Extruder/die characteristics

Equation (4.12) enables the die characteristics to be plotted on Fig. 4.12 and the intersection of the two characteristics is the operating point of the extruder. This plot is useful in that it shows the effect which changes in various parameters will have on output. For example, increasing screw speed, N, will move the extruder characteristic upward. Similarly an increase in the die radius, R, would increase the slope of the die characteristic and in both cases the extruder output would increase. 

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). 

Figure 3.6 Screw and die characteristic curves for a 45 mm diameter extruder with an PE-LD.

The screw extruder is equipped with a die, and the flow rate of the extruder as well as the pressure rise at a given screw speed are dependent on both, as shown in Fig. 6.16. The screw characteristic line at a given screw speed is a straight line (for isothermal Newtonian fluids). This line crosses the abscissa at open discharge (drag flow rate) value and the ordinate at closed discharge condition. The die characteristic is linearly proportional to the pressure drop across the die. The operating point, that is, the flow rate and pressure value at which the system will operate, is the cross-point between the two characteristic lines, when the pressure rise over the screw equals the pressure drop over the die. 

Fig. 9.4 Schematic views of screw characteristic lines for Newtonian fluids and isothermal flow. The points where the screw and die characteristic lines cross are the operating points of the extruder. The effect of the screw speed and the channel depth on the operating points is demonstrated.

Ram extrusion is a process to produce PTFE extrudates of continuous lengths. Granulated resins used in ram extrusion must have good flow characteristics so they can be fed readily to the extruder die tube. Presintered and agglomerated PTFE powders with bulk density ranging typically from 675 to 725 g/1 are used for this process.5... 

In many cases it is preferential to use steam for heating and moistening this technique commonly results in a higher extrusion rate (capacity), increased die life, decreased power consumption, and improved quality of the extrudate. These characteristics are most reliably obtained if conditioning takes place in separate machines in which residence times of 5-30 minutes can be achieved. Figure 345 shows schematically the conditioner of such a system in which material is constantly moved with slowly rotating scrapers and transported from deck to deck while steam is injected and other additives, such as molasses or fat, are incorporated. 

A plot of the flow G as a function of the pressure P is given in Figure 23.6. For a given screw geometry, the first equation, the screw characteristic, is given for two values of the rpm N, while the second equation, the die characteristic, is shown for two levels of the flow resistance in the die, 1/c. Each combination gives rise to an intersection of the two lines, which is the working point of the extruder, from which the values of Q and P can be read. 

Equation (5.25) for the metering section and Eq. (5.26) for the die, can be solved graphically. The solution, lying at the intersection of the two curves (Fig. 5.11), is known as the extruder operating point. The performance of a real extruder at different screw speeds (varying V) and with different dies (varying the pressure flow component) can be used to construct the screw and the die characteristics, and confirm the analysis. 

Melt pumps are most appropriate when the screw and die characteristics combine to give a relatively poor pumping performance by the total system. This can happen when die pressures are low but more often occurs when they are extremely high (5,000-8,000 psi), or when the melt viscosity is extremely low. When pumps are used to increase the production rate by reducing the extruder head pressure without a corresponding increase in the screw speed, the extrudate solids content often is increased. The result is an inferior product. This problem often necessitates additional filtration, which serves only to increase pressure and may counteract many of the benfits expected from the pump, as well as increasing the financial investment even further. 

In contrast to extruder output, die output increases with head pressure (Fig. 5.27). Die output is also enhanced by low-viscosity melts and larger die gaps. The match between extruder and die output shifts with operating conditions. The simple die characteristic curve in Fig. 5.27 shows the optimized processing conditions. However, this curve does not consider extrudate quality. Other lines would be required to locate the onset of surface defects, such as melt fracture, and for incomplete melting. 

It is of course possible to redo the simple extruder analysis with a pressure-dependent viscosity. Equation 4.7b defines the die characteristic equation. Solution of the momentum equation with a moving wall then leads directly to Equation 3.15b for the wall velocity as a function of the flow rate and system geometry, even when the viscosity is pressure dependent. 

We use the general approach [20-22,26-28], first applied to the co-rotating machines here. The output of the twin screw extruder is known, G=pQ, where Q is volume flow rate and p is density. A temperature profile is proposed along the screw axis f(z). From the die characteristics and the melt flow behavior, we may compute the pressure p(L) at the end of the screw. This is... 

In Fig. 4.12 these points are shown as the limits of the screw characteristic. It is interesting to note that when a die is coupled to the extruder their requirements... 

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