Lithographic Printing How Offset Printing Works We will discuss: The Creative Process The Printing Process Pre-Press The Press Run Bindery A Brief History of Printing The earliest known origin of printing is traceable to Chinese scribes in the first century, who developed moveable type.
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How Offset Printing Works
The Creative Process
The Printing Process
The Press Run
Multimedia INFT 140
Every print piece starts with the creative process. Writers, editors, graphic designers and artists are the initial step in the creation of magazines, newspapers, brochures, flyers, catalogues and other print pieces.
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.
To a final colour drawing
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.
Images (words and art) are put on plates which are dampened first by water, then ink. The ink sticks 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.
Before the job can be printed, the document must be converted to film and "plates." 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.
Formatting the page by cutting negatives and placing them in place before the plate is made.
A blueprint of the negative and plate layout, used to check image positions before printing.
There are different materials for plates, including paper (which produces a lower-quality product). The best plate material is aluminium, which is more costly.
Each of the primary colours -- black, cyan (blue), magenta (red), and yellow -- has a separate plate. Even though you see many, many colours in the finished product, only these four colours are used
The printing process used to print our newspapers is called web offset lithography. The paper is fed through the press as one continuous stream pulled from rolls of paper. Each roll can weigh as much as 2,000 pounds (1 ton). The paper is cut to size after printing. Offset lithography can also be done with pre-cut paper in sheet fed presses.
Web presses print at very high speeds and use very large sheets of paper. Press speeds can reach up to 50,000 impressions per hour. An impression is equal to one full press sheet (38 inches x 22 and three quarters), which is 8 pages of the Express & Star.
Even when a 1-ton roll of paper runs out, the presses do not stop rolling. Rolls can be spliced together as the web press is running by using festoons. Festoons are a series of rollers that extend up into a tower. At the moment the splice occurs, the rolls of paper stop rotating for a split second, at which point the paper is taped together automatically. As the newly spliced roll begins to pick up speed, the festoons begin to drop out of the tower at a rate predetermined by the speed at which the press is operating. The press operator never has to adjust the press controls during this operation.
The press has to maintain a constant balance between the force required to move the paper forward and the amount of backpressure (resistance) that allows the paper to remain tight and flat while travelling through the equipment..
Ink and water do not mix -- this is the underlying principle of offset lithography. 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.
The image area of the plate picks up ink from the ink rollers. The water rollers keep the ink off of the non-image areas of the plate. Each plate then transfers its image to a rubber blanket that in turn transfers the image to the paper. The plate itself does not actually touch the paper -- thus the term "offset" lithography. All of this occurs at an extremely high speed.
The paper is left slightly wet by all of the ink and water being applied. Obviously, there is a risk of the ink smudging. The smudging is avoided by having the paper pass through an oven. The oven is gas fired, and the temperature inside runs at 350 to 400 degrees Fahrenheit (176 to 206 degrees Celsius).
Colour and registration control is a process that is aided by the use of computers. Registration is the alignment of the printing plates as they apply their respective colour portion of the image that is being printed. If the plates do not line up perfectly, the image will appear out of focus and the colour will be wrong.
Colour control is a process that involves the way in which the ink blends together, and is tied closely to the plate registration. The amount of ink that is released into the units depends on how much ink is needed to achieve a desired look. The ink is adjusted via the control panel that is part of the overall control console.
The bindery is where the printed product is completed. The huge rolls 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.
The "stitcher" gathers, assembles and staples the magazines (called books) before they are sent for final trimming.