Printing Graphic Overlays with UV Screen Inks

Graphic overlays have become part of everyday appliances and machinery. Most microwave ovens, membrane switches and OEM labels incorporate some form of graphic overlay. Traditionally, these overlays have been printed on polycarbonate and polyester films with solvent-based inks. The demand for more productivity through increased processing speeds is fueling the demand for UV inks which can be successfully used in this challenging application. This article will address many of the issues the printer will face as they move to UV inks for this application.
Graphic overlays consist of a polycarbonate or polyester film of various thickness ranging from 5 to 15 mils. All of the printing is done second surface to provide protection from harsh environments, strong chemicals or cleaning solvents that the overlay may be exposed to in everyday use.
The most difficult challenge for the screen printer will be the degradation of the polycarbonate and polyester films which result from exposure to UV energy. Exposing these films to short wavelength UV light (200-260 nm) results in a degraded surface layer which frequently may result in ink adhesion problems. If the ink is not formulated correctly and cannot successfully penetrate the degraded film layer to fuse with the underlying virgin layer, the resulting adhesion will be poor. The printer should insure that the UV ink they use for the printing on polycarbonate and polyester films has been formulated to successfully overcome this issue. Most graphic overlays will incorporate multiple colors (sometimes as many as 12 to 15) in a single job. Therefore, the ink's intercoat adhesion is absolutely critical.
Since the typical overlay requires multiple colors, the first layers of the ink applied will see multiple exposures to the UV curing lamps, which can lead to a hard ink surface that cannot be etched by the next ink layer. Again the printer should ensure that the UV ink they are using has been formulated to provide an adequate processing window so that multiple passes under the UV lamps will not cause an intercoat adhesion failure.
After printing, the overlay is laminated to the part (such as an instrument panel) with a pressure sensitive adhesive (PSA). The PSA used is typically very aggressive and is designed to prevent the overlay from being stripped off the finished part. The ink must perform as a functional part of the lamination since it is the bonding layer between the PSA and the part. Therefore, the ink's adhesion to the overlay film is paramount, and can not be affected by exposure to extremes of temperatures or humidity. Ink delamination is a common cause of failure due to an improperly designed or applied ink. The overlay may require embossing for use as an actuation or membrane touch switch. This will require that the UV ink have a high degree of flexibility and formability.
Typically, the end user of the actuation panel will require that it be able to pass a specified number of actuations (as much as 1 million or more). If you are producing actuation panels you should ensure that the UV ink being used has been tested to meet or exceed your customer's specification (we recommend ASTM F1578-95). The printer should work closely with their ink supplier to develop a full conversion plan when switching to UV inks to ensure all necessary issues are addressed.
It is strongly recommended that the printer conduct a complete analysis to assess what areas need to be addressed prior to making a conversion to UV ink. Sericol's Needs Assessment Survey provides an excellent format for conducting such an analysis. You can contact Sericol's technical support team direct at 1-800-SERICOL to discuss the Needs Assessment Survey in greater detail.