Why Compression Ratio Matters
One of the terms you hear about is the compression ratio of a screw. Some have assumed that the compression ratio is designed purely with power in mind. Not so. The compression ratio must be set not only to feed enough solid polymer to fill the screw, but also to introduce enough shear heating into the screw to complete fusion. There are a number of variables that go into selecting the appropriate compression ratio. I find that most designs don’t really consider many of them, so this really only becomes a rough estimate.
The compression ratio is simply the ratio H1 ÷ H2 (feed depth ÷ metering depth) and applies to all screws.
Feed is the first consideration, which is a balance between particle geometries, particle friction on screw/barrel surfaces (including feed throat), and particle-to-particle friction. HMWPE, for example, is usually treated with a grooved barrel section because it is very slippery. Add bulk density, angle of repose, particle hardness, and melt density, and you start to get some of the most influential properties. Even the design of the hopper has an influence because the flow of particles governs the filling pressure on the auger. Any “feed assist” device (like a bin) would alter the ideal compression ratio for low bulk density material.
Equally important, shear heating is also a function of compression ratio, as it determines shear rates throughout the screw. The greater the “compression” between the feed section and the rest of the screw, the greater the amount of energy per unit output in the polymer to complete the melt. Each polymer is different in its energy requirements, and you need to consider factors such as polymer viscosity at different shear rates, as well as specific heat and ideal exit temperature.
Screw performance is often compromised if you fail to consider some of the most influential variables that go into selecting the most effective compression ratio.
Different processes, for example, require different melting temperatures, even with the same polymer. For example, the ideal melting temperatures for sheet extrusion and fiber drawing can range from 50°For more. This requires balancing power and fusing requirements to some extent. In other words, there is no ideal compression ratio for a specific polymer without considering many other variables. Assuming a certain feed depth of “H”, a compression ratio of 2:1 will put about half the energy into the polymer as a compression ratio of 4:1. Again, the screw must be full for optimum melting, which involves feed variables.
The technology exists so that all of these properties can be accounted for, but collecting all the data and analyzing it is so complicated that it can only be done with the most advanced computer simulation programs available. Therefore, it’s just not done regularly. Compression ratios have been pretty much standardized over the years based on what generally worked before. For example, PE screws typically use a 3:1 ratio as a starting point, while nylon screws typically start with a 4:1 ratio. This only establishes a starting point and streamlines design effort, but much screw performance is often compromised if you ignore some of the most influential variables that go into rate selection. most effective compression.
Much wider use of the recycle product has made these considerations even more important, as the feed properties of the recycle product may conflict with melting requirements. For example, recycling with low bulk density may require a greater depth of feed to fill the screw; yet the “standard” compression ratio for this polymer would result in insufficient shear heating. Consequently, the feed and the melting must be determined separately, freeing themselves from the so-called standard compression rates.
Even more complicated are mixed polymers which are common in some recycling applications. I recently came across a PP/HDPE blend that required extra thought, especially on the blend calculation, which completely changed the compression ratio. The two recycled polymers were different in both their solid properties and their energy requirements.
ABOUT THE AUTHOR: Jim Frankland is a mechanical engineer involved in all types of extrusion processing for over 50 years. He is now president of Frankland Plastics Consulting LLC. Contact; 724-651-9196; [email protected]