Low-pressure casting method and low-pressure casting device

Abstract

To provide a method of low-pressure casting that realizes stable charging of a molten metal into a mold cavity without affected by fluctuations of atmospheric pressure irrespectively of the size or working accuracy of a mold and a state of the mold, such as pressurization and vacuum decompression.SOLUTION: In a method of low-pressure casting for supplying a pressurized gas into a sealed molten metal holding furnace and injection-charging a molten metal into a mold cavity, an internal-pressure measuring unit for measuring pressure of a pressurized gas in the molten metal holding furnace and an external-pressure measuring unit for measuring atmospheric pressure are provided. Before starting injection-charging of the molten metal, the initial pressure adjustment of the pressurized gas in the molten metal holding furnace is performed so that measurement pressure becomes equal between the internal-pressure measuring unit and the external-pressure measuring unit. After completing the injection-charging of the molten metal, the pressure reduction adjustment of the pressurized gas in the molten metal holding furnace is performed so that measurement pressure becomes equal between the internal-pressure measuring unit and the external-pressure measuring unit.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a low-pressure casting method in which pressurized gas is supplied into a sealed molten metal holding furnace to inject and fill 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, and then injecting the molten metal into the mold cavity via a feed pipe connecting the molten metal holding furnace and the mold cavity (this process is called the injection filling process). The filling speed and amount of molten metal into the mold cavity are adjusted by controlling the pressure of the supplied pressurized gas and the pressurization time. After the injection filling process, a pressure increase process is carried out to increase the filling density of the molten metal by controlling the pressure of the pressurized gas, and a pressure dwell process is carried out to compensate for solidification shrinkage of the molten metal. After the pressure dwell process, the molten metal is cooled 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 in the feed pipe is returned to the molten metal holding furnace. The pressure increase and dwell processes may also be carried out using a pressurization device installed in the mold.

[0003] Unless special treatment such as vacuum decompression or gas pressurization is performed on the mold cavity, the inside of the mold cavity is atmospheric pressure. Therefore, the injection and filling of molten metal into the mold cavity by controlling the pressurization of pressurized gas is carried out based on the pressure difference between the pressure inside the sealed molten metal holding furnace and atmospheric pressure. In other words, the filling condition and amount of molten metal in the mold cavity are determined by this pressure difference. Furthermore, in general, in equipment used for low-pressure casting, the casting mold is placed above the molten metal holding furnace, and the molten metal flows from the lower molten metal holding furnace to the upper casting mold. Therefore, the filling condition and amount of molten metal in the mold cavity are expressed in terms of the distance from the surface of the molten metal in the molten metal holding furnace to the surface of the molten metal flowing inside the mold cavity (called the molten metal fill height).

[0004] Furthermore, atmospheric pressure varies by region and fluctuates daily. For example, if the average atmospheric pressure at point A is 101.325 kPa, it is said to fluctuate by approximately ±2% (approximately ±2 kPa) over the course of a day. If the relationship between the pressure of the pressurized gas and the molten metal filling height is assumed to be 40 mm / 1 kPa, the molten metal filling height will fluctuate by approximately 80 mm over the course of a day, making the impact of atmospheric pressure fluctuations significant and not negligible. Furthermore, under low or high pressure weather conditions, atmospheric pressure fluctuates even more, resulting in even greater fluctuations in the molten metal filling height. For this reason, many proposals have been made to avoid fluctuations in the molten metal filling height due to fluctuations in atmospheric pressure.

[0005] For example, Patent Document 1 proposes storing a mold in a sealed chamber, pressurizing the molten metal storage furnace and the chamber to the same pressure, and then reducing the pressure in the chamber. This is believed to avoid an increase in flow resistance of the molten metal due to atmospheric pressure and to avoid the effects of atmospheric pressure. Furthermore, Patent Document 2 proposes, in a casting method in which the mold cavity is evacuated and depressurized before being filled with molten metal, measuring the atmospheric pressure around the casting machine in advance, offsetting the reading on a vacuum gauge in a control device, and then starting the vacuum depressurization of the mold cavity. This is believed to correct the reading on the vacuum gauge in the control device and avoid the effects of atmospheric pressure. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 63-273561 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-26740 Summary of the Invention [Problem to be solved by the invention]

[0007] The technique described in Patent Document 1 is limited to small casting machines that can house and pressurize molds in a sealed chamber. For example, in large casting machines with a clamping force of 10,000 kN or more, the molds are large, and the chambers required to house these molds are even larger, making this approach impractical. Even if it were possible to implement this technique, it would inevitably increase the size and complexity of the casting machine. Furthermore, it would require pressurizing and depressurizing the large-capacity chamber for each casting shot, significantly reducing workability and productivity. Furthermore, regardless of the size of the mold, fluctuations in the accuracy of pressurization and depressurization within the chamber, similar to fluctuations in atmospheric pressure, would affect the filling of the molten metal into the mold cavity.

