Chemistry Reference
In-Depth Information
Figure 10.6.
Effect of kneading paddle on dissolution pro
les of NP. (a) With kneading paddle
elements. (b) Without kneading paddle elements. Reproduced with permission from Ref. [11].
extruder with no end plate, extruder screws were stopped and exposed mid-
process for sampling of in-process material at multiple regions along the screw.
Kneading elements of speci
c geometry were required to generate a glassy
amorphous phase with the required dissolution characteristics, unlike a reference
con
guration with only feeding elements that generated a dispersion with
residual crystallinity (Figure 10.6). The effect of screw design on the solid
dispersion may directly relate to the additional shear force in a kneading zone, but
other changes in residence time or mixing ef
ciency may also be a factor. For
reasons including scientist health and safety, the dif
culty in representative
sampling, and ease of data interpretation, isolating materials from the extruder in
this way is not recommended as a
first-choice approach.
Third Region (Pre-extrusion):
The
final barrel sections are designed to prepare the
extruder contents for extrusion at the die plate. The section must build up material
and associated pressure to enable mixture passage through the die in a controlled
manner. These functions are typically achieved with screws of wider
flight and
shorter pitch. Low temperature in zones at or close to the die may be used to ensure
that extrudate exiting the extruder is of a viscosity suited to the postextrusion
processing step. A venting port may also be included in this region to allow
controlled removal of air and/or volatile components from the barrel mixture.
Extruder Die:
The
final component of the extruder barrel is the die plate that
contains the die opening(s) of speci
guration is
selected based on the postextrusion process being used. The simplest con
ed shape and size. The die con
gura-
tion is a single round die producing a single extrudate strand, which is compatible
with chill roll cooling, conveyor cooling, cutting, or pelletization downstream
processes. Multiple die openings in the die plate allow larger quantities of
extrudate to simultaneously exit the extruder, a design that is well suited to die
face pelletization or use with a chill roll. Up to 12 die ori
ces are common, as
described in Ref. [12]. Dies with a thin slit opening allow formation of extruded
sheets that may be collected via torque winders as a cooled
film.
