(Quoted from Duane Fast and Sign & Digital Graphics. Full article here.)
Printing with UV-cure inks is a great solution for many printing situations. However, there are some difficult substrates, such as plastics, metals and glass, where adhesion may be poor or non-existent. Many plastics have low-energy surfaces that actually repel inks; metals may have oils in them from the manufacturing process; and glass/ceramics are just too hard and non-porous for just about anything to stick.
Click here to read the full article from Duane Fast and Sign & Digital Graphics