VACUUM CASTING APPARATUS

Abstract: A method of vacuum casting, wherein pressure in the cavity of a mold is reduced beforehand, molten metal is temporarily stored in the molten metal reservoir and, under such conditions as above, a gate having blocked the cavity from the molten metal reservoir is opened so that molten metal may be sucked into the cavity to be cast, in which gas and/or foreign matters are effectively prevented from being dragged and sucked into the cavity. For the above-said pulse, a space is provided in the molten metal reservoir for containing gas or other foreign matters. Furthermore, the space is positioned where none of the contents can be sucked into the cavity.

Description: Technical Field This invention relates to an apparatus of casting molten metal sucked from a molten metal reservoir into a cavity held under a reduced pressure by opening a gate having been blocking the communication between the molten metal reservoir and the cavity Background Art The vacuum casting can suppress the trapping of air in molten metal and this permits obtaining high quality casting with less casting defects such as cavity blanks, blowholes and microporocity. An example of the apparatus to this end is disclosed in Japanese Laid-Open Utility Model Publication No. 3-31058. FIG. 16 shows the apparatus disclosed in the publication. This apparatus comprises a cavity 2 formed in a casting die 1 and a vacuum pump 4 for reducing the pressure in the cavity 2. The cavity 2 can communicate with a molten metal tank 8, in which heatedly molten metal 6 is stored, via a molten metal passage 10. At a sprue 2a of the cavity 2, a gate mechanism 12 is provided for on-off switching the communication between the molten metal passage 10 and the cavity 2. The gate mechanism 12 has an internal axial passage 12b communicated with a pressure reducing pump 20. The passage 12b has an end communicating with the molten metal passage 10 via a vent 12a at a position closer to the molten metal tank 8 than the gate mechanism 12 is. A molten metal reservoir 14 having a predetermined volume is provided in an intermediate portion of the molten metal passage 10 located to be closer to the molten metal tank 8 than the gate mechanism 12 does. In this casting apparatus, molten metal is introduced into the cavity 2 in the following way. First, in a state with the sprue 2a held closed by the gate mechanism 12, the pressure in the cavity 2 is reduced by operating the vacuum pump 4. At this time, the pressure reducing pump 20 is also operated to produce a negative pressure in the molten metal reservoir 14 and molten metal passage 10, thus causing molten metal 6 stored in the molten metal tank 8 to be sucked up to the position of the end of the gate mechanism 12. That is, the molten metal 6 is tentatively stored in the molten metal reservoir 14 and molten metal passage 10. In this state, the gate mechanism 12 is opened, causing the molten metal 6 stored in the molten metal reservoir 14 and molten metal passage 10 to be sucked and fill the cavity 2. In the above prior art casting apparatus, however, the level (or height) of the top of the molten metal reservoir 14 is lower than the level of the opening/closing position of the gate mechanism 12. Therefore, as the molten metal 6 having been stored in the molten metal reservoir 14 and molten metal passage 10 is sucked quickly into the cavity 2, gas and foreign matter which have been floating in an upper portion of the molten metal reservoir 14 are carried along with the molten metal 6 into the cavity 2. Gas which is once sucked into the cavity 2 cannot be removed even by bringing about a high vacuum state of the cavity 2 and thus causes defective product. JP Patent Publication No. 59-9263 discloses a vacuum casting apparatus utilizing an oblique pouring sleeve communicating with a molten metal reservoir, provided by a plunger sleeve, between an upper plunger and a lower plunger disposed therein preparatory to the introduction of the molten metal into the die cavity. Gas can be readily trapped in the molten metal in the reservoir. Residual molten metal also tends to solidify in the reservoir. JP Laid-Open Patent Publication No. 2-155557 discloses a vacuum casting apparatus having a molten metal reservoir adjacent a gate mechanism of the die cavity, to collect gas entrained in the molten metal. However, when the die cavity is evacuated and the gate opened, the gas in the reservoir can be sucked into the cavity with the molten metal under higher suction conditions. JP Laid-Open Utility Model Publication 3-68955 discloses a vacuum casting apparatus having a molten metal reservoir communicating with a gate mechanism controlling entry into the die cavity through a molten metal passage. However, gas and foreign matter in the molten metal can