Processing, thermoplastics mixing sections

Because of the heterogeneous nature of the mixture, and presence of contaminants, the articles produced should have large cross-sections, so that small imperfections do not hinder the mechanical performance of the products. Products can be manufactured by continuous extrusion, compression molding, or by Klobbie-based intrusion processes. Intrusion process is a cross between conventional injection molding and extrusion. In this, the extruder first works and softens the thermoplastic mixture, which is poured into one of the molds without using screen packs or nozzles. Today s recycling equipments are capable of processing most types of mixed thermoplastic material, even with up to 30 to 40 percent contamination as unmelted polymers or nonpolymer materials such... 

Elastomers can be prepared and processed using a variety of mixing and processing equipment, similarly to thermoplastic materials, and these will be addressed in this section. 

The procedure used in this section on thermoplastics processability is to follow the material through the various stages through which it passes from pellet to product. It should be borne in mind that the pellets may not be a single material but a blend of different polymers, or the mix may contain pellets of masterbatched additives as described in Section 3.2. 

Flows can be classified into streamline, when particles in the fluid follow paths (streamlines) that remain constant with time, and turbulent, when vortices cause unpredictable changes in the flow pattern with time. The changeover occurs at a critical value of the Reynolds number, which is defined as the melt velocity, divided by the viscosity times the channel diameter. The high viscosity of thermoplastic melts causes velocities to be low. Hence, the Reynolds number is very low and the flows are streamline. We will consider steady flows, and ignore the start and end of injection and blow-moulding flows, when the melt accelerates and decelerates, respectively. However, in the RIM process (Section 5.6.5), turbulent flow of the low viscosity constituents in the mixing head achieves intimate mixing. 

In reinforced plastics various inorganic materials are dispersed in the polymer. Carbon black reinforced elastomers have already been considered see Section 9.16.2. For fiber composites, two subtypes are important, the short fiber-containing materials, which are thermoplastic, and the continuous filament types, which cannot flow. While short fibers can be melt blended with thermoplastics, they are often embedded in monomeric mixes, followed by polymerization in situ. Continuous fibers are always processed via monomeric mixes which can flow over the beds of fibers. Of course, these monomeric mixes may have polymers or prepolymers dissolved in them, raising the viscosity, and reducing shrinkage on polymerization. An example of the continuous filament type is a tape composite, familiar as the strapping tape used for packaging. 

In melt extrusion, polymer is forced to flow under shear along with helical screw direction, which divided into number of zones based on their functions and the requirement for processing specific combination such as feed section, mixing and melting section, and compression section and takes the shape of the die at the other end. In melt intercalation, biofiller can be directly mixed mechanically with the polymer melt to form a homogeneous mixture. Method is very widely used for thermoplastic nanocomposites and can be applied to nanobiocomposite. Melt intercalation is highly specific for a polymer, which may lead to new hybrids. Moreover, absence of solvent in the technique leads to industrially economical as well as environment friendly by waste point of view (Ray et al., 2005). 

Melt mixing is a common method of improving the properties or appearance of a thermoplastic by incorporating a polymer with a combination of other polymer(s), filler(s), reinforcement(s), functional additive(s), or colorant(s). This process is referred to as compounding. Since SPS in its base form has inferior strength and ductility for most target applications, commercial products of SPS are mostly compounds. In this section, the equipment and processing conditions typically used to create SPS compounds are summarized. 

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