formation by injection molding, will be substantially concave. The concave portion of the mold  10  defines a reservoir having a height  22 . 
   The mold  10  is formed with a pin  14  extending upwardly from its base  16 . The pin  14  on the finished mold  10  will have a base portion  18 , an upper portion  20 , and a height  12 . The upper portion  20  of the pin  14  will be larger in a dimension perpendicular to the base  16  of the mold  10  than the base portion  18  of the pin  14 . The pin  14  will have a surface  24  extending between the upper portion  20  and the base portion  18  of the pin  14 . Ideally, the pin  14  will be frustoconical in shape. 
   By virtue of the fact that the upper portion  20  is larger than the base portion  18  of the pin  14 , the surface  24  will be disposed at an angle to the base  16  that is less than ninety degrees. This means that vector A representing the pressure that would be exerted on casting fluid within the reservoir would have a downward component B that would tend to retain set casting fluid in the mold  10 . 
   The pin  14  may extend beyond the height of the reservoir  22  or be of a height less than the height of the reservoir  22 , as shown. If the pin extends upwardly beyond the height of the reservoir  22 , it is important to have a portion of the surface  24  that is disposed at an angle to the base  16  that is less than ninety degrees located below that height. In such a structure, it would be common for the casting material to be introduced to the mold  10  only to the height of the reservoir  22 . The pin  10  would not exert downward pressure on the set casting material unless that portion of the surface  24  that is disposed at an angle to the base  16  that is less than ninety degrees is in contact with the casting material. 
   The pin  14  as shown in the figures could not be formed by traditional injection molding. Nontraditional methods may be used, however, to obtain a pin of the desired shape. One method of forming such a pin  14  is to remove the mold  10  from an injection molding machine (injection molding machines are well known in the art and not shown) in which it is formed. The pin  14  may be formed in a right cylindrical shape, either hollow or solid, so that the mold can be extracted from the injection molding machine. A right cylindrical shape would have no downward component B in a pressure vector A extending from its surface. One method of forming such a pin  14  is to remove the mold  10  from the injection molding machine in which it is formed while the plastic is still pliable. When the mold  10  is being extracted from the injection molding machine, but while the thermoplastic fluid is still pliable, downward pressure can be exerted on the upper portion  20  of the pin  14 . Because the thermoplastic will be pliable, the right cylindrical pins will expand outwardly under the downward pressure. If allowed to set in this shape, the mold  10 , when set will include a pin  14  having a shape generally as shown in the figures. 
   The pin  14  may be largely two-dimensional in shape. In this instance, a flat pin (not shown) could be formed in a fan shape such that its upper edge is wider than its base. This structure, too, would exert downward pressure on the casting material along the edge between the upper edge and the base. 
   It is possible to provide such a structure in a mold form by methods other than injection molding. Pins having a frustoconical could be affixed to the base of a mold  10  with adhesives, by mechanical connection, or by other means well known in the art. 
   The shape of the pin  14  is not limited to those previously described, but may be of any shape being larger at its upper end that at its lower end. Thus it can be seen that the objects of the invention have been met.