The Barrier Screw Concept

5 Barrier Flight Melting Screws 7.5.1 The Barrier Screw Concept 

Research using conventional extruder screws has shown that  

1) Completion of melting of the solids bed moves towards and beyond the end of the compression/melting zone as screw speed is increased. It may even result in incomplete melting at the screw tip and lumpy extrusions. 

2) The tip of the solids bed often breaks up so that unmelted clusters of granules travel downstream more quickly in the central channel regions. This is where shear strain mixing and melting conditions are at a minimum [6] (See Section 7.3). 

3) The presence and size of masterbatch striations can increase with increasing screw speed as screw mixing capacity is reduced by later melting. 

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It appears that the barrier screw concept originating over 40 years ago has reached maturity without becoming the screw of choice for many applications. Meanwhile, a range of quite different innovative screw designs have evolved in recent years to challenge both the conventional screw and barrier screw including the examples in Section 7.5.5. 

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