JPH11156529A - Differential pressure casting method and molten metal holding method therefor as well as differential casting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cast a steadily quality controlled casting by holding a molten metal in a molten metal supply pipe at a height between a top end of a holding furnace and a gate of a cavity during the repeated casting so that dispersion of a molten metal does not occur. SOLUTION: This differential pressure casting device 1 is a low pressure casting device and a casting mold 3 is arranged above a holding furnace 2. Inside of the casting mold, a cavity is installed and a molten metal 6 in the holding furnace 2 is supplied to the cavity by the molten metal supply pipe. An air intake pipe 22 to send in an air, an air exhausting pipe 23 to exhaust the air, and pressure sensors 7 to measure a furnace inside pressure are installed to the holding furnace 2. Then, a casting is carried out by applying a differential pressure in the cavity and in the holding furnace 2. While castings are repeated, the molten metal in the molten metal supplying pipe is held at a height between the first and the second molten metal perceive sensors 51, 52 by controlling the differential pressure based on an output of the first and the second molten metal perceive sensors 51, 52.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a differential pressure casting method, a method for holding a molten metal in a holding furnace at a predetermined height in a molten metal supply pipe in the differential pressure casting method, and a differential pressure casting apparatus.

[0002]

2. Description of the Related Art In a differential pressure casting method, a differential pressure is applied between a cavity provided in a mold and a holding furnace for holding the molten metal, whereby the molten metal in the holding furnace is injected into the cavity and cast. It is. This casting method is disclosed in, for example, JP-A-53-50018.

In the differential pressure casting method described in this publication,
After the molten metal in the holding furnace is injected into the mold via the molten metal supply pipe, the molten metal descends only in the molten metal supply pipe to a predetermined level near the outlet of the molten metal supply pipe, and holds the level of the molten metal in the molten metal supply pipe. The pressure of the gas in the furnace is measured and the gas inlet and outlet are controlled as a function of these measurements to ensure a predetermined substantially constant pressure rise and fall time. You.

[0004] This publication discloses a method of injecting a molten metal in a holding furnace into a mold and returning the molten metal only to a predetermined level in the molten metal supply pipe as described above. And is described as being very preferred as it prevents oxides of the coating from transferring into the mold.

[0005]

However, in the differential pressure casting method described in the above-mentioned publication, when the molten metal falls down to the above-mentioned predetermined level in the molten metal supply pipe, the molten metal no longer exists in the upper part of the molten metal supply pipe. That part is exposed to the outside air because it is located above the top of the holding furnace. For this reason, the pipe wall at that part of the molten metal supply pipe is cooled by the outside air, and as a result, the inner surface is at a temperature considerably lower than the hot water temperature.

If the inner surface of the molten metal supply pipe is at a low temperature as described above, when the molten metal passes through that portion during casting, it is cooled and causes a variation in the temperature of the molten metal injected into the cavity. . If the temperature of the molten metal injected into the cavity varies, a casting of stable quality cannot be cast.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a differential pressure casting method in which the temperature of molten metal injected into a cavity does not vary and therefore a casting of stable quality can be cast. The present invention also provides a molten metal holding method for holding a molten metal in a holding furnace at a predetermined height in a molten metal supply pipe in the differential pressure casting method, and a differential pressure casting apparatus for performing the differential pressure casting method. Is to provide.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present invention is directed to a holding furnace for holding a molten metal, a mold disposed above the holding furnace, a cavity in the mold, and a holding furnace. In a differential pressure casting method in which a molten metal supply pipe communicating with the molten metal in the inside is provided and casting is performed by applying a differential pressure between the cavity and the holding furnace, the molten metal in the molten metal supply pipe is interposed between repeated castings. It is characterized in that it is held at a height between the top of the holding furnace and the gate of the cavity.

[0009] With this configuration, the tube wall of the portion of the molten metal supply pipe between the top of the holding furnace and the gate of the cavity is constantly exposed to the high-temperature molten metal, and as a result, the temperature of the tube wall is reduced. Less decrease.

According to a second aspect of the present invention, in addition to the first aspect of the present invention, first and second molten metal sensing sensors for sensing the molten metal are provided in the molten metal supply pipe, and between the repeated castings. By controlling the differential pressure based on the outputs of the two molten metal sensing sensors, the molten metal in the molten metal supply pipe is maintained at the height between the first and second molten metal sensing sensors.

