Pad printing (also called tampography) is a printing process that can transfer a 2-D image onto a 3-D object (e.g., a ceramic pottery). This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise difficult to print on products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as 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 to 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.
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 surfaces, such as flat, cylindrical, spherical, compound angles, textures, concave, or convex surfaces.
History
While crude forms of pad printing have existed for centuries, it was not until the twentieth century that the technology became suitable for widespread use. First gaining a foothold in the watch-making industry following World War II, developments in the late 60s and early 70s, such as silicone pads and more advanced equipment, made the printing method far more practical. The ability to print on formerly unprintable surfaces caught the imaginations of engineers and designers, and as a result pad printing exploded into the mass production marketplace.
Today, pad printing is a well established technology covering a wide spectrum of industries and applications.
Process
Cycle
From the home position, the sealed ink cup (an inverted cup containing ink) sits over the etched artwork area of the printing plate, covering the image and filling it with ink. The sealed ink cup moves away from the etched artwork area, taking all excess ink and exposing the etched image, which is filled with ink. The top layer of ink becomes tacky as soon as it is exposed to the air; that is how the ink adheres to the transfer pad and later to the substrate. The transfer pad presses down onto the printing plate momentarily. As the pad is compressed, it pushes air outward and causes the ink to lift (transfer) from the etched artwork area onto the pad. As the transfer pad lifts away, the tacky ink film inside the etched artwork area is picked up on the pad. A small amount of ink remains in the printing plate. As the transfer pad moves forward, the ink cup also moves to cover the etched artwork area on the printing plate. The ink cup again fills the etched artwork image on the plate with ink in preparation for the next cycle. The transfer pad compresses down onto the substrate, transferring the ink layer picked up from the printing plate to the substrate surface. Then, it lifts off the substrate and returns to the home position, thus completing one print cycle.
Plate and ink interface technologies
Open inkwell system
Open ink well systems, the older method of pad printing, used an ink trough for the ink supply, which was located behind the printing plate. A flood bar pushed a pool of ink over the plate, and a doctor blade removes the ink from the plate surface, leaving ink on the etched artwork area ready for the pad to pick up.
Sealed ink cup system
Sealed ink cup systems employ a sealed container which acts as the ink supply, flood bar and doctor blade all at the same time. A ceramic ring with a highly polished working edge provides the seal against the printing plate.
Printing pad
Pads are three-dimensional objects typically molded of silicone rubber. They function as a transfer vehicle, picking up ink from the printing plate, and transferring it to the part (substrate). They vary in shape and diameter depending on the application.
There are two main shape groups: "round pads" and long narrow pads called "bar pads". Pads are also made in other shapes, called "loaf pads". Within each group there are three size categories: small, medium, and large size pads. It is also possible to engineer custom-shaped pads to meet special application requirements.
Image plate
Image plates (also called clichés or print plates) are used to contain the desired artwork "image" etched in its surface. Their function is to hold ink in this etched cavity, allowing the pad to pick up this ink as a film in the shape of the artwork, which is then transferred to the substrate.