A crucible can be defined as a pot in which a metal is held, while it is melt in a furnace. It is generally made of clay graphite and silicon carbide.Crucibles Shapes:
Metal casting crucibles come in two shapes – “A” shape and Bilge shape.
Bilge shaped crucibles have the shape of a barrel. The region in the middle part of the crucible is known as bilge, which is the widest part of the crucible and has the maximum diameter. Bilge has greater diameter than the top of the crucible. According to a thumb rule, the # of a bilge crucible reflects the number of pounds of aluminum, which it can hold. In the case of brass and bronze, three times of the # can be held. For example, a # 5 bilge crucible can hold around 5 pounds of aluminum and around 15 pounds of brass. This thumb rule applies only to bilge shaped crucibles.
“A” shaped crucibles have a narrow bottom and a wider top. This types of crucibles have straight sides, which are bent outwards and the diameter increases from the bottom to the top. “A” shaped crucibles are easier to design than bilge ones and hence have lower cost.
Both Bilge as well as “A” shape crucibles can be manufactured in clay graphite and silicon carbide.
Properties of Crucibles:
A crucible should be designed in a way so that it can handle the extremely high temperatures of the molten metal. For this, the crucible should be made of a material, which has a melting point more than that of the material to be melted in the crucible. In addition, the crucible material must have superior strength even in white hot state.
For melting metals, such as aluminum and zinc, a home made steel crucible can be used. However, in this case, the interior surface of the crucible undergoes scaling or flaking, which may pollute the molten metal. It also quickly decreases the thickness of the crucible walls. To solve this problem, a coating of marcote-7 is applied to the inner surface of the steel crucible. The use of steel crucibles is allowed if the contamination due to scales is not serious.
Materials Used:
To design a crucible, the usual refractory materials (such as carbon bonded silicon carbide and clay graphite) can sustain the extremely high temperatures in a foundry. Moreover, silicon carbide is a highly durable material.
Some types of the clay graphite crucibles can be used up to a temperature of 2750 degrees Fahrenheit. Some “A” shape crucibles can be used up to a temperature of 2000 degrees Fahrenheit. These crucibles can be designed to melt silver, gold, bronze, brass, aluminum and zinc alloys.
“A” shaped crucibles have a rough surface finish and have dents along the rims. However, this roughness and dents does not affect the performance of these crucibles.
For proper handling of crucibles, adequately fitted tongs should be used. Poorly designed tongs may result in the damage or the total failure of the crucible. A disk of cardboard or a Plumbago coating should be placed in between the base of the furnace and the crucible.
