Method for measuring the density of compacted powder

Abstract

To provide a method for simply measuring a density of a green compact.SOLUTION: The present invention relates to a method for measuring a density of a green compact, which comprises subjecting a green compact to a water-repellent coating treatment, and measuring a density of a water-repellent coated green compact by underwater gravimetry.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a method for measuring the density of a compacted powder body.

Background Art

[0002] Materials obtained by compacting metal powders such as magnetic materials, for example, ferrite and rare earth magnets (hereinafter also referred to as "compacted powder bodies") are used in various applications including motors as magnetic materials.

[0003] For example, in Patent Document 1, after forming an oxide film containing 0.2% to 1.6% by weight of oxygen with respect to the total amount on the surface of rare earth-iron-nitrogen-based magnet powder in advance, the magnet powder having the oxide film is pre-compression molded into a predetermined shape in a non-oxidizing atmosphere to obtain a preform having a relative density of 40% or more. Then, the preform is densified at a temperature of 350°C to 500°C in a non-oxidizing atmosphere to obtain a magnet compact having a relative density of 85% or more. A method for manufacturing a rare earth permanent magnet is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When such a compacted powder body is used as, for example, a magnetic material, the density in the physical properties of the compacted powder body is an important factor that determines the magnitude of magnetization and coercive force as a magnetic material. Therefore, it is one of the indicators for quality control in the production of the compacted powder body.

[0006] The density of conventional compacts is measured, for example, according to JIS Z 2501:2000, "Sintered metal materials - Test methods for density, oil content and open porosity." Since compacts are usually water-absorbing, for example, in Patent Document 1, the relative density of a magnet is measured by treating the sample with paraffin.

[0007] However, conventional methods for measuring the density of compacted powder, particularly those using paraffin as a sealing agent, have the following problems: (1) In terms of safety, the temperature required to melt paraffin is high, at approximately 80°C, posing a risk of contact hazards with heated and melted paraffin. (2) In terms of quality, during immersion in paraffin and wiping off excess paraffin after immersion to prevent volume changes, there is a risk of residual paraffin or peeling / damage of the sample (workpiece), which can easily lead to errors in the accuracy of the obtained density measurement results. (3) In terms of productivity, the time required from paraffin melting and immersion to paraffin removal and drying is long, at approximately 60 minutes. Furthermore, the weight of the paraffin-treated compacted powder must be taken into account for the desired density calculation, making automatic calculation by density measuring instruments impossible.

[0008] Therefore, the present invention aims to provide a simple method for measuring the density of compacted powder. [Means for solving the problem]

[0009] As a result of various investigations into means to solve the above problems, the present inventors have found that by applying a water-repellent coating treatment to the compacted powder during density measurement, (1) in terms of safety, the temperature required for the coating treatment can be lowered; (2) in terms of quality, the removal of excess coating agent can be simplified to drying only, preventing peeling and damage to the sample; and (3) in terms of production, since only the surface of the sample needs to be treated with the water-repellent coating, the time required for the coating treatment can be shortened; and furthermore, since the amount of residual coating agent remaining on the compacted powder is small, it is not necessary to consider the weight of the coating agent for the target density calculation, and automatic calculation by a density measuring instrument can be performed, thus completing the present invention.

[0010] In other words, the gist of this invention is as follows: (1) A method for measuring the density of a compacted powder, comprising the steps of applying a water-repellent coating to the compacted powder and measuring the density of the water-repellent coated compacted powder by an underwater gravimetric method. (2) The method according to (1), wherein the compacted powder is a magnetic material. (3) The method according to (1) or (2), wherein the water-repellent coating treatment is carried out by an immersion method which includes an immersion step of immersing the compacted powder in a water-repellent coating agent and a drying step of drying the removed compacted powder. (4) The method according to (3), wherein the water-repellent coating agent is a fluororesin-containing fluorosolvent-type coating agent that includes a fluorosolvent and a fluororesin. (5) The method according to (4), wherein the immersion time is 0.1 seconds to 3.0 seconds, the drying temperature is 30°C to 50°C, the drying time is 180 seconds to 300 seconds, and the amount of fluororesin in the water-repellent coated compact is 0.02% by weight to 0.2% by weight relative to the total weight of the compact. [Effects of the Invention]