collect in a space adjacent the gate mechanism to be sucked into the die cavity when the gate mechanism is opened. Summary of the Invention The present invention is predicted in the development of a technique of preventing gas and foreign matter floating in the molten metal reservoir from being sucked into the cavity when molten metal is sucked thereinto. The invention provides a vacuum casting apparatus in accordance with Claim 1. Preferably, the gate means have an internal cavity constituting the molten metal reservoir. With the casting apparatus according to the invention, while molten metal is stored in the molten metal reservoir, a step of accommodating gas or foreign matter in the accommodation space located at a position from which the gas or foreign matter is not sucked into the cavity, is executed before molten metal is sucked into the cavity. By such a method, it is thus possible to effectively suppress the suction of gas or foreign matter into the cavity. Brief Description of the Drawings FIG. 1 is a sectional view of the essential parts of a casting apparatus according to a first embodiment of the invention; FIG. 2 is an enlarged sectional view of a gate tip used in the casting apparatus of the first embodiment; FIGS. 3(A) to 3(C) are schematic views of a molten metal feeder mechanism used in the casting appratus of the first embodiment; FIG. 4 is a sectional view illustrating the operation of the casting apparatus of the first embodiment; FIG. 5 is a sectional view illustrating the operation of the casting apparatus of the first embodiment; FIG. 6 is a sectional view illustrating the operation of the casting apparatus of the first embodiment; FIG. 7 is a sectional view illustrating the operation of the casting apparatus of the first embodiment; FIG. 8 is a fragmentary sectional view illustrating the level (or height) relation between the cavity and the gate tip; FIG. 9 is a graph comparing the status of generation of blowholes (as a result of bubbles) in the casting obtained with the casting apparatus according to the invention and that in the casting obtained with the prior art casting apparatus; FIG. 10 is a sectional view of the essential parts of a casting apparatus according to a second embodiment of the invention; FIG. 11 is an enlarged-scale sectional view showing a gate tip used in the second embodiment of the casting apparatus; FIG. 12 is a sectional view illustrating the operation of the casting apparatus of the second embodiment; FIG. 13 is a sectionial view illustrating the operation of the casting apparatus of the second embodiment; FIG. 14 is a sectional view illustrating the operation of the casting apparatus of the second embodiment; FIG. 15 is a sectional view illustrating the operation of the casting apparatus of the second embodiment; and FIG. 16 is a sectional view of a prior art casting apparatus. Best Mode of Carrying Out the Invention Now, a first embodiment of the invention will be described with reference to the accompanying drawings. FIG. 1 is a sectional view of the essential parts of a casting apparatus according to the embodiment. This casting apparatus comprises a casting die 50 which comprises an upper die 52 and a lower die 54. In the casting die 50, a central cavity 56 is formed when the upper and lower dies 52 and 54 are engaged. The cavity 56 has a central sprue 56a formed via a dam portion 56b. At the sprue 56a, the cavity 56 is communicated with a molten metal passage 58. The molten metal passage 58 is constituted by a passage 58a formed in the center of the lower die 54 and a cylindrical stalk 58b connected to the lower surface of the lower die 54 in such a manner as to be communicated with the passage 58a. During casting, the free end of the stalk 58b is immersed in a molten aluminum alloy 62 (hereinafter referred to as molten metal 62) stored in a melting furnace 60. An O-ring 64 made of heat-resistant rubber is provided between the engaged surfaces of the lower die 54 and the stalk 58b to ensure airtightness of the molten metal passage 58. The melting furnace 60 serves as a molten metal tank to store molten metal. Designated at 61 is a heater for thermally melting metal. A gate mechanism 70 for opening and closing the sprue 56a of the cavity 56 is provided at the interface between the sprue 56a and the molten metal passage 58. The gate mechanism 70 includes a cylindrical gate tip 70a closed at the top and a hollow coupling member 70b coupling the gate tip 70a and a moving mechanism (not shown). The gate tip 70a is accommodated in a central axial hole 52a formed in the upper die 52 such that it is substantially in close contact with the wall surface of the hole 52a. The gate tip 70a can be displaced vertically through the hole 52a when the moving mechanism is operated. When the gate tip 70a is