What Is Offset Printing?
In Offset Printing (a.k.a. Offset Lithography and/or Plantographic Printing) the printing plates carry both the image and non-image areas on the same level (unlike letterpress where areas are raised to catch ink). The transfer of ink is controlled by an unlikely principle. The principle that oil and water don't mix.
Image areas are photographically transferred to thin metal plates which are treated chemically to accept oil-based ink but repel water on the image areas. Conversely, non-image areas accept water but repel the oil-based ink.
A plate first contacts rollers of a clean solution or water and then is inked by other rollers. The oil-based ink "sticks" to the image area. The inked image is then transferred from the plate to a rubber blanket. The rubber blanket then transfers the image onto the paper's surface.
Because of the resilience of the rubber blanket, it's possible to use offset lithography printing on a wide variety of surfaces and for large quantities. Both spot colors and CMYK colors can be used with this process.
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