[0008] Furthermore, the method disclosed in Patent Document 2 does not guarantee the accuracy of the actual vacuum reduction of the mold cavity, even if the accuracy of the vacuum gauge in the control device is improved. The accuracy of the vacuum reduction varies due to factors such as deterioration over time of vacuum seal members used to maintain the vacuum reduction of the mold, and this affects the filling state of the molten metal into the mold cavity, just like fluctuations in atmospheric pressure.

[0009] Therefore, an object of the present invention is to provide a low-pressure casting method that realizes stable filling of molten metal into a mold cavity without being affected by fluctuations in atmospheric pressure, regardless of the size and processing accuracy of the mold, or the state of the mold, such as pressurization or vacuum decompression. [Means for solving the problem]

[0010] The low-pressure casting method of the present invention comprises: A low-pressure casting method in which pressurized gas is supplied into a sealed molten metal holding furnace to inject and fill molten metal into a mold cavity is characterized in that the method comprises an internal pressure measuring unit that measures the pressure of the pressurized gas inside the molten metal holding furnace and an external pressure measuring unit that measures atmospheric pressure, and an initial pressure adjustment of the pressurized gas inside the molten metal holding furnace is performed before the start of injection and filling of the molten metal so that the pressures measured by the internal pressure measuring unit and the external pressure measuring unit match.

[0011] Further, in the low-pressure casting method of the present invention, After the injection and filling of the molten metal is completed, it is preferable to adjust the pressure reduction of the pressurized gas in the molten metal holding furnace so that the pressure measured by the internal pressure measuring unit and the pressure measured by the external pressure measuring unit coincide with each other.

[0012] Furthermore, in the low-pressure casting method of the present invention, After the pressure reduction adjustment, it is preferable to start injection and filling of the molten metal. [Effects of the Invention]

[0013] According to the present invention, a low-pressure casting method can be provided that realizes stable filling of molten metal into a mold cavity without being affected by fluctuations in atmospheric pressure, regardless of the size and processing accuracy of the mold, or the state of the mold, such as pressurization or vacuum decompression. [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] FIG. 10 is a diagram illustrating the effect of atmospheric pressure fluctuations and initial pressure adjustment. [Figure 3] 2 is a flow diagram showing a low-pressure casting method using the low-pressure casting apparatus shown in FIG. 1. 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.

[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 mold 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 mold 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. 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, an internal pressure measurement unit 35 that measures the pressure of the pressurized gas in the sealed chamber 21, and an external pressure measurement unit 36 ​​that measures atmospheric pressure. While compressed air may be used as the pressurized gas, it is preferable to use an inert gas such as argon or nitrogen to prevent oxidation of the molten metal M in the melt furnace 22. Therefore, it is preferable to use a pressure cylinder that stores 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, or the like may be used as the pressurized gas supply source 31.

[0020] The pressurized gas supplied from pressurized gas supply source 31 is adjusted to a predetermined pressure by pressure adjustment unit 32 and sent to pressurized gas adjustment unit 33. Pressurization control unit 34 operates pressurized gas adjustment unit 33 to adjust the amount of pressurized gas supplied to sealed chamber 21 so that the pressurized gas pressure in sealed chamber 21 measured by internal pressure measurement unit 35 matches a preset pressurization control pattern. As a result, the pressurized gas pressure in sealed chamber 21 increases, pressing against molten metal M in melt furnace 22, causing molten metal M to flow through feed pipe 23 and be injected and filled into mold cavity 15 through mold gate 16. In other words, the amount of molten metal M injected and filled into mold cavity 15 is determined according to the increase in pressurized gas pressure in sealed chamber 21. This relationship will be explained in detail using FIG. 2.

