Offset Lithographic Printing
Following on from the prepress stage, where the operator has transferred the artwork onto metal plates, comes the process of Offset printing. This is where the image transfers (offset) from metal plates to rubber blankets or rollers and then onto paper. This is how the actual ‘printing’ part of the project takes placce:
The Process
The press operator sets up the press by loading and fixing the printing plates onto the plate cylinder, and a new rubber blanket onto the blanket cylinder. The press operator fills the ink trays (note that larger presses may run off a reservoir connected to the press via hoses). For any project using PMS Spot Colours, the press operator will add these inks to an empty station. The operator loads the stacker with paper, this tray will continually rise throughout the print run ensuring a constant supply into the press.
Managing the Press
The press operator uses the digital control station to manage the press. The printer can incrementally adjust the ink flow onto each roller from here. Every print run starts with ‘running up’ the colour, where the press operator will run a volume of sheets through the press, adjusting the ink flow until it reaches the desired density. The paper consumed in this process is recycled. Using a
densitometer, the press operator reads the density of the ink off a run-up sheet (called ‘make-readies’). From this reading, the press operator corrects the colour and ink saturation. Using a magnifying eye-piece, the press operator also checks the registration of each colour on the subsequent run-ups. Once the run-ups are complete and the ink densities are correct—the project is printed.
After Printing
After printing is complete, the sheets are stacked to dry. Once dry, these will be sent for finishing. The press operator begins clean down by washing the press, ready for the next job. Used printing plates and paper go to recycling, other waste consumables are disposed of in an environmentally responsible manner. Most offset inks today are made from eco-friendly soy- and plant-bases. Very little from the process goes to landfill.
Offset Printing Process
- The operator wraps the plate around the plate cylinder and secures it in place.
- The below close-up image, taken at the end of a print run, shows the loaded printing plate (top) and corresponding blue rubber transfer blanket (bottom). You can see the black ink transferred from the plate to the transfer blanket.
- The press operator fills the ink trays.
- And then loads the stacker with paper.
- The press operator uses the control station (the grey desk/monitor in the background) to manage the press.
- A densitometer is used to read the density of the ink from a run-up sheet. This allows the press operator to correct the colour and ink saturation.
- Clean-up begins after the print run is complete. The printery recycles all waste paper and printing plates.
Run-up Sheets and Overprinting
The above image shows the run-up sheets for each process colour printed, using the plates from the previous spread. The press operator uses these run-up sheets to bring each process colour to the correct ink density.
The operator has pulled these run-up sheets (above) from the press during the print run. They show the effect of overprinting as each colour is laid down. Key-Black is usually the first colour laid down, followed by Cyan, then Magenta, and finally—Yellow over the other three colours. This overprinting creates all achievable colours within the CMYK colour model, the result is a full-colour sheet.
