Low-pressure casting method

Abstract

To provide a low pressure casting method executing the safety operation and high response and high precision pressurization control of a pressurization control device feeding a pressurization gas into a sealed molten metal holding furnace by the optimization of a control parameter value set based on a pressure difference.SOLUTION: A low pressure casting method operates the spool position of a servo valve based on a previously set pressurization control pattern, and adjusts pressurization gas pressure in a sealed molten metal holding furnace so as to injection-charge a molten metal into a mold cavity, The low pressure casting method comprises: obtaining a control parameter correction value of the pressurization control pattern based on a pressure difference between the pressurization gas pressure and the target pressure of the pressurization control pattern; and further, adjusting the control parameter correction value based on a positional difference between the spool position of the servo valve and a target position of the pressurization control pattern.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to 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 pressure of pressurized gas in a sealed molten metal holding furnace, thereby injecting and filling molten metal into a mold cavity. [Background technology]

[0002] 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 (called pressure control), and then injecting the molten metal into the mold cavity via a feed pipe connecting the molten metal holding furnace to the mold cavity (called the injection process). Following the injection process, a pressure boosting process is performed to increase the filling density of the molten metal, and a pressure holding process is performed to compensate for 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 boosting and pressure holding processes may also be performed using a pressurizing device installed in the mold. In this case, the pressure control is stopped after the pressurizing device is activated, and the molten metal in the feed pipe is treated in parallel with the pressure boosting or pressure holding process.

[0003] Here, pressurization control adjusts the filling rate, volume, and density of the molten metal in the mold cavity by manipulating the amount of pressurized gas supplied to 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 flow, incomplete filling, missing parts, porosity, and poor transfer. Conversely, if the pressurized gas supply rate or pressure exceeds the target values, the molten metal filling volume will be too high, resulting in casting defects such as flash, product deformation, and damage to insert materials such as sand cores. Furthermore, if the pressurized gas pressure increases too quickly, the flow of the molten metal will be significantly disrupted, leading to casting defects such as gas entrapment, molten metal wrinkles, voids, flash, and flashing from the mold mating surface (called the PL surface) around the mold cavity. If the rate at which the pressurized gas pressure increases is too slow, casting defects such as poor molten metal flow, rough casting surface, inclusion of foreign matter due to solidification of the molten metal, and disturbance of the solidification structure will occur.

[0004] For this reason, for example, control devices equipped with proportional control valves and servo valves that enable high-response and high-precision pressurization control have been proposed. 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 a target value. For example, as shown in Patent Document 1, in pressurization control that operates a proportional control valve based on a target pattern of a time-pressure curve, 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.

[0005] 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, with the proportional flow control valve being operated to adjust the pressure rise rate and the proportional pressure control valve being operated to adjust the pressure.The patent also states that 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 to obtain a correction value, and the correction value is added to the target pattern to operate the proportional pressure control valve.

[0006] 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 a proportional control valve is operated with a command value obtained by adding the correction value to a 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 at the end of the pressure rise section. Also, 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 4-26723 [Patent Document 2] Special Publication No. 4-26724 [Patent Document 3] Japanese Patent Application Publication No. 8-90207 Summary of the Invention [Problem to be solved by the invention]

[0008] In the methods described in Patent Documents 1 and 2, the gain value used to calculate the correction value is a fixed value. Generally, when the gain value is small, the control shaft of the proportional control valve moves slowly, resulting in a delayed pressure rise, a longer time to reach the set pressure, or even failure to reach the set pressure. As a result, the molten metal filling speed and amount decrease, making casting defects more likely to occur. Furthermore, when the gain value is high, the control shaft of the proportional control valve moves too sensitively, 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 life of the proportional control valve is shortened, increasing the risk of failures such as breakage or seizure of the control shaft.

[0009] 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 checking 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 slow or overly sensitive.

[0010] Therefore, an object of the present invention is 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 performs highly responsive and highly accurate pressure control by optimizing control parameter values ​​that are set based on the pressure difference. [Means for solving the problem]

[0011] 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.

