Chemistry Reference
In-Depth Information
Screw length, expressed as the screw length:diameter ratio, typically ranges from
15:1 up to 40:1. Usually the extruder is operated not at the maximum possible feed rate
(volume limited) but at an adequate feed rate and process temperature that results in
both moderate torque (which is proportional to the applied shear stress) and moderate
mean residence time. The mean residence time strongly correlates with the feed rate
and hence a too high feed rate results in a too low mean residence time and poor axial
mixing.
The equipment is built and operated in a modular manner, with multiple tempera-
ture-controlled blocks assembled along the length of the screw shaft to form the barrel of
the extruder. Each modular block has heating and cooling control that allows different
temperature set points, referred to as heating zones, to be maintained along the barrel. In
an optimized extrusion process, the majority of process heat generated in the heating
zones is from frictional forces between feed materials, the rotating screw surfaces, and
the barrel walls [9].
Durability of extrusion equipment depends upon the process conditions applied as
well as on the level of owner care during operation, cleaning, and storage of product
contact parts. Equipment vendors provide barrel and screw components in different
grades of stainless steel that meet high speci
cations against abrasion and corrosion [10].
Single- or twin-screw extruders are available, with twin-screw designs offering
counterrotating or corotating operation. The conveying and mixing dynamics within
these different designs differ profoundly, as well as material output. Corotating twin-
screw extruders are most commonly used for pharmaceutical HME, providing a high
degree of material mixing within the extruder that aligns with the typical process goal of
material melting, dissolution, and extensive molecular interaction of the components. An
additional advantage of the corotating design is high output rate coupled with relatively
low residence time distribution within the extruder.
The rotating screw consists of multiple screw elements with geometric variables
such as screw pitch and
flight (Figure 10.5), the sequence and proportions of which are
de
guration
. Different screw geometries provide separate regions
for material conveying in the forward direction, material conveying in the reverse
direction, material kneading and mixing of varying intensity, and material compacting.
The best practice is to design the screw con
ned as the
screw con
guration based on the material properties
(both feed materials and extrudate) being processed during formulation development. As
such, it is rare for the scienti
c literature or an equipment vendor to specify default screw
configurations. Information is provided in Table 10.1 and in the following text to support
screw con
guration de
nition and selection:
First Region (Feeding and Transport):
Conveying screw elements are required at
the feeding zone and beyond to provide opportunity for ambient temperature feed
material to become distributed onto the screw surfaces in powder form and begin
heating toward softening temperatures. Without correct conditions for material
conveying along the extruder, risks include partial
ow of
powder or sticking of materials in the feeding zone. These scenarios could
desegregate the feed material, or increase the torque requirement for the screw
shafts.
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