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CPP films - an overview

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.


Smartech Global Solutions undertakes market studies, project report assignments, feasability studies and consultancy services for the plastics industry. If you require further details on CPP films or want to post an enquiry you can contact us or send us your queries via email

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