[0021] Here, the molten metal M rises through the feed pipe 23 from the melt furnace 22 below toward the mold cavity 15 above, where it is injected and filled. The amount of molten metal M injected and filled can be expressed as the distance (molten metal filling height H) from the surface of the molten metal M in the melt furnace 22 to the surface of the molten metal M flowing within the mold cavity 15. Figure 2(a) is a graph showing the pressurized gas pressure P in the sealed chamber 21 on the horizontal axis and the molten metal filling height H on the vertical axis. The pressurized gas pressure P and the molten metal filling height H are proportional to each other, as shown in Figure 2(a) as a correlation graph PH. For example, assume that a pressurized gas pressure P of 1 kPa causes the molten metal filling height H to move 40 mm. At this time, if the molten metal filling height H0 required to fill the mold cavity 15 with the molten metal M is 1000 mm, the pressurization control unit 34 operates the pressurization gas adjustment unit 33 to adjust the amount of pressurized gas supplied to the sealed chamber 21 so as to obtain a pressurized gas pressure P0 of 25 kPa. The pressure within the mold cavity 15 before injection and filling with the molten metal M is set as a reference value KP, which is set to zero by the pressurization control unit 34. If this reference value KP is, for example, the average atmospheric pressure at point A (101.325 kPa), then the difference between the pressurized gas pressure P0 and the reference value KP is the pressure difference P0 of 25 kPa required to inject and fill with the molten metal M. This reference value KP is used in the pressurization control pattern set in the pressurization control unit 34.

[0022] As mentioned above, atmospheric pressure varies by region and fluctuates daily. For example, it is said that the average atmospheric pressure at Point A (101.325 kPa) fluctuates by approximately ±2% (±2 kPa) per day. This fluctuation in atmospheric pressure affects the pressure difference required to inject and fill the molten metal M, which in turn significantly affects the molten metal filling height H0. For example, when the atmospheric pressure is high (+2 kPa), the reference value KP1 is offset by approximately +2 kPa, reducing the pressure difference P1 to 23 kPa. As a result, from the correlation graph PH1 starting from the offset reference value KP1, even with the same pressurized gas pressure P0, the molten metal filling height H1 drops to 920 mm, resulting in an 80 mm shortfall in the injection filling volume of molten metal M. This condition can lead to casting defects due to insufficient injection filling volume of molten metal M, such as poor molten flow, voids, and underweight products.

[0023] Conversely, when the atmospheric pressure is low (-2 kPa), the reference value KP2 is offset by about -2 kPa, increasing the pressure difference P2 to 27 kPa. From the correlation graph PH2 starting from the reference value KP2, the molten metal filling height H2 increases to 1080 mm, and even with the same pressurized gas pressure P0, the injection filling volume of molten metal M exceeds the required volume by 80 mm. In this state, casting defects such as casting flash and excessive product weight occur due to excessive injection filling volume of molten metal M. Under low or high pressure weather conditions, the fluctuation range of the molten metal filling height becomes even greater, making it impossible to produce stable casting quality.

[0024] Therefore, an external pressure measuring unit 36 ​​is provided to measure the atmospheric pressure. Before the injection and filling of the molten metal M begins, the pressurization control unit 34 operates the pressurization gas adjusting unit 33 to adjust the pressurization gas pressure (referred to as the initial pressure SP) supplied to the sealed chamber 21 so that the pressure measured by the internal pressure measuring unit 35 and the external pressure measuring unit 36 ​​coincide. The pressurization control unit 34 replaces the reference value KP of the pressurization control pattern with the adjusted initial pressure SP, and the injection and filling of the molten metal begins. For example, as shown in Figure 2(b), when the atmospheric pressure is high (+2 kPa), the initial pressure SP is increased by about +2 kPa to equalize it with atmospheric pressure. Conversely, when the atmospheric pressure is low (-2 kPa), the initial pressure SP is reduced by about -2 kPa to equalize it with atmospheric pressure. As a result, the correlation graph PH between the initial pressure SP and the initial pressure SP is stable, and the required molten metal filling height H0 = 1000 mm can be obtained even if the atmospheric pressure fluctuates.

[0025] [Low pressure casting method] Next, a low-pressure casting method using the low-pressure casting apparatus shown in Fig. 1 will be described with reference to Fig. 3. The adjustment of the initial pressure SP is performed before the start of injection filling of the molten metal M. When adjustment of the initial pressure SP begins, the external pressure measurement unit 36 ​​measures the atmospheric pressure PT, and the internal pressure measurement unit 35 measures the pressurized gas pressure PG inside the sealed chamber 21 of the molten metal storage furnace 20. The measured atmospheric pressure PT and pressurized gas pressure PG are compared and determined by the pressurization control unit 34. When the comparison confirms that the molten metal filling height H matches within an allowable range (PT = PG), the initial pressure adjustment is completed.

