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To orient the longitudinally stretched film in a transverse direction, a stretching
temperature of approx. 160°C is necessary. Since the transverse stretching temperature
is considerably higher than the longitudinal stretching temperature, the film must be
pre-heated to that temperature. For thermo-fixing
of the oriented film, also a heat-
setting zone is required. Finally, the hot film must be cooled down to room temperature.
For
the above reasons, the TDO is subdivided into four sections:
−
Pre-heating section,
−
Stretching section,
−
Heat-setting section,
−
Cooling section.
The film is transported through the TDO by a clip system mounted on a chain
and guided by a chain rail.
At the film inlet into the TDO, the distance between the clips is monitored by
the edge guiding unit according to the width of the longitudinally oriented film.
In the preheating section, the temperatures are set at approx.170 – 180°C. The chain
rail adjustment in this section is mainly parallel, i.e. in the preheating section, the film
must not be stretched at all but transported only.
In the stretching section, the chain rail adjustment becomes continuously wider.
That means that the film is stretched in a transverse direction.
The transverse stretching temperature is approx. 160°C.
If a too low stretching
temperature is chosen, the film will break with a loud noise. In this case, the thermal
shrinkage of the film during heat treatment increases too. If the stretching temperature
is too high, the thickness tolerances are bad. The film becomes hazy and finally breaks.
During transverse stretching the film also becomes thinner concerning the
stretch ratio.
As the
film edges are held by clips, the edges remain unstretched during
transverse orientation. This is why the edges stay thicker.
For this reason, the
unstretched edges must be removed after leaving the TDO.
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In the heat-setting section, the transport system converges to thermo-stabilize
the film by temperature and relaxation. The film width is decreased by approx. 5-10%
compared to the width of the last stretching zone.
The heat-setting temperature is limited since too high temperatures make the
film hazier. The normal heat-setting temperatures are approx. 160 – 165°C.
The last section behind the TDO is a cooling zone. Here, the air at room
temperature is blown onto the film to cool it down. Quick cooling down is necessary
to achieve good optical properties, especially
for the co-extruded film, and to avoid
dimensional changes on the winder by thermal shrinkage.
After
cooling down, the film is released from the clip system for further
conveyance by the pull roll stand.
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