CROSS REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation application of U.S. patent application Ser. No. 09/684,450 filed Oct. 6, 2000, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to the construction of valves such as are used to control the flow of combustible gas-to-gas-fired appliances and more particularly pertains to improvements in the configuration and construction of such valves in order to reduce the cost of their manufacture. 
     Regulating authorities typically mandate that valves that are to be used to control the flow of a combustible gas to a gas-fired appliance, such as for example a furnace, water heater or gas-burning fireplace, must be capable of maintaining their integrity when subjected to high temperatures such as may be encountered during a fire so as to preclude the external leakage of gas. This is intended to prevent a fire from being aggravated by the supply of additional fuel leaking from a fire-damaged valve. A side effect of such requirement is that it also typically serves to limit the type of materials that are used in the construction of such valves. 
     In view of the requirements with respect to fire resistance, valves that are subject to the regulations vis-a-vis high temperature resistance are typically predominantly constructed of metal wherein the melting temperature of such metal exceeds the temperature it is to be exposed to during fire testing. Metal-to-metal contact is relied upon between adjacent components in an assembled valve in order to effect a seal or are adjacent components are sufficiently dimensionally matched such that the failure of any gaskets or seals that may be positioned therebetween would result in minimal, i.e. acceptable, rates of leakage. The failure of gaskets or seals used in the interior of such valves to control the flow of gas through the valve would at worst merely result in the flow of additional gas to the burner where its combustion is adequately provided for and where it would thus not pose an additional hazard during a fire. 
     The construction of such valves has heretofore called for the manufacture of very tightly dimensioned metal components that require the machining of various cast parts to very close tolerances. Many stopcock configurations employ a conical plug that is rotatably received in a conical cavity formed in a valve body. The plug has an orifice formed in its side that is in fluid communication with an opening formed at its narrow end. The valve body has one duct formed therein that extends from an exterior port to an opening formed in the side of the conical cavity and another that extends from an opening in the base of the conical cavity to a second exterior port. Rotation of the plug so as to align the orifice formed on its side with the opening formed in the side of the conical cavity in the valve body establishes a flow path through the valve. Rotation of the plug so as to avoid any overlap between the orifice formed in the side of the plug and the opening formed in side of the conical cavity serves to positively shut off the flow of gas through the valve. 
     The interior surface of the conical cavity thus serves as a valve seat for the exterior surface of the plug whereby an effective seal is achieved with the very precise machining of the two surfaces. Machining of the cast metallic components to within 0.001″ is typically followed by a lapping operation to substantially perfectly match the two surfaces. The use of metals and the machining of the various components significantly contribute to the overall cost of such valve. Even minor variations in the machining process contribute to a high rejection rate that further affects the overall cost of manufacture. 
     A valve is needed that is less expensive to manufacture than heretofore known valves while retaining the ability to control the flow of gas therethrough. More particularly, a valve configuration is needed that obviates the various machining and matching operations that are currently needed to in order to achieve a proper seal. Additionally, a valve configuration is needed that reduces the number of cast metal parts that are needed without compromising the valve&#39;s ability to resist external leakage upon being subjected to the elevated temperatures it is subjected to during a fire test. 
     SUMMARY OF THE INVENTION 
     The present invention provides a gas valve that includes a conical rotatable plug component that is received in a conical cavity formed in a valve body wherein neither the surface of the plug nor the surface of the cavity require machining in order to achieve an effective seal therebetween. Moreover, the configuration of the valve permits the use of molded plastic rather than cast metal in the construction of the plug. The plug serves to establish a flow path through the valve body when rotated so as to align an opening formed in the cavity wall with an orifice formed in the side of the plug. A rotational orientation in which there is no overlap between the opening in the wall and the orifice in the plug shuts off all flow through the valve. 
     An effective seal is achieved between the plug and the wall of the conical cavity by an array of rubber sealing ridges that are overmolded onto the plug. The compliance of the rubber obviates the need for it to engage an ultra-smooth mating surface in order to achieve a seal. As a consequence, the surface of the cavity wall does not require machining and thus remains in its as-cast state. The surface of the plug does not engage the surface of the wall cavity as it merely serves as a support for the overmolding. As a consequence, machining of the plug surface is not necessary and it may be used in its as-cast or as-molded state. Moreover, since direct plug to cavity wall contact is not relied upon to achieve a seal, the dimensional stability afforded by the use of metal in the manufacture of the plug is not necessary and a plastic can be substituted therefor. While the plastic plug is expected to fail when subjected to the elevated temperature of a fire test, the metal valve body and metal cap that completely encapsulate the plug preclude such failure from resulting in any significant external leakage. 
