The pad printing
process has its origins in screen, rubber stamp and photogravure printing
processes. It is a unique process due to its ability to print on
three-dimensional objects and at compound angles. The image to be transferred
is etched into a printing plate commonly referred to as a cliche. The cliché can
be a photopolymer plate or a steel plate. A pad made of silicone rubber is used
to transfer the image to a substrate.
Once mounted in the machine,
the cliche is flooded with ink. The surface of the cliche is then doctored
clean, leaving ink only in the image area. As solvents evaporate from the image
area the ink's ability to adhere to the silicone transfer pad increases. The
pad is positioned directly over the cliche, pressed onto it to pick up the ink,
and then lifted away. Then the pad is shifted and pressed down onto the
substrate, conforming to its shape and depositing the ink in the desired
Pad printing is limited to
relatively small images compared with screen printing. Pad-printable image area
is usually less than 100 square inches. There are two basic types pad printing
machines: open and closed. Open systems feature an ink trough or basin of some
type that is at least partially exposed to the air. Closed systems have "ink
cups" that keep the ink from coming into contact with the air. In addition to
standard solvent-borne pad printing inks, there are baking inks, oxidizing inks
and UV-cured inks that are used in pad printing.
Steel-backed photopolymer plates
are preferred as clichés. Polymer thickness ranges from 30 to 50 microns.
Photopolymer plates are available which can
withstand more than 100,000 impressions, with an image resolution that meets or
exceeds that of etched steel clichés. These plates are also used in letterpress
printing and are generally based on nylon chemistry. They may be developed with
water or alcohol. Manufacturers of photopolymer plates include those
here. One of the finest examples of pad printing is printing on pimpled
surface of golf balls.
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