Abstract:
A spark plug including a ground electrode which has an excellent heat sinking ability. The ground electrode includes a core material therein. Heat received from a combustion chamber during a drive of an internal-combustion engine can be conducted to the core material. More effective heat sinking ability can be achieved because of the core material.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a spark plug used for internal-combustion engines and including a ground electrode which has a metal-made core material excellent in thermal conductivity. 
   BACKGROUND OF THE INVENTION 
   Conventionally, a spark plug is used for igniting an internal-combustion engine. An ordinary spark plug is comprised of: a metal shell radially surrounding and holding a circumference of an insulator in which a center electrode is accommodated in an axial bore; and a ground electrode in which one end thereof is joined to a front end of the metal shell and the other end thereof is bent towards a front end of the center electrode so as to face each other and form a spark discharge gap therebetween. Such a spark plug tends to be exposed at a high temperature because the ground electrode projects to a combustion chamber when the spark plug is attached to an engine head. Thus, since the heat load applied to the ground electrode becomes greater, an improvement in heat sinking ability (thermal conductivity) of the ground electrode has become highly demanded. 
   Japanese Patent Application Laid-Open (kokai) No. 2005-135783 discloses a ground electrode comprised of an electrode base material (e.g., nickel base alloy or the like) having corrosion resistance and oxidation resistance. A core material (e.g., Cu, Ag or the like) having an excellent thermal conductivity is embedded in the electrode base to promptly conduct heat generated during the engine drive to a metal shell. Generally, such a ground electrode is formed through an extrusion molding process to produce an integrated body where a cup-like electrode base material accommodates the core material therein. The thus-produced ground electrode is joined to the metal shell at a rear end portion thereof where a front end side in the extruding direction serves as a front end portion and a rear end side serves as the rear end portion. In the electrode base material, the core material is disposed so as to taper towards the front end side of the ground electrode. 
   However, since the power of an internal-combustion engine has been recently stronger, the heat load applied to a ground electrode has been greater in connection with a fuel combustion temperature in a combustion chamber. When a core material assumes a tapered shape towards a front end side of the ground electrode as mentioned above, the core material is disposed in a vicinity of an axis line and not near an outer circumference face in the front end portion of the ground electrode. Thus, heat that the front end portion of the ground electrode receives is unlikely to be promptly conducted to the metal shell, and the heat sinking ability of the ground electrode tends to be insufficient. 
   The present invention has been developed in view of the above problems, and provides a spark plug including a ground electrode which has an excellent heat sinking ability. 
   SUMMARY OF THE INVENTION 
   In order to solve the above problems, there is provided a spark plug according to a first embodiment, comprising: a center electrode; an insulator having an axial bore that extends along an axial direction of the center electrode and that accommodates the center electrode therein; a metal shell surrounding the insulator in a radial direction so as to hold the insulator therein; and a ground electrode having one end bonded to the metal shell and the other end bent so that a side face of the ground electrode is located opposed to the center electrode, and accommodating a core material which extends from one end to the other end of the ground electrode along a first direction, wherein, when an outline of the core material is defined by projecting the core material onto the side face of the other end of the ground electrode, at least either a second part located on a first segment side in a second direction and close to the edge of the other end of the ground electrode or a third part located on a second segment side in the second direction and close to the edge of the other end of the ground electrode is disposed on a side towards the edge of the other end of the ground electrode with respect to a first part located in a center with respect to the second direction, which is perpendicular to the first direction, on a third segment that connects the first segment and the second segment at the edge of the other end both of which constitute the outline of the core material and extend along the first direction. 
   In addition to the composition of the present invention according to the first embodiment, there is provided a spark plug according to a second embodiment, wherein an electrode tip is bonded to the side face of the other end of the ground electrode. 
   In addition to the composition of the present invention according to the second embodiment, there is provided a spark plug according to a third embodiment, wherein the electrode tip is bonded to the side face of the ground electrode through resistance welding, and wherein, when the outline of the core material and that of a bonding face of the electrode tip bonded to the side face are defined by projecting the core material and the bonding face, respectively, onto the side face of the other end of the ground electrode, a fourth part located in a furthest position away from the edge of the other end of the ground electrode on the outline of the bonding face of the electrode tip is disposed between the first part located on the outline of the core material defined by projecting the core material onto the side face and at least either the second part or the third part in the first direction. 
   In addition to the composition of the invention according to the third embodiment, there is provided a spark plug according to a fourth embodiment, wherein, when the outline of the core material and that of the bonding face of the electrode tip bonded to the side face are projected, respectively, onto the side face of the other end of the ground electrode, the outline of the bonding face of the electrode tip and that of the core material are kept in a noncontact state. 
