Abstract:
An arc tube for a ceramic metal halide lamp includes a hollow first body member ( 40 ) that is tapered along a majority of its length to aid in removing the molded component from a pin during assembly. An open end ( 46 ) of the first body member has a constant diameter ( 60 ) allowing it to be joined to a constant diameter portion ( 72 ) of the second body member or end cap ( 70 ). The mating constant diameter portions ensure that the hollow first body member can be monolithically joined with the end cap with a reduced level of seal voids.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to ceramic tubes and methods of forming same, and more particularly to ceramic arc tubes used in ceramic metal halide lamps.  
           [0003]    2. Discussion of the Art  
           [0004]    Ceramic arc tubes used for discharge lamp chambers were developed to operate at high temperatures on the order of 950° C. and higher. These types of lamps exhibit improved color temperature, color rendering, and luminous efficacies. Typically, ceramic discharge chambers are constructed from a number of individual components that are extruded or die-pressed from a ceramic powder. Prior practice employed a five component construction that included a central hollow cylinder substantially closed at either end by first and second plugs to which first and second legs were joined to the end plugs.  
           [0005]    More recent developments have been directed to minimizing the number of joints, i.e., reducing the number of individual components, to establish an improved sealed arc chamber. For example, U.S. Pat. No. 6,004,503 discloses a method of making a ceramic arc tube for a metal halide lamp comprised of two components, i.e., a hollow body and an end cap. The &#39;503 patent describes a two-part arc tube produced with a hollow body having an open end with a diameter approximately three to six percent (3-6%) greater than the opposite, closed end. That is, the hollow tube is tapered along its length and the patent disclosure is void of any description regarding the hermetic seal formed between the hollow body and the end cap. In the past, these components have been extruded or pressed and subsequently heated or fired to integrally sinter and join the components together.  
           [0006]    Current injection molding practice for molding hollow body parts or cylindrical components employs a taper on a mold pin to aid in removing the part after molding. The degree of taper ranges from about one-half percent to about six percent (0.5%-6%) along the length of the pin. While this may be acceptable for many molded assemblies, the tapered conformation presents problems where a tapered end of one component joins a second component having a non-tapered surface. In attempting to monolithically join the components together, e.g., sealing or bonding the hollow body to the end cap, to form a hermetically sealed ceramic arc discharge tube, the mating of the two non-parallel surfaces has been determined to be very problematic. This is because of potential seal voids forming at the joined interface and precluding a hermetic seal. Thus, improving manufacturing steps, components, and addressing these needs will lead to longer-life lamps having improved monolithic seals between the hollow body and end cap.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    A ceramic arc tube for a metal halide lamp includes a first body portion open at a first end and having a tapered wall extending along its length and a cylindrical region spaced inwardly from the first end and receiving a second body member hermetically sealed along the cylindrical region.  
           [0008]    The first body member has a tapered internal wall that, in a preferred arrangement tapers at a rate of at least approximately 0.5° over its length.  
           [0009]    The tapered wall can is also taper along its external surface to define a substantially constant wall thickness over its length.  
           [0010]    A method of making a ceramic arc tube for a metal halide lamp includes the steps of forming a first body portion having a hollow body region open at a first end. Providing a taper on the hollow body over substantially its entire length and forming a cylindrical internal region at the open first end for receiving a second body portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 generally illustrates a lamp assembly incorporating a ceramic discharge chamber.  
         [0012]    [0012]FIG. 2 is an enlarged, longitudinal cross-sectional view of the present invention.  
         [0013]    [0013]FIG. 3 is an enlarged view similar to FIG. 2 and illustrating a second configuration of the second body member. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 1 illustrates a ceramic discharge lamp  10  that includes a double ended discharge chamber  12  that receives first and second electrodes  14 ,  16 . The electrodes extend into the chamber and a fill material is encapsulated in the discharge chamber so that upon application of an electrical potential difference across the electrodes, an arc is produced that ionizes the film material to produce a plasma in the discharge chamber in a manner well known in the art. For ceramic metal halide lamps, the fill material typically includes a mixture of mercury (Hg), a rare gas such as argon (Ar) or xenon (Xe), and a metal halide such as NaI, TII, or DyI 3 . Other examples of fill materials are well known in the art and do not form a particular part of the present invention so that further discussion herein is deemed unnecessary.  
         [0015]    A central body member  20  includes first and second legs  22 ,  24  extending from opposite ends of the chamber. Lead wires  26  and  28  pass through the legs and extend therefrom for connection with a lamp contact or connector  30 , such as an Edison type base, although other electrical connections can be used without departing from the scope and intent of the present invention. Seals are preferably formed at opposite ends about the lead wires or conductors that extend into the first and second legs. The seals are preferably made with a glass frit that, when the glass is melted, flows into the legs to form a seal between the conductor and the leg.  
