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| Author | Topic: PRESSURE FLUCTUATIONS |
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novadan Senior Member Posts: 21 |
posted September 14, 2009 03:59 PM
  
We are experiencing pressure fluctuations in our extruder and not sure if its normal. It doesnt seem to occur in our other extruder but the other is older and not as much info displayed, so it maybe happening but I do not think so with the info I have. On the newer extruder that we are experiencing trouble with is not a year old yet. It has a barrier type screw installed and runs with no metering pump. The computer that is linked to this unit has a graph diplay a will show you all that is happening in the extruder. The usual pressure in the barrel is 105ish in the barrel, the work pressure is not set because we run in manual mode, but the screw rpm is set to run at a constant speed. with this combo the pressure in the barrel can drop below 100 bar down to 96 and climb anywhere over 118. Is there a normal fluctuation for this type of set up or is there an issue here. I should add that the three pressure zones(security,work and head) all fluctuate at the same time, which I find odd. Shouldnt the fluctuation travel down line rather than all three zones at once. Also, The screw was changed just as I was coming aboard this extruder, my qustion here is, how much effort does it take to install the screw into the barrel. I seen screws being installed before that had to be towed in thru the rear with a winch and driven into place. This screw was pushed into the barrel by hand more or less by leaning on it. Could this be a tolerance issue causing a fluctuation. Keep in mind the screw going in was also new. Alot of info to digest but any info would be greatly appreciated. IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 14, 2009 04:06 PM
The first thing to do is go to the calculator at: www.ExtrusionTechnicalServices.com and report back all of the data, and how your process compares with the calculator. This will let us know how well your screw is running, and how you are running it compared to normal parameters. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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Steve H Moderator Posts: 427 |
posted September 14, 2009 11:59 PM
quote: Set it may be, but is it actually doing so, have you confirmed it with a tachometer? What is happening with the melt temp? ------------------ If at first you don't succeed, skydiving really is not for you. IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 15, 2009 12:32 PM
I have not confirmed this with a tach, other than whats available on the extruder computer. It holds a constant RPM. Are you suggesting confirming this with an external tach? a sort of hand held deal? What would I be looking for in the melt temp as mentioned in the post above? The EXTRUDER MELT is 204c, the MELT HEAD is 164c and although the pump is not installed it registers 208c. zones 1-48c,zone 2-196 zone 3-230c zone 4-225c zone 5-215c zone 6-202c zone 7 thru 15-210c. We have changed PE suppliers and the peaks and valleys are not as great with the PE being used at this point. IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 15, 2009 12:43 PM
There are several technical reasons that extruders surge. RPM control issues is just one of them. The reason I asked you to go to the calculator was to help diagnose what the cause of the surging might be. Without that information, and probably some screw design information, it is difficult to diagnose. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 15, 2009 02:01 PM
Hello Tom, I filled out the calculator with as much info as I have. One of the problems with the calculator is the material we are using is its not listed. we are blending PP/PE. Also we run the extruder at 55% most of the time due to down line equipment not being able to keep up with much higher. We have a 110mm screw thats approx 11ft of flighted area. we are extruding at approx 128kgs/hour with a set RPM of 83.5 which is actual 46.1 due to the 55% line speed. I hope this sheds some light IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 15, 2009 03:27 PM
The calculator predicts that you would get a lot more output than you are getting. Could be the extruder has feeding issues or is worn. Your head pressures are not that high so as to reduce output significantly. When was the last time the screw and barrel were inspected for wear? Try letting the feed throat and zone 1 warm up a bit. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 15, 2009 09:15 PM
As I mentioed above the extruder is less than a year old. Before it was shipped to us, my boss went to visit the facility that manufactured it to make sure it was up to spec. Apparently it wasnt producing the proper material so they replaced the barrier type screw with a single flute type. This is the screw it was set up on our floor with. After some trial work, my boss wanted to reinstall the original barrier screw that was shipped along with the extruder. Its producing the proper material but with the pressure issue. And as I mentioned before the screw slid in very easy. This extrusion business is hard to deal with at times, especially for a black/white type of person as myself who doesnt like the grey areas!! IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 15, 2009 11:04 PM
Extrusion involves mutiple inter-related non-linear processes operating at the same time. There is nothing black and white about it. In fact it was one of the first processes simulated by computer models back around 1970, because nobody could understand it. I try to work with the best technology and knowledge available in order to gain control of the extrusion process. As a manufacturing engineer by training, I too would like to make extrusion a controllable process, and I have made some progress towards that. Single screw extrusion is a trick process, kind of like a 2 stroke engine is a trick engine. If the tricks are working well, then you are OK, if not it runs like crap. There is a 4 stroke version of an extruder, called a twin screw, but your boss likely will not like to pay double the price for the line. So learning the tricks of single screw extrusion is probably your only bet and you are stuck with it. As I said before, using the calculator and a screw drawing I can go down the list of tricks and problems and figure out what is going wrong. Once I know what is wrong, then we can attack the problem. I have solved problems like this more than 10,000 times in my career, so you might want to trust my guidance.