In this way, when the molten metal in the molten metal supply pipe is above the two molten metal sensing sensors during the repeated casting, the molten metal is lowered, or the molten metal in the molten metal supply pipe is reduced by the two molten metal sensing sensors. When it is also below, the molten metal rises. As a result, the molten metal in the molten metal supply pipe is maintained at the height between the first and second molten metal sensing sensors.

[0012] Further, this control is achieved by the invention according to the third aspect of the present invention, in which the molten metal held in the holding furnace is supplied to the cavity by a differential pressure between the cavity formed by the mold and the inside of the holding furnace via the molten metal supply pipe. In the differential pressure casting method, wherein the molten metal in the molten metal supply pipe is maintained at a predetermined height between repeated molten metal pouring operations. The molten metal is once lowered to the holding furnace side from the first molten metal detection sensor installed on the furnace side, and then the first molten metal is detected.
The molten metal is raised to the mold side until the molten metal is detected by the molten metal sensing sensor, and when the molten metal is sensed by the first molten metal sensing sensor, the molten metal is set in the molten metal supply pipe closer to the mold than the first molten metal sensing sensor. It is also possible to temporarily hold the differential pressure at that time in a range where the second molten metal sensor does not detect the molten metal.

According to this structure, the molten metal in the molten metal supply pipe is maintained at a height between the first molten metal sensor and the second molten metal sensor, as in the second aspect of the invention. can do.

According to a fourth aspect of the present invention, there is provided a holding furnace for holding a molten metal, a mold disposed above the holding furnace, and a molten metal supply pipe for communicating a cavity of the mold with the molten metal in the holding furnace. In a differential pressure casting apparatus in which casting is performed by applying a differential pressure between the cavity and the inside of the holding furnace, the molten metal is placed at a height between the top of the holding furnace and the gate of the cavity in the molten metal supply pipe. It is characterized by providing a molten metal sensing sensor for sensing.

With this configuration, it is possible to reliably detect that the position of the molten metal surface in the molten metal supply pipe is at the height between the top of the holding furnace and the gate of the cavity.

Further, the molten metal sensing sensor is provided in claim 5
As in the invention described above, it is preferable to provide them at two different heights in the molten metal supply pipe. By doing this,
It can be sensed that the molten metal surface is located at a height between the two molten metal sensing sensors.

According to a sixth aspect of the present invention, in addition to the fourth or fifth aspect, a heat insulating material layer is provided on a pipe wall of a portion of the molten metal supply pipe above the top of the holding furnace. It is characterized by the following. With this configuration, the heat that is lost to the outside in the portion of the molten metal supply pipe above the top of the holding furnace can be extremely reduced.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a cross-sectional view showing one embodiment of the differential pressure casting apparatus of the present invention, and FIG. 2 is a graph showing an example of a temporal change of the furnace internal pressure in the holding furnace of FIG.

In FIG. 1, a differential pressure casting apparatus 1 is a low pressure casting apparatus. A casting mold 3 for casting a vehicle wheel is disposed above a holding furnace 2. The mold 3 can be divided into four parts: an upper mold 3a, a lower mold 3b and horizontal molds 3c, 3c, and a cavity 31 is provided therein. The number of divisions of the mold 3 may be two or more. Cavity 3
A gate 32 into which the molten metal 6 enters is provided at a lower part of the first metal melt 1.
The sprue 32 is slightly widened on the cavity 31 side, and the molten metal supply pipe 4 is provided vertically below the lower end of the sprue 32.

The molten metal supply pipe 4 penetrates the upper wall of the holding furnace 2 and has a lower end extending to near the bottom of the holding furnace 2. The holding furnace 2 contains a molten metal 6 obtained by heating and melting a metal such as an aluminum alloy. The lower end of the molten metal supply pipe 4 is always immersed in the molten metal 6. At the upper part of the holding furnace 2, an air supply pipe 22 for feeding air into the holding furnace 2, an exhaust pipe 23 for discharging the air inside the holding furnace 2 to the outside, and a furnace pressure are measured. Pressure sensor 7 is provided.

The molten metal supply pipe 4 is provided at the top 2 of the holding furnace 2.
At a height between 1 (furnace lid (not shown) when holding furnace 2 has a furnace lid (not shown)) and gate 32 of cavity 31, molten metal sensing sensors 51 and 52 for sensing molten metal 6 are provided. The molten metal sensing sensors 51 and 52 are provided at two places having different heights, but may be provided at one place or three or more places.
When provided at two locations, they may be provided so as to be exactly overlapped when viewed from above, or at different positions,
For example, they may be provided in different directions based on the axis of the molten metal supply pipe 4.

