JP2889028B2 - Isostatic molding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of isostatic molding, and more particularly, to isostatic molding of metal, carbon, or ceramic powder particles into a uniform, compact, compacted product having a specific density. It is concerned with an improved method for doing so.

[0002]

2. Description of the Related Art Isostatic molding is a pressing method for densifying a powder composition into a compact shape with a pressure sufficient to obtain a density close to a theoretical value. To obtain a high unidirectional green density, the powders and particulate matter are densified under pressure acting through a suitable fluid medium, preferably a liquid.

[0003] Prior art isostatic molding methods include:
It has been found that density affects the final mechanical and physical properties of the pressed product more than any other property. The density is
Determine the strength and physical properties of green billets and final sintered compacts. In the current method,
By setting the pressure target during the molding operation and controlling the pressing speed of the press until this pressure target is achieved, the green density level of the compact is indirectly controlled. The second method includes temperature control, holding time control, and pressure reduction control in the compression-filling step.

[0004]

Unfortunately, experience shows that pressure control, with or without the adjustment of other process variables, as described above, achieves a high degree of uniformity, especially of carbon or ceramic moldings. I can't do it.
Due to the non-uniformity of the molding density, there is a wide variation in molding properties. In order to guarantee the reproducibility of moldings, especially for graphite and ceramic moldings, the density of the moldings must be kept within a limited narrow density range.

[0005]

SUMMARY OF THE INVENTION An isostatic molding process of the present invention comprises the steps of charging a metal, carbon or ceramic powder charge into a mold assembly of known weight and volume; Weighing the mold assembly loaded with the article and the mold set from the difference between the weight of the mold assembly loaded with the powdered charge weighed in the previous step and the known weight of the mold assembly. Calculating the weight (Wm) of the powder charge in a volume, charging the mold assembly and the powder charge into an isostatic pressurized vessel, and filling the pressurized vessel with a fluid medium. Applying pressure to the pressurized container; weighing the total weight of the mold assembly and the powder charge in the pressurized container during the pressurizing; and From the difference between the known weight and the product of the known volume of the mold assembly times the fluid medium density (ρf) , the powder charge The weight of the mold assembly in the fluid medium without the powder charge is calculated and then the weight of the mold assembly in the fluid medium with the powder charge weighed in the previous step and the fluid medium without the powder charge Calculating the weight of the powder charge (Wmf) in the pressurized vessel from the difference from the weight of the inner mold assembly;
Calculating the density of the compressed powder charge (ρp) in a pressurized vessel by the following algorithm;

[0006]

(Equation 2) Where Wm = weight of powder charge in mold assembly outside pressurized vessel; Wmf = weight of powder charge in fluid medium inside pressurized vessel; ρf = density of fluid medium; ρp = pressurized vessel Depressurizing the pressurized vessel when the density of the compressed powder charge therein and the density of the compressed powder charge calculated in the previous step are equal to the target density of the pre-selected compact ; including.

[0007]

EXAMPLE A typical density versus pressure relationship for an isostatic pressure compact is shown in FIG. In a typical isostatic molding process, the relative green density of the molding is determined indirectly by its known correspondence to the pressing pressure. Monitor and vary the pressure in the pressurized container until the target pressure is obtained. Adjustment of the actual pressure in the pressurized container is performed by adjusting the pressurizing speed of the pressurized container, the holding time at a given pressure level, the temperature and the depressurizing speed. However, even under highly controlled pressure cycling conditions, variations in the degree of filling of the mold, the composition of the molding powder and the temperature cause variations in the density of the molding. It is the density of the molding that governs the uniformity of the molding.

The system 10 of the present invention, shown in FIG. 2, is designed to monitor the density of the powder charge directly during in-situ compression. The apparatus itself of this system is conventional and includes a pressurized container 11, a pump 12, and a liquid tank 14 containing an isostatic fluid 15 such as water. A mass flow meter 16 for measuring the amount of liquid pumped into a pressurized container is also desirable for the practice of other embodiments of the present invention, described in further detail below. Pump 12 in control loop 17 including control valve 18
Is arranged so that the pump can be operated during a working cycle. The pressurized container 11 includes a weighing system 20, which allows the removable mold assembly 22 to be weighed during pressing by the process of the present invention. Although the system 10 of the present invention is configured to operate automatically by the computer controller 24, it can be operated manually.