[0011] The present invention provides a simple method for measuring the density of compacted powder. [Brief explanation of the drawing]

[0012] [Figure 1] This graph shows the weight change of the compacted powder (untreated), as well as the compacted powders of Comparative Example 1 and Example 1, with respect to immersion time in water. [Figure 2] This graph shows the relationship between the immersion time in a fluororesin-containing fluorine-based solvent-type coating agent in compacted powder and the resulting amount of the coating agent applied. [Figure 3] This graph shows the relationship between the amount of coating applied before drying and the amount of coating applied after drying in a compacted powder. [Figure 4] This graph shows the relationship between drying time and coating amount in compacted powder. [Figure 5]It is a graph showing the relationship between the drying time and the coating amount in hot air drying when ferrite or magnet is used as the compacted powder. [Figure 6] It is a graph showing the measurement results (N = 3) of the density of the paraffin-treated compacted powder of Comparative Example 2 and the water-repellent coated compacted powder of Example 5.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. The method for measuring the density of the compacted powder of the present invention is not limited to the following embodiments, and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art without departing from the gist of the present invention.

[0014] The present invention relates to a method for measuring the density of a compacted powder, including a step of performing a water-repellent coating treatment on the compacted powder and a step of measuring the density of the water-repellent coated compacted powder by the water immersion weighing method.

[0015] Here, the compacted powder to be measured for density is not limited. The compacted powder means a material obtained by pressure molding a metal powder, for example, a magnetic material. The compacted powder may be a compacted powder obtained by pressure molding and sintering a metal powder, for example, a magnetic material. Examples of the compacted powder include, for example, a water-absorbent magnetic material such as ferrite, rare earth magnet such as SmCo-based rare earth magnet, NdFeB-based rare earth magnet, and SmFeN-based rare earth magnet.

[0016] According to the present invention, even a water-absorbent compacted powder can be simply measured for density.

[0017] The water-repellent coating treatment is not limited. Examples of the water-repellent coating treatment include, for example, a method of bringing the compacted powder into contact with a water-repellent coating agent.

[0018] The water-repellent coating agent is not limited. Examples of water-repellent coating agents include those in which a liquid water-repellent coating agent is dissolved during the water-repellent coating treatment, such as a fluororesin, such as polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), perfluoroethylene propene copolymer (PEA), ethylene tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), or ethylene chlorotrifluoroethylene copolymer (ECTFE), in a volatile solvent, such as a fluorine-based solvent (e.g., hydrofluoroether (HFE), hydrofluorocarbon (HFC)) or an organic solvent (toluene, isobutanol, heptane, etc.) (fluororesin-containing solvent-type coating agent). The fluororesin-containing solvent-type coating agent typically contains 0.2% to 30% by weight, preferably 1% to 3% by weight, of the total weight of the coating agent, a fluororesin, and typically 70% to 99.8% by weight, preferably 97% to 99% by weight, of the solvent, such as a fluorine-based solvent. Examples of water-repellent coating agents include products containing fluororesin, such as SF Coat SFE-DP02H, SNF-DP20H (manufactured by AGC Seimi Chemical Co., Ltd.), Opto Ace WP-100 series (manufactured by Daikin Industries, Ltd.), Fluorosurf FG-3030 series, FG-3020 series, FG-3650 series (manufactured by Fluorotechnology Co., Ltd.), and SURECO CC Series (manufactured by AGC Inc.). For example, a suitable water-repellent coating agent can be selected if, after treating a compacted powder with the water-repellent coating agent, the powder does not absorb water (does not change in weight) for a time required for density measurement by the underwater gravimetric method, usually 5 seconds or less, preferably 30 seconds or less.

[0019] Methods for bringing the compacted powder into contact with the water-repellent coating agent include, for example, spraying or dropping the water-repellent coating agent onto the compacted powder, applying it uniformly to the compacted powder if necessary, and then drying it; or immersing the compacted powder in the water-repellent coating agent, removing it, and then drying it (immersion method). The immersion method is preferred for the water-repellent coating treatment because it can be easily carried out.

[0020] As described above, the immersion method includes the steps of immersing the compacted powder in a water-repellent coating agent (immersion step) and drying the removed compacted powder (drying step).

[0021] The temperature of the immersion process is not limited. From a safety standpoint, the temperature of the immersion process is, for example, room temperature, for example, 5°C to 35°C, preferably 20°C to 30°C.

