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| Author | Topic: Vent Blockage |
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swickham Senior Member Posts: 6 |
posted May 22, 2005 10:32 AM
   
I am running HIPS on a 30:1 4.5" My temp profile is as follows: 190,205,205,160,220, screen/coex/die at 235 all in Celsius. Pressure 1000-1500 psi.Speed about 60 revs. I have leakage through vent. Any ideas?? I have tried reducing conveying in first stage by incresing temp in 2&3 to 220 but no joy. The vent may be badly designed but i have to live with it for now. IP: Logged |
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Tom C Moderator Posts: 546 |
posted May 22, 2005 07:37 PM
Plug the vent? ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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Steve H Moderator Posts: 365 |
posted May 23, 2005 01:51 AM
G'day Steve Welcome to the Single Screw Forum. Has the vent always leaked or is this a recent development. How restrictive is the screen pack you're running, I'd try increasing Adaptor Zone(s) and the Screen Changer to 230C. The second stage of your screw isn't keeping pace with the first, so maybe increase the feed zone temp a little (to retard the amount of material being conveyed into the first stage of the screw) Have you seen this article by Chris Rauwendaal? http://www.plasticstechnology.com/articles/200201ts1.html ------------------ If at first you don't succeed, skydiving is not for you. IP: Logged |
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shayne31 Senior Member Posts: 10 |
posted June 03, 2005 02:52 PM
Swickham, Whay type of vent insert are you using? IP: Logged |
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witnish Member Posts: 1 |
posted June 07, 2005 06:35 PM
I have had this problem with one of our HIPS extrusion lines. You can run for weeks then out of the blue material will start to leak from the vent. Does it happen when the machine has stopped? If so, and you are using a high percentage of regrind? it may contain moisture. IP: Logged |
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swickham Senior Member Posts: 6 |
posted June 08, 2005 04:31 AM
I am running 100% regrind although it is pelletised. I thought it was because it was wet so i dried it for several hours at 90c. However, it still comes out! It is not just a trickle either, it is a flood. I have checked screw design, vent design, pressure and changed the temp profile. I can only deduce that it is material related as if i double pass the material it seems to run ok. Any ideas would be much appreciated.
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zabielski Senior Member Posts: 379 |
posted June 08, 2005 07:32 AM
NEVER Plug a Vent! I've witnessed a catastrope once and I'm not sure if the law suite is yet settled. It's downright dangerous. Verify this with your Extruder Manufacturer too. IP: Logged |
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swickham Senior Member Posts: 6 |
posted June 08, 2005 08:09 AM
Too late. i've plugged it and it seems ok. Not the solution though as i've been drying this material for some time now.
quote: IP: Logged |
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Tom C Moderator Posts: 546 |
posted June 08, 2005 08:27 AM
I'm surprised my short comment would be the solution and cause controversy. Many extruders are run without vents. If you can get away without a vent life can be much easier. I'm a vent fan myself. But given the constraint of having to live with the current design, if a plugged vent will work why not. Ken, The situation you described might be related to conditions other than the vent.
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Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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louis33 Senior Member Posts: 122 |
posted June 08, 2005 08:33 AM
I am going way out on a limb here, but if double pass works, you may be drying the material in that process or even boiling off some volitles. Of course that depends on the pelletizing operation. One assumes that the regrind is made in house so that you have control over what is in it, but that is not always the case. On the issue of plugging the vent - I do have a question. What is the difference between a plugged vent and a non vented extruder? One would think none, other than the screw design, and that shouldn't cause anything to "blow up." What if you were to change to a non-vented barrel on that machine with the same feed screw? Would than cause anything to blow up? Unless we are saying that if you modify a machine and an accident happens, you are liable. That I'll buy - but not that plugging is, in and of itself an unsafe act. IP: Logged |
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swickham Senior Member Posts: 6 |
posted June 08, 2005 09:07 AM
The material has been pelletised off site on a strand line with a water bath. The material ahs picked up moisture which has not been blown off. Sure, granulating it has dried it which is why i put it all through the dryer. Whst i don't understand is why sometimes it leaks badly and other times it doesn't
quote: IP: Logged |
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shayne31 Senior Member Posts: 10 |
posted June 09, 2005 12:19 PM
So, once again, it would be interesting to know what type of vent insert you are using. We run a nearly identical process on a 36:1, 2 stage, vented line. If the 'post-industrial' resin is created off-site, chances are that viscosity shifts are occurring throughout the resin population, not to mention foreign contaminates and varying volatile content. Because it is difficult and / or time consuming to accurately predict the shift direction and material composition, it is important to balance the devolatization zone of the extruder to a large window of variation. We have found that increasing the ID of the insert opening greatly reduced material venting, due to reduced velocity. If safety is a concern, experiment with step vent inserts. It is also important to check the insert for wear; since it is in essence part of the barrel, any clearance between the screw flight and barrel wall (or insert) can result in material pooling, excess shearing etc. ------------------ IP: Logged |
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zabielski Senior Member Posts: 379 |
posted June 13, 2005 09:00 PM
Simply speaking, there is a decompression segment in the screw design that will allow volitiles to excape - providing you run a vacuum. Some who think they are running a vacuum may not be, based on the gauge reliability. Pressure can build up and blow the "PLUG" off, warp it, or even cause severe or minor leakage. No controversy on my part Tom. Check with ANY extruder manufacturer who sells vented extruder's, and simply request a WRITTEN reply that they will assume ALL damages if the vent is plugged, and any body gets hurt or killed. If your going to replace the vented barrel, you should replace the screw - without a devolitization zone. Spend the $$$$ and be safe.
