Usually can be divided into letterpress, gravure, lithography, stencil printing 4 categories. The graphic part of the printing plate used in letterpress printing is uplifted, including engraving, movable type, stereotype, copper-zinc plate, photosensitive resin plate and flexographic printing. Due to the great development of flexographic printing since the 1980s, its press structure and printing substrate are different from general letterpress printing, so some countries regard flexographic printing as an independent printing type. The graphic part of the gravure printing plate is concave, and it is divided into three categories: engraving gravure, photogravure and electronic engraving gravure. The graphic part of the lithographic printing plate and the non-graphical part are basically on the same plane, which usually refers to offset printing, but in fact, lithography (which has been basically eliminated) and lithography (preserved as a unique process) also belong to the category of lithography. Stencil printing is mainly screen printing, that is, the screen is made of a printing plate that can transmit ink through the graphic part instead of the ink-tight part of the graphic part. Transcription version mimeograph is also a stencil printing.
According to the printing process, the above four printing methods are different from direct printing and indirect printing. The layout ink is first transferred to the blanket cylinder, and then the offset printing in which the image and text are transferred to the paper by the blanket cylinder is indirect printing, and the other printing methods (including lithography in lithography) are direct printing. The pictures and texts of the direct printing plate are inverse images, and the pictures and texts of the indirect printing plates are positive images.