Manufacturing: Surface Finishing

Pad Printing

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:

Printing plate
The ink
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.

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.

INKING SYSTEMS

Conventional pad printing machines are divided into three families.

1. 'Open ink trough’ where the ink is held open to the atmosphere in a trough either behind the plate or around the plate.
2. '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.
3. ‘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.