WO2011001868A1 - Compression molding method for powder and device therefor - Google Patents

Abstract

A magnetostrictive actuator (52), as an impact force generating means, is inserted between at least one of the upper and lower rams (2, 5) of a powder molding press and a punch (3). After the compression by a predetermined static pressure, the compression by the impact force is performed to reduce the internal stress. If the downward pressure of the upper ram (2) is released after the compression by the static pressure and the upper ram (2) is dropped by the weight thereof, the molding time can be further reduced.

Description

Powder compression molding method and apparatus

The present invention relates to a powder compression molding method and apparatus for granulating a granulated powder such as ceramics or metal by a vertical press.

A granulated powder produced by mixing a binder such as wax with a ceramic or metal powder is filled in a mold of a press machine and compression-molded. The compression-molded powder is usually fired in a firing furnace to form a cemented carbide chip for machining or a precision machine part.

The method of slowly raising and lowering the upper punch and lower punch with a crank mechanism or hydraulic mechanism as in a normal press machine makes it difficult to form powder with high density due to poor sliding between the powders, and the inside of the molded product The density distribution is not uniform, which is not preferable.

Japanese Patent Application Laid-Open No. 2004-174595 discloses that a predetermined punching is performed by attaching a punch to the upper ram or upper and lower rams via stacked piezoelectric elements, and applying intermittent impact force to the powder filled in the mold. It is described that it is formed into a shape. According to the method described in this patent publication, it is expected that slipping occurs between powders due to impact force, and the above problems are solved.

FIG. 11 shows an example of an impact press described in the above-mentioned Japanese Patent Application Laid-Open No. 2004-174595. Reference numeral 1 denotes a frame, reference numeral 11 denotes an intermediate frame, reference numeral 2 denotes an upper ram, reference numeral 21 denotes an upper ram 2 Ball screw which is a mechanism, reference numeral 23 is a laminated piezoelectric element, reference numeral 3 is an upper punch attached to the upper ram 2 via the laminated piezoelectric element 23, reference numeral 4 is a die fixed to the intermediate frame 11, reference numeral 5 is A lower ram, 51 is a ball screw which is a lifting mechanism of the lower ram 5, 52 is a laminated piezoelectric element, and 6 is a lower punch.

As a piezoelectric element, for example, PZT (Piezo-electric Transducer) using a piezoresistance effect is known. This element is a ceramic that deforms at high speed when a drive voltage is applied.

In the powder molding press described in the above Japanese Patent Application Laid-Open No. 2004-174595, as described in paragraph [0030] of the patent publication, the displacement amount of the piezoelectric element is as small as several μm to several tens μm. In addition, it is not only necessary to laminate a plurality of sheets, but there is also a problem that the effect is not achieved unless a powder whose springback amount (length during compression−length after compression) is smaller than the displacement amount is targeted.

In addition, since the impact force generated in the piezoelectric element is not directional in nature, it is necessary to devise a device in order to concentrate this on the vertical movement.

Further, according to the experiments by the present inventors, if a predetermined pressure is not applied in advance before the impact is applied to the powder, the impact force does not act on the entire powder, leaving a void inside and uniform. It turns out that compression is not achieved.

The object of the present invention is to solve these problems and realize compression molding of a uniform powder with no voids remaining inside by an effective impact force.

In the present invention, an upper punch and a lower punch are respectively arranged above and below the die, and a powder is filled in a space formed by the upper punch, the lower punch, and the die, the lower punch is raised, or the upper punch is In a vertical powder compression molding method in which the powder is compressed and molded by lowering, the lower punch is raised or the upper punch is lowered to compress the powder filled in the space to a predetermined pressure. Then, an impact force generating means provided between the upper punch and the upper ram to which the upper punch is attached, or between the lower punch and the lower ram to which the lower punch is attached. A method for compressing and molding powder, characterized in that an impact force generating means is operated to further compress the powder.

In the present invention, after the impact force generating means is operated to further compress the powder, the lower punch is raised again, or the upper punch is lowered again, It is good also as eliminating the clearance gap which arose by the reduction | decrease of the bulk.

