JP2017196287A - Rice cooker, inner pot of rice cooker, and method of manufacturing inner pot of rice cooker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rice cooker, an inner pot of the rice cooker, and the manufacturing method of the inner pot of the rice cooker, in which the inner pot can be made light in weight, in which the thickness of a magnetic material can be made partly variable, and in which combination strength of a magnetic metal and the inner pot base material is strong.SOLUTION: The rice cooker includes: the main body 2; an inner pot 4 housed in the main body 2; and an induction heating coil 6 that is installed in the main body 2 to induction-heat the inner pot 4. The inner pot 4 is composed of: an inner pot base material 11; a first film part 12 for which magnetic material powders bite in and is laminated on the outer surface of the inner pot base material 11; a second film part 13 which is the film of the magnetic material stuck to the first film part 12 in a manner covering the surface of the first film part 12; and a third film part 14 which is installed to cover the entire surface of the first and the second film parts 12, 13.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a rice cooker, an inner pot of a rice cooker, and a method for manufacturing an inner pot of a rice cooker.

An electromagnetic induction heating rice cooker uses a magnetic material that is induction heated by an induction heating coil on the outside as the material of the inner pot, and the heat from the magnetic material generated by induction heating is applied to the rice inside the inner pot. It is common to use a material with good thermal conductivity so that it can be transmitted uniformly.
As those materials, as shown in Patent Document 1, a material obtained by drawing a clad material with a stainless steel plate that is a magnetic material on the outside and an aluminum plate on the inside into an inner pot shape is often used.

Moreover, as shown in Patent Document 2, a plate-shaped stainless steel plate or the like, which is a magnetic metal material, is laid on the bottom surface of the outer mold having an inner pot shape, and molten aluminum, which is a nonmagnetic metal material, is injected thereon, There is also a type in which the inner hook is molded by pressing from above with high pressure.
Further, as shown in Patent Document 3, magnetic metal iron fine particles are melted and applied to a portion facing an induction heating coil in an inner pot base material processed into an inner pot shape with a material having good thermal conductivity such as aluminum. Some have fine iron particles bound together by a thermal spraying process.

JP-A-5-199934 JP-A-8-24123 JP 11-276341 A

However, in recent years, the inner cooker of rice cookers is also required to be lighter. In addition, in order to improve the taste of the cooked cooked rice, we would like to make it possible to partially change the thickness of the magnetic material of the inner pot at a low manufacturing cost. Furthermore, the above-described thermal spraying method has a problem that magnetic metal iron fine particles cannot be formed on the inner pot base material with a strong bond strength.
Therefore, the present invention can reduce the weight of the inner pot, can also partially change the thickness of the magnetic material, and has a strong bond strength between the magnetic metal and the inner pot base, and the inner pot of the rice cooker. It is another object of the present invention to provide a method for manufacturing an inner pot of a rice cooker.

  In order to solve the above-described problems, an embodiment of the present invention includes an inner hook base material, a first coating portion in which magnetic powder bites into and laminates an outer surface of the inner hook base material, and the first Provided to cover the entire surface of the second film part, which is a magnetic film attached to the surface so as to cover the surface of the film part, and the first film part and the second film part. And a third film portion.

  Another embodiment of the present invention includes an inner hook base material, a first film portion in which magnetic powder bites into an outer surface of the inner pot base material, and the entire surface of the first film portion. And a third film portion provided so as to cover.

  Another aspect of the present invention is a first film part forming step in which a magnetic powder is applied to an outer surface of an inner pot base material by a cold spray manufacturing method to form a first film part on the outer surface; After the first film part forming step, a second film part forming step of forming a second film part so as to cover the surface of the first film part by hitting a non-magnetic material to the first film part by a thermal spraying method. And a third film part forming step of forming a third film part so as to cover the entire surfaces of the first film part and the second film part after the second film part forming step.

  Another embodiment of the present invention includes a first film part forming step of forming a first film part by hitting a non-magnetic powder on the outer surface of the inner pot base material by a cold spray method, and a first film part. After the forming step, a third film portion forming step for forming a third film portion so as to cover the entire surface of the first film portion.