[0012] 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. [Effects of the Invention]

[0013] According to the present invention, by optimizing the control parameter values ​​set based on the pressure difference, it is possible 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 performs highly responsive and highly accurate pressure control. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a conceptual diagram showing a low-pressure casting apparatus according to an embodiment of the present invention. [Figure 2] 2 is a conceptual diagram showing a pressure control device of the low-pressure casting apparatus shown in FIG. [Figure 3] 3 is a diagram illustrating control parameter correction values ​​of the servo motor shown in FIG. 2. FIG. [Figure 4] 4 is a diagram illustrating optimization verification of the control parameter correction values ​​set in FIG. 3. FIG. [Figure 5]FIG. 1 is a flowchart showing a low-pressure casting method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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 of the 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.

[0016] [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.

[0017] The casting mold 10 includes 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 toward and away from the fixed mold 14 in the vertical direction. The fixed mold 14 and the 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 movement, and their movement toward the fixed mold 14 is defined as a mold closing movement. The state in which the fixed mold 14 and the movable mold 12 come into contact with each other is defined as a mold touch point. The movement from the mold touch point to press the movable mold 12 against the fixed mold 14 to apply a clamping force is defined as a pressure increase movement. The position at which the maximum clamping force is generated is defined as a pressure increase completion position. The movement from the pressure increase completion position to the mold touch point is defined as a pressure decrease movement. Note that no clamping force is generated at the mold touch point. The movable mold 12 is also provided with a center pin 17 that opens and closes the gate 16. The center pin 17 also serves to pressurize the molten metal M that has been injected and filled into the mold cavity 15.

[0018] The molten metal holding furnace 20 includes a molten metal furnace 22 that stores the molten metal M, a sealed chamber 21 that houses 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 pressed, 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) that maintains the molten metal M at a predetermined temperature. A melting furnace (not shown) that melts a molding material such as an aluminum alloy to produce molten metal is also provided adjacent to the molten metal furnace 20, and the molten metal M is periodically replenished from the melting furnace to the molten metal furnace 22, and the level of the molten metal M in the molten metal furnace 22 is maintained at a constant level. 1 shows a configuration in which the inside of the sealed chamber 21 is pressurized, but the present invention is not limited to this. For example, the molten metal furnace 22 may have a sealed structure and the inside of the molten metal furnace 22 may be pressurized. Also, a separate pressurizing 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 pressurizing chamber. In either case, the molten metal M is maintained in a sealed state.

[0019] 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 in the sealed chamber 21. The pressurized gas supplied from the pressurized gas supply source 31 is adjusted to a predetermined pressure by a 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 in the sealed chamber 21 measured by the pressure measurement unit 35 matches a preset pressurization control pattern. As a result, the pressurized gas pressure in the sealed chamber 21 increases, pressing against the molten metal M in the melt furnace 22, causing the molten metal M to flow through the melt supply pipe 23 and be injected and filled into the mold cavity 15 through the gate 16. That is, the injection filling amount of the molten metal M in the mold cavity 15 is determined according to the increase in the pressure of the pressurized gas in the sealed chamber 21.

[0020] Here, compressed air may be used as the pressurized gas, but it is preferable to use an inert gas such as argon or nitrogen in order to prevent oxidation of the molten metal M in the melt furnace 22. 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, an adsorption membrane, or the like may be used as the pressurized gas supply source 31.

[0021] The pressurized gas adjustment unit 33 will be described in detail with reference to FIG. 2. The pressurized gas adjustment unit 33 is a servo valve that adjusts the flow rate of pressurized gas by controlling the rotation of a servo motor 333. The unit 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 inside 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 to each other, and pressurized gas is 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 with each other, the pressurized gas in sealed chamber 21 is discharged, thereby 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.

[0022] Here, the movement of the spool 336 in the direction in which the pressurized gas pressure in the sealed chamber 21 increases (positive direction) is defined as forward movement, and the movement in the direction in which the pressurized gas pressure decreases (negative direction) is defined as backward movement. As shown in FIG. 2, a spool position measurement means for measuring the position of the spool 336 moving forward or backward is performed using a detection signal from an encoder 334 that 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, a connecting rod 335 may be protruded 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 an electromagnetic sensor.