Further, in the present invention, a clearance of a predetermined dimension is provided in the lifting mechanism that lifts and lowers the upper punch so that the upper punch can freely fall in the vertical direction, the lower punch is lifted, or the upper punch is The process of lowering and compressing the powder to a predetermined pressure may be performed by the weight on the upper punch side due to the free fall of the upper punch.

In the method of the present invention, compression of the powder to a predetermined pressure by raising the lower punch or lowering the upper punch and further compression by the impact force generating means may be repeated.

Furthermore, a magnetostrictive actuator may be used as the impact force generating means in the method of the present invention.

Furthermore, in the method of the present invention, the further compression stroke by the impact force generating means can be a stroke twice or more the average particle diameter of the powder.

On the other hand, in the apparatus of the present invention, an upper punch and a lower punch are respectively arranged above and below a die, a powder is filled in a space formed by the upper punch, the lower punch and the die, and the lower punch is raised. Or a vertical powder compression molding apparatus that compresses and molds the powder by lowering the upper punch, and between the upper punch and the upper ram to which the upper punch is attached, and the lower punch The powder compression molding apparatus is characterized in that a magnetostrictive actuator as an impact force generating means is provided between at least one of the lower ram to which the lower punch is attached.

The apparatus of the present invention further includes an elevating mechanism for elevating the upper punch, and a vertical gap provided inside the elevating mechanism so that the upper punch can be freely dropped, The upper punch can be configured such that the weight of the portion below the gap acts as the predetermined pressure for compressing the powder.

According to the present invention, the internal stress is reduced by applying an impact force to the powder at the time of compression molding, and an excellent effect is achieved in that the thermal shrinkage in the subsequent baking process is made uniform and the quality is improved.

It is a front view which shows the compression molding apparatus of this invention Example. It is sectional drawing which shows the metal mold periphery which is the principal part of FIG. It is explanatory drawing explaining the compression molding method of this invention. It is a perspective view which shows the workpiece | work in the compression molding of FIG. It is a graph which shows the relationship between the punch movement distance and extraction force in this invention Example. It is a graph which shows the relationship between the relative speed of a punch in this invention Example, and a friction coefficient. It is a graph which shows the relationship between the density and extraction force in an Example of this invention. It is a schematic diagram explaining the effect of this invention Example. It is a fragmentary sectional view near the upper ram lower end in the example of the present invention. It is explanatory drawing explaining the clearance gap of the upper ram part in this invention Example. It is a front view of the impact type press in a prior art.

First, the powder compression molding method of the present invention will be described with reference to FIG.

In FIG. 3, reference numeral 3 indicates an upper punch, reference numeral 6 indicates a lower punch, and reference numeral 4 indicates a die. The punch and die have a cross-section with a radius r (2 mm as an example). The work W, which is a powder, has a cylindrical shape filled in a space surrounded by these dies as shown in FIG. If the upper punch 3 is now driven and the lower punch 6 is stationary, the compression load of the upper punch 3 is P _D , and the stationary load, which is the reaction force of the lower punch 6, is P _S.
P _S = P _D − (2πrh × friction coefficient × internal stress) (1)
It is. The frictional resistance is in the parenthesis on the right side.

In order to lift the lower punch 6 and extract the workpiece after completion of the compression, it is only necessary to overcome the frictional resistance. Therefore, the necessary force, that is, the extraction force _PE is
P _E = 2πrh × friction coefficient × internal stress (2)
It is.

The extraction force can be measured. Therefore, if the friction coefficient is known, the internal stress can be estimated by the equation (2), so that the extraction force can be considered as an index of internal stress, that is, density uniformity inside the green compact.

FIG. 5 is an example of a graph showing the relationship between the punch moving distance and the extraction force during extraction. Up to the last peak value of the proportional portion that rises sharply corresponds to static friction, and the subsequent low portion is dynamic friction, which is about half of static friction.