According to the present invention, the inner pot can be reduced in weight, the thickness of the magnetic material can be partially changed, and the bonding strength between the magnetic metal and the inner pot base is strong, and the inner pot of the rice cooker And the manufacturing method of the inner pot of a rice cooker can be provided.
Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

FIG. 1 is a longitudinal sectional view of a rice cooker that is Embodiment 1 of the present invention. FIG. 2 is a longitudinal sectional view of the inner pot of the rice cooker which is one embodiment of the present invention. FIG. 3 is a flowchart showing an inner hook manufacturing process according to the inner pot manufacturing method according to the embodiment of the present invention. FIG. 4 is an enlarged cross-sectional view of a laminated structure of the inner pot base material, the first film part, the second film part, and the third film part of the inner pot of the rice cooker which is an embodiment of the present invention. It is drawing substitute photograph (a), drawing substitute photograph (b) which becomes the enlarged view of the A part, and drawing substitute photograph (c) which becomes an enlarged view of the boundary part of the inner hook base material and the first film part. FIG. 5 is a drawing-substituting photograph (a), which is a plan view showing fine particles of iron forming the first film part that bites into the inner pot base material of the inner pot according to an embodiment of the present invention, and a sectional view thereof. (B). FIG. 6 is a drawing-substituting photograph (a) corresponding to an enlarged cross-sectional view of an inner hook peeling test according to an embodiment of the present invention, and an inner hook peeling test as a comparative example. It is a drawing substitute photograph corresponding to the enlarged sectional view of time (b). FIG. 7: is a longitudinal cross-sectional view of the inner pot of the rice cooker which is Example 2 of this invention. FIG. 8: is a flowchart which shows the manufacturing process of the inner hook concerning the manufacturing method of the inner hook which is Example 2 of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

[Structure of rice cooker and its inner pot]
FIG. 1 is a longitudinal sectional view of a rice cooker according to the present embodiment.
In FIG. 1, a protective frame 3 having an open upper surface constituting an inner wall of the main body 2 is provided inside a main body 2 of a rice cooker (jar rice cooker) 1, and an inner pot 4 is detachable in the protective frame 3. It is stored in.

An outer lid 5 is attached to the upper surface of the main body 2 so as to be freely opened and closed, and the upper surface opening portions of the main body 2 and the inner hook 4 can be closed. An induction heating coil 6 for induction heating the inner pot 4 is provided on the bottom surface of the protective frame 3.
By the way, in recent years, the inner pot of a rice cooker is required to be light in addition to good heat generation by electromagnetic induction heating and good heat transfer.

The technique shown in Patent Document 1 is drawn into an inner pot shape using a clad material having a stainless steel plate as a magnetic material on the outside and an aluminum plate with good heat conductivity on the inside. In this configuration, all parts outside the inner hook, including the part not facing the induction heating coil, are made of stainless steel, and parts that do not require magnetic metal for electromagnetic induction heating are covered with stainless steel. There was a problem of increasing.
In addition, the thickness of the inner pot is the same in all parts due to the processing of the plate material, for example, it is not possible to change the thickness partially, such as thickening only the bottom part. It was not possible to adjust the heat conduction characteristics by partially changing the thickness.

  The inner pot shown in Patent Document 2 is one in which a stainless steel or the like that is a magnetic material is processed into a dish shape, and aluminum that has good heat conduction is joined by high-pressure melt forging. The outer magnetic material only needs to be provided to oppose the induction heating coil, and the thickness of the aluminum part can be partially changed by the shape of the mold that pressurizes the molten aluminum under high pressure. The problem of literature 1 is solved.

  However, the manufacturing involves many processes, and large equipment for melt forging is required, resulting in a high manufacturing cost and a problem that the inner pot cannot be provided at a low cost. Specifically, in order to firmly bond stainless steel and aluminum, the shape of the magnetic metal dish requires a certain depth, and it is a thermal spray manufacturing method in which fine particles of iron are hit on the inner bottom surface and inner surface. A step is required in which a sprayed layer having an uneven shape is formed, and aluminum melted in the unevenness is inserted to bond both.

