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PLASTICS IN FOOD PACKAGING
211
reduce with time, and hence the specification has to be set higher than the
known minimum requirement.
7.5.3
Thermal lamination
When two webs each have heat-sealing properties, it is possible to join them
together by passing the films through a heated nip roller system. With no adhesive involved, the final weight of the laminate is the same as that of the
original components. This process relies on the films each having a low sealing point, as under tension the films may shrink if heated at a high temperature,
causing creasing, or stretch under tension. As the films approach the elastic
limit, curl may be produced due to the slightly greater shrinkage of one web.
Bond strengths should be high depending on the nature of the original coatings. This form of laminating is not common for the production of laminates in
the food industry. It is widely used in book cover production. A specific type
of thermal lamination is that using a laser to activate the surfaces being bonded
(Potente et al., 1995).
7.6
7.6.1
Printing
Introduction to the printing of plastic films
Printing preferences until recently seemed to be geographical, with a tendency
for gravure presses being popular in Europe and flexographic presses in North
America. This may be historical, as the result of the way the markets
developed. As the quality of flexo printing has increased with the introduction
of photopolymer plates and with the market requiring shorter and shorter print
runs, the flexo process has gained ground in Europe. Combination presses
which incorporate flexo and gravure units have also become popular. The
number of printing stations has grown in number with up to 10 stations now
available on central impression flexographic (CI) machines and on gravure
reel fed presses.
7.6.2
Gravure printing
The gravure press consists of a series of printing stations in-line, each applying
one colour of liquid ink, applying cold seal latex or PVdC emulsion in-line.
A roller is engraved, mechanically, chemically or electrically laser eroded,
into a pattern of small cells. These cells hold the ink which is picked up from
the ink bath in which the gravure roller rotates. The amount of ink is controlled
by the depth and area of the cell, and a doctor blade scrapes off the excess ink.
Film is passed over the gravure roller with backing pressure from a lay-on, or
impression roll, to pull the ink out of the cells. The inked film is passed into