Abstract: An employed rear mold is comprised of a plurality of mold pieces. The plurality of mold pieces include a first mold piece which forms a first partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface, and a second mold piece which is located axially forward of the first mold piece and forms a second partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface. Releasing the molded body includes starting to move the first mold piece rearward in relation to a rear molded portion, and, subsequently to starting to move the first mold piece, starting to move the second mold piece rearward in relation to the rear molded portion, thereby disassembling the rear mold into the plurality of mold pieces.

Abstract: A method for manufacturing an insulator for a spark plug includes a molding process of forming a cylindrical molded product having an axial hole that extends in a direction of an axial line, by means of injection molding using a mold that has a columnar cavity therein and a bar-shaped member disposed in the cavity and extending in the direction of the axial line. In this method, the molding process includes an injection step of injecting a material containing a ceramic. In the injection step, the material is injected into the cavity from a plurality of injection openings that are opened at an inner circumferential surface, of the mold, that forms the cavity. The plurality of injection openings include two or more injection openings located at different positions in the direction of the axial line, or two or more injection openings located at different positions in a circumferential direction.

Abstract: The present invention allows reduction in high frequency noise of a spark plug. A Fe-containing oxide layer is formed on the surface of a beneath-flange rod-shaped portion, of a metal terminal of the spark plug, between a terminal flange portion and an upper end of a metal shell. The surface area of the Fe-containing oxide layer is not less than 10% of the surface area of the beneath-flange rod-shaped portion.

Abstract: Disclosed is a spark plug capable of suppressing high-frequency noise by improving the material of an electrical connection part between a center electrode and a metal terminal within an axial hole of an insulator. A spark plug has an electrical connection part arranged in an axial hole of an insulator to establish electrical connection between a center electrode and a metal terminal. The electrical connection part includes a conductor having a first crystal phase formed of a Fe-containing oxide and a second crystal phase formed of a conductive metal oxide of perovskite crystal structure.

Abstract: A connection structure between a center electrode and a metal terminal of a spark plug having a composite part that contains a plurality of secondary particles each formed of a plurality of primary particles of iron-containing oxide as a magnetic substance, and a conductive material coating the plurality of secondary particles. The iron-containing oxide includes at least one of an oxide represented by M1+AOFe2?AO3 (where ?0.5?A?0.5; and M is at least one kind of element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn and Ca) and an oxide represented by Q3Fe5O12 (where Q is at least one kind of element selected from the group consisting of Y, Lu, Yb, Tm, Er, Ho, Dy, Tb, Gd and Sm). In cross section, an average particle size of the primary particles is 0.5 ?m to 100 ?m, an average particle size of the secondary particles is 0.5 mm to 2.0 mm, and a porosity of the inside of the secondary particles is 5% or lower.

Abstract: An insulator is rendered less breakable. A spark plug insulator is a tube-shaped spark plug insulator having a through hole extending in a direction of an axial line. The spark plug insulator contains alumina as a main component and mullite at at least part of the insulator. Mullite is contained in only an inner circumferential surface of the tube-shaped spark plug insulator and in at least part of the inner circumferential surface of the spark plug insulator in an area extending toward a distal end from a portion having a largest outer diameter.

Abstract: A spark plug includes an insulator, a center electrode, a metallic shell, and a ground electrode. The insulator contains not less than 90.0 mass % and not greater than 98.1 mass % of Al2O3 with respect to a total mass of elements, as reduced to oxides, contained in the insulator, and has a porosity not greater than 5%. Assuming that a porosity of the insulator after a heat treatment is B, a difference (B?A) of the porosities before and after the heat treatment is not greater than 3.5%. In the heat treatment, the insulator is placed in a furnace, and a temperature in the furnace is increased from a room temperature to 1400° C. at a temperature increase rate of 7° C./min and is held at 1400° C. for 30 minutes, and thereafter, the temperature is decreased from 1400° C. to 400° C. at a temperature decrease rate of 10° C. or less per 5 minutes and is decreased to the room temperature.

