Crucible Backing and Sleeves
BACKING materials play an important role in the successful use of ceramic crucibles in a melting campaign; indeed, they are an essential part of the ceramic system. The majority of crucible failures by cracking can be attributed to inadequate backing.
MP BACKING is a magnesia-based with a high thermal expansion designed to hold the crucible in compression, its strongest state, during heating and so minimising the chances of any metal break-out reaching the coil.
The high thermal insulation of the ZA BACKING makes it a highly effective packing medium, retaining heat within the crucible and enhancing its’ life. This, along with the compatibility of dimensional and thermal properties, makes it especially suitable for backing zirconia crucibles which need to be maintained at reasonably high temperature between casts to avoid structural problems.
ZNP backing is a carefully graded zircon mix suitable for small gaps. It is used extensively in the precious metal industry.
The outer casing of the backing material is is either the coil screed or else a backup crucible, usually known as a SLEEVE. As the sleeves are in a non-critical position and do not experience the heat or chemical interaction that the crucible experiences, they can be made from a less refractory and cheaper material; the main qualities should be high strength and good thermal flexibility.
From the Aluminosilicate range, SILLIMANITE S1 is the standard material. It has good thermal cycling capabilities and has adequate strength to withstand the thermal expansion of the crucible and backing material during multiple thermal cycles. The MULLITE ML80 product has a higher strength and a higher temperature capability if needed.
To maintain compatibility with the crucibles used for melting precious metals in terms of thermal behaviour, the sleeve can be supplied as a ZIRCON product. It has good thermal cycling properties and is quite economical to run.