Background Art In general, a low pressure casting process is intended to gradually cast molten metal at low pressure from an a furnace underneath of a mold and then to allow the molten metal to be solidified, differently from a common die casting process.

Such a low pressure casting process is predominantly used to produce engine blocks, cylinder heads, wheels, or the like, because the molten metal produces few casting defects and limits inclusion of extraneous substances, such as oxides, and it is possible to produce precision casting products.

This type of low pressure casting process will be now more specifically described.

First, a mold having therein a molding cavity of a shape the same as that of a desired product, and a molten-metal holding furnace filled at its lower portion with molten metal are prepared. The molten metal received in the molten-metal holding furnace is gently cast into the molding cavity through one or more runners and casting openings in the mold by applying pressure or elevating a level of the molten metal.

Then, a certain pressure must be exerted on the molten metal and maintained until the molten metal in the mold is completely solidified.

In this way, after the molten metal cast in the mold is completely solidified, the molten metal in the runners that is not solidified yet is returned to the molten-metal holding furnace by eliminating the pressure exerted thereon, and then the mold is opened to enable the molded product to be removed.

Although the above-mentioned low pressure casting process advantageously has a high recovery rate of molten metal because molten metal in runners is directly cast into a molding cavity in a mold through casting openings, flow of the molten metal is concentrated at portions in the vicinities of the casting openings of the mold, thereby causing the portions to be heated to high temperature as compared with other portions of the mold.

In particular, heat transfer from molten metal in the runners to the casting openings continues even after molten metal has been completely cast into the mold.

Consequently, time required for solidification of the molten metal is inevitably delayed owing to the heat concentrated at the portions in the vicinities of casting openings.

For instance, though time required for one cycle of a casting operation in the low pressure casting process may vary depending on a shape and a size of a desired casting product, a time period of 15-20 minutes may be required for casting of a cylinder head.

In that time period, only 1-2 minutes is required for supplying molten metal into a mold, for removing a molded. product from the mold, and for cooling the mold, while the remaining time is required for solidification of molten metal, in particular, for complete solidification of molten metal in vicinities of casting openings.

Therefore, the above-described low pressure casting process has disadvantages in that total time required for one cycle of a low pressure casting operation is lengthened because of heat concentrated at vicinities of pouring gates of a mold, and mechanical properties of molded products are deteriorated because crystals of molded products in adjacent to the vicinities of casting openings become coarse or Dendrite Arm Spacing becomes large.

Furthermore, in accordance with the low pressure casting process, since molten metal is cast through casting openings which are directly communicated to runners,

flowing distance of molten metal from the casting openings to a molding cavity must be lengthened in order to completely fill the molding cavity with molten metal. Therefore, it is necessary for molten metal to have sufficient flowability so as to produce normal products. To this end, since a temperature of molten metal must be considerably elevated, time required for solidification of molten metal is further lengthened.

For overcoming the above-mentioned problems, Japanese Patent Laid-Open Publication No. 10-296423 discloses a method for shortening a time required for solidification of molten metal by forcibly cooling portions of a lower mold. According to the prior art publication, molten metal positioned outside a molding cavity, i. e., in contact with a mold can be rapidly solidified, but molten metal disposed at the center of the molding cavity and molten metal disposed at pouring gates, through which molten metal is cast into the molding cavity, is hardly affected by the cooling of the mold, thereby causing the solidification time to still be long.

Furthermore, the method disclosed in the publication has disadvantages in that flowability of molten metal disposed outside of the molding cavity becomes worse to cause casting ability of the molten metal to be deteriorated owing to supercooling of the molten metal, and mechanical properties of resulting products are also deteriorated owing to great differences among solidification times of molten metal disposed inside and outside of a molding cavity and of molten metal disposed at casting openings.

Alternatively, a method for shortening time required for solidification of molten metal by forcibly cooling only portions around casting openings of a mold, through which the molten metal is cast, is also disclosed. However, since only the portions around the casting openings of a mold are forcibly cooled, molten metal disposed above the casting openings is hardly affected by the cooling operation.

In addition, since molten metal disposed at casting openings of a mold is previously cooled before molten metal received in a molding cavity of the mold is completely cooled, pressurizing effect in the mold is lowered, and thus there are fine casting defects of molded products.

Disclosure of the Invention Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a low pressure casting method and apparatus intended to maximize productivity of casting products by shortening total time required for a casting operation.

It is another object of the present invention to provide a low pressure casting method and apparatus intended to improve productivity of casting products by shortening a total time required casting operation.

In order to accomplish the above object, the present invention provides a low pressure casting method and apparatus adapted to cast molten metal received in a molten- metal holding furnace into a molding cavity of a mold through riser tubes at low pressure and then to solidify the molten metal, including the steps of : preparing a mold having a molding cavity communicated to runners via a plurality of branch gates; casting molten metal received in a molten-metal holding furnace into the runners through riser tubes; distributing the molten metal cast in the runners through a plurality of branch gates; introducing the molten metal into the molding cavity of the mold; and allowing the molten metal introduced in the molding cavity to be cooled.

Brief Description of the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a side elevation view showing an apparatus according to the present invention; and Fig. 2 is a top plan view showing a substantial part of the invention.

