SEQUENTIAL COEXTRUSION

Thielen, M., Blow Molding, Sequential Coextrusion Advanced Technology Opens Door into a New World of Tailored Parts, Modem Plastics Encyclopaedia, 1999. 

Sequential coextrusion also offers the possibility of manufacturing a single part made with different regions of hardness, elasticity, strength, or heat deflection temperatures. This opens up a wide variety of applications. Sequential coextrusion can be used, for example, when the ends of a pipe must be flexible (for coimection purposes), but the middle section must be stiff and strong. Such a part, made out of thermoplastic elastomer (TPE), for example, can then be substituted for rubber parts. 

Recently, a novel method to prepare multilayers and nanolayers from a sequential coextrusion process has been introduced. In this method, the number of layers can be largely increased at the expense of reducing the thickness of the coextruded layers. It is also possible to obtain nanodroplet dispersions by annealing such multilayer coextruded ensemble. The following systems have been studied employing this method polyethylene/ polystyrene (PE/PS), polypropylene/polystyrene (PP/ PS), polycarbonate/poly(ethylene terephtalate) (PC/ PET), and poly(ethylene oxide)/ethylene acrilic acid copolymer (PEO/EAA). The crystallization behavior within the nanolayers and within the spheres that are obtained after heating and coalescence of the nanolayers has also been studied [18,33,66,94,104, 111, 140-143]. 

Two basic methods are used in this process to deliver material to the processing units. These are extrusion and injection. In the next step, the preformed material is expanded to form parison. There are many commercial variations on this basic technique some of which include continuous-extrusion-blow-molding, coextrusion-and-sequential-blow-molding, and injection-stretch-blow-molding. Both extrusion and injection molding are the subjects of later discussions below, we will concentrate here on the parison formation, its processing, and the related effects. 

Coinjection n. A process similar in its results to coextrusion but accomplished by modifications of the injection-molding process. By means of various nozzle and valving arrangement, two or more materials, can be injected either simultaneously or sequentially to form an article with an outer shell of one material with certain desired properties, the shell filled with another material to attain other desired properties such as reduced cost. Coinjection, like coextrusion, basically means that two or more different plastics are formed into a composite or laminated structure. 

Another commercial application for flat-die coextrusion is biaxially oriented multilayer films (11) made with the tentering process to improve mechanical properties. Tentered film is biaxially oriented by stretching in the longitudinal and transverse direction, either sequentially or simultaneously, at uniform optimum temperature. In sequential stretching, the multilayer extrudate is cooled to a suitable orientation temperature on a first set of rolls and then stretched in the machine direction between a second set of rolls which is driven faster than the first set. The uniaxially stretched film then enters a tentering frame, which has traveling clips that clamp the edge of the film. The clips are mounted on two... 

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