Manufacturing method for compacted magnetic core

Abstract

The present invention aims to provide a method for manufacturing a compacted magnetic core having an insulating coating. [Solution] The present invention provides a method for manufacturing a compacted magnetic core, characterized by: putting a required amount of magnetic powder covered with an insulating coating into a primary molding die; using the primary molding die to perform primary molding at a low pressure that does not damage the insulating coating to produce a primary molded body; removing the primary molded body from the primary molding die; applying a lubricant to the surface of the primary molded body before inserting it into a secondary molding die; and performing secondary molding using the secondary molding die at a higher pressure than the primary molding.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a compacted magnetic core having an insulating coating. [Background technology]

[0002] Compacted magnetic cores, used in magnetic components such as boost reactors, are manufactured by compressing and annealing magnetic powder covered with an insulating film. However, when compressing magnetic powder, the sliding motion between the molded body and the die can damage the insulating film on the surface of the magnetic powder, leading to a problem where the insulating film on the surface of the molded body peels off.

[0003] Typically, in powder metallurgy, magnetic powder is mixed with a lubricant before being placed in a mold and compressed. When magnetic powder is compressed with a lubricant, the lubricant seeps out during compression molding, suppressing damage to the insulating film caused by sliding against the mold. However, in this manufacturing method, if the pressure applied during compression molding is high, sliding between the inner surface of the mold and the outer surface of the molded body is unavoidable, and partial destruction of the insulating film is inevitable. If the insulating film on the surface of the molded body is damaged, there is a problem that eddy currents flow during use, leading to increased losses.

[0004] Therefore, a method is known in which a lubricant is applied to the mold to suppress sliding resistance and prevent the insulating film on the surface from being damaged. For example, Patent Document 1 describes a technique in which an aqueous solution containing dispersed higher fatty acid-based lubricant particles is applied to promote chemical bonding between the magnetic powder insulating film and the higher fatty acid-based lubricant, thereby forming a metal soap coating. The metal soap coating exhibits superior lubrication performance compared to the higher fatty acid-based lubricant, making it less prone to galling and extending the mold life even when molded under high pressure. Furthermore, Patent Document 2 discloses a method for manufacturing a compression molded body, in which a soft magnetic powder having an insulating coating is pressure-molded to form a material molded body, and a laser is irradiated onto a part of the surface of the material molded body. Laser irradiation can increase the number of disconnected conductive parts on a part of the surface of the material molded body in which the constituent materials of multiple soft magnetic particles conduct electricity, thereby reducing the loss of the compacted molded body. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 4024705 [Patent Document 2] Japanese Patent Publication No. 2012-199568 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The method of applying lubricant to a mold, as described in Patent Document 1, has the problem of low productivity because it requires time for spraying the lubricant onto the mold and time for the lubricant to dry. For example, because of the time required for application and drying, only about two molds can be formed per minute. As described in Patent Document 2, the laser irradiation method does not treat the entire sliding surface of the compacted molded body, but rather treats along the line where the laser is scanned. Therefore, there is a problem of eddy currents flowing in the untreated areas. Consequently, although losses can be reduced compared to untreated products, the losses tend to be greater than those of compression molded products produced by mold lubrication. Furthermore, since compression-molded products inevitably contain air between the filled powder particles, if the molded product is long in the compression direction, there is a problem of lamination cracks occurring if the air is not sufficiently removed during the compression process.

[0007] This invention has been made in view of the circumstances described above, and aims to provide a method for manufacturing powdered magnetic cores that have less damage to the insulating coating on the surface and high surface electrical resistance, and that can be produced with high productivity. [Means for solving the problem]

[0008] (1) The present invention relates to a method for manufacturing a compacted magnetic core, characterized by: putting a required amount of magnetic powder covered with an insulating coating into a primary molding die; producing a primary molded body by primary molding at a low pressure that does not destroy the insulating coating using the primary molding die; removing the primary molded body from the primary molding die; applying a lubricant to the surface of the primary molded body before inserting it into a secondary molding die; and performing secondary molding at a higher pressure than the primary molding using the secondary molding die.