Abstract: An employed rear mold is comprised of a plurality of mold pieces. The plurality of mold pieces include a first mold piece which forms a first partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface, and a second mold piece which is located axially forward of the first mold piece and forms a second partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface. Releasing the molded body includes starting to move the first mold piece rearward in relation to a rear molded portion, and, subsequently to starting to move the first mold piece, starting to move the second mold piece rearward in relation to the rear molded portion, thereby disassembling the rear mold into the plurality of mold pieces.

Abstract: An insulator is rendered less breakable. A spark plug insulator is a tube-shaped spark plug insulator having a through hole extending in a direction of an axial line. The spark plug insulator contains alumina as a main component and mullite at at least part of the insulator. Mullite is contained in only an inner circumferential surface of the tube-shaped spark plug insulator and in at least part of the inner circumferential surface of the spark plug insulator in an area extending toward a distal end from a portion having a largest outer diameter.

Abstract: A method for manufacturing an insulator for a spark plug includes a molding process of forming a cylindrical molded product having an axial hole that extends in a direction of an axial line, by means of injection molding using a mold that has a columnar cavity therein and a bar-shaped member disposed in the cavity and extending in the direction of the axial line. In this method, the molding process includes an injection step of injecting a material containing a ceramic. In the injection step, the material is injected into the cavity from a plurality of injection openings that are opened at an inner circumferential surface, of the mold, that forms the cavity. The plurality of injection openings include two or more injection openings located at different positions in the direction of the axial line, or two or more injection openings located at different positions in a circumferential direction.

Abstract: Disclosed is a spark plug capable of suppressing high-frequency noise by improving the material of an electrical connection part between a center electrode and a metal terminal within an axial hole of an insulator. A spark plug has an electrical connection part arranged in an axial hole of an insulator to establish electrical connection between a center electrode and a metal terminal. The electrical connection part includes a conductor having a first crystal phase formed of a Fe-containing oxide and a second crystal phase formed of a conductive metal oxide of perovskite crystal structure.

Abstract: A spark plug having a resistor disposed within a through hole of an insulator and between a center electrode and a metal terminal so as to be spaced apart from the center electrode in a direction of an axial line; and a conductive glass seal layer provided between the resistor and the center electrode and electrically connecting the resistor and the center electrode to each other, the conductive glass seal layer has a diameter of 3.9 mm or less, and a joined surface of the conductive glass seal layer and the resistor has a convex shape toward the center electrode side. A length ? from a rear end to a front end of the joined surface and a maximum length ? of the conductive glass seal layer in the direction of the axial line meets a relation of ?/??0.4.

Abstract: A resistor element of a spark plug containing ZrO2, wherein a target region is defined by a rectangular region where the size in the direction perpendicular to an axial line is 1800 ?m and the size in the direction of the axial line is 2400 ?m. In the case where the target region is divided into a plurality of square regions having lengths of 200 ?m on a side, a transverse line-shaped region is defined by a region in a line shape that is constituted of nine square regions arranged in the direction perpendicular to the axial line. A first type region is defined by the square region where a proportion of an area of ZrO2 is 25% or more, and a second type region is defined by the square region where a proportion of an area of ZrO2 is less than 25%.

Abstract: A spark plug includes an insulator, a center electrode, a metallic shell, and a ground electrode. The insulator contains not less than 90.0 mass % and not greater than 98.1 mass % of Al2O3 with respect to a total mass of elements, as reduced to oxides, contained in the insulator, and has a porosity not greater than 5%. Assuming that a porosity of the insulator after a heat treatment is B, a difference (B?A) of the porosities before and after the heat treatment is not greater than 3.5%. In the heat treatment, the insulator is placed in a furnace, and a temperature in the furnace is increased from a room temperature to 1400° C. at a temperature increase rate of 7° C./min and is held at 1400° C. for 30 minutes, and thereafter, the temperature is decreased from 1400° C. to 400° C. at a temperature decrease rate of 10° C. or less per 5 minutes and is decreased to the room temperature.

