Effect of Compression Ratio

The compression of the channel depth tends to widen the solid bed, as discussed in Section 7.3. Melting tends to reduce the width of the solid bed. If compression is too rapid, the melting may be insufficient and the solid bed can grow in width. This will generally cause plugging of the channel by the solid bed and should be avoided if at all possible. Plugging will cause output fluctuations, but it may also cause wear in the compression section of the screw, as discussed in Section 8.1.3. It was discussed in Section 7.3 that plugging can be avoided if  

The compression ratio X, is the channel depth in the feed section divided by the channel depth in the metering section. The axial length of the compression section is Lc. Thus, in order to avoid plugging, the length of the compression section should obey the following inequality  

Minimum length of compression section versus compression ratio 

If a large compression ratio is used, the length of the compression section must be quite long to avoid plugging. 

The most dangerous combination in design of the compression section is a large compression ratio and a short compression section length. This will lead very easily to plugging. Rapid compression screws should be avoided for this very reason. One of the myths in screw design is that certain polymers, e.g., nylon, require a very 

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Neuman et al., 1994] Neuman, A., Bakke, M., Heilman, S., and Levitt, H. (1994). The effect of compression ratio in a slow acting compression hearing aid Paired-comparison judgments of quality. J. Acoust. Soc. Am., 96 1471-1478. 

Figure 5. Effect of compression ratio on fuel consumption...

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