[0022] The immersion time is not limited as long as the surface of the compacted powder is coated with a water-repellent coating.

[0023] The immersion time is usually 0.1 seconds or more, preferably 0.5 seconds or more, when the water-repellent coating agent is a fluororesin-containing fluorine-based solvent type coating agent. For example, the object may be immersed in the water-repellent coating agent and then immediately removed from the water-repellent coating agent. Therefore, the immersion time is usually 0.1 seconds or more, preferably 0.5 seconds or more. Furthermore, the longer the immersion time, the greater the amount of water-repellent coating agent, especially fluororesin, applied. For example, when the water-repellent coating agent is a fluororesin-containing fluorine-based solvent type coating agent, the immersion time is usually 3.0 seconds or less, preferably 2.5 seconds or less, and more preferably 2.0 seconds or less.

[0024] By setting the temperature and time of the immersion process within the aforementioned range, the compacted powder can be safely coated with an appropriate amount of water-repellent coating agent precursor.

[0025] The temperature of the drying process is not limited as long as it is within the temperature at which the solvents that may be contained in the water-repellent coating agent, such as fluorine-based solvents, volatilize. The temperature of the drying process is, for example, room temperature (natural drying) to 60°C (hot air drying), for example, 5°C to 60°C, preferably 30°C to 50°C.

[0026] The drying time is not limited to the time required for the solvents contained in the water-repellent coating agent, such as fluorine-based solvents, to evaporate. Therefore, the drying time may also depend on the water-repellent coating agent and the drying temperature. For example, if the water-repellent coating agent is a fluorine-based solvent type coating agent containing fluororesin, it is usually 300 seconds or more for natural drying at approximately 22°C, and usually 180 seconds or more for hot air drying at approximately 40°C. There is no upper limit to the drying time, and the drying process can be completed once the fluorine-based solvents contained in the water-repellent coating agent have evaporated. The drying time can also be determined by plotting the weight of the compacted powder after the immersion process against the drying time, and determining the point at which the weight of the compacted powder has almost stopped changing.

[0027] By setting the temperature and time of the drying process within the aforementioned range, the compacted powder can be safely coated with an appropriate amount of water-repellent coating agent.

[0028] The amount and thickness of the water-repellent coating agent applied to the compacted powder by the water-repellent coating treatment are not limited as long as the surface of the compacted powder exhibits water repellency, but it is preferable to use an amount and thickness that do not affect the density calculation of the compacted powder.

[0029] The amount and thickness of the water-repellent coating agent are, for example, 0.01 g or less of fluororesin and a thickness of 1 μm or less per 7.5 g of compacted powder subject to density measurement, when the water-repellent coating agent is a fluororesin-containing fluorine-based solvent type coating agent. The lower limit of the amount and thickness of the water-repellent coating agent is not limited as long as the surface of the compacted powder exhibits water repellency, but for example, 0.002 g of fluororesin and a thickness of 0.2 μm per 7.5 g of compacted powder subject to density measurement, when the water-repellent coating agent is a fluororesin-containing fluorine-based solvent type coating agent. Therefore, the amount of water-repellent coating agent is, for example, 0.02 wt% to 0.2 wt%, preferably 0.10 wt% to 0.15 wt%, per 7.5 g of compacted powder subject to density measurement, when the water-repellent coating agent is a fluororesin-containing fluorine-based solvent type coating agent.

[0030] By setting the amount and film thickness of the water-repellent coating agent within the aforementioned range, water repellency is provided to the compacted powder while minimizing the impact on density calculations, even without considering the amount, allowing for automatic density calculation.

[0031] Next, the density of the water-repellent coated compacted powder is measured using the underwater gravimetric method.

[0032] The underwater gravimetric method is publicly known in the art and is not limited to that method, and is, for example, based on JIS Z 2501:2000 "Sintered metal materials - Test methods for density, oil content and open porosity".

[0033] Density calculation using underwater gravimetric methods involves, for example, measuring the air weight of a water-repellent coated compacted powder, then measuring the water weight, and using the following formula. Density (g / cm 3 )=weight in air / (weight in air - weight in water) This can be determined by [method].

[0034] The present invention's method for measuring the density of compacted powder can be used for quality control of compacted powder in methods for manufacturing compacted powder.