[0009] (2) In the method for producing a compacted magnetic core according to the present invention, any of iron powder, Sendust powder, FeSi alloy powder, FeNi alloy powder, or permalloy powder can be used as the magnetic powder. (3) In the method for manufacturing a powdered magnetic core according to (1) or (2) of the present invention, it is preferable that the molding pressure during the primary molding is 1 / 2 or less of the molding pressure during the secondary molding. [Effects of the Invention]

[0010] According to the present invention, it is possible to obtain a powder magnetic core that is high in strength while having low eddy current loss in the high-frequency range. [Brief explanation of the drawing]

[0011] [Figure 1] A block diagram showing an example of a method for manufacturing a compacted magnetic core according to the present invention. [Figure 2] A block diagram showing an example of a conventional method for manufacturing compacted magnetic cores. [Figure 3] A microstructure photograph showing the surface condition of the compacted magnetic core of Example 1. [Figure 4] A microstructure photograph showing the surface condition of the compacted magnetic core of Comparative Example 1. [Modes for carrying out the invention]

[0012] The following describes one embodiment of the present invention. In the method for manufacturing the compacted magnetic core according to this embodiment, magnetic powder covered with an insulating coating is put into a primary molding die in a necessary amount in a powder filling step F1 shown in FIG. 1, and in a primary molding step (preliminary molding step) F2 shown in FIG. 1 using the primary molding die, primary molding (preliminary molding) is performed at a low pressure such that the insulating coating is not broken by sliding during demolding, and a primary molded body (preliminary molded body) is produced. Next, after extracting the primary molded body from the primary molding die, a lubricant is applied to the surface of the primary molded body in a lubricant application step F3 shown in FIG. 1. Thereafter, the primary molded body coated with the lubricant is inserted into a secondary molding die (main molding die), and in a secondary molding step (main molding step) F4 shown in FIG. 1 using the secondary molding die, secondary molding is performed at a pressure higher than the pressure during primary molding, and a secondary molded body is produced. Thereafter, the secondary molded body is extracted from the secondary molding die, and annealing is performed by heating at a necessary temperature for a necessary time and then gradually cooling in an annealing step F5 shown in FIG. 1, whereby the target compacted magnetic body can be obtained. Regarding the application of the lubricant, conventionally known application methods such as spray spraying, electrostatic coating, and brush coating can be appropriately used.

[0013] As the magnetic powder used in this embodiment, one or more of pure iron powder, FeSi alloy powder, FeNi alloy powder, Sendust powder, and Permalloy powder can be appropriately used. Many of the soft magnetic powders excluding pure iron powder are harder than pure iron powder. As an example, Sendust powder can be a powder of Sendust having a composition of Fe-9.5%Si-5.5%Al in mass%. As an example, the Fe-Si based alloy powder can be one containing 4.5 mass% or more and 7 mass% or less of Si. As an example, the Fe-Ni alloy powder can be one having a composition of Fe-50Ni. In addition, since FeSi alloy powder, FeNi alloy powder, Sendust powder, and Permalloy powder are known in various composition systems, they are not limited to those having the above-described compositions, and any known composition can be used.

[0014] As an example, a silicone resin can be used as the insulating coating covering the magnetic powder. Silicone resin is a resin whose main backbone is a siloxane bond (Si-O-Si). Silicone resins that can be used include methyl-based, methylphenyl-based, propylphenyl-based, epoxy resin-modified, alkyd resin-modified, polyester resin-modified, and rubber-based types. Among these, silicone resins composed of methyl and phenyl groups can be used. Furthermore, these silicone resins can be mixed with a solvent. The amount of silicone resin added can be 3% by mass or less, for example, about 1-2% by mass, relative to the magnetic alloy powder. Other insulating coatings may include those obtained by applying a coating solution, described later, which is obtained by dissolving or dispersing silicone resin and TEOS (tetraethoxysilane:Si(OC2H5)4:Si alkoxide) in a solvent and then drying it, or by using magnesium oxide insulating coatings.

[0015] Insulated magnetic powder is placed into a primary molding die, and prior to secondary molding (main molding) to obtain the final desired shape and pressure density, primary molding is performed at a low pressure of about half or less of the pressure required for secondary molding. The required molding pressure during secondary molding is 6-12 ton / cm². 2 If we consider a range of values, the molding pressure during primary molding should be 2-5 ton / cm². 2 This can be within a certain range. In this embodiment, the final compacted molded body is formed by a total of two molding processes: primary molding and secondary molding. Therefore, in the following description, primary molding may be referred to as the first molding (preliminary molding), and secondary molding as the second molding (final molding). The pressure during the initial molding was 2 tons / cm². 2 If the molding pressure is less than 5 ton / cm², the molding pressure is too low, resulting in insufficient strength in the primary molded body, making it brittle and prone to cracking when removed from the primary molding die after primary molding. 2 If the pressure exceeds this value, the pressing force for primary molding is too high, and there is a risk that the insulating coating on the surface of the primary molded body will be damaged when the primary molded body is removed from the mold. Considering these factors, the pressure during primary molding should be 2 to 5 ton / cm².2 It is desirable to set it within the range.

