Method for manufacturing a spark plug

Abstract

Method for manufacturing a spark plug (1) comprising a resistance element (8), wherein the method comprises: an electrode positioning step for positioning a central electrode (3) in a shaft hole (21) of an insulator (2); a first filling step to fill a space on a near end side of the central electrode (3) in the shaft hole (21) with a first electrically conductive glass powder (41a); a second filling step to fill a space on the near end side of the first electrically conductive glass powder (41a) in the shaft hole (21) with resistance body composition powder (8a) to form the resistance body (8); a third filling step to fill a space on the near end side of the resistive body composition powder (8a) in the shaft hole (21) with a second electrically conductive glass powder (42a); and a sintering step to sinter the first electrically conductive glass powder (41a), the resistive body composition powder (8a) and the second electrically conductive glass powder (42a) in the shaft hole (21); and a charge matching step prior to the second filling step to bring at least one selected from the insulator and the first electrically conductive glass powder (41a) into an uncharged state or to bring the insulator (2) and the first electrically conductive glass powder (41a) into the same charged state.

Description

Background Technical field

[0001] The present invention relates to a method for manufacturing a spark plug. Related state of the art

[0002] JP 2010-135345A discloses a method for manufacturing a spark plug incorporating a resistive element. In the method for manufacturing a spark plug in JP 2010-135345A, a center electrode is first inserted into a shaft hole of an insulator. Next, the shaft hole is filled with a first electrically conductive glass powder, a resistive element composition powder for forming the resistive element, and a second electrically conductive glass powder, and then a terminal fitting is inserted into the shaft hole.

[0003] The powder is then softened with heat in the insulator's shaft hole, and the terminal fit is pressed towards the distal end (in other words, towards the center electrode) by hot pressing. The softened powder is cooled and solidified, and a first sealing layer, the resistive body, and a second sealing layer, which are sintered bodies, are formed in this order between the center electrode and the terminal fit in the insulator's shaft hole, starting from the distal end.

[0004] However, in the case of the spark plug disclosed in JP 2010-135345A, situations can arise where the insulator becomes charged by friction, for example, during transport of the insulator as a single piece. Thus, if a spark plug is manufactured using the charged insulator, when the insulator's shaft hole is filled with the first electrically conductive glass powder to form the first sealing layer, a portion of the electrically conductive glass powder may be electrically adsorbed onto the insulator's inner wall. If this occurs, there is a risk that the peripheral edge of a near end surface of the first sealing layer may be shaped in such a way that it protrudes significantly towards the near end after sintering.

[0005] If the peripheral edge of the near end surface of the first sealing layer is shaped in such a way that it projects considerably towards the near end, the peripheral edge of the near end surface of the first sealing layer and the resistive body come into close proximity locally, and cases may occur where a desired resistance value is not obtained. More precisely, in the method for manufacturing a spark plug disclosed in JP 2010-135345A, the resistance value of a manufactured spark plug varies between individual products, meaning that the difference between the resistance values ​​of individual pieces is likely to be high.

[0006] In light of the foregoing, it is desirable to have a method for manufacturing a spark plug which prevents the peripheral edge of a near end surface of a first sealing layer formed by sintering a first electrically conductive glass powder from rising significantly towards the near end. Summary

[0007] One aspect of the present invention provides a method for manufacturing a spark plug which includes a resistance element.The process for manufacturing the spark plug includes: an electrode positioning step to position a center electrode in a shaft hole of an insulator; a first filling step to fill a space at a near end face of the center electrode in the shaft hole with a first electrically conductive glass powder; a second filling step to fill a space at the near end face of the first electrically conductive glass powder in the shaft hole with resistance body composition powder to form the resistance body; a third filling step to fill a space at the near end face of the resistance body composition powder in the shaft hole with a second electrically conductive glass powder; and a sintering step to sinter the first electrically conductive glass powder, the resistance body composition powder, and the second electrically conductive glass powder in the shaft hole.

[0008] The process for manufacturing the spark plug includes, prior to the second filling step, a charge-matching step to put at least one selected from the insulator and the first electrically conductive glass powder into an uncharged state or to put the insulator and the first electrically conductive glass powder into the same charged state.

