How UV-LED is used to gel ink droplets
Splat. You can picture a tiny drop of ink thrust from a piezoelectric inkjet nozzle landing on a substrate. The ink droplet will not remain intact for long. Even under calmer conditions—like delicately depositing the drop onto a sheet of paper—it tends to spread out; increasing in diameter and altering its shape. The dimensional change from spreading, called dot or drop gain, is a nagging problem for digital printers. In a world where precise drop size and location determines print quality, drop gain tends to blur images and muddle colors with a loss of resolution, color integrity and gloss control. It is a finicky problem, confounded by a lot of variables, but one that is being helped by a new technology: UV LEDs.
UV LEDs are light sources that allow digital printers to gel or freeze UV ink droplets quickly before they have time to spread out (Figure 1). This is possible because the UV-LED light sources are more compact than conventional curing systems and can be easily positioned between successive print heads. The process of freezing ink droplets between print heads is called pinning and is somewhat different from fully curing the ink (Figure 2). While pinning stops the drop from spreading, it leaves the ink flexible and soft enough for proper inter-coat adhesion to other ink droplets and for further handling.
Full or final curing ordinarily occurs at the end of the process and transforms the ink into a more rigid film with the required final surface properties. All pinned inks must still pass underneath a final cure UV source to be fully cured.
Pinning is also different from so-called inter-deck curing found on some larger offset printing presses, where colors are early completely cured near the drum before other colors are applied downstream. A close-to-full cure is necessary with an offset process because the transfer plates make physical contact with the cured surface during printing. Pinning is possible with inkjet processes because inkjet printing requires no physical contact with the cure surface (Figure 3). The extent of dot or drop gain depends on properties of the ink, substrate, and the process environment.
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