[0035] For example, in the process of manufacturing compacts by molding magnetic materials, after visually inspecting the compacts obtained by press-molding magnetic powder using a press device to ensure there are no chips or cracks, the compacts are subjected to the water-repellent coating treatment described above, and then the density is automatically calculated by measuring the weight in air (sample weight in air) and the weight in water (sample weight in water). Subsequently, it is determined whether the obtained density falls within the set density range for compacts. If an appropriate density is obtained, the production of compacts continues without changing the manufacturing conditions. If it falls outside the appropriate density range, instructions are given to change the manufacturing conditions to obtain the appropriate density.

[0036] According to the present invention, the temperature required for density measurement can be set to around room temperature, and the time required for density measurement can be drastically reduced compared to conventional methods (conventional methods: approximately 60 minutes or more → present invention: 5 minutes or less). Therefore, in the production of compacted powder, even if a problem occurs in the manufacturing conditions, such as the sintering process or the compression process, the problem can be detected safely, quickly, and accurately, thereby suppressing the occurrence of defective products and improving productivity. [Examples]

[0037] The following describes some embodiments of the present invention, but the present invention is not intended to be limited to those shown in these embodiments.

[0038] 1. Water absorption evaluation 1-1. Sample Preparation Comparative Example 1 The compacted powder was treated with paraffin as follows. (1) The compacted powder (diameter φ17mm x total thickness 11.3mm, weight 7.87g, ferrite powder Z10FG) was immersed in paraffin that had been melted by heating at approximately 80°C for 40 minutes. (2) The compacted material immersed in (1) was kept in paraffin for about 20 minutes so that the paraffin could penetrate into the voids present in the compacted material. (3) The compacted powder held in (2) was removed from the paraffin, any excess paraffin adhering to the surface of the compacted powder was wiped off, and it was dried at 22°C for 5 minutes to obtain paraffin-treated compacted powder.

[0039] Example 1 The compacted powder was treated with a water-repellent coating as follows. (1) A compacted powder (diameter φ17 mm x total thickness 11.3 mm, weight 7.82 g, ferrite powder Z10FG) was immersed in a fluororesin-containing fluorine-based solvent-type coating agent (Fluorosurf® FG-3650TH series (solid content concentration 2 wt%)) at 22°C. (2) The compacted powder that was immersed in (1) was then immersed entirely in a fluororesin-containing fluorine-based solvent-type coating agent and held for 1 second. (3) The compacted powder held in (2) was removed from the fluororesin-containing fluorine-based solvent-type coating agent and dried in a 40°C hot air dryer for 180 seconds to obtain a compacted powder with a water-repellent coating.

[0040] 1-2. Evaluation The compacted powder (untreated, φ17mm diameter x 11.2mm total thickness, 7.84g weight, ferrite powder Z10FG), the paraffin-treated compacted powder of Comparative Example 1, and the water-repellent coated compacted powder of Example 1 were immersed in water, and the change in weight (weight in water) with respect to immersion time was evaluated. The results are shown in Figure 1.

[0041] Figure 1 shows that the untreated compacted material began absorbing water immediately after immersion in water, and that the paraffin-treated compacted material of Comparative Example 1 also showed an increase in water absorption over time. On the other hand, the water-repellent coated compacted material of Example 1 did not show any weight increase over time during the observed 30 seconds. Therefore, it was found that the water-repellent coated compacted material of Example 1 did not undergo weight change during weight measurement using the underwater gravimetric method, i.e., it did not absorb water.

[0042] 2. Relationship between immersion time and coating amount 2-1. Sample Preparation

[0043] Example 2 The compacted powder was subjected to a water-repellent coating treatment as described below. This experiment was conducted three times (N=3). (1) The weighed compacted powder (diameter φ17 mm x total thickness 11.3 mm, weight 7.85 g, ferrite powder Z10FG) was immersed in a fluororesin-containing fluorine-based solvent-type coating agent (Fluorosurf® FG-3650TH series (solid content concentration 2 wt%)) at 22°C. (2) The compacted powder that was immersed in (1) was then immersed entirely in a fluororesin-containing fluorine-based solvent-type coating agent and held there for 0.5 seconds. (3) The compacted powder held in (2) was removed from the fluororesin-containing fluorine-based solvent-type coating agent, and the weight of the removed compacted powder was measured.