[0023] Position data of the spool 336 measured by the spool position measuring means (encoder 334) is sent to the pressurization control unit 34. Upon receiving 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 respect to a 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 in the sealed chamber 21 is measured by the pressure measuring 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 in the sealed chamber 21 for each time period to accurately reproduce the pressurization control pattern, enabling highly responsive and highly accurate pressurization control. It is well known that the rotational control of the servo motor 333 and the transmission accuracy of the ball screw mechanism 332 are highly responsive and highly accurate.

[0024] [Servo motor control parameter correction value] Next, the control parameter values ​​of the servo motor 333 shown in FIG. 2 will be explained using 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.

[0025] First, as shown in FIG. 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 the surface of the molten metal M 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 so as not to disrupt 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 the pressure control ending at the start of the cooling process. Note that if the pressure-increasing process and the pressure-holding process are performed using a pressure device (not shown), the pressure control ends at that point.

[0026] Assume that the control parameter values ​​of the servo motor 333 are set for the injection process and the pressure increase process, for example, using a pressure control pattern for the injection process, in which the pressure increase rate is slow and the target pressure MP is low. In this case, the pressure difference P1 between the target pressure MP and the measured pressure KP is small during the injection process, and highly accurate pressure control is performed. However, during the pressure increase process, in which 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, the rotation control of the servo motor 333 is heavily dependent on the control parameter values, and optimization of the control parameter values ​​is required.

[0027] Therefore, as shown in FIG. 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, when the pressure difference is equal to or less than the lower limit PL, the pressure controller 34 sets the control parameter value of the servo motor 333 as the control parameter value of the servo motor 333. When the pressure difference exceeds the upper limit PH, the control parameter correction value PHH is adopted. When 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. That is, in the present invention, the control parameter correction value PH is selected and set based on the pressure difference between the target pressure MP and the measured pressure KP, and the rotation of the servo motor 333 is controlled using the set control parameter correction value PH, and the position of the spool 336 is adjusted to control pressure increase. This allows the control parameter value to be optimized, resulting in highly responsive and highly accurate pressure increase control.

[0028] [Optimization verification of control parameter correction values] Next, the compatibility of the control parameter correction value PH set based on the pressure difference (see 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 explained using 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 device. 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. The waveform used was obtained when the measured pressure KP and the target pressure MP were completely identical.

[0029] First, as shown in Figure 4(a), even though the measured pressure KP and the target pressure MP are the same, 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 continued in this state, the pressure control pattern could be accurately followed and stable casting quality could be achieved. However, this is not a desirable state, as it presents a risk of damage to the servo motor 333, ball screw mechanism 332, spool 336, and other components that are vibrating violently.

[0030] Therefore, in the present invention, the spool position waveforms of the measured waveform and the reference waveform are compared, and based on the results of the comparison, the control parameter correction value PH is adjusted, and the adjusted control parameter adjustment value is 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, and enables stable production of low-pressure casting with absolutely no risk of damage to the servo motor 333, ball screw mechanism 332, spool 336, etc.

[0031] In FIG. 4, the spool position waveform is compared and determined on the assumption that the measured pressure KP and the target pressure MP are perfectly consistent. However, this is not limited to this. For example, the present invention may be applied to cases where the measured pressure KP and the target pressure MP deviate from each other. Furthermore, cases where the spool position waveforms of the measured and reference waveforms are similar but the measured pressure KP and the target pressure MP deviate from each other are also possible. In such cases, 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 modified depending on the specific case. In short, rather than simply using the control parameter correction value PH set based on the pressure difference, it is important to verify its compatibility with the servo motor 333 and adjust the control parameter correction value PH as necessary.

[0032] 4(b), the range of elapsed time from point D to the right indicates 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, and the supplementary filling flow associated with the solidification and shrinkage of molten metal M, are being precisely controlled.

