Sendzimir 20-high cluster mills commonly carry the prefix ZR, which stands for "Zimna," the Polish word for "cold," and "reversing." This prefix was used the first time to describe Silesia's first reduction mill. The numbers, and sometimes letters, following this prefix describe the mill’s geometry, the relative size of its work roll, and the widest strip that the mill can roll.
The number immediately following the "ZR" indicates the mill "section" size comprising a combination of the backing bearings, the work rolls, and so on. For example, a ZR 23 has backing bearings with a diameter of 8.858 inches (225 mm) and work rolls with a diameter of 1.578 inches (40 mm). A ZR 32, on the other hand, has backing bearings of 1.875 inches (48mm) in diameter and work rolls of 0.250 inches (6.5mm) in diameter. The significance of the difference is that a ZR 23 can roll ferrous and nonferrous materials down to a minimum gauge of 0.002 inches (50 microns), while a ZR 32 can roll ultrathin nonferrous foil down to 0.0001 inches (2.5 microns), which is 20 times thinner!