   In addition to the composition of the invention according to any one of embodiments from second to fourth, there is provided a spark plug according to a fifth embodiment, comprising the columnar shape electrode tip with an outer diameter of 2 mm or more, wherein, when the outline of the core material and that of the bonding face of the electrode tip bonded to the side face are defined by projecting the core material and the bonding face, respectively, onto the side face of the other end of the ground electrode, at least either a representation of W 2 &gt;R or W 3 &gt;R is satisfied, where a position of the central axis of the electrode tip is regarded as a location C, a radius of the electrode tip is regarded as R, a distance between the position of second part and the location C in the second direction is regarded as W 2 , and a distance between the position of the third part and the location C in the second direction is regarded as W 3 . 
   In addition to the composition of the invention according to any one of embodiments from second to fifth, there is provided a spark plug according to a sixth embodiment, comprising the columnar shape electrode tip with an outer diameter of 2 mm or more, wherein, when the outline of the core material and that of the bonding face of the electrode tip bonded to the side face are projected, respectively, onto the side face of the other end of the ground electrode, at least either a representation of L 2 &lt;L 1  or L 3 &lt;L 1  is satisfied as is R&lt;L 1 , where a position of the central axis of the electrode tip is regarded as a location C, a radius of the electrode tip is regarded as R, a distance between the position of first part and the location C in the first direction is regarded as L 1 , a distance between the position of second part and the location C in the first direction is regarded as L 2 , and a distance between the position of the third part and the location C in the first direction is regarded as L 3 . 
   In the spark plug according the first embodiment, since at least either the second part or the third part is disposed on a side towards the edge of the other end of the ground electrode with respect to the first part on the third segment that constitutes the outline of the core material defined by projecting the core material onto the side face of the ground electrode, the core material can be located on the further edge side of the front end portion and close to the outer circumference face. With this composition, in the front end portion of the ground electrode, heat received from a combustion chamber during a drive of an internal-combustion engine can be conducted to the core material from the position on the further front end side and close to the outer circumference face. As a result, more effective heat sinking ability of the front end portion of the ground electrode can be achieved. 
   The composition that the core material can be located on the further edge side of the front end portion and close to the outer circumference face is still effective for the case where an electrode tip for improving a durability of an electrode in a spark discharge gap is provided in the front end portion of the ground electrode according to the second embodiment. As mentioned above, in addition to the improvement in the heat sinking ability of the front end portion of the ground electrode, heat that the electrode tip receives can be smoothly conducted to the core material. As a result, the heat sinking ability near the spark discharge gap can be further improved. 
   When such an electrode tip is bonded to the front end portion of the ground electrode through the resistance welding, heat produced in a welding area at the time of bonding is conducted through the core material whereby it is unlikely to obtain sufficient bonding strength. In this case, as in the present invention according to the third embodiment, the fourth part on the outline of the bonding face of the electrode tip which is defined by projecting the bonding face onto the side face of the ground electrode is located between the first part on the outline of the core material and at least either the second part or the third part in the first direction. With this composition, a portion can be reliably provided where the outline of the electrode tip and that of the core material defined by projecting the electrode tip and the core material, respectively, onto the side face of the ground electrode do not overlap each other, thereby preventing heat during the resistance welding from being conducted to the core material. As a result, the electrode tip and the ground electrode can be further effectively bonded together. On the other hand, since at least either the second part or the third part since on the outline of the core material is disposed on the further front end side of the ground electrode with respect to the fourth part on the outline of electrode tip, the core material and the electrode tip are disposed close to each other in the light of the relation between the first part on the outline of the core material and the fourth part on the outline of the electrode tip. Thus, heat that the electrode tip receives can be smoothly conducted to the core material whereby the heat sinking ability near the spark discharge gap can be further improved. 
   Further, as in the present invention according to the fourth embodiment, when the outline of the bonding face of the electrode tip and that of the core material are kept in the noncontact state, both of which are defined by projecting the bonding face and the core material, respectively, onto the side face of the ground electrode, heat during the resistance welding is more effectively prevented from being conducted to the core material, thereby improving the bonding strength. Furthermore, since the core material can extend towards the further front end side of the front end portion by diverting the position of the electrode tip, the heat that the electrode tip receives can be conducted to the core material whereby the heat sinking ability near the spark discharge gap can be further improved. 
