JP7790069B2 - Low-pressure casting method - Google Patents

Description

This invention relates to a low-pressure casting method in which the spool position of a servo valve is manipulated based on a preset pressure control pattern to adjust the pressure of pressurized gas in a sealed molten metal holding furnace, thereby injecting and filling molten metal into a mold cavity.

Low-pressure casting using molten metal such as aluminum alloy involves supplying pressurized gas into a sealed molten metal holding furnace, pressurizing the furnace and compressing the molten metal (this is called pressurization control), and then injecting the molten metal into the mold cavity via the feed pipe connecting the molten metal holding furnace to the mold cavity (this is called the injection process). Following the injection process, a pressure increase process is carried out to increase the filling density of the molten metal, and a pressure hold process is carried out to compensate for the solidification shrinkage of the molten metal. The molten metal then undergoes a cooling process before being removed from the mold cavity. At the same time, the pressure of the pressurized gas in the molten metal holding furnace is reduced, and the molten metal remaining in the feed pipe is returned to the molten metal holding furnace. The pressure increase and hold processes may also be carried out using a pressurization device installed in the mold. In this case, the pressure control is stopped after the pressurization device is activated, and the molten metal in the feed pipe is treated in parallel with the pressure increase or hold process.

Here, pressurization control adjusts the filling rate, volume, and density of molten metal in the mold cavity by manipulating the amount of pressurized gas supplied into the sealed molten metal holding furnace, its pressure, and pressurization time. These adjustments significantly affect casting quality. For example, if the pressurized gas supply rate, pressure, and time do not reach their target values, the molten metal filling volume will be too low, resulting in casting defects such as poor molten metal flow, incomplete filling, missing parts, blowholes, and poor transfer. Conversely, if the pressurized gas supply rate or pressure exceeds the target value, the molten metal filling volume will be too high, resulting in casting defects such as burrs, product deformation, and damage to insert materials such as sand cores. Furthermore, if the pressurized gas pressure rises too quickly, the flow of molten metal will be significantly disrupted, leading to casting defects such as gas entrapment, molten metal wrinkles, voids, burrs, and molten metal flashing from the mold mating surface (called the mold PL surface) around the mold cavity. If the rate at which the pressurized gas pressure rises is too slow, it can result in casting defects such as poor molten metal flow, rough casting surfaces, foreign matter being mixed in as the molten metal solidifies, and disturbances in the solidification structure.

For this reason, control devices equipped with proportional control valves and servo valves have been proposed that enable highly responsive and highly accurate pressurization control. Furthermore, control methods have been proposed that enable more accurate pressurization control by correcting the pressure difference between the actual pressurized gas pressure in a molten metal holding furnace and the target value. For example, in pressurization control that operates a proportional control valve based on a target pattern of a time-pressure curve, as shown in Patent Document 1, the pressure deviation between the pressure in the holding furnace and the target pressure of the target pattern is calculated, this pressure deviation is multiplied by a gain value (also called a control parameter value) to obtain a correction value, and the correction value is added to the target pattern to operate the proportional control valve.

Furthermore, as shown in Patent Document 2, a first gas supply circuit equipped with a large-capacity proportional flow control valve and a second gas supply circuit equipped with a small-capacity proportional pressure control valve are used to perform pressurization control based on a target pattern of a time-pressure curve. The rate of pressure rise is adjusted by operating the proportional flow control valve, and pressure is adjusted by operating the proportional pressure control valve. Furthermore, the pressure deviation between the pressure inside the holding furnace and the target pressure of the target pattern is calculated, this pressure deviation is multiplied by a gain value to obtain a correction value, and the correction value is added to the target pattern to operate the proportional pressure control valve.

Furthermore, as shown in Patent Document 3, in pressurization control performed based on a preset optimal pressurization pattern, a gain value is set according to the pressure gradient, a correction value is calculated from the set gain value, and the proportional control valve is operated with a command value obtained by adding the correction value to the steady-state value. Specifically, the gain is set to a larger value as the pressure gradient becomes steeper in the pressure rise section, and the gain value is gradually reduced before and after the inflection point where the pressure rise section ends. Furthermore, the command value is calculated by subtracting an overshoot correction value that is set to a larger value as the difference in pressure gradient at each inflection point increases.

