1. Field of the Invention
The present invention relates to a metal casting fabrication method applicable for forming a metal housing of a notebook computer, a mobile telephone or the like. The present invention also relates to a metal casting produced by such a method.
2. Description of the Related Art
Mobile devices such as notebook computers and cellular phones should not weight very much. For the purposes of reducing weight (and some other purposes as well), their housings may be made of lightweight metal such as magnesium alloy or aluminum alloy. Since great precision is possible, such a metal housing is often formed by die-casting, whereby molten metal is injected under pressure into a cavity (xe2x80x9cdie cavityxe2x80x9d) defined by dies, or molds. A forming technique by die-casting is disclosed in JP-A-9(1997)-272945 for example.
Though great precision is attained, die-casting has a drawback as follows. Specifically, molten metal injected into the die cavity will harden by being cooled by the cold dies. The problem occurs when the die cavity includes a narrow portion (whose width is smaller than 1.5 mm for example). Since the narrow portion cools the molten metal quickly, the metal impelled into the narrow portion may harden prematurely before it fills the narrow portion. Accordingly, an unfilled space is left in the die cavity.
The above problem may be addressed by a method disclosed in JP-A-2000-223855. In accordance with the teaching of JP-A-2000-223855, a metal object including small-width portions is formed by the combination of a die-casting and a non-die-casting techniques. Specifically, a metal object to be produced may include a first narrow portion and a second narrow portion continuous with the first narrow portion. The second narrow portion has a smaller width than the first narrow portion. To produce this metal object, the second narrow portion is prepared beforehand, separately from the first narrow portion, by a non-die-casting technique. The obtained second narrow portion is placed in the die cavity. Then, molten metal is injected into the die cavity. As a result, the broader first narrow portion will be formed in contact with the inserted second narrow portion.
In the method of JP-A-2000-223855, however, the first narrow portion is still formed by die-casting. Therefore, the above-mentioned problem (the occurrence of an unfilled space) may result in the first narrow portion. Another problem is caused by the direct contact of the second narrow portion with the dies. In this contact arrangement, the heat of the molten metal dissipates easily via the second narrow portion. As a result, the mechanical properties of the first narrow portion fail to be uniform in a region thereof adjacent to the joint between the first and the second narrow portions. Disadvantageously, this makes unstable the connection of the first narrow portion to the second narrow portion.
JP-A-5(1993)-177333, JP-A-7(1995)-255607 and JP-A-11(1999)-104798 also teach methods whereby a metal member is inserted in the die cavity before injection of molten metal is performed. These techniques, however, have been proposed in view of improving the surface condition of magnesium alloy or aluminum alloy, which has poor heat and corrosion resistance, but not for the purposes of forming a thin-walled portion properly by die-casting.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a metal casting fabrication method whereby a thin-walled portion is properly formed without suffering from the occurrence of an unfilled space in a die cavity. Another object of the present invention is to provide a metal casting produced by such a fabrication method.
According to a first aspect of the present invention, there is provided a metal casting fabrication method that includes the steps of: disposing a metal plate in dies for improving mold-filling properties of molten metal; and forming a casting, or molded member, by injecting the molten metal into the dies.
With such an arrangement, it is possible to prevent misruns that would otherwise happen in a thin-walled portion of the molding cavity during a die-casting process.
Preferably, the method of the present invention may further comprise the step of forming a heat insulating layer on a prescribed surface of the metal plate before the metal plate is disposed in the dies. After the heat insulating layer is formed, the metal plate is disposed in the molding dies in a manner such that the insulating layer is held in contact with the dies. With this arrangement, since the metal plate is thermally insulated from the dies, it is possible to prevent the heat of the injected molten metal from being wastefully conducted to the dies via the metal plate. Accordingly, the injected metal is allowed to maintain its flowability and can fill the molding cavity from end to end. In addition, upon coming into contact with the metal plate, the injected metal is not unduly cooled by the plate owing to the heat insulating layer. Accordingly, the resulting molded member (xe2x80x9ccastingxe2x80x9d) is stably attached to the metal plate. Preferably, at the casting-forming step, the molten metal may be brought into contact with a second surface of the metal plate that is opposite to the above-mentioned first surface (upon which the heat insulating layer is formed), so that the casting is reliably fixed to the metal plate.
Preferably, the method of the present invention may further include the step of forming a bonding layer on the second surface of the metal plate before the plate-disposing step is performed. The bonding layer is designed to improve the bonding strength between the metal plate and the casting so that they are reliably fixed to each other.
The metal plate to be used for the present invention may be made of a light metal (whose density is no greater than 5 g/cm3) such as aluminum, magnesium and titanium, or made of a light metal alloy based on these metals, so that the resulting metal casting can be small in weight. Preferably, the thickness of the metal plate to be used may be 0.1xcx9c1.0 mm.
