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. Figure 1 shows a coextrusion cast film line.
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 3 (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.
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.