Screw shear rate

Shear rate When, a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner and cavity, or die wall, it is subject to a shearing force. As the screw speed increases, so does the shear rate, with potential advantages and disadvantages. The advantages of an increased shear rate are a less viscous melt and easier flow. This shear-thinning action is required to move the melt. 

TPEs are quite fluid at shear rate above 500 s. A moderate speed is always preferred. A screw-back pressure below 1.38 MPa is sufficient for homogenization. 

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... 

The shear rate in the channel contains contributions from the rotational motion of the screw and the pressure-driven flow. The calcuiation of the shear rate, 7, using Eq. 1.24, is based on the rotational component only and ignores the smaller contribution due to pressure flow. For the calculations here, Eq. 1.24 can be used. 

Finally, the concept of viscosity and resistance to flow of polymers as a function of shear rate will be discussed because there is often a misconception regarding the dissipation in an extruder at high shear rates when the viscosity is in the power law region. As previously discussed, the viscosity decreases as a function of increasing shear rate as shown in Fig. 3.23. Often this reduction in viscosity is misinterpreted as a reduction in the amount of power needed when the polymer is sheared at high rates. For an extruder, the misinterpretation would be that less motor power would be required to operate the machine at higher screw speeds. It... 

The results presented here are encouraging but only qualitative and have been produced using this first-order model. Current limitations of the model are the use of a constant-viscosity function independent of temperature and shear rate. Also, the dynamic local temperature of the barrel and screw (Section fO.lO) must be incorporated into the model they are currently set as constants. An enhanced model for the film thickness at both the barrel and screw surfaces should be added to the current model along with flows induced by pressure gradients. 

Eqs. 7.22 and 7.24 represent the velocities due to screw rotation for the observer in Fig. 7.9, which corresponds to the laboratory observation. Eq. 7.25 is equivalent to Eq. 7.24 for a solution that does not incorporate the effect of channel width on the z-direction velocity. For a wide channel it is the z velocity expected at the center of the channel where x = FK/2 and is generally considered to hold across the whole channel. The laboratory and transformed velocities will predict very different shear rates in the channel, as will be shown in the section below relating to energy dissipation and temperature estimation. Finally, it is emphasized that as a consequence of this simplified screw rotation theory, the rotation-induced flow in the channel is reduced to two components x-direction flow, which pushes the fluid toward the outlet, and z-direction flow, which tends to carry the fluid back to the inlet. Equations 7.26 and 7.27 are the velocities for pressure-driven flow and are only a function of the screw geometry, viscosity, and pressure gradient. 

The combined mass flow for rotation-driven and pressure-driven flow is given by Eq. 1.28 and is the expected rate of the process. The average shear viscosity is calculated using the average shear rate in the channel for screw rotation and the bulk temperature. This method is also known as the generalized Newtonian method. 

The base case pressure flow calculation and the above viscosity function require that the shear rate be calculated from the screw rotation equations using Eq. 7.41 ... 

Since historically the dissipation is evaluated using the local velocity at the boundary and the shear stress is evaluated as the product of the viscosity and the shear rate at the boundary, it follows that if the velocity is not frame indifferent then the dissipation will not be frame indifferent. As discussed previously in this chapter, rotation of the barrel at the same angular velocity as the screw are the conditions that produce the same theoretical flow rate as the rotating screw. Because the flow rate is the same and the dissipation is different, it follows that the temperature increase for barrel and screw rotation is different. This section will demonstrate this difference from both experimental data and a theoretical analysis. 

If n = 1 then this is a Newtonian fluid and K is the Newtonian viscosity. The shear stress, T, is the viscosity times the shear rate. The reader is encouraged to go to references [4-6, 45] to become familiar with the literature development of the screw surface velocities. 

The secondary variables, such as shear rate, mean residence time, power consumption, throughput rate, etc., are expressed as a function of the primary variables. For example, the shear rate (or material displacement rate) in the screw channel is a function of the primary variables D, N, and H... 

The whole set-up can either be mounted into a ram extruder for low shear rates (0.5—15 sec-1) or onto a screw extruder for high shear rates (up to 103 sec-1). The unit can also be mounted in a reverse position, so that the entrance is only 1.5 cms from the windows W. In this way it can be checked whether the flow near the windows is really of the steady state type, i.e. independent of the distance from the entrance. For this check and for a series of interesting measurements not reported in Section 1.5 for brevity, reference is made to the original paper of Wales (40). 

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