JP3445112B2 - Powder metallurgy in powder metallurgy, lubrication of molding dies and stamping dies - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lubrication method for a powder molding die (pressing die) in the field of powder metallurgy, which reduces friction when extruding a green compact from the pressing die,
It facilitates the molding of high-density sintered parts.

[0002]

2. Description of the Related Art When a metal powder is compressed in a die, friction is generated between the die wall and the powder particles, or between the powder particles, so that the required density of the green compact (green compact density) is increased. Molding pressure is required. Further, the larger the friction between the green compact and the wall surface of the die, the larger the pressing force required. As a result, the residual stress in the green compact increases, so that the occurrence of defects such as cracks when extruding the green compact from the outer die increases. . Therefore, it is necessary to minimize friction in the die in terms of product quality and facility cost.

As a lubrication method for reducing friction, a "push-type lubrication method" in which a lubricant is applied to a die such as an inner surface of an outer die or a surface of a core rod, and a powdery lubricant is added and mixed to a raw material powder. There is a "mixed lubrication method". In the Japanese Industrial Standard (JIS Z2500-1960) regarding powder metallurgy terms, the lubricant applied to the die is called the die lubricant, and the lubricant mixed with the raw material powder is called the powder lubricant. However, stearic acid and its metal soaps and waxes are generally used.

In the case of the pressing lubrication method, a powder lubricant is often dissolved in an organic solvent and then applied and dried. However, in order to avoid environmental hygiene problems associated with the handling of organic solvents, recently, for example, JP-A-8-100203, a method for directly and electrostatically adhering a powdery lubricant to a die is being developed. .

Conventionally, a mixed lubrication method, which is easy to carry out and suitable for mass production, is generally used. However, the addition of a powder lubricant reduces the fluidity of the raw material powder, the green compact density and the green compact strength. There is a problem. Comparing the pressing lubrication method with the mixing lubrication method, the powder density is higher in the mixing lubrication method when the molding pressure is low, but is higher in the pressing lubrication method as the molding pressure is higher. Further, the pushing force when the green compact is pushed out from the outer die is smaller in the case of the pressing lubrication method than in the case of the mixed lubrication method. Therefore, in order to obtain a high-density product, the pressing lubrication method is essentially suitable. By the way, it is said that the combined use of the mixed lubrication method and the pressing lubrication method is less advantageous in terms of the obtained green density and the pressing force. The present invention relates to this pressing lubrication method,
This is a further improvement.

[0006]

As described above, in order to obtain a high-density sintered component free from defects such as cracks, a pressing wall surface (mainly an outer mold and a core rod, if any) is used when the green compact is extruded. Also, the friction with) must be minimized. However, while the demand for higher density and higher strength of sintered parts is increasing, the conventional method is used especially when the contour shape of the product is complicated, when the diameter is high (long), or when high molding pressure is required. In the case of the lubrication method, the friction is not sufficiently reduced, which causes problems such as an increase in the pressing force of the green compact, galling of the pressing wall surface, and deterioration of the surface roughness of the product. Therefore, the development of a better lubrication system has been required.

[0007]

In order to reduce friction, it is important to improve the material of the lubricant, but in parallel with this, the inventors have studied various actions of the lubricant on the wall surface of the die, and as a result, Solid lubrication is suitable for reducing friction (static friction) at the initial stage of extrusion, that is, at the stage where the green compact has not yet started to move. On the other hand, for reducing friction (dynamic friction) after the green compact has started to move, liquid lubrication is used. I found that the better.

The present invention is based on the above finding, and is provided with a cooling means and a heating means in an outer die to which the die-lubrication method is applied,
The main idea is to control the temperature of the inner wall surface of the outer mold so that the lower inner wall of the outer mold on which the green compact is formed is below the melting point of the lubricant used, and above that is above the melting point.

As a result, the lubricating coating formed on the inner wall surface of the outer die maintains a solid state at the position where the green compact is formed, and the static friction is reduced by the solid lubricating action at the initial stage of the green compact extrusion. On the other hand, above that, the lubricating coating is in a molten state, and the dynamic friction of the green compact that has started to move is reduced by the liquid lubricating action. In this way, the required pushing force is remarkably reduced as compared with the prior art throughout the entire process of pushing out the green compact. In view of the above, the present invention relates to a pressing lubrication method in which a lubricating coating having a liquid upper portion and a solid lower portion is provided on an inner wall surface of an outer die, and a powder molding die equipped with heating means and cooling means for forming such a lubricating coating. , And a powder compacting method in which extrusion of the compacted green compact is performed initially with solid lubrication and then with liquid lubrication.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The structure of the main part of a powder molding die according to the present invention is schematically shown in FIG. In the figure, reference numeral 10 denotes an outer die, and a reinforcing ring 11 is fitted around the outer die to apply a compressive stress to the outer die 10 to suppress distortion of the outer die at the time of powder compaction.
The outer die (and the reinforcing ring) is attached to the outer die plate of the die set, and the lower punch is fitted in the outer die, but they are not shown.