Best Mode for Carrying Out the Invention

Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.

A low pressure casting method according to the invention will be first described in detail as follows.

The invention provides a low pressure casting method adapted to cast molten metal received in a molten-metal holding furnace into a molding cavity of a mold through riser tubes at low pressure and then to solidify the molten metal.

A mold having a molding cavity communicated to runners via a plurality of branch gates is prepared. Molten metal received in a molten-metal holding furnace is cast into the runners through riser tubes. Subsequently, the molten metal cast in the runners is distributed through a plurality of branch gates, and then introduced into the molding cavity of the mold. Then, the molten metal introduced in the molding cavity is allowed to cool.

According to the present invention, molten metal in, the molten-metal holding furnace is cast into the runners through the riser tubes under pressure, and then the molten metal in the runners is distributed into the plurality of branch gates and indirectly introduced into the molding cavity. Therefore, since flow volume of the molten metal introduced into the molding cavity is distributed into a plurality of casting openings, each of the casting openings is not excessively heated owing to the reduced flow volume of molten metal. In addition, since heat from hot molten metal in the riser tubes is not directly transferred to the casting openings of the molding cavity to prevent a rise in temperature of the casting openings, time required for solidification of molten metal in the casting openings can be shortened, and molten metal in the molding cavity is uniformly and rapidly cooled, thereby enabling mechanical properties of molded products to be excellent.

Subsequently, a low pressure casting apparatus for embodying the invention will be described with reference to the accompanying drawings, i. e., Figs. 1 and 2.

The apparatus according to the invention includes a mold 10 having a molding cavity of a certain shape, a molten-metal holding furnace 20 receiving molten metal

therein, and riser tubes 30 for casting the molten metal in the molten-metal holding furnace 20 into the molding cavity 15 at low pressure.

The molding cavity 15 of the mold 10 is communicated to runners 40 via a plurality of branch gates 42 disposed around the molding cavity 15. The runners 40 are communicated to at least one riser tube 30.

The runners 40 are provided around the molding cavity 15.

Reference numeral 16 denotes casting openings.

Function and operation of the low pressure casting apparatus according to the invention will be now described.

First, upon applying pressure to inside of the molten-metal holding furnace 20, the molten metal received in the molten-metal holding furnace 20 is raised through the riser tubes 30 and then cast into the runners 40 of the mold 10.

As pressure is applied to the molten-metal holding furnace 20, the molten metal cast in the runners 40 is distributed through the plurality of branch gates 42 and then indirectly filled into the molding cavity 15 of the mold 10.

At this point, since the molding cavity 15 is filled with molten metal through the plurality of branch gates 42 disposed around the molding cavity, flowing distance of molten metal can be shortened.

Therefore, since the molten metal is introduced into the molding cavity 15 through the plurality of casting openings 16, overheating of the casting opening portions is alleviated. Furthermore, since the molten metal in the riser tubes 30 is indirectly introduced into the molding cavity 15 through the runners 40, heat transfer from the riser tubes 30 is diminished to cause time required for solidification of molten metal to be shortened.

In addition to this, since flowing distance from each of casting openings 16 to the molding cavity 15 can be shortened, there is no need to excessively elevate temperature of molten metal in order to enhance its flowability, and thus time required for solidification of molten metal can be further shortened.

Additionally, since it is possible to prevent overheating of the casting openings 16 caused by molten metal introduced therethrough, cooling velocity of the cast molten

metal becomes uniform and rapid, and defects of molded products become fewer.

Furthermore, mechanical properties of the molded products are improved due to achievement of fine grain crystals and reduction of Dendrite Arm Spacing.

In this way, when solidification of molten metal in the casting openings 16 as well as in the molding cavity 15 of the mold 10 is completed, the application of pressure is stopped to return molten metal remaining in the riser tubes 30 and the runners 40 to the molten-metal holding furnace 20, and then the molded product is removed from the molding cavity 15.

(Example) A conventional method (wherein riser tubes are communicated. to a molding cavity) and the present invention (wherein a molding cavity is communicated at its both sides to runners through ten branch gates, and the runners are communicated to four riser tubes) are tested so as to compare both methods.

In the test, molten metal of AC4A aluminum alloy which is heated to the temperature of 680°C is cast at the flow rate of 0. 2m/sec under the pressure of 8kPa.

[Figure 1] Result from the conventional method Pressure [l'). S Smin ICmfn SGmin Time (t) [Figure 2] Result from the present invention <BR> Pressure (P)

Time (t) As can be seen from Figure 1, according to a conventional method, time required until removal of pressure exerted to molten metal was 20 minutes and Dendrite Arm Spacing was about 50-60, u m.

As can be seen from Figure 2, according to the present invention, time required until removal of pressure exerting to molten metal was 5minutes and Dendrite Arm Spacing was about 25 A m or less.

Industrial Applicability As described above, the present invention is capable of shortening time for production of molded products to maximize productivity by preventing overheating of casting openings owing to flow of molten metal concentrated thereto and by lowering temperature of molten metal by shortened flowing distance of molten metal.

Furthermore, defects of molded products are reduced due to uniform and rapid solidification of molten metal, and reliability of molded products is maximized due to fine grain crystals thereof.