Extruder zones

Crahtree, S. L., Spalding, M. A., and PavUcek, C.L., Single-Screw Extruder Zone Temperature Selection for Optimized Performance, SPE ANTEC Tech. Papers, 54, 1410 (2008)... 

FIGURE 18.15 Sketch details of screw and extruded zones. (From Seymour, R., Modern Plastics Technology, Reston Publishing, Reston, VA, 1975. With permission.)... 

Conveying mixing elements operate most effectively in the filled extruder zones, i.e., they should operate against pressure. This guarantees that the product flows back through the gaps in the flights and mixes with the product in the screw channel upstream. 

Cl whose decomposition temperature is below that of the pol3mier processing (including all extruder zones and die head) must not be used for extrusion of inhibited polymer compositions into films. 

Takamura et al. [127] cross-linked PLLA using various types of peroxides under constant mole ratios of peroxide-derived radicals to PLLA during reactive extrusion in a single-screw extruder (D = 20 mm, L/D = 25) with a fixed temperature profile for the extruder (Zone 1/Zone 2/Zone 3/Die = 180 =C/185°C/190°C/190°C). 

A twin extruder is used to compound a thermoplastic polymer with a whisker suspension. Such suspension is added in an extruder zone where the PLA is melted by means of a peristaltic pump. The extmder has a venting system that is used to remove the liquid phase of the whisker suspension. Kamel [81] has compounded PLA-malic anhydride and poly (ethylene glycol) with cellulose whiskers... 

From mechanical mixtures with a given weight ratio of the components films with the thickness of 60 5 or 200 10 micrometers were molded by the single screw extruder ARP-20 (Russia) with a screw diameter of 20 mm and a diameter to length ratio 25. Temperature of the extruder zones was varied from 150 to 190°C. 

FIGURE 8.33 Degassing in a single-screw extruder. Zone 1 consists of a typical plasticating screw section zone 2 is a deep channel section with a vacuum port zone 3 is a metering section. The channels in zone 2 are partially full to facilitate degassing. 

That the compound is not allowed to stagnate in heated zones of the extruder and thus decompose. The life of many compounds at processing temperatures is little more than the normal residence time of the material in the extruder. 

The particular features of the nylons should also be taken into account in extrusion. Dry granules must be used unless a devolatilising extruder is employed. Because of the sharp melting point it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. 

Basically an extruder screw has three different zones. 

The pressure build-up which occurs along a screw is illustrated in Fig. 4.2. The lengths of the zones on a particular screw depend on the material to be extruded. With nylon, for example, melting takes place quickly so that the compression of the melt can be performed in one pitch of the screw. PVC on the other hand is very heat sensitive and so a compression zone which covers the whole length of the screw is preferred. 

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

As discussed in the previous section, it is convenient to consider the output from the extruder as consisting of three components - drag flow, pressure flow and leakage. The derivation of the equation for output assumes that in the metering zone the melt has a constant viscosity and its flow is isothermal in a wide shallow channel. These conditions are most likely to be approached in the metering zone. 

K. Schnieder, Technical Report on Plastics Processing in the Feeding Zone of an Extruder, Institute of Plastics Processing (IKV)TH, Aachan (1969). 

This makes it possible to conclude that the intensity of fiber filler destruction depends on the zone of processing installation where the filler is introduced (Fig. 7). As the Figure shows, with glass fibers introduced into the loading zone, their destruction occurs rapidly (curve 1). When fibers are introduced into the melt zone of an extruder the fiber destruction is less intensive. 

Fig. 7. Relationship of average fiber length L/Lc and screw length 1/L0 of extruder 1 — introducing fibers into melt zone 2 — introducing fibers into leading zone [47]...

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