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The importance of periodic audits of extrusion performance

Modified on Saturday, 31 January 2015 02:17 AM by mpieler Categorized as Consultants Corner
Whether or not sophisticated controls are employed in an extrusion operation, it can be extremely important to periodically conduct a comprehensive audit of the extrusion process, equipment, feed stocks, and product quality. It is amazing how many plants don’t even know what screw was used in the extruder a year ago for the same or similar product. Many plants keep records over the years of average daily output, but quite a few plants don’t have records of the instantaneous output capability (correcting for down time) at a measured actual screw speed with all other process conditions firmly documented. It is so easy to forget something like changes in feed stock characteristics (including percent regrind, moisture content, temperature, particle size distribution, additive content, etc.).

Obviously there are many extrusion processes where it is impractical to record everything significant every day. Computers interfaced with sophisticated process control instrumentation can be a big help where economically justified to keep track of almost everything important, providing feedback control to correct some problems and extensive data analysis to help avoid other problems. Nevertheless, rarely is everything important in the extrusion process covered in this manner.

However, for most extrusion processes it is entirely practical to have a skilled process engineer or well-trained foreman periodically make a comprehensive performance audit, even if only once a year, so that long-term trends can be properly interpreted. It can be very costly to allow some aspect of the operation to deteriorate. Obviously, the extrusion process should give at least the same rate and product quality from year to year (within normal control limits) unless something bad is being neglected. However, even if the audit shows a 5 or 10 percent improvement, consider the possibility that there should have been much greater improvement because of some new developments incorporated into the process. Perhaps other operating conditions or equipment have been allowed to deteriorate so that the full benefits of the new developments cannot be realized.

What kind of periodic audit is justified? Because many extrusion lines have unique differences and often downstream operating conditions and equipment can be extremely significant, the audit program should be set up with considerable skill. To effectively initiate the audit program it may be helpful to utilize the services of a consultant well qualified in commercial extrusion technology. In all cases at least several sets of measurements should be taken on each extrusion line under nor. mal (hopefully standard) operating conditions for the major products manufactured on the line. It is preferable to include at least one set of data for each operating shift and more than a single day period. Product quality measurements and equipment condition should be noted during the audit. For some extrusion lines a much more elaborate or frequent audit may be justified, but in many cases the approach described above will be adequate if results are reasonably uniform and within the predicted range, based on prior experience of the skilled person performing the audit. Needless to say, the audit is no substitute for good day-to day operating controls and records.

It is amazing how many plants operate with broken pressure gauges, improperly seated thermocouples, worn screw and barrels, and other significant equipment defects. These problems usually can be directly or indirectly detected during the audit. Also, operators or foremen may have gained more skill or may have been replaced by less experienced people since the last audit. New operating practices or equipment features may have been introduced (for better or worse). The feed stock may have changed — The change may be a simple as changing to finer screens in the recycle scrap grinder (possibly adversely affecting feeding characteristics while improving dispersion of additives).

All things considered, periodic audits of every aspect of extrusion line performance can be recommended to help avoid problems and assure greater profitability.

- George A. Kruder

See also:
  • Calculating extruder performance
  • Don't always blame the extruder
  • Process uniformity
  • Shop foremen
  • The programmable calculator

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