Most products will run acceptably on a twin-screw extruder under a wide variety of conditions. For instance, on a 50 mm extruder, a given formulation might be able to run at 400 lb/h (180 kg/h) using any RPM from 200 to 600. To the average operator, there may seem little or no difference in the mixing or material quality. However, we know that there has to be a difference, because the shear imparted to the material will be different from low to high degrees of fill. The way to answer this question is by running a test grid, which is a basic SPC (statistical process control) technique. The grid should be set up to test combina- tions of low, medium, and high values for RPM and throughput rate, within a realistic range. The grid also needs to have columns for key physical properties and quality parameters. For example, Table 1 clearly shows the best combi- nation of properties (in bold), running 500 lb/h at 400 RPM. But if nobody takes the time to do the tests, the product could be extruded for years under conditions which produce less than optimum properties.
Summary An extruder seems like a very simple machine at first glance, but there are many interactions of components and sub-systems involved. If the operator has an awareness of how these details affect the overall process performance, he or she will have the capability to tune the factors to optimize the process.
About the author Bert Elliott is engineering manager at Leistritz Corpora- tion in New Jersey, USA. If you missed the first part of his Top tips for twin-screw extrusion feature, then it can be found in the May issue of Compounding World – http://bit.ly/leistritz. For more information on Leistritz’s twin-screw extruders, visit: ❙ www.leistritz.com
References
1. G. Elliott, Guidelines for Installation, Operation, and Maintenance of Twin-screw Extruders, (1996)
2. D.B. Todd, Plastics Compounding Equipment and Processing, Hanser, New York, (1998)
3. V.L. Streeter and E.B. Wylie, Fluid Mechanics, McGraw- Hill, New York, (1975)
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