Blow 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. 

Conventional extmsion or coextrusion may be performed on vertical or horizontal rotary or shuttle mold configurations. In shuttie blow mol ding the extmder and die are in fixed horizontal and vertical position two or more molds shuttie into and out of position beneath the die. By reciprocating in two planes, the mold may remove a parison and permit the extmder to function continuously. 

Laminar materials are produced by a variety of techniques. Coextrusion blow-molding produces a number of common food containers that consist of multilayers such as layers consisting of PP-adhesive-PVA-adhesive-adhesive-PP. 

Multilayered articles can be made by coinjection blow-molding or coextrusion methods. A three-layer system generally contains a barrier layer sandwiched between two exterior layers. These are actually laminar products. In the coextrusion sequence, several extruders can be used to place the material into the mold. The multilayer container is then produced from blowing air into the preform. 

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). 

It is possible to blow mold several layers of material by coextrusion blow molding processes. By appropriate material selection, the various parts of the structure can be optimized for the best balance between properties and cost. 

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). 

Figure 6.2 Example of a 3-layer coextrusion parison blow mold head with die profiling [left) and example of a 5-layer eoextrusion parison blow mold head with die profiling (courtesy of Graham Machinery Group)...

Castillo, R., Coextrusion Die Design Doubles Number of Layers for Packaging Films and Blow Molding Parisons, SPE ANTEC, 2001. 

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

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. 

Fina . [FinaChem.] Polystyrene resins for inj. molding, coextrusion, blow molding, extruded foam, thermoforming, lamination, film, consumer prods., me al and laboratory parts, tumblers. 

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. 

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. 

A container with a volume of about 2 liters was produced by coextrusion blow-molding according to the following procedure in aBekum BM-401 blowmolding machine (produced by Bekum America Corp., Williamston, Michigan). The fluoropolymer... 

Table 6.44. Blow Molding Coextrusion Operating Conditions ...

Figure 6.59 Blow-molding head for four- or six-layer parison coextrusion. (Courtesy Bekum America Corp.)...

EVOH can be extruded in films, blow molded, or injection molded. It can also be processed by coextrusion or lamination, in combination with PE, PP, PET, nylons, and other resins. Applications in packaging include flexible structures and rigid containers, as shown in Table 4.12. Typical applications are ketchup and barbecue sauce bottles jelly, preserves, vegetable juice, and mayonnaise containers and meat packages. Nonfood applications include packaging of solvents and chemicals. 

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%. 

Technology to successfully produce multilayer injection blow molded bottles is much newer than that to produce multilayer extrusion blow molded bottles. Heinz Inc. s ketchup bottle was the first U.S. example (Fig. 12.22). The use of PET bottles made by coinjection blow molding has grown rapidly. As is the case for coextrusion blow molding, the key is production of the parison. Once the multilayer parison is produced, the remainder of the process is essentially the same as for single layer materials. Even stretch blow molding can be used. 

Shaping of material variable Coextrusion/thermoforming of multilayers, blow molding... 

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