Blow molding coextruded

A similar problem is presented by vehicle tires and certain blow molded bottles, which must be virtually impermeable to air and other gases. An example of the use of a very impermeable elastomers is butyl rubber. Because of its impermeability to gases, butyl rubber is used as a roof coating. With plastic bottles, different layers of both coinjected and coextruded plastics (Chapter 8) can be used to fabricate the bottle to make it impermeable to different vapors and gases depending on the barrier plastic included. 

Figure 4. SEM Micrograph of Extrusion Direction Cross-Section of Coextruded Blow Molded Bottle Showing "Sine Wave" Pattern of Mica Platelets.

Table VII. Effect of Platelet Orientation on Barrier Enhancement - Coextruded Blow Molded Bottles...

PC can be processed by injection molding, extrusion, coextrusion, and blow molding. Coextrusions with EVOH or polyamides are carried out with the help of adhesives. PC can be laminated or coextruded to PP, PE, PET, PVC, and PVDC. PC is a hydrophilic polymer, and at ambient conditions can reach moisture levels of 0.35%. 

The counter-rotating die of Figure 11.5 can be used for film, tubing, and blow-molded containers, as well as multilayer coextruded products. The machinery, processing, and applications are described in subsequent sections of this chapter. Specifically, section 3 will describe the importance of stretching, combined with shear, to control orientation. 

Once the coextruded LCP-thermoplastic multilayer parison is formed, it can be blow molded into bottles and other containers. The orientation in the LCP layer is controlled by the counter-rotating action of the cylinders in the die, and the LCP orientation remains in the finished bottle because of the very long relaxation time of LCP. While other polymers being coextruded with LCP may... 

Both blow molding and injection molding are downstream extensions of the extruder and are discussed separately. Profile extrusion is the direct manufacture of a product from the extruder die. The products are a continuous length with a constant cross section. Examples of profile extrusions include plastic pipe, garden hose and PVC window framing. Profile extrusion is the method now being used to create plastic lumber from mixed plastics. Extruders can also be arranged as coextruders where different layers... 

Barrier layer n. In multiplayer films, coextruded sheet, and blow-molded containers, a layer of polymer having very low permeability to the gases and/or vapors of interest for the application of the film, sheet, or container. 

A growing trend in extrusion blow molding is to coextrude parisons that contain up to seven layers of different materials. However, as of this time, coextrusion is limited to machines that only use one (1) parison as on a wheel and the continuous-type machine. Since the different materials combine in the head tooling (see Fig. 35), the use of manifolds for multiple cavity is not feasible, nor are accumulator machines (see Coextrusion). 

Coextrusion blow-molding machines are very expensive and demand skilled operators. Coextrusion is used to produce containers having a view stripe and more recently for 55-gal drums and the plastic gasoline tanks that are in passenger cars, sport utility vehicles, or pick-up trucks. The plastic gasoline tanks are produced from a coextruded parison that contains six layers of different resins. For example, HDPE/adhesive/EVOH/adhesive/regrind/HDPE (see Fig. 35). 

Ethylene-vinyl acetates (EVA) copolymers are in the polyolefin family of thermoplastics. They are used in all processes, particularly for extrusion, injection molding, and blow molding. They are used either alone or are coextruded or coinjected and used in compounds to provide unique properties. They approach elastomeric materials in their softness and flexibility. 

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. 

Just as in the production of film, coextrusion is now an essential technique in the production of high performance blow molded containers. The parison is coextruded with a number of different layers, each of which contributes an important property to the finished package. 

Coextruded blow moldings commonly include from two to six layers, although more can be accommodated if necessary. The construction usually includes one or more barrier layers. These are polymers with a particular resistance to the transmission of water vapor or gases such as oxygen or carbon dioxide. Examples are... 

Fuel tank applications utilizing coextruded blow-molded layers of ethylene vinyl alcohol (EVOH) in conjunction with high-density polyethylene (HOPE) have been in place for decades. In these applications, the EVOH layer is adhesively bonded to the HOPE with an acrylic copolymer adhesive. The EVOH acts as an impermeable membrane to gasoline and additives such that the anission permeation requirements for fuel tanks is met. 

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