JP2000058226A - Manufacture of insulator for spark plug, press pin using it, and spark plug - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of an insulator for a spark plug capable of easily pulling a press pin out of a molded body in molding of raw material powder. SOLUTION: When an insulator is molded in a rubber press, a hard carbon base mold release film 60 mainly comprising amorphous carbon is formed on the surface of a press pin 50. Releasing property of the press pin 50 from a molded body becomes excellent and the press pin 50 can smoothly be pulled out of the molded body without generating cracks or break. As a result, since lubricant coating to the press pin 50 can be eliminated or reduced, a molding process is simplified, and yield of molded bodies is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing an insulator for a spark plug, a press pin used for the same, and a spark plug.

[0002]

2. Description of the Related Art In a spark plug used for an internal combustion engine such as an automobile engine, alumina (Al ₂ O ₃ ) is used.
An insulator mainly composed of, for example, is used. The insulator is generally formed in a vertically long shape, and at the center thereof, an elongated through hole for inserting a center electrode for forming a spark gap and a terminal electrode for applying a high voltage to the center electrode is formed. Such an insulator is produced by firing a compact of raw material powder, and as a compacting method, for example, a kind of cold isostatic pressing method called a rubber press method is often used. In this method, a raw material powder is filled in a cavity of a rubber mold provided with a press pin for forming a through hole, and a liquid pressure is applied from outside the rubber mold to obtain a powder molded body.

[0003]

In the above-mentioned molding method, the raw material powder compressed in the cavity is molded so as to be integrated with the press pin.
A step of removing the press pin from the molded body is required. However, the press pins used in the production of spark plugs are generally very long and slender, and if the releasability from the molded body is poor, defects such as cracks and chips are likely to occur in the molded body at the time of extraction, and the yield is reduced. There are easy problems. In order to prevent this, a mold release agent such as oil is applied to the press pin each time molding is performed. However, the work of applying the release agent requires a lot of man-hours, and the release property is also low. In many cases, it cannot be improved sufficiently. In addition, when the application amount of the release agent becomes excessive, there is a concern that the performance of the insulator may be deteriorated (for example, the withstand voltage may be reduced) due to mixing of components.

Further, in order to facilitate the removal of the press pin, a gentle taper (so-called punch taper) is applied to the outer peripheral surface of the press pin. However, when a press pin having a punched taper is used, the obtained through hole of the insulator necessarily has a tapered surface. When the amount of this taper is large, the gap between the center electrode inserted into the through hole and the inner surface of the hole increases, and it becomes difficult for the electrode to heat through the insulator when using the spark plug, leading to a shortened life. There is. In this case, it is not preferable to form a taper corresponding to the center electrode because the process becomes complicated and the manufacturing cost increases.

A first object of the present invention is to provide a spark which can easily remove a press pin when a raw material powder is formed, thereby simplifying the forming process and improving the yield of a formed body. An object of the present invention is to provide a method for manufacturing a plug insulator and a press pin used for the same. Further, the second problem is that the gap between the center electrode and the inner surface of the hole can be reduced while the taper based on the extraction taper on the press pin side is formed in the through-hole of the insulator, and thus the insulator is interposed. An object of the present invention is to provide a spark plug having a long life, in which heat is easily drawn from an electrode.

[0006]

The method of manufacturing an insulator for a spark plug according to the present invention is directed to a method of manufacturing an insulator in which a through hole for inserting a center electrode and a terminal electrode is formed in an axial direction. A powder filling step of filling a raw material powder into a cavity of a molding die in which a press pin is arranged; a pressurizing step of pressing and molding the raw material powder in the cavity; and removing the press pin from the obtained molded body. Accordingly, the method includes a press pin removing step of forming a pin hole to be a through hole, and a firing step of firing the formed body after removing the press pin as it is or after processing the outer surface thereof. The first is that, as a press pin, one in which at least a part of the outer surface of a portion located in the cavity is covered with a hard carbon release coating mainly composed of amorphous carbon is used. Features.

A press pin according to the present invention is used for manufacturing an insulator for a spark plug in which a through hole for inserting a center electrode and a terminal electrode is formed in an axial direction. In the cavity of the mold in which the cavity filled with the raw material powder is formed, the cavity is arranged to form a pin hole to be a through hole in the obtained molded body, and at least an outer surface of a portion located in the cavity is formed. It is characterized in that a part thereof is covered with a hard carbon type release coating mainly composed of amorphous carbon.

[0008] By forming the release coating as described above on the press pin, when the raw material powder is formed, the releasability of the press pin becomes extremely good, and the press pin is removed without cracking or chipping. Can be performed smoothly. As a result, the application of the release agent to the press pins can be omitted or reduced, so that the molding process can be simplified and the yield of the molded body can be improved.

