When people ask us at Semco what is so special about graphite, we usually describe the amazing versatility of graphite materials. The applications that are optimized by using graphite materials are only limited by the imaginations of engineers. One such application, of both ISO-molded and extruded graphite, is a casting method called continuous casting. This casting method is 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 molten metal is poured. The graphite mold is typically water cooled. This cooling method allows the molten metal to solidify within the die. The custom solid metal form is then extracted from the mold and pulled 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 from graphite. In these types of applications, the types of graphite materials used are ISO-molded types. Small porosities and good machinability are required properties, and ISO-molded graphite has those properties. In rare instances, such as where large ingots are produced, extruded graphite is used. Extruded graphite has the benefit of being more economical that ISO-molded graphite but is not always a good choice for many applications. 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 ideal as a 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 important factor why graphite materials are ideal for use in continuous casting is self-lubrication. Graphite is a solid lubricant that allows for low friction between the mold and the cast material. If what you were molding in a cast were muffins, you would liberally grease the muffin pan with butter or oil so that when you pull the muffins out of the oven, you could easily extract them from the pan to cool. Graphite works similarly to the grease in this comparison, allowing for smooth extraction of the casting. No one wants their gold ingots sticking to the pan.
There are various factors determining the lifetime of a graphite mold. From the composition of the casted materials, to the casting temperature, to the cooling rates — all of these affect the life of the graphite mold. In general, graphite molds can operate continuously in excess of 100 hours. Another benefit of graphite molds is that they can be re-machined to smooth surfaces and then be used again.
The specific graphite grade used in a mold is also determined by a multitude of factors. The main factor is the composition of the alloy to be cast. For example, for a gray iron, or a high-nickel alloy, a graphite resistant to wear is required, while for brass, a relatively dense graphite with enough open porosity to allow zinc to evaporate would be a better choice.
Designing graphite molds for continuous casting requires consideration of all these factors and applying them to your specific application. Grade selection is of the greatest 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. Our graphite engineers at Semco have answers for you about the difficult decision of which graphite grade to use for your application.