Special Publication No. 4-26723 Special Publication No. 4-26724 Japanese Unexamined Patent Publication No. 8-90207

Here, the means shown in Patent Documents 1 and 2 use a fixed gain value to calculate the correction value. Generally, when the gain value is small, the movement of the control shaft of the proportional control valve becomes slow, resulting in a delayed pressure rise, a longer time to reach the set pressure, or the set pressure not being reached at all. As a result, the filling speed and amount of molten metal decrease, making casting defects more likely to occur. Furthermore, when the gain value is high, the movement of the control shaft of the proportional control valve becomes too sensitive, resulting in a phenomenon known as hunting, in which the control shaft vibrates violently. This results in turbulence in the flow of molten metal, resulting in casting defects. Furthermore, the lifespan of the proportional control valve is shortened, and the risk of failure such as control shaft breakage or seizure increases.

In contrast, the means shown in Patent Document 3 sets the gain value based on the pressure gradient or pressure difference, which is thought to eliminate the problems anticipated in Patent Documents 1 and 2. However, there is no means for confirming whether the set gain value is appropriate for the proportional control valve. As a result, it is quite possible that the movement of the control shaft of the proportional control valve will become sluggish or overly sensitive.

The present invention aims to provide a low-pressure casting method that enables safe operation of a pressure control device that supplies pressurized gas into a sealed molten metal holding furnace, and provides highly responsive and highly accurate pressure control, by optimizing control parameter values set based on the pressure difference.

The low-pressure casting method of the present invention comprises:
A low-pressure casting method in which the spool position of a servo valve is operated based on a preset pressurization control pattern to adjust the pressurized gas pressure in a sealed molten metal holding furnace and inject and fill the molten metal into a mold cavity, is characterized in that it includes a pressure measurement unit that measures the pressurized gas pressure in the molten metal holding furnace, calculates a pressure difference between the pressure measured by the pressure measurement unit and a target pressure of the pressurization control pattern, and when the pressure difference exceeds a preset allowable value, corrects a control parameter value of the pressurization control pattern based on the pressure difference, and operates the spool position of the servo valve using the corrected control parameter correction value.

In the casting method of the present invention,
The present invention is characterized in that it comprises a spool position measurement means for measuring a spool position of the servo valve, the measurement result of the spool position measurement means is displayed as a spool position waveform, the spool position waveform is compared with a reference waveform having a preset tolerance range, the control parameter correction value is adjusted based on the comparison result, and the adjusted control parameter adjustment value is used to operate the spool position of the servo valve.

The present invention provides a low-pressure casting method that optimizes control parameter values set based on the pressure difference, enabling safe operation of a pressure control device that supplies pressurized gas into a sealed molten metal holding furnace, and highly responsive and highly accurate pressure control.

1 is a conceptual diagram showing a low-pressure casting apparatus according to an embodiment of the present invention. FIG. 2 is a conceptual diagram showing a pressure control device of the low-pressure casting apparatus shown in FIG. 1 . 3 is a diagram illustrating control parameter correction values of the servo motor shown in FIG. 2. FIG. 4 is a diagram illustrating optimization verification of the control parameter correction values set in FIG. 3. FIG. FIG. 1 is a flowchart showing a low-pressure casting method according to an embodiment of the present invention.

Preferred embodiments for carrying out the present invention will be described below with reference to the drawings. Note that the following embodiments do not limit the inventions according to the claims. Furthermore, not all combinations of features described in the embodiments are necessarily essential to the solutions of the inventions according to the claims. Furthermore, in the present embodiments, the scales and dimensions of each component may be exaggerated, and some components may be omitted.

[Low-pressure casting equipment]
First, a low-pressure casting apparatus according to an embodiment of the present invention will be described with reference to Fig. 1. The low-pressure casting apparatus 100 shown in Fig. 1 includes a casting mold 10, a molten metal holding furnace 20, and a pressure control device 30. The casting mold 10 is disposed above the molten metal holding furnace 20.