[0009] The method for manufacturing the spark plug according to the preceding aspect or embodiment includes, prior to the second filling step for filling the shaft hole with the resistive body composition powder, a charge-matching step to bring at least one selected from the insulator and the electrically conductive glass powder into an uncharged state or to bring the insulator and the electrically conductive glass powder into the same charged state. Therefore, the electrical adsorption of a portion of the electrically conductive glass powder onto the inner surface of the insulator, due to the fact that the insulator and the electrically conductive glass powder are charged with opposite polarities, can be prevented after the charge-matching step before the shaft hole is filled with the resistive body composition powder.This prevents the peripheral edge of the near end surface of the first sealing layer from being formed in such a way that it protrudes significantly towards the near end in a manufactured spark plug.

[0010] As just described, according to the above aspect, it is possible to provide a method for manufacturing a spark plug in which the peripheral edge of a near end surface of a first sealing layer, which is formed by sintering a first electrically conductive glass powder, is prevented from rising significantly towards the near end.

[0011] It should be noted that the reference numerals in parentheses, with which elements in the claims and the abstract are provided, indicate relationships to specific elements in the embodiments that will be described later, and these do not limit the technical scope of the present invention. Brief description of the drawings

[0012] It shows / shows: Fig. 1 a cross-sectional view which includes a representation of a central axis of a spark plug according to a first embodiment; Fig. 2 a cross-sectional view to illustrate a method for manufacturing the spark plug according to the first embodiment; Fig. 3 an enlarged cross-sectional view of the spark plug around a resistance body according to the first embodiment; and Fig. 4 An enlarged cross-sectional view of a spark plug around a resistor body according to a comparative embodiment. Description of specific embodiments

[0013] Exemplary embodiments of the present invention will below be described in detail with reference to the accompanying drawings, in which the same reference numerals denote the same or similar elements, and their duplicated description will be omitted. First embodiment

[0014] An embodiment of a method for manufacturing a spark plug 1 is described with reference to the Fig. 1 to 3 described.

[0015] The method for manufacturing the spark plug 1 according to the present embodiment is a method for manufacturing the spark plug 1 which includes a resistance element 8. First, with reference to Fig. 1. The structure of the spark plug 1 obtained by the manufacturing process according to the present embodiment is described. It should be noted that in this description, a direction in which the central axis of the spark plug 1 extends is referred to as an axial direction X of the plug, a direction along the axial direction X of the plug in which a central electrode 3 projects from an insulator 2 is referred to as the distal end face, and the direction opposite to this is referred to as the near end face.

[0016] The spark plug 1 comprises: a cylindrical housing 6; the cylindrical insulator 2, which is held inside the housing 6; the center electrode 3, which is arranged inside the insulator 2; a first sealing layer 41; the resistance body 8; a second sealing layer 42; and a terminal fitting 5.

[0017] The insulator 2, for example, is made from a sintered ceramic body, which is obtained by forming an insulating ceramic material, such as aluminum oxide, into a predetermined shape and then firing it. The insulator 2 has a shaft hole 21 formed at its center point, extending along the axial direction X of the candle. Starting from the distal end, the shaft hole 21 comprises, in sequence, a small-diameter section 211, a stepped section 212, and a large-diameter section 213.

[0018] Both the small-diameter section 211 and the large-diameter section 213 have a constant inner diameter in the axial direction X of the candle. The inner diameter of the large-diameter section 213 is larger than the inner diameter of the small-diameter section 211. The stepped section 212 is designed to connect the small-diameter section 211 and the large-diameter section 213 and is tapered, with the diameter increasing towards the near end.

[0019] The inner diameter of the large-diameter section 213 is less than or equal to 3.8 mm. This is considered the maximum inner diameter of the large-diameter section 213 that can be formed on the insulator 2, taking into account its stiffness and dielectric strength. The insulator 2 can be attached to the housing 6, which includes a mounting screw section 61 with a thread diameter of at most M12, as described later. This belongs to a class of spark plugs in which the inner diameter of the large-diameter section 213 is relatively small. For some years now, this has been accompanied by increases in efficiency and output, respectively.The output of machines presents a growing need for a reduction in the diameter of a spark plug, which is used to ensure flexibility in machine design, leading to a need for a reduction in the diameter of the insulator 2 and also a reduction in the diameter of the shaft hole 21 in order to reduce the diameter of the spark plug.

