Fundamentals of cast film extrusion technology

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Fundamentals of cast film extrusion technology

  1. 1. Fundamentals of Cast Film Extrusion Technology The cast film extrusion process is gaining increased popularity and enjoying sustained growth worldwide. New lines are being installed in a significant number and the market segments penetrated by this technology are on the rise. This article identifies the main components of a cast extrusion line and presents the fundamental aspects that require consideration when approaching cast film technology. Applications of Cast Extrusion Cast films are used for food and textiles packaging, flower wrapping, as photo album page protectors, as coating substrates in extrusion coating processes or laminated to other materials in the formation of more complex films, among others. Typically, the cast film process involves the use of coextrusion, which is a simultaneous extrusion of two or more materials from a single die to form a multi-layered film. This is because in many cases the final application of the plastic film demands a performance that cannot be achieved if the film is composed of only one material. For example, in many instances food packaging applications require the use of films with oxygen barrier capabilities. To meet the requirement a high oxygen barrier material like EVOH is combined with polyolefin materials in a multi-layered structure. Coextruded films typically contain up to seven layers; however, the use of more layers is becoming more common. The number of layers, their position in the coextrudate and their individual thickness are all variables that change depending on the particular application of the film. Benefits/Limitations of Cast Extrusion Unlike the blown film process, the cooling of the film with cast extrusion is highly efficient. This allows for higher production line speeds resulting in higher production rates with superior optical properties of the product. The degree of draw and orientation is significantly lower in the cast film process than in the blown film process. This is the reason why the thickness distribution in the machine cross direction is more uniform with cast processes (with variations that could be as low as ± 1.5%). However, the film mechanical properties in the machine cross direction are lower when compared to those obtained with the blown film process due to the higher level of orientation that the film experiences in the blown process. In cast extrusion the edges of the film are trimmed due to dimensional irregularities and/or poor layer distribution. As a result, the process can be negatively affected if the trimmed material cannot be recycled. Recent flat die system technology has minimized this problem by significantly reducing the amount of wasted material in coextrusion processes. This subject will be covered to some extent in a subsequent section. Basic Concepts of Cast Extrusion In the cast film extrusion process, the molten polymer travels through a flat die system to adopt its final flat film shape. The die system is formed by the die and feedblock (if the process requires coextrusion) or simply the die, if the process is that of mono-layer extrusion. The process starts with the feeding of plastic resins by means of a gravimetric feeding system to one or more extruders. The materials are then melted and mixed by the extruders, filtered and fed to the die system. Immediately after exiting the die, the molten curtain enters the cooling unit where its temperature is lowered with a water cooled chill roll to “freeze” the film. The film is then passed downstream where the edges are trimmed, corona treatment is applied (if a fabrication process such as printing or coating is required) and the film is wound into rolls. A description of the main components of a typical cast film line is presented below. Cast Film Line Components Gravimetric Feeding System Gravimetric feeding systems control the amount of material that is fed into the extruders by weight, not volume. The system is more precise than its volumetric counterpart and features a reduced error tolerance in the order of ± 0.5%. In many cases, the film is fabricated with materials that are blends of a base polymer with one or more secondary components. In state of the art production lines, this blending is carried out inline. Special care is needed to prevent premature melting of the pellets, especially when materials with low melting temperatures are processed, or when the pellet size is small. Vibration and cooling of the feeding hoppers are options recommended to alleviate this problem. It is also important to ensure that the material being fed carries no moisture that could give rise to the appearance of small bubbles, also known as “fish eyes”, in the final film. In some cases, drying of the material is required. This may be performed by a separate unit or by a highly sophisticated feeding system with built-in drying capabilities. Extruder The main functions of an extruder are to melt the plastics pellets and mix the resulting molten polymer to achieve a homogeneous melt. This is done by conveying the material along a heated barrel with a rotating screw. Commercially used extruder barrels are typically 3½” (90 mm) to 6” (150 mm) in diameter. The screws are tailored to the specific characteristics of the extruded materials and process parameters. The length of the screw is heavily influenced by their diameter. Screw length to diameter (L/D) ratios commonly lie in the range of 26:1 to 30:1. It is critical to ensure that the flow exiting the extruder is well controlled and constant with variations on the screw's rotational speed not exceeding ±1%. A failure to accurately control the screw speed typically results in undesired pulsating flow that can cause periodic changes in film thickness in the machine direction. The metering section, or final section of the extruder, is designed to guarantee a precise dosing of material from the extruder. In order to achieve the above, the gap between the screw and the barrel is very small. This creates another challenge since it is difficult to maintain a constant gap between the rotating screw and the barrel. To overcome the above-mentioned potential problems, a melt pump is commonly employed downstream of the extruder. The pump is a positive displacement device that produces a consistent flow regardless of the discharge pressure of the extruder (Figures 2 and 3). The pump alleviates the workload on the extruder by taking on the job of generating pressure. The reduced extruder head pressure translates into energy consumption savings, a drop in the melt temperature and less wear between the barrel and the screw. In coextrusion lines, the number of extruders depends on the number of different materials being extruded and not necessarily on the number of layers. This is because the existing feedblock technology allows the flow from one extruder to be split into two or more layers in the final coextrudate. Filtration System The objective of the filtration system is to prevent downstream passage of melt impurities and/or gels that are formed during the extrusion process. Proper control at this stage is imperative to prevent melt contamination. The most common filters are those containing a metallic mesh. The case hosting the filter media has to be capable of bearing the forces exerted by the polymer flow when subjected to the maximum pressure allowed by the extrusion process.
