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The following paragraphs describe how ink is transferred from the image plate to the part to be printed on any standard type of pad printing machine.
Inking the Image
The ink cup pushes ink across the image plate (or cliche) that contains the engraved image, filling the image with ink. When the ink cup returns in the opposite direction, it removes the remaining ink (and outside the etching of course) from the plate. The solvents in the ink that remains in the etching evaporates. The evaporation of the ink solvents inside the etching causes the surface of the ink in the etching to get tacky.
Picking Up the Image
The pad comes down to the plate and makes contact with the pad printing ink inside the etching. As the pad is compressed, it spreads, or rolls out, toward the edges of the image. This motion pushes aside the air that is above the ink film so the pad comes in contact with the ink. If the air cannot escape, air pockets remain between the pad and the picked-up ink film, which become visible as pin holes in the print.
As the pad goes up, part of the tacky ink film inside the etching is picked up by the pad. The rest of the ink stays inside the etching.
Transferring the Image to the Substrate
Once the pad picks up the image from the cliche, the pad moves horizontally toward the part to be printed (called the substrate). The pad is stopped in a horizontal position above the part to be printed.
While the pad is moving into position above the part, more solvents in the ink sticking on the pad evaporate, which makes the ink film get tacky again. It is this tackiness that makes the ink adhere to the substrate.
The pad is pressed down on the substrate and the ink sticks to the surface to be printed.
Returning to the Starting Position
The pad returns to its original position over the plate and is ready for the next print.
NEXT: About the terminology – Pad Printing Defined