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Tom C Moderator Posts: 546 |
posted June 13, 2005 09:54 PM
Ken, If a vendor sells you a 10K PSI rated barrel, with a vent hole, and a plug for it, I would think they would be smart enough to design the plug to resist 10K PSI no matter what the screw design. Then again I've seen a lot of crazy stuff in extrusion. If the user makes a DIY vent plug, well that is a whole other story. Like playing with dynamite. For those who are interested in screws and vent diverter designs that eliminate venting, please send me an email.
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Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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louis33 Senior Member Posts: 122 |
posted June 14, 2005 09:08 AM
quote: Is the bulk density of the single pass much different from that of the double pass material? IP: Logged |
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swickham Senior Member Posts: 6 |
posted June 14, 2005 10:17 AM
quote: Yes, The single pass is small pellet and the double is 2mm ground sheet through a 10mm granulator seive. IP: Logged |
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louis33 Senior Member Posts: 122 |
posted June 14, 2005 11:01 AM
quote:
Do you mix the second pass with pellets? IP: Logged |
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swickham Senior Member Posts: 6 |
posted June 14, 2005 11:16 AM
quote: IP: Logged |
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griffex Senior Member Posts: 28 |
posted June 27, 2005 11:50 PM
I was intrigued by a topic that had so many posts, so I tuned in, and it wasn't until the end that it all made sense. The plug issue depends on who makes the plug and how it is held in. Lots of vented machines run plugged with two stage screws. However, you want to vent so let's talk about how to do it, not about new screws or plugs. I am obsessively dedicated to using existing equipment wherever possible. Material comes up the vent when the rear pushes in more than the front can take away. The rear doesn't know about screen clogging or anything else that happens up front, so it should start out OK with clean screens. A pressure of 1500 at screw tip before the screens is reasonable. Now the dirt builds up, and if you do nothing, there will be extrusion up the vent in a well-designed machine around 2500 psi. In fact, that is how some operators decide when to change the screens, via a little microswitch in the vent hole! Not a good idea, because it won't always happen when convenient. Watch the pressure gauge instead, and schedule your change as needed, but before you ever get to 2500 or whatever it is for you. Or get a continuous changer and run constant pressure "forever." Experienced operators with broken pressure gauges watch the ammeter, and it works for them if they know how many amps represents the 2500 limit. Again not a good idea, because no-one lasts forever. Deal with the numbers. Back to the problem. If you are running close to or beyond the pressure limit, that will do it. You can use coarser screens but that will allow more contamination. You can run the die hotter, to reduce back pressure, but that may cuse cooling problems. I'd rather go back to the feed. Measure the bulk densities. All you need is a cup and a scale. Weight of plastic over weight of same volume of water is BD in g/cc (subtract cup weights). Virgin pelletized HIPS is around 0.65. If the rear zone bites off too much it will more easily flood the vent. If the reprocessed sheet is fluffier, e.g., BD around 0.50-0.55, there is your answer. It doesn't bite so much. You probably have to run more rpm, too, to get same production. But you don't want to run everything twice, so the real question is how to cut back on feed. The cheapest way is to change rear barrel temperature to reduce inpush. Watch amps and measure output per rpm to see what is happening. I can't say for sure to go up or down in RBT. Try both, with big differences (25-50 F) and make sure the barrel responds -- don't assume that just because you set a temp you're going to get a temp. If this doesn't work, and HIPS isn't so sensitive as PP to this, you may have to (ugh) buy another piece of equipment -- a controlled feeder -- which will certainly resolve the question. A gear pump will solve the problem too, with the added benefit of close linear thickness control and maybe a lot of resin ($) saved, but that's a major surgery, while a feeder is minor surgery. Coming after everyone else is a little unfair, as you guys asked all the right questions, but I thought it worthwhile to sum it all up in digestible form. Hope it all helps. Allan Griff IP: Logged |
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Steve H Moderator Posts: 365 |
posted June 28, 2005 03:51 AM
quote: But always nice to have you visit Allan,thanks for a concise summation. Best regards to you and Nancy ------------------ If at first you don't succeed, skydiving is not for you. IP: Logged |
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