[0044] Example 3 In Example 2, the experiment was conducted in the same manner as in Example 2, except that the holding time in the fluororesin-containing fluorine-based solvent-type coating agent (2) was changed from 0.5 seconds to 1.0 seconds.

[0045] Example 4 In Example 2, the experiment was conducted in the same manner as in Example 2, except that the holding time in the fluororesin-containing fluorine-based solvent-type coating agent (2) was changed from 0.5 seconds to 2.0 seconds.

[0046] 2-2. Evaluation Figure 2 shows the relationship between the immersion time in the fluororesin-containing fluorosolvent-type coating agent for compacted powder and the resulting amount of fluororesin-containing fluorosolvent-type coating agent applied. The amount applied was calculated by subtracting the weight of the compacted powder itself from the weight measured in (3). Figure 2 shows that increasing the immersion time in the fluororesin-containing fluorosolvent-type coating agent proportionally increases the amount of fluororesin-containing fluorosolvent-type coating agent applied. Therefore, it was found that the desired amount of fluororesin-containing fluorosolvent-type coating agent can be achieved by adjusting the immersion time in the fluororesin-containing fluorosolvent-type coating agent.

[0047] 3. Relationship between the amount of fluororesin-containing fluorine-based solvent-type coating agent applied and the amount of fluororesin applied. 3-1. Sample Preparation After measuring the application amount (weight before drying) of a fluororesin-containing fluorine-based solvent type coating agent (Fluorosurf® FG-3650TH series (solid content concentration 2% by weight)) alone at 22°C under seven conditions, the samples were dried in a 40°C hot air dryer for 30 to 300 seconds, and the weight (weight after drying) was measured again. The application amount of the fluororesin-containing fluorine-based solvent type coating agent (application amount before drying) and the application amount of fluororesin (application amount after drying) on ​​the treated compacted powder were then calculated.

[0048] 3-2. Evaluation Figure 3 shows the relationship between the amount of coating applied before drying and the amount applied after drying in a compacted powder. From Figure 3, it was found that there is a proportional relationship between the amount of coating applied before drying and the amount applied after drying. Therefore, it was found that the amount of fluororesin applied in the coating agent can be controlled by adjusting the immersion time in the fluororesin-containing fluorine-based solvent-type coating agent to adjust the amount of coating agent applied.

[0049] 4. Examination of the drying process in the immersion method 4-1. Measurement A compacted powder (diameter φ17mm x total thickness 8.45mm, weight 5.88g, ferrite powder Z10FG) was weighed and immersed in a fluororesin-containing fluorine-based solvent-type coating agent (Fluorosurf® FG-3650TH series (solid content concentration 2% by weight)) at 22°C to create a compacted powder with approximately 0.3g of the fluororesin-containing fluorine-based solvent-type coating agent applied. The change in the amount of the coating agent applied with respect to drying time was measured when the compacted powder was dried by natural drying (22°C) or hot air drying (40°C). The amount of coating agent applied was calculated by subtracting the weight of the compacted powder itself from the weight after drying.

[0050] 4-2. Evaluation Figure 4 shows the relationship between drying time and coating amount in the compacted powder. From Figure 4, it was found that in both natural drying and hot air drying, the coating amount decreased with time, and after about 300 seconds in natural drying and about 180 seconds in hot air drying, the coating amount became constant at 0.01g or less. In other words, both natural drying and hot air drying can be used in the drying process, but it was found that using hot air drying is preferable from the viewpoint of shortening the drying time. In this experiment, when the immersion time in a fluororesin-containing fluorine-based solvent type coating agent (solid content concentration 2 wt%) was adjusted from 0.5 seconds to 2.0 seconds for 5.88g of compacted powder, and the coating amount of fluororesin-containing fluorine-based solvent type coating agent was adjusted from 0.25g to 0.35g, it was found that a coating amount of 0.01g of fluororesin could be achieved in about 300 seconds in natural drying and about 180 seconds in hot air drying.

[0051] Figure 5 shows the relationship between drying time and coating amount when using ferrite (diameter φ17 mm × total thickness 8.45 mm, weight 5.88 g, ferrite powder Z10FG) or magnetic material (diameter φ17 mm × total thickness 4.9 mm, weight 6.39 g, SmFeN magnetic powder Z12) as the compacted material with hot air drying. From Figure 5, it can be seen that even when using ferrite or magnetic material as the compacted material, the drying process can be completed in a similar drying time.