On the other hand, it is known that the relationship between the friction coefficient and the relative speed of the punch is an exponential function. In other words, if it is represented by a semilogarithmic graph, it is a straight line descending to the right, but it is as shown in FIG. A value in contact with the vertical axis, that is, a value at a speed of 0 is a static friction coefficient, and a value on the right side corresponds to a dynamic friction coefficient. In a normal press machine, the punching speed is about 10 to 100 mm per second, but in an impact press, it reaches 1 m per second. Therefore, the impact press is a fraction of that of a normal press in terms of friction coefficient.

In addition, according to experiments by the present inventors, it was found that the friction coefficient varies depending on the type of powder, but the same powder does not change before and after compression.

FIG. 7 is a graph showing the relationship between the density and the extraction force between normal compression molding performed without generating an impact force by changing the type of powder, and compression molding with an impact force applied, (a) Tungsten carbide (WC) granulated powder, (b) is the case of alumina powder.

Tungsten carbide is a fine powder of about 10 μm, but it is too fine to be filled easily, so a binder is mixed to make a size of about 50 μm. This is called granulated powder.

In all the graphs in FIG. 7, the dashed line is normal compression molding, and the solid line is compression molding with impact applied. The graph rises to the right. When the density is increased by compression, the extraction force also increases. However, when compared at the same density, the extraction force decreases by about 25 to 45% by applying an impact, and the density is The higher the value, the greater the effect.

By the way, the impact force is not effective just by adding this. According to the experiments by the present inventors, it is preferable to apply the impact force after compressing the powder by a conventional method to a predetermined pressure (preload). Otherwise, the corner impact force will not be transmitted to the entire powder, and only the surface will be struck. A preferable preload value is generally in the range of 4.9 to 14.7 MPa (50 to 150 kg / cm ² ) although it depends on the size of the mold and the kind of powder. If it is lower than this, there are too many internal voids and even if an impact force is applied, there is no effect, and if it is higher than this, the internal voids are confined, which is not preferable.

The stroke during compression by impact force is also an important factor. The average particle size of the powder such as ceramics is about 50 μm, but the stroke needs to be at least twice this, that is, 100 μm or more. Small strokes smaller than this are not different from compression by normal static pressure, and there is no effect of impact force. On the other hand, a larger stroke is preferable.

In this respect, what is preferable as the impact force generating means is a magnetostrictive element or a magnetostrictive actuator. One is a rod having a length of about 50 mm, and when a coil arranged around it is excited, a deformation of 200 μm is instantaneously generated. When two of these are used in series, a large stroke of 400 μm can be easily realized. The impact force when effectively acting is 98 MPa (1 ton / cm ² ) or more.

On the other hand, when PZT is used as the impact force generating means, the amount of deformation is as small as about 0.5 μm with respect to the thickness of 1 mm, so some kind of device for increasing the stroke is required.
Example 1

Subsequently, a first embodiment of the powder compression molding method and apparatus of the present invention will be described with reference to the drawings.

FIG. 1 is a front view showing the compression molding apparatus of the first embodiment, FIG. 2 is a sectional view showing the periphery of a mold as the main part, and each reference numeral is the same as that used in FIG. Denotes a guide bar for raising and lowering the upper and lower rams, 24 denotes a pressure sensor for measuring an extraction force and the like, and 52 denotes a magnetostrictive actuator which is deformed by excitation. Although an example in which the pressure sensor 24 is provided on the punch 3 side is illustrated, in the present invention, the pressure sensor 24 may be provided on the lower punch 6 side. In short, it is provided according to the pressure to be measured. I just need it.

In this compression molding apparatus, the magnetostrictive actuator 52 is inserted between the lower punch 6 and the lower ram 5, but the magnetostrictive actuator may be inserted on the upper punch 3 side, or may be provided on both the upper and lower sides. Absent.

Subsequently, the compression molding method in the first embodiment will be described.

