Graphite Molds or Steel Molds?

Graphite Molds or Steel Molds?

Graphite molds are less common in permanent mold casting than steel molds. Why?

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The choice depends on several factors such as material compatibility, durability, and cost.

Material Compatibility

Steel molds are the preferred choice for casting nonferrous alloys such as aluminum (A356) and zinc (ZA5 or ZA12). But, since the mold is made of steel, you cannot use it to cast steel alloys.

Graphite molds can cast carbon steels and low alloy steels.

Durability

Steel molds are high-strength and durable. Steel molds can withstand multiple casting cycles without significant degradation.

Graphite molds are not as durable as steel molds. They degrade more quickly overtime with repeated use. Therefore in high production runs, they must be more frequently replaced than steel molds.

Graphite is also relatively brittle compared to steel. This means graphite molds are more prone to cracking and breaking than steel molds.

Cost

Graphite molds are generally cheaper than steel molds. The exact tooling cost depends on the casting size, complexity, feeding, and material cost.

However, keep in mind the lifespan of each type of mold and production volume. If you have a larger production run, investing in one steel mold may be more cost effective than investing in multiple graphite molds.

Advantages of Graphite Molds

Although less common in permanent mold casting, graphite molds are still used for certain applications.

The most common reason for using a graphite mold instead of a steel mold is in situations where the molten metal would have reacted with a steel mold (e.g. casting steel alloys).

Graphite molds are good for small to medium production runs but must be maintained and replaced as production volume increases (due to lower durability).

Advantages of Steel Molds

Steel molds are often preferred over graphite molds for permanent mold casting.

Steel molds are strong and durable, lasting several casting cycles. The casting is repeatable, with the same dimensional accuracy and high-quality surface finish every time. This makes steel permanent molds ideal for moderate volume production.

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At Batesville Products, we use tool steel for all our permanent molds.

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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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