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181
PLASTICS IN FOOD PACKAGING
For increased strength and improved barrier properties, film can be stretched
to realign, or orient, the molecules in both the machine direction (MD), and
across the web in the transverse (TD) or cross direction.
In the Stenter-orienting process, transverse stretching of the cast flat sheet is
carried out using clips which grip and pull the film edges, so that the width
increases. Stretching in the MD can be achieved with several sets of nip rolls
running at faster speeds.
With the blown, or tubular, film process, orienting is achieved by increasing
the pressure inside the tube to create a tube with a much larger diameter
(Fig. 7.4).
Film stretched in one direction only is described as being mono-oriented.
When a film is stretched in both the directions, it is said to be biaxially orientated. Packing the molecules closer together improves the gas and water
vapour barrier properties. Orientation of the molecules increases the mechanical
strength of the film.
Cast films and sheets which are not oriented are used in a range of thicknesses and can be thermoformed by heat and either pressure or vacuum to make
the bottom webs of pouches and for single portion pots, tubs, trays or blister
packs.
Cast films are also used in flexible packaging because they are considered
to be tough; if one tries to tear them, they will stretch and absorb the energy,
even though the ultimate tensile strength may be lower than that with an
oriented equivalent.
Annealing
zone
Extrusion
die
ter
ten
gs
ratin
le
e
Acc
s
clip
Cooling
roll
Extruder
Casting
rolls
Acc
eler
atin
g st
ente
r cli
ps
Heated oven
Wind-up
Figure 7.4 TD orientation by Stenter and MD orientation by acceleration in machine direction
(courtesy of The Institute of Packaging).
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