  The technique shown in Patent Document 3 is a thermal spray manufacturing method in which a magnetic metal iron fine particle is melted and applied to a portion facing an induction heating coil in an inner pot base material processed into an inner pot shape with a material having good thermal conductivity such as aluminum. Thus, the iron fine particles are bonded to the inner pot base material. The portion on which the thermal spray coating of iron is formed by the thermal spraying method can be arbitrarily selected, and for example, only the bottom surface can be formed. In addition, it is possible to make a shape with any thickness change depending on the spraying location and spraying time, such as increasing the thickness at the center of the bottom surface, and adjusting the heat conduction characteristics of the inner pot to finish it into delicious rice.

However, there is a problem that the bonding strength between the inner pot base material such as aluminum and the iron layer generated by the thermal spraying manufacturing method is weaker than the manufacturing methods of other conventional examples. As a countermeasure, it is conceivable that the inner pot base material is blasted to improve the bonding strength between the inner pot base material and the iron layer. However, due to the difference in thermal expansion and contraction between aluminum and iron, it is necessary to bond both of them fairly firmly, and the blasting process requires high cost and high technology.
Therefore, the inner hook 4 of this embodiment can be reduced in weight, can be partially variable in thickness of the magnetic material, and has the following configuration so that the coupling strength between the magnetic metal and the inner hook base material is increased. It is said.

  FIG. 2 is a longitudinal sectional view of the inner hook 4. The inner pot base 11 is made of aluminum. In the portion where the outer surface of the inner pot base 11 is subjected to induction heating, that is, below the bottom and the side, magnetic substance powder, for example, iron powder fine particles, bite into the outer surface of the inner pot base 11 and are laminated. The first film portion 12 is formed. Moreover, the 2nd film | membrane part 13 which is a magnetic body affixed on the said surface so that the surface of the 1st film | membrane part 12 may be covered, for example, an iron film | membrane is further formed. The 1st membrane | film | coat part 12 and the 2nd membrane | film part 13 are induction-heated by the induction heating coil 6 as a magnetic body.

Furthermore, the third film part 14 is provided so as to cover the entire surfaces of the first film part 12 and the second film part 13. The third film part 14 is a protective film that protects the first film part 12 and the second film part 13 from being corroded. The third coating 14 can be made of a non-magnetic material, for example, a material whose main material is at least aluminum. More specifically, a material in which silicon is partly mixed with aluminum can be used as the third coating portion 14.
Alternatively, the third coating portion 14 may be formed in such a manner that nonmagnetic powder, for example, almost 100% aluminum powder bites into the second coating portion 13 and is laminated.

And the antirust coating 15 is given to the whole outer surface of the inner hook 4 formed as mentioned above. Further, a fluororesin coating 16 is applied to the entire inner peripheral surface of the inner hook 4.
In addition, the thickness of the 1st membrane | film | coat part 12, the 2nd membrane | film part 13, the 3rd membrane | film part 14, the antirust coating 15, and the fluororesin coating 16 is exaggerated for convenience and is shown in FIG.
Next, the effect of the rice cooker 1 of a present Example is demonstrated.

  In FIG. 1, first, the user puts an appropriate amount of rice and water into the inner pot 4, accommodates the main body 2, and closes the outer lid 5. Next, when cooking is started by operating an operating means (not shown), a current flows through the induction heating coil 6, a high-frequency magnetic field is generated, and the first and second bottom surfaces of the inner pot 4 facing the induction heating coil 6, Heat is generated by Joule heat by the current generated in the second coating parts 12 and 13.

  This heat is transmitted from the second coating portion 13 to the inner pot base material 11 through the first coating section 12 and is transferred to the rice and water immediately above the induction heating coil 6, and at the same time, the aluminum of the inner pot base material 11. It is transmitted to the side of the inner hook 4 due to high thermal conductivity. In addition to the convection of water from the bottom of the inner pot 4, heat is transferred to the side of the inner pot 4 by heat conduction of the inner pot 4 to heat the rice and water, so that the whole is heated uniformly and the cooked rice is evenly delicious. Can be finished.

