There are three types of image carriers in flexography, two of which can be categorized as plates:
Rubber Plates. A negative of the image to be printed is placed on top of a metal alloy coated with a light-sensitive acid resist. When exposed to light, the resist hardens in the exposed image areas, and remains soft and soluble in the unexposed, non-image areas. The unhardened resist is washed away after exposure, and an etchant is applied to the surface, which engraves those areas not protected by the hardened resist. The result is a metallic relief plate. A mold—or matrix—is then made of the relief plate. After cooling the mold, a rubber sheet is pressed into the matrix which, after cooling, will be a rubber relief plate. Various finishing operations optimize the plate for flexographic printing.
Photopolymer Plates. Manufactured either from sheet photopolymer or liquid photopolymer materials, a photographic negative is placed on top of the photopolymeric material and exposed to ultraviolet light, which hardens the photopolymer in those areas through which it passes (the image areas), leaving the unexposed regions unhardened. After exposure, washout procedures remove the unhardened photopolymer from the non-image areas, leaving the image areas in relief.
Plates are mounted on the plate cylinder either by an adhesive backing or by other means, such as plate clamps. See Plate: Flexography.
The third type of image carrier is called a design roll, which consists of a layer of vulcanized rubber applied as an unbroken “jacket” on the surface of the plate cylinder itself. The imaging of the plate is commonly performed using high-energy lasers, which atomize the non-image portions of the rubber surface, leaving the image areas in relief. Design rolls, due to their seamlessness, are useful for printing continuous background patterns such as those found in packaging, wrapping, and other forms of decorative printing applications. They are also capable of higher print runs than conventional plates, with which they are occasionally used in tandem. (See Design Roll.)