[0009] In the present specification, the term "hard carbon-based release coating mainly composed of amorphous carbon" means that the skeleton structure of carbon, which is the main component of the film, is amorphous and its Vickers hardness is 1500 kg. / Mm ² or more. The hardness of the coating can be measured using, for example, a dynamic ultra-micro hardness tester (for example, NHT manufactured by CSEM, Switzerland). Such coatings are already widely used in various fields, for example, to improve the lubricity of sliding members such as magnetic heads and bearings, and to improve the wear resistance of ceramic tools and the like. As the method of producing the coating, those disclosed in various publications or literatures can be adopted. Examples thereof include JP-A-58-91100, JP-B-6-60404, JP-T-63-501237, and JP-A-63-501237. 63-162870, JP-A-63-162873
JP-A-63-222314, JP-A-5-2962
No. 48 and the like. In these methods, a film is formed by a vapor phase film forming method using the principle of a physical vapor deposition method or a chemical vapor deposition method. Specifically, a hydrocarbon as a raw material gas or a mixed gas of hydrocarbon and hydrogen is introduced into a decompressed atmosphere, and thermally decomposed using high-frequency plasma or a resistance heating filament to form a substrate (press in the present invention). Pin) A coating is obtained by depositing on the surface.

In the coating, a diamond bond (covalent bond by sp ³ hybrid orbital) is formed in a bond forming a carbon skeleton structure.
Is also referred to as a diamond-like carbon coating (hereinafter, abbreviated as a DLC coating) and is very hard.
In addition, it has a surface property with very low adhesion to the insulating raw material powder such as alumina, and the releasability from the molded body becomes extremely good even when molding is performed under high pressure. this is,
It is presumed that dangling bonds generated in the carbon skeleton structure are blocked by hydrogen atoms derived from the raw material gas, thereby realizing a surface structure in which hydrogen bonding which causes the raw material to adhere is difficult to occur. In addition, the entry of hydrogen atoms into dangling bonds of carbon atoms stabilizes the amorphous structure by closing the carbon chain, and creates a harmful graphite structure that causes a decrease in film strength and adhesion to the base. Is considered to contribute to the realization of a DLC film having high strength and high adhesion (see, for example,
No. 162872).

The hard carbon release coating used in the present invention may contain additional elements such as phosphorus, silicon, tungsten, and chromium for the purpose of improving the film hardness. A method for producing a film containing these additional elements is described in, for example,
JP-A-162870, JP-A-63-162871 or JP-A-63-162872 may be employed.

[0012] One or more intermediate layers can be formed between the hard carbon release coating and the press pin base material as the base for the purpose of improving the adhesion strength of the coating. . For example, an intermediate layer composed of a lower layer mainly composed of chromium or titanium and an upper layer mainly composed of silicon or germanium disclosed in Japanese Patent Publication No. 6-60404 is an iron-based metal or a cemented carbide. (WC-Co composite material) and the like, and there is a great effect in increasing the adhesion strength between the press pin base material and the hard carbon release coating.

Next, a second method of manufacturing an insulator for a spark plug is to use a press pin in which a rib-shaped or groove-shaped pin-side spiral portion is formed on the outer peripheral surface in the pressurizing step. By forming a molded body-side spiral portion having a shape obtained by inverting the pin-side spiral portion on the inner surface of the pin hole of the molded body, and in the press pin removing step, the press pin is relatively rotated around the axis with respect to the molded body, so that the pin side The press pin is screwed in the axial direction with respect to the molded body based on a screw action caused by the engagement between the spiral part and the spiral part on the molded body side, and the press pin is extracted. In this case, the compact may be fixed and the press pin may be rotated, or the press pin may be fixed and the compact may be rotated. Further, both the press pin and the compact may be rotated in opposite directions.

If the press pin integrated into the molded body is to be removed at once by drawing or the like, a large pulling force is required to overcome the frictional force acting on the entire contact surface between the inner surface of the pin hole and the outer surface of the press pin. Becomes As a result, an excessive force is inevitably applied to the molded body, and the probability of occurrence of defects such as cracking, chipping or breakage increases.
Therefore, as described above, if a rib-shaped or groove-shaped pin-side spiral portion is formed on the outer peripheral surface of the press pin, the screw action by the engagement between the pin-side spiral portion and the molded body-side spiral portion is used. The press pin can be smoothly and reliably removed from the molded body, and the above-described problems are less likely to occur. In addition, by combining this with the above-mentioned first method of the present invention, the removal of the press pin from the molded body can be further facilitated.

Next, a spark plug according to the present invention is provided to solve the second problem, and has a metal shell disposed outside the center electrode, and a center electrode having one end coupled to the metal shell. And a ground electrode arranged to face the
Between the center electrode and the metal shell, an insulator is disposed so as to cover the outside of the center electrode, and an insulator in which a through hole for inserting the center electrode and the terminal electrode is formed in the axial direction,
In the through hole of the insulator, a first portion having a substantially cylindrical inner surface through which the center electrode is inserted is formed on the tip side, and a substantially cylindrical second portion having a larger diameter than the first portion is formed behind the first portion. It has a stepped surface shape, at least the inner peripheral surface of the second portion is provided with a taper having a smaller diameter toward the rear side,
On the other hand, the inner peripheral surface of the first portion is not tapered, or is tapered at a smaller angle than the second portion.

On the other hand, a third method of the present invention for producing an insulator for a spark plug is a method for producing an insulator used for the above-described spark plug, wherein the press pin has a first portion formed on the tip end side. A first shaft portion for, and a second shaft portion for forming a second portion in a form following the rear side of the first shaft portion is formed, and at least the outer peripheral surface of the second shaft portion,
An extraction taper for extracting the press pin by moving the press pin relatively rearward in the axial direction with respect to the formed body is formed at least on the outer peripheral surface of the second shaft portion, and is formed on the outer peripheral surface of the first shaft portion. Is there a punching taper formed?
Alternatively, a taper with a smaller angle than the second shaft portion is formed.

