A rotary printing press is a printing press in which the images to be printed are curved around a cylinder. Printing can be done on large number of substrates, including paper, cardboard, and plastic. Substrates can be sheet feed or unwound on a continuous roll through the press to be printed and further modified if required. There are three main types of rotary presses; offset, rotogravure, and flexo. Offset lithography uses a chemical process in which an image is chemically applied to a plate. Lithography is based on the fact that water and oil do not mix, which enables the planographic process to work. Gravure is a process in which small cells or holes are etched into a copper cylinder, which are able to be filled with ink. All the colors are etched in different angles, thus while printing every colour is placed in proper position to give the appropriate image. Flexography is a relief system in which a raised image is created on a typically polymer based plate.