Cast fused silica from Blasch, 87% to 99% SiO2, for launders, nozzles, stopper rods and kiln furniture.
Pour molten metal into a cold launder and most ceramics crack.
Fused silica barely notices, because it hardly grows.
Fused silica ceramic survives extreme thermal shock because its thermal expansion is close to zero: when one side heats faster than the other, the part barely changes size, so almost no stress builds up. It also insulates and keeps its dimensions, which suits launders, nozzles and stopper rods in metal handling. Blasch fused silica compositions range from 87% to 99% SiO2 and withstand up to 3,000 °F (1,649 °C), but repeated cycling above 1,000 °C is usually not recommended.
Fused silica is silicon dioxide that has been melted and cooled to a glass, without the crystal structure of quartz. A fused silica ceramic is made from this glass as grains, cast into shape and fired, so the part keeps the low expansion of the glass.
That is the difference from quartz glass in a laboratory: a fused silica ceramic is a porous, cast refractory part, not a clear glass body.
Blasch casts fused silica in its own net shape process to a tolerance of about 0.5%, so complex shapes such as launders with spouts or nozzles with threads come out of the mold. How the Blasch process works
Thermal shock is a fight between the hot side and the cold side of a part, and fused silica avoids it by hardly growing at all.
Near zero expansion: a part cracks under thermal shock when its hot face grows more than its cold back. With fused silica that difference stays tiny, even under very fast heating, as when metal is poured onto a cold surface.
Insulation: fused silica conducts heat poorly. It keeps heat in the metal and in the launder, which helps where heat retention matters, and shields metal parts behind it.
Dimensional stability: the part keeps its shape and size through heating and cooling, which matters for nozzle bores and stopper rods that have to seal.
Chemical stability and low electrical conductivity: fused silica resists many acids and slags and does not conduct.
Blasch fused silica withstands up to 3,000 °F (1,649 °C). Above about 1,000 °C, however, the glass slowly starts to crystallize into cristobalite. Cristobalite changes volume sharply when it cools through a lower temperature range, so a part that is cycled repeatedly above 1,000 °C loses the thermal shock resistance it was chosen for.
For long service at high temperature with temperature cycling, mullite or a silicon carbide grade is the better choice. For contact with molten aluminum over many cycles, aluminum titanate lasts longer than fused silica in riser tubes.
Blasch values: Blasch fused silica material page and product information. Riser tube comparison: Blasch riser tube product information. Glass structure, expansion and devitrification of silica: W. D. Kingery, H. K. Bowen, D. R. Uhlmann, Introduction to Ceramics, 2nd ed., Wiley, 1976.
Parts where thermal shock and heat retention matter more than hot strength.
Troughs and tundishes that take molten metal without preheating.
Down spouts and pour spouts that keep their bore through thermal cycles.
Rods and pins that seal reliably while hot metal flows past them.
Low expansion plates, saggers and setters for firing furnaces.
Fused silica is also used for saggers, boats and setter plates and for riser tubes in low pressure die casting. Most fused silica parts go to molten metal operations and to industrial process heating. For kiln parts that cycle up to 1,000 °C, see also cordierite ceramics.
Chemically yes, both are silicon dioxide. Quartz is crystalline; fused silica is the glassy form, and that is why it expands so little.
Launders, nozzles, stopper rods, riser tubes and kiln furniture, wherever a part sees fast heating and needs to keep its shape.
Blasch fused silica withstands up to 3,000 °F (1,649 °C), but repeated thermal cycling above 1,000 °C is usually not recommended.
It is often a lower cost alternative to higher end materials, as long as the temperature profile stays within its limit.
Send the dimensions, the metal and how the part is heated. A Blasch engineer checks whether fused silica or another grade fits better.
