Extruder barrel

The barrel must be extremely strong in order to withstand the high liquid polymer pressures developed it has characteristics analogous to those of the barrel of a field gun, which is the origin of the name. The size of the extruder is defined by the internal diameter, which is normally in the range 2.5 to IS cm. 

The working of the plastic by the screw generates additional heat which can be of such magnitude, for example, to permit extrusion to continue even if the power to the heaters is switched off. Because of the heat produced by mechanical working, many extruders today are equipped with a barrel cooling system the barrel must be provided with means to both add and extract heat. 

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The copolymers have been used in the manufacture of extruded pipe, moulded fittings and for other items of chemical plant. They are, however, rarely used in Europe for this purpose because of cost and the low maximum service temperature. Processing conditions are adjusted to give a high amount of crystallinity, for example by the use of moulds at about 90°C. Heated parts of injection cylinders and extruder barrels which come into contact with the molten polymer should be made of special materials which do not cause decomposition of the polymer. Iron, steel and copper must be avoided. The danger of thermal decomposition may be reduced by streamlining the interior of the cylinder or barrel to avoid dead-spots and by careful temperature control. Steam heating is frequently employed. 

If the extruder is to be used to process polymer melts with a maximum melt viscosity of 500 Ns/m, calculate a suitable wall thickness for the extruder barrel based on the von Mises yield criterion. The tensile yield stress for the barrel metal is 925 MN/m and a factor of safety of 2.5 should be used. 

Solution The maximum pressure which occurs in the extruder barrel is when there is no output. Therefore the design needs to consider this worst case blockage situation. As given by equation (4.11)... 

The main design criteria of most TPE dies are to ensure that changes in flow channel diameter from the extruder barrel bore to the die exit are equal. Most of the viscoelastic materials exhibit a die swell on exit from a die. TPEs tend to show die swell significantly lower than that of typical thermoplastics. This swell must be taken into consideration in designing dies and adjusting extrusion condition to achieve a perfect profile. The die swell normally increases with increasing hardness and shear rate and decreasing temperature. 

A schematic view of an extruder is shown in figure 1. The extruder barrel is essentially a ferrous alloy cylinder, with aluminum block heaters attached to the outside. There are several temperature control zones along the length of the extruder. Measurement thermocouples are installed in the extruder barrel itself. Barrel temperature is used to control the temperature of the polymer melt. Energy from the heaters is conducted both radially and axially in the barrel. Below, figure 2 shows a sketch of the extruder barrel, with the heaters and the temperature measurement points used in this paper marked. 

Figure 2. Sketch of cylindrical extruder barrel zone used in mathematical model.

A first principle mathematical model of the extruder barrel and temperature control system was developed using time dependent partial differential equations in cylindrical coordinates in two spatial dimensions (r and z). There was no angular dependence in the temperature function (3T/30=O). The equation for this model is (from standard texts, i.e. 1-2) ... 

Based on the shapes of the responses to step changes in controller output, and reasoning from the physical configuration of the extruder barrel, a reduced order dynamic model of the process was postulated. One can think of the Topaz program as order 80 (the number of nodes in the finite element subdivision), and the reduced model of order 4 (the number of dynamic variables). The figure below illustrates the model. 

Why is it sometimes necessary to cool an extruder barrel, even though you need to maintain high processing temperatures ... 

A term applied to a variety of vessels used in vulcanising mbber articles. See Autoclave and Dry Heat Cure. Also refers to heater bands used on extruder barrels and similar machines. 

A Uniroyal patent originated the principle of using pins protruding into the extruder barrel to disrupt the laminar flow of compound, and this principle was further developed by Troester in their QSM (cross-flow mixing) machines. The protrusion of pins into the barrel necessitated a modification to the screw design to accommodate them. Many papers can be found in the literature dealing with this type of pin extruder. 

Kramer, W. A., Extruder Barrel Cooling, SPE ANTEC Tech. Papers, 33, 34 (1987)... 

To illustrate the barrel optimization process [22], an extrusion process was optimized first for rate and then for discharge temperature. Each of the experiments was conducted at a screw speed of 50 rpm. The initial extruder barrel tempera-... 

In the Redaimator, a bigb pressure extruder, fiber-free mbber is heated to 175—205°C with oils and other ingredients. High pressure and shear between the mbber mixture and the extruder barrel walls effectivdy devulcanize the mixture in one to three minutes. In the Lancaster-Banbury method, bigb temperature, pressure, and shear are applied to the mbber in a batch process that is otherwise similar to the Redaimator process. In another high pressure process, scrap mbber is devulcanized at 5.5—6.9 MPa (54—68 atm) for ca five minutes. The product is milled, baled, or pelletized as in other processes. 

Kinsella (16), in his recent review on texturized proteins, described the texturization process as follows the globular proteins (glycinins) in the aleurone granules become hydrated within the extruder barrel, are gradually unravelled, and are stretched by the shearing action of the rotating screw flites. 

Temperature development in an extruder channel during melting. In this example, we illustrate reduction in dimensionality of the energy equation to find an equation that would reveal the change of the melt temperature through the gap between the solid bed and extruder barrel during melting, as schematically depicted in Fig. 5.10. To simplify the problem, we assume to have constant properties and a Newtonian viscosity. 

Figure 10.15 3D single screw extruder barrel and mixing head discretization. White element delineations define the mixing head surface, and the black lines define the barrel surface representation. The mixing head surface is represented with triangular as well as quadrilateral elements. 

Figure 4.37 Ring extruder barrel and screw shafts [231...

Choice of Materials for Extruder Barrel and Screw Elements... 

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