Fully filled zone

If a twin-screw extruder is stopped and opened, several zones can be clearly distinguished [9]. Depending on whether the extruder is fed with a solid or a liquid material, two different situations occur. In the case of a solid feed (a polymer or solid monomer), the channels near the feed hopper are more or less filled with solids. This material melts, resulting in a zone where the channels are only partly filled. At the end of the screw, close to the die, a zone exists where the channels are completely filled with polymer. If the extruder is fed with a liquid monomer, the first part does not necessarily need to be partly empty but, as will be explained later, for reasons of stability it is advisable to create a zone where the channels are not fully filled. In particular, the fully filled zone is very important for correct functioning of the extruder. In this zone the pressure is built up, the mixing and kneading mainly... 

Fig. 12.1 Different zones in a twin-screw extruder (A) fully filled zone (pump zone), (B) partially filled zone. (C) melting zone. (D) feed zone.

In order to explain the existence of the fully filled zone, we will have to realize that the different zones in a twin-screw extruder cannot be viewed separately, but are interconnected. This can be shown by the throughput. The actual throughput of a twin-screw extruder is determined by the feeding... 

According to theory increasing the throughput causes a direct reduction in residence time on the one hand but an increase in die pressure and lengthening of the fully filled zone on the other hand. As a result, the residence time and therewith the conversion of the reaction should hardly be influenced by a change in throughput. In practice however, the throughput... 

For equal average residence times of fluid elements in the fully filled zone it is required that the movement of the channel remains constant. For closely intermeshing extruders this is easy to see because this is the time needed for a chamber to move from the beginning of the fully filled zone to the end of the screws however, this equation holds also for scale-up of other types of extruders. 

The pressure gradient in the fully filled zones can be calculated from the equation for the throughput and equals for an iso-viscous process ... 

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