   As in the present invention according to the fifth embodiment or the sixth embodiment, when a positional relationship between the outline of the bonding face of the electrode tip and that of the core material both of which are defined by projecting the bonding face and the core material, respectively, onto the side face of the ground electrode is more specifically defined, the bonding strength between the electrode tip and the ground electrode can be sufficiently secured as well as improving the heat sinking ability of the front end portion of the ground electrode including heat conduction from the electrode tip to the ground electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial sectional view of spark plug  100 . 
       FIG. 2  is an enlarged sectional view showing around the ground electrode  30 . 
       FIG. 3  is a sectional view showing the ground electrode  30  seen from the arrow direction in a two-dot chain line S-S of  FIG. 2 . 
       FIG. 4  is a diagram showing a positional relation between an electrode tip  91  and a core material  35  whose outline is defined by projecting the core material  35  onto an inner face  33  of the ground electrode  30  from the thickness direction. 
       FIG. 5  is a perspective view showing an outline of the core material  35  embedded in a front end portion  31  of the ground electrode  30  so as to show a positional relation between the core material  35  and the electrode tip  91 . 
       FIG. 6  is a partial sectional view showing a composition of a ground electrode base material  130  which serves as a base for the ground electrode  30 . 
       FIG. 7  is a partial sectional view showing an extrusion molding process of the ground electrode base material  130  which is performed using a dice  200 . 
       FIG. 8  is a sectional view of the dice  200  seen from the arrow direction in a single dotted-line X-X of  FIG. 7 . 
       FIG. 9  is a sectional view of the dice  200  seen from the arrow direction in a single dotted-line Y-Y of  FIG. 7 . 
       FIG. 10  is a sectional view of the dice  200  seen from the arrow direction in a single dotted-line Z-Z of  FIG. 7 . 
       FIG. 11  is a diagram showing a way how to obtain the ground electrode  30  by cutting the ground electrode base material  130  formed by an extrusion molding. 
       FIG. 12  is a diagram showing a positional relation between the electrode tip  91  and a core material  335  whose outline is defined by projecting the core material  335  onto an inner face  333  of a ground electrode  330  in the thickness direction according to the modification. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Hereafter, an embodiment of a spark plug embodying the present invention will be described with reference to the drawings. First, referring to  FIG. 1 , a composition of a spark plug  100  will be explained.  FIG. 1  is a partial sectional view of the spark plug  100 . It is noted that, in the axial direction “O”, a side where a center electrode  20  is accommodated in an axial bore  12  of an insulator  10  is regarded as a front end side of the spark plug  100 , and a side where a terminal metal fitting  40  is held is regarded as a rear end side of the spark plug  100  in the specification. 
   As shown in  FIG. 1 , the spark plug  100  is comprised of: an insulator  10 ; a metal shell  50  provided in a generally central portion of the insulator  10  in the longitudinal direction and holding the insulator  10 ; a center electrode  20  accommodated in an axial bore  12  of the insulator  10  in the axial direction; a ground electrode  30  having one end (a base portion  32 ) welded to a front end face  57  of the metal shell  50  and the other end (a front end portion  31 ) bent towards a front end portion  22  of the center electrode  20 ; and a terminal metal fitting  40  provided at a rear end portion of the center electrode  20 . 
   First, the insulator  10  constituting an insulating body of the spark plug  100  will be described. The insulator  10  is a tubular insulating member including the axial bore  12  in the axial direction “O”, which is formed by sintering alumina or the like as is commonly known. A flange portion  19  having the largest outer diameter is formed in a generally center with respect to the axial direction “O”, and a rear end side body portion  18  is formed at the rear end side of the flange portion  19 . Further, a corrugate portion  16  used for extending a creepage distance is formed in the rear end side of the rear end side body portion  18 . A front end side body portion  17  having a smaller outer diameter than that of the rear end side body portion  18  is formed at the front end side of the flange portion  19 . A long leg portion  13  having a smaller outer diameter than that of the front end side body portion  17  is formed at further front end side of the front end side body portion  17 . The long leg portion  13  tapers off toward the front end side, and the long leg portion  13  is exposed to the combustion chamber when the spark plug  100  is assembled in an internal-combustion engine (not shown). 
