Current Status
The first line for Cast PP films was imported from
Reifenhauser about 15 years back. The line was imported for a dedicated
application of twist wrap for candies, as a replacement of conventionally
used Cellophane paper and to a extent, PVC films. Since then, at
least six more, mainly 3-Layer co-ex lines, have been installed
as of today and at least one more line is just 10-12 months away.
The potential for CPP Films was estimated around 10,000 tpa by the
year 2000 to cover the segments such as wrappers, bags, lamination
film, soft blister film, twist wrap, stationery products, health-care
products. However, the manufacturers of CPP Films have not been
able to generate the estimated demand for various short-listed end-uses.
One major bottleneck; as I have myself experienced,
has been non-availability of the required materials in small quantities
for trials from the ready stocks. The second has been the comparatively
very much lower priced TQ film, which could grab many applications
of CPP Films. The third major factor was the drop in prices of PET
films and BOPP films. And also important was lack of sustained efforts
to develop CPP Films usage.
All the grades and sub-grades of PP - Homo, Copolymer,
Random Copolymer, Metallocene PP - are required for one or the other
application, either as such or as one of the layers in the co-extruded
films.
Manufacture of CPP Films and their correct usage
is based on a proper selection of a PP grade and employment of the
Precise Processing Parameters based on the polymer, additives, equipments
and the end-use requirement.
Let us briefly look at the Manufacture of CPP Films.
Cast film grades of PP generally have a mfi range
of 5.0 to 12.0 gms/10 min and contain slip and anti-block additives
to prevent problems in handling particularly on very high speed
packaging machines.
The plastics web is extruded from a T-Die to fall
on water-cooled chill roll, where the inlet temperature of water
is maintained between 8OC and
12OC to have effective cooling.
It is also important to have uniform surface temperature over the
entire surface and that there is no dew formation. The roll stack
is generally vertical but the new trend is to use horizontal or
inclined roll stack.
Film thickness is partially regulated by the gap
between the die lips but also by the rotational speed of the chill
roll which is arranged so as to draw down and reduce thickness of
the melt web. The die gap, therefore, is set a little higher than
the desired film thickness. Typical die-gap settings as a very general
reference are; since they vary with polymer, equipment and the processing
parameters, 0.4 mm for film upto 0.25 mm thick, and 0.75 mm for
films in the range of 0.25 and 0.6 mm. Generally films are produced
in the thickness range of 14 microns to 250 microns. It is very
important to precisely control the film thickness over the entire
width, except the edges, which are thicker and are continuously
trimmed off, ground and fed back to the hopper, by adjusting the
points provided across the Die Width. On the new equipments, thickness
indicators such as Beta Gauges are provided to continuously show
on the monitor variations across the width to enable the operator
to make precise adjustment. On some of the latest equipments, there
is automatic adjustment of the Die Lips to monitor and control the
film thickness. Film Roll quality suffers if the traverse tolerance
exceeds + 5% of the set thickness. This will result in uneven
winding, creases, non-uniform treatment level and higher wastage
in slitting and further processing.
The barrel temperatures are set between 180OC
/240OC and 300OC
to get better optical properties. A Die Temperature may be slightly
higher to allow for the cooling due to exposure to lower ambient
temperature. A constant temperature of the Die across the entire
width is very important so that the film draw down rates and physical
properties remain constant across the entire web. Any playing with
the set temperature profile across the Die for controlling the film
thickness will disturb these factors and adversely affect the film
quality. The Die is kept as close to the chill roll as possible,
say between 40 and 80 mm, so that the web which has low melt strength
remains unsupported for the minimum possible distance and time.
If the Die is too close, there is no sufficient space for thickness
drawdown and width-wise neck-in to take place in a precise way.
The web flows on to the water cooled chill roll
with a temperature of around 240OC
or even more before passing to a second chill roll and then proceeding
to edge trimming, tensioning and winding.
The first chill roll considerably influences the
process quality. The cooling capacity must be adequate to chill
the film eve at high output rates and the temperature gradient across
the width of the roll should not exceed + 1 OC
The actual roll temperature depends on film thickness, line speed
and roll diameter, the common set temperature being around 200 OC.
The chill roll drive speeds must be very precisely
controlled to control film draw-down and the final thickness of
the film. The chill rolls have a high mirror finish but embossed
matt finish rolls are preferred for production of Stationery film.
Embossed cast film can by produced be replacing
the air knife with a pressure roll in conjuction with an engraved
or etched chill roll. The pressure roll forces the hot melt into
the surface texture of the chill roll to produce film with an appropriately
embossed appearance.
Neck-in Phenomenon
Neck-in is usually defined by the difference between
the width of the die slit and that of the film. The larger the neck-in,
thicker the edges of the film and therefore, the yield goes down
according to the increase of the edge trimmings. The neck-in is
related to surface tension and elastic modulus of the molten film
and then is caused by contraction of the film. The degree of neck-in
is related to the characteristics of the PP Polymer with respect
to density and Melt Index, temperature of the film emerging from
the Die, the length of air gap and width of the die slit. It is
now understood that the longer air gap and a higher temperature
gives a large neck-in and that the air-gap has the greater effect.
In general, the neck-in and stretchability show an approximately
contrary tendency. A molten film with large neck-in can be cast
and thinned in higher take-up speed.
Edge Bead Phenomenon
Edge beads are generally caused by surface tension,
Die Swell and an edge stress effect. The predominant cause of edge
beads is an edge stress effect, which occurs when films is stretched
between the die and the roll. The edge elongates in uniaxial stress
while the center material elongates in plane strain.
Note:
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