In the above-mentioned spark plug, the through hole of the insulator is provided with the first portion on the tip end side through which the center electrode is inserted,
It consisted of at least two parts, the second part having a large diameter following the rear side. And, in the press pin for manufacturing this, the second shaft portion for forming the second portion is subjected to a draft taper, and the first portion for forming the first portion does not have a draft taper, Even when formed, the angle was smaller than the second shaft portion. In the resulting insulator, the inner peripheral surface of the first portion of the through hole is not tapered or has a smaller angle than the second portion, reflecting the formation state of the punch taper of the press pin. It has a tapered shape.

Thus, the taper angle of the first portion of the insulator through hole into which the center electrode is inserted can be reduced.
The gap between the inner surface of the hole and the center electrode can be reduced, and the heat can easily be drawn through the electrode via the insulator, and a long-life spark plug can be realized. On the other hand, by giving the second shaft portion, which is the corresponding portion on the press pin side, a larger taper than the first shaft portion, the press pin can be smoothly removed from the molded body, and the formed punch taper can be obtained. Affects only the second part of the through-hole where the gap amount between the center electrode is not a problem,
The effect on heat drawing and the like can be reduced.

In this case, at least the hard carbon release coating used in the first step of the method of the present invention is formed on the outer peripheral surface of the first shaft portion where the punching taper cannot be made so large with respect to the outer surface of the press pin. With this configuration, smooth release of the press pin is ensured even in the first shaft portion, even though the draft taper is not large (or the draft taper is not substantially provided). As a result, the occurrence of cracks and chips in the compact during the removal of the press pins is more effectively reduced, and the yield of the compact (and thus the insulator) can be further improved. It should be noted that the third method of the present invention described above can be applied together with this, so that the press pin can be more smoothly and reliably extracted from the molded body.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the present invention will be described below with reference to the drawings. A spark plug 100 as an example of the present invention shown in FIG. 1 has a cylindrical metal shell 1, an insulator 2 fitted inside the metal shell 1 so that the tip protrudes, and an insulator 2 with the tip protruded. One end is connected to the center electrode 3 provided inside 2 and the metal shell 1 by welding or the like, and the other end is bent back to the side,
A ground electrode 4 and the like are disposed such that the side faces the tip of the center electrode 3.

A spark discharge gap g is formed between the ground electrode 4 and the center electrode 3. The metal shell 1 is formed of a metal such as low-carbon steel in a cylindrical shape, constitutes a housing of the spark plug 100, and has a screw portion 7 on its outer peripheral surface for attaching the plug 100 to an engine block (not shown). Are formed. 1e is
A tool engaging portion for engaging a tool such as a spanner or a wrench when attaching the metal shell 1, and has a hexagonal axial cross-sectional shape. The center electrode 3 and the ground electrode 4 are made of a Ni alloy or the like, and a core material 3a such as Cu or a Cu alloy for promoting heat radiation is embedded as needed.

Next, the insulator 2 has a through hole 6 into which the center electrode 3 is fitted along its own axial direction, and is made of an insulating material mainly composed of alumina or the like.
The terminal electrode 13 is inserted and fixed on one end side, and the center electrode 3 is inserted and fixed on the other end side. Further, a resistor 15 is arranged between the terminal electrode 13 and the center electrode 3 in the through hole 6. Both ends of the resistor 15 are electrically connected to the center electrode 3 and the terminal electrode 13 via conductive glass seal layers 16 and 17, respectively. The resistor 15 is formed of a resistor composition obtained by mixing glass powder and conductive material powder (and ceramic powder other than glass as required) and sintering the mixture by hot pressing or the like.

The axial sectional diameter of the center electrode 3 is set smaller than the axial sectional diameter of the resistor 15. And the through hole 6
Is a substantially cylindrical first portion 6a through which the center electrode 3 is inserted.
And a substantially cylindrical second portion 6b formed on the rear side (upper side in the drawing) of the first portion 6a. As shown in FIG. 1, the terminal electrode 13 and the resistor 1
5 is accommodated in the second portion 6b, and the center electrode 3 is inserted into the first portion 6a. At the rear end of the center electrode 3, an electrode fixing projection 3a is formed to protrude outward from the outer peripheral surface thereof. A convex portion receiving surface 6c for receiving the electrode fixing convex portion 3a of the center electrode 3 is formed in a tapered or round shape at a connection position between the first portion 6a and the second portion 6b of the through hole 6. Have been.

The inner peripheral surface of the second portion 6b of the through hole 6 has a taper Δ1 whose diameter decreases toward the rear side in the axial direction.
Is given. The size of the taper is, for example, 5 /
It is about 1000 to 5/100 (about 2/100 in this embodiment). On the other hand, the inner peripheral surface of the first portion 6a is provided with a taper Δ2 having a smaller angle than that of the second portion 6a, or has a shape in which the taper is not substantially provided.
Note that the overall length of the insulator 2 is, for example, 30 to 75 mm,
The average inner diameter of the second portion 6b of the through hole 6 is 2 to 5 mm, and the average inner diameter of the first portion 6a is 1 to 3.5 mm. Further, when the length of the first portion 6a is L1 and the length of the second portion 6b is L2, the value of L1 / (L1 + L2) is 0.18.
It has been adjusted to the range of ~ 0.40.