The casting mold 10 comprises a movable mold 12 supported by a movable platen 11 and a fixed mold 14 supported by a fixed platen 13. The fixed platen 13 and movable platen 11 are arranged vertically, and a clamping device (not shown) moves the movable platen 11 and movable mold 12 vertically toward and away from the fixed mold 14. The fixed mold 14 and movable mold 12 are closed together to form a mold cavity 15. Regarding the movements of the movable platen 11 and movable mold 12, their movement away from the fixed mold 14 is defined as a mold opening operation, and their movement toward the fixed mold 14 is defined as a mold closing operation. The state in which the fixed mold 14 and movable mold 12 come into contact is defined as a mold touch point. The operation of pressing the movable mold 12 against the fixed mold 14 from the mold touch point to apply a clamping force is defined as a pressure increase operation. The position where maximum clamping force is generated is defined as a pressure increase completion position. The operation from the pressure increase completion position to the mold touch point is defined as a pressure decrease operation. Note that no clamping force is generated at the mold touch point. The movable mold 12 also has a center pin 17 that opens and closes the gate 16. This center pin 17 also serves to pressurize the molten metal M that has been injected and filled into the mold cavity 15.

The molten metal holding furnace 20 comprises a molten metal furnace 22 for storing molten metal M, a sealed chamber 21 for housing the molten metal furnace 22, and a molten metal supply pipe 23. The molten metal supply pipe 23 is hollow, with one end immersed in the molten metal M in the molten metal furnace 22 and the other end connected to the gate 16 of the casting mold 10. By pressurizing the sealed chamber 21 of the molten metal holding furnace 20 with pressurized gas or the like, the molten metal M in the molten metal furnace 22 is compressed, flows through the molten metal supply pipe 23, and is injected and filled into the mold cavity 15 via the gate 16. The molten metal furnace 22 is equipped with a temperature control means (not shown) to maintain the molten metal M at a predetermined temperature. A melting furnace (not shown) is also installed adjacent to the molten metal furnace 20 to melt molding materials such as aluminum alloys and produce molten metal. Molten metal M is periodically replenished from the melting furnace to the molten metal furnace 22, and the surface height of the molten metal M in the molten metal furnace 22 is maintained at a constant level. In Figure 1, the sealed chamber 21 is pressurized, but this is not limited to this. For example, the molten metal furnace 22 may be sealed and the interior of the molten metal furnace 22 may be pressurized. Alternatively, a separate pressurized chamber connected to the molten metal furnace 22 may be provided, and the molten metal M may be injected and filled into the mold cavity 15 via the molten metal supply pipe 23 by adjusting the pressure in the pressurized chamber. In either case, the molten metal M is maintained in a sealed state.

The pressurization control device 30 includes a pressurized gas supply source 31, a pressurized gas adjustment unit 33 that adjusts the amount of pressurized gas supplied to the sealed chamber 21, a pressurization control unit 34 that operates the pressurized gas adjustment unit 33, and a pressure measurement unit 35 that measures the pressurized gas pressure within the sealed chamber 21. The pressurized gas supplied from the pressurized gas supply source 31 is adjusted to a predetermined pressure by the pressure adjustment unit 32 and sent to the pressurized gas adjustment unit 33. The pressurization control unit 34 operates the pressurized gas adjustment unit 33 to adjust the amount of pressurized gas supplied to the sealed chamber 21 so that the pressurized gas pressure within the sealed chamber 21 measured by the pressure measurement unit 35 matches a preset pressurization control pattern. As a result, the pressurized gas pressure within the sealed chamber 21 increases, pressing against the molten metal M in the molten metal furnace 22, causing the molten metal M to flow within the feed pipe 23 and be injected and filled into the mold cavity 15 through the gate 16. In other words, the injection filling amount of molten metal M in the mold cavity 15 is determined according to the increase in pressurized gas pressure in the sealed chamber 21.

Here, compressed air may be used as the pressurized gas, but to prevent oxidation of the molten metal M in the molten metal furnace 22, it is preferable to use an inert gas such as argon or nitrogen. Therefore, it is preferable to use a pressure cylinder storing an inert gas as the pressurized gas supply source 31. Alternatively, a nitrogen gas generator that separates and stores only nitrogen gas from air using a separation membrane, adsorption membrane, etc. may be used as the pressurized gas supply source 31.