[0020] Within the shaft hole 21, the center electrode 3, the first sealing layer 41, the resistance body 8, the second sealing layer 42, and the connection fitting 5 are arranged in this order, starting from the distal end. The center electrode 3 is rod-shaped and has a distal end section that projects from the shaft hole 21. A near end section 31 of the center electrode 3 is supported on the stepped section 212 of the shaft hole 21, thus positioning the center electrode 3 within the shaft hole 21. On the near end of the center electrode 3, within the shaft hole 21 (in other words, on the large-diameter section 213), the first sealing layer 41, the resistance body 8, and the second sealing layer 42 are arranged in this order, starting from the distal end.

[0021] The first sealing layer 41 is sintered from a body of electrically conductive glass powder obtained by mixing glass with a metal powder, such as copper powder. The first sealing layer 41 is in close contact with the inner wall surface of the shaft hole 21 to ensure airtightness within the shaft hole 21. The first sealing layer 41 is in contact with the central electrode 3 at its distal end and with the resistance body 8 at its near end, and the central electrode 3 and the resistance body 8 are electrically connected to each other via the first sealing layer 41.

[0022] The resistive body 8 is made from a sintered body formed by sintering resistive body composition powder, which is obtained by mixing glass with a resistive body material powder such as carbon. The resistive body 8 is used to prevent the occurrence of radio noise generated by the transmission of a spark discharge from the spark plug 1 to the spark plug 1. The resistivity of the resistive body 8 in the axial direction X of the spark plug is greater than the resistivity of both the first sealing layer 41 and the second sealing layer 42.

[0023] The second sealing layer 42 is sintered from a body of a second electrically conductive glass powder. Similar to the first electrically conductive glass powder, the second electrically conductive glass powder is a copper-glass powder (more precisely, a powder obtained by mixing glass with a metal powder such as copper powder). The second sealing layer 42 is in close contact with the inner wall surface of the large-diameter section 213 of the shaft hole 21 to ensure an airtight seal in the shaft hole 21. The second sealing layer 42 is in contact with the resistive body 8 at the distal end and with the terminal fit 5 at the near end, and the resistive body 8 and the terminal fit 5 are electrically connected to each other via the second sealing layer 42.

[0024] The boundary between the first sealing layer 41 and the resistance body 8, as well as the boundary between the resistance body 8 and the second sealing layer 42, can be checked on a cross-section that includes the central axis of the spark plug 1. In the shaft hole 21, the connection fit 5 is located adjacent to the near end of the second sealing layer 42.

[0025] The connection fitting 5 is electrically connected to the center electrode 3 via the second sealing layer 42, the resistance body 8, and the first sealing layer 41. The connection fitting 5 comprises: an inserted connection section 51, which is inserted into the shaft hole 21; and a protruding connection section 52, which extends from the shaft hole 21 towards the near end. The second sealing layer 42 is in close contact with the distal end of the inserted connection section 51. The distal end of the inserted connection section 51 is threaded to improve adhesion to the second sealing layer 42. Furthermore, the outer diameter of the inserted connection section 51 is smaller than the inner diameter of the shaft hole 21, and the second sealing layer 42 is also positioned between the distal end of the inserted connection section 51 and the shaft hole 21.The excellent connection section 52 is a connection used to connect the spark plug 1 to an ignition coil.

[0026] The insulator 2 is held in the housing 6. The housing 6 is cylindrical and is manufactured from a heat-resistant metal material, such as an iron-based alloy. A mounting screw section 61 for screwing the housing 6 into an internally threaded hole provided in a spark plug hole of a cylinder head of an internal combustion engine is formed on an outer peripheral section of the housing 6.

[0027] A grounding electrode 7 is connected to the distal end of the housing 6. The grounding electrode 7 is positioned facing the center electrode 3 in the axial direction X of the spark plug and forms a discharge path G between the center electrode 3 and the grounding electrode 7 in the axial direction X of the spark plug. Applying a high voltage to the center electrode 3 causes a spark discharge in the discharge path G, thus igniting an air-fuel mixture in a combustion chamber.