Hereinafter, an example of the steps of the method for manufacturing the insulator 2 will be described. First, as a raw material powder, alumina powder (average particle size of 1 to 5 μm) and Si as a sintering aid are used.
Ingredients, such as components, Ca component, Mg component, Ba component or B component, are blended in a predetermined ratio, and a hydrophilic binder (for example, PVA or acrylamide binder) and water are added and mixed to form. Make a base slurry. In addition, as for each additive element-based raw material, for example, Si component is SiO ₂ powder, Ca component is CaCO ₃ powder, Mg component is MgO powder, Ba component is BaCO ₃ powder, and B component is H ₃ BO ₃ powder (or aqueous solution may be used). )).

The molding base slurry is spray-dried by a spray drying method or the like to obtain a molding base granule GP (raw material powder). Then, the press-formed body serving as the original form of the insulator 2 (FIG. 1) is produced by subjecting the green body granulation GP for molding to rubber press molding. 5 to 9 schematically show the steps of rubber press molding. As shown in FIG.
A cylindrical inner rubber mold 82 having a cavity 83 penetrating in the axial direction therein is arranged substantially concentrically in a cylindrical outer rubber mold 81 arranged in the molding machine main body 80. In addition,
The lower opening of the cavity 83 is the bottom lid 84 and the lower holder 8.
Blocked by 5.

In this state, the cavity 83 is filled with a predetermined amount of the base granule GP for molding, and the press pin 50 is inserted in the axial direction here as shown in FIG. Press pin 5
7, the upper holder 86 is integrated with the base end side of FIG.
As shown in the figure, the upper part of the cavity 83 is fitted and closed in a sealed state. The press pin 50 can rotate relative to the upper holder 86 about the axis.

Then, as shown in FIG.
Through the pressurized liquid passage 80a formed at zero
Is applied in the radial direction to the outer peripheral surface of the outer rubber mold 81 to reduce the cavity 83, and the filled granules GP are compressed via the outer rubber mold 81 and the inner rubber mold 82 (that is, granulated). The product GP (raw material powder) is pressurized and compressed by indirectly applying liquid pressure via the rubber molds 81 and 82). As a result, a green (compact) PC is formed integrally with the press pin 50.

Next, when the application of the hydraulic pressure is released, the outer rubber mold 81 and the inner rubber mold 82 elastically return, and the reduced cavity 83 returns to the original shape. As a result, a gap is formed between the green PC formed by compression and the inner surface of the cavity 83. Then, as shown in FIG.
Is pulled up relative to the rubber molds 81 and 82 in the axial direction, whereby the press pin 50 is pulled out of the cavity 83 with the green PC attached.

Here, the raw material powder, ie, the base granulated material for molding G
P is adjusted so as to contain water within a range of 1.5% by weight or less by adjusting conditions at the time of spray drying (for example, drying temperature and spraying speed). Due to moisture content,
It loosens the bonding force of the powder particles in the granulated particles, promotes the disintegration of the granulated particles during pressing, and swells the hydrophilic binder blended in the molding base to effectively improve the caking properties. The main purpose is to increase the strength of drawers and green PCs.

The lower limit of the water content varies depending on the particle size distribution of the raw material powder and the like, but is appropriately set to such an extent that the above effect is not insufficient. If the water content exceeds 1.5% by weight, the fluidity of the granulated material may be poor and the granulated material may be difficult to handle.
The water content is more desirably adjusted within a range of 1.3% by weight or less.

The blending amount of the hydrophilic binder in the green body granulated material for molding GP is preferably adjusted in the range of 0.5 to 3.0% by weight. When the compounding amount of the hydrophilic binder is less than 0.5% by weight, the green strength is insufficient and handling becomes difficult, and cracks and chips may be easily generated. On the other hand, if the content exceeds 3.0% by weight, the binder removal time during firing becomes longer, which leads to a reduction in the efficiency of the production of the insulator. In addition, the residual amount of impurity components (eg, carbon) derived from the binder in the insulator. In some cases, leading to a decrease in performance (for example, withstand voltage).

On the other hand, the pressing pressure (liquid pressure) is 30 to 10
It is preferable to adjust in the range of 0 MPa. Press pressure is 30
If it is less than MPa, the green strength becomes insufficient and handling becomes difficult, and cracks and chips may easily occur. On the other hand, when it exceeds 100 MPa, the life of the rubber mold is shortened, which may lead to an increase in cost. Also,
Because of the high pressure molding, the cavity inner wall portion is bitten from the outer surface of the molded product so as to penetrate between the powder particles, and the smooth elastic return of the rubber mold (in this case, the inner rubber mold 82) at the time of unloading is easily hindered. As a result, vibration due to rapid elastic return of the rubber mold is likely to occur, and the green may be easily damaged.

Returning to FIG. 9, it is necessary to remove the press pin 50 from the green PC thus formed. As shown in FIG. 8, the forming granulated material GP in the cavity 83 is compressed in the radial direction and strongly pressed against the outer peripheral surface. The substrate tends to adhere. As described above, there is no problem because the amount of water in the base granulated material for molding GP is adjusted within the range of 1.5% by weight or less. More likely to occur.

