Buzz: Oh yes, Skippy; another thought on the discussion so far about extrusion screw and barrel wear and "surging".
As you know, we have been involved with extrusion companies in two broad camps -
a) those that religiously measure and rebuild screws and replace barrels when they expect wear to be too much of an impact on reliable extrusion, and
b) those that have machines that you can rattle the screws around in worn barrels to a good degree
and yet because they can continue with process updates and changes produce product profitably, these companies couldn't care less about the thought of "measuring the wear" and moving directly to repair/replacement.
Skippy: - but how can this be? If both can achieve steady state extrusion with a wide variety of virgin and regrind materials in new and worn equipment, and both can exhibit machine conditions where "surging" exists - there must as Paul Harvey says be a "the rest of the story" -
Buzz: well yes. We have some thoughts on this that we would like to share with our readers and use to develop some discussion on solutions as well as discuss why this TYPE of question is so difficult - once we've had a little tryptophan er um TURKEY to help marshall our thoughts, we shall continue.
Skippy: - what direction is the discussion likely to go?
Buzz: well, I'd like to see us confirm that there is essentially no right or wrong answer here, but due to the system or environment in which it is answered, it could go either way.
BECAUSE of that, perhaps it is time for us to spend a few sessions in a much more compressed time span talking about a number of the other extrusion variables that can contribute to "surging" as well, how to spot them, how to eliminate or at least control them, and certainly the importance of being aware of them. In this way, our readers hopefully will jump in with additional thoughts and our dialog will be that much more productive -
Skippy - can't wait - see you in a day or two . . . Happy Thanksgiving to all!
(to be continued)
Showing posts with label wear. Show all posts
Showing posts with label wear. Show all posts
Wednesday, November 25, 2009
Saturday, November 14, 2009
Worn Screw/Barrel - Extruder Surging - more thoughts
Skippy: Our first stab on this question was put out there and a question came back on our preliminary thoughts below - concerning whether this was seen on single or twin screws. Others have observed more wear at the start of the life cycle on singles -
Buzz: Oops, engaged fingers faster than brain. The intent is not to be aggregating total screw and barrel wear as being a one side or the other phenomenon; regret it appearing to be so one sided - to continue;
In singles, we did see wear in a brief spike at the beginning of each screw and barrel rebuild for a short duration while all of the items "wore in" so to speak (remember, we were looking at situations where multiple screws and materials went into each machine).
Skippy: What happened after this initial break in period?
Buzz: We would observe a long period of wear which would be fairly predictable when on lines committed to more or less one material, and then accelerate nearer the end as the wear began to accumulate enough to cause significantly greater changes to process 'output' and we applied more and more resultant torsion to the screw and barrel interfacial region with other process variables. Our feeling was that since the screws were captive at the hub and near the gate, the torsion resulted in a greater arc in the "bend" to the screws and opened up the clearances at an accelerating rate in the 'middle' of the processing area.
Skippy: Did machine size or rate expectations play into this at all?
Buzz: Other factors coming into play did include the over-all rate as a percentage of "top end" capability since the observations were made on 2 1/2 and 3 1/2" machines and our goals did include increasing rates to "gain contribution per hour." I guess the anecdotal way to communicate this concept is the same as saying that when painting, using an 18" roller versus a 9" doesn't double the output at the same intended quality with out other factors being affected.
The other major difficulty area for our data may have been the constant change from one material to another, the various screw designs with and without various mixing elements in differing locations down the screw. Rather than having to overcome "wear" in one part of the screw and barrel with ONE screw and barrel, things were changing up and down the inter-facial area probably from just after the feed section until well into the metering sections from a number of the combinations.
Skippy: so in the end, wear begins to accumulate and . . .
Buzz: Well, once we began to be confronted with the results of leakage back over the flights, and the corresponding reduced outputs etc, the natural tendency was to try to put a toe or two out over the edge without leaping into the abyss - increase screw speed.
Skippy: Hmm. Most find that dealing with the resultant increased shear, and "residence" time for a portion of the material along with the general increase in melt temperature requires rethinking how to pump the material down a screw and barrel (with increasing wear) in differing manners versus suffering the resultant degradation, adhesive versus cohesive, flow related and sometimes aesthetic issues related to a hotter melt.