   Next, the center electrode  20  will be explained. The center electrode  20  is a rod-shaped electrode wherein a metal core  23  for facilitating heat sinking and made of Cu, Ag or the like as a elemental substances, or an alloy containing Cu, Ag or the like as a main component is embedded in a center portion of an electrode base material  21  made of nickel-system alloy or the like such as INCONEL (trade name) 600 or 601. A part of the front end portion  22  of the center electrode  20  projects from a front end face of the insulator  10  and tapers off toward the front end side. A columnar electrode tip  90  made of, for example, a noble metal, such as Pt, is welded through resistance welding to a front end face of the front end portion  22  so as to align its column axis with an axis of the center electrode  20 . The center electrode  20  is electrically connected to the upper terminal metal fitting  40  through a sealing body  14  and a ceramic resistance  3  provided inside the axial bore  12 . A high-tension cable (not shown) is connected to the terminal metal fitting  40  through a plug cap (not shown), to which high voltage is applied. 
   Next, the metal shell  50  will be described. The metal shell  50  holds the insulator  10  to fix the spark plug  100  to an engine head of the internal-combustion engine (not shown). The metal shell  50  holds the insulator  10  so as to surround the flange portion  19 , the front end side body portion  17  and the long leg portion  13  from the rear end side body portion  18  which is close to the flange portion  19  of the insulator  10 . The metal shell  50  is comprised of a low-carbon-steel material and includes a tool engagement portion  51  to which a spark plug wrench (not shown) is fit at the rear end side, and a screw portion  52  which screws to an engine head provided at an upper part of the internal-combustion engine (not shown). 
   Annular ring members  6 ,  7  are interposed between the tool engagement portion  51  of the metal shell  50  and the rear end side body portion  18  of the insulator  10 . Further, talc powder  9  is filled between the ring members  6 ,  7 . A sealing portion  53  is formed at the rear end side of the tool engagement portion  51 . The insulator  10  is forced toward the front end side in the metal shell  50  through the ring members  6 ,  7  and the talc  9  by sealing the sealing portion  53 . A step portion  15 , formed between the front end side body portion  17  and the long leg portion  13  of the insulator  10 , is supported by a step portion  56  formed in the inner periphery of the metal shell  50 . A packing  8  is disposed between step portion  15  and step portion  56 . As a result, the metal shell  50  and the insulator  10  are integrated. Airtightness between the metal shell  50  and the insulator  10  is maintained by the packing  8 , which prevents combustion gas from flowing out from the combustion chamber (not shown) through spark plug  100 . A flange portion  54  is formed in the center portion of the metal shell  50 , and a gasket  5  is inserted in and fitted to the vicinity of the rear end side of the screw portion  52  (upper portion in FIG.  1 )—i.e., fitted to a seat surface  55  of the flange portion  54 . 
   Next, the ground electrode  30  will be described with reference to  FIGS. 1 to 5 .  FIG. 1  is a partial sectional view of the spark plug  100 .  FIG. 2  is an enlarged sectional view showing around the ground electrode  30 .  FIG. 3  is a sectional view taken along lines S-S of  FIG. 2  showing the ground electrode  30 .  FIG. 4  is a diagram showing a positional relation between an electrode tip  91  and a core material  35  whose outline is defined by projecting the core material  35  onto an inner face  33  of the ground electrode  30  from the thickness direction.  FIG. 5  is a perspective view showing an outline of the core material  35  embedded in the front end portion  31  of the ground electrode  30  so as to show a positional relation between the core material  35  and the electrode tip  91 . 
   The ground electrode  30  shown in  FIG. 1  generally has the rear end portion  32  joined to the front end face  57  of the metal shell  50 . The front end portion  31  of ground electrode  30  is bent so as to face the front end portion  22  of the center electrode  20 . The electrode tip  91  made of a noble metal, such as Pt, is bonded to the inner face  33  of the ground electrode  30 , which is one of the side faces and is located opposed to the center electrode  20 . 
   The ground electrode  30  shown in  FIG. 2  is comprised of: an electrode base material  34  made of a nickel alloy, such as INCONEL (trade name) 600 or 601, and having an excellent corrosion resistance; and the core material  35  for facilitating the heat sinking having a better thermal conductivity than that of the electrode base material  34 . As shown in  FIG. 3 , the ground electrode  30  assumes a generally plate-like rectangular shape in the cross-section perpendicular to its axis line P. As shown in  FIG. 2 , in the ground electrode  30 , one of two wide side faces, identified as the inner face  33 , is located so as to be opposed to the center electrode  20 . The rear end portion  32  of ground electrode  30  is joined to the front end face  57  of the metal shell  50 . The front end portion  31  of ground electrode  30  is bent toward the inner face  33  side and forms a spark discharge gap between an electrode tip  91  bonded to the inner face  33  and an electrode tip  90  of the center electrode  20 . For the sake of convenience, when referring to the side faces of the ground electrode  30 , a direction perpendicular to the axis line P in a wide side face is referred to as a width Q direction of the ground electrode  30 . (See  FIG. 4 ). A direction perpendicular to the axis line P in a narrow side face is referred to as a thickness direction of the ground electrode  30 . 
