Prepress - Platemaking

The photomechanical plate making method begins with making an engraving. This is accomplished by exposing a metal plate through a negative and processing the exposed plate in an acid bath. The metal engraved plate is used to make a master which is molded out of bakelite board. The engraving is placed in a mold press. The mold is produced by applying heat & pressure to the mold material (bakelite board), which can be either plastic or glass, against the engraving under controlled temperature and pressure. The bakelite board fills the engraving on the metal plate. When its cooled you end up with a master mold for the plastic or rubber compound that will be pressed into the mold under pressure and elevated temperature to produce the flexible printing plate with raised areas that will transfer the ink.

The second method of flexo plate making is relief plates. This utilizes a solid or liquid photopolymer. The sheet of photopolymer is exposed to light through a negative. The unexposed areas are then washed away with solvent or water wash. This is fast becoming the most common method of making plates.

The process differs depending on whether solid sheets of photopolymer or liquid photopolymer are used, though the two processes are similar in general outline. In both processes the plates are made in ultraviolet exposure units. A negative of the job is placed between the photopolymer and the ultraviolet light source. The photopolymer sheet or liquid is then exposed to ultraviolet light, hardening the image area. Lastly, the plate is processed to remove the unhardened non-image area. Photopolymer plates are replacing rubber plates because they offer superior quality and performance at a lower cost.

Flexographic printing plates may be made by laser engraving, which is called direct digital platemaking. In this process an image is scanned or computer generated. Then a computer-guided laser etches the image onto the printing plate.