lowered upon operation of the moving mechanism to bring its end face into contact with the surface of the lower die 54, the sprue 56a of the cavity 56 is closed by the outer surface of the gate tip 70a. At this time, the inner space in the gate tip 70a is communicated with the molten metal passage 58. When the gate tip 70a is raised so that its end face is separated from the surface of the lower die 54, the sprue 56a of the cavity 56 is opened. As a result, the inner space in the gate tip 70a is communicated with both the molten metal passage 58 and cavity 56. An O-ring 72 made of heat-resistant rubber is provided between the outer surface of the gate tip 70a and the inner wall surface of the hole 52a of the upper die 52 to prevent deterioration of the seal due to vertical frictional movement of the gate tip 70a. As shown in the enlarged sectional view of FIG. 2, the gate tip 70a has a heat insulation layer 70e, which is formed by spraying a ceramic material onto the inner wall surface of its cylindrical part (with an inner diameter of 80 mm and a height of 220 mm). Its top ceiling has a central sintered vent 70d, which does not pass the molten metal 62 but passes gas only. To the center of the top surface (i.e., ceiling surface) of the gate tip 70a is connected the lower end of the cylindrical coupling member 70b coupling the gate tip 70a to the moving mechanism noted above. The pin 70b, as shown in FIGS. 2 and 3, is coaxial with the gate tip 70a and has an inner axial exhaust passage 70c. The lower end of the exhaust passage 70c is communicated with the interior of the gate tip 70a via the permeable sintered vent 70d. The upper end of the exhaust passage 70c is connected to a pressure reducing unit (not shown). When the pressure reducing unit is driven while the sprue 56a of the cavity 56 is closed by the gate tip 70a, a negative pressure is produced in the molten metal passage 58, causing the molten metal 62 stored in the melting furnace 60 to be sucked through the stalk 58b into the gate tip 70a. The extent of reduction of the pressure in the stalk 58b is set to about 400 mm Hg (5.332 x 10 4 Pa). Inside the gate tip 70a a molten metal temperature sensor 74 is disposed at a predetermined position, as shown in FIG. 3(A). The output signal of the molten metal temperature sensor 74 is inputted to a controller 76. In the controller 76, the temperature detected by the molten metal temperature sensor 74 is compared to a reference temperature (which is set to be lower than the molten metal temperature). If the detected temperature is above the reference temperature, it is judged that the molten metal 62 has reached the position of the molten metal temperature sensor 74. When the position of the molten metal temperature sensor 74 is reached by the molten metal 62, the controller 76 outputs a signal for closing a valve 80 provided on the exhaust passage 70c. When the exhaust passage 70c is closed by the valve 80, the suction of the molten metal 62 through the stalk 58b is stopped even if the pressure reducing unit is operative, and thus the level of the molten metal in the gate tip 70a is held at the position of the molten metal temperature sensor 74 as shown in FIG. 3(C). The position of the molten metal temperature sensor 74 is set such that the heat of molten metal comes to be in a proper range of molten metal head position as shown in FIG. 2. The proper range of molten metal head position is such that the molten metal 62 stored in the gate tip 70a does not contact the sintered vent 70d and also that the molten metal 62 in excess of the amount to be charged into the cavity 56 can be stored at a level higher than that of the cavity 56. More exactly, a volume in excess of the cavity volume is secured at a level higher than the level of the upper end of the sprue 56a opened and closed by the gate mechanism 70. With the molten metal head level thus held in the proper range, the molten metal 62 does not contact the sintered vent 70d, and thus there is no possibility of clogging of the sintered vent 70d. This status of the inside of the gate tip 70a is illustrated in FIG. 8. It is to be appreciated that the inner space of the gate tip 70a serves as a molten metal reservoir and that the exhaust passage 70c, pressure reducing unit, molten metal temperature sensor 74, controller 76 and valve 80 constitute a molten metal supply mechanism for supplying molten metal to the molten metal reservoir. In the molten metal supply mechanism of this embodiment, the molten metal 62 is supplied by suction to the gate tip 70a. However, this is by no means limitative, and it is possible to pressurize the molten metal 62 in the melting furnace 60 and supply it to the inner space of the gate tip 70a, as will be described in detail in connection with a second embodiment. As