   As shown in  FIGS. 2 and 3 , the core material  35  embedded in the electrode base material  34  has a double structure, and is comprised of: an outer core  36  made of a metal containing Cu, Fe, Ag, Au or the like as an elemental substance, or an alloy containing Cu, Fe, Ag, Au or the like as a main component; and an center core  37  located inside the outer core  36  and made of a metal containing Ni or Fe as an elemental substance or an alloy containing Ni or Fe as a main component. As shown in  FIGS. 2 to 5 , the core material  35  is embedded in the electrode base material  34  so as to align with the axis line P of the ground electrode  30 , extends like a flat plate shape so as to align with the plate-like ground electrode  30  and extends to a vicinity area where the electrode tip  91  of the front end portion  31  is bonded to. 
   As shown in  FIG. 4 , when the core material  35  is seen from the thickness direction of the ground electrode  30 , the core material  35  is divided into two forks in the front end portion  31  and extends towards an edge  38  of the front end portion  31 . An outline defined by projecting the core material  35  on the inner face  33  of the front end portion  31  of the ground electrode  30  is generally comprised of: two segments (a first segment and a second segment) extending along the axis line P; and a third segment connecting the first segment and the second segment at the edge  38  of the front end portion  31 . The first segment and the second segment are a segment AB and a segment DE, respectively, extended generally in parallel to the axis line P (this direction corresponds to a “first direction” in the invention), and are equivalent to the outline of side edges of the core material  35  extending to the rear end portion  32  of the ground electrode  30  (not illustrated in  FIG. 4 ). Further, the third segment is a segment BFGHE which connects the segments AB and DE at the edge  38  of the front end portion  31  of the ground electrode  30  in the width Q direction (this direction corresponds to a “second direction” in the invention). The segment AB, the segment DE and the segment BFGHE correspond to the “first segment”, the “second segment” and the “third segment”, respectively, in the invention. 
   The segment BFGHE constituting the outline of the core material  35  assumes a generally “M” shape in the embodiment. More particularly, points F, G and H on the segment BFGHE satisfy the following conditions. First, a point on the segment BFGHE located in the center with respect to the width Q direction is regarded as the point G. A point located at the segment AB side with respect to the point G and nearest to the edge  38  of the front end portion  31  is regarded as the point F. Similarly, a point located at the segment DE side with respect to the point G and nearest to the edge  38  of the front end portion  31  is regarded as the point H. At this time, the segment BFGHE assumes a shape in which the positions of the points F, H are nearest to the edge  38  of the front end portion  31  with respect to the point G in the axis line P direction. The points G, F and H are referred to as “a first part”, “a second part” and “a third part”, respectively, in the invention. 
   The electrode tip  91  bonded to the inner face  33  of the front end portion  31  of the ground electrode  30  assumes a columnar shape in the embodiment. One side perpendicular to an axis line of the electrode tip  91  is in contact with the inner face  33  of the ground electrode  30  as a bonding face and, with this state, welded to the front end portion  31  through resistance welding. On the inner face  33  of the ground electrode  30  in the embodiment, the positional relation between the contact face of the electrode tip  91  before bonding and the outline of the core material  35  defined by projecting the core material  35  onto the inner face  33  is specified as follows. 
   First, before bonding the ground electrode  30  and the electrode tip  91 , the outline of a contact face (the bonding face) of the electrode tip  91 , which is in contact with the inner face  33 , is not in touch with the outline of the core material  35  defined by projecting the core material  35  onto the inner face  33 . That is, the position of the core material  35  and that of the electrode tip  91  does not overlap each other in the thickness direction of the ground electrode  30 . Next, a point on the outline of the bonding face of the electrode tip  91  bonded to the inner face  33  which is the furthest position away from the edge  38  in the axis line P direction is regarded as a point I. At this time, in the axis line P direction, the point I is located in a position at least either between the point G and the point F or between the point G and the point H. That is, a part of the outline (including the point I) of the bonding face of the electrode tip  91  is located in a valley of the “V” shaped segment FGH, which is constituted by the points F, G and H on the segment BFGHE. The point I corresponds to a “fourth part” in this invention. 