[0016] After primary molding, the primary molded body is removed from the primary molding die. At this time, the outer surface of the primary molded body slides against the inner surface of the primary molding die. However, the pressure during primary molding is lower than the pressure during secondary molding, which will be explained later. Assuming that the primary molded body is in close contact with the inner surface of the primary molding die, the adhesion force is lower than in the main molding process described later. Therefore, the risk of the insulating coating on the outer surface of the primary molded body peeling off when removed from the die is reduced, and the risk of the insulating coating on the outer surface being damaged is also reduced.

[0017] Once the primary molded body has been removed from the primary molding die, the required amount of lubricant is applied to the outer surface of the primary molded body. The lubricant used here can be any lubricant commonly used during molding. For example, lubricants such as zinc stearate, lithium stearate, magnesium stearate, calcium stearate, ethylene bisamide, erucamide, melamine cyanurate, and mixtures thereof can be used as appropriate. There are no particular restrictions on the amount of lubricant applied here, but it is preferable to apply a thin layer of lubricant to the entire surface of the molded body's side surface that slides against the mold.

[0018] After applying lubricant to the primary molded body, the lubricated primary molded body is placed in the secondary molding die, and a molding rate of 6-12 tons / cm² is applied. 2 The main molding is performed under a certain range of pressure. This secondary molding (main molding) allows for obtaining a secondary molded body (compacted body) of the desired size. Once a secondary molded body is obtained, the desired compacted magnetic core can be obtained by annealing the secondary molded body by heating it at 500-700°C for several tens of minutes to 1 hour (for example, about 30 minutes) in an inert gas atmosphere such as an N2 gas atmosphere, followed by slow cooling. This compacted magnetic core can be obtained as a compacted magnetic core with high surface electrical resistance that is free from defects such as cracks. Furthermore, compacted magnetic cores with high surface electrical resistance have the characteristic of low eddy current loss in the high-frequency range.

[0019] According to the manufacturing method described above, the primary molding is performed with a pressure lower than that used during secondary molding, and with a pressure that does not damage the insulating coating on the surface when removed from the primary molding die. Therefore, the insulating coating present on the surface of the primary molded body can be used for secondary molding without damaging it. In the secondary molding process, a sufficient amount of lubricant is applied to the surface of the primary molded body before it is placed in a secondary molding die and pressure molded. This suppresses damage to the insulating coating on the outer surface of the secondary molded body after secondary molding. As a result, a powder compacted body with high electrical resistance on its outer surface that is free from defects such as cracks can be obtained. Powder compacted bodies with high electrical resistance on their outer surface have low eddy current losses and excellent magnetic properties.

[0020] Figure 2 is a block diagram showing the conventional process for manufacturing compacted magnetic cores. In the conventional manufacturing process shown in Figure 2, a lubricant is applied to the inner surface of the mold in the lubricant application step f1, magnetic powder is filled into the mold in the powder filling step f2, and in the molding step f3, for example, 6 to 12 tonns / cm³ is filled. 2 The material is compressed and molded under a certain pressure, and then annealed in the annealing process f4, which involves heating at, for example, 600-700°C for several tens of minutes to an hour, followed by slow cooling. After this annealing process, the compacted product is removed from the mold at a rate of 6-12 tons / cm³. 2 Because the compacted body is formed under relatively high pressure, when it is removed from the mold, the circumferential surface of the compacted body rubs against the inner surface of the mold, resulting in damage and peeling of the insulating coating on the circumferential surface of the compacted body. Alternatively, adjacent particles on the outer surface of the compacted body are stretched and adhered together by plastic deformation. If there is no insulating film on the outer surface of the compacted body, adjacent powder particles will connect with each other after molding, resulting in a decrease in electrical resistance and thus a large eddy current loss in the compacted body. In contrast, if the compacted magnetic core is manufactured using the manufacturing method described above based on Figure 1, the magnetic powder particles constituting the outer surface of the compacted magnetic core can maintain an insulating state from each other, thus increasing the surface electrical resistance and reducing eddy current loss. [Examples]

[0021] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. As magnetic powders, we prepared pure iron powder (average particle size D50 = 50 μm), Sendust powder (Fe-9.5-5.5Al: average particle size D50 = 50 μm), Fe-Si alloy powder (Fe-6.5Si), and Fe-Ni alloy powder (Fe-50Ni: average particle size D50 = 50 μm). An insulating coating was formed on the surface of each powder by adding silicone resin.