shown in FIG. 1, the cavity 56 is communicated with a pressure reduction passage 52c formed in the upper die 52 via a gap 53 formed between the engagement surfaces of the upper and lower dies 52 and 54. The pressure reduction passage 52c is connected to a vacuum pump (not shown). An O-ring 53a made of a heat-resistant rubber is provided between the engagement surfaces of the upper and lower dies 52 and 54 adjacent the edge thereof to ensure airtightness between the cavity 56 and the outside of the casting die 50. With this arrangement, when the vacuum pump is operated with the sprue 56a held closed by the gate mechanism 70, the pressure in the cavity 56 is reduced down to a predetermined pressure (below 20 Torr (2.666 x 10 3 Pa)). Above the cavity 56, a pressurizing piston 52d is provided to pressurize the molten metal 62 charged in the cavity 56. The pressurizing piston 52d is capable of vertically sliding along a cylinder 52e formed in the upper die 52, and it is operated by a piston drive mechanism (not shown). The pressure applied by the pressurizing piston 52d is set to 200 to 1,000 kg/cm 2 (1.96 x 10 7 to 9.8 x 10 7 Pa). The pressure reduction passage 52c and the vacuum pump constitute pressure reducing means for reducing the pressure in the cavity. The operation of this casting apparatus will now be described with reference to FIGS. 4 to 8. First, in a state shown in FIG. 4, with the sprue 56a of the cavity 56 held closed by the the gate mechanism 70, the vacuum pump is operated to reduce the pressure in the cavity 56. Meanwhile, the pressure reduction unit is operated to reduce the pressure in the molten metal passage 58 and the gate tip 70a, thus causing the molten metal 62 stored in the melting furnace 60 to be sucked through the stalk 58b into the gate tip 70a. At this time, the molten metal temperature sensor 74, controller 76 and valve 80 are operated to hold the head level of the molten metal 62 sucked into the gate tip 70a in the proper range as noted above. That is, the molten metal 62 stored in the gate tip 70a doesnot contact the sintered vent 70d, and the molten metal 62 in excess of the amount to be charged into the cavity 56 is stored at a level higher than the level of the cavity 56 (more accurately the height of the upper end of the sprue 56a). In this state, the gate mechanism 70 is then raised to open the sprue 56a of the cavity 56, as shown in FIG. 6, thus causing the molten metal 62 having been stored in the molten metal passage 58 and the gate tip 70a to be sucked quickly into the cavity 56. If the molten metal is sucked slowly into the cavity 56 at this time, the molten metal head in the gate tip 70a is not substantially lowered because the molten metal supplied to the cavity 56 is sucked from the melting furnace 60 through the stalk 58. However, when the molten metal is sucked quickly as in the casting apparatus of this embodiment, the amount of molten metal fed through the molten metal passage 58 is insufficient. This insufficiency is made up for by the molten metal stored in the gate tip 70a. Therefore, the molten metal head in the gate tip 70a is lowered. However, the molten metal 62 in excess of the amount to be charged into the cavity 56 is stored in a portion of the gate tip 70a higher in level than the sprue 56a of the cavity 56. For this reason, when the molten metal stored in the gate tip 70a is supplied to the cavity 56, the molten metal head in the gate tip 70a does not become lower than the sprue 56a of the cavity 56. Thus, even when the molten metal 62 is sucked quickly into the cavity 56, gas floating above or foreign matter floating on the molten metal head in the gate tip 70a is not sucked into the cavity 56. In other words, in this embodiment an upper space in the gate tip 70a serves as an accommodation space to accommodate the gas or foreign matter, and the accommodated gas or foreign matter is not sucked into the cavity 56. When the cavity 56 is filled with the molten metal 62 in this way, the gate mechanism 70 is lowered to close the sprue 56a of the cavity 56 again, as shown in FIG. 7. Then, the molten metal 62 in the cavity 56 is pressurized with a predetermined pressure by the pressurizing piston 52d. Concurrently, the exhaust passage 70c is opened to atmosphere to cause the molten metal 62 having been stored in the molten metal passage 58 and the gate tip 70a to be returned to the melting furnace 60. FIG. 9 is a graph comparing the status of generation of blowholes (as a result of bubbles) in the casting obtained with the casting apparatus of this embodiment and that of the casting obtained with the prior art casting apparatus. In the graph, the ordinate is taken for the amount of blowholes, and the abscissa is taken for the number of shots. The blowhole generation status was evaluated according