The feature of this outer mold is that it is provided with means for cooling the lower part of the outer mold and means for heating the upper part above the melting point of the pressing lubricant. In this embodiment, the electric heater 12 is provided on the upper part of the reinforcing ring. Is provided as a heating means, and on the other hand, a spiral refrigerant passage 13 is built in the lower part as a cooling means.
The cooling effect can be enhanced by winding the spiral roughly upward and densely downward. Due to the action of both means, the upper part of the inner surface of the outer mold becomes a heating zone where the lubricant film is melted, and the lower part becomes a cooling zone which maintains a solid state, but the distribution of both zones is adjusted according to the position where the green compact is formed. To be done. It should be noted that providing a shallow annular groove of the order of μm as an oil reservoir on the inner surface of the heating zone is effective in preventing oil film breakage.

FIG. 2 shows one cycle of powder molding in the present invention. In the figure, (a) First, the lower punch 20 is positioned at the bottom dead center of the vertical movement, and the lubricant is applied to the inner surface of the outer die 10. Here, an acetone solution of zinc stearate was sprayed and a zinc stearate film was formed by volatilization of the solvent, but the lubricant may be electrostatically attached as powder. The coating film of the lubricant is uniformly in the solid state in the case of the conventional pressing lubrication method, but in the present invention, the upper portion of the outer die is heated and the lower portion is cooled. The lower part is in a solid state and the upper part is in a molten state. (B) Next, the lower punch is raised to a predetermined filling depth, the raw material powder is scraped and filled into the cavity formed by the outer die and the lower punch, and (c) the formed green compact is lubricated with a coating. Is placed in the solid state zone, the lower punch is pulled down to move the powder to the bottom of the outer mold. (D) From this state, the upper punch 30
Is pressed to pressurize the powder, and at the same time, the lower punch 20 is lifted to compress and powder the powder between the upper and lower punches at a predetermined molding pressure.

(E) When the upper punch is returned to the upper position, the green compact thus obtained remains on the lower part of the outer die while being placed on the lower punch. As described above, the green compact is located in the cooling zone at this point, that is, in the zone where the lubricating coating is in the solid state.
(F) Next, if the lower punch is raised to the upper edge of the outer die, the green compact is extruded from the outer die through the cooling zone, the heating zone, that is, the zone in which the lubricating coating is in a molten state, and one cycle of powder molding is performed. Is completed. After that, if the green compact is removed and the lower punch is lowered to the bottom dead center, the state returns to (a).

The former of the two methods of die set, that is, the outer die fixing method, the lower punch operating method and the lower punch fixing method, the outer die operating method is illustrated above. The relative position between the lower punch and the outer die is illustrated. Since there is no difference in the relationship, the present invention can be applied as it is to the latter lower punch fixing and outer die operating method.

In conventional powder molding apparatus, since the outer mold is not intentionally heated or cooled intentionally, the lubricant coating remains uniformly solid. Therefore, in the case of the mixed lubrication method and the conventional pressing lubrication method, the steps (b) to (c) of moving the raw material powder can be omitted, but here, compared with the present invention under the same conditions with the same pushing distance of the green compact. Therefore, in the case of the mixed lubrication method, the same is applied except that the lubricant is not applied to the outer mold in (a). Also in the present invention, when the temperature gradient applied to the outer mold is set to be narrow in the heating zone and wide in the cooling zone due to the relationship of the shape and size of the product, the steps (b) to (c) also move accordingly. Can be shortened or omitted.

(Example 1) First, a copper powder is used in a weight ratio of 1.5.
%, Graphite powder 1% and the balance of iron powder are prepared, and in the case of the pressing lubrication method, this mixed powder (apparent density 2.85 g / cm
³ ) was used as it was, and in the case of the mixed lubrication method, 0.8% of zinc stearate powder having a melting point of 122 ° C. was added to this mixed powder to obtain a raw material powder (apparent density: 3.28 g / cm ³ ).
This zinc stearate is the same as the press type lubricant used in the press type lubrication method.

