Thursday, December 3, 2009

Feed Throat Cooling - Surging

Buzz: Good morning Skip - we've been having some discussion in other formums about "screw and barrel wear and it's possible affect(s) on surging". To that end, we have gotten a lot of good feedback, and are in process of organizing it into a better report out format and to give credit where we should.

In going over the material, I couldn't help noting that the overlap of experience will indeed be of great value to the "newer" extrusion techs.

Skippy: - Well, there IS a lot of good training material out there, but sometimes in the editing process it can become so terse, abbreviated, or worse situational as to be little to no help to our "newer" team mates.

Buzz: - right; for instance, one of the thoughts central to the surging discussion comes back to the simple "feed throat cooling". This important area should be cool enough to allow material not to stick in the feed throat on the way into the feed section of the screw, but not so cold as to

a) be cooler than the dew point in the plant resulting in water condensation on and in the throat being carried down into the extruder and
b) be cold enough to impact temperature sensing devices on zone one controls.
c) warm enough not to pull heat unnecessarily out of pre-conditioned (heated or dried or both) hopper materials.

Skippy: you bet. I remember once dealing with a problem on a particular make of extruder that used TWO thermocouples per barrel zone - one in the approximate center of the barrel, and one a scant few thousands of an inch from the barrel wall. The idea was that the comparison between the net change of the two was call for 'control intervention'.

Buzz: what happened?

Skippy: It was the darnest thing - in real operation, with regrinds in the material mix and a drying system in place (and feed throat water not under very good control,) we were finding that the net temperature of the mix delivered was moment to moment changing in the feed section and disturbing the thermocouple nearest the wall only a couple of degrees. However, with this 1-2 degree change happening so fast, the machine interpreted huge swings in temperature average range occurring. The machine controls in response would apply heat and then (water) cooling in a progressively expanding band of ripples which would throw the machine into real dismay. The output eventually would surge (at best) or stop with melt blockage on the screw (at worst).

Buzz: Was there a fix?

Skippy: - you bet. We put a small 1/2" long metal dowel in the bottom of the hole, and THEN the thermocouple to move it out just a bit from the wall. In this way, the mass of the measured area was not so quick to be "affected" by a momentary change in the mix. The problem was solved and never occurred again. This repair was actually applied to all additional similar equipment in the plant to overcome the same problem.

Buzz: what about the feed throat water?

Skippy: Well, the material was CAB and very hydroscopic. We used a lot of cold water cooling baths, and found that under the right circumstances, our chilled water could be applied too liberally to the feed throat and actually sweat water out of the air into the feed section. Water in clear materials ends up looking like "moisture" trapped in the material, so you can mistakenly be looking for a dryer problem where none really exists. Water driven out in the feed section as steam ends up going up the hopper stack and re-hydrating dried materials and can come back down through the throat again etc - all in all not a good process to support.

Buzz: what was your approach?

Skippy: The global answer was to equip each feed throat with a gallons per hour meter on the OUTPUT side of the loop. In other words, you want to trap the water in the feedthroat and only let out enough volume at what ever temperature you are putting in at what ever rate you are running (since many plants approach it differently) to yield a net temperature that is measured (on the same place of the feed throat each time)as part of the operations once or twice a shift after start up and steady state.

The temperature of the feed throat can be different for different materials and will be affected by different hopper dryer temperatures, rates and shear heat curve in the barrel etc. of course, so a "once size fits all" temperature isn't correct, but a one process variable fix to get to a repeatable measurable temperature is -

Buzz: - good stuff.

Anyway, over the next day few days, we will be consolidating more of this excellent input from the various groups for a report out for all how have contributed - stay tuned

Skippy and Buzz