Chemistry Reference
In-Depth Information
Figure 10.5.
Design and geometry descriptions of a conveying screw. Reproduced with
permission from Breitenbach [9].
Second Region (High-Shear Mixing and Transport):
At suf
cient barrel length
after which powder feeding and liquid feeding (if applicable) are complete and
partial melting is anticipated, one or more kneading zones may be introduced. A
kneading zone will generate localized torque on the screw shaft, high-shear
forces between feed material and equipment surfaces, causing intense melting
and mixing of materials. Different kneading screw elements are available that
affect material dynamics, from forwarding (30 or 60
offset) to neutral (90
°
°
offset) to reversing (reverse 30
offset). The kneading element offset also affects
the degree of shear, with larger offset angles (forwarding or reversing) providing
greater shear.
More than one kneading zone may be included in the second barrel region,
separated by some conveying screw elements. Such a design may be selected for
multiple reasons, for example, to ensure that the maximum torque for the extruder
equipment is not reached, to reduce the material residence time in the extruder, or
to minimize the risk of localized overheating of materials in the kneading zone
due to too high local shear. The contribution of kneading elements in producing
drug
°
polymer solid dispersion was demonstrated by Nakamichi et al. by
comparing solid-state and dissolution properties of materials made with and
without kneading elements in the screw design [11]. Using a clamshell design
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