As the molten metal sensing sensors 51 and 52, for example, a sensor in which a thermocouple is housed in a ceramic protective tube can be used, and the tip of the protective tube is slightly inserted from the tube wall of the molten metal supply tube 4 into the tube. Use it protruding. According to the molten metal sensing sensors 51 and 52, it is possible to detect that the distal end is immersed in the molten metal 6 by a temperature change when the molten metal 6 comes into contact with the distal end of the protective tube.

The molten metal sensing sensors 51 and 52 are
It is not limited to a thermocouple as long as it can detect the molten metal 6 in the molten metal supply pipe 4. For example, a pair of electrode rods can be used. According to such an electrode rod, it is possible to sense that the distal ends of the pair of electrode rods are immersed in the molten metal 6 by electrical conduction when both the distal ends are immersed in the molten metal.

In addition, the molten metal supply pipe 4 of this embodiment includes:
A heat insulating material layer 4 is formed on the tube wall in a portion above the top 21 of the holding furnace 2.
1 is provided. The heat insulating material layer 41 is concentrically lined with the inner peripheral portion of the tube wall. The heat insulating material used for the heat insulating material layer 41 is not particularly limited, but a ceramic heat insulating material, for example, a material mainly containing calcium silicate can be used.

Further, a safety device is provided on the upper part of the differential pressure casting device 1 in case of leakage of molten metal around the above-mentioned gate.
It is preferable to provide an infrared sensor (not shown) capable of sensing the temperature. Further, in order to more reliably grasp the pressure difference between the cavity 31 and the inside of the holding furnace 2, for example, a level sensor (not shown) is provided from the ceiling of the holding furnace 2 to the level 61, and the inside of the holding furnace 2 is provided. It is preferable that the height of the molten metal surface 61 can be sequentially measured. An electrode rod or the like can be used as this level sensor, and the height of the level is measured by raising and lowering the electrode rod by an air cylinder or the like.

In the differential pressure casting apparatus 1 according to the present embodiment configured as described above, first, as shown in FIG. Thereafter, the holding furnace 2 and the mold 3 are sealed, and then the holding furnace 2 is
Air and inert gas are fed into the inside. Then, the furnace pressure increases, the molten metal 6 rises in the molten metal supply pipe 4, and after the first molten metal detection sensor 51 detects the molten metal 6, the molten metal surface 62 reaches the level a. In this case, the furnace pressure is next p _a, the molten metal surface 61 in the holding furnace 2 becomes level s ₁ slightly lowered.

When air is further fed into the holding furnace 2 to increase the pressure, the molten metal 6 in the molten metal supply pipe 4 rises, and after the second molten metal sensor 52 detects the molten metal 6, the molten metal surface is melted. 62 sequentially reaches level b and level c. The level b is the height of the entrance of the cavity 31, and the level c is the height of the cavity 31.
The height of the highest part. When the molten metal surface 62 reaches the level c, the cavity 31 is filled with the molten metal 6. In this process, the furnace internal pressure rises to p _c from p _b. Hot water surface 62
Correspond to the subscripts _{a, b, c of} p indicating the pressure.

The furnace pressure is then raised further to p _d for feeder, a certain period of time held at that pressure. Thus, the casting of the first shot is completed. Therefore, the air supply pipe 22
Of the air by the exhaust pipe 23
Evacuated rapidly to below p _b .

As a result, the furnace pressure changes from time t _e to time t _{e in} FIG.
It falls rapidly as shown between t _f . Thereafter, at a time t _g after a lapse of a little time, the mold 3 is divided and opened to the atmosphere, and the casting inside is taken out. After that,
The mold 3 is closed again, and the casting of the second shot is started from the time _ta2 .

In this case, if the pressure difference between the inside of the cavity 31 and the inside of the holding furnace 2 becomes lower than a certain value between the times t _{e and} t _a2 , the molten metal 6 filled up to the level of the cavity 31 falls by its own weight. there is a possibility. In particular, at time t _g when the mold 3 is divided, a large amount of air enters the gate 31, so that the molten metal 6 in the molten metal supply pipe 4 surely descends. However, in the differential pressure casting method of this example, the molten metal surface 62 in the molten metal supply pipe 4 is maintained at the level a even in this case.