[0009] To press the powder composition into a compact, compact form, the desired powder composition is charged into a conventional elastomeric mold or "bag" (not shown) of known weight. The mold is then sealed and the mold is loaded into a conventional support structure (not shown) to form a mold assembly 22 to prevent ingress of the isostatic fluid. The weight of the powder charge in the mold assembly is calculated simply by subtracting the mold weight with and without the powder charge. Next, the mold assembly 22 is placed in the pressure vessel 11. Before being introduced into the pressurized container 11, the mold assembly 2
Record the weight and volume of all components of 2. Weight and volume data are transmitted to the computer controller 24 by the operator via the modem 26 or manually.
It is charged inside.

A weighing system 20 mounted in the pressurized container 11 comprises a conventional weighing platform (not shown) built at the bottom of the press with a conventional load cell (not shown).
Consists of The load cell must be able to accurately measure the weight of the mold assembly 22 in water. In a preferred embodiment, the mold assembly 22 is weighed during the entire pressure cycle. Therefore, in order to accurately weigh the mold assembly under pressure, the load cell must be hydrostatically compensated. Still water compensated load cells are currently commercially available.

The sequence of steps that occur during a density controlled press cycle is as follows.

1. The mold assembly 22 is placed on the weighing platform of the weighing system 20 in the pressurized container 11. Mold assembly 22
Can optionally be warmed before or during introduction into the pressurized vessel.

2. The pressurized container 11 is filled with water and sealed.

3. The pressurizing pump 12 is started, and the controller 24 is used.

4. Controller 24 continuously calculates the in-situ density of the molding during the press cycle as described below.

Molded product density = powder weight / powder volume Here, powder weight = (mold assembly + weight of powder) −mold assembly weight.
This is measured outside the press.

Powder volume = total volume of (mold + powder) −mold volume. Total volume of (mold + powder) = mold outside the press + weight of powder (ie, dry weight) −total weight of mold and powder in press (measured by load cell) / pressurized fluid density.

Pressurized Fluid Density = f (Temperature, Pressure and Fluid Composition) The calculation of the target density of the compressed powder charge is expressed by the following algorithm, which is derived from the above relationship: .

[0019]

(Equation 3) Where Wm = weight of powder charge in mold assembly outside pressurized vessel; Wmf = weight of powder charge in fluid medium inside pressurized vessel; ρf = density of fluid medium; ρp = pressurized vessel Density of the compacted powder charge inside. From the difference between the weight of the mold assembly outside the pressurized container and the product of the known volume of the mold assembly times the fluid density, the weight of the mold assembly without the powder charge was determined, and then the powder charge was determined. The weight of the powder charge in the pressurized container (Wmf) is calculated from the difference between the weight of the mold assembly in the fluid medium containing, and the weight of the mold assembly in the fluid medium without the powder charge.

[5] Pressurized container while monitoring molded object density
The pressure at 11 is increased at a controlled speed. The pressure is increased manually or continuously by an automatic command from controller 24.

6. When the density of the molded product measured by the computer reaches a predetermined target density, the pressure in the pressure vessel 11 is reduced. The predetermined target density is selected so that billet bounce occurs upon decompression.

7. When the predetermined density is reached, the pressure in the pressurized container 11 is reduced at a control speed. The pump 12 is configured such that the discharge flow rate always returns to the suction side through the control valve 18. Therefore, the control loop for pressurization and decompression can be easily adjusted.

8. The molding is removed from the press and removed from the mold assembly.

The real-time calculation of the in-situ compact density is:
It is based on Archimedes' buoyancy principle, which makes the buoyancy of the fluid medium equal to the weight of the fluid to be rejected by objects immersed in the liquid. The compact density is equal to powder charge weight / powder charge volume. The weight of the powder charge outside the mold assembly is easily calculated. The density of an isostatic fluid is determined by the composition of the fluid,
Because it is known based on temperature and pressure, the in situ powder volume is simply weight related, and the in situ density can be calculated directly from the weight and volume data. The above-mentioned algorithm of the density of the molded article is calculated from the above-mentioned analytical relationship, and can be expressed in various forms.