     The plug component of the valve of the present invention is an injection-molded plastic part that is overmolded with a pliable rubber-like material. The overmolding is configured to provide an array of raised ridges that extend both circumferentially as well as longitudinally about the surface of the plug. Circumferential ridges about the wide and the narrow end of the conical plug prevent the escape of gas into the space between the plug and cavity wall. Longitudinally oriented ridges prevent the escape of gas from the space between the plug and cavity wall into the outlet port. A closely spaced second ridge may extend along each of the vertical and horizontal ridges for the purpose of redundancy and for enhancing the robustness of the valve. 
     These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the valve of the present invention. 
         FIG. 2  is a slightly enlarged cross-sectional view of the valve taken along lines II—II of  FIG. 1 . 
         FIG. 3  is an enlarged perspective view of the plug element of the valve of the present invention. 
         FIG. 4  is a cross-sectional view of the plug element taken along lines IV—IV of  FIG. 3 . 
         FIG. 5  is an enlarged cross-sectional view of the section of the plug element within circle V of  FIG. 4 . 
         FIG. 6  is a cross-sectional view of the plug element taken along lines VI—VI of  FIG. 4 . 
         FIG. 7  is an enlarged cross-sectional view of the section of the plug element within circle VII of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The valve of the present invention is especially suited for controlling the flow of a combustible gas to gas-fired appliances such as a furnace, water heater or gas-burning fireplace. The valve configuration obviates the need to rely on machined surfaces in order to establish seals between moving parts within the valve and further allows the use of plastic in the construction of certain components within the valve. 
       FIG. 1  is a perspective view of a valve of the present invention. The valve  12  includes a valve body  14 , an inlet port  16  and an outlet port  18 . Control knob  20  is rotatable and depressible to enable manual actuation of the valve. In the particular embodiment shown, a thermocouple-powered electromagnet is insertable into a bore  22  formed at the base of the valve body. 
       FIG. 2  is a cross-sectional view of the valve shown in  FIG. 1  taken along lines II—II. A conical cavity  24  is formed in the valve body  14  and is dimensioned to receive rotatable plug element  26  therein. Endcap  28  is attached to the top of the valve body to serve as an end wall for cavity  24  and to thereby enclose plug element  26 . An aperture  30  formed in the center of the endcap permits the extension of stem  32  therethrough so as to be rotatable and longitudinally shiftable. O-ring  34  achieves a positive seal between stem  32  and end cap  28 . Push rod  36  is attached to stem  32  and extends through the center of the plug element through which it is longitudinally shiftable. Compression spring  38  is nested in the interior of the stem and serves to bias plug element  26  into the cavity  24 . A hole (not visible) and conduit formed in the side of conical cavity is in fluid communication with outlet port  18 . The base of the conical cavity opens into chamber  40 , which is in fluid communication with inlet port  16 . Valve  42  seals off chamber  40  from cavity  24  when urged against valve seat  44  by spring  46 . Bore  22  is configured to receive an electromagnet for holding valve  42  in its open position when energized by a thermocouple. 
       FIG. 3  is an enlarged perspective view of plug element  26 , while  FIG. 4  is a cross-section thereof. The plug has a conical outer surface and a hollow interior that is divided into an open lower chamber  50  and open upper chamber  51 . An orifice  53  formed in the side of the plug extends into its hollow interior. An array of sealing ridges are formed on the exterior surface of the plug including a circumferential pair of sealing ridges  54  about the wide end of the plug and a circumferential pair of sealing ridge  56  about the narrow end of the plug. At least two longitudinally oriented pairs of sealing ridges  58 ,  60  extend between the two pairs of circumferential sealing ridges. Additional sealing ridges (e.g.  61 ) may be formed on the surface of the plug. The sealing ridges are formed as part of an over-molded layer  62  of silicon rubber that covers most of the plug. The overmolded layer is at least 0.015″ thick while the ridges extend outwardly beyond the conical surface defined by the overmolded and exposed portions of the plug element to a height of approximately 0.008″. Single as well as paired ridges may be employed to form the required seals, as may different types of rubber and rubber-like materials. 
       FIG. 5  is an enlargement of the circled portion shown in  FIG. 4 . The illustration shows the cross-sectional configuration of the paired sealing ridges  54  and further shows an undercut  64  formed in the plug itself. The undercuts extend across the surface of the plug directly below each of the sealing ridges to ensure that a positive bond and a mechanical interlocking with the plastic is achieved and further serve to enhance the pliability of the sealing ridges. Additional undercuts may be formed at various locations about the surface of the plug to provide further anchoring points for the overmolding. 