   The electrode tip  91  of the embodiment assumes a columnar shape and has an outer diameter of 2 mm or more. More particularly, the positional relation between such an electrode tip  91  and the core material  35  will be specified as follows. First, on the inner face  33 , a point corresponding to a center axis of the bonding face of the electrode tip  91  is regarded as a location C, and a radius of the bonding face is regarded as R. In the axis line P direction, a distance between the point G and the location C is regarded as L 1 , the distance between the point F and the location C is regarded as L 2  and the distance between the point H and the location C is regarded as L 3 . Further, in the width Q direction (i.e., upper side to lower side direction in  FIG. 4 ), the distance between the point F and the location C is regarded as W 2 , the distance between the point H and the location C is regarded as W 3 . At this time, the positional relation between the electrode tip  91  and the core material  35  satisfy an expression of R&lt;L 1  and at least either expression of W 2 &gt;R or W 3 &gt;R, and further satisfying at least either the expression of L 2 &lt;L 1  or L 3 &lt;L 1 . 
   Thus, in the ground electrode  30 , the core material  35  is divided into two forks in the front end portion  31  and extends toward the edge  38  so as to avoid an area in the thickness direction where the electrode tip  91  is disposed. With this construction, the core material  35  can be disposed nearest to the edge  38  of the front end portion  31 , as well as closer to an outer circumference face of the ground electrode  30 . As a result, the heat which the ground electrode  30  receives from the combustion chamber can promptly be conducted to the core material  35 , thereby efficiently conducting the heat to the metal shell  50  through the core material  35 . On the other hand, when extending the core material  35  to a position nearer to the edge  38  of the front end portion  31 , the core material  35  is disposed so as to avoid the position of the electrode tip  91 . As a result, the heat required for the resistance welding is unlikely to be drawn through the core material  35  when welding the electrode tip  91  to the front end portion  31  by the resistance welding, thereby preventing a poor bonding between the ground electrode  30  and the electrode tip  91 . Of course, when the electrode tip  91  is bonded to the ground electrode  30  through laser welding instead of resistance welding, it is possible to avoid the poor bonding therebetween. However, since the electrode tip  91  according to this embodiment has the outer diameter of 2 mm or more and assumes the columnar shape, an area not in contact with the ground electrode  30  may remain in the central area of the bonding face when the laser welding is used for bonding such a large bonding face of the electrode tip  91  to the ground electrode  30  because the laser welding is performed to a peripheral edge of the bonding face. In the ground electrode  30  which receives the heat from an engine drives, the electrode tip  91  is likely to drop out due to the long-term use of the spark plug. Thus, the columnar electrode tip  91  having the outer diameter of 2 mm or more is preferably bonded with the entire bonding face to the ground electrode  30  by the resistance welding as mentioned above. 
   To explain the positional relation between the electrode tip  91  and the core material  35 , the bonding face in the invention means a contact face being in contact with the inner face  33  of the ground electrode  30  at the time of the resistance welding of the electrode tip. Since the contact face after the resistance welding is melt with the electrode base material  34  of the ground electrode  30 , it is difficult to identify the outline of the electrode tip. In this case, in order to identify the outline of the electrode tip  91 , an area defined by a virtual line which extends from the outer circumference face of the electrode tip  91  and is perpendicular to the inner face  33  is deemed to be a bonding face when, for example, the electrode tip  91  assumes a columnar shape according to the embodiment and has a bonding face perpendicular to the axis line of the electrode tip  91 . Similarly, when the electrode tip  91  assumes a prismatic shape or a disc shape, an area defined by a virtual line perpendicular to the inner face  33  and extending from the outer circumference face, which forms the outline of the contact face, is deemed to be the bonding face. 
   The virtual line deemed to be the outline of the contact face should not overlap with the outline of the core material  35  on the inner face  33 . In this case, the outline of the core material  35  may be identified by, for example, an X-ray of the inner face  33  of the ground electrode  30  or the cross-section of the ground electrode  30  in the thickness direction. Although a part of melting portion of the electrode tip  91  resulting from the welding may overlap with the thus-identified outline of the core material  35 , a sufficient effect can be obtained as long as the virtual line deemed to be the outline the bonding face of the electrode tip  91  does not overlap with (in a noncontact state) the outline of the core material  35 , in the light of the prevention of a deterioration in the bonding strength caused by the core material  35  that is likely to draw the heat produced during the resistance welding. 
