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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: - Pad
Printing plate
- Pad Printing Inks
- Silicone
rubber printing pad
- Doctor blade or ink cup
- Machine
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.
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
affect pad printing. Etch depth, ink condition, ambient conditions, pad shape,
surface finish, hardness and machine speed are the key factors.
INKING
SYSTEMS
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.
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