- February 01, 2001, Richard M. Podhajny
Although this column primarily covers conventional printing processes, I do occasionally receive questions about processes that are less common, such as pad printing.
Pad printing, which has its roots in intaglio and flexography, is a unique process that can be used to print on 3-D objects. Examples of products that utilize this printing technology are golf balls, syringe barrels, untreated silicon, and polyolefin plastics.
Pad printing has been used for more than a hundred years, and it is one of the most versatile of the printing processes. The best way to describe it is that it resembles offset gravure; i.e., an ink is used to flood an engraved plate, which is doctored to remove the excess ink in the land areas, and then physically transferred by contact to a compressible pad, which then deposits the ink onto a substrate.
The five essential steps involved in pad printing are as follows:
Step 1: Flooding the engraved plate with the ink: Pad inks are typically much slower drying than gravure inks, since the overall pad printing process is much slower.
Step 2: Doctoring the ink: Like rotogravure and flexo, this process involves doctoring the excess ink by use of a metal doctor blade.
Step 3: Transferring the ink from the engraved plate to the pad: The ink in the engraved plate is transferred by physically contacting the engraved plate surface with a compressible pad that picks up the ink and transfers it to the desired substrate. Any solvent loss of the ink on the pad will increase its tack. This "tack increase" is controlled, in some cases, by a heater that is in the proximity of the pad(s) and can be adjusted within certain limits of temperature and press speed.
Step 4: Depositing the ink onto the substrate: Because this requires complete release from the pad and good wetting and transfer to the substrate, the pad is usually made out of silicone. The pad lifts away from the substrate and assumes its original shape.
Step 5: Drying of the ink: Since the ink often is composed of slow solvents, solvent removal requires heat.
There are basically two types of pad printers: rotary and single-stroke machines. The majority of pad printing machines are single-stroke vertical or horizontal machines. The other main types are rotary pad printers. These also can be vertical or horizontal but utilize a rotogravure cylinder rather than a flat engraved plate.
For difficult substrates, the machines can be equipped with an in-line corona treater similar to what is used in conventional flexographic film printing. Flame or plasma treating is used in some applications for surface enhancement.
This process has been primarily developed with solvent-based inks and is limited to relatively small image areas. The pad printing process can print multiple colors, but most of the vertical machines are one-color printers.
Pad inks are typically solvent-based inks composed of binders, pigments, thinners, and additives. The complexity of the pad printing process requires careful attention to the ink drying and tack buildup.
End-use requirements often necessitate the use of adhesion promoters, matting agents, waxes, and antistatic additives. In addition, various ink chemistries can be used, including inks that dry by oxidation, UV-cured inks, or catalyzed systems.
Two of the key advantages of pad printing are that you can print irregular shapes and you can print on a variety of different substrates. It is a versatile process that can handle some of the most unusual and difficult printing requirements. It's not going to replace conventional printing processes, but it might be the answer you are looking for in certain applications.