[0026] When the result of the comparison determination is determined to be inconsistent (PT≠PG), the pressure control unit 34 operates the pressurized gas adjustment unit 33 to adjust the pressure of the pressurized gas in the sealed chamber 21 so that the atmospheric pressure PT and the pressurized gas pressure PG become equal. For example, when the atmospheric pressure PT is higher than the pressurized gas pressure PG (PG<PT), pressurized gas is supplied to increase the pressure in the sealed chamber 21. Conversely, when the atmospheric pressure PT is lower than the pressurized gas pressure PG (PG>PT), the pressurized gas is exhausted to reduce the pressure in the sealed chamber 21. In the pressure increase adjustment or pressure reduction adjustment, the sealed chamber 21 is blocked from the outside air and maintained in a sealed state, and an inert gas is used as the pressurized gas to prevent oxidation of the molten metal M in the molten metal holding furnace 20. When the pressurized gas pressure PG and the atmospheric pressure PT match by the pressure increase adjustment or pressure reduction adjustment, the adjustment of the initial pressure SP is completed.

[0027] After the adjustment of the initial pressure SP, the reference value KP of the pressure control pattern set in the pressure control unit 34 is replaced with the adjusted initial pressure SP (initial pressure SP setting). After the setting of the initial pressure SP, pressure control is performed based on the pressure control pattern. The pressure control unit 34 operates the pressurized gas adjustment unit 33 to supply pressurized gas into the sealed chamber 21, and inject and fill the molten metal M into the mold cavity 15 via the hot water supply pipe 23. Thereafter, a pressure increase process for increasing the filling density of the molten metal and a pressure holding process for compensating for the solidification shrinkage of the molten metal are performed. From the injection filling process to the pressure holding process, the pressure control unit 34 operates the pressurized gas adjustment unit 33 to supply pressurized gas into the sealed chamber 21 and continue to replenish the molten metal M from the molten metal holding furnace 20. Thereafter, through the cooling process, the casting mold 10 is opened, and the casting is taken out from the casting mold 10.

[0028] After the pressure holding process, the pressure control is terminated. Note that the mold gate 16 may be closed using the gate closing means provided in the mold, and the pressure increase process and the pressure holding process may be performed using the pressurizing means provided in the mold. In this case, the pressure control is terminated after the mold gate 16 is closed.

[0029] After the pressurization control, pressure reduction adjustment is initiated to return the molten metal M remaining in the supply pipe 23 back into the molten metal furnace 22. The pressurization control unit 34 operates the pressurization gas adjustment unit 33 to discharge the pressurized gas and reduce the pressurized gas pressure in the sealed chamber 21. The pressing force on the molten metal M in the molten metal furnace 22 decreases, and the molten metal M in the supply pipe 23 descends and returns to the molten metal furnace 22. During this pressure reduction adjustment, the external pressure measurement unit 36 ​​measures the atmospheric pressure PT and the residual gas pressure PZ in the sealed chamber 21, and adjustments are made so that the atmospheric pressure PT and the residual gas pressure PZ are equal. When the pressurization control unit 34 confirms that the atmospheric pressure PT and the residual gas pressure PZ are equal (PT = PZ), the pressure reduction adjustment is completed. Note that even during the pressure reduction adjustment, the sealed chamber 21 is isolated from the outside air and filled with inert gas. Furthermore, by adjusting the pressure reduction so that the atmospheric pressure PT and the residual gas pressure PZ of the pressurized gas remaining in the sealed chamber 21 are equal, the initial pressure adjustment is completed. As a result, the initial pressure adjustment can be omitted from the next shot of casting onwards.

[0030] [effect] This system is equipped with an internal pressure gauge that measures the pressure of the pressurized gas and an external pressure gauge that measures atmospheric pressure. Pressurized gas is supplied into a sealed molten metal holding furnace, and before the start of casting, the molten metal is injected and filled into the mold cavity, the initial pressure of the pressurized gas inside the molten metal holding furnace is adjusted so that the pressures measured by the internal and external pressure gauges match. This simple adjustment, simply matching the pressures measured by the internal and external pressure gauges, prevents fluctuations in the amount of molten metal injected into the mold cavity due to fluctuations in atmospheric pressure. Furthermore, it prevents casting defects such as poor molten metal flow, molten metal wrinkles, voids and blisters, poor transfer, casting flash, and mold damage caused by fluctuations in the injection filling volume, thereby achieving stable production of high-quality castings.

[0031] Furthermore, unlike conventional technologies, this method does not require the use of cumbersome auxiliary equipment, such as a large, sealed container called a chamber that houses the casting mold and performs pressurization and depressurization, or a vacuum depressurization device that vacuums and depressurizes the mold cavity. Therefore, there are no issues with fluctuations in the injection filling volume of molten metal into the mold cavity, similar to fluctuations in atmospheric pressure, caused by fluctuations in the accuracy of pressurization and depressurization in these auxiliary equipment. Furthermore, by using an inert gas such as argon or nitrogen as the pressurizing gas and filling the molten metal holding furnace with inert gas, oxidation of the molten metal can be reliably prevented, contributing to the stable production of high-quality castings by injecting and filling clean molten metal.

