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| Author | Topic: welex 4.5 pet extrusion line |
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curiousgeorge Senior Member Posts: 7 |
posted July 05, 2007 04:30 PM
  
Hi, my company has a welex 4.5 inch extruder which extrudes only pet. this particular machine is rated at 1550 lb/hr output, however we have upgraded the motor from 250hp to 300hp and have set our goal at 2100 lb/hr. Currently we are running a 90% motor load with zone temperatures in the 660s and output at about 1800 lb/hr. Any thoughts on our current condition, as to why our zones are so high or how to improve production? screw standard welex barrier screw IP: Logged |
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Tom C Moderator Posts: 868 |
posted July 05, 2007 05:12 PM
Are your zones set to 660F, or overriding to 660F. What are your RPMs? ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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curiousgeorge Senior Member Posts: 7 |
posted July 06, 2007 09:16 AM
zones are overode to 660s with resin and 610 with repelet do not know rpms but the extruder speed is generally around 100% and gear pump around 70 % IP: Logged |
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Tom C Moderator Posts: 868 |
posted July 06, 2007 10:52 AM
Barrel zones over-ride becasue you are not removing enough heat. It does not necessarily reflect the bulk melt temperature at that point on the screw. Is the extruder water cooled or air cooled? Ultimate output depends on a lot of factors; L :D, screw design, motor power, resin tolerance for overheating, available RPMs, discharge pressure, melt quality requirements, pre-heating. You are certainly in the range of ultimate output on a 4 1/2". ------------------ Tom Cunningham [This message has been edited by Tom C (edited July 06, 2007).] IP: Logged |
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curiousgeorge Senior Member Posts: 7 |
posted July 06, 2007 11:28 AM
i am sorry i am mistaken ( i am new at this) we set our zones. we have a watercooled extruder kept at around 75 degrees. i guess my first question would be why we have to run so high in the zones. my train of thought is that maybe we went over the bell curve of max plastic friction. we have tried bringing down the zone temperatures by about ten to fifteen degrees and the amps on our extruder always climb and cause us to slow down the extruder. P.S. thanks for all your help so far sorry i am very new to this and am trying to figure it out. Also if it would help i have been taking settings from the extruder for a couple weeks and could send you them. IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 71 |
posted July 09, 2007 02:19 PM
PET requires about 0.10 to 0.12 hp-hr/lbm. So, 300 hp is enough hp for up to 2500 lbm/hr, if the proper screw is used. You probably need a new screw to provide the desired flow rate and product melt temperature within the speed range of your drive. A proper screw design will accomplish this with minimal heating/cooling by the barrel. ------------------ IP: Logged |
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curiousgeorge Senior Member Posts: 7 |
posted July 10, 2007 12:19 PM
Well speak of the devil we are changing the screw out this week so hopefully that will settle things down a bit. thanks for all the info. one other question where would you think our zones should be at when we start running again, surely not in the 660s again right? IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 71 |
posted July 12, 2007 10:24 PM
Curious G Yes, 660 F is way too high for normal PET extrusion. Are the zone heaters on or off (cooling or heating?) Solids conveying should be about melting point. The rest of the barrel should be a little lower than the product temp. What IV is you PET? PET pellets? Are you using a barrier screw or a two-stage? ------------------ IP: Logged |
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curiousgeorge Senior Member Posts: 7 |
posted July 16, 2007 09:49 AM
we are heating to 660 to overcome high amps. material has targeted iv of .78 to .83. we are running a barrier screw which has just been replaced. sadly we are still having the same problems if not worse, our amps are through the roof! IP: Logged |
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Tom C Moderator Posts: 868 |
posted July 16, 2007 10:31 AM
What is the starting IV? Are you pre-drying? What is the temperature of the material entering the extruder? What are the screw RPMs? What are the maximum screw RPMs avaialble? Would you be interested in an on site visit from a consultant? ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 71 |
posted July 16, 2007 01:08 PM
The iv is higher than I assumed. So, the hp-hr/lbm will be higher than the 0.10 to 0.12 that I previously quoted. At 300 hp and 2100 lbm/hr, the maximum hp-hr/lbm you have to work with is about 0.15. This is likely not adequate for the iv that you are running. So, you add energy and lower viscosity with a hot barrel. According to your previous setup, 250 hp and 1550 lbm/hr, your hp-hr/lbm was a little over 0.16. So, based on that, at 300 hp you could expect to be able to approximately process 300/0.16 = 1875 lbm/hr. ------------------ IP: Logged |
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curiousgeorge Senior Member Posts: 7 |
posted July 16, 2007 02:22 PM
you make a lot of sense. is there anything we could try to help us run better? or do you think that we are actaully optimized for what we are doing? IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 71 |
posted July 16, 2007 10:48 PM
Curious George-- The screw you have now needs to run at a high speed to obtain the rate you want. Excessive strain and stress are generated because the speed is high and the channels not deep enough This makes the the motor amp out. So, the first thing is to make the screw channels deeper. This will lower the screw speed for your rate. Less speed, larger channel depth mean lower energy generation in the polymer. The net energy needed to process PET is about 0.10 hp-hr/lbm. Now you generate about 0.16 hp-hr/lbm. Therefore, lowering the energy generation with the deeper screw can still provide enough energy to process the PET, usually about 0.12 hp-hr/lbm for the motor. However, the screw design must be done correctly. If you make the screw too deep, then it will run so slowly at your rate that insufficient energy (less than 0.10 hp-hr/lbm) is devloped to process the PET. Excessive barrel heat will then be required to make up the difference. A different gear ratio may also be needed. However, just a proper screw may do the trick. Try that first. Allot can be done with the screw design (adjust lead length) to make it run with existing speed capability. Perhaps, you can just recut an existing screw? Cheap and fast. ------------------ IP: Logged |
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curiousgeorge Senior Member Posts: 7 |
posted July 17, 2007 09:18 AM
Thanks for all the info guys. I am curious as to the cost of recutting a screw and to the gains that you think we could get. IP: Logged |
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Tom C Moderator Posts: 868 |
posted July 17, 2007 09:32 AM
George, The cost of changing the screw profile depends on a number of factors like; whether it has been chrome plated or nitrided or other. I depend on computer simulation to predict the outcome of various screw design and process changes. It involves some work, but the results are good. Much quicker and less expesnive than the cut and try method. Some knock computer simulation. I have spent years perfecting the use, and have tools which few others have. My proof is happy, repeat customers. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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