Find out how FIM functions in a variety of high-end applications and determine whether it’s a fit for your business.
By Neil Bolding
Delamination failure of the film substrate The film surface pulls away with the ink from the rest of the film. Delamination failure is usually a feature of the film itself. PET can tend to fail in this way under extreme test conditions due to the internal film structure created by crystallization of the polymer during the manufacturing process. This occurs at test forces so high that this failure mode is unlikely to occur in normal use and should not be regarded as a cause for rejection.
Trapped solvents These may weaken the film and create forming problems such as stress cracks or blisters. The solvents may even pass all the way through the film and have undesirable effects on the hard coating.
Mirror inks require special consideration. These inks rely upon a completely flat surface on which to allow the pigment plates to lie in order to create a high-quality mirror effect. FIM films with a smooth second surface must be used. Even on the gloss print surface of gloss/gloss PC films, care is needed to ensure that the print surface is not etched by the ink solvents and that overprinting of the mirror by other inks does not disrupt the mirror effect.
For rapid drying, a lower print deposit is better suited for FIM than a higher deposit. Thinner coatings dry more quickly and give less opportunity for solvent attack on the PC film. However, care should be taken to ensure that coverage is sufficient to prevent pinholes—especially in backlit applications. This is most important when matte-second-surface grades are used, as the rough print surface of these products must be completely flooded. It is usually better to use two or more thin layers of ink than one thicker layer.
Drying of each layer of ink must take place immediately after printing. Forced hot-air drying (jet drying) drives the solvent efficiently from the ink without skinning the surface, which can trap solvent and make drying more difficult. A typical drying sequence for each print pass is 2 min at 80°C. The volume of air used in the dryer is also very important in preventing skinning. Air volumes and velocities should be as high as can be achieved on the equipment without causing damage to the wet ink surface.
Thorough baking of the inks is normally required after the final print pass in order to achieve the best possible resistance to ink-wash problems during injection molding. Typical conditions are between 60-80°C for 3-6 hr. Racking of sheets is recommended for baking, as this allows maximum opportunity for trapped solvent to escape. It is important, however, to ensure that the hot rack does not directly contact the film, as this may cause localized heating which results in line defects caused by flow in the coating. The film should, therefore, be placed onto a support sheet that will insulate the film from the rack.
Baking rolls of printed product does not allow ink solvents to escape as easily as they can in a racked sheet. Therefore, thorough drying of the prints after the final print cycle is even more important in roll-to-roll printing. With most FIM inks, baking is still required in order to develop the full melt-resistance properties of the print.
Neil Bolding is quality, technical, and marketing manager for MacDermid Autotype.
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