TWI848558B - Core manufacturing method - Google Patents

Abstract

The present invention provides a core manufacturing method, which can improve the quality and manufacture any shape, and can use a general core manufacturing device. The core is composed of at least two layers, a first layer and a second layer. The method comprises: after manufacturing a first layer core 10, manufacturing a second layer core 11 on the outer peripheral surface 10a of the first layer core 10; wherein the outer peripheral surface 10a of the first layer core 10 has a property of improving the connection with the second layer; When the second-layer core 11 is manufactured on the outer peripheral surface 10a of the first-layer core 10, the second-layer core 11 is manufactured while the first-layer core 10 is held by the second-layer core forming mold 3, and the ratio of the cross-sectional area of the first-layer core 10 to the second-layer core 11 is: the cross-sectional area of the second-layer core 11/the cross-sectional area of the first-layer core 10 ≦1.0.

Description

Core manufacturing method

The present invention relates to a core manufacturing method.

As a conventional core manufacturing method, a core manufacturing device as described in Patent Document 1 is known. This core manufacturing device is formed of an outer layer and an inner layer composed of thermosetting resin coated sand with different particle size indexes. It is used to manufacture a core with an outer layer having a larger particle size index than the inner layer. It includes a pair of molds that can rotate around a horizontal rotation axis, a heating means for heating the mold to a required temperature, a movable frame that moves above the mold in the same direction as the mold contact and separation direction, an outer layer sand trough and an inner layer sand trough that are arranged at a fixed interval on the upper side of the movable frame in front of and behind the moving direction of the movable frame, and an outer layer sand blowing unit and an inner layer sand blowing unit that can move forward and backward between the sand receiving position directly below each of the two sand troughs and the sand blowing position directly above the two molds.

Prior art literature

Patent Literature

Patent document 1: Japanese Patent No. 5042055

It is worth mentioning that, as in the above-mentioned core manufacturing device, after the outer layer of thermosetting resin coated sand is filled in the molding chamber of the mold, the mold is flipped by the mold reversal drive mechanism so that the opening is facing downward, and the uncured outer layer of thermosetting resin coated sand is discharged downward from the opening, thereby forming the outer layer of the core in the molding chamber of the mold. However, when the mold is reversed in this way to discharge the uncured outer layer of thermosetting resin coated sand, there is a problem of poor quality due to unsmooth discharge. In addition, there is a problem that the shape of the core is limited due to the need to reverse the mold to discharge the sand. Furthermore, there is also a problem that the general core manufacturing device cannot be used.

Therefore, the present invention is made in view of the above problems, and aims to provide a core manufacturing method that can improve the quality while being able to manufacture any shape and use a general core manufacturing device.

The above-mentioned purpose of the present invention is achieved by the following means. In addition, although the reference symbols of the following embodiments are attached in parentheses for explanation, the present invention is not limited to them.

According to the invention of claim 1, a method for manufacturing a core composed of at least two layers, a first layer and a second layer, comprises: after manufacturing the first layer core (10), manufacturing the second layer core (11) on the outer peripheral surface (10a) of the first layer core (10); wherein the outer peripheral surface (10a) of the first layer core (10) has a shape that improves the closeness with the second layer core (11); when manufacturing the second layer core (11) on the outer peripheral surface (10a) of the first layer core (10), the second layer core (11) is manufactured in a state where the first layer core (10) is held by a predetermined mold (a mold for forming a second layer core 3).

According to the invention of claim 2, a method for manufacturing a core composed of at least two layers, a first layer and a second layer, comprises: after manufacturing the first layer core (10), coating the outer peripheral surface (10a) of the first layer core (10) (refer to FIG. 5), and manufacturing the second layer core (11) on the outer peripheral surface (10a) of the first layer core (10) after coating; wherein the first layer core (10) after coating The outer peripheral surface (10a) of the first-layer core (10) has a shape that improves the close fit with the second-layer core (11). When the second-layer core (11) is manufactured on the outer peripheral surface (10a) of the first-layer core (10) after coating, the second-layer core (11) is manufactured while the first-layer core (10) is held by a predetermined mold (a mold for forming the second-layer core 3).

