Die casting method

Abstract: This invention relates to a die casting method characterized in that powder-like thermal insulation agent is coated on the cavity surfaces of a die when a product is to be casted by using a die casting machine, thereafter molten metal is filled in said cavity at a low speed, and upon completion of filling operation, a high pressure is applied to the molten metal.

Description: Field of the Invention and Related Art Statement Although there are various types of casting methods such as a gravity casting method, a die casting method, a low pressure casting method and the like, each of them has merits and demerits. That is, in case of a gravity casting method or low pressure casting method, molten metal fills a cavity at a low pressure and a low speed, resulting in that a minute casting having a superior mechanical characteristic and anti-pressure characteristic can be obtained. On the contrary, this type of casting has a certain limitation in its product shape and product thickness, and productivity is poor. In turn, in case of die casting method, since the molten metal fills the cavity at a high speed and under a high pressure, it is possible to obtain castings of a high size accuracy at a high productivity. On the contrary, this casting method may result in gas in the injecting sleeve or within a cavity to be taken into the casting so as to easily make pin holes therein and has the disadvantage that it is hard to cast castings having a uniform high quality with high reliability. Thus, in U.S. Patent 3,472,308 there is disclosed a method and apparatus for permanent mould casting in which a product is cast by charging a quantity of molten material into a part of an open mould, and by moulding the charge by a rapid and non-turbulent filling of the mould cavity, by bringing the mould parts together. This is a high-speed high pressure method, and it has the disadvantage that the mould cavity initially embodies air which has to be displaced as the mould parts are brought together and some of said air may become entrapped in the moulded material and subsequently create pinholes therein. Applying a thermally resistant oxide coating on the interior surfaces of the cavity in order to influence solidification as disclosed in the said U.S. Patent may reduce such defects. It, however, remains difficult to obtain consistently good casting products. Object and Summary of the Invention It is an object of the present invention to provide a new die casting method in which castings of high quality with less defects such as pin holes can be cast with high productivity. According to the invention there is provided a die casting method comprising the steps of: providing a powder like insulation agent, applying said powder like insulation agent on the interior surfaces of a die cavity of a die casting machine to form a crushable, porous layer on said surfaces; pouring a molten metal at a slow pace into said die cavity having said porous layer so as fill the cavity; and mechanically pressurising said molten metal, after completion of the filling of said die cavity, so as to crush and reduce the thickness of said porous layer so that said molten metal sinks into and passes through said crushed porous layer to reach said interior surfaces of said die cavity. Brief Description of the Drawings Fig.1 is a schematic illustration showing the condition in which powder-like thermal insulation agent is coated on a cavity surface. Fig.2 is a photograph showing the solidification structure of a casting cast by a casting method of the present invention. Fig.3 is a photograph showing the solidification structure which results when a high pressure is not applied to molten metal filling the cavity. Fig.4 is a photograph showing the solidification structure of a casting cast by a conventional type of high pressure die casting method. Detailed Description of Preferred Embodiments The die casting method of the present invention is composed of steps of coating powder-like thermal insulation agent to the cavity surfaces of a fixed die and a movable die (hereinafter merely called as a die) set in a die casting machine. Thereafter the cavity is filled by injection of a molten metal into the above-mentioned cavities at a low speed and then a high pressure is applied to the molten metal filled in the above-mentioned cavity. That is, a thermal insulation layer composed of powder-like thermal insulation agent and air is formed on the cavity surfaces of the dies by coating the powder-like thermal insulation agent on the cavity surfaces of the dies ( a coating step) and thereafter the cavity is filled with molten metal at a low speed (an injecting step); Thereby the molten metal injected into the cavity is not directly contacted with the cavity surfaces at first, so that solidification of the molten metal filling the cavity is restricted by the heat insulation action provided by the above-mentioned thermal insulation layer. Upon completion of filling of molten metal into the cavity, a high pressure is applied to the molten metal (a pressurizing step) to cause the above-mentioned thermal insulation layer to be thin crushed and reduced in thickness