By rotating the ball screw 51 by a motor (not shown), the lower punch 6 is raised to form a recess with the lower punch 6 at the bottom in the center of the die 4, and in this space formed by the lower punch 6 and the die 4. The powder is filled to the surface height. Subsequently, the upper punch 3 is lowered by rotating the ball screw 21 with another motor (not shown) to compress the powder with static pressure until it reaches a predetermined pressure (the aforementioned “preferred preload”), and then the magnetostrictive actuator 52. Is applied and an impact force is applied once to the powder surrounded by the upper and lower punches 3 and 6.

The impact force is generated by applying a voltage to the magnetostrictive actuator 52 instantaneously. For example, a pulse voltage of about 200 μsec is applied at 300 V and 100 A by a power supply device (not shown).

Since the powder is compressed and the volume is reduced, the upper punch 3 or the lower punch 6 is moved again and compressed again with static pressure until it reaches a predetermined pressure again, and then the magnetostrictive actuator 52 is operated to apply an impact force. .

This operation is repeated as many times as necessary, for example 10 to 20 times.

Finally, the lower punch 6 is raised and the workpiece W is extracted. The spring back of the extracted workpiece W is also ½ or less compared to the case of only compression by static pressure.

When the whole is compressed evenly, the volume is reduced to 1/2 of the initial filling in the case of ceramic powder, and to 1/3 in the case of tungsten carbide granulated powder, but the internal voids disappear, Even if a baking treatment is performed in the process, a high-quality intermediate product is obtained that does not cause defects such as cracks and chips due to shrinkage.
Example 2

Next, a powder compression molding method and apparatus according to the second embodiment of the present invention will be described with reference to the drawings.

When the powder is instantaneously compressed by the impact force generating means, the punch on the side provided with the impact force generating means immediately returns to the original position by the electric signal, but the compressed powder is slightly restored to the original volume by the spring back. Try to return. FIG. 8A is a schematic diagram in which this situation is changed from left to right in time series.

First, the upper punch 3 is lowered to compress the powder to a predetermined pressure with a static pressure. Next, the powder is compressed by impact force generating means provided on the lower punch 6. At the next moment, the lower punch 6 returns to the original position in a time of about 1 / 10,000 second, but since the powder is compressed and the volume is reduced, a gap is formed. The spring back of the powder gradually progresses later than this, and the gap decreases, but the movement of the powder at this time is static friction against the wall surface, so the resistance is large and it takes time and density. Non-uniformity occurs. A gap remains as much as the powder is finally compressed. Therefore, the cycle from the lower punch 6 being lifted up to eliminating this gap is one cycle, after which it returns to the left end state and a second impact force is applied.

As is clear from the above explanation, an operation of moving the punch is necessary to eliminate the gap generated by one compression, and it takes time to repeat this operation.

In this second embodiment, a vertical gap is provided between the upper ram drive mechanism and the upper ram in order to eliminate this problem. FIG. 9 is a partial cross-sectional view near the lower end of the upper ram drive mechanism for explaining this situation. Reference numeral 2 is the upper ram, reference numeral 21 is a ball screw (the front end portion) for driving the upper ram, and reference numeral 22 is the upper ram 2. Reference numeral 3 denotes an upper punch, and reference numeral 31 denotes a punch holder for holding the upper punch 3 on the upper ram 2.

Such a locking structure is for raising the upper ram 2 by means of the ball screw 21. In this embodiment, a clearance of the dimension g is provided in the locking portion in the vertical direction.

The effect will be described with reference to FIG. This is a schematic diagram that is changed from the left to the right in time series as in FIG. 8A.

First, the upper punch 3 descends and the powder is compressed to a predetermined pressure with static pressure as in the case of FIG. Next, the ball screw 21 is reversely rotated to loosen the lower ram 2 so that the upper ram 2 floats.

