Bopp film Manufacture: Metallization


The metallization of a BOPP film is a process that allows the deposit of a thin layer of metal (about 200 Å) on the film surface through the condensation of vapours.

This process doesn't alter the original properties of the materials. It increases the opacity to light and to UV radiations, as well as improves the barrier properties to oxygen and water vapour, and gives the film a glossy and reflecting appearance. Thus  considerably enhancing the aesthetic effect and the outcome of the packaging presentation.

The  metallization process takes place in semi-continuous machinery and consists of alternating "cycles" of pure metallization (vacuum) with cycles of simple and methodical preparation (in air) of the machine for the following working phase. 

The Aluminium, under ultra vacuum conditions (vacuum level required: ~ < 4 x 10-4 mbar), is brought to vapor state inside the machine in crucibles usually composed of Boron Nitride (BN) and Borides of Titanium (TiB2), that reach 1500 °C temperature.

The aluminium wire (1,8 mm diameter) is fed and carried on the crucible where the metal sublimation takes place. The aluminium vapors then condense on the BOPP film as it is being cooled.

Measures on metallized film

The parameters that are always measured on BOPP metallized film intended for food packaging are the following:

  • Metal-film adhesion
  • Gas transmission rates to the film (oxygen and water vapour)
  • Optical density (indirect measurement of the aluminium quantity deposited)
  • Optical density measurement

The crucible is heated by applying a voltage at the two ends (i.e. an electric current is passed through it)

A unidirectional beam of light is directed perpendicularly on a film sample. The light passing through the film is measured.
The optical density is linked to the light transmission (%) by the following equation:
Optical density = log10 (Li/Lt) = log10 (100/light transmission (%))
The following table relates the optical density values with the light transmission values (%)

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