Abstract: A connection portion connecting a center electrode and a terminal metal fixture together in a through hole of the insulator includes a resistor and a magnetic substance structure including a magnetic substance and a conductor and being disposed on a leading end side or a rear end side of the resistor while being positioned away from the resistor. The connection portion further includes a first conductive sealing portion, a second conductive sealing portion and a third conductive sealing portion. The first conductive sealing portion is disposed on a leading end side of a first member and is in contact therewith. The second conductive sealing portion is disposed between the first member and a second member and is in contact with the first and second members. The third conductive sealing portion is disposed on a rear end side of the second member and is in contact therewith.

Abstract: A connection portion connecting a center electrode and a terminal metal fixture together in a through hole of the insulator includes a resistor and a magnetic substance structure including a magnetic substance and a conductor. The connection portion further includes a first conductive sealing portion, a second conductive sealing portion and a third conductive sealing portion. The first conductive sealing portion is disposed on a leading end side of a first member and is in contact therewith. The second conductive sealing portion is disposed between the first member and a second member and is in contact with the first member and the second member. The third conductive sealing portion is disposed on a rear end side of the second member and is in contact therewith.

Abstract: A connection portion connecting a center electrode and a terminal metal fixture together in a through hole of the insulator includes a resistor and a magnetic substance structure including a magnetic substance and a conductor and being disposed on a leading end side or a rear end side of the resistor while being positioned away from the resistor. The connection portion further includes a first conductive sealing portion, a second conductive sealing portion and a third conductive sealing portion. The first conductive sealing portion is disposed on a leading end side of a first member and is in contact therewith. The second conductive sealing portion is disposed between the first member and a second member and is in contact with the first and second members. The third conductive sealing portion is disposed on a rear end side of the second member and is in contact therewith.

Abstract: A connection portion connecting a center electrode and a terminal metal fixture together in a through hole of the insulator includes a resistor and a magnetic substance structure including a magnetic substance and a conductor The connection portion further includes a first conductive sealing portion, a second conductive sealing portion and a third conductive sealing portion. The first conductive sealing portion is disposed on a leading end side of a first member and is in contact therewith. The second conductive sealing portion is disposed between the first member and a second member and is in contact with the first member and the second member. The third conductive sealing portion is disposed on a rear end side of the second member and is in contact therewith.

Abstract: A spark plug includes an insulator having an axial hole, a center electrode inserted into a forward portion of the axial hole, a terminal electrode inserted into a rear portion of the axial hole, and an interelectrode insert which contains glass and electrically conductive carbon and is disposed in the axial hole between the center electrode and the terminal electrode. The interelectrode insert has a resistance of 1.0 k? to 3.0 k?, and the interelectrode insert has a carbon content of 1.5% by mass to 4.0% by mass at a forward portion located forward of a center point between the rear end of the center electrode and the forward end of the terminal electrode. Furthermore, the forward portion is lower in resistance than a rear portion of the interelectrode insert located rearward of the center point.

Abstract: A spark plug having an insulator formed of an alumina-based sintered material which contains an Si component and a rare earth element component that satisfy the following conditions (1) and (2): (1) an oxide-reduced amount of the Si component (mass %) is greater than or equal to 0.05 and less than or equal to 0.45; and (2) a ratio of an oxide-reduced amount of the rare earth element component to an oxide-reduced amount of the Si component is greater than or equal to 2.5 and less than or equal to 4.5, and which has alumina grains and an alumina grain boundary phase including a complex oxide crystal phase satisfying the following condition (3): (3) the complex oxide crystal phase contains at least two components selected from among an Al component, an Si component, a Group 2 element component, and a rare earth element component.