Next, the powder molding die described with reference to FIGS. 1 and 2 is prepared, and in the case of the die lubrication method of the present invention, the heating means and the cooling means are made to function so that the temperature of the outer die is 130 at the upper end thereof.
℃, 25 ℃ at the bottom, the middle temperature is shown in Figure 2 (d),
The heating current, the flow rate of cooling water, and the water temperature were adjusted so that the formation position of the green compact shown in (e) was kept lower than the melting point of the pressing lubricant. Next, the mold is set in the state of FIG. 2 (a), a lubricant film is formed on the inner surface of the outer mold, and thereafter (a) to (b).
Filling of raw material powder and molding pressure 58 according to the steps of (f)
The compression molding at 8 MPa (6 t / cm ² ) and the extrusion of the green compact were performed, and the pressing force of the green compact between (e) and (f) was continuously measured. The result is as shown in the graph (solid line) of FIG. 3, and the obtained green compact has a diameter of 2 mm.
0 mm, 10 mm high columnar shape with a powder density of 7.12 g
/ Cm ³ . Here, the pushing force is indicated by the load of the lower punch required for pushing out the green compact.

Next, also in the case of the conventional pressing lubrication method, the same mold was used, and the comparison was made in the same manner as in the pressing lubrication method of the present invention except that the heating means and the cooling means of the outer die were both stopped. The test was conducted. The result is shown in the graph (broken line) of FIG. The obtained green compact has the same shape and compact density as those of the present invention.

Next, in the case of the mixed lubrication method, as in the case of the conventional pressing lubrication method described above, a die in which both the heating means and the cooling means of the outer die are stopped is used, and the outer die lubrication step (a) in FIG. ) Was omitted and the raw material powder mixed with the powder lubricant was filled to a predetermined filling depth, and thereafter, the same operation as in the case of the present invention was carried out and a comparative test was conducted. The result is shown in the graph of FIG. 3 (dashed line). The green compact obtained had a height of 10 mm and a green density of 7.0 g / cm ³ .

Considering the graph of FIG.
In this graph, the horizontal axis represents the time until the green compact is completely discharged from the outer die (f) after the lower punch 20 starts to rise from the state of FIG. The magnitude of the load detected from) is on the vertical axis. Since the green compact does not move within a certain minute time after the lower punch is started to rise due to the frictional resistance (static friction) with the outer die, the pushing force sharply increases to a certain value in a substantially straight line during this period. However, illustration is omitted from 0 point to here.

When the green compact starts to move, the pushing force suddenly decreases and then continues to increase at a gentle slope in the case of the mixed lubrication method (one-dot chain line), at the time when the green compact comes out from the edge of the outer die. The output changes to 0 and the output becomes 0 at the same time as the extrusion is completed. In the case of the conventional pressing lubrication method (broken line), the tendency is similar to this, but the pushing force is somewhat smaller than in the case of the mixed lubrication method throughout the entire process of powder compact extrusion.

On the other hand, in the case of the pressing lubrication method (solid line) of the present invention, unlike this, the pressing force suddenly decreases at the same time when the powder compact starts to move, and then continues to decrease with a gentle inclination, and from the above-mentioned inflection point, As the powder comes out, it decreases abruptly as before, and the pushing force becomes 0 as soon as the pushing out is completed. As described above, the pressing lubrication method of the present invention has a higher green compact density and a significantly smaller pressing force than the conventional mixed lubrication method, but the results are the same as those generally introduced at the beginning.

Furthermore, the pressing lubrication method of the present invention has a significantly smaller pushing force than the conventional pressing lubrication method.
The result of both methods is that the inner surface of the outer die is conventionally uniformly solid-lubricated in the past, whereas in the present invention, the stationary position of the green compact is solid lubrication and the moving position is liquid lubrication. It is judged from the fact that liquid lubrication is superior to solid lubrication in the area of dynamic friction, and at the same time, it is judged that the present invention is effective in controlling the temperature gradient of the outer die. It

When the green compact discharged from the die in this manner was examined for defects such as cracks, microcracks, and rough skin, it was 0.07% in the case of the mixed lubrication method and in the case of the conventional die lubrication method. Has a defect of 0.04%,
In the case of the press-type lubrication method of the present invention, none was found. In addition, a hard alloy powder having a composition of Fe-3Cr-0.3Mo-0.3V is used as a raw material powder, and an outer die (3 pieces) made of a cemented carbide having an inner surface finish of 0.2 μm is prepared and lubricated. As a result of investigating the relation between the outer mold and the wear of the outer mold, the surface roughness of the outer mold after molding each 30,000 pieces is Rmax 1.0 μm in the case of the mixed lubrication method, 0.8 μm in the case of the conventional pressing lubrication method, 0.4 μm in the case of the stamping lubrication method of the present invention
m, the present invention was also excellent in terms of die life.