to the area of blowhole projection through X-ray photography of each casting. As is seen from the graph, with the casting obtained with the prior art casting apparatus the blowhole amount is large and fluctuates considerably. In contrast, with the casting obtained with the casting apparatus of this embodiment the blowhole amount is small and stable. This is so because substantially no gas is sucked into the cavity. A second embodiment of the invention will now be described with reference to FIGS. 10 to 15. The second embodiment is different from the preceding first embodiment in the molten metal supply mechanism for supplying molten metal to the molten metal reservoir. More specifically, while in the first embodiment the molten metal 62 is introduced into the molten metal reservoir by the pressure reducing unit connected to the top of the gate tip 70a, in the second embodiment the introduction is effected by applying a pressure to the surface of the molten metal 62 stored in the melting furnace 60. In addition, the second embodiment is different from the first embodiment in the shape of the gate tip. FIG. 10 is a sectional view of the essential parts of a casting apparatus according to the second embodimnet. This casting apparatus comprises a casting die 150 including an upper die 152 and a lower die 154. In the casting die 150, a central cavity 156 is formed when the upper and lower dies 152 and 154 are engaged. The cavity 156 has a central sprue 156a formed via a dam portion 156b. At the sprue 156a, the cavity 156 is communicated with a molten metal passage 158. The molten metal passage 158 is constituted by a passage 158a formed in the center of the lower die 154 and a cylindrical stalk 158b connected to the lower surface of the lower die 154 in such a manner as to be communicated with the passage 158a. During casting, the free end of the stalk 158b is immersed in a molten metal 162 stored in a melting furnace 160. An O-ring 164a made of heat-resistant rubber is provided between the engaged surfaces of the lower die 154 and the stalk 158b to ensure airtightness of the molten metal passage 158. The melting furnace 160 has its top opening closed by a lid 160h having a central hole 160k which is penetrated by the stalk 158b. Between the lid 160h and the lower surface of the lower die 154, a substantially cylindrical seal member 160s is disposed in such a manner as to surround the stalk 158b. O-rings 164b made of heat-resistant rubber are provided for sealing between the seal member 160s and the lower die 154 and also between the seal member 160s and the lid 160h. A further O-ring 164b made of heat-resistant rubber is provided for sealing between the lid 160h and the melting furnace 160. With this arrangement, the interior of the melting furnace 160 is held air-tight. In FIG. 10, designated at 161 is a heater for thermally melting metal. To the melting furnace 160 is connected a piping 160a which is communicated with a pressurizing unit (not shown). When pressurized gas is forced from the pressurizing unit through the piping into the melting furnace 160, a predetermined pressure is applied to the surface 162a of the molten metal 162 in the melting furnace 160, whereby the molten metal 162 is partly raised into the molten metal passage 158 via the stalk 158b. The pressure applied to the molten metal 162 is set according to the height to which molten metal is to be raised. In the pressurizing unit, an inner pressure vent valve (not shown) is mounted in such a manner as not to apply a pressure above 0.45 kg/cm 2 maximum (with respect to 0 kg/cm 2 as atmospheric pressure) to the melting furnace 160. During casting, usually a pressure of 0.25 kg/cm 2 is applied, as will be described later. A gate mechanism 170 for opening and closing the sprue 156a of the cavity 156 is provided at the interface between the cavity 156 and the molten metal passage 158. The gate mechanism 170 includes a cylindrical gate tip 173 and a mechanism (not shown) for axially displacing the gate tip 172. The gate tip 172 is accommodated in a central axial hole 152a formed in the upper die 152 such that it is substantially in close contact with the wall surface of the hole 152a. The gate tip 172 can be displaced vertically through the hole 152a when the moving mechanism is operated. When the gate tip 172 is lowered upon operation of the moving mechanism to bring its end face 172s into contact with the surface of the lower die 154, the sprue 156a of the cavity 156 is closed by the outer surface of the gate tip 172. At this time, the inner space of the gate tip 172 is communicated with the molten metal passage 158, and the molten metal passage 158 is open to atmosphere. When the gate tip 172 is raised so that its end face 172s is separated from the surface of the lower die 154 