Therefore, at least when the mold 3 is divided,
The exhaust pipe 23 is substantially closed, and air is supplied from the air supply pipe 22 to maintain the furnace internal pressure at _pa2 . However, the start of the air infeed to the exhaust pipe 23 closed and the holding furnace 2 of, after the time t _e, preferably in the time the furnace pressure is equal to or less than p _a. The above-mentioned furnace pressure p _a2 is adjusted to match the drop because the casting level of the first shot lowers the molten metal surface 61 in the holding furnace 2 from s ₁ to s ₂ , and the drop from the level a increases. And p _a <p _a2 . The same applies to the third and subsequent shots.

However, since the furnace pressure p _a2 varies depending on the difference between the heights of s ₁ and s ₂ and the internal dimensions of the holding furnace 2, it is actually difficult to know accurately. Therefore, in the differential pressure casting method of the present invention, the molten metal sensing sensors 51 and 52 disposed at two different heights are used.

That is, when the molten metal 6 is filled in the molten metal supply pipe 4, the two molten metal sensing sensors 51 and 52 both sense the molten metal 6. After time t _e, when the molten metal supply pipe 4 bath level 62 drops, firstly, the second molten metal detection sensor 52 located above the level a no longer senses the molten metal.

Immediately at that time, the exhaust pipe 23 is almost closed and, at the same time, the supply of air is started. When this operation is performed properly, the molten metal surface 62 can be stopped at the level a as it is. In this case, the second molten metal sensing sensor 52 does not sense the molten metal 6, and the first molten metal sensing sensor 51 located below the level a is in a state of sensing the molten metal 6. Therefore, by adjusting the Iriryou or / and the exhaust amount feeding of air to hold the furnace pressure p _a2 at this time, thereby holding the molten metal surface 62 of the melt feed pipe 4 to the level a. At this time, it is preferable that an exhaust control valve be separately provided in the exhaust pipe 23 in addition to the exhaust valve.

However, in order to just stop the molten metal surface 62 at the level a, a relatively sophisticated control is required. Therefore, the following control can be performed instead of this control. That is, when the molten metal surface 62 of the melt feed pipe 4 after the time t _e is lowered temporarily to move the melt surface 62 to below the first first molten metal detection sensor 51.
In this state, the first and second molten metal sensing sensors 51, 52
Do not sense the molten metal 6.

Next, at the same time as the exhaust pipe 23 is almost closed, the air supply is started at the same time as the furnace, and the furnace pressure is gradually increased, and the molten metal surface 62 is raised until the first molten metal sensor 51 detects the molten metal 6. Let it. When the first molten metal sensor 51 detects the molten metal 6, the molten metal surface 62 is detected.
Has reached level a. Therefore, in this state, the amount of air supplied and / or the amount of exhaust air is adjusted so that the second molten metal sensor 52 does not detect the molten metal 6, and the furnace pressure at that time is maintained. Thereby, the molten metal surface 62 in the molten metal supply pipe 4 is maintained at the level a.

When the molten metal sensing sensor is provided at only one position of the molten metal supply pipe 4 (not shown), for example, the molten metal surface 62 can be controlled as follows. That is, in this case, the decrease Δs of the molten metal surface 61 of the holding furnace 2 when the molten metal 6 is reduced by the capacity of the cavity 31 is previously calculated from the change in the cross-sectional area of the holding furnace 2 in the depth direction. . Also,
Taking into account the specific gravity of the molten metal 6,
Is constant, the furnace pressure increase Δp that must be increased when the molten metal surface 61 of the holding furnace 2 decreases by Δs
Is also calculated.

At the first shot, first, the furnace pressure is gradually increased from zero to raise the molten metal surface 62 in the molten metal supply pipe 4. Then, when the molten metal detection sensor senses the molten metal 6, to measure the furnace pressure p ₁ at that time, then the furnace pressure slightly lower than, retains its furnace internal pressure it was no longer senses the melt 6. In this case, the height s of the first shot of the previous holding furnace 2 of the molten metal surface 61, previously determined by calculation from the furnace pressure p _1. After that, the first shot is cast.

