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)
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
Breaker Plate clean up suggestion
Buzz: - Hey Skippy, the holidays continue to close in on us and we had a quick question on breaker plate maintenance -
[Flex PVC user] "Is there an energy efficient way to clean breaker plates?"
Skippy: As a big believer in (M)inimum (N)ecessary (C)hange or correction should be applied for (M)aximum (D)esired (R)esult, how about this -
Assuming that you are pulling and tidying them up "hot" after running flex PVC (even highly loaded), an operator with sufficient vocational prep time should be able to use compressed air and have them spic and span by simply pulling them clean most of the time. This does require a little practice with experienced personnel and the appropriate non scratching tools - most houses make brass tooling available specifically for this type of clean up. Once fully cleared, they should be dipped in an acid neutralizing solution, rinsed, air dried to remove moisture and either stored with/in a light lubricant, or preheated and reused on the next line -
Buzz: there are of course all sorts of other cleaning methods - (old) salt baths, fluidized beds, ultrasonics, etc . . .
Skippy: Yes of course. Generally plants with processes requiring breaker plates to convert ribbon to plug flow also have a supply of compressed air on hand as part of the operations, and may need to avoid the extra expense of these additional energy consuming devices. You may need to re-engineer your tooling with one or more die "swing gates" or multiple breaker plate positions on a push through system to keep the line up and running again during clean up/change over with a minimum of lost time. The activity to avoid appears to be pulling the plates out and leaving them to cool down, then trying to "clean them" in an "energy efficient manner"; which seems like the start of a wasteful process -
Just our two cents -
[Flex PVC user] "Is there an energy efficient way to clean breaker plates?"
Skippy: As a big believer in (M)inimum (N)ecessary (C)hange or correction should be applied for (M)aximum (D)esired (R)esult, how about this -
Assuming that you are pulling and tidying them up "hot" after running flex PVC (even highly loaded), an operator with sufficient vocational prep time should be able to use compressed air and have them spic and span by simply pulling them clean most of the time. This does require a little practice with experienced personnel and the appropriate non scratching tools - most houses make brass tooling available specifically for this type of clean up. Once fully cleared, they should be dipped in an acid neutralizing solution, rinsed, air dried to remove moisture and either stored with/in a light lubricant, or preheated and reused on the next line -
Buzz: there are of course all sorts of other cleaning methods - (old) salt baths, fluidized beds, ultrasonics, etc . . .
Skippy: Yes of course. Generally plants with processes requiring breaker plates to convert ribbon to plug flow also have a supply of compressed air on hand as part of the operations, and may need to avoid the extra expense of these additional energy consuming devices. You may need to re-engineer your tooling with one or more die "swing gates" or multiple breaker plate positions on a push through system to keep the line up and running again during clean up/change over with a minimum of lost time. The activity to avoid appears to be pulling the plates out and leaving them to cool down, then trying to "clean them" in an "energy efficient manner"; which seems like the start of a wasteful process -
Just our two cents -
Friday, November 13, 2009
Plastics in the news - cutting through the noise -
Welcome one and all - our list of visitors is growing, and we are always interested in new discussion topics.
Interested in keeping up with a wide varitey of plastics information worth investigating?
Check out Matt Defosse's blog at Plastics Today!
http://www.plasticstoday.com/blog/4942
Skippy and Buzz -
Interested in keeping up with a wide varitey of plastics information worth investigating?
Check out Matt Defosse's blog at Plastics Today!
http://www.plasticstoday.com/blog/4942
Skippy and Buzz -
Thursday, November 12, 2009
Worn extrusion equipment and surging
Skippy: Hey Buzz - happy almost Thanksgiving - we saw an interesting question in another plastics forum -
"Can a worn extruder screw cause surging? I've heard this for years but can't see how a linear variation with a period of a minute or two can be caused by a slow radial change over many weeks or months or even years. If anyone can come up with a reasonable and technically sound explanation, I would like to see it. ALG"
Buzz: - Well Skippy, I tend to agree with most that the slow evolution of “wear” is not likely to create a situation where we have good melt pumping on Tuesday, only to have it fall apart on Wednesday. Screw melt pumping conditions DO change over time with wear, and particularly when aggressive screw and screw tip cooling are employed to “throw material around” in profile tools.
Skippy: To be fair, don't we have to acknowledge here that there are a number of critical process areas that contribute perhaps as much to "surging" based on material delivery (aspect ratio of material components, mix of materials, conveying method/stability, existence of fines, drying temperature, air flow, residence time in the dryer, feed throat temperature control, zone 1 and or 2 temperature control, etc etc etc) to the feed section of a screw that are ignored or not very well understood in too many shops?
Buzz: Of course, although that conversation is way beyond the scope of this particular discussion and will be investigated more fully in the future. On the other hand, in custom plant environments where screw and barrel wear were faithfully measured by quarter on extrusion lines that ran a variety of materials from week to week (aka not just one material from beginning to end of each screw/barrel rebuild life) we observed the following –
The most amount of wear from quarter to quarter occurred at the END of the screw and or barrel life measurement period, not at the beginning. (by production hours)
R&D staff tend to prefer to “run in new items on new screws and barrels” and in short order, the “process” is/was not as "stable" to established run conditions in future runs and required “tweaking”
Many US firms have gotten caught in the trap of increasing rates to “increase contributions per hour” (false economy of course if no new sales fill the new empty machine time) at the expense of significantly closing the ‘window of process ability’ with any particular brand of elements
Skippy: I'm wondering whether the economic climate we're experiencing contributes in any way as well -
Buzz: well, we have observed that batch to batch variation in raw materials from manufacturers quite frankly (despite their “certifications” on lab materials) DO exist, sometimes in great measure and
Business conditions (and perhaps lack of cash flow?) in custom extrusion in poorly run shops affects the run to run use of varying raw material virgins and regrinds –
Skippy: How does that affect running an extrusion line?
