JP2747082B2 - How to find the coefficient of friction during powder molding - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for determining a coefficient of friction at the time of molding a powder.

[Conventional Metal] In the field of manufacturing a product by sintering a metal powder or a ceramic powder by using a metal mold and sintering, there are strict requirements for the shape and dimensional accuracy of the product. In order to meet such demands or to improve the production yield, it is important to reduce various disturbance factors in each process such as production, molding, and sintering of powder raw materials. For this purpose, it is necessary to always manage important process control factors in each process and keep them at a certain level.

The shape and dimensions of the sintered product are determined by the density distribution of the compact and the temperature distribution during sintering.

There are many factors that affect the density distribution of a compact. For example, the particle size distribution of the raw material powder, the lubricant, the material of the mold, the pressing method, and the like. Conventionally, the relationship between the change of these factors and the shape and dimensional accuracy of the sintered product has been investigated, and the optimum operating conditions for these factors have been empirically found.
However, the effects of these various factors were complex and it was quite difficult to quantify the effects. From another point of view, the density distribution of the compact is mechanically determined by the stress-strain behavior of the powder in the die and the friction behavior of the die wall (for example, “Plasticity and Processing”, Vol. 27, No. No. 308, (198
6-10), pp. 1255-1131, and "Spring Lecture on Plastic Working in 1989" (1989-5), pp. 195-198). Therefore, in order to make it easier to understand the effects of the various factors on the shape and dimensions during sintering, the various factors first enhance the effect on the stress-strain relationship and the friction coefficient, and then increase the stress-strain.
It is appropriate to consider the effect of the strain relationship and the coefficient of friction on the shape and dimensions during sintering.

[Problems to be Solved by the Invention] In order to adopt the above method, it is necessary to obtain a stress-strain relationship and a friction coefficient. The stress-strain relationship can be determined from the relationship between the load and the density during CIP (Cold Isostatic Pressing), or when compressing a single-shaft die under conditions where lubrication is improved and friction is almost negligible. This method is known (see the above-mentioned literature).

It is not always known how to determine the coefficient of friction during molding. Conventionally, the coefficient of friction has been determined, but there is no guarantee that the coefficient of friction is the same as the coefficient of friction during molding ("The International Journal of Powder Metallurg").
y ”vol.23, No23, p83-93, and“ Journal of Powder &
Bulk Solid Technology ", 11 (1987) 2; 1-5).
For example, this "Journal of Powder & Bulk Solid Te
In "chnology", the friction coefficient is determined by sandwiching a powder between two plates and sliding the plates in opposite directions while applying pressure, but the deformation mode is greatly different from that of die molding.

The inventors of the present invention have studied various methods for measuring the coefficient of friction and found that the following method is optimal.

[Means for Solving the Problems] A method for obtaining a friction coefficient according to the present invention is a method for producing a single-axis compression of a powder, wherein one or more portions of a die wall constituting an inner surface of a die are separated from a die body. With the structure separated, the force P perpendicular to the wall surface which the separated die wall (movable die) receives from the powder in the die and the frictional force F in the wall in the pressing direction are measured by a load cell attached to the movable die. In addition, the friction coefficient μ between the powder and the wall surface is obtained from the relationship μ = F / P.

The principle will be described with reference to FIGS. FIG. 1 is a transverse sectional view of a square-shaped single-shaft die press, and FIG. 2 is a longitudinal sectional view. 1 is a die main body, 2 is a part of the die, but is separated from the main body 1 and is called a movable die. The movable die is separated from the die body because the surface pressure P and the frictional force F applied to the portion 2 ′ in contact with the powder on the inner surface of the die.
It is for measuring. If these values are known, the coefficient of friction μ can be determined from the relationship μ = F / P. Hereinafter, a press operation method will be described. First, a prestress f is applied from the back of the movable die. This force is applied to the surfaces 7, 7 ', 8, 8' where the movable die and the die body contact. In this state, the surface pressure P is set to 0. f is a force applied to absorb backlash and to prevent the powder from first entering the contact surfaces 7, 7 ', 8, 8' of the movable die and the die body, so that a large force is not required.

