Skippy: Good morning Buzz! The last go around, we talked about feed throat cooling.
Buzz: Right; it is more important than many operators give credit to to have a feed throat temperature control ‘plan’ that is measurable and repeatable for this key area on an extrusion since it can impact
a) Zone One (feed section) of the barrel overall temperature variation and the machines controls for (potential over) compensation and
b) Moisture content of hopper flood fed In-rush of material – aka too cold in a humid production environment could sweat out moisture to end up falling into the feed stream
Skippy: The generic improvement suggested was adding a gallons per hour meter to the feed throat cooling loop. It could be necessary depending on larger machinery for say sheet etc running thousands of pounds per hour to require a gallons per minute meter, but that will be rate dependent.
Buzz: Yes, make an observation(s) at the line running in control as to water flow at various usage rates and select a gage that has the RANGE of use occupying same about the middle 50% of the gages working range.
Skippy: Let’s not forget safety issues –
Buzz: Of course, it’s a good idea to start the water flow at a small rate at start up so as not to flood the feed throat with water (which will turn to STEAM – OUCH) if you heat up the line and forget to have the feed throat coolant on until the barrel has potentially over-heated that area. Your set up procedure should include having the gage OPEN at start up to allow pressure relief as the system is started up then closed down to the range of coolant flow necessary for operation.
Skippy: also, as a reminder, remember to add a measurement process to your processing records for each rate requirement which can be impacted by incoming material temperatures (from cold boxes off of freezing trucks in the winter to preheated and dried materials) and shear generated during extrusion.
Buzz: Again, it is important to measure the flow OUT of the feed loop – keeping the loop filled, and at a constant temperature pulling out only a given amount of heat once equilibrium is achieved is key –
Skippy: ok, so having dealt with the feed throat area, what is this we continue to hear about “Hammer Rash”?
Buzz: You mean “hoppers that have been terribly abused and mercilessly beaten” in the interests of improving flow?
Skippy: Exactly.
Buzz: Well, we’ve always tried to avoid “reinventing the wheel”. There IS a lot of good information out there on the internet and discussions about material handling always get around to HOPPER FLOW.
In a general search on the internet for some good information about hopper flow, we’ve found a number of good sources and among them, found AJAX EQUIPMENT’s site at
http://www.ajax.co.uk/index.htm
Where there are a number of excellent articles and videos describing typical problems with feed related issues. For anyone interested in improving flow related issues in hoppers for plastics - check it out!
Skippy: Great - With a good grounding on hopper related issues and potential corrections, we can turn to another big element in feed related surging – material presentation to the screw –
Stay tuned –
Skippy and Buzz
Friday, December 11, 2009
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
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
Wednesday, November 25, 2009
Worn screws/barrels and extruder surging
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)
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)
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
Subscribe to:
Posts (Atom)
Posts
