Graphite Mold Advantages and Applications in Different ...

Continuous casting and semi-continuous casting  

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The promotion of advanced manufacturing methods such as direct continuous (or semi-continuous) manufacturing of bars or pipes from molten metal has increased in recent years, both domestically and internationally. This method has been adopted in China for the production of copper, copper alloys, aluminum, and aluminum alloys. Continuous casting or semi-continuous casting molds for non-ferrous metals are made of artificial graphite, which is considered to be the most suitable material. As evidenced by production practice, the graphite mold not only improves the casting speed due to its high thermal conductivity (thermal conductivity is a property that determines the solidification speed of a metal or alloy) and good self-lubricating performance, but also allows for direct processing in the following process due to the ingot’s accurate size, smooth surface, and uniform crystal structure. As a result, not only does the yield increase significantly, but the loss of waste products is reduced, and the product quality is significantly improved. 

Pressure casting  

It has been proven that artificial graphite can be used successfully in the pressure die casting of non-ferrous metals. Among other things, zinc alloy and copper alloy castings made with pressure casting molds made of artificial graphite materials have been used in automobile parts and other applications. 

Centrifugal casting 

Centrifugal casting has been successfully accomplished with the use of graphite molds. It has been decided in the United States to use an artificial graphite mold with a wall thickness greater than 25mm for centrifugal casting of the bronze casing. Certain anti-oxidation measures can be implemented in order to reduce the risk of artificial graphite mold catching on fire. In the event that it is discovered that the inner surface of the mold has been burned after casting a certain number of castings, the inner hole of the mold can be increased in size to allow for the casting of a large-size casing. 

Glass forming 

Because of the chemical stability of the stone graphite material, it is difficult to be infiltrated by the molten glass and will not alter the composition of the glass once it has been molten. The heat-resistant impact performance of graphite is excellent, and the material’s size changes only slightly with temperature. In recent years, it has emerged as an essential mold material in the glass manufacturing industry, and it can be used to create molds for glass tubes, bends, funnels, and other custom-shaped glass bottles, among other applications. 

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Sintering 

It is possible to manufacture the sintering mold and support for transistors by taking advantage of the characteristics of very small thermal deformation of artificial graphite material. It has a long history of use. It has evolved into a critical component in the advancement of the semiconductor industry. As well as molds for cast iron, graphite molds are also used in the production of wear-resistant molds for a wide range of non-ferrous metals, as well as molds for heat-resistant metals (such as titanium and zirconium). Thermit welding molds are also used in the production of rail welds.

Continuous casting is a casting method used in high volume production of metals with a constant cross-section. The method uses an open ended graphite die which is surrounded by a copper jacket. Through which is poured molten metal. The graphite mold is typically water cooled. This allows the molten metal to solidify within the die, the custom solid metal form is then extracted from the mold and pulled/passed through rollers and water sprays. This process removes the heat from the metal, and gradually solidifies it.

Graphite is a widely used material in continuous casting. The die molds are generally produced out of graphite. In these types of applications, the types of graphite used are ISO-Molded types. Small porosities and good machinability are required properties, and ISO-molded graphite has those. In rare instances, where large ingots are produced, extruded graphite is used. There are two types of continuous casting processes, a vertical casting and a horizontal casting. Graphite molds are used in both types of applications.

A wide variety of alloys are fabricated using continuous casting in graphite molds. Gray cast irons, copper-nickel alloys, nickel silvers, aluminum bronzes or gold and silver alloys, are  just some of the alloys being produced with graphite molds.

The coefficient of thermal expansion is an important factor that determines why graphite is used as mold material. The casting mold is a medium with very un-uniform temperatures, from hot inside due to the molten material, to cold on the outside due to the cooling process applied to it. The low coefficient of thermal expansion that graphite exhibits, allows the mold to function without deforming or cracking.

 Another very important factor is self-lubrication. Graphite is a solid lubricant that allows for low friction between the mold and the cast material. Subsequently graphite allows for smooth extraction of the casting.

There are various factors determining the lifetime of the mold. From the composition of the casted materials, to the casting temperature, to the cooling rates, all affect the life of the graphite mold. In general, graphite molds can operate continuously in excess of 100 hours. Also, graphite molds can be re-machined to smooth surfaces and used again.

The specific graphite grade used is also determined by a multitude of factors. The main factor is the composition  of the alloy to be casted. For example, for a gray iron, or a high nickel alloy a graphite resistant to wear is required, while for brass a relative dense graphite with enough open porosity to allow zinc to evaporate, will be a better choice.

Designing graphite molds for continuous casting requires taking in consideration all these factors and applying them to your specific application. Grade selection is of the most importance, and should include input from foundry men, graphite manufacturers, and machine shops specialized in machining graphite. Even then, when designing a new system, actual trials could be needed, before an actual graphite grade is selected.

 Attached is a video showing the precious metal strip casting process using an IRM caster fitted with Semco Carbon graphite components. 

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