       FIG. 6  is a cross-sectional view taken perpendicular to the longitudinal axis of the plug  26 . This view clearly shows the longitudinal ridge pairs  58 ,  60  that are positioned on either side of orifice  53 . An additional ridge pair  61  is shown disposed therebetween. Multiple undercuts  64  are visible at numerous locations about the plug&#39;s surface so as to ensure a secure bond and mechanical interlocking with the overmolding  62  as well as to impart additional pliability to the sealing ridges. The overmolding is at least 0.015″ thick and substantially thicker within the undercuts  64 . The ridges  58 ,  60 ,  61  extend beyond the conical surface by 0.008″. 
       FIG. 7  is a further enlarged cross-sectional view of the section circled in  FIG. 6 . Ridge pairs  60  protruding from the surface of the overmolding  62  and beyond the surface of the plug  26  are clearly visible as is undercut  64  that is positioned directly below the sealing ridges. 
     The valve of the present invention is manufactured using well known metal casting, plastic molding and overmolding techniques. The valve body  14  and end cap  28  are cast of an aluminum alloy. The conical cavity  24  is cast into the valve body and is used in its as-cast condition without any machining of its interior surface. The plug element  26  is formed of a molded plastic such as a polyphthalamide (e.g. AS-1566 HS) which is selected for its low shrink rate, its ability to withstand high temperatures of up to 520° F. and the strong bond it forms with silicone rubber. The molded plug element is subsequently subjected to the overmolding process without any machining of the plug elements exterior surface. The preferred overmolding material is a silicone rubber, which is selected for its ability to bond with the plastic plug, its imperviousness to methane, and its pliability throughout a wide temperature range. Various silicone rubbers may be used as well as other rubber-like materials. During assembly, a lubricant that is compatible with both the plastic and the rubber sealing ridges is preferably applied to the plug to provide lubrication and to prevent galling. 
     In use, the valve  12  positively precludes the passage of gas from the inlet port  16  to the outlet port  18  by rotation of the plug element  26  via knob  20  to a position in which there is no overlap between the opening formed in the side of the conical cavity  24  and orifice  53  formed in the side of the plug. Sealing ridges  54 ,  56 ,  58 ,  60  completely surround the opening while the bias generated by spring  38  ensures that sealing ridges firmly come to bear on the cavity wall. The circumferential sealing ridges  54 ,  56  in concert with the vertical sealing ridges  58 ,  60  and any additional ridges that may extend between the circumferential sealing ridges ensure that no gas can reach the hole formed on the side of the conical cavity  24 . The use of ridge pairs rather than a single ridge configuration enhances the robustness of the valve and provides redundancy in the sealing mechanism. 
     When knob  20  is depressed, push rod  36  transfers pressure to the supplemental valve  42  to overcome the force exerted by spring  46  and allow the supplemental valve to open. Further rotation of the knob serves to rotate the plug  26  to a position wherein orifice  53  overlaps with the opening formed in the side of the cavity wall. A flowpath through the entire valve is thereby established to set the inlet valve  16  into fluid communication with the outlet valve  18 . Gas will freely be admitted from inlet port  16  and chamber  40  into the interior of plug  26 , out through orifice  53  and on into outlet port  18 . Once a flame has been established at the burner and a thermocouple or thermopile is able to generate sufficient power to energize an electromagnet received in bore  22  to hold valve  42  in its open position, pressure on knob  20  can be released and the valve will remain fully open until either the knob is rotated back into its closed position or when power to the electromagnet is discontinued. 
     In the event the valve is subjected to extreme temperatures such as may be encountered during a fire, failure of all non-metallic parts including the plug element  26  can be expected. However, in view of the fact that the interior of the plug is entirely encapsulated by metallic elements, no appreciable external leakage will result. The tight tolerances between the valve body  14  and end cap  28  as well as the end cap  28  and stem  32  will ensure that only acceptable rates of leakage may occur despite the failure of gasket  29  or O-ring  34 . Such gasket and O-ring serve to ensure zero-leakage during the normal service life of the valve. 
     While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except by the appended claims.
A valve for controlling the flow of a gas therethrough which requires no machining in order to attain an effective seal between the valve seating surface and a rotatable plug element. The valve seat surface remains as cast while the plug consists of an overmolded plastic. The overmolding defines an array of pliable sealing ridges. The use of overmolding facilitates the use of cost-saving plastic materials, and obviates the need for expensive machining of the cooperating surfaces.