The Offset Printing Process

The Offset Printing Process

The next time you read your favourite magazine or go through the latest mailer that arrives in your mailbox, stop for a moment and think about how that publication came to be.

First, writers, editors and designers participate in the creative process. Printers take that creative work and turn it into the publications you read every day. Printing is a fascinating process involving high-speed machines, large amounts of paper, computers, metal plates, rubber blankets and guillotines.

In this article, we'll look at offset printing, the most commonly used printing process and one that encompasses modern computer technology. We will take a look in detail of the three production steps: pre-press, press run and bindery.

The Creative Process

Every printed article starts with a creative process. Writers, editors, graphic designers and artists are the initial step in the creation of magazines, newspapers, brochures, flyers, catalogues and other printed articles.

In the case of magazines and newspapers, topics for articles are identified and writers are assigned. Strict guidelines for word length and a reliance on custom graphics keep articles short, informative and entertaining. Editors help focus copy and keep the whole process moving.

Once the text has been developed, graphics are created. Many discussions between the author, illustrator and director of design move the work from conceptual drawings to final art.

When each article is written, edited and approved with final art, the pieces are sent electronically to the director of graphic design for page layout. The director determines what page a story will appear on, where art will be in relation to words and, in some publications, where advertising will appear. Often, there are difficult decisions to make about how best to fit the pieces of art and text into very limited space. As in the making of a movie, some materials must be left on the "cutting room floor."

Finally, after the layout of every page has been completed, edited and proofread, a digital "printer's file" is created for the entire document. This is usually done by burning a CD, but can also be done with Zip files or File Transfer Protocol (FTP).

In the case of smaller run print jobs, the prepress process may be limited to one or two people, depending on the size of the company undertaking the work. This work is largely carried out on Macintosh computers using software such as Quark Express, Pagemaker, In Design, Freehand and Illustrator.

The Printing Process

There are nine main types of printing processes:

• offset - what we are exploring in this article
• engraving - think fine stationery
• thermography - raised printing, used in stationery
• reprographics - copying and duplicating
• digital printing - limited now, but the technology is exploding
• letterpress - the original Guttenberg process (hardly done anymore)
• screen - used for T-shirts and billboards
• flexography - usually used on packaging, such as can labels
• gravure - used for huge runs of magazines and direct-mail catalogs

The offset process is the workhorse of printing. Almost every commercial printer does it. But the quality of the final product is often due to the guidance, expertise and equipment provided by the printer.

Offset printing works on a very simple principle: ink and water don't mix. Images (words and art) are put on plates, which are dampened first by water, then ink. The ink adheres to the image area, the water to the non-image area. Then the image is transferred to a rubber blanket, and from the rubber blanket to paper. That's why the process is called "offset" -- the image does not go directly to the paper from the plates, as it does in some other forms of printing.

Now, let's look at the steps in the printing process.

Pre-Press Production

Before the job can be printed, the document must be placed on printing plates. There are now two main ways in which this achieved. The older system involves producing film negatives and these are now largely created from digital files in an imagesetter. Images from the negatives are transferred to printing plates in much the same way as photographs are developed. A measured amount of light is allowed to pass through the film negatives to expose the printing plate. When the plates are exposed to light, a chemical reaction occurs that allows an ink-receptive coating to be activated. This results in the transfer of the image from the negative to the plate. The newer system now dispenses with the film stage and plates are produced directly from improved imagesetting systems.

There are different materials for plates, including paper (which produces a lower-quality product). The best plate material is aluminum, which is more costly. Each colour requires a separate plate and if colour photgraphs are required, this is done with a process of using three primary colours, cyan (blue), magenta (red), and yellowand black. Even though you see many, many colors in the finished product, only these four colours are used (you'll also hear this called process colour printing).

In the case of photographs, as only a limited number of colours are used to produce images that comprise thousands of shades or colours, a system of using small dots to represent the percentage of ink coverage of a given area is used. This system is known as half tone screening and is covered in another artice at http://www.welmac.org.nz/articles/desktop.htm.

The Press Run

The printing process used to print the majority of printed paper articles is done on offest printing presses. These presses print at high speeds and can use very large sheets of paper. Press speeds can reach up to 20,000 impressions per hour. some presses are able to print a number of colours at one time and this makes the effective speed of offset printing much faster than any other process.

When books, magazines and smaller sized items are printed, it is common to print more than one page or copy on a single sheet of paper. In the case of books and magazines, the layout of the various pages is critical and the process of doing this is called imposition. Basic forms of imposition can be done in desktop publishing applications such as Pagemaker, however more complex impositions require either hand laying up or expensive software.

The Inking Process

Ink and water do not mix -- this is the underlying principle of offset printing. The ink is distributed to the plates through a series of rollers. On the press, the plates are dampened, first by water rollers, then ink rollers. The rollers distribute the ink from the ink fountain onto the plates.

Bindery

The bindery is where the printed product is completed. The stacked sheets of now-printed paper are cut and put together so that the pages fall in the correct order. Pages are also bound together, by staples or glue, in this step of the process.

In the case of books and magazines, the folded printed paper (called press signatures) are collated into the correct order and then a machine called a stitcher stiches (staples) them together.

The final component of the process is done with a guillotine which trims the paper to the final delivered size. The product is then ready to be delivered to the end destination.