   Next, a method for manufacturing the ground electrode  30  having the two-fork shaped core material  35  in the front end portion  31  will be described with reference to  FIGS. 6 to 11 .  FIG. 6  is a partial sectional view showing a composition of a ground electrode base material  130  which serves as a base for the ground electrode  30 .  FIG. 7  is a partial sectional view showing an extrusion molding process of the ground electrode base material  130  which is performed using a dice  200 .  FIG. 8  is a sectional view of the forming die  200  seen from an arrow direction in a single dotted-line X-X of  FIG. 7 .  FIG. 9  is a sectional view of the forming die  200  seen from the arrow direction in a single dotted-line Y-Y of  FIG. 7 .  FIG. 10  is a sectional view of the forming die  200  seen from the arrow direction in a single dotted-line Z-Z of  FIG. 7 .  FIG. 11  is a diagram showing a way how to obtain the ground electrode  30  by cutting the ground electrode base material  130  formed by an extrusion molding. 
   As shown in  FIG. 6 , in the manufacture process of the ground electrode  30 , a cylindrical nickel alloy material serving as a base for the electrode base material  34  is formed into a bottomed cylindrical shape through a cold forging process to thereby form an electrode base material  134 . A columnar center core base material  137  serving as a base for the center core  37  is inserted in a cylindrical outer core base material  136  serving as a base for the outer core  36  so as to form an integrated body. The thus-produced integrated body is formed into a columnar core base material  135 , serving as a base for the core material  35 , with a flange portion so as to engage with a concave portion of the electrode base material  134  through the cold forging process or a cutting process. The core base material  135  is inserted in and fitted to the concave portion of the electrode base material  134  to thereby form the ground electrode base material  130 . 
   Next, the ground electrode base material  130  is inserted in an aperture formed in a die  200  from the cylindrical bottom side of the electrode base material  134  to perform an extrusion molding using a punch  250 . As shown in  FIG. 8 , the die  200  has an inner circumference face  201  at the side where the ground electrode base material  130  is inserted, and the inner circumference face  201  assumes a circular sectional shape so as to match with the outer circumference of the electrode base material  134 . As shown in  FIG. 10 , an inner circumference face  203  at the side from which the ground electrode base material  130  is extracted is formed into a generally rectangular shape (refer to  FIG. 3 ) so as to match with the sectional shape of the ground electrode  30 . Further, as shown in  FIG. 9 , an inner circumference face  202  connecting the inner circumference face  201  and the inner circumference face  203  is formed into a tapered shape. As shown in  FIG. 7 , the ground electrode base material  130  is inserted in the die  200  and subjected to the extrusion molding using the punch  250 . Then, the electrode base material  130  is extended in the axis line P direction to thereby form a columnar body which the core base material  135  and the electrode base material  134  are adjacently joined. 
   The ground electrode base material  130  assumes a circular shape in the sectional view perpendicular to the axis line P. The ground electrode base material  130  is crushed flatly so that the cross-sectional shape thereof matches to the shape of the inner circumference face  203  of the die  200 . Thus, in the sectional view of the ground electrode  30  shown in  FIG. 3 , a portion corresponding to the center with respect to the width Q direction is compressed the most in the thickness direction. Since a material forming a bottom portion of the bottomed cylindrical electrode base material  134  occupies the most of the center area in the ground electrode  30  in the width Q direction after forming the ground electrode  30 , the core base material  135  in the center area with respect to the width Q direction is prevented from being extruded compared to the case of both ends of the core base material  135  with respect to the width Q direction. For this reason, in the front end portion  131  of the ground electrode base material  130 , the core base material  135  is divided into two forks towards the front direction where the ground electrode base material  130  is extruded when the core base material  135  is projected onto the inner face  33  in the thickness direction. 
   The rear end side of the thus-extrusion molded ground electrode base material  130  is cut after being extended to a predetermined length to thereby complete the ground electrode  30 . The rear end portion  32  at the rear end side of the extrusion direction (the side to be cut) is joined to the front end face  57  of the metal shell  50  produced through a separate process. At this time, the ground electrode  30  is joined so that a side thereof in the thickness direction serves as the inner face  33  and faces the central axis of the metal shell  50 . Then, the electrode tip  91  is bonded to the inner face  33  of the front end portion  31  through the resistance welding. Since the core material  35  is formed into the two-fork shape as mentioned above, and the core material  35  and the electrode tip  91  do not overlap each other in the thickness direction of the ground electrode  30 , the heat produced during the resistance welding is unlikely to be drawn by the core material  35 , thereby preventing the deterioration in the bonding strength. Further, the insulator  10  produced through a separate process and integrally holding the center electrode  20  and the terminal metal fitting  40  is inserted in the metal shell  50  and subjected to caulking. The ground electrode  30  has one face in the thickness direction which serves as the inner face  33  and is bent so that the inner face  33  faces an inner side and is opposed to the front end portion  22  of the center electrode  20 . As a result, the spark plug  100  having a spark discharge gap between the electrode tip  91  of the ground electrode  30  and the electrode tip  90  of the center electrode  20  is completed. 
