JPH0569578B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an apparatus for molding various ceramic powders, uranium dioxide powder, mixed oxide powder of plutonium and uranium, etc. into pellets.

[Conventional technology]

Taking the example of green pellets, in order to produce nuclear fuel pellets with a uniform and consistent sintered density, the density of the green pellets obtained by molding nuclear fuel powder must be uniform. To briefly explain a conventional molding device as shown in FIG. 9, the through hole 11 of the floating die 3 is inserted into the fixed lower punch 4, and the through hole 1
1, an upper punch 2 held by the upper ram 1 to which a hydraulic damper 5 is attached is inserted into the through hole 11.
The powder filled in the through hole 11 of the floating die 3 above the lower punch 4 is pressed by the upper punch 2 and the lower punch 4 to form green pellets. The powder filling amount is adjusted by moving the floating die 3 up and down by rotating the adjustment motor 9 in the forward or reverse direction. Compare the value measured by the detector 6, the height setting device 7, and the value set in the height setting device 7, and if it is out of a certain range, a forward/reverse operation signal is sent. and a comparator 8 which feeds back the information to the motor 9. When the movement of this molding device is summarized using the cam diagram shown in FIG. 10, and are the trajectories of the lower surface of the upper punch 2, the upper surface of the floating die 3, and the upper surface of the lower punch 4, respectively. In the powder filling process, the depth of the inner part of the through hole 11 filled with the powder to be molded is indicated by the filling amount a, which is the difference between In the first stage compression process, the upper punch 2 descends by an amount b of upper pressure to apply upper pressure to the stationary floating die 3, and in the second stage compression process, the upper punch 2 and the floating die 3 The die 3 descends to the bottom dead center of the press while maintaining a constant relative positional relationship, and then comes to rest. This is the same as pushing the lower punch 4 by the amount of downward pressure c, so it means that downward pressure is applied. In the extrusion process, the floating die 3 is lowered by the extrusion amount e until the upper surface of the floating die 3 matches the upper surface of the lower punch 4, while the upper punch 2 escapes upward. The height h of the green pellet compressed by the amount of descent of the upper ram 1 is detected by the height detector 6, and compared with the set value by the comparator 8, the height h of the green pellet is detected by the motor 9.
is rotated forward or backward to move the floating die 3 in the vertical direction to adjust the filling amount a. At that time, if the change in the pellet height h is within the permissible range Y of the hydraulic damper 5, when excessive pressure is applied to the upper punch 2, the hydraulic damper 5 will move as shown in the hydraulic damper operating area shown by the hatched area. This allows the pressure to be released and molding to be performed at a constant molding pressure. However, the above-described molding apparatus is incapable of dealing with changes in the density of the green pellets, and the operator has no choice but to manually adjust the molding pressure based on the results of measuring the density of the green pellets.

[Problems to be Solved by the Invention] Generally, the particle size distribution of the powder filled into a die changes due to drifting and deflection, so even if pressed at a constant molding pressure, the density of the obtained green pellets will change. . In the past, workers manually adjusted the molding pressure based on the density measurement results of the green pellets as described above, but this work was rather troublesome, and the hydraulic damper method In addition to not being able to change the pressure easily, there was a time delay in releasing the pressure, which resulted in pressure exceeding the specified value being applied to the green pellets. Furthermore, in conventional molding equipment, when the floating die 3 is slightly raised to increase the powder filling amount a because the green pellet density has decreased or to increase the height of the green pellets, the lower pressurization amount is increased. As c increases, the downward pressure by the lower punch 4 increases, and the density at the bottom of the green pellet increases locally, so if it is sintered in this state, the shrinkage rate at the bottom is low and the pellet is distorted in the shape of a truncated cone. There was a drawback that. The purpose of the present invention is to obtain a density balance between the upper and lower parts of the molded green pellets even when the amount of powder packed is changed, and at the same time to maintain the height of the green pellets almost constant. The object of the present invention is to provide a molding device that can also ensure uniformity.