The pressurized gas adjustment unit 33 will be described in detail using Figure 2. The pressurized gas adjustment unit 33 is a servo valve that adjusts the pressurized gas flow rate by controlling the rotation of the servo motor 333. It includes the servo motor 333, a valve body 331, and a ball screw mechanism 332. The valve body 331 also has three connection ports, one of which, connection port 338, is connected to the pressurized gas supply source 31 via the pressure adjustment unit 32. Connection port 337, located opposite connection port 338, is connected to the sealed chamber 21 of the molten metal storage furnace 20. A slidable spool 336 is incorporated within the valve body 331, and this spool 336 has an adjustment hole 336P. By manipulating the position of the spool 336, the connection port 338, the adjustment hole 336P, and the connection port 337 are connected, allowing pressurized gas to be supplied from the pressurized gas supply source 31 into the sealed chamber 21. The flow rate of pressurized gas to the sealed chamber 21 can be adjusted by changing the position of the spool 336. Furthermore, by manipulating the position of spool 336 so that connection port 337, adjustment hole 336P, and connection port 339 are in communication, the pressurized gas in sealed chamber 21 is discharged, lowering the pressurized gas pressure in sealed chamber 21. In other words, by manipulating the position of spool 336, the pressurized gas pressure in sealed chamber 21 can be adjusted as desired.

Here, the movement of the spool 336 is defined as forward movement in the direction in which the pressurized gas pressure in the sealed chamber 21 increases (+ direction), and as backward movement in the direction in which the pressurized gas pressure decreases (- direction). As shown in FIG. 2, the spool position measurement means that measures the position of the spool 336 as it moves forward or backward uses the detection signal of the encoder 334, which detects the rotation direction and rotation speed of the servo motor 333. The amount of movement of the spool 336 per one rotation of the servo motor 333 is determined in advance. Note that the spool measurement means is not limited to this. For example, the connecting rod 335 may be extended from the valve body 331, and the amount of movement of this connecting rod 335 may be directly measured using a position sensor or the like. Alternatively, the position of the spool 336 may be measured from outside the valve body 331 using a non-contact position sensor such as an ultrasonic sensor or electromagnetic sensor.

The position data of the spool 336 measured by the spool position measurement means (encoder 334) is sent to the pressurization control unit 34. In response to the detection signal from the encoder 334, the pressurization control unit 34 performs feedback control of the rotational movement of the servo motor 333 to ensure no error with the preset pressurization control pattern. The feedback-controlled rotational movement of the servo motor 333 is converted into linear movement by the ball screw mechanism 332, and the forward and backward movement of the spool 336 is adjusted via the connecting rod 335. The pressurized gas pressure within the sealed chamber 21 is measured by the pressure measurement unit 35 and sent to the pressurization control unit 34. The pressurization control unit 34 feedback-controls the forward and backward movement of the spool 336 to ensure no error occurs between the target pressure and the measured pressure for each time period of the pressurization control pattern. This allows the pressurized gas pressure within the sealed chamber 21 for each time period to accurately reproduce the pressurization control pattern, enabling highly responsive and highly accurate pressurization control. The rotational control of the servo motor 333 and the transmission accuracy of the ball screw mechanism 332 are well known to be highly responsive and highly accurate.

[Servo motor control parameter correction value]
Next, the control parameter values of the servo motor 333 shown in FIG. 2 will be described with reference to FIG. 3. FIG. 3(a) shows the waveform of an example of a pressurization control pattern for low-pressure casting. The horizontal axis represents the elapsed time from the start of filling the mold cavity 15 with the molten metal M to the completion of filling, and the vertical axis represents the pressurized gas pressure. The solid line represents the waveform of the target pressure MP of the pressurization control pattern, and the dashed line represents the waveform of the measured pressure KP of the pressurized gas pressure in the sealed chamber 21 measured by the pressure measurement unit 35. FIG. 3(b) shows the control parameter values for controlling the rotation of the servo motor 333 of the pressurized gas adjustment unit 33, which are set in the pressurization control unit 34. The horizontal axis represents the pressure difference between the target pressure MP and the measured pressure KP, and the vertical axis represents the control parameter correction value PH based on the pressure difference.

First, as shown in Figure 3(a), the pressure is gradually increased from the start of filling to point A. The molten metal M in the molten metal holding furnace 20 rises through the feed pipe 23 until its surface reaches the gate 16. The pressure is then gradually increased to point B, and the molten metal M is filled into the mold cavity 15 (injection process). At this time, the rate at which the pressurized gas pressure increases is precisely adjusted to prevent disruption of the flow of the molten metal M. The pressure is then rapidly increased to point C, creating a high-pressure state. A pressure-increasing process is performed to fill the unfilled portion of the mold cavity 15 with molten metal M and increase the density of the filled molten metal M. This is followed by a pressure-holding process and a cooling process, with pressure control ending at the start of the cooling process. Note that if the pressure-increasing and pressure-holding processes are performed using a pressure device (not shown), pressure control ends at that point.