Buzz: Well sometimes, rather than running a standard mix of regrinds back into product from the beginning of the run until the end, some houses accumulate and “rev the meter” from nearly 0 to nearly 100% which further requires "tweaking" in general to re-achieve proper melt thixotropy with varying amounts of regrind.
This can result in some rather aggressive approaches to achieving a stable melt pumping mechanism at the correct melt temperature and pressure to have the material “be one” with the intended tooling flow paths –
Skippy: So all that translates to what?
Buzz: In the case(s) above, we find that the requirements for process adjustment in a process where we KNOW that all else (listed in the opening above) is in good working order and under “control” end up being “justified” due to “changing outputs”. The bottom line is that although a good deal of data can be accumulated in “screw and barrel wear” verus "surging" it is meaningless, unless the contribution to wear can some how be tied back to the requirements for any number of the destabilizing run approach problems above in the plant as well.
Your question is an excellent one, because many shops quite frankly do not do a very good job of monitoring, mixing, drying etc the input of raw materials and then “change the process and blame the extruder” aka worn barrel and screw.
Skippy: is this likely to be a standard answer for extrusion in general?
Buzz: Quite frankly, we would guess that the pipe folks who will routinely rebuild screws and change out barrels on an “X” pounds schedule on lines running the same product 24/7 for months on end would have a totally different answer than the “custom gang” but that's just our two cents
"Can a worn extruder screw cause surging? I've heard this for years but can't see how a linear variation with a period of a minute or two can be caused by a slow radial change over many weeks or months or even years. If anyone can come up with a reasonable and technically sound explanation, I would like to see it. ALG"
Buzz: - Well Skippy, I tend to agree with most that the slow evolution of “wear” is not likely to create a situation where we have good melt pumping on Tuesday, only to have it fall apart on Wednesday. Screw melt pumping conditions DO change over time with wear, and particularly when aggressive screw and screw tip cooling are employed to “throw material around” in profile tools.
Skippy: To be fair, don't we have to acknowledge here that there are a number of critical process areas that contribute perhaps as much to "surging" based on material delivery (aspect ratio of material components, mix of materials, conveying method/stability, existence of fines, drying temperature, air flow, residence time in the dryer, feed throat temperature control, zone 1 and or 2 temperature control, etc etc etc) to the feed section of a screw that are ignored or not very well understood in too many shops?
Buzz: Of course, although that conversation is way beyond the scope of this particular discussion and will be investigated more fully in the future. On the other hand, in custom plant environments where screw and barrel wear were faithfully measured by quarter on extrusion lines that ran a variety of materials from week to week (aka not just one material from beginning to end of each screw/barrel rebuild life) we observed the following –
The most amount of wear from quarter to quarter occurred at the END of the screw and or barrel life measurement period, not at the beginning. (by production hours)
R&D staff tend to prefer to “run in new items on new screws and barrels” and in short order, the “process” is/was not as "stable" to established run conditions in future runs and required “tweaking”
Many US firms have gotten caught in the trap of increasing rates to “increase contributions per hour” (false economy of course if no new sales fill the new empty machine time) at the expense of significantly closing the ‘window of process ability’ with any particular brand of elements
Skippy: I'm wondering whether the economic climate we're experiencing contributes in any way as well -
Buzz: well, we have observed that batch to batch variation in raw materials from manufacturers quite frankly (despite their “certifications” on lab materials) DO exist, sometimes in great measure and
Business conditions (and perhaps lack of cash flow?) in custom extrusion in poorly run shops affects the run to run use of varying raw material virgins and regrinds –
Skippy: How does that affect running an extrusion line?
Buzz: Well sometimes, rather than running a standard mix of regrinds back into product from the beginning of the run until the end, some houses accumulate and “rev the meter” from nearly 0 to nearly 100% which further requires "tweaking" in general to re-achieve proper melt thixotropy with varying amounts of regrind.
This can result in some rather aggressive approaches to achieving a stable melt pumping mechanism at the correct melt temperature and pressure to have the material “be one” with the intended tooling flow paths –
Skippy: So all that translates to what?
Buzz: In the case(s) above, we find that the requirements for process adjustment in a process where we KNOW that all else (listed in the opening above) is in good working order and under “control” end up being “justified” due to “changing outputs”. The bottom line is that although a good deal of data can be accumulated in “screw and barrel wear” verus "surging" it is meaningless, unless the contribution to wear can some how be tied back to the requirements for any number of the destabilizing run approach problems above in the plant as well.
Your question is an excellent one, because many shops quite frankly do not do a very good job of monitoring, mixing, drying etc the input of raw materials and then “change the process and blame the extruder” aka worn barrel and screw.
Skippy: is this likely to be a standard answer for extrusion in general?
Buzz: Quite frankly, we would guess that the pipe folks who will routinely rebuild screws and change out barrels on an “X” pounds schedule on lines running the same product 24/7 for months on end would have a totally different answer than the “custom gang” but that's just our two cents
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