  2. 2. It is highly recommended to use continuous screen changers, in which the mesh is continuously regenerated, to minimize the replacement time of the screen pack. Flat Die System It can be said that the die system is the heart of any coextrusion line. The die system is formed by the coextrusion feedblock, the flat die and the melt transfer adapters that transport the different molten polymers from the extruders to the feedblock inlet ports. The quality of the coextruded film and the productivity of the process are greatly dependent on the design and performance qualities of the die system. The primary function of the die system is to form a multi-layered film that is uniformly distributed across the width of the die with thickness variations on the film and thickness variations on each individual layer within industry accepted tolerances (not to exceed ±2.5% for the total thickness and within ±15 to ±20% for each layer). Upstream from the feedblock are the melt transfer adapters. The design criteria of this capillary system must consider parameters such as material residence time, pressure drop and temperature control. For instance, an excessive pressure drop could be addressed by increasing the pipe diameter; however, this in turn would increase the residence time of the material and increase the possibility of material degradation. Also, accurate wall thickness sizing and proper heater specifications are necessary to prevent the pipes from heating or cooling the melts that they transport. It is the task of the designer to find the proper balance between all these variables. The coextrusion feedblock arranges the different melt streams in a predetermined layer sequence and generates as many melt streams as layers are to be in the final coextrudate. Once this is done, each stream adopts a planar geometry, meets its neighbouring layers and the final planar coextrudate is formed. Coextrusion feedblocks are grouped into two categories: Fixed and variable geometry blocks. In the upstream section of these blocks the so-called selector plug or selector spool is found. This cylindrical shaped removable part is responsible for routing each melt stream into its final position in the coextrudate. The plug, if required, also splits those streams with a material that feeds more than one layer in the structure. If a different layer sequence is required, it can be achieved by simply changing the plug. Fixed geometry blocks are most effective when the production line is devoted to only a few different products that are similar in their rheological behavior. However, it is worth noting that these blocks have removable flow inserts that could be machined or replaced if required to process a wider spectrum of materials. Variable geometry feedblocks are ideal for the coextrusion of high added value materials or when the scope of the production line is more diversified. In general, these blocks feature movable internal components that can adjust the width distribution of an individual layer prior to meeting with neighboring layers and/or its velocity, which in turn affects its shear rate and viscosity. Thus, problems inherent to coextrusion such as that of layer distortion and interfacial instability can be overcome with adjustments of the feedblock. In spite of all the capabilities of coextrusion feedblock technology to address flow anomalies inherent to coextrusion flows, the production of an optimal coextrudate is only possible if the feedblock operates in conjunction with a die conceived and properly designed to process a coextrusion flow. The perfect synergy between the die and the feedblock is what will guarantee a high quality product. A well designed die must guarantee that in the process of spreading the coextrudate coming from the feedblock the flatness of each individual layer is maintained within a tolerance of ±15 to ±20%. It must also be designed so that the residence time is not excessive in order to prevent degradation problems or in some cases to prevent undesired heat transfer between layers. The die must also be designed so that the pressure drop is kept at a level that is normal within the extrusion process. It is also critical that the die has the appropriate size, sufficient mass of steel and proper mechanical design to guarantee thermal stability and to minimize the so-called clam-shelling problem that manifests itself as an excessive deformation of the die lips when the die is subjected to the high pressures inherent to the extrusion of thin films. Recent advances in die technology have boosted the productivity of cast film production lines. Special reference can be made to the socalled internal deckles. Inserted on both ends of the die, the deckles allow changes to the film width and the consequent reduction of trim. They can be fixed or adjustable and their length can exceed 20 inches. Edge encapsulation technology has been introduced in recent times to reduce the negative financial impact of material waste caused when the trim of the coextrudate is not recyclable. The previous figure shows a band of a single material being coextruded side by side with a coextrudate. The encapsulation material is of low cost, recyclable and has high mechanical properties. The encapsulation material mainly forms the trim, which allows for its reinsertion into the production process and great savings in material cost. In addition, edge encapsulation technology is fully compatible with the internal deckle technology. Cooling Unit The cooling unit is comprised of a primary quenching roll, a secondary roll, a motorized roll positioning system for proper vertical and cross machine direction alignment of the rolls, and in many cases a vacuum box and/or air knife. The rolls are typically chrome plated to achieve a better surface finish and to enhance the heat transfer process during film cooling. The cooling agent is commonly water that circulates inside the rolls. The primary quenching roll cools one side of the film while the secondary roll cools the opposite side of the film. The die is positioned above the primary quenching roll at an angle that varies from 45° to 90°. The distance between the die lips exit and the roll ranges from 0.8 to 2 inches. The cooling system allows the line to operate at high speeds. As the line speed requirement increases, so do the diameters specified for the rolls. The rolls must be perfectly aligned with the web to guarantee a uniform tension and to minimize thickness variations across the width of the film. In addition, the angular velocity of the rolls must be well controlled to prevent film thickness fluctuations in the machine direction. The use of a vacuum box, connected to the die fixed body, is necessary in certain applications, like that of Cast PP, that require a more efficient cooling. PP materials, if not cooled aggressively, tend to form crystals that ultimately give rise to hazy films. The vacuum box removes entrained air between the primary quenching roll surface and the film to minimize the air barrier between the hot web and the roll. This air barrier, if not reduced, acts as a thermal insulation cushion that impedes the film cooling process. The box also reduces the amount of necking in the film and the air gap and allows higher line speeds to be utilized. The vacuum box can be combined with an air knife or an air chamber to further enhance the web cooling. Automatic Gauge Control System 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
  3. 3. 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: • • • • • • A good 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. 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-
  4. 4. 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.

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