The process of Pad printing is the most versatile of all printing processes with its unique ability to print on three-dimensional objects and compound angles. The theory behind the pad printing process was derived from the screen, rubber stamp and photogravure printing processes.
This is how the process works:
Step 1: Flooding:
The image to be transferred is etched into a printing plate commonly referred to as a cliche'. 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.
Step 2: Pick Up:
The pad is positioned directly over the cliche', pressed onto it to pick up the ink, and then lifted away. The physical changes that take place in the ink during flooding (and wiping) account for its ability to leave the recessed engraving in favor of the pad.
Step 3: Print Stroke:
After the pad has lifted away from the cliche' to its complete vertical height, there is a delay before the ink is deposited on the substrate. During this stage, the ink has just enough adhesion to stick to the pad (it can easily be wiped off, yet it does not drip). The ink on the pad surface once again undergoes physical changes: solvents evaporate from the outer ink layer that is exposed to the atmosphere, making it tackier and more viscous.
Step 4: Ink Deposit:
The pad is pressed down onto the substrate, conforming to its shape and depositing the ink in the desired location. Even though it compresses considerably during this step, the contoured pad is designed to roll away from the substrate surface rather than press against it flatly. A properly designed pad, in fact, will never form a 0-degree contact angle with the substrate; such a situation would trap air between the pad and the part, resulting in an incomplete transfer.
Step 5: Pad Release:
The pad lifts away from the substrate and assumes its original shape again, leaving all of the ink on the substrate. The ink undergoes physical changes during the head stroke and loses its affinity for the pad. When the pad is pressed onto the substrate, the adhesion between the ink and substrate is greater than the adhesion between the ink and pad, resulting in a virtually complete deposit of the ink. This leaves the pad clean and ready for the next print cycle.
Limitations of Pad Printing
Versatile as it is, the process does have a few limitations. For example:
1. Image transfer is much more efficient when solvent-based inks are used. The use of water-based or UV curable inks are not recommended.
2. The object to be printed needs to have a higher surface energy than the pad. This rules out printing on silicone and other non-stick materials, as well as wet, greasy parts. Additionally, some types of plastic require pre-treatment.
3. Pad printing is limited to relatively small /images/ compared to screen-printing. Pad printable /images/ are usually less than 100 square inches. Large opened areas (>4.0 sq. in) can be difficult to cover with special, screened cliché’s.
4. Pad printing produces a finished ink film thickness of approximately .00025" to .0003" with a single pass. By screen printing standards, this is very thin.
Examples:
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