[0028] Next, with reference to Fig. 2 a method for manufacturing the spark plug 1 according to the present embodiment is described.

[0029] The present embodiment describes a method for forming the central electrode 3, the first sealing layer 41, the resistance body 8, the second sealing layer 42 and the connection fit 5 within the insulator 2.

[0030] First, a preparatory step is carried out to prepare the insulator 2, which is produced, for example, at a location different from the location where the present procedure is implemented, as in (A) of Fig. Figure 2 shows the problem or concern that the insulator 2 becomes charged, for example, by friction occurring during transport from the manufacturing site. Therefore, the preparation step is followed by a charge-matching step, which is shown in (B) of Fig. 2 is shown.

[0031] The charge matching step is performed to prevent the insulator 2 and the first electrically conductive glass powder from being charged with opposite polarities. As shown in (B) of Fig. As shown in Figure 2, the charge matching step in the present embodiment serves to neutralize the insulator 2 in order to bring it into an uncharged state. The charge neutralization of the insulator 2 is carried out using a charge neutralizer 11. The charge neutralizer 11 can be a device that includes a needle electrode and generates ions around the needle electrode by applying a voltage to it. Furthermore, the charge neutralizer 11 can diffuse the generated ions using a jet of air A.

[0032] For example, a known ionizer or ion generator operating on pulsed alternating current, alternating current, direct current, or pulsed direct current can be used as the charge neutralizer 11. Using such a charge neutralizer 11, the amount of charge in the insulator 2 can easily and rapidly be neutralized to near 0 nC (nanocoulombs). It should be noted that a method other than charge neutralization can be used.

[0033] Charge neutralization is performed on the inner wall of the shaft hole 21 of the insulator 2. More precisely, the ions generated by the charge neutralizer 11 are sprayed onto the inside of the shaft hole 21 using a jet of air A to neutralize the insulator 2. The charge matching step is performed on the separately mounted insulator 2, to which no other components, such as the central electrode 3, have been attached. Thus, the air jet A can be sprayed onto the inside of the insulator 2 in such a way as to pass through the shaft hole 21, which facilitates the charge neutralization of the insulator 2. The charge matching step is followed by an electrode positioning step, as shown in (C) of Fig. 2 is shown.

[0034] As in (C) of Fig. As shown in Figure 2, during the electrode positioning step, the center electrode 3 is inserted into the shaft hole 21 of the insulator 2, starting from the near end of the shaft hole 21. The center electrode 3 is held in the shaft hole 21, with the near end section 31 of the center electrode 3 resting on the step section (compare reference numeral 212 in Figure (B) of Figure 2). Fig. 2) of the shaft hole 21 is stored. Next, a first filling step is carried out, as shown in (D) of Fig. 2 is shown.

[0035] As in (D) of Fig. As shown in Figure 2, in the first filling step a space on the near end face of the central electrode 3 in the shaft hole 21 is filled with a first electrically conductive glass powder 41a (copper glass powder) to form the first sealing layer 41. At this time, the insulator 2 is in an uncharged state as a result of the charge matching step, and thus the first electrically conductive glass powder 41a is not electrically adsorbed on the inside of the shaft hole 21 of the insulator 2.

[0036] A rod clamping device, not shown in detail in the drawings, is inserted into the shaft hole 21 and pressurizes the first electrically conductive glass powder 41a towards its distal end. To enable insertion into the shaft hole 21, the clamping device is designed such that its diameter is slightly smaller than the inner diameter of a section of the shaft hole 21 where the first electrically conductive glass powder 41a is positioned.It should be noted that in a method for manufacturing a spark plug which, unlike the method according to the present embodiment, does not include a charge matching step, in a case where a proportion of the electrically conductive glass powder is electrically adsorbed to the shaft hole, even if the clamping device is inserted into the shaft hole, the electrically conductive glass powder that is adsorbed to the shaft hole cannot be removed.

[0037] Furthermore, in the present embodiment, a second filling step is carried out after the first electrically conductive glass powder 41a is applied using the clamping device, as shown in (E) of Fig. 2 is shown.

