Continuous casting transforms molten metal into solid on a continuous basis and includes a variety of important commercial processes. These processes are the most efficient way to solidify large volumes of metal into simple shapes for subsequent processing. Most basic metals are mass-produced using a continuous casting process, including over 500 million tons of steel, 20 million tons of aluminum, and 1 million tons of copper, nickel, and other metals in the world each year. Continuous casting is distinguished from other solidification processes by its steady state nature, relative to an outside observer in a laboratory frame of reference. The molten metal solidifies against the mould walls while it is simultaneously withdrawn from the bottom of the mould at a rate which maintains the solid / liquid interface at a constant position with time. The process works best when all of its aspects operate in this steady-state manner. Relative to other casting processes, continuous casting generally has a higher capital cost, but lower operating cost. It is the most cost- and energy- efficient method to mass-produce semi-finished metal products with consistent quality in a variety of sizes and shapes. Cross-sections can be rectangular, for subsequent rolling into plate or sheet, square or circular for long products, and even “dog-bone” shapes, for rolling into I or H beams. Many different types of continuous casting processes exist. Vertical machines are used to cast aluminum and a few other metals for special applications. Curved machines are used for the majority of steel casting and require bending and / or unbending of the solidifying strand. Horizontal casting features a shorter building and is used occasionally for both nonferrous alloys and steel. Finally, thin strip casting is being pioneered for steel and other metals in low-production markets in order to minimize the amount of rolling required.