Cast or Blown Film

Blow and cast film extrusion are the common methods in film production. Blow film extrusion is a major method for producing bags from LDPE, LLDPE, and FIDPE. 

Their excellent optical properties, notably gloss and haze, gauge tolerance, and the lower capital investment of a cast film extrusion line (less than half that of a biaxially oriented PP (BOPP) line) have enabled cast film to maintain its market presence, despite the superior mechanical properties of oriented blown film. The disadvantages of cast films are their tendency to split and very low transverse strength. 

More recently there has been extensive growth in cast stretch films, based on LLDPE resins, particularly in the area of pallet stretch wrap, for which pre-stretching of up to 300% and even higher is required. 

Redrawn from C.M. Wong, H.H. Shih, C.J. Huang and A.M. Sukhadia in Metallocene-Catalyzed Polymers - Materials, Properties, Processing and Markets, William Andrew Publishing/Plastics Design Library, 1998. Copyright William Andrew Publishers, 1998. 

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As cast or blown films we encounter polyethylene in packaging films, bags, cling film, greenhouse skins, diaper back sheets, and high altitude scientific balloons. In these applications... 

SBC film is growing into more and more film applications. It is proving to be well suited for fresh cut produce packaging, candy twist wrap, shrink films, flexible medical and decorative films. SBC can be extruded into either cast or blown film for use in flexible packaging applications. While some applications employ a monolayer SBC film, many films are co-extruded with SBC along with several layers of other materials in order to bring a combination of properties to the final application. 

Multilayer structures are often used in packaging in order to take advantage of the properties of several polymers at once. Multilayer structures are made by lamination, coating, and/or coextrusion. Because of the multiple handling of materials and the thickness of the layers, laminations (see Chapter 8) may be relatively expensive materials. Coextrusion can be used to reduce the cost of multilayer plastic film and sheet. Coextruded packaging materials are multilayer plastic sheet or film constructions produced from more than one plastic resin in a single step, using either the cast or blown film process. In coextrusion, the materials never exist as separate webs. 

Processing Methods Grilon BM 20 SBG can be readily converted using cast or blown film equipment, and can be oriented using most systems. ... 

Film Processing Methods Cast or blown films. 

In making multilayer structures, two or three melt streams from separate extruders can be combined and cast to form a multilayer structure. Alternatively, a melt stream, and a premade cast or blown film web or a quenched and oriented film web, may combined on a chilled drum providing the web with new surface functionality (Fig. 12,58). This method is known as the extrusion coating process [2],... 

An important application for polypropylene is film tape. This is made by slitting unoriented film (cast or blown) into tapes 2 or 3.5 mm wide and stretching under heat about seven-fold. With cast film the orientation is more completely monoaxial and there is a tendency for the film to split along its length (fibrillate). Tubular film does not self-split so easily and also has a somewhat softer feel. Such tapes may be woven into sacks and these have... 

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film, which is drawn down along the extrusion direction only. 

Orientation by drawing during film casting or blowing or after it has cast or blown. 

Antimicrobial Edible films were prepared from natural fiber of pectin and other food hydrocolloids for food packaging or wrapping by extrusion followed by compression or blown film method. Microscopic analysis revealed a well mixed integrated structure of extruded pellets and an even distribution of the synthetic hydrocolloid in the biopolymers. The resultant composite films possess the mechanical properties that are comparable to films cast from most natural hydrocolloids that consumed as foods or components in processed foods. The inclusion of polyethylene oxide) alters the textures of the resultant composite films and therefore, demonstrating a new technique for the modification of film properties. The composite films were produced in mild processing conditions, thus, the films are able to protect the bioactivity of the incorporated nisin, as shown by the inhibition of Listeria monocytogenes bacterial growth by a liquid incubation method. 

Each type of resin is melted in an individual extruder, and the melts are carefully brought together prior to or in the die, in a manner that keeps them in homogeneous layers, without mixing. The process used to combine the polymers is usually different in the cast and blown film processes. In cast processes, as illustrated in Fig. 7.12, the polymers are typically combined in an adapter, called a feed block, before they enter the coat hanger die. This permits a simpler design for the die itself. Multimanifold dies are used when plastics with widely different flow properties are to be combined, as such systems provide a shorter multilayer flow path before solidification, and thus minimize distortion of the interface. 

Film Processing Method Polypropylene film can be cast or blown.i ... 

Virtually every polymer can be used in film form. Most thermoplastic polymer films are prepared by conventional extrusion techniques based on calendering, casting, and blown-film, or tenter-film systems. Other polymers, which cannot be easily melted, are formed into films by solvent casting. In the selection of a film for a particular application, the properties of the poljuneric materials must be considered in view of the application. Thermal properties, molecular characteristics, and crystallinity of the polymer affect processing and film properties. Additives influence extrusion and orientation processes and improve film properties. 

With a given die cross-sectional area, there is only 1 ratio of puller speed to extruder throughput rate that produces a product with the correct cross-sectional dimensions. If the extruder throughput is increased, the puller speed must be increased proportionally to maintain the same finished product dimensions. Likewise if the throughput is decreased the puller speed must be decreased proportionally to maintain the same finished product cross-sectional area. The draw ratio and molecular orientation can only be increased or decreased by changing the die cross-sectional area relative to the puller speed assuming the final product dimensions are kept constant. This is easily done with sheet dies, cast film dies, or blown film dies that have adjustable die lips. Profile and cross head dies... 

Numerous types of dies are available depending on the extrusion process and the type of product being produced. Compounders use strand dies to make continuous strands that are chopped into pellets. Sheet and cast film producers use flat dies. A schematic of a flat film or sheet die is shown in Fig. 12.53. This produces a flat web of a specific thickness that passes through a three-roll stack or lays flat on a cast roll. Blown film producers use circular dies as shown in Fig. 12.54. Tubing and pipe cross head dies are similar to that shown in Fig. 12.55A where... 

An important development in the extrusion of film is the so called co-extrusion technique, where two or more layers of film are extruded simultaneously to produce a laminate with improved barrier, appearance or mechanical properties. Each layer is fed from a separate extruder. Both cast and blown film techniques are used the composite tubular film may be rotated in the same way as in simple lay extrusion whilst the flat film die allows individual adjustment of the melt streams. 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

Union Carbide Corp produced the first of the synthetic papers in the late 1960s. Since that time other examples of synthetic papers include DuPont s Tyvek nonwoven paper and Van Leer s Valeron cross-laminated film. This market is now dominated by a few large, worldwide ventures with proprietary processing techniques that extend the use of single and multilayer extruded blown film or cast film. 

Polypropylene film is found in a variety of applications. Films are thin plastic sheets with thicknesses less than 250 pm. Applications for these films include candy mappers, plastic label material, electrical capacitors (which requires very pure polymer), and fiber glass insulation backing. There are two general categories of polypropylene films unoriented and oriented. Films are manufactured through either a cast or a blown film extrusion process. Oriented films exhibit improved strength relative to unoriented films. This means that they can be... 

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