After completion of the casting, the furnace pressure is reduced until the furnace pressure becomes p ₁ + Δp. Then, the molten metal surface 62 in the molten metal supply pipe 4 descends to a position slightly higher than the molten metal sensor. Therefore, the furnace pressure is slightly lowered, and when the molten metal 6 is no longer detected, the furnace pressure is maintained. Then, the second shot is cast.
In this way, between the first shot and the second shot,
The molten metal surface 62 in the molten metal supply pipe 4 can be maintained at a height immediately below the molten metal sensing sensor.

As described above, when the molten metal sensor is provided only at one location, the molten metal sensor is substantially equivalent to the second molten metal sensor 52 when the molten metal sensor is provided at two locations. Can be
It is not limited to this control method.

In addition, in the case where the molten metal sensing sensors are provided at three or more locations of the molten metal supply pipe 4 (not shown), the above-mentioned 2 is used.
As in the case of providing at two locations, it can be detected that the molten metal surface 62 is located between any two adjacent molten metal sensing sensors. Therefore, the height of the molten metal surface 62 can be more accurately sensed, and even if one of the three or more molten metal sensors fails, the position of the molten metal surface 62 is not completely lost. Therefore, in order to prevent the molten metal from spouting from the gate, it is necessary to provide three or more sensors directly below the gate to control the pressure so that the pressure is reduced when the sensor detects the molten metal. Can also.

As described above, according to the differential pressure casting method of this embodiment, the pipe wall of the portion of the molten metal supply pipe 4 between the top 21 of the holding furnace 2 and the gate 32 of the cavity 31 is always in a state. The tube is exposed to the high-temperature molten metal 6, and as a result, the temperature of the tube wall is less reduced. For this reason, the temperature of the molten metal 6 injected into the cavity 31 through the molten metal supply pipe 4 does not vary, and a casting of stable quality can be cast.

Further, since the heat insulating layer 41 is provided on the pipe wall of the above-mentioned molten metal supply pipe 4 above the top 21 of the holding furnace 2, the heat lost to the outside in the molten metal supply pipe 4 is extremely small. Can be reduced. Therefore, the temperature drop of the tube wall is further reduced.

Further, there is a concern that the molten metal may erroneously overflow from the gate, etc. if the inside of the holding furnace is constantly pressurized, but in the differential pressure casting apparatus 1 of this embodiment, Since the molten metal detection sensors 51 and 52 are provided in the supply pipe 4, the molten metal detection sensors 51 and 52 can also serve as safety devices. That is, in the event of an abnormality, the furnace pressure can be immediately reduced to zero or an alarm can be issued by the molten metal sensing sensors 51 and 52. For this reason, the safety of the differential pressure casting device 1 is further enhanced.

It should be noted that the present invention is not limited to the above example. For example, in the differential pressure casting method of the present invention, a differential pressure is applied between a cavity provided in a mold and the inside of a holding furnace. Any casting method may be used as long as the molten metal in the holding furnace is injected into the cavity and cast.

Accordingly, the differential pressure in this case may be a differential pressure in a state where both the cavity and the holding furnace are pressurized, a differential pressure in a state where both the cavity and the holding furnace are depressurized, or a pressure difference in the cavity. A differential pressure in which the pressure in the holding furnace is increased while maintaining the atmospheric pressure, or a differential pressure in which the cavity is depressurized while maintaining the atmospheric pressure in the holding furnace may be used. Further, the present invention is not particularly limited to the values of the pressurization and decompression.

Further, the differential pressure casting apparatus of the present invention is not limited to the above embodiment, but applies a differential pressure between the cavity provided in the mold and the inside of the holding furnace, thereby forming the molten metal in the holding furnace. May be injected into the cavity to perform casting.

Therefore, in the differential pressure casting apparatus, both the cavity and the holding furnace are closed and pressurized at the time of casting, or the cavity and the holding furnace are both closed and depressurized at the time of casting. The thing, the cavity is open to the atmosphere and the holding furnace is sealed and the holding furnace is pressurized, and the one where the holding furnace is open to the atmosphere and the cavity is closed and the cavity is depressurized included. Further, the present invention is not particularly limited to the values of pressurization and decompression.

[0049]

As described above in detail, according to the first aspect of the present invention, the pipe wall of the portion of the molten metal supply pipe between the top of the holding furnace and the sprue of the cavity is formed. It is always exposed to a high-temperature molten metal, and as a result, there is little decrease in the temperature of the tube wall. Therefore, the temperature of the molten metal injected into the cavity through the molten metal supply pipe does not vary, and a casting of stable quality can be cast.

