Optimizing for Melt Conveying

The melt conveying theory discussed in Section 7.4 can be used to determine the optimum screw geometry for melt conveying. This optimum geometry will not normally be used in the metering section of the extruder screw. The optimum channel depth for output can be determined from  

The volumetric melt conveying rate for a Newtonian fluid, neglecting the effect of the flight flanks, is given by Eq. 7.198. Considering that the channel width W = (rt D sincp/p) -w, down-channel barrel velocity Vb = 7t D N coscp, and down-channel pres- 

57tDNH(7tDsin pcos(p- pwcostp)--------(nDsin (p-pwsintp) 

The polymer melt viscosity ri will depend on the local shear rate in the screw channel. If it is assumed that the representative shear rate in the channel is the Couette shear rate and that the polymer melt behaves as a power law fluid, which melt viscosity can be represented by  

The optimum channel depth for output rate H can now be determined from Eqs. 8.28 through 8.30. 

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