Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
to create a cell. The melt temperature has to be carefully controlled at this point,
therefore, because it affects the melt strength.
Films produced in this way have some of the attributes of paper, e.g. appearance
handle, stiffness and some retention of creasing. Densities are of the order of 600 g/l
(37.5 lb/ft 3 ).
2.7 VACUUM METALLIZATION:
Although not strictly a printing process, vacuum metallization is a means of
decoration. The fundamental principle of this process is the heating of aluminum to a
high enough temperature to vaporize it. In practice this necessitates temperatures in
the range 1500—I800C. One way of achieving this is to hang short pieces of
aluminum wire onto a tungsten filament which is then heated electrically. This is only
suitable for short heating cycles and so is used for metalizing plastics moldings on a
batch system. For longer cycles such as are needed for the metallization of film, the
aluminum wire is fed onto a block of metal, usually titanium. The tantalum is heated
by holding it in a carbon crucible across which is arced a high voltage current.
Vaporization of the aluminum causes minute particles to be ejected from the surface
in all directions. The operation must be carried out in a vacuum in order that the
metallic particles can reach the surface to be metalized
Certain films may have to be de-gassed before vacuum metalizing because they
contain moisture or other volatile constituents, such as plasticizers, which could
otherwise cause difficulties during the process. If the normal vacuumisation does not
completely remove them before metallization commences then their continued out
given interferes with adhesion of the aluminum giving dull and incompletely anchored
coatings. Even if the volatile constituents are completely removed they can make
vacuumisation difficult and cause contamination of the pumping system. Films which
need degassing include regenerated cellulose and cellulose acetate. Since this
degassing may also remove other, perhaps important, constituents of the film a
preferable method of solving the problem may be to seal the surface of the film with a
lacquer. It is important, of course, that the lacquer itself should not contain anything
likely to interfere with the vacuum. If colored metallic effects are required, the film can
be self-colored or a colored lacquer can be applied after metalizing.
One of the most important things to note about vacuum metallisation is that any faults
in the underlying film are not hidden by the very thin film of metal deposited. Indeed,
faults may even be enhanced. The lacquer film can help to hide substrate faults but
can, of course, contribute faults of its own as, for example, ‘orange peel’, or ‘fish
eyes’. Correct tensioning during unwind or re-reeling is also important, especially
with very thin plastics films because creasing will cause uneven deposition of the
aluminum. Cracking and wrinkling of the aluminum layer can also occur when a
metalized film is used to form a heat sealable material by extrusion coating it with
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