Since the press pin 50 is for forming the elongated through hole 6 in the insulator 2, the average outer diameter of the portion located in the cavity 83 (FIG. 7) is D,
Similarly, when the length is L, an extremely large aspect ratio of 6 ≦ L / D ≦ 26 is used. Therefore, FIG.
As shown in FIG.
0 is pulled out of the green PC, the adhered molding material DP is peeled off, and the inner walls of the pin holes 6b 'and 6a' formed after the press pin 50 comes off, or a defect occurs in the green PC. In some cases, a natural force acts to cause a failure such as breakage.

Therefore, in this embodiment, various improvements are made to the press pin 50 in order to improve the releasability.
FIG. 2 shows details of the press pin 50. The press pin 50 has a first portion 6a of the through hole 6 in FIG.
Are formed, and a second shaft portion 52 for forming the second portion 6b of the through-hole 6 is formed in a shape following the rear side of the first shaft portion 51. . For example, the whole is made of a material having high rigidity, for example, a cemented carbide or alloy tool steel, so that deformation during molding is minimized.

The outer peripheral surface of the second shaft portion 52 has a draft taper Δ whose diameter decreases toward the rear side in the axial direction.
1 'is given. The size of the taper Δ1 ′ is, for example, about 5/1000 to 5/100 (in the present embodiment, 2/1000).
100). On the other hand, the outer peripheral surface of the first shaft portion 51 is
The second taper portion 52 is provided with a smaller taper Δ2 ′ than the second shaft portion 52 or has a shape in which the taper is substantially not provided. The average outer diameter d1 of the first shaft portion 51
Is 1.2 to 4 mm, and the average outer diameter d2 of the second shaft portion 52 is
Is 3.6 to 5.7 mm. Further, when the length of the first shaft portion 51 is L1 'and the length of the second shaft portion 52 is L2', the value of L1 '/ (L1' + L2 ') is 0.18 to 0.4.
It is adjusted to the range of 0. The average outer diameter D of the portion of the press pin 50 located in the cavity 83 (FIG. 7)
Is approximately {L1 ′ / (L1 ′ + L2 ′)} × d1 + {L2 ′
/ (L1 '+ L2')｝ d2, whose value is 3.7
It is about 4.9 mm.

A flange-shaped end surface forming portion 55 forming the base end surface of the green PC (FIG. 8) is integrated with the base end side of the second shaft portion 52 of the press pin 50, and further rearward thereof. On the side, a head portion 56 having an internal thread portion 57 formed in the axial direction is formed. As shown in FIG. 10, an upper holder 86 is rotatably fitted to the outside of the head 56.

As shown in FIG. 2, a rib-shaped pin-side spiral portion 54 is formed on the outer peripheral surface on the base end side of the second shaft portion 52. Note that the spiral winding direction of the pin-side spiral portion 54 is opposite to the spiral winding direction of the female screw portion 57. When molding is performed using such a press pin 50, as shown in FIG.
On the inner surface of the portion 6b 'corresponding to the second shaft portion 52 of the pin hole of the green PC (formed body), a formed body side spiral portion 20a having a shape obtained by inverting the pin side spiral portion 54 (that is, a groove shape) is formed. The Rukoto. In addition, this molded body side spiral portion 20a
Is often removed by cutting or the like, but if not removed, as shown in FIG.
It remains as a spiral part 20.

On the other hand, as shown in FIG.
A hard carbon release coating 60 mainly composed of amorphous carbon is formed on at least the outer peripheral surface of the second shaft portion 52, in this embodiment, substantially the entire surface except the inner surface of the female screw portion 57. In this embodiment, as shown in FIG.
0 and a hard metal base material 52a, an intermediate layer 61 composed of two layers is formed. Of these, the lower layer 61b is mainly composed of chromium (or titanium), and the upper layer 61a is mainly composed of silicon (or germanium). Such a multilayer coating structure is disclosed in, for example, Japanese Patent Publication No. 6-60404.
Can be formed. The outline of the processing is as follows.

First, after the surface of the base material 52a is degreased and washed, the lower layer 61b and the upper layer 61a are sequentially formed by a known vacuum evaporation method, ion plating method, sputtering method or the like. Next, this is set on the cathode side in a vacuum chamber of the plasma polymerization film forming apparatus. Then, the inside of the vacuum chamber is evacuated, and a hydrocarbon gas (for example, methane, ethylene, benzene, or the like;
Hydrogen may be mixed. In this embodiment, methane is used), and the pressure is adjusted to, for example, about 0.1 torr. Then, a high frequency voltage of, for example, about 3.56 MHz is applied between the cathode and the anode in the vacuum chamber to generate plasma. As a result, the hydrocarbon is decomposed and deposited on the upper layer 61a in the form of amorphous carbon while taking in hydrogen, thereby forming the hard carbon-based release coating 60.

Here, the thickness of the lower layer 61b is 0.1 to 0.1.
4 μm (for example, 0.2 μm in this embodiment), upper layer 61 a
Has a thickness of 0.1 to 0.4 μm (for example, 0.1 μm in this embodiment).
2 μm), and the thickness of the hard carbon-based release coating 60 is 0.1 to
The thickness is about 1.5 μm (for example, 0.4 μm in this embodiment). The Vickers hardness of the hard carbon-based release coating 60 is 1500 kg / mm ² or more (in this embodiment, 3000 V / mm ^2).
０００4000 kg / mm ² ).

