The pad printing process is an indirect gravure printing, developed to supplement existing printing systems (for example silk-screen printing, embossed printing). Different materials (also materials sensitive to mechanical stresses, like glass or eggs) can be printed nearly independent of shape and surface structure (for example concave, convex, curved surfaces). It is possible to print in cavities of 3 D work pieces. In addition to ink also other liquids such as glue, varnish and separating agents etc. can be transferred.
Printing pads (also printing stamps or printing balls) out of silicone-rubber were developed about 30 years ago. Before pads were produced out of different materials (for example gelatine). Their printing format was not necessarily worse but the durability was insufficient. The result was that the printing process had to be interrupted in short intervals for changing the printing pad. The origin of the gelatine pads lies in the Swiss watch industry, which has already made greater experiences in the printing of clock faces.
Function of the printing pad
The task of the printing pad is the transfer of the printing format from the printing cliché to the printing substrate. It is pushed on the printing cliché, deforms and hereby takes the ink out of the etched cavities of the printing cliché. The ink film is now on the bottom side of the pad which is subsequently driven to the printing substrate. The printing pad is now pushed on the printing substrate, deforms and transfers the sticking printing format to the part. Now the ink film sticks to the printing substrate due to the adhesiveness of the solvent evaporation. In these operations the pad has to meet various mechanical requirements to guarantee a precise printing. The flexible deformability and the geometry of the printing pad have to be adjusted to the special printings tasks.