According to the invention of claim 3, in the core manufacturing method described in claim 1 or 2, the shape for improving the tightness includes at least one of a concave portion, a convex portion, a concave stripe portion, or a convex stripe portion.

According to the invention of claim 4, in the core manufacturing method described in claim 1 or 2, the first layer of the core (10) is formed into a solid or hollow shape.

According to the invention of claim 5, in the core manufacturing method described in claim 1, the second layer of core (11) is formed to cover a part or all of the outer peripheral surface (10a) of the first layer of core (10).

According to the invention of claim 6, in the core manufacturing method described in claim 2, the second layer of core (11) is formed to cover a part or all of the outer peripheral surface (10a) of the first layer of core (10) after coating.

According to the invention of claim 7, in the core manufacturing method described in claim 1, when manufacturing the second core (11) on the outer peripheral surface (10a) of the first core (10), the first core (10) is preheated to a temperature range of 300°C above room temperature and below.

According to the invention of claim 8, in the core manufacturing method described in claim 2, when manufacturing the second core (11) on the outer peripheral surface (10a) of the first core (10) after coating, the first core (10) is preheated to a temperature range of 300°C above room temperature and below.

According to the invention of claim 9, in the core manufacturing method described in claim 1, the predetermined mold (second layer core forming mold 3) is capable of fixing and retaining the first layer core (10).

According to the invention of claim 10, in the core manufacturing method described in claim 2, the predetermined mold (second layer core forming mold 3) is capable of fixing and retaining the first layer core (10) after coating.

The effects of the present invention will be described below with reference to the reference symbols in the drawings. In addition, although reference symbols of the embodiments described below are attached in parentheses for description, the present invention is not limited thereto.

According to the invention of claim 1, since it is not necessary to reverse the mold to discharge the sand as in the conventional art, the quality of the core can be improved. In addition, since the shape of the core is not limited as in the conventional art, any shape can be manufactured. Furthermore, since a core manufacturing device as in the conventional art is not required, a general core manufacturing device can be used.

On the other hand, the shape for improving the adhesion preferably includes at least one of the concave portion, convex portion, concave stripe portion, or convex stripe portion as described in claim 3, and the first layer of the core (10) is preferably formed into a solid shape or a hollow shape as described in claim 4.

In addition, as described in claim 5, the second layer core (11) is preferably formed to cover a part or all of the outer peripheral surface (10a) of the first layer core (10), and as described in claim 6, the second layer core (11) is preferably formed to cover a part or all of the outer peripheral surface (10a) of the first layer core (10) after coating.

On the other hand, as described in claims 7 and 8, if the first layer core (10) is preheated to a temperature range of 300°C above room temperature and below, the manufacturing time of the second layer core (11) can be shortened.

In addition, if the configuration described in claim 9 and claim 10 is used, the problem that the second layer core (11) may not be successfully manufactured due to the position deviation of the first layer core (10) can be solved.

1: Mold for forming the core of the first layer

1a,1b,3a,3b:Mold

2: Cavity of the mold for forming the first layer with a core

2a: Upper surface of cavity 2

2b: Lower surface of cavity 2

4: Cavity of the mold for forming the second layer with a core

4a: Upper surface of cavity 4

4b: Lower surface of cavity 4

3: Mold for forming the core of the second layer (predetermined mold)

3A: First mold

3Aa: Lower surface of the first mold 3A

3B: Second mold

3Ba: Upper surface of the second mold 3B

3Ba1: concave hole

5:Fix the support parts

5a: Support part

5b:Fixing pin

6: Fixed support part

10: The first layer of core

10A: Body

10Aa: Outer surface of the body

10Aa1: Fixing hole

10B: Feet

10C: horizontal stripes

10a: The outer surface of the first layer of the core

10AA: First layer of paint

11: The second layer of core

11a: The outer surface of the second layer core

11A: Second layer of paint mold

Figure 1 is an explanatory diagram for explaining the steps of a core manufacturing method of an embodiment of the present invention.