The real-time calculation of the in-situ molded article density can be performed by other methods. The method includes an initial calculation of the powder density at or before the application of the initial scheduled press pressure. Once this initial powder density is calculated, each pound of incompressible fluid pumped into the press results in a one pound weight gain as a powder densifier. Thus, the controller can utilize the initial density calculation and calculate the amount of fluid (pounds) added to the pressurized vessel during the press cycle. The added fluid can be measured by a mass flow meter 16 or a level drop in a fluid feed tank. The controller 24 then determines the initial state , ie, the weight of the powder charge at or before the application of the predetermined press pressure , ie the fluid medium at a certain reference pressure.
The weight of the powder charge in (a) and the first measurement was made
From time to time , "Wmf" is recalculated by simply adding the weight (b) of the fluid added into the pressurized vessel to calculate the molded article density. Once "Wmf" is calculated, the final compact density is calculated by the algorithm described above. Each of the above
When recalculating Wmf by adding the weights (a) and (b), the algorithm is as follows.

[0026]

(Equation 4) Here, Wmf in the initial state of the atmospheric pressure or the predetermined pressure equal to or higher than the predetermined atmospheric pressure does not include the weight of the mold assembly in the fluid medium including the powder charge and the powder charge as described above. Calculated from the weight difference of the mold assembly in the fluid medium. Here, the weight of the mold assembly without the powder charge is determined by the difference between the weight of the mold assembly outside the pressurized vessel and the product of the known volume of the mold assembly times the fluid density. Wmf (a + b) in the final state is equal to Wmf in the initial state plus the weight of the fluid added into the pressurized container.

FIGS. 4 and 5 show normal density and pressure relationships with respect to time, respectively. This density versus time non-linear graph results from indirect density control using pressure as a control variable. In contrast, in the method of the present invention, the density itself is the control variable, which varies linearly with time, resulting in a non-linear pressure versus time graph as shown in FIGS. In another embodiment of the invention, the density is measured and controlled in situ by adjusting the fluid applied to the pressurized container. This method can be implemented in combination with the main embodiment, or can be used only to check the results obtained by implementing the main embodiment. Also, the direct density control method is
The density versus time relationship can be followed to any desired pattern, including curves, where the density is kept relatively constant, for example, until it approaches the final density and / or for a period of time during the depressurization period.

Example of In Situ Density Control In this example, a molding powder was prepared from a particulate carbonaceous filler mixed with a coal tar pitch binder.

The mold used was a cylindrical rubber molded bag having a weight of 7.392 pounds and a density of 57.811 bs / ft ³ and a top lid. Weight 20.40 pounds, volume 0.
The bag was placed in a 1176 ft ³ stainless steel cylindrical holder. The vacuum applied to the exterior of the holder caused the bag to be pulled into tight contact with the holder as the powder was loaded into the bag. During filling of the bag, the powder was degassed by rocking the assembly vertically 71 turns. Next, a rubber lid containing a stainless steel valve with a flange was sealed to the bag with a hose clamp. The weight of the valve is 0.95461B, the volume was 0.00225ft ^3.

The thus-installed and sealed mold assembly was evacuated by a vacuum pump. The mold assembly was then warmed to the desired molding temperature and then placed in the press.

The computer control system of the press asks the operator for the weight of the forming bag, the identification letter of the stainless steel holder, in this case "E", and the final weight of the loaded mold assembly. Therefore, the computer calculates these three constants used for controlling the press cycle.

Constant # 1 = (total weight−metal frame weight−
Bag weight) /62.428 This is the molding powder weight lbs
等 し い 62.428; ie, in this case = 1.72
4.

