Transfer pad printing, commonly
known as pad printing, is an "indirect
offset gravure" printing process directly
related to a process invented several hundred
years ago in Europe.
The main elements of the
pad printing process are:
Silicone rubber printing pad
Doctor blade or ink cup
The image to be printed is created on
the printing plate, normally produced
by chemical etching. The plate is generally
steel or a nylon photopolymer material.
EXAMPLE OF PAD PRINTING
Pad printing inks contain
solvents. The evaporation of the solvents
from the ink is the main mechanism that
enables the process to operate. When the
ink is contained within the etched image
area this evaporation of solvents causes
the surface of the ink to become tacky.
The shape of the pad is such that when it
compresses on the plate the printing surface
of the pad rolls across the plate, as it
does so it comes into contact with the tacky
surface of the ink. The ink sticks to the
pad, as the pad lifts the printing surface
rolls away from the plate and lifts up ink
from the etching.
Whilst the pad moves towards
the object to be printed the solvent continues
to evaporate from the ink on the pad and
the outside surface of the ink becomes tacky.
As the pad makes contact with the object
and compresses, the print surface of the
pad rolls across the object and the tacky
surface of the ink attaches itself to the
surface of the object. The pad then lifts
and the printing surface of the pad rolls
away from the surface of the object and
releases the ink leaving it laying on the
surface. Whilst this is happening the etched
portion of the plate is recharged with ink,
and the pad returns to pick up another image
from the plate.
A whole range of variables
will effect pad printing. Etch depth, ink
condition, ambient conditions, pad shape,
surface finish and hardness and machine
speed are the key factors.
Conventional pad printing
machines are divided into three families.
- 'Open ink trough
where the ink is held open to the atmosphere
in a trough either behind the plate or
around the plate.
- 'Closed cup' or sealed
cup, where the ink is contained
in an inverted cup with its opening in
contact with the plate. Being enclosed
in the cup inhibits the evaporation of
solvent from the ink.
- Rotary system
where a rotary drum type silicone pad
is used in conjunction with a steel cylindrical
plate. The ink is either held in an open
trough type system or in a sealed chamber
rather like the inverted cup.