Skippy: Hey Buzz; Happy 2010!
Buzz: Howdy Skip - same to you. Glad to get 2009 behind us and start moving forward again -
Skippy: - right. To kick off the year - how about a quick processing question - the material? PETG or glycol modified PET for us extruders -
The question goes:
"Can anyone recommend a heat profile for extruding a PETG Tube 3" in Dia with a wall of 1 mm? Experiencing too much sag between the Die and the water tank in a vac sizer."
Buzz: the real answer of course is a heat profile that yields the correct clarity, and the least adhesive version of the material with the most hot guts you can get. This would of course also have to take into account the type of screw, compression, screens, tooling die pressure drop and other items. Not to mention that a heat profile while extruding at rate may or may not be the same as during string up, etc etc etc -
Skippy - well, yes, hopefully a dialog will ensue that will prompt a few more details. Do we have any thoughts that we can share assuming that the heat profile that is being used now is correct (and it just seems too hard to handle?)
Buzz: - you bet - although we can not be sure by the question as asked is whether we are asking in this way due to a surface issue, control of ovality or inability to string up. Will assume the worst -
There are a couple of more important questions for glycol modified PET -
1) What is the DRAW DOWN of the material - draw down orientation can be very useful; we used to produce our tooling approximately 50-80% greater than the size of the drawn down part for 'some' dependable orientation between the pin/shell and the calibration kiss.
2) Do you have an open air passage to the inside of the tube through the pins (perhaps through one or more spider legs) to allow air pressure inside the tube as you string it up?
3) is your tank in a position to give you the best shot at string up and running? Normally, you have the tank on center line in both plan and elevation to balance out "kiss", but this material is ADHESIVE versus just COHESIVE at correct melt temperature, so the KISS has to be at the last possible instant - you can’t just rub it all over and around the entrance of the sizing; it will want to set up too quickly, so you need a fair amount of draw down.
4) What is the nature of your calibration design - wafers, solid sleeve, rifled sleeve etc. as well as the surface texture - smooth, glass beaded, chromed etc.
Skippy: What about this "sag" business?
Buzz: At the same time as you are drawing down, the tube is wanting to "sag" due to gravity; recognizing, planning and depending on the "sag" that you mention for good clarity can be advantageous - so for this reason, the vacuum sizing tank as a whole is inline from a plan and elevation view as well as level for start up but after the string up, you will run it LOWER (still level; drop BOTH ends) than the die and backed away to point that still allows a kiss, but minimal contact prior to going down the "barrel" of the tubing calibration unit - and the tube does some controlled "sagging" as it falls from the die face down to a lower level calibration entrance level, then is pulled in evenly into the calibration unit.
Skippy - Cool; any more pointers?
Buzz: Assuming you are using a vacuum tank, I prefer a vacuum chamber that is 2' or less to get the line up into a tube quickly. Mount a fogging sprayer in the lid of the tank that can be used to spray the material with a mist of water (which will harden it pretty quickly at low speeds). You use this during the vacuum chamber sealing and water fill, then turn it off once a regular vacuum is achieved.
Skippy: What about gasketing?
Buzz: Be sure to have an adequate seal at the down stream exit of the vacuum chamber - one sealing material preferred is a silicone rubber (smooth orange) that is pretty friendly to sticky PETG. Use a few test swatches - you are looking for a supple material that will stretch to seal off as pulled downstream but doesn't stick and release. In general, the gasket material might ben1/8" thick and cut about 1/4-3/8" smaller on center than the OD of the final tube depending on suppleness. It is extremely important to cut the inside circle of the gasket on center with the supports and the tube itself. The seal material should be supported on BOTH sides of the seal with semi rigid to hard supports. The UPSTREAM support ring should be the OD of the tube plus a smidgeon (maybe 1/8" but not more than one thickness of the seal material larger than the final tube OD). Not larger than one times the thickness of the seal material, or the seal can be sucked upstream through the support easily. The DOWNSTREAM support ring should be the OD of the tube Plus 3 times the thickness of the seal material and a smidgeon more, so as not to be pinched too tightly by the tube when it is pulled round and the friction between it and the tube pulls it evenly down stream to minimize drag/release marks.
Skippy - gee the gasket and tube sealing technique described there sounds like a good practice on just about any tubing -
Buzz - Right!
In addition, you will need cooling that is MEASUREABLE in gallons per minute depending on line speed and calibration design for duplication in the future (see another post about water use across a plant), and a water level that has the cooling around the calibration deep enough to prevent air being cavitated into the cooling stream turbulence.
At string up, use a bit of silicone mold release on the shell and pin face that is clean to avoid material sticking to these surfaces and melting into "drool". Try to control your pin/shell temperatures to the extent that you can cool down the shell slightly to below the melt temperature which will result in a duller looking matte surface (melt fracture as the material slows down at the wall) at string up. This will give you a little more in the way of "hot guts" with a material that is very soupy at melt. When the Shell is a bit colder than the Pin, the material will bloom AWAY from the pin face once trimmed off. Spray a bit of silicone mold release on the pin face, then work to string up the line through the calibrator and avoiding touching the die face with the PETG as you pull it into the calibration towards the take off. Some of the sticky material may have stuck to the Shell face during the bloom once you get the line strung, you can scrape the face of the shell as necessary to clean up any material (being careful not to cut the material off the pin etc).
Skippy - is it hard to handle down line towards the take off?
Buzz: Once you are pulling the material down line, twist the material as you pull it into a 'rope" in water that has been brought up to the bottom edge of the calibration. The transition point is so abrupt that it will harden very quickly at this point; the twisting into the rope keeps it straight -
This next part is a little tricky - it takes a time or two to get the hang of it; read and visualize it a couple times before trials:
Once in the take off, you need to seal up the vacuum chamber -
a) move the tank forward to an adequate "kiss point", and
b) will open the drain to the chamber slightly (only a crack - this will help prevent pulling too much vacuum too quickly) and you
c) will apply vacuum to the calibration chamber, while you
d) turn the water misting/fogger lightly on (don't blow it out the face of the die) to set the tubing up as a "tube" while you simultaneously use a wet rag in the exit side seal area to "seal" the vacuum chamber around the rope until the "tube" arrives and is sealed with the gasketing. You do turn on the general fill water to be bring up the water level as well - just enough to fill but not flood out the front and hit the die - the light vacuum being maintained with the wet rag at the back and the drain cracked should allow you to make a "tube" in the sizer and keep water from going forward to the die -
Skippy: So . . . you want to be pulling enough vacuum to encourage the air pressure in the tube to expand it to fill the calibration unit and be raising the general water level to coincide with covering the tube when the mostly filled tube gets to the rear seal. Once you have good vacuum and the tube covered, close off the drain, bring your water level on up and level out your vacuum in the chamber?
Buzz: Perfect! Get things up to rate, drop your tank slightly and move the kiss back to a sweet spot and run with it.
These thoughts should get you up and running - good luck.
Just our two cents -
Skippy and Buzz
Wednesday, January 6, 2010
Friday, December 11, 2009
Extrusion Surging - potential trouble areas
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
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
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 -
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