For example, assume that the control parameter values for servo motor 333 are set for the injection process and pressure increase process using a pressure control pattern for the injection process, where the pressure increase rate is slow and the target pressure MP is low. In this case, during the injection process, the pressure difference P1 between the target pressure MP and the measured pressure KP is small, and highly accurate pressure control is performed. However, during the pressure increase process, where the pressure increase rate is fast and the target pressure MP is high, the control parameter values may not match, resulting in a large pressure difference P2. In other words, rotation control of servo motor 333 is heavily dependent on the control parameter values, and optimization of the control parameter values is required.

Therefore, as shown in Figure 3(b), the control parameter value is set based on the pressure difference between the target pressure MP and the measured pressure KP. For example, if the pressure difference is equal to or less than the lower limit PL, the pressure controller 34 sets the control parameter value for the servo motor 333 as the control parameter value for the servo motor 333. If the pressure difference exceeds the upper limit PH, the control parameter correction value PHH is used. Furthermore, if the pressure difference is between the lower limit PL and the upper limit PH, the control parameter correction value PH is selected and set according to the pressure difference. In other words, this invention is characterized by selecting and setting the control parameter correction value PH based on the pressure difference between the target pressure MP and the measured pressure KP, controlling the rotation of the servo motor 333 using the set control parameter correction value PH, and adjusting the position of the spool 336 to control pressure increase. This allows for optimization of the control parameter value, enabling highly responsive and highly accurate pressure increase control.

[Optimization verification of control parameter correction values]
Next, the compatibility of the control parameter correction value PH set based on the pressure difference shown in FIG. 3 with the servo motor 333 of the pressurized gas adjusting unit 33 is verified (referred to as optimization verification). This optimization verification will be described with reference to FIG. 4. FIG. 4 shows a waveform of a portion of the pressurization control pattern for low-pressure casting, with the horizontal axis representing elapsed time and the vertical axis representing the pressurized gas pressure and the position of the spool 336 of the servo motor 333. FIG. 4(a) shows the spool position waveform (referred to as measured waveform) before optimization verification, and FIG. 4(b) shows the spool position waveform (referred to as reference waveform) after optimization verification. Here, the position of the spool 336 is measured using a spool measurement means. Specifically, the spool position waveform is generated using a spool monitor value SM, which is obtained by multiplying the output signal of the encoder 334, which detects the rotation state of the servo motor 333, by a coefficient and converting it into position data. Furthermore, a waveform in which the measured pressure KP and the target pressure MP are completely identical is used.

First, as shown in Figure 4(a), even though the measured pressure KP and target pressure MP match, the spool monitor value SM is oscillating (hunting) violently up and down with an abnormal amplitude SS. This indicates that the control parameter correction value PH, which was set based on the pressure difference, and the characteristics of the servo motor 333 are not appropriate. If low-pressure casting were to continue in this state, the pressure control pattern could be accurately followed and stable casting quality could be achieved, but this is not a desirable state, as there is a risk of damage to the servo motor 333, ball screw mechanism 332, spool 336, etc., which are vibrating violently.

In this invention, the spool position waveforms of the measured waveform and the reference waveform are compared, and the control parameter correction value PH is adjusted based on the results of this comparison. The adjusted control parameter adjustment value is then used to control the rotation of the servo motor 333. This process is repeated, and ultimately, the control parameter correction value PH is optimized so that the vibration of the spool monitor value SM falls within the appropriate amplitude KS range. This ensures the tracking ability of the pressure control pattern, enabling stable production of low-pressure casting with absolutely no risk of damage to the servo motor 333, ball screw mechanism 332, spool 336, etc.

In Figure 4, the spool position waveform is compared and determined on the assumption that the measured pressure KP and the target pressure MP are completely consistent. However, this is not limited to this, and the method may also be applied to cases where the measured pressure KP and the target pressure MP differ, for example. It is also possible for the spool position waveforms of the measured waveform and the reference waveform to be similar, but the measured pressure KP and the target pressure MP differ. In this case, optimization processing may be performed by adjusting the control parameter correction value PH to correct the measured pressure KP, or the optimization processing may be adjusted depending on the specific case. The key is not to simply use the control parameter correction value PH set based on the pressure difference, but to verify its compatibility with the servo motor 333 and adjust the control parameter correction value PH as necessary.