Eventually, when the rice sucks water and the water in the inner pot 4 runs out, the temperature at the bottom of the inner pot 4 suddenly rises, and this is detected by a temperature detection means (not shown) to energize the induction heating coil 6. Is stopped and heating is completed. Then, steaming for a fixed time is performed and cooking rice is complete | finished.
Due to the heating during rice cooking, the inner pot 4 generates a stress on the bonding surface due to the difference in thermal expansion between the aluminum of the inner pot base 11 and the iron of the second coating portion 13 and the first coating portion 12.

However, since the 1st membrane | film | coat part 12 bites into the inner pot base material 11 and both are firmly joined, peeling does not arise.
Since the iron can be firmly bonded to the inner pot base 11 by the first coating 12, it is possible to join the iron to an arbitrary position such as the bottom or the side of the inner pot base 11 at an arbitrary thickness. The cooked rice can be cooked by adjusting the thickness of the first film part 12 and the second film part 13 to adjust the heat conduction characteristics of the inner pot 4.
Moreover, it is not necessary to provide iron in places which do not contribute to induction heating, such as the upper part of the side surface of the inner pot 4, and the lightened inner pot 4 can be provided.

[Inner pot manufacturing method]
FIG. 3 is a flowchart showing the manufacturing process of the inner hook 4.
In this manufacturing process, first, the inner pot base 11 is manufactured (S1). The inner pot base 11 is manufactured by drawing an aluminum plate having a predetermined thickness, which is a highly heat-conductive nonmagnetic metal, into an inner pot shape.

  Next, the 1st membrane | film | coat part 12 is formed by the cold spray manufacturing method on the bottom part and side part lower part of the outer surface of the inner pot base material 11 (S2) (1st membrane | film | coat part formation process). That is, a magnetic substance, for example, iron powder fine particles are beaten on the inner hook base material 11 at a speed higher than the speed of sound (for example, 1500 km / h or higher) at a temperature lower than the melting point, and the iron powder is applied to the outer surface of the inner pot base material 11 The first film portion 12 is formed by stacking. The thickness of the 1st membrane | film | coat part 12 is about 100 micrometers, if an example is given.

  Next, the thermal spraying process is performed on the first coating portion 12 in a slightly narrower range closer to the bottom than the bottom and side portions of the inner pot base 11 provided with the first coating portion 12. Then, the second film portion 13 is formed using a magnetic material, for example, iron (S3) (second film portion forming step). In the thermal spray production method, for example, an iron wire is melted by an electric discharge of about 2000 ° C., and in the case of arc welding, the melted particles are struck onto the inner pot base material 11 at a speed of, for example, 300 to 400 km / h. For example, the second film portion 13 has a thickness of about 400 μm.

  Next, the third film part 14 is formed using a nonmagnetic material as a material so as to cover the entire first film part 12 and the second film part 13 (S4) (third film part forming step). . The third coating portion 14 can be formed by a thermal spraying method using, for example, a material mainly composed of aluminum. This is, for example, a material in which silicon is partially mixed in aluminum. In the thermal spraying method, a material mainly composed of aluminum melted at a temperature equal to or higher than the melting point of aluminum is struck onto the first coating portion 12 and the second coating portion 13.

The third coating portion 14 can also be formed by a cold spray manufacturing method using a non-magnetic material, for example, a material using substantially 100% aluminum (fine particles of aluminum powder). That is, fine particles of aluminum powder having a temperature lower than the melting point of aluminum are struck onto the exposed first coating portion 12 and second coating portion 13 at a speed higher than the speed of sound by high-pressure gas injection.
Next, the rust preventive coating 15 is applied to the entire outer surface of the inner pot base 11 after the formation of the third film portion 14, and the fluororesin coating 16 is applied to the entire inner peripheral surface of the inner pot base 11 (S5). ). The inner hook 4 is completed through the above steps.

  FIG. 4 is a drawing substitute that becomes an enlarged cross-sectional view of a laminated structure of the inner pot base material 11, the first coating portion 12, the second coating portion 13, and the third coating portion 14 formed by the above manufacturing method. It is a photograph. In addition, the 3rd film | membrane part 14 is an example using the said thermal spraying manufacturing method. From FIG. 4A, it can be seen that the second film portion 13 is formed by laminating iron particles. FIG. 4B is a drawing-substituting photograph that is a partially enlarged view of the inner hook base material 11, the first coating portion 12, and the second coating portion 13 in the portion A of FIG. In the boundary part between the inner pot base 11 and the first film portion 12 shown in FIG. This is because the iron fine particles hit the inner pot base 11 at the speed of sound and bite into the inner pot base 11 in the step S2. FIG. 4 (c) is a drawing-substituting photograph that is an enlarged view of the boundary portion between the inner hook base material 11 and the first film portion 12, and the irregularities on the surface of the inner hook base material 11 can be clearly seen.

