Hot foil stamping mainly involves the transfer of foil by means of a heated stamping die; it can be regarded as a form of letterpress printing. The basic differences between hot foil stamping and letterpress are the use of stamping foil instead of viscous ink as printing medium and the indirect heating of the printing form. Due to the combined effect of temperature and pressure, a layer of foil is released from the carrier at the points where the stamping die has raised areas and then transferred to the substrate, in which it is permanently and indelibly embedded. Hot foil stamping tends to be used e.g.to print text and motifs on a passport cover (e.g. gold stamping,silver stamping). Hot foil stamping is also used for the application of holograms and kinegrams®, etc.
Source : Click Here