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Figure 8.10 The extrusion and extrusion laminating process (courtesy of Iggesund Paperboard).
Ionomer resin (Surlyn™), a polyolefin with high resistance to fat, including
essential oils in citrus fruit, and moisture with very good sealing properties, is
used as a tie layer on aluminium foil when applying PE to foil.
The process of extrusion is often extended to include extrusion lamination
so that a structure, such as paper or paperboard/PE/aluminium foil/PE, can be
produced in one operation on an extruder with two extruding units.
Note: By special selection of polymers, e.g. for lids and trays, it is possible
to provide easy-open peelable heat-seals.
Printing and varnishing
Usually, printing and varnishing are associated with the appearance of the pack
with respect to the visual impact of the pack through colour, information, text
and illustration. There are also important functional aspects of printing and
varnishing which are important for food packaging.
All the main printing processes are used – Gravure, Flexographic, Letterpress,
Silk Screen and Lithographic. Paper and paperboard can also be printed by the
recently introduced digital process. Choice is influenced by the appearance and
performance (functional) needs and commercial aspects such as order size, lead
time and price.
The inks and varnishes may be those described as traditional for the process
concerned, based on pigment, resin and vehicle. The vehicle, which transports
the pigment and resin from the ink or varnish reservoir to the substrate via the
printing plate, varnish pick-up roll etc., may be an organic solvent, water or
a drying oil. For some processes, pigments are replaced by dyes. In recent years
inks and varnishes cured by UV radiation have also become popular and these