------------------ Tom Cunningham [This message has been edited by Tom C (edited September 17, 2009).] IP: Logged |
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Steve H Moderator Posts: 427 |
posted September 16, 2009 04:02 PM
quote: Exactly, I'd also confirm that barrel heating/cooling controls are functioning correctly too. quote:Changes in temp, is your melt temp probe immersed in the melt, or is it flush with the interior surface. If the latter is the case, it's measuring the wall temp and the actual melt temp could be hotter (and varying) All the grey areas in extrusion are what makes it interesting [This message has been edited by Steve H (edited September 17, 2009).] IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 17, 2009 02:30 PM
I found a blueprint of the screw in our extruder. Our extruder is a 100mm. the screw drawing shows an outside diameter of 99.8 That would give a flight to wall clearance of .2mm. so in a perfect situation that would give a clearance of .1mm even all the way around the screw. Is this logic right? If so those seem to be vey tight tolerances! IP: Logged |
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Steve H Moderator Posts: 427 |
posted September 17, 2009 03:18 PM
quote: Welcome to the world of extrusion  , on a 100mm extruder that's the clearance you'd be looking for.A screw should be a "fit" to it's barrel, depending on the diameter, some effort will be needed to slide it home. But if it has to have the crap beaten out of it to do so, suggests that there were some mechanical issues that should have been addressed. We know the pressure is varying, are you also getting accompanying variation in extrudate dimension? IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 17, 2009 04:25 PM
Yes, we are, and thats why I'm concerned. The other extruder has a metering pump installed and there is very little difference in the extrudate, well within tolerences. But the extruder we are having issues with has no metering pump and any fluctuation shows up down line. IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 17, 2009 04:42 PM
Is the extruder vented? ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 17, 2009 06:14 PM
Vented?? I dont mean to sound ignorant, but I am just beginning to learn. The extruder is used for pp/pe yarn extrusion. I dont think it vented. What would I be looking for to tell me? The denier od our yarn is all over the map!! IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 17, 2009 07:04 PM
If it was vented you would have a hole in the side of the barrel. Your output seem about right for a 24:1 L :D screw, and low for a 32:1. Are you sure about the length of the flighted portion of the screw?
Tom Cunningham [This message has been edited by Tom C (edited September 17, 2009).] IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 17, 2009 07:46 PM
No, after seeing the bluprints of the screw I now have more info. The extrude is a 100mm and not 110 as I mentioned before for starters. I'll check the rest of the info in the morning. Im at home now. Thanks for your help. nighty nite. IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 18, 2009 09:48 AM
After looking over the drawing, I can say the length of the threaded area of the screw is 3197mm or 10.48ft. and a dia 99.8mm. Our extruder is not vented. wouldnt material be released thru the vent? what happens to the material if so? IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 18, 2009 03:08 PM
So your screw is nominally 100MM dia., 32:1 L:D, non vented. Output is less than typical for the material being run and the screw length. Likely rate reducing issues are a short barrier section, poor barrier transition, short transition section, high head pressure, or poor feeding. Of the above, given your screw speed and surging, starving is the most likely source. Resolving the starving issues requires manipulation of the screw and feed throat temperatures, or a geometry change if that does not work. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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Steve H Moderator Posts: 427 |
posted September 24, 2009 04:26 AM
quote: Extruders purpose built to run Hydroscopic materials are constructed with a barrel vent. Effectively, they are two extruders in one, the first stage melts the material and pumps it into a decompression zone. The vent is located at this point, because the screw channel is considerably deeper under the vent, the melt pressure is low, and polymer tends to get swept along with the screw flight, rather than ejected out the vent (at least in an ideal world that's the case). Once the polymer is past the vent section, the screw channel begins to get shallow again, recompressing the melt (the second extruder) Now, because we live in a less than ideal world, vented extruders can have issues. Getting the feed rates balanced between first and second stages being one of them. If the first stage is delivering more material to the vent section than the second can pump away, will result in material spewing out the vent. IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 24, 2009 11:44 AM
Ahhh, the perfect world! Never been, but trying to get there. IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 24, 2009 11:48 AM
we have changed the PE brand, slight difference in fluctuation, somewhat less. Ive increased the temp in the first zone. no difference could be seen.n When the RPM is increased the fluctuations get worse on the graph, but I have to work it out to see the % of diff. I wonder who would be more of the culprit, the PP or PE?? hmmmm. IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 24, 2009 12:09 PM
