Screw extruder feedstock sizes

[fractional@willmar.com]

Hello,

   Do screw extruders briquetting straw require a mix of sizes to bind
(or feed) well?  Or will chopped straw/hay/grass pass and bind as well
as grinder processed material?  1" nuggets/1" chop?

  Would the screw and case have a big bearing on this?  I have seen
designs with a strong taper others with  none.  Shallow flights and
deep.  Fat and narrow pitches.  Channeled walls and smooth bore.

  In addition to feeding, wear with different materials varies
drastically.  The big FAO review has some in various Indian tests
lasting only 2 hours.  One of those reports had 44 hours as the longest
run on a screw before it had to be resurfaced with tungsten carbide
rod.  One Thai machine reported repair weekly, it didn't state if it ran
24 hours a day.
  Jim Durham mentioned his ran 120 hours or so.  Some say 300 hours.
Anything less than 120 hours would seem to be uneconomic at the low
production rates these machines are scaled to.  If wear is such a
drastic problem the screw and case should be designed for the best
throughput for a given feedstock, any ideas what geometry is best for
straw?

Thanks,

Alan