The Dray and Lawrence Screw

The screw geometry is very similar to the Barr screw with the one major difference being an abrupt change in helix angle in the main flight at the point where the barrier flight is introduced. This allows the width of the solids channel to stay just as wide as the full channel width of the feed section. Obviously, this is done in an attempt to maintain the solids channel as wide as possible. However, this also causes an abrupt change in the direction of the solid bed velocity, and this can lead to instabilities. 

If the effect of the change in helix angle on Q, is neglected, the melting length can be shown to be [27]  

In this case, however, the down-channel melting length does not provide a good basis for comparison because the helix angle is different along the screw. The total axial melting length can be expressed as  

If typical values are used for cpb and cpf, the melting length of the Dray and Lawrence screw will about 10 to 20% longer than the ideal compression screw. The melting performance of the Dray and Lawrence screw is thus about the same as the Barr screw and slightly better than the Maillefer screw. A patent on this barrier screw 

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The Kim screw is basically an improvement of the Dray and Lawrence screw. A picture of the Kim screw is shown in Fig. 8.61. 

This means that the melting performance of the Kim screw is slightly lower than the Dray and Lawrence screw and the Barr screw. It has an advantage over the Dray and Lawrence screw in that the transition from feed to barrier section occurs more smoothly. However, at the end of the barrier section the same difficulty arises as with the Dray and Lawrence screw. 

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