Inline measuring and control of film thickness distribution across its width is the function of the gauge control system or APC (Automatic Profile Control). When the flexible lip on the die is manually controlled and the production process is well tuned, film thickness variations will be in the range of ±3 to ±5%. In automatic mode, it is possible to reduce these variations by half. The figure below shows an automatic die with the automatic control module mounted on the flex body of the die. The so-called thermal translators or thermal bolts form the module. The distance between the bolts is typically 1.125 inches.
The gauge control system includes a radiation emission unit and a control console. The radiation unit travels in the machine cross direction, scanning the film in cycles (measured in minutes). Commonly, the radiation originates from a beta ray source; although, x-ray and infrared sources can also be used. In general terms, the film thickness is determined as a function of the film radiation rate of absorption. Thus, variations on the absorption rate translate into film thickness variations.
The control console is the interface between the control system and the automatic die. Each adjustment point or thermal translator on the die is spatially correlated with a position on the film. This is called mapping.
The control system applies power to the thermal translators, as required, and the lip gap is regulated via thermal expansion of the adjustment element. An important variable associated with APC is the time constant. It is defined as the time needed for an adjustment element to elongate 62.3% of its maximum elongation. The shorter the time constant the more responsive the system is, translating to gains in productivity.
Corona Treatment
In order to facilitate the adherence of inks or coatings onto the film surface it is necessary to apply a surface treatment. Corona treatment is the most commonly used of the existing methods. Corona treatment increases the surface energy of the film and consequently its surface tension. The system includes a power source and the treatment station. The power source transform 50/60 Hz plant power into much higher frequency power in a range of 10 to 30 KHz. This higher frequency energy is supplied to the treatment station and is applied to the film surface by means of two electrodes, one with high potential and the other with low potential, through an air gap that typically ranges from 0.5 inches to 1 inch. The surface tension on the film surface is increased when the high potential difference that is generated ionizes the air.
Corona treatment can be done inline or as a separate downstream process once the film is produced. If performed inline, special consideration must be given to the potential generation of toxic ozone. In some cases, it is necessary to provide a ventilation system in the production area.
Winder
In simple words winders are used to convert the extruded film into rolls of material. The winding process has to be such that the film preserves its properties and dimensions when these rolls are unwound and converted in other downstream processes.
There are three basic types of winders; surface winders, turret or center winders, and center/surface winders. Surface winders wind film through the contact between a large diameter drum and a winding shaft that is pressed against the drum with variable pressure. Turret winders or center winders are any style of winding machine that use a driven shaft running through the center of the building roll or on chucks supporting the core to drive the building roll. Finally, in the combination approach of a center/surface winder (or gap winder) a small gap is maintained between the surface winding roll or lay on roll and the winding roll. A center drive system drives the winding roll independently of the surface drum.
Films can be tacky or have some degree of slip, have high or low elasticity, thin or thick, the required roll diameter can be large or small; rolls can be narrow or wide, soft or hard. Winder technology is complex and the proper type of winder used in a particular application depends on all of the above variables.
The use of turret or center winders is typical in cast film applications. With this type of winder the web tension decreases as the roll diameter increases. This is controlled by the rotational speed of the winding spindle.A lay on roll prevents or allows the entrapment of small amounts of air between the layers. The latter is recommended for winding films with high tack or for winding soft rolls.
In order to evenly distribute defects on the extruded film (thickness variations) a randomizer is used. The randomizer moves the film back and forth, as it is slit and wound. An alternative approach is to move the slitter and winder back and forth relative to the film.
Computerized Supervisory and Control System
The main components of a cast extrusion line have been enumerated and described. These components do not act on their own but are integrated and governed by a computerized supervisory and control system.
The main computer is the brain that couples and drives the controls of all the line components in an orchestrated way.
The main tasks of the computer are:
To control start-up, shutdown and speed of the line;
To monitor the weight of material fed into the extruders and to control the speed of the extruders in order to maintain a constant throughput;
To control all temperature zones and the temperatures of all the materials;
To coordinate the interaction between the gauge control system, the response of the automatic die and the line speed;
To control the web tension; and
To store and handle all product recipes, store operational data and control the alarm system.
A good control system must provide operators with an easy to operate graphical interface or monitor system.
Specialty Applications
This section describes some complex coextruded structures that include high added value materials that are a growing demand in the international food packaging markets.
The table below shows the specifications of these coextruded films. In the structures, EVOH is used to provide the oxygen barrier, the presence of PP as a skin layer facilitates the thermoformability of the film, and the PE used as a skin layer acts as a heat-sealing material. Combining PVdC with EVOH is an effective way to address the potential loss of oxygen barrier capabilities experienced by the EVOH when exposed to moisture like in the case of meat packaging. Nylon material is used in combination with EVOH to provide added barrier when the film is to be thermoformed and the rigidity of the EVOH limits the thickness of the EVOH layer.
As seen, the specification process of these structures is not a simple task and multiple variables need to be considered. Companies wishing to diversify their product portfolio with the inclusion of specialty films need to be aware that the high cost of added value resins and the constantly changing market are factors that demand the use of high technology process equipment that is sufficiently flexible to be effectively used in the production of both commodities and specialty films.
Concluding Remarks
This article has enumerated and provided the basic functioning parameters of all the main components of a cast film production line. The technology of each component is complex, as is also their interaction and functional integration in the line.
In order to prevent premature technological obsolescence of the equipment, special consideration needs to be given before purchasing your equipment.
It is imperative to establish a clear understanding of what product, and its application, is to be produced on the line. The idea of an all-encompassing "universal" line may be attractive, but in reality no such line exists. The more generalized a line is designed, the less optimized the product can be manufactured because the line components may not be suitable for product-specific process requirements. In addition, industrial sized cast lines are built for long production runs that are not well suited for frequent product changes - the operation of cast lines regularly requires a significant number of process adjustments. Production of complex and sophisticated films often consume large amount of time for fine-tuning, especially during the development of a film structure. Even with Macro's comprehensive software, which aids the process engineer to predict the behavior of multilayer structures, many trials are typically required to achieve the targeted mechanical, physical, optical and technological parameters.
All cast line components will affect the overall performance of the complete line. In order to get a first class line that is suitable of producing an excellent quality product, each of the individual components or systems must be of equally high quality.
It is expected that the concepts provided above serve the purposes of introducing cast film technology to those new to it and of solidifying the knowledge base of those already familiar with this production process.