Further, the surface roughness of the substrate 52a is 0.05
It is preferable to adjust within the range of 5 μmRa. After the formation of the coating, the surface roughness of the hard carbon release coating 60 is preferably adjusted in the range of 0.05 to 5 μmRa.
When the surface roughness of the release coating 60 becomes 5 μmRa or more,
The releasability of the press pin 50 from the green PC may be deteriorated. On the other hand, when the surface roughness is less than 0.05 μmRa, the surface of the press pin 50 and the inner surface of the pin hole of the green PC may be too close to each other, and the mold releasability may worsen.

The hard carbon release coating 60 and the substrate 52
In the case where the adhesive force with a can be sufficiently ensured, as shown in FIG. 3B, only one intermediate layer 61 is used (for example, mainly composed of silicon), or as shown in FIG. As described above, the intermediate layer 61 can be omitted. In particular,
In the case of the base material 52a containing a large amount of a carbon component such as a cemented carbide, there is an advantage that the affinity with the hard carbon-based release coating 60 is strong and the adhesion of the film is easily secured.

FIG. 10 shows how such a press pin 50 is extracted from the green PC. That is,
Green P pulled up from cavity 83 (FIG. 9)
While holding C by an air chuck (not shown), the rotating shaft 87 screwed into the female screw hole 57 of the press pin 50 is rotated by a driving source such as a motor (not shown) so as to be tightened into the female screw hole 57. As a result, the press pin 50 rotates around the axis with respect to the green PC, and the pin-side spiral portion 5
The press pin 50 advances and rises in the pulling-out direction based on the screw action caused by the engagement between the screw 4 and the molded body-side spiral portion 20a.

That is, since the press pin 50 slowly rises while rotating due to the screwing action of the screw, an excessive frictional force is less likely to be generated between the press pin 50 and the inner wall surface of the pin hole, and thus the green PC is smooth without damaging the green PC. The press pin 50 can be withdrawn. Press pin 50
Since the second shaft portion 52 is formed with a taper, the pin 50 can be easily released from the pin hole 6b 'by only slightly rising. On the other hand, the first shaft portion 51 is not provided with a large taper, but the surface thereof is hard carbon-based release coating 60.
Is formed, so that the mold release from the pin hole 6a 'can be performed smoothly. It goes without saying that the hard carbon-based release coating 60 formed on the first shaft portion 51 makes the release of the shaft portion 51 easier. As shown in FIG. 4, the pin-side spiral portion 54 may be formed at the tip of the press pin 50 (for example, when the aspect ratio of the green PC is relatively small).

As shown in FIG. 11, the green PC from which the press pins 50 have been removed is subjected to grinding or the like on its outer surface to finish the outer shape corresponding to the insulator 2 shown in FIG. ~ 1650 ° C
Fired. Thereby, each pin hole 6a ', 6b'
Are the first portions 6a and 6b of the through hole 6 in FIG. After that, it is further baked with glaze and finished. The spark plug 100 using this is attached to the engine block at the screw portion 7 and used as an ignition source for the air-fuel mixture supplied to the combustion chamber.

For example, when the first shaft portion 51 of the press pin 50 is excessively tapered in the through hole 6 of the insulator 2, as shown in a somewhat exaggerated manner in FIG. A relatively large gap GW is likely to occur in the first portion 6a which is a portion. This impedes the dissipation of heat from the center electrode 3 through the insulator 2 (so-called heat drawing), leading to problems such as electrode wear-out and shortening of the life of the plug. However, by employing the above-described manufacturing method, it is possible to reduce the amount of application of the draft taper to the first shaft portion 51. This facilitates the heat removal of the center electrode 3 and extends the life of the spark plug 100.

[0049]

EXAMPLES The following experiments were conducted to confirm the effects of the present invention. (Example 1) Alumina powder (purity 9) was used as a raw material powder.
9.5%, average particle size of 3.0 μm), SiO ₂ (purity 99.5%, average particle size of 1.5 μm), CaCO ₃ (purity 99.9%, average particle size of 2.0 μm), MgO (Purity 9
9.5%, average particle size 2 μm), BaCO ₃ (purity 99.
5%, average particle size 1.5 μm), H ₃ BO ₃ (purity 99.
0%, an average particle size of 1.5 μm) in a predetermined ratio, and the total amount of the mixed powder is 100 parts by weight,
3 parts by weight of PVA as a hydrophilic binder and water 103
By weight and wet mixing, a green body slurry for molding was prepared.

Next, these slurries having different compositions were dried by a spray-drying method, respectively, to prepare spherical molding base granules GP. The granules GP are sieved to a particle size of 50 to 100 μm. The granules GP are mixed with water so as to have various contents of 0.5 to 1.5% by weight, and are formed at various pressures by a rubber press method described with reference to FIGS. Green PC having the shape shown in FIG. The dimensions of each part are as follows: l1 = 85 mm, r1 = 20 mm, r2 =
13 mm, r3 = 20 mm, δ1 = 5 mm, δ2 = 3 m
m.