[0052] 5. Density measurement and evaluation 5-1. Sample Preparation Comparative Example 2 The density of the compacted powder (magnetic material) was measured three times by paraffin treatment according to the procedure shown below (N=3). (1) The weight of the compacted powder (diameter φ17mm x total thickness 4.9mm, weight 6.39g, SmFeN magnetic powder Z12) was measured. (2) The compressed powder whose weight was measured in (1) was immersed in paraffin that had been melted by heating it at approximately 80°C for 40 minutes. (3) The compacted powder immersed in (2) was held in paraffin for approximately 20 minutes so that the paraffin could penetrate into the voids present in the compacted powder. (4) Remove the compacted powder held in (3) from the paraffin, wipe off any excess paraffin adhering to the surface of the compacted powder, and dry at 22°C for 5 minutes. (5) The weight of the paraffin-treated compacted powder dried in (4) was measured. (6) The weight of the paraffin-treated compacted powder measured in (5) was measured in water. (7) The density was calculated based on the following formula. Density (g / cm 3 ) = (1) air weight / ((5) air weight - (6) underwater weight)

[0053] Example 5 The density of the compacted powder (magnetic material) was measured three times after applying a water-repellent coating according to the procedure shown below (N=3). (1) The weight of the compacted powder (diameter φ17mm x total thickness 4.65mm, weight 5.80g, SmFeN magnetic powder Z12) was measured. (2) The compacted powder whose weight was measured in (1) was immersed in a fluororesin-containing fluorine-based solvent-type coating agent (Fluorosurf® FG-3650TH series (solid content concentration 2% by weight)) at 22°C. (3) The compacted powder that was immersed in (2) was then immersed entirely in a fluororesin-containing fluorine-based solvent-type coating agent and held there for 1.0 second. (4) The compacted powder held in (3) was removed from the fluororesin-containing fluorine-based solvent-type coating agent and dried in a 40°C hot air dryer for 180 seconds to obtain a compacted powder treated with a water-repellent coating. (5) The weight of the water-repellent coated compacted powder, which had been dried in (4), was measured. (6) The water weight of the water-repellent coated compacted powder, whose weight was measured in (5), was measured. (7) The density was calculated based on the following formula. Density (g / cm 3 ) = (1) air weight / ((5) air weight - (6) underwater weight)

[0054] 5-2. Evaluation Figure 6 shows the density measurement results (N=3) for the paraffin-treated compacted material of Comparative Example 2 and the water-repellent coated compacted material of Example 5. In Figure 6, the maximum, average, and minimum values ​​are shown for each experimental example.

[0055] As shown in Figure 6, in the density measurement of Comparative Example 2 using paraffin treatment, the obtained density result was the same as the density of the compacted powder used, calculated as follows: Weight (6.39g) / Volume (3.14 × 1.7 × 1.7 / 4 × 0.49 = 1.11cm³) 3 )=density(5.75g / cm 3 ) In contrast, it was found that the error was large and the variability was large. On the other hand, in the density measurement using the water-repellent coating treatment in Example 5, the obtained density result was the same as the density of the compacted powder used, calculated as follows: Weight (5.80g) / Volume (3.14 × 1.7 × 1.7 / 4 × 0.465 = 1.05cm³) 3 )=density(5.50g / cm 3 ) In contrast, it was found that the error was small and the variability was almost nonexistent.

Claims

[Claim 1] The process includes the steps of applying a water-repellent coating to a compacted magnetic material and measuring the density of the water-repellent coated compacted magnetic material by an underwater gravimetric method, The water-repellent coating treatment is carried out by an immersion method that includes an immersion step in which the compacted powder is immersed in a water-repellent coating agent, and a drying step in which the removed compacted powder is dried. The water-repellent coating agent is a fluororesin-containing fluorosolvent-type coating agent that includes a fluorosolvent and a fluororesin. The immersion process time is between 0.1 seconds and 3.0 seconds. The drying process temperature is 30°C to 50°C. The drying process takes 180 to 300 seconds. A method for measuring the density of a magnetic material compact, wherein the amount of fluororesin in the water-repellent coated compact is 0.02% to 0.2% by weight relative to the total weight of the compact.

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