Figure 2 is a longitudinal cross-sectional view showing the state in which the core of the first layer is fixed by the second layer core forming mold in the same embodiment.

Figure 3 is a semi-longitudinal cross-sectional view showing the state in which the core of the first layer is fixed by the second layer core forming mold in the same embodiment.

Figure 4 is an exploded perspective view showing another embodiment in which the core of the first layer is fixed by the second layer core forming mold.

Figure 5 is a partial longitudinal cross-sectional view of a two-layer core showing another embodiment.

An embodiment of the core manufacturing method of the present invention will be described in detail below with reference to the drawings. In addition, in the following description, when the up, down, left, and right directions are indicated, they are based on the up, down, left, and right directions when viewed from the front of the drawing.

The core manufacturing method of this embodiment is mainly a method for manufacturing a sand model core for aluminum die casting, and is carried out through the steps shown in Figure 1. Specifically, as shown in Figure 1 (a), first, a first-layer core forming mold 1 for forming the first-layer core is prepared. This first-layer core forming mold 1 is formed in a rectangular shape in the figure and can be divided into two halves, and in Figure 1 (a) it is shown as a state of being spliced by a pair of molds 1a and 1b. Then, a cylindrical mold cavity 2 having an open upper surface 2a and a closed lower surface 2b is formed in the first-layer core forming mold 1. Then, the first-layer casting sand (for example, resin-coated sand, etc.) is blown into the mold cavity 2 thus formed from the upper surface 2a. Thus, as shown in Figure 1 (b), the first layer of the core 10 (a solid circle is used as an example in the figure) is manufactured.

Then, the first layer core 10 manufactured as shown in FIG. 1(b) is taken out from the first layer core forming mold 1 as shown in FIG. 1(c). In addition, when taking it out, the first layer core 10 can be taken out by separating the first layer core forming mold 1, that is, a pair of molds 1a and 1b.

Then, as shown in FIG. 1 (d), a second-layer core forming mold 3 for forming the second-layer core is prepared. The second-layer core forming mold 3 is formed in a rectangular shape in the figure and can be divided into two halves, and is shown in FIG. 1 (d) as a state of being spliced by a pair of molds 3a and 3b. Next, a cylindrical cavity 4 having an open upper surface 4a and a closed lower surface 4b is formed in the second-layer core forming mold 3. Then, the first-layer core 10 is placed in this cavity 4, and the second-layer casting sand (for example, resin-coated sand, etc.) is blown into the cavity 4 from the upper surface 4a. Thereby, as shown in FIG. 1 (e), the second-layer core 11 is manufactured to cover the outer peripheral surface 10a of the first-layer core 10.

Furthermore, when the first-layer core 10 is placed in such a cavity 4, the first-layer core 10 can be fixed in the cavity 4 to maintain the posture of the first-layer core 10. As a specific example, as shown in FIG. 2, the second-layer core forming mold 3 can be composed of a first mold 3A and a second mold 3B, wherein the first mold 3A can be divided into two halves. In addition, the second mold 3B can be inserted into the first mold 3A from the lower surface 3Aa of the first mold 3A, and is formed into a convex shape when viewed from a longitudinal section, and the upper surface 3Ba of the second mold 3B is formed with a plurality of concave holes 3Ba1 (two in the figure).