The press pin 50 shown in FIG. 2 was used. However, the substrate is made of cemented carbide, and the finished surface roughness of each outer peripheral surface of the first shaft portion 52 and the second shaft portion 51 before coating is 0.2 μm Ra by mirror polishing.
Two levels were adopted, one of about 4 μmRa and about 4 μmRa by grinding. Note that the pin-side spiral portion 54 is not formed. A lower layer 61b made of chromium and an upper layer 61a made of silicon were each formed to a thickness of 0.2 μm by a known high-frequency sputtering method. Further, the hard carbon release coating 60 was formed by the plasma polymerization method described above. However, methane was used as the source gas, the gas flow rate was 30 cm ³ / min, the pressure was 0.1 torr,
The high frequency power was 100 W. The surface roughness after forming the hard carbon-based release coating 60 was 0.2 μmR for the mirror-polished product.
a, The grinder ground product was 4 μmRa, and the Vickers hardness of the coating was approximately 3500 kg / mm ² .

For comparison, an experiment using a press pin which did not form the hard carbon-based release coating 60 or the like only after the base material surface was mirror-polished was performed. The press pin was used by applying a release agent (spindle oil) as necessary.

With respect to the obtained green, the following items were checked.・ Releasability of press pin: easy release, no adhesion was good (○), slight adhesion was acceptable (△), firmly fixed, large adhesion, Alternatively, a molded article that was damaged was judged as unacceptable (x). Table 1 shows the above results.

[0054]

[Table 1]

That is, in the tests (Nos. 13 to 30) of the examples using the press pins with the hard carbon release coating,
It can be seen that good releasability was obtained under any of the test conditions of the finished surface accuracy of the base material, the molding pressure, and the moisture content. On the other hand, in the tests (Nos. 1 to 12) of the comparative examples using the press pins without the hard carbon-based release coating, it can be seen that the releasability was poor. In this case, the use of a release agent can improve the performance somewhat, but still does not reach the level of the embodiment.

[Brief description of the drawings]

FIG. 1 is an overall front sectional view showing an example of a spark plug of the present invention.

FIG. 2 is an explanatory view showing an example of a press pin used for manufacturing the insulator.

FIG. 3 is a conceptual diagram illustrating the form of a coating applied to the surface of a press pin.

FIG. 4 is a schematic view showing an example in which a pin-side spiral portion is formed at the tip of a press pin.

FIG. 5 is an explanatory view of a manufacturing process of an insulator.

FIG. 6 is an explanatory view following FIG. 5;

FIG. 7 is an explanatory view following FIG. 6;

FIG. 8 is an explanatory view following FIG. 7;

FIG. 9 is an explanatory view following FIG. 8;

FIG. 10 is an explanatory view following FIG. 9;

FIG. 11 is an explanatory view following FIG. 10;

FIG. 12 is an explanatory diagram exaggeratingly showing that a taper on an inner surface of an insulator through-hole forms a gap between itself and a center electrode.

FIG. 13 is a view for explaining a problem caused by a press pin having poor releasability.

FIG. 14 is a perspective view showing the outer shape of a green produced in the example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 metal shell 2 insulator 3 center electrode 4 ground electrode g spark discharge gap 6 through hole 6a first portion 6b second portion 13 terminal electrode 50 press pin 51 first shaft portion 52 second shaft portion 54 pin side spiral portion 60 hard carbon System release coating 81 Outer rubber mold 82 Inner rubber mold 83 Cavity GP Base granule for molding (raw material powder) PC Green (Molded body) 6a ', 6b' Pin hole

Continued on the front page (72) Inventor Satoshi Iio 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi F-term in Japan Special Ceramics Co., Ltd. 4G053 AA15 BA06 BB17 BC02 BC04 BD02 BE04 BF01 CA15 CA16 CA23 EA08 EA27 EA43 EB01 EB17 4G054 AA06 AB07 AC00 5G059 AA04 AA10 FF02 FF06 FF14

Claims (14)