(Embodiment 2) In Embodiment 1, the case where the entire inner surface of the outer die is solid lubricated by the pressing lubrication method is compared with the present invention. Therefore, in Embodiment 2, the entire inner surface of the outer die is liquid lubricated. By comparing the case of the present invention with the present invention, the relationship between the configuration of the present invention and the effect was clarified. First, the solid line graph shown in FIG. 4 is for the method of the present invention and is the same as the graph (solid line) in FIG. On the other hand, in the mold of the present invention, only the heating means of the outer die is allowed to function, the cooling means is stopped, and the entire inner surface of the outer die is kept at 130 ° C. After that, the result of measuring the pressing force of the green compact (comparative example) is shown by a broken line in FIG. The measured value after intersecting with the graph (solid line) of the present invention in the figure almost overlaps the graph of the present invention.

As is apparent from this graph, the pushing force of the lower punch required to move the green compact is considerably larger in the comparative example than in the case of the present invention, and decreases as the green compact moves. After the movement is stabilized, it overlaps with the case of the present invention, but the difference between them is clear. This fact indicates that the effect of pressing lubrication (lubrication film) that overcomes the frictional resistance with the outer die to move the powder compact is that solid lubrication is superior to liquid lubrication in the area of static friction. It is shown. Thus, from the results of Examples 1 and 2, the effectiveness of the present invention was sufficiently confirmed.

As described above, in the above embodiment, the pressing lubricant was dissolved in the solvent and applied to form the lubricating coating on the inner surface of the outer mold. However, the pressing lubricant may be applied in the form of powder, In that case, the electrostatic adhesion method is preferable from the viewpoint of uniformity of adhesion state and adhesion strength. In this method, an electric charge is applied to a powder-type pressing lubricant by frictional charging, and electrostatically attracted and attached to a grounded external mold. As the pressing lubricant, a fatty acid amide wax such as ethylene bis-stearamide, or a metal soap such as zinc stearate or lithium stearate is suitable.

[0028]

As described above in detail, according to the present invention, the pushing force at the time of discharging the green compact compacted from the mold from the mold becomes significantly smaller than the conventional one, resulting in cracks and rough skin. It is possible to easily obtain a high-density molded product without defects such as.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view schematically showing a configuration of a main part of a powder molding die according to the present invention.

FIG. 2 is a diagram illustrating one cycle of a powder molding process.

FIG. 3 is a graph showing a relationship between a pressing die lubrication method and a pressing force of a green compact in Example 1.

FIG. 4 is a graph showing the relationship between the lubrication method of the die and the pressing force of the green compact in Example 2.

[Explanation of symbols]

10 ... Outer mold, 11 ... Reinforcing ring, 12 ... Electric heater (heating means), 13 ... Refrigerant passage (cooling means), 20 ... Lower punch, 30 ... Upper punch

Claims (8)

(57) [Claims] 1. A powder molding method in which a raw material powder filled in a cavity formed by an outer die and a lower punch is compression-molded between upper and lower punches, and the obtained green compact is extruded from the outer die by a lower punch. A lubrication film on the inner surface of the outer mold with a lubricant,
A powder molding method in powder metallurgy, characterized in that the powder compact is extruded in the solid state at the position where the green compact is formed and in the liquid state above it. 2. A powder molding configured such that a raw material powder filled in a cavity formed by an outer die and a lower punch is compression-molded between upper and lower punches, and the obtained green compact is extruded from the outer die by the lower punch. In a metal mold, a powder in powder metallurgy, comprising means for heating the upper part of the outer mold to a temperature above the melting point of the pressing lubricant and means for cooling the lower part of the outer mold below the melting point of the pressing lubricant. Mold for molding. 3. When powder molding is carried out by a pressing lubrication method, the upper part of the outer mold is heated to a temperature above the melting point of the pressing lubricant and the lower part is cooled below the melting point of the pressing lubricant so that the lower part of the inner surface of the outer mold is cooled. A method for lubricating a die in powder metallurgy, comprising forming a solid-state lubrication coating made of a die-casting lubricant and a liquid lubrication coating made of a die-pressing lubricant on the top. 4. The powder molding method in powder metallurgy according to claim 1, wherein the formation of the lubricating coating on the inner surface of the outer die is performed by electrostatically applying a powdery pressing lubricant. 5. The method for lubricating a die in powder metallurgy according to claim 3, wherein the formation of the lubricating coating on the inner surface of the outer die is performed by electrostatically applying a powdery die-type lubricant. 6. The powder molding method in powder metallurgy according to claim 1, wherein the raw material powder is moved downward after the outer mold is filled with the raw material powder and before the start of compression molding. 7. The powder molding method in powder metallurgy according to claim 1, 4 or 6, wherein a fatty acid amide wax or a metal soap is used as the pressing die lubricant. 8. The method according to claim 3, wherein a fatty acid amide wax or a metal soap is used as the pressing lubricant.
A method for lubricating a die in powder metallurgy according to.