According to the present invention, the molten metal to be used may be the above-mentioned light metals whose density is no greater than 5 g/cm3, or light metal alloys. Preferably, the metal plate and the molten metal may have the same or common properties (in composition, main component, etc.), so that they are properly welded to each other. In addition, when the metal plate and the resulting molded member (casting) are made of the same or similar material, they exhibit the same or similar thermal properties. When the metal plate and the molded member have the same coefficient of thermal expansion for example, the final product composed of these elements will not be deformed unduly nor broken even in a heated atmosphere.
Preferably the heat insulating layer may have a heat conductivity of 0.01xcx9c0.1 cal/(cmxc3x97degxc3x97sec) for a temperature range of 300xcx9c600xc2x0 C. Such a heat insulating layer may be made of aluminum oxide, silicon dioxide, or magnesium oxide. The heat conductivity of these elements is advantageously small (about one-tenth or even smaller than that of an ordinary metal).
Preferably, the heat insulating layer may be formed to cover the entirety of the first surface of the metal plate to reliably check the heat conduction from the molten metal to the dies. The thickness of the insulating layer may be 0.01xcx9c50 xcexcm, more preferably 0.01xcx9c10 xcexcm.
The heat insulating layer may be formed by spraying a heat insulator-dispersed liquid on the first surface of the metal plate. This dispersion liquid may be prepared by mixing powder of the above-mentioned metal oxide (average particle diameter of 0.01xcx9c2 xcexcm) into a solvent (water or silicone oil for example) to the concentration of 5xcx9c15 wt %. The heat insulating layer may also be formed in the following manner. First, powder of the above-mentioned metal oxide (average particle diameter of 0.01xcx9c2 xcexcm) is mixed with a resin binder, and this mixture is dissolved into an organic solvent (such as N-methyl-2-pyrrolidinone [NMP]) to the concentration of 5xcx9c15 wt %. Then, the obtained liquid is applied to the metal plate by spraying or brushing for example. Finally, the applied material is solidified at a prescribed curing temperature to provide the desired insulating layer. The resin binder to be used may be epoxy resin or polyimide resin. The curing temperatures for the epoxy resin and the polyimide resin may be 100 xc2x0 C. and 200 xc2x0C., respectively. The insulating layer may also be formed by ceramic coating (e.g., vapor deposition [PDV or CVD] or thermal spraying) of a heat insulating material.
Preferably, the bonding layer may be formed by thermal spraying, plating or vapor deposition of a metal selected from a group of aluminum, magnesium, titanium and zinc. The bonding layer may also be formed by thermal spraying, vapor deposition, spin-coating, brush-application, etc., of a ceramic material.
Further, the bonding layer may be formed by applying a resin material to the second surface of the metal plate and then causing either one of a fibrous material and a porous material to be supported by the resin material. With such an arrangement, the molten metal flows into the fine structure of the fibrous or porous material. Thus, the resulting molded member (casting) can be strongly fixed to the metal plate. The bonding layer may be made of a resin material only. Preferably, the fibrous or porous material may be xe2x80x9creactivexe2x80x9d to the molten metal. For instance, when magnesium is used as the molten metal, the fibrous (or porous) material is called as xe2x80x9creactivexe2x80x9d when it causes the molten magnesium to undergo deoxidization. More specifically, when use is made of molten magnesium for the bonding layer containing silica, MgO or Mg2Si is produced by deoxidization, thereby providing a strong connection.
Preferably, the metal plate may be dissolved into the molten metal to cause depression of freezing point of the molten metal. To this end, the molten metal is injected into the dies at a temperature high enough to melt the metal plate.
With such an arrangement, the injected metal can stay in the molten state for a longer period of time than otherwise, so that it can fill the cavity without leaving any portion thereof unfilled. For lowering the freezing point of the molten metal, the metal plate may be made of aluminum, magnesium, zinc or tin for example, or made of an alloy containing one of these elements as the main component.
According to a second aspect of the present invention, there is provided a metal casting that includes: a metal plate provided with a first surface and a second surface opposite to the first surface; a heat insulating layer formed on the first surface of the plate; and a molded member attached at least to the second surface of the plate.
Preferably, the metal casting of the present invention may further include a boding layer disposed between the second surface of the plate and the molded member for the purposes of improving the bonding strength between the metal plate and the molded member.
Preferably, the heat insulating layer may dominantly contain a metal oxide selected from a group of aluminum oxide, silicon dioxide and magnesium oxide.
Preferably, the bonding layer may be made of a metal selected from a group of aluminum, magnesium, titanium and zinc, or made of a ceramic material. Preferably, the bonding layer may contain a resin material and either one of a fibrous material and a porous material attached to the resin material.
Preferably, the molded member may include a functional portion attached at least to the second surface of the plate. The functional portion may comprise a rib, a boss or a frame for example.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.