Processing factors that affect the film properties are generally as below :

Key
D : Drops / Lower             S - Negligible effect
H - Higher / Improves      I - Improve / Higher

Control of Thickness Profile

Thickness variation influences quality of the film in two ways - economic and technical. A reduction of thickness variations certainly means direct resin saving. Also, the more uniform the film thickness, less are the Winding and Wound Roll problems such as deformation of the film in the reel. If the film unwound is not flat, it causes problems in lamination, printing and bag-making.

The film thickness is influenced by the die gap other parameters such as melt throughput, film speed and film width are constant. The elements generally used for adjusting the die lip gap are:

• Pressure screws (manual)
• Thermal bolts
• Piezo translators
• Mechanical setting units
• Hydraulic setting units

The new trend is to have automatically operated dies which is done in combination with a continuous measuring device, infrared or beta (gauge) radiation which measures and projects the thickness profile traverse to the machine direction and a microprocessor interprets the data and produces the signals for the setting adjustments. The thickness data is shown on monitor as a profile. Most machine suppliers today are having this as their standard provision on the Cast Film Line offered by them.

The accuracy of the film thickness may not be necessarily better than the manually controlled die, but it certainly uniformly maintains this round the clock.

The automatic dies can be used for multiplayer films also by having instruments that can measure individual layers of the structure so that the total thickness can be calculated.

LINE SPEEDS

The CPP film lines outputs vary from as low as 100 Kgs/h to as high as almost one Ton/h and so also the line speeds of the equipments. At line speeds higher than 30 m/minute, there is a tendency for a thin cushion of air to become trapped between film and the chill roll. This results in slow and uneven chilling which in turn affects the film appearance and its properties. This problem is overcome by providing an air knife, which delivers a streamline jet of filtered air that impinges on the film just beyond the point of first contact with the chill roll and press it against the roll. An correct air pressure improves the film quality and gloss. The other way of overcoming the problem of air cushion is to provide a vacuum box to remove any condensates that are deposited on the roll by the film and sucks the air between film and the roll.

Some Common Problems

1. Poor Clarity
   Cooling rate too low. Try Nucleating Agent / Metallocene PP.

2. Variable Optical properties accross the width
   Coolant supply inadequate due to inlet water temperature and/or blocking of cooling circuits.

3. Plate-out
   Due to insufficient film contact with chill roll. Adjust air knife.

4. Flow Marks
    Melt temperature too low.

5. Brittle Seals
    Melt temperature too high.