Extrusion shear rate limitations

When the extrusion is a profile with sections of different thickness the shear rates and die swell will be higher in the thinner sections than elsewhere—so drawing again may be of limited value. The thinner sections may have a shorter die parallel (to ensure that the rate of extrusion is constant over the entire cross-section), and this will alter die swell even further. 

Recently a new freezing method, low temperature extrusion, has been developed to overcome the self-limiting nature of the factory freezer, and to avoid the need for hardening. Ice cream from the factory freezer at about — 5 °C is passed through a screw extruder with refrigerated walls and cooled to about —15 °C (Figure 4.18). The extruder applies a higher shear stress (and lower shear rate) to the ice cream than a... 

One further limitation is that the extrapolation not be taken too far. Capillary rheometric measurements (Fw = 10 up to 1(F [1/s]) have shown that the flow curves level off in that area. Thus, ceramic extrusion compounds count as structurally viscous across wide shear-rate ranges. Since the measured data in question stay within the extruder s operating range, the material-data yield provides a solid foundation for die- or extruder design as well as compound developping. 

Finally, we shall mention the slip-stick melt fracture phenomenon — a phenomenon often observed in polymer extrusion — which is much related to the relative positions of the /Ltg(t, E) and pcit) processes. The phenomenon is a well-known problem in the polymer processing industry because it limits the output of polymer through an extruder. In a capillary flow, the decline of the viscosity observed as the shear rate (or flow rate) increases from the Newtonian region is much related to the damping factor... 

Increasing the barrel diameter will increase the shear rate—other factors being constant. This will increase viscous dissipation and melt temperatures as discussed earlier. Another problem with larger barrel diameters is that the heat transfer surface area increases with the diameter squared, while the channel volume increases with the diameter cubed. As a result, the heat transfer becomes less effective with larger diameter extruders. It is well known in the extrusion industry that the ability to influence melt temperature by changes in barrel temperature is very limited for large extruders. 

High stock temperatures are likely to be a problem in extrusion operations where the extruder is run at high screw speed and where the polymer melt viscosity is high. The main screw design variable that affects viscous heating is the channel depth. Increasing the channel depth will reduce the shear rate and thus the viscous heating. There are limits to how deep the screw can be cut. One limit is the physical... 

Another consideration in the selection of technology platform is the polymer chosen for the amorphous solid dispersion formulation. The fact that many pharmaceutical polymers degrade, crosslink, or lose functionality at high temperatures has already been discussed. However, the melt viscosity of a polymer is critical to the ability to extrude the amorphous solid dispersion within the capabilities of the extrusion equipment. The melt viscosity as a function of temperature and shear rate varies considerably across pharmaceutical polymers (Chokshi et al. 2005). Formulation melt viscosities in the range of 10-100,000 Pa s are generally acceptable for HME, although the range depends heavily on the torque limit capability of the particular extruder. 

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