Solid bed breakup

For processes with relatively low compression rates such that the air entrained between the pellets is not readily pushed back out of the hopper, solid bed breakup will eliminate a pathway back to the hopper. In this case the entrained air will discharge with the extrudate and often create defects in the product. This type of problem is presented in Section 10.2.2. 

Flow from melt Film D to the melt pool 

Degraded Resin at the Screw Root Where the Flow Streams Merge 

The degradation ribbon at the merger of the flows occurs because of the crosschannel flow of material from the region between the solid bed and the screw root to the melt pool. As shown by Fig. 6.35, this flow is relatively large. As previously stated, the flow occurs because of pressure-induced flow and the dragging of fresh material under the solid bed by the backwards motion of the screw root. This process is consistent with the physics presented for screw rotation. The flow fields developed for a barrel rotation system would not create the low-flow region such as shown in Fig. 6.37. 

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Compression rates typically vary between 0.0025 and 0.0055 for pellet feedstocks. If the compression rate is too low, then the compaction rate of the solid feedstock may not be high enough to force the entrained gas out through the hopper. If the compression rate is too high, poor melting performance and solid bed breakup can occur. For example, LDPE resins can be melted very easily using a screw with a compression rate of 0.0055, while LLDPE resins perform best with a compression rate near 0.0030 [3, 4]. If an LLDPE is extruded using a screw with a compression rate of 0.0055, then solid bed breakup and solid polymer particles in the extrudate are likely to occur. 

It should be noted that melting takes place along most of the extruder. Indeed, the production capacity of plasticating extruders is frequently determined by their plasticating capacity. Further visual analysis of the experimental results reveals a tendency of the melt pool to penetrate under the solid bed and, occasionally, to completely surround it the continuity of the solid bed is frequently broken and a melt filled gap appears (e.g., turn 15.5, Fig. 9.23). This tendency for solid bed breakup seems to originate in the tapered sections of the extruder, and it appears to be a source of surging (i.e., fluctuation in time of temperature, pressure, and flow rate) of the extmdate at the die, as well as a source of entrapping some air bubbles into the melt stream. 

An important development in screw design was the barrier screw. The primary reason for a barrier screw is to eliminate the problem of solids bed breakup for more efficient melting. They have been around for over a quarter century. Original developments were for extrusion, but latter they were used to solve problems in injection and blow molding. There are many different patented barrier screw designs that under the broad claims of the Geyer or Uniroyal U.S. Patent No. 3,375,549 that expired in 1985.3>143... 

Solid bed breakup is generally associated with an inadequate melting rate of the solids, which leads to plugging of the screw channel. For example, the compression ratio of the screw may be so high that the volume available for material at positions further downstream is so reduced that the screw channel may not be able to accommodate both the remaining solids and the melt pool. In this case, the solid bed continues to be broken and dislodged. 

In single screw extrusion there is a commonly held belief that solid bed breakup should be avoided if at all possible. Most barrier extruder screws are based on this premise. They are designed to minimize the chance of solid bed breakup. The type... 

The advantage of the Maillefer screw is primarily the physical separation of the melt pool and the solid bed. As a result, there is less chance of formation of a melt film between the solid bed and the screw and, therefore, there is less chance of solid bed breakup. Thus, the melting process can occur in a more stable fashion but not necessarily at a higher rate. 

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