In addition, in Figure 4(b), the range of elapsed time to the right of point D represents the process of filling the unfilled portion of the mold cavity 15 with molten metal M and increasing the density of the filled molten metal M. The fact that the spool monitor value SM fluctuates up and down within this range indicates that the flow of molten metal M to fill the unfilled portion and increase the density, as well as the supplementary filling flow associated with the solidification and shrinkage of molten metal M, are being precisely controlled.

[Low pressure casting method]
Next, a low-pressure casting method using the low-pressure casting apparatus 100 shown in Fig. 1 will be described with reference to Fig. 5. Fig. 5 explains an optimization process for adjusting the control parameter correction value PH based on a comparison of the amplitudes (SS, KS) of the spool monitor value SM of the measured waveform and the reference waveform. Other aspects of the optimization process will not be described here.

First, the most appropriate pressurization control pattern is selected based on information such as the type and fill amount of molten metal M, the shape of the mold cavity 15, the arrangement of the gates 16, the expected casting quality, and potential casting defects, and set in the pressurization control unit 34. Next, based on the selected pressurization control pattern, control parameter values for controlling the rotation of the servo motor 333 of the pressurization gas adjustment unit 33 are set in the pressurization control unit 34, thereby initiating pressurization control for low-pressure casting. Based on the selected pressurization control pattern, the pressurization control unit 34 operates the pressure adjustment unit 32 to adjust the pressure of the pressurized gas from the pressurized gas supply source 31 and the pressurization gas adjustment unit 33 to adjust the amount of pressurized gas supplied into the sealed chamber 21 of the molten metal holding furnace 20. The pressurized gas pressure within the sealed chamber 21 increases in accordance with the amount of pressurized gas supplied, compressing the molten metal M in the molten metal furnace 22. The compressed molten metal M rises within the melt supply pipe 23 and is injected and filled into the mold cavity 15 via the gate 16 (injection process).

At the same time as pressurization control begins, the position of the spool 336 and the pressure of the pressurized gas within the sealed chamber 21 are measured. The position of the spool 336 is measured using an encoder 334 that detects the rotational state of the servo motor 333, and the spool position waveform is displayed as the spool monitor value SM. In addition, the pressure of the pressurized gas is measured using the pressure measurement unit 35, and the pressure waveform is displayed superimposed on the spool position waveform. These two waveform displays are saved as measurement waveforms in the pressurization control unit 34.

Next, the pressure control unit 34 calculates the pressure difference between the measured and reference pressure waveforms and performs a comparison. If the pressure difference exceeds the range of a preset tolerance value KY, the control parameter value is corrected based on the pressure difference, and the corrected control parameter correction value PH is reset in the pressure control unit 34. The reset control parameter correction value PH is then used to perform low-pressure casting of the next shot, and the measured waveform is similarly compared with the reference waveform.

If the reference waveform and measured waveform were compared, the control parameter values were corrected, and the control parameter correction value PH were replaced simultaneously during the low-pressure casting process, the following problems would occur. For example, if the pressure difference is such that the measured waveform is greater than the reference waveform, the supply of pressurized gas would be stopped and the pressurized gas in the sealed chamber 21 would be vented to eliminate the pressure difference. This would temporarily halt the flow of molten metal M within the mold cavity 15, resulting in casting defects such as poor molten metal wrinkles and poor molten metal flow. Conversely, if the measured waveform is less than the reference waveform, the supply of pressurized gas would be suddenly increased to eliminate the pressure difference. This would disrupt the flow of molten metal M, resulting in casting defects such as air entrapment and burrs. For this reason, in this invention, simultaneous correction is not performed during the low-pressure casting process, and optimization processing is performed in the next low-pressure casting shot after correction.

Returning to the explanation. If the pressure difference is within the allowable value KY, the optimization process for the reset control parameter correction value PH proceeds. If the comparison result for the amplitude of the spool position waveform during low-pressure casting is that the measured waveform SS is greater than the reference waveform KS, it is determined that the compatibility between the reset control parameter correction value PH and the servo motor 333 is unsatisfactory, and based on the comparison result, the reset control parameter correction value PH is adjusted to determine the control parameter adjustment value PG, which is then reset in the pressurized gas adjustment unit 33. The next shot of low-pressure casting is then performed using the reset control parameter adjustment value PG, and the measured waveform is similarly compared with the reference waveform. Again, for the reasons described above, simultaneous adjustment and molding of the control parameter adjustment value PG during low-pressure casting is not performed.