  FIG. 5A is a drawing-substituting photograph that is a plan view showing fine particles of iron forming the first film portion 12 that has bitten into the inner pot base material 11, and FIG. 5B is a drawing-substituting drawing that is a sectional view thereof. It is a photograph. The first film portion 12 is biting in the direction of the white arrow.

  FIG. 6B shows the inner shell base material 111 (the same material as the inner hook base material 11) subjected to the blasting process (reference numeral 112 is the processing surface) and then the same as the second coating portion 13. 5 is a drawing substitute photograph corresponding to an enlarged cross-sectional view when an iron coating 113 is formed on an inner pot base 111 by a thermal spraying method. This is a comparative example with respect to the present example, and is a result of a peeling test in which the iron film 113 is peeled off from the inner hook base material 111. The iron film 113 is peeled off from the inner pot base 111, and reference numeral 114 denotes a space created between the two.

On the other hand, FIG. 6A is a photograph for a drawing table corresponding to an enlarged cross-sectional view when a peeling test is performed in the case of this example. A space 41 is formed by peeling off between the first film portion 12 and the second film portion 13.
In the present embodiment of FIG. 6A, the inner hook base 11 and the first film portion 12 are not peeled even with a force about three times that of the peeling test in which the peeling of FIG. As a result, the first film part 12 and the second film part 13 were peeled off (reference numeral 41 is a space created at that time).

That is, compared with the comparative example of FIG. 6B, in the case of the present embodiment of FIG. 6A, the magnetic body (the first coating portion 12 and the second coating portion 13) and the inner pot base material 11 are used. And the bond strength can be increased.
Moreover, since the 1st membrane | film | coat part 12 is formed by the cold spray manufacturing method and the 2nd membrane | film | coat part 13 is formed by a thermal spraying manufacturing method, the 1st membrane | film | coat part 12 and the 2nd membrane | film part 13 are made thick partially. It can be easily made thin. That is, the weight of the inner hook 4 can be reduced, and the thickness of the magnetic material can be partially changed.

Furthermore, when the cold spray manufacturing method is used for manufacturing the third coating portion 14, the thickness of the third coating portion 14 can be easily controlled.
In addition, the outer surface of the inner pot 4 is provided with a rust preventive coating 15, but even if a part of the rust preventive paint 15 is peeled off due to wear or scratches, it is covered with the third film portion 14, It is possible to prevent the first coating portion 12 and the second coating portion 13 from rusting.

  In the present embodiment, the same members and the like as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  [Structure of rice cooker and its inner pot]

  FIG. 7 is a longitudinal sectional view of the inner pot of the rice cooker of the present embodiment. The rice cooker of the present embodiment is the same as the rice cooker 1 of the first embodiment. The difference between the present embodiment and the first embodiment lies first in the structure of the inner hook. Unlike the inner hook 4, the inner hook 51 of the present embodiment shown in FIG. In place of the second coating portion 13, the first coating portion 12 is also formed in the formation portion. That is, the magnetic body to be induction-heated by the induction heating coil 6 is a first in which fine particles of magnetic powder, for example, iron powder, are stacked on the bottom surface and the lower side of the outer surface of the inner pot base 11 of the inner pot 4. It is formed only by the film part 12.

The 1st membrane | film | coat part 12 and the inner pot base material 11 are couple | bonded firmly as mentioned above. Therefore, as compared with the first embodiment in which the second film portion 13 is also laminated, this embodiment can further strengthen the bond between the inner hook base material 11 and the magnetic body.
Moreover, since the formation process of the 2nd membrane | film part 13 can be abbreviate | omitted, compared with Example 1, a manufacturing cost can be reduced.

