Aluminum is uniquely suited to this application. Its strength resists scratching and dimensional change on the press and register can be maintained throughout the press run. Aluminum takes a fine sharp grain which produces the high quality half tones so important in color printing. The grained aluminum surface is naturally water receptive and the non-image areas respond more readily to desensitizing treatment. Therefore, plates stay cleaner on long press runs and require less attention by the pressman. The light weight of aluminum makes the plates easier to handle and store without damage, a particular advantage on large presses, on which sheet aluminum plates are used. This combination of properties, strength, stability, durability, light weight and surface response makes aluminum the ideal material.
Aluminum rolling mills have developed techniques to produce wide sheet and foil economically with virtually defect-free surfaces in alloys offering good tensile strength, wear resistance, fatigue strength, creep resistance, and corrosion resistance in fountain solutions. Foil is used in the applications requiring lesser physical strength, due to limited press runs, and lends itself well to lamination to strength improving backup materials where advantageous. Most used alloys for litho plates are 1100 and 3003.
Because of strength, flatness, and corrosion resistance, aluminum sheet and foil are also finding broad application in flexographic printing, which is a modification of letter press or raised image printing. In this case, aluminum serves as a base for a relatively heavy photopolymer coating which carries the printing surfaces. This type of foil plate is generally used for shorter runs of line work, while the sheet versions are used for more complex and longer runs.