[Means to solve the problem]

In the molding device of the present invention, a fixed lower punch is slidably inserted in the lower part of the through hole of the floating die which can be moved up and down by an adjustment motor, and a lower punch held by the upper ram is inserted in the upper part of the through hole. The pellet molding device is the same as the conventional pellet molding device in that it is equipped with a press that allows the upper punch to be pushed in and the powder filled in the through hole to be compressed using the upper punch and the lower punch, but the press bottom dead center A hydraulic unit is provided to apply a constant upward pressing force F _U to the upper punch. The control device for the hydraulic unit is comprised of a pellet density measuring device, a density setting device, and a comparator that feeds back a hydraulic control signal to the hydraulic unit based on the comparison result between the measured value and the set value. The control device for the motor for adjusting the floating die includes a load cell installed below the lower punch, and a lower pressing force detected by the load cell just before the upper punch starts applying constant pressure at the bottom dead center of the press. The F _L /F _U measuring device measures the ratio of F _L and the above upper pressing force F _U , and the F _L /F _U setting device sets the motor control signal above based on the comparison result between the measured value and the set value. It is configured with a comparator that provides feedback to the adjustment motor. The values of the upper limit H and lower limit L input to the density setter and F _L /F _U setter described above are as follows: H=A+√ L=A−√. However, A is the target value, and V is the unbiased variance of the pellet density or the average value of F _L /F _U. When the measured value is within the range between the upper limit H and the lower limit L, it is better not to perform feedback. Further, it is preferable that the control device for the floating die adjustment motor constantly monitors the upper limit of the amount of downward pressurization, and automatically stops the molding device when the upper limit is reached.

[Effect]

For example, if the properties of the powder change during molding and the pellet height h is low (high) because it is molded with a low (high) density, the lower pressure F _L will decrease (increase) from the normal value, and the upper pressure Since F _U is constant,
The F _L /F _U value decreases (increases). In this way F _L /F _U
Since the value is correlated with the density of the upper and lower parts of the pellet and the pellet height, the comparison result between the measured value and the set value regarding the F _L /F _U value is fed back to the motor for lifting and lowering the floating die. Raising and lowering the pellets equalizes the density of the upper and lower parts of the pellets, and at the same time equalizes the pellet height. Further, since the hydraulic unit generating pressure control device changes the molding pressure according to the pellet density, the pellet density is made uniform.

