Printing processes such as offset lithography use printing plates to transfer an image to paper or other substrates.
The plates may be made of metal, plastic, rubber, paper, or other materials. The image is put on the printing plates using photomechanical, photochemical, or laser engraving processes. The image may be positive or negative.
The offset lithography process works by first transferring an image photographically to thin metal, paper, or plastic printing plates. Unlike other forms of printing, the image on the printing plate in offset lithography is not recessed or raised. Rollers apply oil-based ink and water to the plates. Since oil and water don’t mix, the oil-based ink won’t adhere to the non-image areas. Only the inked image portion is transferred to a rubber blanket (cylinder) that then transfers the image onto the product as it passes between it and another cylinder beneath the product. The term offset refers to the fact that the image isn’t printed directly to the product from the plates, but is offset or transferred to another surface that then makes contact with the product.
The printing plates used depend on the type of press, the printing method, and size of the print run. A plate is prepared for each color used, or four plates in the case of 4-color (CMYK) process printing. In general, metal plates are more expensive but last longer and have greater accuracy. Paper plates are usually more suitable for shorter runs without close or touching colors.