The press is made up of three sections. The feeder that places paper into the press, the printing units that actually print the sheet, and the delivery that stacks the sheets (see diagram).
The printing unit has three large cylinders. A plate cylinder, a blanket cylinder
and an impression cylinder. The ink is transferred from the ink
fountain down to the plate cylinder rollers that split the ink into
a thin film before going onto the plate.
An aluminum plate wrapped around the plate cylinder takes the ink and transfers it onto the rubber blanket which is mounted on the blanket cylinder. The sheet then passes between the blanket and the impression cylinder to complete an impression. But wait a minute you ask. If the ink covers the entire plate cylinder how does a picture show up? Good question.
When the plates are made for the press, the image becomes ink receptive while the background becomes water receptive. Water? I’m glad you are paying attention. On the back of each printing unit is a group of rollers known as the dampening unit. This unit produces a thin film of water to wash over the plates, so as to "clean up the plate", and make the images visible. When the correct amount of ink and water is on the plate the operator has achieved ink and water balance - a critical element in making the print job a success.
The printing plates are .012 inches thick. The blankets are made of rubber with a thickness of .075 inches. The amount of squeeze between each of the cylinders is around .004 of an inch, so you can see that accuracy is essential for a quality printing job.
A four colour press has four separate sets of rollers, blankets, and cylinders - one for each of the four colours used to re-create the full colour spectrum. By combining the colours of cyan, magenta, yellow, and black, we can produce just about any other colour. When the sheet passes through the press, it picks up a little of each colour, in a specific pattern and quantity, to reproduce a colour page rich in detail and variety.
Upon completion of the printing cycle the sheets are stacked in the delivery. The paper is given a light layer of powder on top of each sheet to keep the sheets from sticking together.
When the run is completed it will need to be backed up, or
printed on the other side. Remember, only perfecting presses can print on both sides at
once, and this is not very common for four colour work. To turn the sheets
over to print on the other side use to be a significant amount of work.
Enter our indispensable skid turner. The finished skid of paper is driven into the skid-turner where big jaws clamp together. This allows it to flip the skid without spoiling the sheets. Now the unprinted side is up and can be loaded back into the press for its final pass.
When preparing to run a job, the press operator refers to colour proofs to match on press. The operator is responsible for signing off on the sheet if the customer is not at the plant for the press approval. By signing it off, we mean that the colour on the printed sheet matches the proof, the pictures are in register, any changes requested by the customer have been made, the pages are squared and in sequence, and the pages line up front to back. Once the job is printed, samples are stored for reference in the event of a reprint.
When a customer sends in colour pictures to be scanned they are separated into the four colours that our presses use: cyan, magenta, yellow, and black. After the proof has been o.k’d by the customer, each colour is given a separate plate so that when it goes on press we can essentially reassemble the picture one unit at a time. So simple yet so complicated.