By controlling the operating conditions so that the friction coefficient determined by the above procedure and the stress-strain relationship curve determined by a known method are always the same, a compact having the same density distribution can be obtained, If the temperature distribution at the time is controlled to be constant, a product having the same shape and dimensions can be obtained. The advantage of this method is
This is because the friction coefficient and the stress-strain relationship are easily related to various process controlling factors such as various material property values of the powder and mold conditions.

In the above description of the principle, a mold having a square inner surface is used as an example, but it is clear that the same measurement can be performed with a circular shape or a polygonal shape.

Embodiment FIG. 3 and FIG. 4 show an example of the apparatus. The die wall corresponding to the two opposing sides of the four sides constituting the inner surface square (20 mm opening) is a movable die. FIG. 3 is a longitudinal sectional view, and FIG.
It is -B sectional drawing.

1 is a die body, 2 is a movable die, 3 is an upper punch, 4
Is a lower punch, 5-1,5-2,5-4,5-5 is a load cell for measuring the surface pressure P perpendicular to the inner wall of the die, 5-3,5-6
Is a load cell for measuring the frictional force F on the inner surface of the die. Reference numerals 6-1 and 6-2 denote load cells for measuring the loads P _U and P _D of the upper punch and the lower punch, respectively. Reference numeral 9 denotes a ball screw for applying a prestress to the movable die, and reference numeral 10 denotes a ball screw for reducing friction of a sliding surface between the movable die and the die body to zero.

FIG. 5 shows a measurement example of a load when a material obtained by mixing 5% by weight of graphite with alumina powder is put into a 20-g die and compressed. The horizontal axis is time t, and the vertical axis is load P
It is. a is when the upper punch is lowered, b is when stopped, and c and d are when ascended. P _U is the upper punch load, P = P ₁ + P ₂ is the normal force,
P ₃ is a friction force.

FIG. 6 shows the relationship between the friction coefficient μ = P ₃ / (P ₁ + P ₂ ) and the aspect ratio X / D of the mixture of alumina powder and graphite powder. The graphite amounts of the material A, the material B, and the material C are 0.5 and 50% by weight, respectively. 30g, 20g, etc. in the figure indicate the amount of powder to be charged into the die. μ is the average value of the coefficient of friction. X is the distance from the die bottom to the upper punch surface, and D is the die inner diameter.

FIG. 7 shows a similar example in the case of soft ferrite powder. The coefficient of friction μ = P ₆ / (P ₄ + P ₅ ) was almost the same value. The value of the coefficient of friction obtained as described above has a reasonable change with respect to the amount of the lubricant (graphite), and is a reasonable value compared with the conventional empirical value of the bulk material. .

[Brief description of the drawings]

FIG. 1 is a transverse sectional view of a square-shaped single-shaft die press, and FIG. 2 is a longitudinal sectional view. FIG. 3 and FIG. 4 show an example of the apparatus used in the embodiment. FIG. 5 shows a measurement example of a load when a material obtained by mixing 5% by weight of graphite with alumina powder is put into a 20-g die and compressed. FIG. 6 shows the relationship between the friction coefficient μ of the mixture of alumina powder and graphite powder and the aspect ratio. FIG. 7 shows a similar measurement example of the coefficient of friction in the case of soft ferrite powder. 1 ... die body, 2 ... movable die, 3 ... upper punch, 4 ... lower punch, 5-1, 5-2, 5-4, 5-5 ... surface pressure P perpendicular to the inner wall of the die Load cell for measurement, 5
-3, 5-6: load cells for measuring the friction force F on the inner surface of the die, 6-1 and 6-2: load cells for measuring the loads P _U , P _D of the upper punch and the lower punch, respectively. 7,7 ′,
8, 8 ': surface where the movable die and the die body are in vertical contact, 9: ball screw for applying prestress to the movable die, 10: friction of the sliding surface between the movable die and the die body Ball screw to bring the value closer to 0

Claims (1)

(57) [Claims] 1. A die for uniaxial compression of powder, wherein one or more portions of a die wall constituting an inner surface of a die are separated from a die body, and the separated die wall ( The force P perpendicular to the wall surface received by the movable die) from the powder in the die and the frictional force F in the wall surface in the pressing direction are measured by a load cell attached to the movable die, and the friction coefficient μ between the powder and the wall is μ = A method of determining a friction coefficient at the time of molding a powder, which is determined from an F / P relationship.