【Example】

In the embodiment of the present invention shown in FIG.
The same parts as in the figure are given the same reference numerals. The non-simultaneous three-stage compression type with-draw mechanical press used has a floating die 3 that can be moved up and down by an adjustment motor 9, with a through hole 11 located below it.
The fixed lower punch 4 is slidably inserted, and the upper punch 2 held by the upper ram 1 can be pushed down into the through hole 11, so that the upper punch 2 and the lower punch 4 can press the through hole 11. Not only is it possible to compress the powder filled therein, but also a hydraulic unit 12 is provided to apply a constant upward pressure F _U to the upper punch 2 at the bottom dead center of the press. The movement of this press is shown in Figure 2, and the cam diagram is shown in Figure 3.
As shown in the figure, and compared with Fig. 9, there are no changes in the powder filling process, first-stage compression process, second-stage compression process, and extrusion process, but following the second-stage compression process, , floating die 3
At the bottom dead center of the press, where the upper punch 2 is at rest, the upper punch 2 is pushed by a constant pressure (P _O Kg/cm ² ) generated by the hydraulic unit 12 and lowers by a constant pressurization amount d, and continues to be pressed for a constant pressurization time t. There is a third compression stage. The operating range of the upper punch 2 that guarantees a constant pressurization amount d is 4~
The pressure is about 6 mm, and the constant pressurizing time t is about 0.5 to 5 seconds. In the figure, X is a constant pressure operation range that can be changed as appropriate in the range of about 2 to 6 tons/cm ² during the molding cycle, which usually takes about 4 to 6 seconds, by the hydraulic unit pressure control device 13, which will be described later, and the shaded area is constant. This is the pressurized operation area. Pressure control device 13 of the above-mentioned hydraulic unit 11
As shown in FIG. 1, there is a density measuring device 14 for measuring the density of the molded pellets, a pellet density setting device 15, and the output of the density measuring device 14 and the value set in the density setting device 15. and a comparator 16 that compares the data at each predetermined sampling point and feeds back a hydraulic control signal to the hydraulic unit 12 according to the result. The initial value of the pellet density input to the density setting device 15 is determined by setting the unbiased variance of the average value of the pellet density to V.
(This unbiased variance is due to the measurement result over a short period of time when it is assumed that there is no change in the average value over time.) When the target value is A, the upper limit H and lower limit L are calculated from the following formula. shall be. H=A+√ L=A−√ The fifth diagram shows the relationship between pellet density and molding pressure.
In the example shown in the figure, one sampling point is taken every 50 punches. To explain with the flowchart of the hydraulic control shown in FIG. 6, it is determined in step 101 whether or not it is a sampling point, and when the sampling point is reached, one of the densities of the green pellets measured by the density measuring device 14 is detected. Alternatively, a plurality of average values are calculated in step 102, and the results are sent to the comparator 16 and compared with the initial value range (H, L) of pellet density input to the density setting device 15 in steps 103 and 104. If so, in steps 105 and 106, a control signal for increasing or decreasing the molding pressure by one step is fed back to the hydraulic unit 12, thereby adjusting the pressure P _O Kg/cm ² generated by the hydraulic unit 12. In the example in Figure 5, 2
Since the pressure exceeds the set value H at the second sampling point, the molding pressure is decreased by one step from the initial value. In addition, in the example shown in Figure 5, the setting range (H,
L) is within. In such a case, it is better not to perform feedback since it is essentially a control-unnecessary range in order to improve the stability of the control system. Note that FIG. 5 also shows the relationship between the lower pressurizing force/upper pressurizing force and the lower pressurizing amount, but in this case, one sampling point is taken every five punches. The pellet density setting value inputted to the density setting device 15 is not limited to the upper limit H and lower limit L mentioned above. In short, H, H ₁ , L, ...L,
L _{1 .} . . may be further subdivided to perform multi-stage feedback control. To illustrate, the setting values when divided into three groups (H, L) (H ₁ , L)
(H ₂ , L ₂ ) is A±√ A±2√ A±3√ Its step amounts (1), (2), and (3) are 0.81√ 2.09√ 3.21√. FIG. 7 is a subdivided flowchart in which steps 107 to 110 are increased. Returning again to FIG. 1, the control device 17 for the floating die adjustment motor 9 will now be described. This control device 17 has an F _L /F _U measuring device 19
Then, the F _L /F _U setting device 20 compares the output of the above F _L /F _U measuring device and the value set in the F _L /F _U setting device at each predetermined sampling point, and depending on the result, and a comparator 21 that feeds back a motor control signal to the adjustment motor 9. Here, the lower pressing force F _L refers to the fourth
In the figure, it refers to the output sampled just before the second-stage compression process reaches the bottom dead center of the press and the third-stage compression process starts when the upper punch 2 starts applying constant pressure. Further, the upper pressurizing force F _U refers to the constant pressurizing force applied to the upper punch 2, that is, the hydraulic unit 1
The product of the pressure P _O Kg/cm ² generated at 1 and the head area S _O cm ² (constant) of the upper punch 2 is defined as P _O ×S _O (Kg). The upper limit H and lower limit L input to the F _L /F _U setting device 20 are determined by the above formula, where V is the unbiased variance of the average value of F _L /F _U , and the target value A is the diameter of the pellet after sintering. The F _L /F _U value is set so that the vertical difference is close to zero and the upper pressing force F _U and the lower pressing force F _L are balanced. P _O ×S _O and F _L /F _U are calculated by a microcomputer within the device. To explain with the flowchart of lower pressurization amount control shown in Fig. 8, F _L /F _U
One or more average values measured by the measuring device 19 are calculated in step 202, and the results are sent to the comparator 21 to set the initial value range (H, L) input to the F _L /F _U setting device 20. and steps 203 and 204
If it is outside the range, a control signal for increasing or decreasing the lower pressure amount by one step is fed back to the motor 9 in steps 205 and 206, and the floating die 3 is raised and lowered to change the filling amount. Adjust the amount of lower pressure. In this example, the step
207, the upper limit of the lower pressure amount c is constantly monitored, and if the lower pressure amount c reaches the upper limit, it is determined that it means that there is no more powder to be supplied to the molding device, and the molding device is is automatically stopped and terminated. The feedback of the molding pressure and lower pressure (filling amount) described above operate independently without interfering with each other, and if the operating point deviates from the optimum value, they work together to return it to its original value. . All of the above uses a single die set, but in the case of a molding machine having a plurality of die sets, feedback control of the density vertical balance may be independently provided for each die set. This suppresses variations in filling amount between die sets.

