Wednesday, 14 November 2012

What is design for print: Offset lithography

Offset printing is a commonly used printing technique where the inked image is transferred (or "offset") from a plate to a rubber blanket, then to the printing surface.

It is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film (called "fountain solution"), keeping the non-printing areas ink-free.

Offset printing uses the process colour or four colour known as CMYK.

'C' stands for Cyan (a colour in the blue/green spectrum),
'M' stands for Magenta (a colour midway in the red/blue spectrum), 'Y' stands for Yellow and 'K' stands for Key (the black key plate).

As a rule CMYK is also the order in which a full colour image is printed.

Advantages of offset printing compared to other printing methods include:
Consistent high image quality.
Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
Quick and easy production of printing plates.
Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface.
Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.

Disadvantages of offset printing compared to other printing methods include:
Slightly inferior image quality compared to rotogravure or photogravure printing.
Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation) and print in non-image/background areas when developed plates are not cared for properly.
Time and cost associated with producing plates and printing press setup. This makes smaller quantity printing jobs impractical. As a result, smaller printing jobs are now moving to digital offset machines. 


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