[0022] Using pure iron powder, a U-shaped powder core for a boost reactor with dimensions of 70 mm in length, 40 mm in width, and 30 mm in height was fabricated by one or two molding processes. When producing a compacted magnetic core using pure iron powder through two molding processes, the molding pressures for the first and second processes were selected from the values ​​shown in Table 1 below. In the first molding process, a U-shaped core with dimensions of 70 mm in length, 40 mm in width, and 37.6 mm in height was created.

[0023] When producing a compacted magnetic core using pure iron powder through two molding processes, the primary molded body was produced at the low pressure shown in Table 1 during the first molding process, without mold lubrication, assuming that the insulating coating would not be damaged. The molding speed during the first molding process was 10 (pieces / minute). After producing the primary molded body, a zinc stearate lubricant was applied to the surface of the primary molded body. The lubricated primary molded body was then placed into a mold for secondary molding, and the secondary molded body was produced again using the pressure and molding speed shown in Table 1. This secondary molded body was removed from the mold and subjected to an annealing process in which it was heated to 640°C for 0.5 hours in an inert gas atmosphere and then slowly cooled to obtain a compacted magnetic core.

[0024] When producing a powder magnetic core in a single molding process using pure iron powder, the molding pressure and molding speed were set to those shown in Table 1. For single-molding production of the powder magnetic core, lubrication methods included either applying zinc stearate lubricant to the inner surface of the mold or placing ethylene bisamide lubricant inside the mold together with the magnetic powder. Even for the molded bodies produced in a single molding process, after removing the bodies from the mold, an annealing process was performed, involving heating at 640°C for 0.5 hours in an inert gas atmosphere followed by slow cooling, to obtain the powder magnetic core. For each compacted magnetic core manufactured using pure iron powder under the various conditions shown in Table 1, the surface electrical resistance (Ω) was measured on the outer surface of the compacted magnetic core that slid against the inner surface of the mold during removal from the mold, and the presence or absence of crack formation on the outer surface of the compacted magnetic core was observed. Surface electrical resistance was measured using the four-terminal method. Examples 1-6 show compacted magnetic cores produced under desirable conditions by two molding processes using pure iron powder, while comparative examples show compacted magnetic cores produced by a single molding process. Even with two molding processes, examples that deviate from the desirable conditions are also indicated as comparative examples.

[0025] For Fe-Si alloy powder, Sendust powder, and Fe-Ni alloy powder, U-core shaped powder cores for boost reactors were fabricated by two molding processes. The size of the primary and secondary molded bodies when using these powders was set to be equivalent to that when using pure iron powder. Using these powders, the first and second molding pressures were set to the values ​​shown in Table 2, similar to the case using pure iron powder. The lubrication method, amount of lubricant, and molding speed were also set as shown in Table 2. A secondary molded body was prepared in the same manner as when using pure iron powder, and the compacted magnetic core was produced by annealing under the same conditions as when using pure iron powder. Examples using Fe-Si alloy powder are shown as Example 6, examples using Sendust powder as Example 7, and examples using Fe-Ni alloy powder as Example 7 are shown in Table 2. For each compacted magnetic core obtained using each magnetic powder under the various conditions shown in Table 2, the surface electrical resistance (Ω) was measured using the four-terminal method on the outer surface that slid against the inner surface of the mold when removed from the mold, and the presence or absence of crack formation on the outer surface of the compacted magnetic core was observed.

[0026] Table 1 below shows the manufacturing conditions of the compacted powder cores according to Examples 1 to 6 and Comparative Examples 1 to 5 prepared using pure iron powder, the measurement results of the surface electrical resistance, and the presence or absence of crack generation. Table 2 below shows the manufacturing conditions of the compacted powder cores according to Examples 6 to 8 prepared using various powders, the measurement results of the surface electrical resistance, and the presence or absence of crack generation.