[0032] Furthermore, when adjusting the pressure reduction to return the molten metal remaining in the feed pipe to the molten metal holding furnace after casting, the remaining pressure of the pressurized gas in the molten metal holding furnace is adjusted so that the pressure measured by the internal pressure measuring unit matches that of the external pressure measuring unit. This makes it possible to accurately reproduce the state after the initial pressure adjustment, which eliminates fluctuations in atmospheric pressure, and omits the initial pressure adjustment from the next shot onwards, achieving high productivity.

[0033] 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]

[0034] 10 Casting mold 11 Movable plate 12 Movable mold 13 Fixed plate 14 Fixed mold 15 mold cavity 16 Mold gate 20 Molten metal holding furnace 21 Closed room 22 Melting furnace 23 Hot water pipe 30 Pressure control device 31 Pressurized gas supply source 32 Pressure adjustment section 33 Pressurized gas adjustment unit 34 Pressure control section 35 Internal pressure measurement section 36 External pressure measurement unit M molten metal H, H0, H1, H2 Molten metal filling height P, P0, PG pressurized gas pressure PH, PH1, PH2 correlation graph KP, KP1, KP2 reference values P0, P1, P2 pressure difference SP Initial Pressure PT Atmospheric Pressure PZ Residual gas pressure 100 Low-pressure casting equipment

Claims

1. A low-pressure casting method in which pressurized gas is supplied into a sealed molten metal holding furnace to inject and fill molten metal into a mold cavity, an internal pressure measuring unit that measures the gas pressure of the pressurized gas inside the molten metal holding furnace, and an external pressure measuring unit that measures atmospheric pressure; Before the injection and filling of the molten metal begins, comparing the gas pressure PG measured by the internal pressure measuring unit with the atmospheric pressure PT measured by the external pressure measuring unit; If the gas pressure PG and the atmospheric pressure PT do not coincide with each other, the pressure of the pressurized gas in the molten metal storage furnace is adjusted so that they coincide with each other. A low-pressure casting method characterized in that injection and filling of the molten metal is started with the gas pressure PG, which is equal to the atmospheric pressure PT, as an initial pressure.

2. After the injection and filling of the molten metal is completed, comparing the gas pressure PZ measured by the internal pressure measuring unit with the atmospheric pressure PT measured by the external pressure measuring unit; 2. The low-pressure casting method according to claim 1, wherein if the measured gas pressure PZ and the measured atmospheric pressure PT do not match, the pressure of the pressurized gas in the molten metal holding furnace is reduced so that they match.

3. 3. The low-pressure casting method according to claim 2, wherein the gas pressure PZ, which is equal to the atmospheric pressure PT, is used as an initial pressure to start injection and filling of the molten metal for the next shot.

4. A low-pressure casting device supplies pressurized gas into a sealed molten metal holding furnace to inject and fill molten metal into a mold cavity. an internal pressure measuring unit for measuring the gas pressure of the pressurized gas inside the molten metal holding furnace; an external pressure measuring unit for measuring atmospheric pressure; a pressure control device that controls the pressure of the pressurized gas inside the molten metal storage furnace based on the gas pressure PG measured by the internal pressure measuring unit and the atmospheric pressure PT measured by the external pressure measuring unit, The pressure control device includes: Before the injection and filling of the molten metal begins, Comparing the gas pressure PG with the atmospheric pressure PT, If the gas pressure PG and the atmospheric pressure PT do not coincide with each other, adjust the pressurized gas in the molten metal holding furnace so that they coincide with each other. A low-pressure casting apparatus characterized in that injection and filling of the molten metal is started with the gas pressure PG, which is equal to the atmospheric pressure PT, as an initial pressure.

5. The pressure control device includes: If the atmospheric pressure PT is high, the gas pressure PG is increased relative to a reference value KP, which is the pressure in the mold cavity before the molten metal is injected and filled, or 5. The low-pressure casting apparatus according to claim 4, wherein if the atmospheric pressure PT is lower than the reference value KP, the gas pressure PG is reduced by a pressure reduction adjustment, thereby making the gas pressure PG and the atmospheric pressure PT equal to each other.

6. The pressure control device includes: In the pressure increase adjustment, the pressurized gas is supplied into the molten metal holding furnace; 6. The low-pressure casting apparatus according to claim 5, wherein the pressurized gas in the molten metal holding furnace is exhausted during the pressure reduction adjustment.

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