Flow in the Extruder

An extruder can be of the piston type or screw t3q e. The flow in the piston t3q e can be modeled by the flow in a die, discussed in Section 

SCREW ROOT a. Drag flow and pressure flow 

FIGURE 13.22 ( a) Schematic of an extruder for ceramics, (b) Detail of the screw action on the ceramic mix. (c) Velocity due to drag and pressure with the net flow. Ideas for this composite figure were taken from Mutsuddy [62]. 

---

There is a continuing interest to improve and extend the fimctional properties range of dairy proteins to provide both health benefits and their characteristic physical behaviors under different temperature, moisture, and pH conditions so that they may be included in foods that ordinarily do not contain them. One such research area is the extrusion texturization of whey proteins, which have resulted in dairy proteins with new characteristics imparted by a controlled texturization process, depending on the application desired (Hale et al., 2002 Manoi and Rizvi, 2008 Onwulata, 2009 Onwulata et al., 1998). Protein texturization is a two-step process that involves, first, the unfolding of the globular structure (denaturation) and, second, the alignments of the partially unfolded structures in the direction of mass flow in the extruder. The surface characteristics are imparted at the extruder die as the molten mass exits (Onwulata et al., 2003a). 

As far as heat transfer is considered, Fenner [27] made a detailed comparison of the thermally fully developed flow and thermally developing flow. He indicated that the thermally developed flow will not be achieved when heat conduction effects become significant [34]. Bruker et al. [35] experimentally verified that the thermally developing flow analysis provided a more accurate description of the flow in the extruder. 

The target discharge temperature for the extrusion is 240 °C, and the maximum screw speed is 130 rpm (N = 2.167 rev/s). What will be the expected production rates for both resins at 130 rpm At 240 °C the melt density of the resins is 735 kg/mT As presented in Sections 1.4 and 7.4, the net flow in the extruder is the difference between the rotational flow and the flow induced by the pressure gradient Qp. The data in Table 7.2 was calculated from the example in Section 1.5.1 and Table 7.1 ... 

Figure 3.20 Extensional flows in the extruder and during processing of polymer melts...

The reaction kinetics depend on the local values of temperature and residence time in turn, their evolution changes the viscosity of the material, which affects the flow characteristics. To handle this interdependence, Vergnes and Berzin [167] developed a procedure that consists in performing an initial simulation of the flow in the extruder, to estimate the residence times, temperatures, and reaction extents. 

This effect is very important during the preparation of the nanocomposite via melt blending, as observed by Gilman et al. for a PS nanocomposite, where gel permeation chromatography analysis of the samples, extruded without a nitrogen flow in the extruder, showed some evidence of degradation in the form of lower molecular weight. ... 

In summary, to identify the optimal composition of the filler in the matrix that results in the desired performance or multi-functionality, we propose a single experiment with a compositional gradient instead of conducting several experiments with each composition. A known step input of the filler to a steady polymer melt flowing in the extruder would result in a compositional gradient that can be rapidly predicted using equation (4). Efforts are underway to characterize the properties of the nanocomposites to complete establishing the process-stracture-property relationships. 

When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

Important for polymer processing is the fact that when the concentration of a hard filler is increased in the composite, the unsteady flow (in the sense of large-scale distortions) of the extrudate occurs at higher shear rates (stresses) than in the case of the base polymer [200, 201,206]. Moreover, the whirling of the melt flow is even suppressed by small additions of filler [207]. 

The first area covers low volumetric flow rates, and entrance pressures below Pcr. This sector of two-phase flow in the molding machine is characterized by a complex non-linear dependence of reduced pressure on reduced volumetric flow rate. The structure of foam plastics obtained in this way was called shell structure by the authors in [20, 21] — the extrudate contains huge shell bubbles which are comparable to its section. As CBA concentration increases, or medium volumetric flow rate is increased at low CBA concentration, small bubbles materialize in the melt around the shell bubbles, and the structure becomes shell-bubble . Increase of the volumetric flow rate and the concentration of flowing agent neutralizes the difference in bubble size their lateral dimensions become smaller than their longitudinal ones. 

Even higher shear rates in the extruder cannot prevent laminar flow in the screw flights and therefore resultant unmixed particles being carried over the shearing sections. Lengthening of the residence time in the barrel also has to be restricted to limit unacceptable temperature build-up, which would result in scorched compound. It is thus necessary to have an effective means of... 

When devolatilization processes are conducted in screw extruders, the screw channels are only partially filled with the polymeric solution to be stripped of the volatile component (see Fig. 5) while the unoccupied portion of the screw channel serves to carry away the evaporated liquid. Because the barrel has a component of motion Vbz in the down channel direction, the solution is caused to flow from the extruder inlet to the outlet, which, in this case, is out of the plane of the paper. The crosschannel component of the barrel motion, Vtx, has two effects. First, it causes a circulation of the fluid in the nip and because of the continual... 

On the basis of the simplified view of the flow patterns just described, a model for predicting mass transfer rates can be developed using penetration theory and the fact that mass is transferred simultaneously from both the nip and the wiped film. We can therefore write that the total molar mass transfer rate from an element of fluid over a length dk in the extruder is... 

Another application based on a polymer s increased ease of flow in the presence of ultrasound is the atomisation of polymers. Here the particles are fed through an extruder then atomised (Fig. 5.51). The size of droplets (D) are governed by Eq. 5.44, where X is the surface tension, p is the density and F is the exciting frequency. 

In order to understand the mechanisms for fluid flow in the metering channel, it is important to understand the reference frames used in the mathematical analysis of the section. As discussed in many chapters leading up to this point, the extruder is a cylindrical structure, as shown in Fig. 7.1(a), that has a helical channel formed... 

---