[0038] As in (E) of Fig. As shown in Figure 2, the second filling step serves to fill a space on the near end of the first electrically conductive glass powder 41a in the shaft hole 21 with resistive body composition powder 8a to form the resistive body 8. Subsequently, a rod clamping device, not shown in detail in the drawings, is inserted into the shaft hole 21 and pressurizes the resistive body composition powder 8a towards the distal end. Next, a third filling step is carried out, as shown in Figure (F) of Fig. 2 is shown.

[0039] As in (F) of Fig. As shown in Figure 2, the third filling step serves to fill a space on the near end of the resistive element composition powder 8a in the shaft hole 21 with a second electrically conductive glass powder 42a. Subsequently, a rod clamping device, which is not shown in detail in the drawings, is inserted into the shaft hole 21 and pressurizes the second electrically conductive glass powder 42a towards its distal end. Thus, the first electrically conductive glass powder 41a, the resistive element composition powder 8a, and the second electrically conductive glass powder 42a are stacked in this order on the near end of the central electrode 3 in the shaft hole 21.

[0040] After the third filling step, the connecting fit 5 is inserted into the shaft hole 21, starting from the near end. In this state, the connecting fit 5 is located relative to the insulator 2 at the near end of its final position. In the next sintering step, the connecting fit 5 is pressed towards the distal end and is located in the final position in the completed spark plug 1.

[0041] As in (G) of Fig. As shown in Figure 2, the sintering step serves to sinter the first electrically conductive glass powder 41a, the resistive body composition powder 8a, and the second electrically conductive glass powder 42a. The sintering step is carried out, for example, in a heating furnace 10 at a temperature greater than or equal to the softening temperatures of the glass materials of the first electrically conductive glass powder 41a, the resistive body composition powder 8a, and the second electrically conductive glass powder 42a. Thus, the first electrically conductive glass powder 41a, the resistive body composition powder 8a, and the second electrically conductive glass powder 42a are softened and become fluid. In this fluid state, the connection fitting 5 is pressed towards the distal end.

[0042] As a result of pressing the connection fitting 5 towards its distal end, the fluid of the second electrically conductive glass powder 42a flows into a gap between the shaft hole 21 and the outer periphery of the inserted connection section 51 of the connection fitting 5, thus improving the adhesion between the connection fitting 5 and the fluid of the second electrically conductive glass powder 42a. Furthermore, the temperature in the heating oven 10 is reduced, causing the fluid of the first electrically conductive glass powder 41a, the fluid of the resistive body composition powder 8a, and the fluid of the second electrically conductive glass powder 42a to solidify, resulting in the first sealing layer 41, the resistive body 8, and the second sealing layer 42, as shown in (H) of Fig. 2 is shown. More precisely, the first electrically conductive glass powder 41a is sintered into the first sealing layer 41, the resistive body composition powder 8a is sintered into the resistive body 8, and the second electrically conductive glass powder 42a is sintered into the second sealing layer 42.

[0043] It should be noted that, even in the present embodiment, pressing the connection fit 5 in the insulator 2 towards the distal end during the sintering step can result in the formation of an ascending section 100, which projects slightly towards the near end, on both the peripheral edge of the near end surface of the first sealing layer 41 and the peripheral edge of the near end surface of the resistance body 8, as shown in Fig. 3 is shown schematically.

[0044] As described above, the central electrode 3, the first sealing layer 41, the resistance body 8, the second sealing layer 42 and the connection fitting 5 can be formed inside the insulator 2.

[0045] Next, the effects achieved with the present embodiment will be described.

[0046] The method for manufacturing the spark plug 1 according to the present embodiment includes, prior to the second filling step for filling the shaft hole 21 with the resistive body composition powder 8a, a charge-matching step to bring the insulator 2 into an uncharged state. Therefore, the electrical adsorption of a portion of the electrically conductive glass powder onto the inner surface of the insulator 2, due to the fact that the insulator 2 and the electrically conductive glass powder are charged with opposite polarities, can be prevented after the charge-matching step before the shaft hole 21 is filled with the resistive body composition powder 8a. Thus, it can be prevented that the peripheral edge of the near end surface of the first sealing layer 41 is formed in such a way that it protrudes significantly towards the near end in the manufactured spark plug 1.