If the comparison of the measured waveform with the reference waveform shows that the pressure difference is less than or equal to the allowable value KY and the amplitude SS is less than or equal to KS, the initially set control parameter value, or the reset control parameter correction value PH, or the third reset control parameter adjustment value PG is determined to be optimized and is set correctly as the control parameter value for the low-pressure casting molding.

[effect]
In this low-pressure casting method, the pressurized gas pressure in the molten metal holding furnace is adjusted by operating a servo valve based on a pressurization control pattern to inject and fill the molten metal into the mold cavity. The control parameter value for controlling the rotation of the servo valve is corrected based on the pressure difference between the reference waveform and the measured waveform, and low-pressure casting is performed using the corrected control parameter value. This enables highly responsive and highly accurate pressurization control. As a result, casting defects caused by insufficient pressurized gas pressure supply, such as poor molten metal flow, incomplete filling or defects, blowholes, poor transfer, casting burrs, product deformation, and damage to insert materials such as sand cores, can be reliably prevented. Furthermore, casting defects caused by the supply rate of pressurized gas pressure, such as gas entrapment, molten metal wrinkles, voids, casting burrs, molten metal ejection (flash) from the mold PL surface, poor molten metal flow, rough casting surface, foreign matter contamination due to solidification of the molten metal, and disturbed solidification structure, can be reliably prevented, thereby ensuring a stable supply of high-quality castings.

Furthermore, the system performs optimization processing on the control parameter correction values corrected based on the pressure difference. Specifically, it compares the servo valve spool positions of the reference waveform and the measured waveform, adjusts the control parameter correction values as necessary, and performs low-pressure casting using the adjusted control parameter adjustment values. This further improves the responsiveness and control accuracy of pressure control, and also enables proper operation of the servo motor. As a result, it enables safe operation and extended lifespan of low-pressure casting equipment, enabling a stable supply of high-quality castings over a long period of time.

The above describes a preferred embodiment of the present invention, but the technical scope of the present invention is not limited to the scope described in the above embodiment. Various modifications and improvements can be made to the above embodiment.

100 Low-pressure casting apparatus 10 Casting mold 11 Movable platen 12 Movable mold 13 Fixed platen 14 Fixed mold 15 Mold cavity 16 Gate 17 Center pin 20 Molten metal holding furnace 21 Sealed chamber 22 Molten metal furnace 23 Melt supply pipe M Molten metal 30 Pressurization control device 31 Pressurized gas supply source 32 Pressure adjustment unit 33 Pressurized gas adjustment unit 331 Valve body 332 Ball screw mechanism 333 Servo motor 334 Encoder 335 Connecting rod 336 Spool 336P Adjustment hole 337, 338, 339 Connection port 34 Pressurization control unit 35 Pressure measurement unit MP Target pressure KP Measured pressure P1, P2 Pressure difference A, B, C, D Points PL Lower limit value PH Upper limit value PH, PHL, PHH Control parameter correction value PG Control parameter adjustment value SM Spool monitor value SS, KS Amplitude KY Allowable value

Claims (1)

A low-pressure casting method in which a spool position of a servo valve is operated based on a preset pressurization control pattern to adjust the pressurized gas pressure in a sealed molten metal holding furnace, thereby injecting and filling molten metal into a mold cavity,
a pressure measuring unit that measures the pressure of the pressurized gas in the molten metal holding furnace;
a pressure difference between the pressure measured by the pressure measuring unit and the target pressure of the pressurization control pattern is calculated, and when the pressure difference exceeds a preset allowable value, a control parameter value of the pressurization control pattern is corrected based on the pressure difference, and a spool position of the servo valve is operated using the corrected control parameter correction value ;
a spool position measuring means for measuring a spool position of the servo valve;
a measurement result of the spool position measuring means being displayed as a spool position waveform, the spool position waveform being compared with a reference waveform having a preset tolerance range, and if the comparison result shows that the amplitude of the spool position waveform is greater than the amplitude of the reference waveform, the control parameter correction value is adjusted based on the comparison result, and the adjusted control parameter adjustment value is used to operate the spool position of the servo valve .