   The present invention is not particularly limited to the embodiments described above but may be changed or modified in various ways. For example, although the electrode tip  91  assumes a columnar shape in the embodiment, it may assume a square pillar, a pyramid or a cone shape, as well as a disk or a rectangular plate shape. Further, the electrode tip  90  is provided on the center electrode  20 , and the electrode tip  91  is provided on the ground electrode  30  in the embodiment. However, the electrode tip may be provided only on either of the sides—i.e., it is not necessarily for the electrode tips  90 ,  91  to be provided on both the center electrode  20  and ground electrode  30 , respectively, as in the above embodiment. 
   Furthermore, in the embodiment, although the outline of the core material  35  defined by projecting the core material  35  onto the inner face  33  of the front end portion  31  of the ground electrode  30  in the thickness direction assumes a two-fork shape and extends towards the edge  38 , the outline of the core material  35  does not necessarily assume the two-fork shape. For example, in a ground electrode  330  as shown in  FIG. 12 , an outline of a core material  335  defined by projecting the core material  335  onto an inner face  333  in the thickness direction (i.e., front page to back page direction where  FIG. 12  is shown) is comprised of: a segment AB and a segment DE which are, as similar to the embodiment, deemed to extend generally in parallel to the axis line P; and a segment BFGHE which connects the segment AB and the segment DE at an edge  338  of a front end portion  331 . A point on the segment BFGHE located in the center with respect to the width Q direction and perpendicular to the axis line P is regarded as a point G. A point on the segment BFGHE located at the segment AB side with respect to the point G and nearest to the edge  338  of the front end portion  331  is regarded as a point F. Further, a point on the segment BFGHE located at the segment DE side with respect to the point G and nearest to the edge  338  of the front end portion  331  is regarded as a point H. At this time, while the position of the point F on the segment BFGHE is nearer to the edge  338  than that of the point G in the axis line P direction, the position of the point H may be the same as that of the point G or away from the point G with respect to the edge  338 . That is, the segment BFGHE which constitutes the outline of the core material  335  may assume a shape which protrudes towards the edge  338  on either the segment AB side or the segment DE side from the center with respect to the width Q direction. 
   Similar to the embodiment, on the outline of the bonding face of the electrode tip  91  defined by projecting the bonding face onto the inner face  333  (or a virtual outline regarded as the outline of the bonding face), a point I located in the furthest position away from the edge  338  in the axis line P direction is preferably between the point G and the point F, and the outline (or a virtual outline regarded as the outline of the bonding face) of the bonding face of the electrode tip  91  preferably does not overlap (noncontact state) with the outline of the core material  335  in the thickness direction of the ground electrode  30 . More particularly, the following conditions are preferably satisfied. In the axis line P direction, a distance L 1  between the point G and a location C of the center axis of the electrode tip  91  is longer than a radius R of the bonding face of the electrode tip  91 , a distance L 2  between the point F and the location C is shorter than the distant L 1  and a distant W 2  between the location C and the point F is longer than the radius R in the width Q direction. In this way, the outline of the core material  335  extends towards the edge  338  on the inner face  333  of the front end portion  331  of the ground electrode  330 , while avoiding overlapping with the outline (or a virtual outline regarded as the outline of the bonding face) of the bonding face of the electrode tip  91 . Thus, heat can be successfully conducted from the front end portion  331  of the ground electrode  330 , thereby preventing the deterioration in the bonding strength of the electrode tip  91 . 
   However, the above description will not limit the state where the outline (or a virtual outline regarded as the outline of the bonding face) of the bonding face of the electrode tip  91  defined by projecting the bonding face onto the inner face  33  and the outline of the core material  35  are not in contact with each other. As in the embodiment, even if the outline of the electrode tip  91  overlaps with that of the core material  35  in the thickness direction, the proportion of the core material  35  occupying in the outline of the electrode tip  91  can be lowered by way of forming at least either the point F or the point H on the outline of the core material  35  defined by projecting the core material  35  onto the inner face  33  so as to extend towards the front end side of the ground electrode  30  with respect to the point G. That is, even in such a composition, the heat generated at the time of the resistance welding is unlikely to be drawn by the core material  35 , thereby preventing the deterioration in the bonding strength.