What is pad printing

Pad printing is a printing process that can transfer a 2-D image (you like) onto a 3-D object (you want). This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the printing plate (cliché) via a silicone pad onto a substrate (surface to be printed).

Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, electronics, appliances, sports equipment and toys. It can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants.

Physical changes within the ink film both on the cliché and on the pad allow it leave the etched image area in favor of adhering to the pad, and to subsequently release from the pad in favor of adhering to the substrate (material being printed).

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 unique properties of the silicone pad enable it to pick the image up from a flat plane and transfer it to a variety of surface (i.e. flat, cylindrical, spherical, compound angles, textures, concave surfaces, convex surfaces).

　

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. 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.

Open and Closed Systems

There are two basic categories or types of systems under which all pads printing machines fall: open and closed.Open systems feature an ink trough or basin or some type which is at least partially exposed to the air. Closed systems have "ink cups" that keep the ink from coming into contact with the air.

Basically open systems allow you to print larger /images/, since more of the cliché’s surface area can be used for the image. Closed systems allow better process control, especially for longer runs, since the ink isn't exposed to the air and doesn't require the frequent addition of thinner. Closed systems typically use less ink, allow for faster cliche' and color changes, and are easier to keep clean.