Casting investment

Investment casting, lost foam casting and shell moulding 

Equipment and materials are more expensive than those required in sand casting and therefore require the production of larger quantities to be economic. The higher costs for small quantities can, however, often be offset by savings as a result of the ability to produce a high degree of dimensional accuracy and surface finish, which can reduce or eliminate subsequent machining operations. 

Any metal capable of being cast can be cast by these methods, although advantage is most readily gained when using those difficult to work by other methods. Metals for investment casting are the subject of British Standard BS 3146. 

The investment-casting process - also known as the lost-wax process - is one of the oldest casting processes, having been practised for thousands of 

The process starts with the production of an expendable wax pattern of the shape required which is then coated with a refractory material to produce the mould, which is allowed to dry. 

The investment-casting process - also known as the lost-wax process -is one of the oldest casting processes, having been practised for thousands of years. Today highly sophisticated plant, equipment and materials are employed to produce a large variety of components in materials ranging from high-temperature nickel- or cobalt-based alloys - known as superalloys - to the non-ferrous aluminium and copper alloys. 

The process starts with the production of an expendable wax pattern of the shape required which is then coated with a refractory material to produce the mould, which is allowed to dry. The mould is heated, melting the wax, which is allowed to run out and so produce a cavity. Further heat is applied to fire the mould before pouring the casting metal to fill the cavity left by the melted wax. When the molten metal has solidified, the refractory shell is broken away to release the casting. The casting is cut away from any runners and is dressed or fettled and finished as required. 

The expendable wax pattern, exactly the shape and size of the required casting, with allowances for pattern contraction and contraction 

Injecting wax Wax pattern ejected into mould from mould 

Turbine blades Machine tool parts Aerospace components Valve and pump casings Pipe fittings 

Very complex castings with unusual internal configurations possible. 

Wax pattern must be easily removable from its mold. 

Complex shapes assembled from several simpler shapes. 

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Alloys developed by processing through the investment casting process had high I 5r strength and design flexibiUty, which led to many further ... 

Magnesium alloys are available in a variety of metal forms, including cast ingots, slabs, and billets sand, permanent-mold, die, and investment castings forgings extmded bars, rods, tubes, stmctural and special hoUow and soHd shapes and roUed sheet and plate. Magnesium alloys are used widely in a great variety of appHcations. 

Castings Investment Casting Institute 8521 Clover Meadow Dallas, Tex. 75243 American Die Casting Institute 2340 Des Plaines Ave. Des Plaines, lU. 60018 American Poundrymen s Society Gulf and Wolf Roads Des Plaines, lU. 60016 Steel Pounders Society of America 455 State Street Des Plaines, lU. 60016 Cement and Concrete Cement Statistical and Technical Association Mahno Sweden American Concrete Institute P.O. Box 19150 Detroit, Mich. 48219... 

Transparent fused silica can be formed at a temperature of 1200°C and a pressure of 13.8 MPa (2000 psi) from silica powder consisting of 15 nm ultimate particles (92) or by electric arc fusion of pure silica sand having low iron and alkali metal contents. The cooled product is ground to the desired particle size. Fused sihca is primarily manufactured by C-E Minerals, Minco, and Precision Electro Minerals in the United States by Chuo Denko, Denki Kagaku Kogyo, NKK, Showa Denko, and Toshiba Ceramics in Japan. Based on 1988 data and projected growth, an estimated 135,000 metric tons of fused siUca were used in 1994 as a sacrificial component or investment casting in the manufacture of metals and as a component in refractory materials (62). 

Casting Investments. Casting investments are used to form molds into which molten metal may be cast. The cavity for receiving the metal is formed by the lost wax process. The composition of investments used for alloys cast from low (<1100° C) temperatures are different from those used for alloys cast from higher (>1300° C) temperatures. 

Sheet nd Sha.pe Waxes. Sheet and shape waxes are used to produce patterns from which complete or partial dentures are cast of gold or base metal alloys. They are used to fabricate the restoration prototype directly upon a refractory investment cast. 

These super-alloys are remarkable materials. They resist creep so well that they can be used at 850°C - and since they melt at 1280°C, this is 0.72 of their (absolute) melting point. They are so hard that they cannot be machined easily by normal methods, and must be precision-cast to their final shape. This is done by investment casting a precise wax model of the blade is embedded in an alumina paste which is then fired the wax bums out leaving an accurate mould from which one blade can be made by pouring liquid super-alloy into it (Fig. 20.4). Because the blades have to be made by this one-off method, they are expensive. One blade costs about UK 250 or US 375, of which only UK 20 (US 30) is materials the total cost of a rotor of 102 blades is UK 25,000 or US 38,000. 

Fig. 20.4. Investment casting of turbine blades. This produces a fine-grained material which may undergo a fair amount of diffusion creep, and which may fail rather soon by cavity formation.

A silver replica of a holly leaf is to be made by investment casting. (A natural leaf is coated with ceramic slurry which is then dried and fired. During firing the leaf burns away, leaving a mould cavity.) The thickness of the leaf is 0.4 mm. Calculate the liquid head needed to force the molten silver into the mould cavity. It can be assumed that molten silver does not wet the mould walls. 

Normal reheat treatment can partially restore blade properties however, it does not appear to be capable of full property recovery, although the miscrostructures are comparable to new blades. This shortcoming implies that cavitation may be present and was not removed by conventional reheat treatment. Hot isostatic press (HIP) processing is an alternative that ensures void removal. It has demonstrated its ability to remove even gross internal shrinkage porosity in investment castings. The results of HIP treatment... 

Radial vaned impellers are investment cast to ensure precise contours and dynamically balanced for a smooth operation. 

This process is used to produce intricate, thin-section parts with great dimensional accuracy, fine detail, and very smooth surfaces. All ferrous and nonferrous alloys can be cast in investment molds. Investment casting begins with expendable wax patterns that are assembled into clusters, then coated with a series of successively coarser ceramic slurries. The assembly is then fired in a furnace to dry and harden the ceramic shell and to melt out the wax, leaving a cavity into which molten metal is poured to form the casting. 

Ammen, C. W. (1979), Lost Wax Investment Casting, McGraw-Hill, New York. 

Uses. Dielectric in capacitors and transformers investment casting processes heat exchange fluid hydraulic fluid no longer produced in the US... 

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