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Long stoppers are refractory for controlling the flow of molten steel from a tundish to water-cooled molds.
By moving them up and down, you can control the aperture between the stopper and a submerged entry nozzle and eventually, level of molten steel in the mold.
Some stoppers have 'plugs' for blowing inert gas into molten steel.

Their requirements include:
- Abrasion resistance
- Erosion resistance
- Clogging resistance
- Strength

The strength is most important because the breakage of the stoppers could result in serious accidents (ex. Molten steel breakout).

Image: Long Stopper