[0033] [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 between the measured waveform and the reference waveform. Other aspects of the optimization process will not be described here.

[0034] First, an optimal pressurization control pattern is selected based on information such as the type and 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 is set in the pressurization control unit 34. Next, based on the set 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, and pressurization control for low-pressure casting begins. Based on the set 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 operates 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, pressing against the molten metal M in the molten metal furnace 22. The pressed molten metal M rises through the molten metal supply pipe 23 and is injected and filled into the mold cavity 15 via the gate 16 (injection process).

[0035] Simultaneously with the start of pressurization control, the position of the spool 336 and the pressure of the pressurized gas inside the sealed chamber 21 are measured. The position of the spool 336 is measured using an encoder 334 that detects the rotation state of the servo motor 333, and a spool position waveform is displayed as a spool monitor value SM. In addition, the pressure of the pressurized gas is measured using a pressure measurement unit 35, and a pressure waveform is displayed superimposed on the spool position waveform. These two waveform displays are saved in the pressurization control unit 34 as measurement waveforms.

[0036] Next, the pressurization control unit 34 calculates the pressure difference between the measured waveform and the reference waveform, and performs a comparison and determination. If the pressure difference exceeds a preset allowable 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 pressurization control unit 34. Thereafter, the next shot of low-pressure casting is performed using the reset control parameter correction value PH, and the measured waveform is similarly compared and determined with the reference waveform.

[0037] If the comparison of the reference waveform with the measured waveform, the correction of the control parameter value, and the substitution of the control parameter correction value PH were performed 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 the molten metal M in 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 result in disruption of the flow of the 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.

[0038] Returning to the explanation, if the pressure difference is within the allowable range KY, the process proceeds to optimization of the reset control parameter correction value PH. If the comparison result of the spool position waveform for low-pressure casting shows 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. Based on the comparison result, the reset control parameter correction value PH is adjusted to obtain the control parameter adjustment value PG, which is then reset to the pressurized gas adjustment unit 33. Then, the next shot of low-pressure casting is performed using the reset control parameter adjustment value PG, and the measured waveform is compared with the reference waveform in the same manner. Again, for the reasons mentioned above, simultaneous adjustment and simultaneous molding of the low-pressure casting and the control parameter adjustment value PG are not performed.

[0039] If the result of comparing the measured waveform with the reference waveform is 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 thirdly 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.

[0040] [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 servo valve rotation 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 pressurized gas pressure supply rate, such as gas entrapment, molten metal wrinkles, voids, casting burrs, molten metal ejection (flash) from the PL surface of the mold, poor molten metal flow, rough casting surface, foreign material contamination due to solidification of the molten metal, and disturbed solidification structure, can be reliably prevented, ensuring a stable supply of high-quality castings.

[0041] Furthermore, the system optimizes the control parameter correction values ​​based on the pressure difference. Specifically, the system 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 values. This further improves the responsiveness and control accuracy of pressure control and ensures proper operation of the servo motor. As a result, the system enables safe operation and extended life of the low-pressure casting equipment, enabling a stable supply of high-quality castings over a long period of time.

[0042] Although the preferred embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the scope described in the above-described embodiments. Various modifications and improvements can be made to the above-described embodiments. [Explanation of symbols]

[0043] 100 Low-pressure casting equipment 10 Casting mold 11 Movable plate 12 Movable mold 13 Fixed plate 14 Fixed mold 15 mold cavity 16 Gates 17 Center pin 20 Molten metal holding furnace 21 Closed room 22 Melting furnace 23 Hot water pipe M molten metal 30 Pressure control device 31 Pressurized gas supply source 32 Pressure adjustment section 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 ports 34 Pressure control section 35 Pressure measurement section MP target pressure KP measurement pressure P1, P2 pressure difference A, B, C, D points PL lower limit pH upper limit PH, PHL, PHH control parameter correction value PG control parameter adjustment value SM Spool Monitor Values SS, KS amplitude KY tolerance

Claims

[Claim 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.

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