Solids Bed Break-up

In addition to a melt film existing between solids bed and barrel surface, a melt film can also form between the solids bed and screw surface. This can promote a phenomenum called solids bed break-up . It was demonstrated by Edmondson and Fenner [6] where in addition to using both screw jacking and split barrel techniques, the melting rate was continuously monitored by observing pressure traces from a number of melt pressure transducers fitted along the length of the barrel. 

This behaviour tended to occur towards the end of melting and could be prevented by applying screw cooling to freeze the melt film in contact with the screw. Screw cooling had been shown to improve mixing in earlier papers [1, 3] but this was a remedy used more often in the days when extruder screw length to diameter ratios (L/D) were typically 15 1 and the consequential reduced output rate due to the cooling was evidently acceptable. 

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While there are various barrier screw designs, all ensure complete melting of the solid bed. This prevents flow instabilities or surging produced when the solid bed breaks up. However, the solids channels are more likely to plug if melting cannot keep pace with the reduction in channel size in the screw. The plugging, in turn, causes surging. 

Modifying the screw design, material or operating conditions can result in a completely different solid bed profile as shown in Fig. 12.1(b) [4]. In this case, the simulation predicts that the solid bed width reduces much slower and that melting is not completed until the very end of the screw. This is clearly a less desirable condition than the one shown in Fig. 12.1(a) because there is the potentiai for some materiai not to melt and mix properly before leaving the extruder. An extreme case of this can result if the reduction in the solid bed is slower than the compression rate of the screw. In this case, the volume of the channel is reducing faster than the voiume of the solid bed. This forces the solid bed to expand, accelerate, or break-up. Premature solid bed break-up can lead to extrudate surging and a poorly mixed material. 

With a demand over the intervening years for higher output rates, the possibility of incomplete melting due to solids bed break-up needed attention. This lead to the development of melting devices and barrier screws. 

This restores the melting function to the compression zone with the Maddock element catching and melting solids bed break-up fragments and other material not quite fully melted. It also avoids blockage from unmelted polymer and consequent reduction in output rate. 

Figure 5 Evidence of solid bed break-up (SBB) as a mechanism of extrusion instability (surging) for metering screw. Pressure traces of transducer 2 at the transition zone at 150 rpm are shown, (a) long term cyclic pressure variation (top) and (b) content of solids channel varies from full solids to lull melt over time (SBB)...

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