Graphic Design & Printing TechnologyPlate making process
There are mainly four types of flexographic plates. They are molded rubber
plates, photopolymer plates, sheet photopolymer plates and liquid photopolymer
plates.
Molded rubber plates or rubber stereo plates:-Molded rubber plates are created
in a multi step process. A sheet of metal alloy coated with a light sensitive
emulsion is placed in a specially designed vacuum frame. The emulsion is not
only light-sensitive, it is also an acid resist. A negative of the job is placed
over the coated sheet and light is passed through the negative. Where light
strikes the emulsion, the acid resist is hardened. During processing, the
unhardened resist in the non-image areas is washed away, leaving hardened
resist only on the image areas. The metal sheet is then etched, which lowers
the non image areas, leaving the image areas raised. The remaining resist is
washed off. The completed engraving is then moved to a molding press where
a matrix (mold) of the engraving is made by pressing matrix material against
the engraving with controlled heat and pressure. The matrix material sinks
into the metal engraving to form a mold. The rubber plate is made from the
matrix by pressing a rubber sheet into the matrix again under controlled heat
and pressure. The thickness selected depends on the job to be printed.
Photopolymer plates:- These plates are made from light sensitive polymers
which are hardended by ultra-violet light. Photopolymer plates are made
from both sheet and liquid materials.
Sheet photopolymer plates:- The plates are cut to the required size and placed
in an ultraviolet light exposure unit. One side of the plate is completely exposed
to uv light to harden or “cure” the base of the plate. The plate is then turned
over, a negative of the job is mounted over the uncured side, and the plate is
again exposed toUVlight, which hardens the plate in the image areas. The
plate is then processed to remove the unhardened photopolymer from the non-
image areas thus lowering the plate surface in the non-image areas. After
processing, the plate is dried and given a post exposure ofUV light to cure the
whole plate.
Liquid photopolymer plates:- These plates are made in a specialUVlight
exposure unit. In this process, a clear plastic protective cover film is mounted
over a negative transparency which is placed emulsion-side up on the exposure
unit. A layer of liquid photopolymer is then deposited over the transparency
and the cover film. A substrate sheet, which is specially coated on one side to
bond with the liquid photopolymer and to serve as the back of the plate after
exposure is placed over the liquid.
Exposure is made first on the substrate side of the plate. This exposure hardens
a thin base layer of the liquid photopolymer and causes it to adhere to the plate