On the other hand, the first-layer core 10 is composed of a main body 10A having a rectangular front surface and a pair of legs 10B provided integrally with the lower surface of the main body 10A. In this case, the main body 10A is provided in the mold cavity 4, and the pair of legs 10B are inserted into a plurality of recessed holes 3Ba1 provided in the lower surface 4b of the mold cavity 4. Next, the first-layer core 10 is provided with a pair of fixing holes 10Aa1 on one side of the outer peripheral surface 10Aa of the main body 10A as shown in FIG. 3, and the fixing pins 5b of the fixing support member 5 provided in the second-layer core forming mold 3 (first mold 3A) are inserted into the fixing holes 10Aa1 as shown in FIG. 3. In other words, the fixed support member 5 has a rectangular support portion 5a as shown in FIG3, and a pair of fixing pins 5b protruding inward are provided on both sides of the support portion 5a. In this way, by inserting a pair of fixing pins 5b into a pair of fixing holes 10Aa1, the first layer of core 10 can be fixed in the mold cavity 4 while maintaining its posture. In this way, when the second layer of casting sand (such as resin-coated sand, etc.) is blown into the mold cavity 4 from the upper surface 4a, the posture of the first layer of core 10 can be maintained. Therefore, when the second layer of casting sand is blown into the mold cavity 4 from the upper surface 4a, the problem that the second layer of core 11 may not be successfully manufactured due to the positional deviation of the first layer of core 10 can be solved.

On the other hand, when the first-layer core 10 is fixed in the cavity 4, the method is not limited to the above method, and the method shown in FIG. 4 may be used. In other words, as shown in FIG. 4, when the first-layer core 10 is composed of a body 10A, a pair of legs 10B integrally provided with the lower surface of the body 10A, and a rectangular cross bar 10C integrally provided outward from both side surfaces of the body 10A, a fixed support portion 6 for fixedly supporting the cross bar 10C is provided in the second-layer core molding die 3 (first die 3A). In other words, the fixed support portion 6 is formed in a concave shape that is approximately the same shape as the cross bar 10C, and the cross bar 10C is fixedly supported by the fixed support portion 6 by inserting the cross bar 10C into the fixed support portion 6. In this way, since the cross bar 10C is fixedly supported by the fixed support part 6, the first layer of the core 10 can be fixed in the mold cavity 4 while maintaining its posture. Therefore, the above-mentioned arrangement can also fix the first layer of the core 10 in the mold cavity 4. In addition, since the cross bar 10C is fixedly supported in the fixed support part 6, the cross bar 10C will not be blown by the second layer of casting sand. Therefore, the outer peripheral surface of the cross bar 10C will not be covered by the second layer of the core 11.

Therefore, as shown in FIG. 1 (e), when the second-layer core 11 is manufactured by covering the outer peripheral surface 10a of the first-layer core 10, it is taken out from the second-layer core forming mold 3 as shown in FIG. 1 (f). In addition, when taking it out, the second-layer core forming mold 3, that is, the pair of molds 3a and 3b are separated, and the second-layer core 11 manufactured by covering the outer peripheral surface 10a of the first-layer core 10 can be taken out.

Thus, through the above manufacturing steps, a core consisting of at least two layers of the first layer and the second layer can be manufactured. In addition, although only two layers are used as an example in this embodiment, the same method can also be used to manufacture more than the third layer.

In addition, the outer peripheral surface 10a of the first-layer core 10 may have at least one of a concave portion, a convex portion, a concave stripe portion, or a convex stripe portion in order to improve the close fit with the second-layer core 11. In other words, at least one of a concave portion, a convex portion, a concave stripe portion, or a convex stripe portion may be formed in the mold cavity 2 of the first-layer core molding mold 1, or as shown in FIG. 1 (c), after the first-layer core 10 is taken out of the first-layer core molding mold 1, at least one of a concave portion, a convex portion, a concave stripe portion, or a convex stripe portion may be formed by post-processing with a file or the like, or by mechanically or chemically roughening the surface. Thus, the outer peripheral surface 10a of the first layer core 10 can have at least one of the shapes of a concave portion, a convex portion, a concave stripe portion, or a convex stripe portion, thereby improving the closeness with the second layer core 11. In addition, any method other than the above can be used to give a shape to the outer peripheral surface 10a of the first layer core 10 as long as the purpose can be achieved.

