Processing, thermoplastics coextrusion

Extrusion is the principal process for producing film and sheet. It is also used to create open (weather stripping) and closed (soda straws) profiles. Dual extruders can be used to produce laminates, a process called coextrusion. This is often done to provide barriers to ultraviolet light or moisture vapor. Two colors can also be extruded side-by-side for interesting effects. All thermoplastics can, theoretically, be extruded including foamed and reinforced materials. However, in practice, the highest melt temperature thermoplastic extruded in any volume is polycarbonate. Thermosets can also be extruded on a limited basis as they require special equipment and are slow to process. 

THV Fluoroplastic can be processed by virtually any method used generally for thermoplastics, including extrusion, coextrusion, tandem extrusion, blown film extrusion, blow molding, injection molding, vacuum forming, and as skived film and solvent casting (only THV-220). 

Generally, processing temperatures for THV are comparable to those used for most thermoplastics. In extrusion, melt temperatures at the die are in the 230°C to 250°C (446°F to 482°F) range. These relatively low processing temperatures open new options for combinations of different melts (coextrusion, cross-head extrusion. 

U.S. Pat No. 6,827,995 [53] discloses a single coextrusion process of making a WPG as a hollow profile comprising a weatherable outer layer made of a first polymeric material, a core layer made of a thermoplastic polymeric foamed composition including a wood component, and an inner layer made of a third thermoplastic material. The first and second polymeric materials are PVC or acrylonitrile-styrene-acrylic polymer or a combination thereof. The third polymeric material is PVC. 

U.S. Pat. No. 6,844,040 [54] discloses a coextrusion process of making reinforced WPC structural members comprising hollow profiles formed from a thermoplastic-cellulose fiber composite material and reinforcing sections bonded to the hollow profile. 

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. 

Coextrusion is based on repeated coextmsion of a powder-filled thermoplastic polymer to form ceramics with textured microstmcture or with fine-scale features [199, 200]. A mixture of ceramic particles, thermoplastic polymer, and processing aid, containing about 50 vol% ceramic powder is formed into a rod-like feed material, i.e., feed rod, with the required arrangement of the ceramic phase, through... 

Various types of multilayer dies, such as the TRD, have successfully produced coextruded LCP-thermoplastic film, sheet, and thermoformed articles. These products are expected to penetrate the high barrier plastics market valued at more than 700 million per year for a variety of food, beverage, medical, and industrial packaging applications. Because of their very low permeability to oxygen and water vapor combined with high temperature and chemical resistance, coextruded LCP films should find applications in this market. The process for coextruded LCP thermoplastic multilayers overcomes the key problems previously limiting LCP coextrusion ... 

Heat sealing is commonly used with a continuous belt process for sealing plastic bags, film, etc. If the plastic film has too high a melt temperature or if it is a themoset material, thin thermoplastic films have been applied by coating or coextrusion to act as a hot-melt adhesive for seating films. A heat-sealing adhesive is a thermoplastic film adhesive that is melted between adherend surfaces by apptication of heat to one or both of the surfaces. 

Multilayer coextrusion of thermoplastic film and sheet has developed into an important plastic fabrication process, providing large growth opportunities for the plastics industry. Coextruded multilayer plastics are challenging such traditional materials as metals, glass, paper, and textiles. 

The LC polyesters are known to be processed using all the techniques commonly used for more conventional thermoplastics including injection molding, extrusion, coextrusion, blow molding, etc. 

An early description of the cast film process was published in Modem Plastics in 1952 [26] which was very similar to the cast-film scheme shown in Figure 6.19 which represents the process used today. The cast film process involves the extrusion of polyethylene through a die to form a thin molten layer of material that is drawn down to a thinner gauge based on the difference between the rate the molten polymer exits the extruder and the rate that the molten polymer is cooled onto a chill roll. The film cools very rapidly on the chill roll and film orientation is only in the machine direction. Optical properties are usually better with cast film as compared to blown film and line rates may be higher with the cast film process. A cast film line may also involve coextrusion of several layers of different types of thermoplastics to fabricate specialty films with improved properties. A... 

Co-injection molding is a process in which injection molding of parts with a skin of one thermoplastic and a core of another compatible thermoplastic. Like coextrusion, co-injection molding can offer cost advantages by molding the base layer out of a less expensive material and the top layer or surface layer out of another material that offers the appearance and weatherabihty. The base layer does not require the expensive pigmentation package. 

Studies of the processing behaviour of thermoplastic elastomers carried out by Berstorff are reported, and developments by the Company in processing methods and machinety for these materials are examined. These include extmsion compounding machinery, extmders for the production of solid profiles and for the coextrusion of profiles combining solid and foamed thermoplastic elastomers with mbbers, and a process for the extmsion of foamed profiles using water as blowing agent. 

Types of extruders manufactured by Berstorff for the processing of thermoplastic elastomers are reviewed. These include extmsion compounding lines and machineiy for the extmsion and coextrusion of water blown foam profiles. 

Dry-processing variants are produced with starches of 5-30% moisture, plastic components and additives by coextrusion imder conditions that favor the thermoplastic behavior of the blend. 

Many variations on the basic film-blowing process exist. Some of the more common variations include the use of rotating dies and coextrusion. Rotating dies serve to even out thickness variations around the circumference of the film, ensuring that rolls of film have no cumulative thick spots that would cause distortion. Coextrusion is used when it is desired to produce a film made up of layers of different polyethylene resins or of polyethylene in combination with other thermoplastics. In coextrusion, two or more separate extruders feed a single die designed in such a way that their outputs are combined concentrically. 

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