[0047] On the other hand, unlike the present invention, the insulator 2 in a spark plug 9 produced by a manufacturing process that does not include a charge-matching step can, for example, be charged before the insulator 2 is filled with the first electrically conductive glass powder, which forms the first sealing layer 41, as shown in Fig.Figure 4 shows that in this case, a portion of the first electrically conductive glass powder is electrically adsorbed on the inside of the shaft hole 21 if the inside of the insulator 2 is filled with the first electrically conductive glass powder. Accordingly, in the manufactured spark plug 9, there is a risk that a rising section 900, which projects considerably further towards the near end than the surroundings, may form on the peripheral edge of the near end surface of the first sealing layer 41. In the case where such a large rising section is formed, the resistance between the rising section of the first sealing layer 41 and the second sealing layer 42 may have a locally small or low value, resulting in a failure to obtain a desired resistance value.In other words, the resistance value of the manufactured spark plug 9 can vary between individual products, which means that the difference between resistance values ​​of individual pieces is likely to be high.

[0048] Therefore, it can be prevented that the peripheral edge of the near end surface of the first sealing layer 41 is formed in such a way that it protrudes considerably towards the near end when the charge matching step is carried out before the second filling step, as in the present embodiment, and it is likely that the products will have the same resistance value.

[0049] Furthermore, the charge-matching step serves to neutralize at least one selected component from insulator 2 and the first electrically conductive glass powder 41a. Therefore, in the charge-matching step, it is sufficient that at least one selected component from insulator 2 and the first electrically conductive glass powder 41a is neutralized; thus, the step can be simplified in a straightforward manner.

[0050] The charge neutralization of the insulator 2 is carried out on the inner wall of the shaft hole 21 of the insulator 2. Therefore, the occurrence of the peripheral edge of the near end surface of the first sealing layer 41, which increases considerably towards the near end, is more likely to be reduced. More precisely, in the case where the inner wall of the shaft hole 21 of the insulator 2 is charged, a portion of the first electrically conductive glass powder 41a is readily electrically adsorbed onto the inner wall of the shaft hole 21 when the shaft hole 21 is filled with the first electrically conductive glass powder 41a. Thus, the inner wall of the shaft hole 21 is neutralized in such a way that the peripheral edge of the near end surface of the first sealing layer 41 in the manufactured spark plug 1 does not protrude significantly towards the near end due to the electrically conductive glass powder that is adsorbed to the shaft hole 21.

[0051] Furthermore, a section (namely the large-diameter section 213) of the shaft hole 21, in which the first electrically conductive glass powder 41a is provided, has an inner diameter of no more than 3.8 mm. In other words, the inner diameter of the large-diameter section 213, as previously explained, is less than or equal to the maximum inner diameter of the shaft hole 21 that can be formed on the insulator 2, taking into account its stiffness and dielectric strength. The insulator 2 can be attached to the housing 6, which includes the mounting screw section 61 with a thread diameter of at most M12. Thus, the large-diameter section 213 is relatively small, which allows for a reduction in the diameter of the spark plug 1.

[0052] On the other hand, in the first filling step for filling the shaft hole 21 with the first electrically conductive glass powder 41a, the smaller the large-diameter section 213 is, the more likely it is that each part of the electrically conductive glass powder will approach the inner wall of the shaft hole 21. This raises further concerns regarding a significant rise in the peripheral edge of the near-end surface of the first sealing layer 41 towards the near end in the manufactured spark plug 1. Even in the case of the spark plug 1, which is so problematic, the peripheral edge of the near-end surface of the first sealing layer 41 can be effectively prevented from forming in such a way that it protrudes significantly towards the near end by performing the charge-matching step before the second filling step, as previously described.

[0053] As just described, according to the present embodiment it is possible to provide a method for manufacturing a spark plug in which the peripheral edge of a near end surface of a first sealing layer is prevented from rising significantly towards the near end. Second embodiment

[0054] The present embodiment is obtained by changing the charge matching step in the first embodiment.

[0055] The charge matching step according to the present embodiment serves to bring the insulator 2 and the first electrically conductive glass powder 41a into the same charged state before the second filling step. For example, both the inner surface of at least the shaft hole 21 of the insulator 2 and the first electrically conductive glass powder 41a are positively or negatively charged during the charge matching step.