to open the sprue 156a of the cavity 156, the interior of the gate tip 172 is communicated with both the molten metal passage 158 and cavity 156. An O-ring 173 made of heat-resistant rubber is provided between the outer surface of the gate tip 172 and the surface of the hole 152a of the upper die 152 to prevent deterioration of the seal due to vertical sliding of the gate tip 172. The gate tip 172 is a cylindrical member having a height of 700 mm. As shown in FIG. 11, the gate tip 172 includes an outer cylinder 172a made of a metal and an inner cylinder 172b made of a ceramic material. The inner cylinder 172b has a flange 172f formed adjacent its top. Its outer diameter is less than the inner diameter of the central axial passage 158a formed in the lower die 154. Thus, the inner cylinder 172b is not in contact with the lower die 154 when the end face 172s of the gate tip 172 is in contact with the surface of the lower die 154, and it is not readily broken although it is made of a ceramic material and fragile. The outer cylinder 172a covers the inner cylinder 172b, and its inner diameter is made to be substantially equal to the outer diameter of the inner cylinder 172b. Its top surface is formed with a recess 172m for accommodating the flange 172f of the inner cylinder 172b. With the flange 172f of the inner cylinder 172b accommodated in the recess 172m, a ring-like retainer 172r for retaining the flange 172f is secured by bolts 172n to the top surface of the outer cylinder 172a. With this arrangement, the inner cylinder 172b is reliably secured by the flange 172f to the outer cylinder 172a. Packing members 172c are provided for sealing between the flange 172f of the inner cylinder 172b and the ring-like retainer 172r and also between the flange 172f and the recess 172m of the outer cylinder 172a. In this embodiment, the interior of the gate tip 172 serves as a molten metal reservoir. The cavity 156 formed in the casting die 150, as shown in FIG. 10, is communicated with a pressure reduction passage 152c formed in the upper die 152 via a gap 153 formed between the engagement surfaces of the upper and lower dies 152 and 154. The pressure reduction passage 152c is connected to a vacuum pump (not shown). An O-ring 153a made of heat-resistant rubber is provided between the engagement surfaces of the upper and lower dies 152 and 154 adjacent the edge thereof to ensure airtightness between the cavity 156 and the outside of the casting die 150. With this construction, when the vacuum pump is driven with sprue 156a held closed by the gate tip 172, the pressure in the cavity 156 is reduced to a predetermined extent (i.e., to 20 Torr (2.666 x 10P3 Pa) or below). Above the cavity 156, a pressurizing piston 152d is provided to pressurize the molten metal 162 charged in the cavity 162. The pressurizing piston 152d is capable of vertically sliding along a cylinder 152e formed in the upper die 152, and it is operated by a piston drive mechanism (not shown). The pressure applied by the pressurizing piston 152d is set to 200 to 1,000 kg/cm 2 (1.96 x10 7 to 9.8 x 10 7 Pa). The operation of this casting apparatus will now be described with reference to FIGS. 12 to 15. First, as shown in FIG. 12, with the sprue 156a held closed by the gate mechanism 170, the pressure in the cavity 156 is reduced by the vacuum pump. Also, a pressure of about 0.25 kg/cm 2 is applied to the surface 162a of the molten metal 162 in the melting furnace 160 with the supply of pressurized gas from the pressurizing unit to the melting furnace 160, thus causing the molten metal 162 to be raised into the gate tip 172 via the stalk 158b, as shown in FIG. 13. When a pressure of 0.25 kg/cm 2 is applied to the melting furnace 160, the surface of the molten metal in the gate tip 172 is raised to a height of about 250 mm from the surface of the lower die 154 (i.e., parting surface). With the molten metal surface at the height of 250 mm from the surface of the lower die 154, the molten metal 162 in excess of the amount to be charged into the cavity is stored in a portion of the gate tip 172 higher in level than the the upper end level of the sprue 156a of the cavity 156. If a pressure of 0.45 kg/cm 2 (i.e., maximum pressure) is applied, the molten metal surface is raised to a height of 450 mm from the surface of the lower die 154. However, since the gate tip 172 has a height of 700 mm as noted above, the molten metal 162 does not overflow from the top of the gate tip 172 even when the maximum pressure is applied to the melting furnace 160. When the molten metal 162 is supplied to the gate tip 172 up to a predetermined level while the pressure in the cavity 156 is reduced to a predetermined extent, the gate tip 172 is raised to open the sprue 156a of the cavity 156. Thus, the molten metal 162 stored