and at the same time the molten metal oozes through and out of the above-mentioned thermal insulation layer and contacts with the cavity surfaces, resulting in that the molten metal is rapidly solidified and cast. As powder-like thermal insulation agent to be coated on the cavity surfaces of the dies, it may be possible to apply powder which does not react with the molten metal. For example, powder having an electrical charging characteristic such as boron or talc or the like, powder such as metal oxide or metal sulfide, metal nitride etc., or powder mixed with resin powder and the like may be used. In particular, it is preferable to use a powder having a self-lubricating characteristic when in powder form in order to improve the die removal characteristic of the die cast product from the cavities. Further, as a practical powder-like thermal insulation agent, it is possible to apply stearate reacted between stearic acid and each of sodium, magnesium, zinc, calcium or the like; resin powder such as fluorine resin, phthalocyanine, polyethylene and polypropylene or the like; indium, lead, black lead, molybdeum disulfide or metal oxide such as Na₂O, BeO, MgO, Al₂O₃, SiO₂, CaO, TiO₂, Cr₂O₃, MnO₂, Fe₂O₃, FeO, MnO, PbO or the like; talc, spinel, mullite etc. or mixtures of these oxides; single substance or a plurality of mixtures such as WC, TiN, TiC, B₄C, TiB, ZrC, SiC, Si₃N₄, BN etc. As a practical particle diameter of the powder-like thermal insulation agent it is preferable to have a value of 0.2 mm or less as particles with a greater diameter may cause the powder coated on the cavity surfaces to be easily peeled off. For coating powder-like thermal insulation agent on the cavity surfaces of the dies, there are several methods such as a spraying method in which gas such as air is applied as carrier, an electrostatic coating method utilizing static electricity or a method in which powder-like thermal insulation agent, for example, found in a rosin bag is filled in a cloth bag, and then the bag is rubbed and struck against them to coat the agent on the surfaces. In these methods, it is the most preferable to provide an electrostatic coating process in which powder-like thermal insulation agent may easily be coated in uniform manner without any irregular thickness as well as without having any relation with the temperature of the die. Although the thickness of the powder-like thermal insulation agent to be coated on the cavity surfaces of the dies, in other words, the thickness of the thermal insulating layer formed by the powder-like thermal insulation agent and air is not specifically limited, as to particle diameter of the powder-like thermal insulation agent, it is preferable to set a thickness as small as possible so as to enable the molten metal supplied which fills the cavity of the die to be kept for a period (several seconds at the longest) before the pressurizing step is performed. In Fig.1 is illustrated a schematic illustration showing the powder-like thermal insulation agent coated on the cavity surfaces of the dies. In this figure, 1 denotes a cavity, 2 a powder-like thermal insulation agent, 3 air and 4 a thermal insulation layer formed by the powder-like thermal insulation agent 2 and air 3. In this way, powder-like thermal insulation agent is coated on the cavity surfaces of the dies in every casting cycle so as to form a thermal insulation layer composed of the powder-like thermal insulation agent and air at the cavity surfaces and thereafter the molten metal is injected from an injection sleeve at a low speed into the cavity. The powder-like thermal insulation agent is coated on the inner surface of the injection sleeve in advance, whereby the molten metal fed into the injection sleeve is prevented from being solidified for a period until the molten metal is injected into the cavity of the die (several seconds at the longest) and further can be kept in the cavity without being solidified, resulting in that even if an injecting speed substantially less than that of the conventional type (for example, 0.05 m/s to 1 m/s), a better movement of molten metal is assured and thus a cast product having a high quality can be obtained in a stable manner. The molten metal is injected through the injecting sleeve and filled into the cavity gradually at a low speed of less than about 1 m/s substantially in the same manner as that of the conventional gravity casting process or a low pressure casting process. If the filling speed is made too fast, gas in the cavity is easily drawn into the molten metal and at the same time the thermal insulation layer (powder-like thermal insulation agent) formed at the cavity surfaces may be peeled off under the force of the flowing molten metal. After the molten metal fills the cavity within the dies, the pouring gate is closed and a high pressure is applied by pushing a pin etc. to the molten metal. Then, the thermal insulation layer formed at the cavity surfaces of the dies is crushed by pressure of the molten metal and made thin, and simultaneously the molten metal oozes through and out of the thermal insulation layer and contacts with the cavity surfaces, resulting in that the molten metal filling the cavity is rapidly solidified and cast. In addition, in case of applying the high pressure to the molten metal within the cavity, a setting of pin at the gate part for use in applying a high pressure to the molten metal enables the cutting off of the pouring gate after casting, to be facilitated. As described above, the die casting method of the present invention is performed such that powder-like thermal insulation agent is coated on the cavity surfaces of the dies, and thereafter the molten metal is injected into the cavity at a low speed to fill same and a high pressure is applied to the molten metal upon completion of filling the cavity with molten metal, resulting in that the following effects can be attained. 