[0056] The other details are the same as in the first embodiment or are similar to them.

[0057] It should be noted that in the case of reference numerals used in the second embodiment and subsequent embodiments, reference numerals which are the same as those used in the previously described embodiment, structural elements and the like which are the same as or similar to those in the embodiment described above, unless otherwise specified.

[0058] In the present embodiment, the charge matching step for bringing the insulator 2 and the first electrically conductive glass powder 41a into the same charged state is carried out before the second filling step. Therefore, the repulsive force between the inner wall of the insulator 2 and the first electrically conductive glass powder 41a prevents the peripheral edge of the near end surface of the first sealing layer 41 in the manufactured spark plug 1 from rising significantly towards the near end due to the first electrically conductive glass powder 41a, which is electrically adsorbed on the inside of the shaft hole 21 in the first filling step.

[0059] In addition, essentially the same effects are achieved as in the first embodiment. Third embodiment

[0060] The present embodiment is obtained by changing the charge matching step in embodiments 1 and 2.

[0061] In the present embodiment, the charge matching step serves to store the insulator 2, for example in an electrically conductive container or a metal mesh, after the insulator 2 has been manufactured, and to avoid charging the insulator 2 from the outset.

[0062] The other details are the same as in the first embodiment or are similar to them.

[0063] In the present embodiment, essentially the same effects are achieved as in the first embodiment. Other embodiments

[0064] The present invention is not limited to each of the embodiments described above and can be applied to various embodiments without departing from the basic ideas and spirit of the present invention.

[0065] For example, charge neutralization can be carried out starting from the outside of the insulator 2 by spraying the jet of air A from the charge neutralizer 11 onto the outside of the insulator 2, although in the first embodiment, the charge neutralization of the insulator 2, as the charge matching step, is carried out on the inner wall of the shaft hole 21. In this case, it may take longer to neutralize the inner wall of the shaft hole 21 than in the case where the inner wall of the shaft hole 21 is neutralized directly, but the insulator 2 can be neutralized starting from the outside.

[0066] Furthermore, this is not limiting as long as the charge matching step is performed before the second filling step for filling the shaft hole 21 with the resistive body composition powder 8a, although in each of the embodiments described above, the charge matching step is performed before the electrode positioning step for positioning the center electrode 3 in the insulator 2. For example, the charge matching step can be performed immediately after the electrode positioning step, or it can be performed after the first filling step for filling the shaft hole 21 with the first electrically conductive glass powder 41a and before the second filling step.

Claims

[1] Method for manufacturing a spark plug (1) comprising a resistance element (8), wherein the method comprises: an electrode positioning step for positioning a central electrode (3) in a shaft hole (21) of an insulator (2); a first filling step to fill a space on a near end side of the central electrode (3) in the shaft hole (21) with a first electrically conductive glass powder (41a); a second filling step to fill a space on the near end side of the first electrically conductive glass powder (41a) in the shaft hole (21) with resistance body composition powder (8a) to form the resistance body (8); a third filling step to fill a space on the near end side of the resistive body composition powder (8a) in the shaft hole (21) with a second electrically conductive glass powder (42a); and a sintering step to sinter the first electrically conductive glass powder (41a), the resistive body composition powder (8a) and the second electrically conductive glass powder (42a) in the shaft hole (21); and a charge matching step prior to the second filling step to bring at least one selected from the insulator and the first electrically conductive glass powder (41a) into an uncharged state or to bring the insulator (2) and the first electrically conductive glass powder (41a) into the same charged state. [2] Method for producing the spark plug (1) according to claim 1, wherein the charge matching step comprises bringing at least one selected from the insulator (2) and the first electrically conductive glass powder (41a) into the uncharged state by charge neutralization. [3] Method for manufacturing the spark plug (1) according to claim 2, wherein the charge neutralization is carried out on an inner wall of the shaft hole (21) of the insulator (2). [4] Method for manufacturing the spark plug (1) according to any one of claims 1 to 3, wherein a section of the shaft hole (21) in which the first electrically conductive glass powder (41a) is provided has an inner diameter of not more than 3.8 mm.

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