Coextruded parts

Pinch-off edges are critical, in particular, for coextruded parts with wall structures of different materials, as well as for products made from Selar RB (DuPont). 

Since PVC is a thermoplastic, it can be recycled using technology that is comparable to that of the other regularly recycled commodity materials. By far, more postindustrial PVC is recycled than postconsumer due to the durability of PVC applications. Depending on the application, recycled PVC can be used as a raw material in the same application, as a layer in a coextruded part, or as a raw material for an entirely different application. Thermoform scrap can be reused in new sheet wire scrap becomes garden hose window lineal cut-offs become pipe siding scrap becomes the base layer laminated with an appearance film. Pipe goes back into more pipe. 

Fig. 8-51 Bonding two coextruded thermofor-med parts to produce a gasoline fuel tank.

They are amorphous, opaque, terpolymers produced by suspension, emulsion, or continuous mass polymerization. Properties are similar to ABS, with the addition of weatherability or UV protection for outdoor use. These materials are usually coextruded over ABS. Typically applications have been exterior automotive and RV parts, truck caps, pool steps, outdoor signs, camper shells, and sidings. 

Films are either coextruded [12] or printed on the back of a very transparent material. The binder of these screen inks or gravure inks has to have a high melting point or crosslinking to form an elastic film to avoid being replaced by the hot injected polymer. It is also possible to coat the backside of the film with an adhesion promoter, which prevents direct contact between the printed area and the polymer. This coating can also improve the adhesion between the film and the injection-molded part [13]. 

Some packaging applications may be particularly sensitive to styrene or other volatile components. SBC and crystal polystyrene blends, even with low residual monomer levels in the polystyrene, may be unacceptable for these applications. In this case one possible solution may be to co-extrude a thin food contact layer on the surface of the blended sheet. Studies have indicated that by co-extruding a thin layer of crystal polystyrene on the surface of an SBC and crystal polystyrene blend, the styrenic volatiles and the odor can be significantly reduced. However, the downside is that the added layer of crystal polystyrene can serve to make the blended part more brittle. Another option that could reduce odor, but not have a deleterious effect on impact, is a thin coextruded layer of PETG [20]. 

The primary method for dispersing PFIEP is by impregnating supports with alcoholic solutions of soluble PFIEP in the acid form. Alternatively, the polymeric sulfonyl fluoride precursor is a thermoplastic and can be extruded into thin films or blended with a powder support and extruded into various shapes (69). In addition, the sulfonyl fluoride precursor can be coextruded with a polyethylene resin to form differently shaped parts (70). 

Plastic parts with a thin black layer on one side can be continuously welded in place. For example, if the black layer is one part of a bUayer, coextruded sheet, it could be unroUed and welded in place. The heat needed for welding would be generated precisely where it is needed, minimizing damage to the part and allowing the joining of thin polymer films. Thin films, particularly when coated on a metal, that are hard to join by other methods should he readily and rapidly joined by infrared welding. 

Other techniques for preventing UV degradation include the alloying or blending of two different polymers (one with better UV stability), surface coating the plastic part with more stable materials (such as an acrylic), or laminating or coextruding an exterior weatherable skin onto parts made from sensitive plastics. 

Figure 7-71. The bonding of two thermoformed parts from coextruded sheets here resulted in a fuel tank. The tank s inside has a nylon surface to resist gasoline the outside uses PP to provide a combination of low cost and the required support strength.

In addition to bottles, a rapidly growing application for blow molding is the production of technical parts, such as automotive components—bumpers, ducts, and other fluid containers. For fuel tanks, a coextruded parison has polyethylene as the structural material and special barrier layers to prevent escape of polluting fumes. 

Figure 9 shows the configuration of the multilayer coextruder built as part of this project. 

In this relatively new process, two different types of raw materials are extruded from one die. These raw materials can come from two extruders or one extruder equipped with one pump. Coextruded snacks have two recognizable parts an external cover and an internal filling. 

Coextrusion is a process that allows for extrusion of two or more materials. Typically, there is a surface layer and a core layer. The core layer often uses a less expensive material and often process regrind material is considered for this appUcation. The surface layer material is chosen to meet the appearance and performance requirements of the part. The surface layer material is usually more expensive because it has the colorants and property enhancement additives to meet the part requirements. With coextrusion process, the thickness of the surface layer is typically minimized to reduce the cost. This has made coextrusion a cost-effective method for mold-in-color parts compared to straight extrusion or coating. The coexftusion process has been successfully used for profile coextruded body-side molding applications. 

Eilms (0.2 to 1 mm) and sheets (2 to 5 mm) are coextruded. Coextruded sheets consisting of multilayers are thermoformed and inserted into an injection mold. This process is also referred to as overmolding. It is well established for automotive interiors. There are a number of promising developments for exterior applications. It is applicable for simple geometry parts with large surface area. See Figures 2 (1) and 3 (7). 

A totally new technology to produce complex metal parts which are partial multi walled at specific areas was developed. By integrating such Rex Ring sleeves into pipe dies the flow channel geometry can be fine tuned to further reduce the thickness tolerances in plastics pipes. This is achieved due to the linear elastic deformation which is realized by the multi-walled flow channel walls. For the first time a pipe die technology is available which enables to establish a close loop control for the total wall thickness of the pipe. It is even possible to establish a close loop control for individual layers of coextruded pipes. But prerequisite for this that it is possible to... 

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