Pad printing is widely used for printing content, such as text, images, logos, or patterns, on different types of substrate surfaces. For example, the substrate surface may be flat or curved. A curved surface may be convex or concave, and may have a cylindrical or spherical geometry. Objects to which pad printing may be applied include bottles or other containers, balls, handles, tubes, or other objects.
In a pad printing system, a printing plate or cliché is provided on which the content to be printed is engraved or etched. Thus, cliché includes a set of recesses or grooves in the form of the content to be printed. The cliché may be made of stainless steel, ceramic, photopolymer, or another suitable material. For example, the grooves of the cliché may be formed by chemical etching using photolithography techniques. Such chemical etching may form a rough surface within the etched groove. As another example, grooves of the cliché may be formed by, or may be further processed by, laser engraving.
During pad printing, the grooves of the cliché are filled with ink. Typically, ink is deposited on a surface of the cliché by a liquid ink supply assembly. For example, the ink may be provided from a closed ink cup or from an open inkwell via a flood bar. A doctor blade or other structure travels across the cliché surface to spread the ink so as to cover the groove structure. After the ink is spread, doctor blade is moved across the surface (e.g., in the opposite direction) to remove any excess ink that remains outside the grooves of the cliché surface.
A deformable pad is pressed against the cliché plate with the ink-filled grooves. The ink in the grooves adheres to and is transferred to the pad surface, forming a pattern of ink that duplicates the pattern formed by the grooves. For example, the pad surface may be made of silicone rubber.
The deformable pad surface with the adhered pattern of ink is pressed against the printing surface of an object or substrate. When pressed, the ink is transferred from the pad surface to the printing surface. A pad may be designed for a particular concrete type of object printing surface.
Resolution of a printed image may be limited by the quality of the grooves of the cliché, by deformation of the pad when transferring the image from the cliché to the object surface, or by adhesion of ink to the grooves of the cliché. For example, when a cliché is formed by standard techniques, groove resolution may be limited to about 20 μm. Accuracy in the depth of the grooves may be limited to about 10%. Wall roughness within the grooves may be about 0.7 μm and pattern edge roughness may about 10%.
In recent years, patterning techniques have been developed to overcome the resolution limits of photolithography. For example, microcontact printing typically utilizes a soft polydimethylsiloxane (PDMS) mold or stamp to print ink patterns. Nanoimprint lithography forms a thickness contrast by deforming a thermoplastic polymer film under high pressure at elevated temperature. In reversal imprinting a polymer film is spin coated onto a patterned mold or stamp and is then transferred to a substrate at suitable temperature and pressure.
The size of printed area on curved surfaces may be limited by the flexibility of the pad. Thus, multiple pads may be utilized to print a single image or pattern, or a specialized pad may be made to match a particularly shaped surface. Patterns that are defined by wide grooves on the cliché (e.g., wider than about 50 μm) often result in distorted images when transferred to the object printing surface.
During pad printing, the pad, which is typically made of silicone rubber or similar polymer material, is subjected to wear that may limit its lifetime. For example, during printing, the pad may be pressed and rubbed against rough surfaces of the cliché, against sharp edges of the grooves, and against the object printing surface. Use of conductive ink or paint (e.g., DuPont 9169 silver conductor), e.g., in printing conductive grids for electromagnetic interference shielding or for photovoltaic cells, may limit the lifetimes of the pads.