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
The inks used for the FIM process should be chosen carefully, as conventional screen-printing inks are not generally formulated to withstand the higher temperatures and mechanical stresses imposed during thermoforming and injection molding. Use inks qualified by the ink supplier for second-surface FIM applications. Most inks are solvent-based, but UV formulations are available. The inks must be processed precisely according to the manufacturer’s recommendations.
Printing onto plastic substrates requires care. Some plastics films are soluble in common ink solvents. This can give good adhesion by allowing the ink molecules to intermingle with the polymer at the interface, but penetration of ink solvent into the bulk film can make the film difficult or impossible to dry thoroughly. If solvent is trapped inside the film, then it can be vaporized during forming or molding with catastrophic effects on cosmetics (bubbles in the film) or adhesion (gross delamination). This is true of PC, PMMA, and even PC/PBT blend films
Long bake cycles are used in FIM processes to prevent such problems, and it is the diffusion time through the film that limits the drying rate. Higher temperatures and high airflows help little once the solvent is deep inside the film. Correct ink and ink-solvent selection is crucial for such films. Too powerful a solvent can even cause stress cracking of the film that might not become visible until long after the part is completed. Too weak a solvent may give insufficient surface etching to allow good adhesion. Ink and film suppliers cooperate well in this market to provide optimal solutions, so it is essential to ensure they are engaged with processors to optimize the performance.
PET film is extremely resistant to solvents. For that reason, it is not susceptible to solvent penetration. Thorough drying is easier on PET, even though it may appear to take longer for the surface to feel dry. This is because solvent can only escape from the film from the top surface and not through the film. High temperatures and high airflows that scour the ink surface during drying ensure good solvent removal because the solvent never gets deeper then that thickness of the ink. This high resistance to solvents is also a huge benefit in the final application as PET films give excellent resistance to common chemicals and reagents.
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