A pin extruder is a type of single-screw extruder that is commonly used in starch processing. There are two designs of this extruder: one has stationary pins and a rotating screw with interruptions of the flights, called the expander, while the other has a rotating and axially oscillating screw, also known as a co-kneader. The latter type of machine is commonly used in the rubber industry and in processes that require intensive mixing. Despite being discovered in 1945 and widely used in industry, the co-kneader’s application is far beyond theoretical understanding.
The co-kneader also consists of a single screw with interrupted flights. Its working principle is based on the rotation and axial oscillation of the screw, which causes transportation and mixing. Mixing is enhanced by stationary pins in the barrel, which promote dispersive mixing through their local weaving action, and distributive mixing through the reorientation introduced by the pins.
The expander extruder, on the other hand, is a relatively simple machine that uses stationary pins to provide stable throughput and improve mixing. The mixing is increased compared to an ordinary single-screw extruder. Pin extruders provide both distributive and dispersive mixing, with distributive mixing caused by the geometry of the screw flights and interruptions. However, the relatively high shear can easily lead to degradation of the starch, and it is not possible to use this machine for pressure build-up due to the large interruptions of the flights.
In the co-kneader, the screw flights slide along the pins and barrel due to the rotating and oscillating movement of the screw, resulting in high shear stresses and dispersive mixing. The co-kneader has good self-cleaning properties, which is much less the case in expander extruders. Temperature can be controlled by the thermostatted barrel and screw, and the large area/volume ratio, together with the radial mixing, contribute to good heat transfer capacities. The mixing mechanism in the co-kneader is rather complex.
Overall, pin extruders are well-suited for processes that require good dispersive mixing, while co-kneaders are commonly used in processes that require intensive mixing.