Tour Stop 5: The Platemaker
Once the film negative is ‘stripped up’ (the pages laid out on goldenrod ‘flats’ in the correct sequence), the plate-exposure unit transforms film negatives into metallic plates that are used in the printing process. The negative is ‘burned’ by exposure to extremely bright light (the equivalent of a street lamp) in tandem with vacuum pressure, to make a plate positive. The chief physiochemical principle behind offset printing is that water and oil don’t mix; when a printing plate is made, the printing image is rendered grease-receptive and water-repellent, while the non-printing areas are rendered water-receptive and ink-repellent. On the press, the plate is mounted on to the plate cylinder which, as it rotates, comes in contact with rollers wet with water (or dampening solution) and rollers wet with ink. The dampening solution wets the non-printing areas and prevents the ink from touching these areas. The ink sticks to burned image areas, which are transferred to the intermediate blanket cylinder. The paper picks up the image as it passes between the blanket cylinder and the impression cylinder.