Constant # 2 = −1 × (metal frame volume + rubber bag volume). This is equal to the negative number of the volume ft ³ other than the molding powder. In this embodiment, = -0.2311
7. Constant # 3 = total weight lb of mold assembly; in this example = 135.4. During the press cycle, the computer calculates the density gram / cc of the molded powder billet as follows.

The computer measures the total immersion weight including the underwater load cell. The computer calculates the pressurized water density lbs / ft ³ by measuring the press temperature and pressure using the following known relationship. Water density lbs / ft ³ = 62.8356-1.3688
× 10 ^-2 (T ° C) -1.9029 × 10 ^-4 (T ° C) ² +
2.6281 × 10 ⁻⁷ (T ° C.) ³ × Exp [3.447
X ^10-6 x pressure (psig)] The computer control program is divided into 7 segments. After loading the assembly into the press and extracting air from the system, the computer program is started. Segment # 1 During the first 90 seconds of the program, the assembly is supported by hydraulic cylinders rather than load cells. At this point, the computer stores the load cell reading with no load as the "tare" weight. The weight is then lowered until it is supported by the load cell.

When the pump is started, the computer controls the press pressure to 5 psig for 5 minutes. After 5 minutes, the computer checks whether ± 2 psi stability control from the set point has been implemented. If stable control has been obtained, the computer stores the current density reading as the initial billet density and advances the program to segment # 2. Segment # 2 During segment # 2, the billet density is calculated by three different methods. This is a feature of the pilot plant that aims to compare the accuracy of each method. Only one method is needed to perform accurate density control.

Since load cell zero shift is usually linear and reproducible, method # 1 uses a stored factor based on zero shift calibration to correct the load cell reading. In this method, in this embodiment, the corrected load cell reading is the raw load cell reading-the tare weight stored in segment # 1-
(0.001 x system pressure, psig).

In method # 2, no calibration is stored in the computer. Instead, the zero shift is determined directly by subtracting the weight from the load cell when the system is under pressure in segment # 3. This is implemented by the computer taking advantage of the underwater hydraulic connection during segment # 3. Segment # 3 is triggered when the billet density approaches its target value. A corrected load cell reading according to this method is obtained as follows. • From segment # 1 to segment # 3: corrected reading = raw display-tare weight of segment # 1 • From segment # 4 to segment # 7: corrected reading = raw display-segment # 3 tare weight In method # 3, Only the load cell reading is used to obtain the initial density in segment # 1. The billet density is then calculated by measuring the amount of water pumped into the press. This corresponds to the load cell reading. This is because each pound of water pumped (excluding any water leakage) increases the dip weight of the billet by one pound.

The first billet density calculated by method # 1 is selected near (directly below) the desired final target density.
Segment # 3 is triggered when the target density level is reached. Segment # 3 This program segment lasts 2 minutes. While the program is in segment # 3, it locks in the calculated density value at the end of segment # 2. The pressure set point is also kept constant. The computer makes use of the hydraulic connection that removes the tare weight from the load cell. The computer holds off the weight for 90 minutes while the tare weight is updated for the density calculation according to method # 2. The tare weight of method # 1 is left unchanged. When the new tare weight is locked in, the weight returns to its position on the load cell.

At the end of segment # 3, segment # 4 begins and the density is again calculated by all three methods. Segment # 4 During this segment, the pressure is increased at a rate of 10 psi / min. The billet density is calculated continuously and when the billet density reaches the final target value, the computer proceeds to segment # 5. The final target value density is selected so that the desired value is obtained after "bounce". "Bounce" is a small drop in density that occurs when pressure is released. Segment # 5 Pressure is reduced to 35 psi at a rate of 100 psi / min. Segment # 6 Pressure is held at 35 psi for 3 minutes. In this way, a stable low pressure reading of the final billet can be obtained with sufficient pressure to bring the rubber molded bag into close contact with the billet. Segment # 7 In this segment, all pressure is released and the computer prints the result of the pressurization cycle. Billet inspection The billet is removed from the press, removed from its frame and inspected. The billet is then weighed in water and out of the water to check the billet density.

[Brief description of the drawings]

FIG. 1: Cold isostatic pressing process “CIP”
5 is a graph showing a typical density-pressure relationship of FIG.