The “floating state” will be described with reference to FIG. 10A shows a standby state before the press enters compression processing, and the upper ram 2 is hung from the tip of the ball screw 21, and there is a gap between the tip of the ball screw 21 and the upper ram 2. Has occurred. (B) is a state when the ball screw 21 is actuated to reduce the pressure. The tip of the ball screw 21 is in close contact with the upper ram 2. (C) is a state in which the ball screw 21 is reversed to loosen the pressure reduction of the upper ram 2. No force acts between the tip of the ball screw 21 and the upper ram 2, and the upper ram 2 is in a floating state. In the state shown in (b) and the state shown in (c), a reaction force from the powder to be compressed acts on the upper punch 3 upward. It is due to this reaction force that the upper ram 2 floats.

That is, when the ball screw 21 is raised by the gap g in FIG. 9, the upper ram 2 is in a floating state, and the upper punch 3 is placed on the powder with the weight of the upper portion. If it is the predetermined pressure and the weight of the upper ram 2 is insufficient for the preferable preload, the weight of the upper ram 2 may be added.

* Apply impact force in this state. In that case, since the weight of the upper punch 3 is sufficiently large, the powder W and the upper punch 3 are not lifted up, and a sufficient compressive force acts on the powder W. As in the case shown in FIG. 8 (a), the lower punch 6 returns to the original position instantly and a gap is generated for a moment, but the upper punch 3 placed on the powder falls by its own weight. As a result, the spring back of the powder and the lowering of the upper punch 3 occur simultaneously, and no gap remains. In addition, since the powder is moved in the state of dynamic friction by the lowering of the upper punch 3, there is little resistance and there is almost no load difference between the upper and lower punches 3 and 6, and the lower punch 3 as shown in FIG. Since the operation of moving is not necessary, the cycle time is shortened and the productivity is improved.

As is clear from the above description, the gap g provided in the engaging portion with the ball screw 21 corresponds to the gap generated by the impact force, and is preferably about 0.2 mm, for example.

Claims (8)

An upper punch and a lower punch are arranged above and below the die, respectively, and a powder is filled in a space formed by the upper punch, the lower punch and the die, and the lower punch is raised or the upper punch is lowered to In the compression molding method of vertical powder that compresses and molds powder,
The lower punch is raised or the upper punch is lowered to compress the powder filled in the space to a predetermined pressure,
Then, an impact force generating means provided between the upper punch and the upper ram to which the upper punch is attached, or an impact force provided between the lower punch and the lower ram to which the lower punch is attached. A method for compressing and molding powder, wherein the generating means is operated to further compress the powder. After the impact force generating means is actuated to further compress the powder, the lower punch is raised again, or the upper punch is lowered again, resulting in a gap caused by a decrease in the bulk of the powder. The powder compression molding method according to claim 1, wherein: A gap of a predetermined dimension is provided in the lifting mechanism that lifts and lowers the upper punch so that the upper punch can freely fall in the vertical direction, the lower punch is raised, or the upper punch is lowered to lower the powder. The powder compression molding method according to claim 1 or 2, wherein the processing to compress to a predetermined pressure is performed by the weight on the upper punch side due to the free fall of the upper punch. 3. The method according to claim 1, wherein compression of the powder to a predetermined pressure by raising the lower punch or lowering the upper punch and further compression by the impact force generating means are repeated. A method for compression molding of the described powder. 3. The powder compression molding method according to claim 1 or 2, wherein the impact force generating means includes a magnetostrictive actuator. 3. The powder compression molding method according to claim 1, wherein the further compression stroke by the impact force generating means is at least twice the average particle diameter of the powder. An upper punch and a lower punch are arranged above and below the die, respectively, and a powder is filled in a space formed by the upper punch, the lower punch and the die, and the lower punch is raised or the upper punch is lowered. In a vertical powder compression molding apparatus for compressing and molding the powder,
Magnetostriction as an impact force generating means between at least one of the upper punch and the upper ram to which the upper punch is attached and between the lower punch and the lower ram to which the lower punch is attached. An apparatus for compression molding powder, wherein an actuator is provided. An elevating mechanism for elevating the upper punch;
A vertical gap provided inside the lifting mechanism so that the upper punch can be freely dropped,
The powder according to claim 7, wherein the weight of a portion below the gap in the upper punch is configured to act as the predetermined pressure for compressing the powder. Compression molding device.

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