Buzz: Right. This tug of war over the abyss was generally continued until we would be delivering sufficiently high rate, continual output in terms of rate but dealing with stratified material coming out of the metering section in the 'plug flow' transformation at the breaker plate after the screw tip with a lower level of melt homogeneity. Instead of relying on the breaker plate and screens to simply help move ribbon to plug flow, we found ourselves relying on these to play an increasing role in melt mix distribution as well.
Sorry, all this once again is beyond the scope of this discussion. The short course was that it was (well past?) time for a screw and or barrel rebuild or replacement; and remember first that as processors, we need to look holistically at all the the interelated variables and manage them as just that - a whole.
Skippy: What about twin screws?
Buzz: A couple of things; on twins - yes we saw the most amount of wear at the end of the process since the forces exerted in the conveying mechanism were significantly different and other control options like screw tempering and differing feeds like starve versus flood feeding in combination with screw rpm and fusion rate modifications with the raw material supplier on formulations gave us a good many more options over the life of the screws and barrels -
Just our two cents
Buzz: Oops, engaged fingers faster than brain. The intent is not to be aggregating total screw and barrel wear as being a one side or the other phenomenon; regret it appearing to be so one sided - to continue;
In singles, we did see wear in a brief spike at the beginning of each screw and barrel rebuild for a short duration while all of the items "wore in" so to speak (remember, we were looking at situations where multiple screws and materials went into each machine).
Skippy: What happened after this initial break in period?
Buzz: We would observe a long period of wear which would be fairly predictable when on lines committed to more or less one material, and then accelerate nearer the end as the wear began to accumulate enough to cause significantly greater changes to process 'output' and we applied more and more resultant torsion to the screw and barrel interfacial region with other process variables. Our feeling was that since the screws were captive at the hub and near the gate, the torsion resulted in a greater arc in the "bend" to the screws and opened up the clearances at an accelerating rate in the 'middle' of the processing area.
Skippy: Did machine size or rate expectations play into this at all?
Buzz: Other factors coming into play did include the over-all rate as a percentage of "top end" capability since the observations were made on 2 1/2 and 3 1/2" machines and our goals did include increasing rates to "gain contribution per hour." I guess the anecdotal way to communicate this concept is the same as saying that when painting, using an 18" roller versus a 9" doesn't double the output at the same intended quality with out other factors being affected.
The other major difficulty area for our data may have been the constant change from one material to another, the various screw designs with and without various mixing elements in differing locations down the screw. Rather than having to overcome "wear" in one part of the screw and barrel with ONE screw and barrel, things were changing up and down the inter-facial area probably from just after the feed section until well into the metering sections from a number of the combinations.
Skippy: so in the end, wear begins to accumulate and . . .
Buzz: Well, once we began to be confronted with the results of leakage back over the flights, and the corresponding reduced outputs etc, the natural tendency was to try to put a toe or two out over the edge without leaping into the abyss - increase screw speed.
Skippy: Hmm. Most find that dealing with the resultant increased shear, and "residence" time for a portion of the material along with the general increase in melt temperature requires rethinking how to pump the material down a screw and barrel (with increasing wear) in differing manners versus suffering the resultant degradation, adhesive versus cohesive, flow related and sometimes aesthetic issues related to a hotter melt.
Buzz: Right. This tug of war over the abyss was generally continued until we would be delivering sufficiently high rate, continual output in terms of rate but dealing with stratified material coming out of the metering section in the 'plug flow' transformation at the breaker plate after the screw tip with a lower level of melt homogeneity. Instead of relying on the breaker plate and screens to simply help move ribbon to plug flow, we found ourselves relying on these to play an increasing role in melt mix distribution as well.
Sorry, all this once again is beyond the scope of this discussion. The short course was that it was (well past?) time for a screw and or barrel rebuild or replacement; and remember first that as processors, we need to look holistically at all the the interelated variables and manage them as just that - a whole.
Skippy: What about twin screws?
Buzz: A couple of things; on twins - yes we saw the most amount of wear at the end of the process since the forces exerted in the conveying mechanism were significantly different and other control options like screw tempering and differing feeds like starve versus flood feeding in combination with screw rpm and fusion rate modifications with the raw material supplier on formulations gave us a good many more options over the life of the screws and barrels -
Just our two cents
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