All films made from thermoplastics share a common problem: special production techniques must be used to achieve a uniform thickness. Unlike extruded tubes and sheets, extruded films can have large variations in thickness if special steps are not used. Even now, the quality of the films produced in large quantities is rather poor as far as thickness distribution is concerned. In any case, too many resins are used for this type of film and its application is limited by its low quality. The better films gain market share and control higher prices, which shows that sophisticated production lines with good measuring and control technology are effective.


Cast films

1. Thickness variation and film quality
Thickness variation affects the quality of cast film in two ways: economically and technically. If the film sold is guaranteed to have a minimum thickness, then reducing thickness variation will directly lead to savings in resin. The more uniform the film thickness, the better the film rolls up. Thickness variation can lead to permanent deformation of the rolled-up film. The unwound film will not be flat for further processing, which will result in irregularities in lamination, printing, bag making, etc. This means that such a thickness variation is not a sign of poor quality, but rather a defect in the film's flatness.


2. The formation of thickness variations
Cast film die heads have hanger-shaped runners, usually heated by cartridge-type electric heating elements At present, the runners are designed for the point of operation. This is a compromise solution and represents an optimum solution for the desired operating range. It is therefore clear that the cast film die head requires some control of the melt flow. Irregularities in melt flow can arise from inhomogeneities and heating disadvantages, and it follows that adjustments should also be made. In addition, film thickening at the edges of the film caused by melt shrinkage between the die head and the calender rollers should be controlled.



3. Influence of film thickness
The film thickness is changed by adjusting the live die lip at the die head gap. Local changes of ±50% or more in the melt flow are entirely possible, as the die gap causes considerable back pressure on the melt flow. This back pressure can be changed considerably by adjusting the live die lip. The average thickness of the film cannot be changed by the position of the live die lip, but is only influenced by the amount of melt passing through, the film speed and the film width.
4. Adjustment of the live die lip
The adjustment elements acting on the live die lip at approximate intervals of 25 mm may be:
-pressure screws for manual adjustment;
-Thermal moving bolts;
-mechanical adjustment devices with stepper motors acting on the screws

-hydraulic regulating devices for remote and automatic adjustment.

Generally, the regulating element is pressed against the lip of the live die, but push/pull elements are rarely used. For the automatic adjustment of the live die lip, thermobolts and piezoelectric converters are usually used. The thermobolt is made of steel and adjusts the lip by thermal expansion. The bolt is electrically heated to elongate. In order to shrink quickly, the bolts are cooled by air. The piezoelectric converters are made of porcelain discs, arranged in layers. The thickness of these discs varies with the applied voltage and cumulatively results in a change in the length of the converter. The thermodynamic bolts take a certain amount of time to elongate and contract, whereas the piezoelectric converters act without any significant lag. The hydraulic and mechanical systems also act very quickly. In comparison, thermally actuated bolts are used with the most energy in the production of cast films.



For example, a die head with a live die lip is actuated by a piezoelectric converter on the die head as follows: In the non-operating state (no material in the die head) the die head opening is 0.8 mm. The die head opening is uniformly reduced to 0.6 mm by mechanically actuated adjusting elements. The driving force for the opening of the die head is the combined force of the melt pressure in the die head and the elastic deformation of the live die lip itself. The cross-section of the live die lip itself must be optimally characterised. The die lip must be able to deform locally, but must not be distorted between the regulating elements due to the melt pressure.


5. Control system and control measures
A die head with a live die lip can gradually be operated automatically. The automatic operation is only possible because of the incorporation of a measuring device (infrared or beta radiation) that measures the thickness distribution of the film in the production direction of the transverse machine. A microprocessor processes the parameters and sends command signals to the regulating elements. The automatic control of the different production steps of the cast film line is based on the thickness control. Standard production equipment is equipped with a thickness controller and a film speed controller. The melt throughput can be controlled by weighted feeders or gear pumps. The manufacturers of measuring and processing equipment have developed a wide range of patented technologies, which are constantly being expanded by extrusion companies.

Some of the features of these instruments are:



-Quick and accurate thickness control;
-The identification and elimination of measuring errors;
-the identification of thickness variations in the machine direction and the exclusion of these variations in thickness when adjusting the transverse thickness distribution

-consideration of the condition of adjacent adjustment elements during the adjustment of the live die lip;
-predictable function of the entire die lip (no deviation towards "closed" or "open")
-the objective of adjusting specific thickness distributions that do not correspond to the required "uniform thickness".
Of course, the thickness parameters can be displayed as a distribution on the fluorescent screen to determine the trend and to obtain information on the condition of the regulating element. A typical illustration is given in Figure 2.37.


6. Operating results
The increasing use of automatically operated cast film die heads proves that the good quality of the film and the reliability of the installations justify this input. The hundreds of units in use benefit users worldwide. The accuracy of film thickness is not necessarily better with an automatically operated die than with an operator-controlled die, but it must be stressed that the results obtained with an automatically operated die are good for 24 hours a day and do not require an operator.



7. Multi-layer films
Of course, an automatically operated die head can also be used for multi-layer film extrusion, as the live die lip acts on each layer simultaneously. One problem, however, is the metering of the thickness. In this case, it is necessary to use an instrument that can measure the layers of film with different densities, so that the total film thickness can be calculated. For determining the thickness of the film, this is not suitable for distinguishing between layers of equal density, which may be important in other respects.

Nanjing Sumino Precision Machinery Co., Ltd. has professional cast film production lines, the company's main cast film manufacturing lines are: TPU hot melt adhesive film production line, CPP/CPE cast film production line, EVA solar encapsulation adhesive film production line, PE hygiene breathable film production line, PVDF cast film production line, PP three-dimensional film production line, medical blood purification film production line, multi-layer co-extrusion barrier film production line, LLDPE winding film production line, etc.