Processing of bioresorbable and other polymers for medical applications - Durability and Reliability of Medical Polymers - page 52

3.3 Pin screw (from Osswald and Hernández-Ortiz, 2006).

Pin path

3.4 Self-cleaning single-screw extruder design (from Osswald and
Hernández-Ortiz, 2006).

as already noted, when dispersive mixing is required, the mixture must
be subjected to large stresses in order to break up agglomerates. in this
instance, barrier-type screws can be used, because the mixture is forced
through narrow gaps, inducing high stresses. it must also be noted that any
disruption to flow causes a pressure drop and viscous heating during extrusion.
Another example of a mixing extruder is a single-screw extruder with pins
on the barrel surface and a screw that oscillates in an axial direction (Fig.
3.4). Pins on the barrel wipe the entire surface of the screw, making this
design the only self-cleaning single-screw extruder (Osswald and Hernández-
Ortiz, 2006). The reduced residence time is advantageous when operating
with thermally sensitive materials. The pins disrupt the solid bed, creating
dispersed melting which improves the overall melting rate.
Twin-screw extruders are continuous-mixing devices and can be classified
as follows (Fig. 3.5; Tadmor and Gogos, 2006):

￿ ￿ ￿ ￿ ￿ ￿

∑ intermeshing or not intermeshing
∑ co-rotating or counter-rotating.

The intermeshing twin-screw extruder has a self-cleaning effect that reduces