EXTRUSION: Finding the Sweet Spot in Screw Design
Each screw design is an exercise in finding the “sweet spot.” Each polymer, each extruder, and each process requires a different design to optimize performance. First to be considered are the capabilities of the extruder, including horsepower, screw speed, bore size, L/D, etc. Then the requirements for the process come into play, such as the desired melt temperature, output, need for devolatilization, homogeneity of the melt, and the stability of output. Finally the thermal, rheological, and solid particle properties of the polymer must be considered. I’ve designed thousands of single screws and can say emphatically that there are few exact duplicates among them, due to all these considerations.
As an exercise, I once tried to write down every consideration and piece of data, no matter how minor, that went into the design of a screw for a new extruder, process and polymer. Many of them are things that become automatic, and you don’t really actively consider them; but if you try to list them, they quickly exceed 100 items.
In a column I wrote back in December 2012, I noted some of the shortcomings of using compression ratio (C/R) as an important criterion in screw design. Compression ratio was commonly used as a primary design parameter for single-screw design back in the very early stages of screw design technology. Today the depth ratio between the feed flights and the metering flights is more useful as an estimate of how efficient you expect the solids conveying to be. That is based primarily on empirical information gathered from numerous designs and polymers.
If it’s suspected (or known) that the solid polymer particles do not feed efficiently because of their bulk density, frictional properties, packing characteristics, or solids flow, the feed flights are deepened to allow more polymer into the screw. This is intended to offset the feeding inefficiencies due to these particle variables, so that the output from the solids feed section is adequate to fill the remainder of the screw.
The compression rate is a more critical parameter, and it is the rate or angle of the compression section. That angle determines how aggressively you are trying to transfer energy to the solids. Too aggressive an angle causes plugging and too gentle an angle causes solids bed breakup and poor melt quality and/or plugging farther downstream. Naturally, different polymers have their ideal melting rates or “sweet spot.”
Designers often discuss whether the C/R should be 2:1, 3:1, and so on, as if that’s all you need to know. The polymer absorbs the shear energy from the rotating screw, and the lower the output, the more energy or melt-temperature increase goes into each unit of output, and conversely. For example a 3.5-in. screw with a feed depth of 0.500 in. and a metering depth of 0.200 in. has a C/R of 2.5. But so does a screw with a feed depth of 0.375 in. and a metering depth of 0.150 in. Are these two designs expected to perform the same?
The second design would be expected to have approximately 75% of the output of the first, and due to the input power being approximately the same, it would have a higher temperature rise because there is less polymer to absorb the shear heating. As a result, simply stating the C/R does not indicate much about the screw’s performance.
With the popularity of barrier screws, which completely separate the melting and conveying functions, the C/R has even less meaning. Many barrier sections have a 10:1 ratio of feed depth to discharge depth, yet they provide lower melt temperature at the same output than a conventional flighted screw with a 3:1 ratio.
C/R was often used in the past as a basis for adjusting melt temperature, and polymers with higher process temperatures were run with higher compression ratios. That does not take into account the polymers’ degree of shear-thinning behavior, their specific heat, delay in melting, screw L/D, viscosity, and so on.
Each polymer and each process has a “sweet spot” where the polymer exiting the barrel has the ideal flow and melt characteristics for that process. Screws are designed for a specific speed, usually based on the highest expected output that is within the capability of the extruder and the anticipated head pressure. Although the screws typically have an extended range of operating speeds, it’s not an infinite range and they can operate poorly at speeds much higher or lower than their design speeds. An example is a screw for high-speed extrusion coating where the melt temperature (±3° F) is critical to develop the correct bond to the substrate, yet a specific output is required to match the desired line speed.
I am asked all the time for “general-purpose” screw designs, though in a column I wrote in October 2013, I noted that there is no such thing, as the “sweet spot” is a moving target. In fact, as I was writing this current column, I received a request for a general-purpose screw design to process rigid PVC, flexible PVC, and HDPE. There is no “sweet spot” for that design.
ABOUT THE AUTHOR
Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact
email@example.com or (724) 651-9196.
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