Why Compression Ratio is Important

March 13, 2023

One of the terms you hear talked about is the compression ratio of a screw. Some have assumed the compression ratio is designed just with feeding in mind. Not so. The compression ratio has to be established not just to feed enough solid polymer to fill the screw, but also to introduce enough shear heating in the screw to complete melting. There are quite a number of variables that go into selecting the proper compression ratio. I find that most designs don’t really consider many of them, so it truly becomes only a rough estimate.

The compression ratio is simply the ratio of H1 ÷ H2 (feed depth ÷ metering depth) and applies to all screws.

Feeding is the first consideration, which is a balance of particle geometries, friction of the particle to the screw/barrel surfaces (including the feed throat), and the particle-to-particle friction. HMWPE, for example, is usually processed with a grooved barrel section because it is so slippery. Add in the bulk density, angle of repose, particle hardness and melt density, and you are starting to get some of the more influential properties. Even the hopper design has an influence because particle flow governs the filling pressure on the screw. Any “feed assist” device (like a crammer) would change the ideal compression ratio for a low-bulk-density material.

Just as important, shear heating is also a function of the compression ratio, as it determines the shear rates throughout the screw. The greater the “compression” between the feed section and the remainder of the screw, the greater the amount of energy per unit output going into the polymer to complete melting. Every polymer is different in its energy requirement, and you have to consider such things as polymer viscosity at differing shear rates along with specific heat and the ideal exit temperature.

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