[Claims] 1. A method of manufacturing an insulator for a spark plug in which a through hole for inserting a center electrode and a terminal electrode is formed in an axial direction, wherein a raw material powder is placed in a cavity of a molding die in which a press pin is arranged. Forming a pin hole to be the through-hole by removing the press pin from the obtained molded body, and a pressing step of pressing and molding the raw material powder in the cavity. A press pin removing step, and a firing step of firing the molded body after removing the press pin as it is or after processing the outer surface thereof, and firing the press pin as a part of the portion located in the cavity. A method for producing an insulator for a spark plug, wherein at least a part of an outer surface is covered with a hard carbon release coating mainly composed of amorphous carbon. 2. The press pin according to claim 1, wherein D is an average outer diameter of a portion located in the cavity, and L is a length of the press pin.
2. The method for producing an insulator for a spark plug according to claim 1, wherein one having 6 ≦ L / D ≦ 26 is used. 3. The pressurizing step uses a rubber mold as the molding die, and pressurizes the raw material powder in the cavity by indirectly applying a liquid pressure through the rubber mold. The method for producing an insulator for a spark plug according to claim 1 or 2, wherein the method is a rubber pressing step of compressing. 4. The rubber mold is formed in a cylindrical shape, and the press pin is inserted in the cylindrical rubber mold in an axial direction, and the hydraulic pressure is applied in a radial direction with respect to an outer peripheral surface of the rubber mold. The method for manufacturing an insulator for a spark plug according to claim 3, which is added. 5. The spark plug according to claim 3, wherein the raw material powder is mainly composed of alumina and contains water in a range of 1.5% by weight or less. Manufacturing method of insulator. 6. A first portion having a substantially cylindrical inner surface through which a center electrode is inserted at a tip end of the through hole of the spark plug insulator to be manufactured. The press pin also has a first shaft portion for forming the first portion on the tip end side, and a first shaft thereof. And a second shaft portion for forming the second portion in a shape following the rear side of the portion, and at least an outer peripheral surface of the second shaft portion is provided with the press pin in an axial direction with respect to the molded body. A withdrawal taper for removing this by relatively moving to the rear side is formed at least on the outer peripheral surface of the second shaft portion, and the withdrawal taper is formed on the outer peripheral surface of the first shaft portion. Not at all, or a small angle Method for manufacturing an insulator for a spark plug according to any one of claims 1 Pa is formed 5. 7. The method for manufacturing an insulator for a spark plug according to claim 6, wherein said hard carbon-based release coating is formed on at least an outer peripheral surface of said first shaft portion of said press pin. 8. In the pressing step, by using a rib-shaped or groove-shaped pin-side spiral portion formed on an outer peripheral surface of the press pin, an inner surface of the pin hole of the molded body is provided with the pin-side spiral portion. Forming a molded body-side spiral portion having a shape in which the spiral portion is inverted, in the press pin removing step, by relatively rotating the press pin around the axis with respect to the molded body,
8. The press pin according to claim 1, wherein the press pin is screwed in the axial direction with respect to the molded body, and is removed from the molded body based on a screw action caused by engagement between the pin-side spiral portion and the molded body-side spiral portion. A method for producing an insulator for a spark plug according to any one of the above. 9. The method for manufacturing an insulator for a spark plug according to claim 8, wherein the pin-side spiral portion is formed on a base end side of the second shaft portion. 10. The method for manufacturing an insulator for a spark plug according to claim 1, wherein said press pin is made of a cemented carbide. 11. A method of manufacturing an insulator for a spark plug in which a through hole for inserting a center electrode and a terminal electrode is formed in an axial direction, wherein a raw material powder is placed in a cavity of a molding die in which a press pin is arranged. Forming a pin hole to be the through-hole by removing the press pin from the obtained molded body, and a pressing step of pressing and molding the raw material powder in the cavity. A press pin removing step, and a firing step of firing the molded body after removing the press pin as it is or after processing the outer surface of the molded body. By using a rib-shaped or groove-shaped pin-side spiral portion, a molded-body-side spiral portion having a shape obtained by inverting the pin-side spiral portion is formed on the inner surface of the pin hole of the molded body. In the press pin removing step, by relatively rotating the press pin around the axis with respect to the molded body,
A spark plug, wherein the press pin is screwed in the axial direction with respect to the molded body and removed therefrom based on a screw action caused by engagement between the pin-side spiral portion and the molded body-side spiral portion. Manufacturing method of insulator for industrial use. 12. A method of manufacturing an insulator for a spark plug in which a through hole for inserting a center electrode and a terminal electrode is formed in an axial direction, wherein a raw material powder is placed in a cavity of a molding die in which a press pin is arranged. Forming a pin hole to be the through-hole by removing the press pin from the obtained molded body, and a pressing step of pressing and molding the raw material powder in the cavity. And a firing step of firing the molded body after removing the press pin as it is or after processing the outer surface thereof, and firing the spark plug insulator to be manufactured. The hole has a stepped portion in which a first portion having a substantially cylindrical inner surface through which a center electrode is inserted is formed on the distal end side, and a substantially cylindrical second portion having a larger diameter is formed behind the first portion. Having a shape, the press pin has a first shaft portion on the distal end side for forming the first portion, and a second shaft portion for forming the second portion in a shape following the rear side of the first shaft portion. A biaxial portion is formed, and at least an outer peripheral surface of the second axial portion has a removal taper for removing the press pin by moving the press pin relatively rearward in the axial direction with respect to the molded body. Is formed at least on the outer peripheral surface of the second shaft portion, and the outer peripheral surface of the first shaft portion is not formed with the draft taper, or has a draft taper with a smaller angle than the second shaft portion. A method of manufacturing an insulator for a spark plug, wherein the insulator is formed. 13. A press pin for use in manufacturing an insulator for a spark plug, wherein a through hole for inserting a center electrode and a terminal electrode is formed in an axial direction. In the cavity of the mold in which the cavity is formed, at least a part of the outer surface of a portion located in the cavity is formed to form a pin hole to be the through hole in the obtained molded body, A press pin characterized in that it is covered with a hard carbon release coating mainly composed of amorphous carbon. 14. A metal shell disposed outside the center electrode, a ground electrode having one end coupled to the metal shell and arranged to face the center electrode, and An insulator in which a through-hole for inserting the center electrode and the terminal electrode is formed in an axial direction, the insulator being provided so as to cover the outside of the center electrode, A stepped surface shape in which a first portion having a substantially cylindrical inner surface through which the center electrode is inserted is formed on the distal end side, and a substantially cylindrical second portion having a larger diameter is formed on the rear side of the first portion. And at least the inner peripheral surface of the second portion is provided with a taper having a smaller diameter toward the rear side, while the inner peripheral surface of the first portion is not tapered, or Indicates that a small angle taper is provided. Spark plug to be a sign.