in the molten metal passage 158 and gate tip 172 is sucked quickly into the cavity 156. As the molten metal 162 in the molten metal passage 158 and the gate tip 172 is sucked into the cavity 156, the molten metal surface in the gate tip 172 is lowered. However, the molten metal 162 in excess of the amount to be charged into the cavity 156 has been stored in a portion of the gate tip 172 higher in level than the sprue 156a of the cavity 156. Thus, with the supply of part of the molten metal stored in the gate tip 172 to the cavity 156, the level of the molten metal surface in the gate tip 172 does not become lower than the spruce 156a of the cavity 156. That is, gas present or foreign matter floating atop the molten metal surface in the gate tip 172 is not sucked into the cavity 156. In this case, the space in the gate tip 172 over the molten metal surface serves as the accommodation space for accommodating the gas or foreign matter. It will be seen that the accommodation space need not be isolated from atmosphere. When the molten metal 162 has been charged into the cavity 156 in the above way, the gate piston 172 is lowered again to close the sprue 156a of the cavity 156 as shown in FIG. 15. Then, the molten metal 162 in the cavity 156 is pressurized with a predetermined pressure applied by the pressurizing piston 152d. Concurrently, the interior of the melting furnace 160 is opened to atmosphere, whereby the molten metal 162 having been stored in the molten metal passage 158 and the molten metal reservoir 180 is returned to the melting furnace 160. As has been shown, in this embodiment the gate tip 172 (i.e., molten metal reservoir) has sufficient volume and height, and thus compared to the previous first embodiment the control of the position of the molten metal surface in the gate tip 172 may be rough, and also the construction of the mechanism for supplying molten metal 162 to the molten metal reservoir may be simplified. Further, the gate tip 172 may be simplified in construction and thus reduced in cost. Further, the gate tip 172 may be replaced simply because it can be taken out from above the upper die 152.

Claims: A vacuum casting apparatus comprising a cavity (56) formed inside a casting die (50), pressure reducing means (52c) for reducing the pressure of the cavity, a molten metal passage (58), capable of being placed in communication with and leading molten metal to the cavity, gate means (70) for on-off switching the communication between the cavity and the molten metal passage, and a molten metal reservoir (70a) for temporarily storing the molten metal to be sucked into the cavity, said molten metal reservoir being formed by making the volume of a portion of said molten metal passage higher in level than the level of said gate means, to be greater than the volume of said cavity so that gas and foreign matter floating atop the molten metal stored in said molten metal reservoir does not enter said cavity even when a major portion of the molten metal stored in said molten metal reservoir is sucked into said cavity, wherein the molten metal passage (58) is capable of being placed in communication with both the cavity (56) and a molten metal tank (60) and leading molten metal in the molten metal tank to the cavity, the molten metal reservoir includes gas-discharging means (70c) set at a level higher than the level of a surface of the molten metal (62) temporarily reserved in said molten metal reservoir (70a), and wherein said molten metal reservoir extends vertically so that the gas is not trapped during introduction of the molten metal into said molten metal reservoir. A vacuum casting apparatus according to Claim 1 wherein said gate means (70) has an internal cavity constituting said molten metal reservoir (70a). A vacuum casting apparatus according to Claim 2 wherein said gate means (70) includes a cylindrical gate tip (70a) which defines a wall of said molten metal reservoir as well as providing a gate closure member. A vacuum casting apparatus according to any one of Claims 1 to 3 including a molten metal temperature sensor (74) disposed in said molten metal reservoir below the upper end thereof, and control means (76,80) responsive to said sensor (74) to control said gas-discharging means (70c) so that the level of molten metal in said reservoir is held at the level of said sensor (74).

Inventors: ARAKAWA YASUYUKI, ONISHI HIROKAZU, OTA ATSUSHI, TAMURA SHIGEKI, UOZUMI MINORU, ARAKAWA, YASUYUKI, ONISHI, HIROKAZU, OTA, ATSUSHI, TAMURA, SHIGEKI, UOZUMI, MINORU, ARAKAWA, YASUYUKI 2, MURAKUMO-CHO 6-CHOME, OTA, ATSUSHI 3-14, ITSUTSUGAOKA 2-CHOME, TAMURA, SHIGEKI 46, MINAMIIYAMA 17-CHOME UKIGAI, UOZUMI, MINORU 12-16, SHIRATORI 2-CHOME
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1992
Main classification: B22D18
IPC codes: B22D18