Furthermore, in order to ensure good disintegration of the core 10 of the first layer of the core composed of at least two layers of the first layer and the second layer, the ratio of the cross-sectional area of the core 10 of the first layer to the core 11 of the second layer is preferably: the cross-sectional area of the core 11 of the second layer/the cross-sectional area of the core 10 of the first layer ≦1.0. In other words, if the cross-sectional area of the core 10 of the first layer is larger than the cross-sectional area of the core 11 of the second layer, the core 10 of the first layer is easier to disintegrate than the core 11 of the second layer. Therefore, the ratio of the cross-sectional area of the core 10 of the first layer to the core 11 of the second layer is preferably: the cross-sectional area of the core 11 of the second layer/the cross-sectional area of the core 10 of the first layer ≦1.0. In addition, the cross-sectional area in this article includes the cross-sectional area of various cross-sections such as the transverse cross-section and the longitudinal cross-section.

In addition, the second layer of cast sand is formed to have a larger particle size index than the first layer of cast sand.

The second layer of casting sand is designed to have a relatively higher strength than the first layer of casting sand because it needs to withstand the strength of metal liquid pouring. The specific bending strength is more than 30Kgf/ ^cm2 . The amount of thermosetting resin used to constitute the second layer of casting sand is determined by the refractory particles (type and particle size) and thermosetting resin (type) selected in consideration of the required strength and other casting properties (low thermal expansion, etc.). It is usually in the range of 1~10% by weight relative to the refractory particles. If the ease of manufacturing and quality stability are further considered, the preferred range is 2~6% by weight.

In addition, the particle size index of the second layer of casting sand is generally above 80. If the ease of manufacturing is further considered, it is preferably in the range of 90 to 160. In addition, a large particle size index indicates finer grains, and a small particle size index indicates coarser grains.

In addition, the second layer of casting sand can be any casting sand used in the past, and there is no particular limitation. However, from the perspective of thinning the second layer and tightly bonding the first layer, it is preferably a slowly curing thermosetting resin coated sand with a curing time of more than 80 seconds, especially more than 90 seconds. In addition, the curing time is defined as the time from the time when the second layer of casting sand is filled in the annular mold (50mmΦ×5mm) placed on a 250℃ hot plate until the surface is no longer pierced by a needle.

On the other hand, the first layer of casting sand is designed to have a relatively lower strength than the second layer of casting sand in order to give the casting mold good disintegration properties, and the specific bending strength is less than 30Kgf/ ^cm2 , preferably less than 20Kgf/ ^cm2 . The amount of thermosetting resin used in the first layer of casting sand is determined by the refractory particles (type and particle size) and thermosetting resin (type) selected in consideration of the disintegration properties of the casting mold and gas defects (suppression of the amount of generation), and is usually less than 2% by weight relative to the refractory particles, and its lower limit is about 0.5% by weight from the perspective of the reinforcement effect of the second layer. In addition, from the viewpoint of gas defects (high permeability), the particle size index of the first layer of cast sand is generally set to be smaller than that of the second layer of cast sand (less than 80), preferably in the range of 20 to 50.

Therefore, the first layer of cast sand and the second layer of cast sand can be manufactured by melting and coating and/or adhering the thermosetting resin to the surface of the refractory particles through known kneading coating methods such as dry heat coating, semi-hot coating, cold coating and powder solvent method. Among them, the dry heat coating method has more advantages in production and quality. In addition, the first layer of cast sand and the second layer of cast sand can also be combined with various additives as needed, such as mold disintegrators, hardening accelerators, anti-caking agents, mold release agents, deodorants, red iron oxide, iron sand, graphite, etc.

On the other hand, refractory particles are the base for forming the casting mold. As long as they have sufficient refractory properties to withstand casting and a particle size suitable for forming the casting mold, there is no particular limitation on the type of refractory particles. Examples of such refractory particles include special sands such as silica sand, olivine sand, zirconia sand, ferrochrome sand, and alumina sand, ferrochrome slag, ferrochrome slag such as NE sand (trade name), slag particles such as converter slag, porous particles such as Naigai Cerabeads#1700 (trade name), iron sand, carbon particles, glass particles, ceramic particles, and regenerated particles or dust of the above particles. These can be used alone or in combination of two or more.

