Polymer moulding extrusion

Polystyrene and closely related thermoplasties such as the ABS polymers may be proeessed by sueh techniques as injection moulding, extrusion and blow moulding. Of less importance is the processing in latex and solution form and the... 

Fig. 9a-h. Different design patterns for superimposition of rotation, reciprocating-rotary and oscillating vibrations on the main flow of molten polymer under extrusion (a-d) and pressurized moulding (e-h). Explanation of flow diagrams is given in the text,... 

In addition to the deliberate monoaxial or biaxial orientation carried out to produce an oriented filament or sheet, orientation will often occur during polymer processing, whether desired or not. Thus in injection moulding, extrusion or calendering the shearing of the melt during flow will cause molecular orientation. 

Admixing an additive-type flame-retardant to a polymer is not always easy. A plain blending of a granulated resin with the additive before the conventional processing (injection moulding, extrusion, etc.) does not provide appropriate homogenity in most cases unless special techniques (e.g. an internal mixer) are used beforehand. 

Abstract Injection moulding, extrusion and other processes for manufacturing polymer components are discussed. Several case studies are presented where manufacture of polymer components is crucial to product performance, such as injection-moulded polycarbonate coimectors in a catheter, cylinder guard fractures, sight tubes, a crutch failure, and ozone cracking of tubing and a condom. 

Modern blow moulding is separated into two major sub-divisions -extrusion blow moulding and injection blow moulding. Extrusion blow moulding was formerly by far the most dominant technique. Injection blow moulding has in recent years emerged as a major section for the production of bottles for carbonated drinks, using especially poly(ethylene terephthalate), PET, polymer. 

A unique situation exists in the polymer industry whereby a substantial proportion of the effort of the large manufacturing polymer producers involves working in conjunction with the users. This technical service is necessary because of the various applications of the polymers, i.e. they are effect chemicals. Thus, a fabricator will co-operate with the manufacturer as to the optimum method (and, in some cases, which polymer to use) for producing the finished article. Space does not permit discussion of the various techniques used, but they include injection and blow moulding, extrusion and the production of films, vacuum forming, etc. 

Processing operations such as injection and compression moulding, extrusion, fibre spuming, etc. are likely involved for the preparation and commercialization of polymer/clay nanocomposites. The flow applied during processing play a decisive role in crystallization kinetics and in the morphology obtained. For neat polymers, it has been demonstrated that the flow accelerates crystallization kinetics when compared with the quiescent melts (Keller and Kolnaar 1997 Kumaraswamy et al. 1999) and some crystallization mechanism has been proposed (Somani et al. 2000 Seki et al. 2002), whereas for polymer/clay nanocomposites, studies are still lacking and most of results have been reported for iPP and nylon 6/clay nanocomposites. 

The use of two isomeric acids in the synthesis results in an irregular chain which precludes crystallization. Thus the material can be processed at much lower temperatures than would be possible with a crystalline polymer derived from only one of the acids. Polyarylates can, in fact, be processed by standard methods such as injection moulding, extrusion and blow moulding. 

Made by esterification of pure cellulose with acetic acid and sulphuric acid as catalyst in solvents of medium polarity. This produces readily soluble resins with varying acetate content depending on the process. The polymers are processible by injection moulding, extrusion and blow moulding. Principle cellulose polymers are cellulose acetate (CA), cellulose acetobutyrate (CAB) and cellulose propionate (CP). Ethyl cellulose forms the other group of polymers. Applications are limited by environmental and chemical stability. 

Fig. 24.3. (a) Extrusion polymer granules ore heated, mixed and compressed by the screw which forces the now molten polymer out through a die. (b) Injection moulding is extrusion into a mould. If the moulding is cooled with the pressure on, good precision and detail ore obtained. 

Deformation of a polymer melt—either thermoplastic or thermosetting. Processes operating in this way include extrusion, injection moulding and calendering, and form, in tonnage terms, the most important processing class. 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

Polymers of a-methylstyrene have been marketed for various purposes but have not become of importance for mouldings and extrusions. On the other hand copolymers containing a-methylstyrene are currently marketed. Styrene-a -methylstyrene polymers are transparent, water-white materials with BS softening points of 104-106°C (c.f. 100°C for normal polystyrenes). These materials have melt viscosities slightly higher than that of heat-resistant polystyrene homopolymer. 

Structurally viscous grades are based on branched polymers (branching being effected by the use of tri- or higher functional phenols). These polymers exhibit a sharp decrease in viscosity with increasing shear rate which makes them particularly suitable for extrusion and blow moulding and also, it is claimed, in reducing drip in case of fire. 

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