According to the second aspect of the invention, in addition to the effect of the first aspect of the invention, the molten metal in the molten metal supply pipe is connected to the first molten metal sensor and the second molten metal sensor between the repeated castings. It can be held at a height between the molten metal sensing sensors.

According to the third aspect of the present invention, the molten metal in the molten metal supply pipe is easily held at the height between the first molten metal sensor and the second molten metal sensor between repeated castings. can do.

According to the fourth aspect of the present invention, it is possible to reliably detect that the position of the molten metal surface in the molten metal supply pipe is at a height between the top of the holding furnace and the gate of the cavity. .

According to the invention of claim 5, in addition to the effect of the invention of claim 4, it is possible to sense that the molten metal surface is located at a height between the two molten metal sensing sensors, Thereby, the height of the molten metal surface can be grasped more accurately.

According to the sixth aspect of the present invention, in addition to the effect of the fourth or fifth aspect, the heat lost to the outside in the portion of the molten metal supply pipe above the top of the holding furnace. Can be significantly reduced. For this reason, the temperature drop of the tube wall is further reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of a differential pressure casting apparatus according to the present invention.

FIG. 2 is a graph showing an example of a temporal change of a furnace internal pressure in the holding furnace of FIG.

[Explanation of symbols]

1 ... Differential pressure casting device 2 ... Holding furnace 21 ... Top 22 ... Supply pipe 23 ... Exhaust pipe 3 ... Mould 3a ... Upper mold 3b Lower mold 31 Cavity 32 Gate 4 Melt supply pipe 41 Thermal insulation layer 51 First molten metal sensor 52 ························································· Pressure sensor a ----- level b ----- level c ----- level s ----- bath level height (holding furnace) s ₁ · · · · bath level height (the holding furnace) s ₂ ··· Hold level (in the holding furnace) s ₃ ··· Hold level (in the holding furnace)

Continued on the front page (72) Inventor Shunichi Itaya 776-2, Yagima-cho, Shimizu-shi, Shizuoka

Claims (6)

[Claims] 1. A holding furnace for holding a molten metal, a mold disposed above the holding furnace, and a molten metal supply pipe communicating between a cavity in the mold and the molten metal in the holding furnace are provided. In the differential pressure casting method in which casting is performed by applying a differential pressure to the inside of the furnace, between repeated castings, the molten metal in the molten metal supply pipe is held at a height between the top of the holding furnace and the gate of the cavity. Differential casting method. 2. A method according to claim 1, further comprising the steps of: providing a first and a second molten metal sensing sensor for sensing the molten metal in the molten metal supply pipe; and controlling a differential pressure based on the outputs of the two molten metal sensing sensors during repeated casting. 2. The differential pressure casting method according to claim 1, wherein the molten metal in the supply pipe is held at a height between the first and second molten metal sensing sensors. 3. A molten metal held in a holding furnace is poured into the cavity through a molten metal supply pipe by a differential pressure between a cavity formed by a mold and the inside of the holding furnace, and the molten metal is interposed between repeated pouring operations. In the molten metal holding method of the differential pressure casting method in which the molten metal in the supply pipe is held at a predetermined height, between the above-described pouring, the first molten metal sensing sensor installed on the holding furnace side of the molten metal supply pipe. The molten metal is once lowered to the holding furnace side, and then the molten metal is raised to the mold side until the molten metal is detected by the first molten metal sensing sensor. When the molten metal is sensed by the first molten metal sensing sensor, the first molten metal is detected. A method for holding a molten metal by a differential pressure casting method, wherein a differential pressure at that time is temporarily held within a range in which a second molten metal detection sensor provided on a molten metal supply pipe closer to a mold than a molten metal detection sensor does not detect molten metal. 4. A holding furnace for holding a molten metal, a mold disposed above the holding furnace, and a molten metal supply pipe for communicating a cavity of the mold with the molten metal in the holding furnace. In a differential pressure casting apparatus in which casting is performed by applying a differential pressure to the inside, a molten metal sensing sensor for sensing molten metal is provided at a height between the top of the holding furnace and the gate of the cavity in the molten metal supply pipe. A differential pressure casting apparatus characterized by the above-mentioned. 5. The differential pressure casting apparatus according to claim 4, wherein said molten metal sensing sensors are provided at two different heights. 6. The differential pressure casting apparatus according to claim 4, wherein a heat insulating material layer is provided on a pipe wall of a portion of the molten metal supply pipe above the top of the holding furnace.