Just Hit Print… Offset

Today I’m covering offset lithography or offset printing.

Offset uses the resistance method of printing. The basic principle of resistance printing is that oil and water don’t mix, so the same surface can be used for ink and un-inked surfaces.

Images and text are transferred to photosensitive plates (made of zinc or aluminum) photographically. The plate is then placed on a cylinder on the press. The plate obtains ink from the ink rollers, while the non-printed areas attract a water-based film (fountain solution), keeping the non-printing areas ink-free. The image is then rolled on to a rubberized offset cylinder, which in turn, inks the paper. The advantage of offset printing is the ability to print thousands of images from one plate.

Below is a side view of the offset process.

offset1

Offset Presses

There are two types of paper feeds on offset presses: sheet fed and web fed.

Sheet fed presses feed one sheet at a time through the rollers. It is commonly used for short-run jobs like brochures, letterhead, business cards, etc. Short-runs are considered less then 10,000 to 20,000 impressions.

Sheet fed presses also come in different sizes. In smaller print shops the presses are usually 2-head presses. That is that they can run 2 inks at a time. Larger print shops have 6+ head presses. These presses can run numerous inks and coatings at the same time.

Web fed presses feed off a roll of paper. Web fed presses are commonly used for large jobs like newspapers, catalogs, magazines, etc.  Jobs run on web presses exceed 10,000 to 20,000 impressions. Web presses generally have 4+ heads for ink.

Spot versus CMYK

When press people talk about ink it seems to be in a strange code: of spot, CMYK, PMS, etc. What they are talking about is ink.

Spot color is a solid ink color (versus a dot-pattern of CYMK). Letterheads and business cards are the most common users of spot color.

CMYK stands for cyan, magenta, yellow and black. CMYK is also called process color. Process color creates images using a mixture of dots from all four inks. CMYK is used in magazines, brochures, etc.

PMS stands for Pantone Matching System. Pantone creates solid and CYMK inks.

Digital Printing

Since the 1990s digital printing has slowly began to replace some offset printing practices. Digital presses do not use a photographic plate. The image can be sent directly from the computer to the press, eliminating the plate and the plate making processes.

The advantages of digital presses are a lower cost in printing. Images can be changed “on the fly”. This is convenient when only one or two aspects of the image are changed. Less wasted chemicals and paper.

Below are examples of spot color offset printing:

offset2

Below are examples of process (CYMK) offset printing:

offset3

Hope you enjoyed this little snippet about Offset Printing, next up Screenprinting.

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2 Responses to “Just Hit Print… Offset”

  1. Thank you for this very clear and thorough explanation of offset printing! I never knew!

    [inkihandmade.blogspot.com]
    [inkihandmade.etsy.com]

  2. Very cool and well written description of some of the differences in printing techniques. I love learning and reading about the various ways and means to make images, great post.

    And thanks for including my Fillmore poster for Brett Dennen too. Cheers!

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