FIG. 2 is a block diagram of the isostatic molding system of the present invention.

FIG. 3 is a time-density characteristic curve of a manufacturing process in an example described in this specification.

FIG. 4 is a graph showing a typical density versus time relationship when pressure is varied non-linearly to control green density in prior art isostatic molding.

5 is a pressure vs. time graph corresponding to FIG. 4;

FIG. 6 is a graph of density versus time for isostatic molding according to the present invention.

FIG. 7 is a graph of the pressure versus time corresponding to FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Isostatic molding system 11 Pressurized container 12 Pump 14 Liquid tank 15 Liquid 16 Flow meter 17 Control loop 18 Control valve 20 Weighing system 22 Mold assembly 24 Computer controller 26 Modem

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) B22F 3/02, 3/04 B28B 3/00 102 B30B 11/00

Claims (11)

(57) [Claims] 1. An isostatic method for molding metal, carbon or ceramic particles into a dense and compact form of a selected density, comprising: (a) charging a metal, carbon or ceramic powder charge to a known weight and volume; (B) weighing the mold assembly loaded with the powder charge; and (c) weighing the powder charge weighed in step (b) above. Is the difference between the weight of the mold assembly charged with and the known weight of the mold assembly?
Et, and calculating the weight of the powder charge in the mold assembly (Wm), the steps of charging (d) is type assembly and powder charge into an isostatic pressure vessel, (e ) Filling the pressurized container with the fluid medium; (f) applying pressure to the pressurized container; and (g) during the pressurization, the sum of the mold assembly and the powder charge in the pressurized container. Weighing; and (h) determining the powder charge from the difference between the known weight of the mold assembly outside the pressurized vessel and the product of the known volume of the mold assembly times the fluid medium density (ρf). the weight of the mold assembly in the fluid medium which does not contain calculated, then the weight of the mold assembly in a fluid medium comprising the (g) powder charge weighed stage, does not include a powder charge Calculating the powder charge weight (Wmf) in the pressurized vessel from the difference from the weight of the mold assembly in the fluid medium; and (i) compressing in the pressurized vessel. Density of the powder charge (ρ
calculating p) by the following algorithm: Where Wm = weight of powder charge in mold assembly outside pressurized vessel; Wmf = weight of powder charge in fluid medium inside pressurized vessel; ρf = density of fluid medium; ρp = pressurized vessel the density of the compressed powder charge in and (j) compressed powder charged calculated in step (i)
Isostatic molding method comprising the steps of decompressing the pressurized container when the density is equal to the target density of the preselected molding of the object. 2. The isostatic method according to claim 1, wherein said mold assembly includes a resilient mold for the powder charge and support means. 3. The isostatic method according to claim 2, wherein the pressurized container is pressurized at a limited speed. 4. The isostatic method of claim 2, wherein the pressurized vessel is pressurized to create a controlled compact density versus time relationship. 5. The isostatic method according to claim 3, wherein the mold assembly in the pressurized container is intermittently weighed during pressurization. 6. The isostatic method according to claim 5, wherein the pressure in the pressurized container is reduced at a controlled rate. 7. The isostatic method according to claim 1, wherein the charged mold assembly is heated to a predetermined temperature before being placed in the pressure vessel. 8. The method according to claim 8, further comprising the step of pressurizing the pressurized container to a first specified pressure, further comprising calculating the density of the powder charge in said pressurized container at said first specified pressure using said step (i). Measuring the amount of fluid pumped into the pressurized container; and measuring the amount of fluid pumped into the pressurized container after reaching the first specific pressure. Calculating the final density of the compact charge. 9. The isostatic method according to claim 8, wherein said first specific pressure is atmospheric pressure. 10. The isostatic method according to claim 8, wherein said first specific level is equal to or higher than the atmospheric pressure. 11. The final density is first determined at an initial condition corresponding to the first specified pressure, and then at a final condition based on the weight of fluid added into the pressurized vessel to calculate the final density. The isostatic method of claim 10, wherein the final density is calculated according to step (i) using the calculated Wmf.