Let's differentiate between the feed throat and the first barrel zone. Have you tried letting the feed throat warm up? What is the range of temperatures you have tried in the first zone? Do you have screw cooling running or available? When you first start the extruder does it run OK or bad for a while, and then settles into a pattern? You should want to try and fix this working with the feed section, as the next solution level calls for a new screw which is big $$$ (Canadian or US). ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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novadan Senior Member Posts: 21 |
posted September 24, 2009 02:59 PM
The first zone on our extruder I assume has a cooling unit installed. Its a PIOVAN unit. It looks it cools with an oil. It is set at 48c. There is two gauges plumbed into what appears to be the inlet and the outlet(temp gauges). One gauge reads 53c and the other 56c. Now the zone I was playing with would be zone two(Its the first heating band. I had this up to approx 196c and zone three the second band up to 230c. Question 1. Should zone one (the cooling zone) be any higher than 48c? Question 2. How high can zone two be heated before we have trouble with burned pellet? We had other issues today so I had to pull the loading unit off the top of the extruder. I could get a good visual of where the feed section of the screw actually started. So I started doing some measuring according to the drawing of the screw. The feed section extends to the very start of the first heating band(zone two) this area is double threads or flights equally spaced and the same height. one of the flights end and it becomes a single flight which extends approx 3" from the end of zone two. In this area there are horizontal lines draw, maybe to represent flats(?). At the end of this section, thats where it looks the double flights start again, only close together and one slightly lower than the other and the spacing increases as it travels down the screw. (this is what a "barrier screw" is?) IP: Logged |
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Steve H Moderator Posts: 427 |
posted September 24, 2009 03:27 PM
The feed zone, is that part of the barrel where your material gets fed into the screw. Sometimes called the feed block, it is usually a hollow/gallery filed casting, through which oil or water is circulated to maintain a set temperature. What we're trying to achieve with the set temp, is the ideal one where the heat of the wall causes the pallets to stick to it, BUT not the screw. At this point, solid pallets will feed into the screw at an optimum rate. Another way of approaching this, is to cool the screw at this point and do things in reverse, with this approach, we have to cool the screw and prevent pellets from sticking to it. Here we rely on the friction of the pallets being pushed along, and heat transference from the first barrel zone to achieve a sticky wall/slippery screw operating condition. So to get your solids feeding issue sorted, you have to determine which beast you have. If it's the first one, as Tom said above, try heating that zone- if you are cooling the screw (is the drive end of the screw tapped and fitted with a double rotary union, with piping/hoses leading back to the Piovan unit), then try cooling that zone. The complicating factor with this, is that you're running a blend of PE and PP, and the ideal throat temp for each is different. IP: Logged |
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Tom C Moderator Posts: 977 |
posted September 24, 2009 03:36 PM
A lot of information. Let's break it down.
This seems an unusual setup. Most extruders use a slow flow of water. Often a hotter feed throat is better, but I would generally start with something like 25C.
Typical first zone temperatures for PE run 170-190C Question 1. Should zone one (the cooling zone) be any higher than 48c? Start with 25C and work your way up to about 90C max. Question 2. How high can zone two be heated before we have trouble with burned pellet? Very high, it will have trouble with running befroe it burns. The pellets are cold at that point. heat transfer is not that fast. We had other issues today so I had to pull the loading unit off the top of the extruder. I could get a good visual of where the feed section of the screw actually started. So I started doing some measuring according to the drawing of the screw. The feed section extends to the very start of the first heating band(zone two) this area is double threads or flights equally spaced and the same height. one of the flights end and it becomes a single flight which extends approx 3" from the end of zone two. The design sounds bad up to this point, and likely to create feeding problems that you can't resolve with temperature changes. In this area there are horizontal lines draw, maybe to represent flats(?). At the end of this section, thats where it looks the double flights start again, only close together and one slightly lower than the other and the spacing increases as it travels down the screw. (this is what a "barrier screw" is?) Perhaps a barrier section. For thermoplastic polymers there should be a single flighted screw with deep feed for about the first 6 diameters from the feed pocket. Anything different is bound to result in a poor feeding process for a variety of technical reasons. Contact me if you would like a nice running screw. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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stevet Member Posts: 3 |
posted February 10, 2010 11:30 AM
Hi just had a quick read of your problem.The pressure fluctuations ,in my experience ,are not massive but higher than you might expect. Hope this helps Stevet IP: Logged |
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