The type of thermosetting resin is not particularly limited as long as it has a binder function of bonding and holding refractory particles by thermal curing in the presence or absence of a crosslinking agent. Examples of such thermosetting resins include phenolic resins, urea resins, melamine resins, unsaturated polyester resins, and epoxy resins. These may be used alone or in combination of two or more.

Therefore, according to the present embodiment described above, it is not necessary to flip the mold to discharge sand as in the prior art, so the quality of the core can be improved. In addition, since the shape of the core is not limited as in the prior art, any shape can be manufactured. In addition, since a core manufacturing device as in the prior art is not required, a general core manufacturing device can be used.

On the other hand, after manufacturing the first layer of the core 10 shown in FIG. 1 (c), the first layer of the core 10 can be heated to a temperature range of 300°C above room temperature and below. In this way, when manufacturing the second layer of the core 11 covering the outer peripheral surface 10a of the first layer of the core 10, the second layer of the core 11 can be cured by heat conduction from the outer peripheral surface 10a of the first layer of the core 10, thereby shortening the manufacturing time. In addition, the heating temperature here is preferably above room temperature and below 300°C. Because if it does not reach room temperature, it is difficult to promote curing, and if it is higher than 300°C, the deterioration of the thermosetting resin may cause a decrease in properties such as strength.

In addition, the core manufacturing method shown in this embodiment is only an example, and various modifications/changes can be made without departing from the scope of the gist of the present invention as described in the scope of the patent application. For example, although the first layer of the core 10 is manufactured as a solid shape in this embodiment, it is not limited to this and can also be manufactured as a hollow shape.

In addition, in this embodiment, although FIG. 1 shows that the second-layer core 11 is manufactured by covering the entire outer peripheral surface 10a of the first-layer core 10, the present invention is not limited thereto, and the second-layer core 11 may be manufactured by covering only a portion of the outer peripheral surface 10a of the first-layer core 10.

In addition, as shown in FIG. 5, after applying the first layer coating 10AA to the outer peripheral surface 10a of the first layer core 10, the second layer core 11 is covered on the outer peripheral surface 10a of the first layer core 10 after applying the first layer coating 10AA, and the second layer coating 11A is applied to the outer peripheral surface 11a of the second layer core 11. In other words, in aluminum die casting, even if the mold is coated, aluminum is easily sandwiched between the sand grains of the sand core due to casting pressure, etc. This may be caused by the fact that air escapes through the sand core, so when the casting pressure is applied, the aluminum is easily squeezed by the coating. Therefore, as shown in FIG. 5, by also applying the first layer coating 10AA to the outer peripheral surface 10a of the first layer core 10, the amount of air escape can be reduced, thereby solving the above problem.

In addition, when manufacturing the core as shown in FIG. 5, after manufacturing the first layer core 10 as shown in FIG. 1 (c), the first layer coating 10AA is applied to the outer peripheral surface 10a of the first layer core 10. In addition, the outer peripheral surface 10a of the first layer core 10 has at least one of the shapes of concave part, convex part, concave stripe part, or convex stripe part as described above, so the first layer coating 10AA is applied to the outer peripheral surface 10a of the first layer core 10 along this shape.

In this way, the first-layer core 10 with the first-layer coating 10AA applied to the outer peripheral surface 10a is fixed in the cavity 4 as described above, and the second-layer casting sand is blown into the cavity 4 from the upper surface 4a. Thereby, the outer peripheral surface 10a of the first-layer core 10 with the first-layer coating 10AA applied can be covered by the second-layer core 11. Then, if the second-layer coating 11A is applied to the outer peripheral surface 11a of the second-layer core 11, a core as shown in FIG. 5 can be manufactured. In addition, although only two layers are used as an example in this embodiment, the third layer or more can be manufactured using the same method. In addition, as described above, the ratio of the cross-sectional area of the first-layer core 10 to the second-layer core 11 is preferably: the cross-sectional area of the second-layer core 11 / the cross-sectional area of the first-layer core 10 ≦ 1.0.