[0027]

Table 1

[0028]

Table 2

[0029] As shown in Table 1, the samples of Examples 1 to 6 in which the molding pressure at the first molding was 2 to 3 ton / cm 2 , and the molding pressure at the second molding was 6 to 10 ton / cm 2 had a higher surface electrical resistance than the samples of Comparative Examples 1 to 5 and no cracks were observed. Since the samples of Examples 1 to 6 have a high surface electrical resistance, it is considered that a compacted powder core with low eddy current loss can be obtained. In addition, since it is necessary to perform molding twice to produce the samples of Examples 1 to 6, it is estimated that the manufacturing time is longer than the manufacturing method of the comparative example samples that produce the compacted powder core by one molding. However, since the molding time of Examples 1 to 6 is shorter than the molding time required for Comparative Examples 1 to 3, it can be seen that the samples of Examples 1 to 6 can be manufactured without reducing the molding speed even if two moldings are performed. Therefore, the method of manufacturing Examples 1 to 6 is considered to be superior in productivity compared to the manufacturing methods of Comparative Examples 1 to 3.

[0030] Regarding the compacted powder core of Example 1, an image obtained by magnifying and photographing its outer surface with a microscope is shown in Figure 3. The length of the white line displayed at the lower right of Figure 3 indicates 50 μm. As shown in Figure 3, the outer surface of the compacted magnetic core in Example 1 shows that the particle shape of the pure iron powder is individually maintained, indicating that the pure iron powder particles are densely and independently present. This is thought to be the reason why the surface electrical resistance is high.

[0031] The samples in Comparative Examples 1 and 2 were molded in a single process (molding pressure of 8 tons / cm²). 2 In one example, a powdered magnetic core was manufactured by mold lubrication or internal lubrication, but in both cases, the surface electrical resistance was low and cracks occurred. Figure 4 shows an image of the outer surface of the compacted magnetic core of Comparative Example 1, magnified with a microscope. The length of the white line shown in the lower right of Figure 4 is 50 μm. As shown in Figure 4, the outer surface of the compacted magnetic core of Comparative Example 1 exhibits a structure in which adjacent particles are connected as a result of individual plastic deformation of the pure iron powder particles. Because of this structure, the surface electrical resistance of the compacted magnetic core of Comparative Example 1 is considered to be low.

[0032] The sample in Comparative Example 2 was internally lubricated compared to the sample in Comparative Example 1, but its surface electrical resistance decreased significantly, and cracks occurred. Comparative Example 3 is an example where the molding speed was slower than in Comparative Examples 1 and 2, reducing the sliding speed between the inner surface of the mold and the outer surface of the sample during molding. Although no cracks were observed, the surface electrical resistance decreased significantly. Comparative Example 4 is a sample in which a compacted magnetic core was prepared by two molding processes, with the first molding pressure being 5 ton / cm². 2 Therefore, the surface electrical resistance was significantly lower compared to Example 1. In Comparative Example 4, it can be presumed that the insulating coating on the surface of the molded body was damaged during extraction after molding. Comparative Example 5 is an example where the initial molding pressure was too low, resulting in the product not achieving the desired shape after molding.

[0033] Examples 6-8, shown in Table 2, all exhibited high surface electrical resistance levels comparable to Examples 1-6, because the first molding pressure was less than half of the second molding pressure. Furthermore, no cracks were observed. Based on the results shown in Tables 1 and 2, it is considered desirable to employ a method for manufacturing compacted magnetic cores in which magnetic powder covered with an insulating coating is placed in a mold, a primary molded body is produced by primary molding at a low pressure that does not damage the insulating coating using the mold, the primary molded body is removed from the mold, a lubricant is applied to the surface of the primary molded body, and then it is inserted into the mold and subjected to final molding at a higher pressure than the primary molding using the mold. [Explanation of symbols]

[0034] F1...Powder filling process, F2...Primary molding process (preliminary molding process), F3...Lubricant application process, F4...Secondary forming process (main forming process), F5...Annealing process.

Claims

1. A method for manufacturing a powder magnetic core, characterized by: loading a required amount of magnetic powder covered with an insulating coating into a primary molding die; using the primary molding die to perform primary molding at a low pressure that does not damage the insulating coating to produce a primary molded body; removing the primary molded body from the primary molding die; applying a lubricant to the surface of the primary molded body before inserting it into a secondary molding die; and performing secondary molding using the secondary molding die at a higher pressure than the primary molding.

2. The method for manufacturing a compacted magnetic core according to claim 1, characterized in that the magnetic powder is one of iron powder, Sendust powder, FeSi alloy powder, FeNi alloy powder, or permalloy powder.

3. A method for manufacturing a powdered magnetic core according to claim 1 or 2, characterized in that the molding pressure during the primary molding is 1 / 2 or less of the molding pressure during the secondary molding.

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