         [0016]    As indicated above, it is desirable to reduce the number of components that comprise the discharge chamber and similarly reduce the number of bonds or joints between the components. This expedites the assembly of the discharge chamber and reduces the number of potential bond defects during manufacture, as well as reduces the possibility of breaking the discharge chamber at a bond region during handling. Accordingly, and by way of example, two part ceramic metal halide arc tubes are preferred to reduce the number of seals in the monolithic joining areas.  
         [0017]    Die designs for injection molding require a wall taper be built into the dies in order to remove the molded components. Thus, as is evident in FIG. 2, body portion  20  includes a hollow first body member  40  that has a tapered wall  42  that extends over a substantial portion of the length of the first body member. The internal taper  42  is preferably matched by an external taper  44  to define a generally constant wall thickness over a substantial length of the first body member. A first end  46  of the first body member is open while the second end  48  defines an integral end wall or cap having an integral leg portion  50  extending therefrom that ultimately defines one of the legs  22 ,  24  of the discharge chamber assembly. It will be further appreciated that opening  52  is provided in the leg to receive the lead or conductor assembly that provides electrical connection to the electrodes  14 ,  16 .  
         [0018]    In accordance with the present invention, a hollow cylindrical portion  60  extends inwardly a predetermined dimension from the open first end to a location  62 . The hollow cylindrical portion  60  cooperates with a second body member  70 , and particularly a cylindrical portion  72  thereof Shoulder  74  is adapted to abuttingly engage the outer end of the first body member and provide a positive fit and insertion of the second body member into the first body member. Preferably, the second body member or end cap includes an integrally formed leg  76  having an opening  78  adapted to receive the other lead/conductor/electrode assembly as is apparent to one skilled in the art.  
         [0019]    As illustrated in FIG. 2, the right-hand end of the hollow portion of the first body member  40  has a first cross-sectional dimension or diameter  90 . The wall taper  42 , for example on the order of one-half degree (0.5°) proceeds or enlarges to a second dimension at location  62  shown by the second cross-sectional dimension or diameter  92 . From location  62  to the open end, i.e., leftwardly as shown, the hollow cylindrical portion  60  has a substantially constant diameter for advantageously joining to the non-tapered surface  72  of the end cap  70 . The taper angle (on the order of 0.5° or where the open end has a diameter approximately three to six percent (3-6%) greater than the closed end) is identified by reference angle  94  and proceeds along the substantial or major length of the hollow body member as referenced by longitudinal dimension  96 . The axial length identified by reference numeral  98  represents the constant diameter portion. It is contemplated that the axial length  98  is dimensioned to fully receive the non-tapered surface  72 .  
         [0020]    In contrast to a tapered portion of a first body member engaging a tapered portion of the second body member as shown in the 6,004,503 patent, the two surfaces  60 ,  72  mated in accordance with the present invention are parallel. This ensures that the hollow cylindrical part can be efficiently ejected off the pin and still achieve the desired differential shrinkage and monolithic join produced with the end cap as illustrated in FIG. 2. Also, using a non-tapered plug or end cap is desirable in achieving an interference fit and dimensional control during sintering. As the outside tube shrinks around a tapered plug, it may have a tendency to push the plug outwardly fomr the tube. This would not occur with the arrangement of the present invention. Seal voids associated with the prior designs are avoided and the joined interface provides the desired hermetic seal between the components.  
         [0021]    [0021]FIG. 3 shows a different conformation of the second body member or end cap  70 ′ that is received in the open end of the first body member  40 ′ and sealed thereto. For purposes of consistency and brevity, like components are identified by like reference numerals with a primed suffix (′) and new components are identified by new numerals. Unless specifically noted, the structure and function is substantially identical to the embodiment of FIG. 2. The mating surfaces are monothlically joined and the parallel interface extending over a few millimeters reduces the level of seal voids.  
         [0022]    The invention has been described with reference to the exemplary embodiment. Modifications and alterations will occur to others upon reading and understanding this specification. For example, the concepts of the present invention may be applicable to single ended ceramic discharge lamps and methods. Likewise, the configuration of the first and second body member may be additionally altered from the arrangements shown in FIGS. 2 and 3, without departing from the present invention. In any event, the novel arrangement of providing a taper along a substantial portion of the hollow cylindrical member that is parallel at one end to provide an acceptable monolithic join will still be achieved. It will also be appreciated that the body member and the end cap can adopt a wide variety of configurations and are not limited to the conformations shown in the drawings. For example, curved recesses can be integrally formed in the body member and/or end cap, or the legs can be formed in one of the body member and end cap. The invention is intended to include such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.