【Effect of the invention】

The present invention feeds back the comparison results between the measured value and set value of F _L /F _U to the motor for raising and lowering the floating die to raise and lower the floating die to increase or decrease the filling amount. Even if the properties of the powder change, it not only contributes to suppressing the density deviation between the upper and lower parts of the green pellets, in other words, it contributes to reducing the diameter distortion of the sintered pellets, but also contributes to keeping the height of the pellets at a predetermined level. It is extremely superior in that it can approach the value at the same time. Furthermore, in the present invention, the comparison result between the measured green pellet density value and the set value is fed back to the hydraulic unit to adjust the molding pressure, so green pellets with uniform density can be obtained. . Both of these effects can improve the yield of products.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the pellet molding device according to the present invention, Fig. 2 is an explanatory diagram showing the molding process in order, Fig. 3 is its cam diagram, and Fig. 4 is a diagram showing the output of the load cell. , Figure 5 is a diagram showing the relationship between the density of green pellets and molding pressure during the molding process, and the relationship between lower pressurizing force/upper pressurizing force and lower pressurizing amount,
Figure 6 is a flowchart for pellet density control, Figure 7 is a flowchart when the control in Figure 6 is performed in multiple stages, Figure 8 is a flowchart for equalizing the density of the upper and lower parts of the pellet, and Figure 9 is a flowchart for equalizing the density of the upper and lower parts of the pellet. The figure is an explanatory diagram of a conventional pellet molding device, and FIG. 10 is a cam diagram thereof. 1... Upper ram, 2... Upper punch, 3... Floating die, 4... Lower punch, 9... Adjustment motor, 11... Through hole, 12... Hydraulic unit,
13... Hydraulic unit control device, 14... Density measuring device, 15... Density setting device, 16... Comparator, 17... Floating die adjustment motor control device, 18... Load cell, 19... Bottom Pressure force F _L /upper pressure F _U measuring device, 20...F _L /F _U setting device, 21... Comparator.

Claims (1)

[Claims] 1. A fixed lower punch is slidably inserted into the lower part of the through hole of the floating die which can be moved up and down by an adjustment motor, and a lower punch held by the upper ram is inserted above the through hole. In a pellet molding device equipped with a press in which the upper punch can be pushed in and the powder filled in the through hole can be compressed by the upper punch and the lower punch, a constant upper pressing force is applied to the upper punch at the bottom dead center of the press. A hydraulic unit is installed to provide F _U ,
The control device for the hydraulic unit is composed of a pellet density measuring device, a density setting device, and a comparator that feeds back a hydraulic control signal to the hydraulic unit based on the comparison result between the measured value and the set value. The control device for the motor for adjusting the floating die uses a load cell installed below the lower punch, and the lower pressurizing force F L and the lower pressurizing force F _L detected by the load cell just before the upper punch starts applying constant pressure at the bottom dead center of the press. The F _L /F _U measuring device that measures the ratio to the above upper pressing force F _U and the F _L /F _U setting device output the motor control signal to the above adjustment motor based on the comparison result between the measured value and the set value. A pellet molding device characterized by comprising a comparator that provides feedback to the pellet. 2 The upper limit H and lower limit L setting values input to the density setter and F _L /F _U setter are when A is the target value and V is the unbiased variance of the average value of pellet density or F _L /F _U The molding apparatus according to claim 1, obtained by the following formula. H=A+√ L=A−√3 The molding apparatus according to claim 1, wherein the feedback is not performed when the measured value is within the range between the upper limit H and the lower limit L. 4. The molding apparatus according to claim 1, wherein the control device for the floating die adjustment motor constantly monitors the upper limit of the amount of downward pressurization and automatically stops the molding apparatus when the upper limit is reached.