Starved Feeding to Avoid Agglomerate Formation

The investigation discussed in Section 13.1 confirmed Smith s advice that the best way to avoid the presence of agglomerates is to avoid their formation in the first place. The requirement is therefore to forward polymer granules and additive powder with minimal compaction forces. This has led to several investigations into the use of starved feeding, i.e., controlling the rate of addition in the extruder feed zone such that the screw channel is less than full. 

Finally, under the influence of die back pressure, polymer will become fully melted during passage though the last few turns of the screw. 

In fact this idea was not completely original, but a recognition that this starved feeding behaviour illustrated by Maddock [8] in screw jacking experiments could be suitable for the avoidance of agglomerates generated by normal full channel extrusion. 

This influence of starved feeding on melting and mixing was just one of many situations included in trials with colour pellet mixtures by Maddock in 1959 to investigate overall melting and mixing mechanisms [8]. 

Both Thompson and co-workers [10] and Elemans and van Wunnik [11] carried out starved feeding trials using barrier screw extruders with the objective of achieving good dispersion of additive powders. 

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