With such a configuration, the quality of the core can be improved because the mold does not need to be reversed to discharge the sand as in the conventional technology. In addition, since the shape of the core is not limited as in the conventional technology, any shape can be manufactured. Furthermore, since a core manufacturing device as in the conventional technology is not required, a general core manufacturing device can be used.

In addition, the first-layer core 10 with the first-layer coating 10AA applied to the outer peripheral surface 10a can be heated to a temperature range of 300°C or above room temperature. In this way, when manufacturing the second-layer core 11 with the outer peripheral surface 10a of the first-layer core 10 with the first-layer coating 10AA applied to the outer peripheral surface 10a, the second-layer core 11 can be cured by heat conduction from the outer peripheral surface 10a of the first-layer core 10, thereby shortening the manufacturing time. In addition, the heating temperature here is preferably 300°C or above room temperature. If it does not reach room temperature, it is difficult to promote curing, and if it is higher than 300°C, the strength and other properties may be reduced due to the deterioration of the thermosetting resin.

In addition, although FIG. 5 shows an example of manufacturing the second-layer core 11 by covering the entire outer peripheral surface 10a of the first-layer core 10, the present invention is not limited thereto, and the second-layer core 11 may be manufactured by covering only a portion of the outer peripheral surface 10a of the first-layer core 10.

1: Mold for forming the core of the first layer

1a,1b,3a,3b:Mold

2: Cavity of the mold for forming the first layer with a core

2a: Upper surface of cavity 2

2b: Lower surface of cavity 2

4: Cavity of the mold for forming the second layer with a core

4a: Upper surface of cavity 4

4b: Lower surface of cavity 4

3: Mold for forming the core of the second layer (predetermined mold)

10: The first layer of core

11: The second layer of core

Claims (10)

A core manufacturing method is composed of at least two layers of a first core and a second core, the method comprising: after manufacturing the first core, manufacturing the second core on the outer peripheral surface of the first core; wherein the outer peripheral surface of the first core has a shape that improves the closeness with the second core, and when manufacturing the second core on the outer peripheral surface of the first core, the second core is manufactured in a state where the first core is held by a predetermined mold. A core manufacturing method is composed of at least two layers of a first-layer core and a second-layer core, the method comprising: after manufacturing the first-layer core, coating the outer circumference of the first-layer core, and manufacturing the second-layer core on the outer circumference of the first-layer core after coating; wherein the outer circumference of the first-layer core after coating has a shape that improves the closeness with the second-layer core, and when manufacturing the second-layer core on the outer circumference of the first-layer core after coating, the second-layer core is manufactured in a state where the first-layer core is held by a predetermined mold. A core manufacturing method as described in claim 1 or 2, wherein the shape that improves the tightness includes at least one of a concave portion, a convex portion, a concave stripe portion, or a convex stripe portion. A core manufacturing method as described in claim 1 or 2, wherein the first layer of core is formed into a solid or hollow shape. The core manufacturing method as described in claim 1, wherein the second layer of the core is formed to cover a part or all of the outer peripheral surface of the first layer of the core. The core manufacturing method as described in claim 2, wherein the second layer of the core is formed by covering a part or all of the outer peripheral surface of the first layer of the core after coating. The core manufacturing method as described in claim 1, wherein when manufacturing the second layer of core on the outer peripheral surface of the first layer of core, the first layer of core is preheated to a temperature range of 300°C above room temperature and below. The core manufacturing method as described in claim 2, wherein when manufacturing the second layer of core on the outer peripheral surface of the first layer of core after coating, the first layer of core is preheated to a temperature range of 300°C above room temperature and below. A core manufacturing method as described in claim 1, wherein the predetermined mold is capable of fixing and retaining the first layer of core. The core manufacturing method as described in claim 2, wherein the predetermined mold is capable of fixing and retaining the first layer of the core after coating.