[Inner pot manufacturing method]
FIG. 8 is a flowchart illustrating a method for manufacturing the inner hook according to the present embodiment. The present embodiment is different from the manufacturing method of Embodiment 1 in that the second coating part forming step of S3 is omitted. As a result, the magnetic body formed on the inner pot base 11 is only the first film portion 12.

As described above, even in the example of FIG. 6A in which the peel test was performed with a force about three times that in which the peel test was performed in the example of the comparative example of FIG. 12 was not peeled off from the inner pot base 11. That is, it can be seen that the magnetic body that is induction-heated by the induction heating coil 6 is formed by only the first film portion 12 and does not easily peel off from the inner pot base 11 even when compared with the first embodiment.
Moreover, the manufacturing cost can be reduced compared with Example 1 by omitting the 2nd film | membrane part formation process of S3.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Moreover, it is also possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

DESCRIPTION OF SYMBOLS 1 Rice cooker 2 Main body 4 Inner pot 6 Induction heating coil 11 Inner pot base material 12 1st film part 13 2nd film part 14 3rd film part S2 1st film part formation process S3 2nd film part formation Step S4 Third coating part forming step

Claims (12)

The body,
An inner pot stored in the main body,
An induction heating coil provided on the main body for induction heating the inner pot;
The inner pot is
An inner pot base material;
A first film portion in which magnetic powder penetrates and is laminated on the outer surface of the inner pot base;
A second film part that is a film of a magnetic material attached to the surface so as to cover the surface of the first film part;
A rice cooker comprising: a first film part and a third film part provided to cover the entire surface of the second film part. The body,
An inner pot stored in the main body,
An induction heating coil provided on the main body for induction heating the inner pot;
The inner pot is
An inner pot base material;
A first film portion in which magnetic powder penetrates and is laminated on the outer surface of the inner pot base;
A rice cooker comprising: a third coating portion provided to cover the entire surface of the first coating portion.   3. The rice cooker according to claim 1, wherein the third film part is formed by a non-magnetic material layered into the first film part and the second film part.   The rice cooker according to any one of claims 1 to 3, wherein the first film part and the second film part are made of iron, and the third film part contains at least aluminum. . An inner pot base material;
A first film portion in which magnetic powder penetrates and is laminated on the outer surface of the inner pot base;
A second film part that is a film of a magnetic material attached to the surface so as to cover the surface of the first film part;
An inner pot for a rice cooker, comprising: a first film part and a third film part provided to cover the entire surface of the second film part. An inner pot base material;
A first film portion in which magnetic powder penetrates and is laminated on the outer surface of the inner pot base;
An inner pot for a rice cooker, comprising: a third coating portion provided to cover the entire surface of the first coating portion.   3. The inner pot of the rice cooker according to claim 1, wherein the third film portion is formed by a non-magnetic material biting into and laminating the first film portion and the second film portion.   The rice cooker according to any one of claims 5 to 7, wherein the first film part and the second film part are made of iron, and the third film part contains at least aluminum. . A first film portion forming step of forming a first film portion on the outer surface by hitting magnetic powder on the outer surface of the inner pot base material by a cold spray manufacturing method;
After the first film part forming step, a second film part that forms a second film part so as to cover the surface of the first film part by hitting a non-magnetic material to the first film part by a thermal spraying method. Forming process;
Of a rice cooker comprising a third film part forming step of forming a third film part so as to cover the entire surface of the first film part and the second film part after the second film part forming step. Manufacturing method of inner pot. A first film part forming step of forming a first film part by hitting nonmagnetic powder on the outer surface of the inner pot base material by a cold spray manufacturing method;
A method for producing an inner pot of a rice cooker, comprising: a third film part forming step for forming a third film part so as to cover the entire surface of the first film part after the first film part forming process.   The third coating portion forming step includes forming a third coating portion by hitting a non-magnetic material on the first coating portion and the second coating portion by a cold spray manufacturing method. Item 11. A method for producing an inner pot of a rice cooker according to Item 9 or 10.   In the first film part forming step and the second film part forming process, iron is used as a material for the first film part and the second film part, and in the third film part forming step, The method for producing an inner pot for a rice cooker according to any one of claims 9 to 11, wherein aluminum is used as a material for the coating portion of No. 3.