Sheet coextrusion

The flat film and sheet coextrusion analyses mentioned earlier can be easily extended to the treatments of the wire coating and film blowing coextrusion processes. 

Uses Barrier resin in sheets, films, pkg. materials, gas till pkg., pkg. for oily foods, edible oils, min. oils, agric. pesticides, org. soivs. suitable for film extrusion, sheet coextrusion, bbw molding coextrusion, tube profile coextrusion, extrusion coating, and laminating procedures Features Thermoplastic... 

A third approach combines the feedblock and multi-manifold types, and provides further processing alternatives as the complexities of coextrusion Increase. This approach has been used successfully in barrier sheet coextrusions where requirements preclude other alternatives. The feedblock is placed on the manifold or manifolds, permitting the combination of materials of similar flow characteristics in the feedblock while feeding dissimilar materials directly into the die. 

In addition to packaging markets, sheet coextrusion is applied in construction, recreational vehicle, and sanitaryware markets where weatherable or chemically resistant layers are coextruded with low cost polymers. 

Figure 4.11 Tensile stress amplitude versus cycles to failure for SABIC Innovative Plastics Geloy CR7020 ASA for sheet coextrusion over ABS [6],...

Coextrusion. An increasingly popular technique to produce tailored film or sheet products is to coextmde one or more polymer types in two or more layers of melt (6). In this fashion the benefits of specific polymer types or formulations may be combined. Thus high cost barrier resins may be combined with a low cost thicker layer of standard resin to achieve an optimum barrier film at lower cost. Thin sUp-control layers may be used on the surface of a bulk layer of opticaUy clear resin to obtain an aesthetic film with good handleabUity. Lower melting outer layers may be used to provide heat sealing for polymers that seal with difficulty by themselves. 

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

Multi-layer sheets for use in the mannfacture of food packaging containers were produced by the coextrusion of PP foam in combination with PE, an EVOH barrier layer and tie layers. Results are presented of studies of the mechanical and morphological properties, density and cell stmctnre of foam layers based on different types of PP. 8 refs. 

Extrusion. Sheet, film, and profiled articles are made by extrusion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextrusions, different combinations of plastics are separately but concurrently extruded to form layered sheet or film. In the packaging industry, specialty resins such as high barrier ethylene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer are additive, as opposed to the "averaging of property" in blends. Multilayers are also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

In the past few years, we have seen trends where more complex manufacturing systems are developed that manufacture parts using various materials and components such as coextrusion of multilayer films and sheets, multi-component injection molding, sandwiched parts or hollow products. 

Multilayer flat films and sheets can be formed by coextrusion, consisting of two layers (AB), three (such as ABA), five (such as ABCBA), and up to eleven layers. Each layer and particular multilayer configuration is selected for its contribution to one or more film/sheet product properties, such as gas barrier, adhesion, abrasion, and chemical resistance. 

Figure 5.1 8 Schematic of a basic three layered coextrusion sheet or film system...

Counter-rotating twin screw extruders are used almost exclusively for the production of uPVC pipes, profiles and sheet because they treat material more gently than single screws. Parallel screw and conical twin screw extruders are compared. Cincinnati Milacron Austria introduced its Argos parallel twin screw extruders last year. These machines are intended to complement the company s Titan conical screw range. Coextrusion of foamed core pipes is also described. 

With sheet or pipe, multilayer coextrusion can be used. Solid outer-solid core coextmsion can place an ABS grade on the outside that has special attributes such as color, dullness, chemical resistance, static dissipation, or fire-retardancy over a core ABS that is less expensive or even regrind. 

Fig. 8 Coextmsion feed block manifold and sheet die (A) (1) sheet die with flow restriction (2) adapter plate (3) feed block asembly (4) core material layer inlet (5) upper material layer inlet (6) lower material layer inlet and coextrusion multimanifold sheet die (B) (1) lower melt channel (2) upper melt channel (3) lower choker bar (4) lower choker bar adjustment bolt (5) upper choker bar (6) upper choker bar adjustment bolt (7) flex lip.

The single-layer coextrusion process based on the use of low-volatility UV absorbers has been adopted for thin-walled sheet and profile manufacture, both in Europe and worldwide. Coextrusion coating using PC is important also from the point of view of recycling, because single-polymer systems increasingly are being required. 

The fabrication or conversion processes for plastics vary according to what is being produced, i.e. films, sheets, laminates, components or containers. Films or sheets are produced by similar processes. Laminates may be produced by adhering films to other materials or by coextrusion. The use of the latter, a process more ideal when high quantities are required of certain constructions, is increasing. Coextrusion can therefore combine the properties of several materials in thinner gauges, and frequently more economically than other alternative processes. However, normal lamination techniques have to be used if plies of paper or foil are involved (see Chapter 9). 

Lotryl . [Atochem N. Am.] Ethylene/ acrylic ester copolymers for films, coextrusion, gloves, sheets, coating, hot melts, compounding. 

Novatec -AP. [Mitsubishi Kasei] Extrudable adhesive polyolefin resins for coextrusion, film, botde, tube, sheet barrier and thermoplastic bonding. 

Polystyrene. [Novacor Ltd.] Crystal or inqract polystyrene for medical nxdd-ii housewares, sheet glazing, coextrusion, oriented sheet, inj. blow molding, inj. foam, thermoformed drink-ware, rigid pkg., audio cassettes, cosmetic molding, electronic pkg., lab-ware, toys, closures blending resin. 

Coextrusion is the process of forming an extru-date composed of more than one thermoplastic melt stream. The process came about because some service demands, particularly from the packaging industry, could not be satisfied by a single polymer although they could be met by a combination of polymers. Coextrusion was first practiced in the production of cast film and is now also used in blown film and sheet extrusion. The intention is normally to produce a laminar structure in which each layer contributes a key property to the overall product performance. Coextruded films may be very complex structures composed of many different functional layers, including tie layers whose purpose is to bond neighboring layers of limited compatibility. Five layers are not uncommon. However, side-by-side coextrusion is also possible. Fluoroplastics can be coextmded with other polymers such as ETFE and nylon. 

The best way for the reader to learn is to study actual examples of parts made by the blow molding of fluoropolymers. A majority of such parts have a multilayer construction to combine the properties of other plastics in a composite structure. The most common technique for producing multilayer sheet/film is coextrusion. The chemical resistance of fluoropolymers makes them attractive materials for inner layers of containers that come in contact with aggressive chemicals that can swell or degrade thermoplastics. 

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