1 ○ When molten metal fills the cavity of the dies, the molten metal does not directly contact the cavity surfaces, but a thermal insulation temperature keeping action provided by the thermal insulation layer formed by the powder-like thermal insulation agent and air may also act against it and so a rapid solidification of the molten metal filled in the cavity can be restricted. Accordingly, the circulation of the molten metal is improved and no seizure of molten metal is produced. Even a cast product having a complex shape or a thin cast product may be cast in a stable manner and further even if the filling speed is substantially reduced, it is possible to cast a cast product having a superior cast surface with less defects. 2 ○ Since it is possible to dampen a rapid shock in temperature at the cavity surfaces in the dies, it is also possible to extend substantially the life of the dies. 3 ○ As powder-like thermal insulation agent, powder having a self-lubricating characteristic is applied, whereby it is possible to eliminate a mold releasing agent coating step for the die cavity and an air blowing step and so it is also possible to shorten the casting cycle and at the same time a conventional type of mold releasing agent using liquid carrier is not required, resulting in that poor circulation due to a mold releasing agent, gas sucking due to a carrier contained in the mold releasing agent and poor remained water due to a lack of blown air are not being generated and so it is possible to improve the quality of product. 4 ○ Since molten metal is filled in the cavity in the dies at a low speed, no sucking in of gas during filling operation takes place and so it is possible to perform with high reliability the stable casting of cast products having less pin holes and having a high quality. 5 ○ In the case of performing a low speed filling operation, a range of proper filling time and filling speed was extremely limited in the conventional process due to a possibility of producing a poor circulation of molten metal. However, in the process of the present invention, since it is possible to restrict rapid solidification of molten metal filled in the cavity, the range of proper filling time and filling speed can be wide. 6 ○ When the high pressure is applied to the molten metal after completion of filling of molten metal in the die cavity, the thermal insulation layer formed by powder-like thermal insulation agent formed on the cavity surfaces and air is crushed by the pressure on the molten metal and is made thin and at the same time the molten metal oozes through and out of the thermal insulation layer and is contacted with the cavity surfaces. The molten metal is rapidly solidified, resulting in that the entire casting cycle time can be set to the same degree as that of the high pressure die casting process and then as apparent from the photograph of structure of the accompanying drawings, it is possible to make a fine cast product as highly accurate sized as that of the high pressure die casting process. 7 ○ In brief, according to the die casting process of the present invention, it is possible to cast with high reliability a fine cast product having superior mechanical characteristics, superior anti-pressure characteristics with less defects, which characteristics are advantages of the conventional type of gravity casting process and the low pressure casting process. The products may have a complex shape, which is an advantage of the high pressure die casting process, and may be obtained with a superior cast surface, a high productivity and an accuracy in size.

Claims: A die casting method comprising the steps of: providing a powder like insulation agent, applying said powder like insulation agent on the interior surfaces of a die cavity of a die casting machine to form a crushable, porous layer on said surfaces; pouring a molten metal at a slow pace into said die cavity having said porous layer so as to fill the cavity; and mechanically pressurising said molten metal, after completion of the filling of said die cavity, so as to crush and reduce the thickness of said porous layer so that said molten metal sinks into and passes through said crushed porous layer to reach said interior surfaces of said die cavity.

Inventors: AOYAMA SHUNZO, AOYAMA, SHUNZO
Applicants: AHRESTY CORP, AHRESTY CORPORATION
Application year: 1989
Main classification: B22D27
IPC codes: B22D17, B22C9, B22D18, B22D27