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| Author | Topic: Pebax Extrusion Problems |
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Technosavy Member Posts: 3 |
posted October 18, 2005 11:58 AM
   
Hello, We are running Pebax extrusion (Pebax 5533 with 24% BaSO4)for medical tubing applications. We ran it using melt pump and had very good success running it whole of last year. All of a sudden now we are having trouble maintaining size. The OD fluctuates a lot causing the ID to vary. The pump pressure seems to be holding fairly consistent and have tried changing to different pumps, different pullers but to no success. We run it free flow using Iris. Any suggestions to correct the problem would be appreciated. Thanks, Technosavy IP: Logged |
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Tom C Moderator Posts: 631 |
posted October 18, 2005 02:58 PM
Pebax seems to have very different flow characteristics depending on temperature. Make sure all of your die heaters are working, and raise the temperature of the entire system. See if that helps to stabilize things. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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griffex Senior Member Posts: 31 |
posted October 23, 2005 03:23 PM
Further to Tom's answer, do you have a melt thermocouple in the stream? If not, consider getting/installing one in the adapter, preferably but not necessarily after the pump. Do not use a dual device for temp and prerssure at screw tip. Pressure reading may be OK, but temp is not. In the meantime, use an infrared gun to see if temp is fluctuating. Most likely temp to vary is die, especially if there is a separate heater at end, which can cause "hotlips disease," where the heater goes on and off and the cycle matches the cycle of OD variation. Solutions include insulating die so heater doesn't have to go on at all, or supplying the heater through a variable resistor and taking it off control. I would have suggested checking the volumetric efficiency of the pump (compare real output per rpm with theoretical displacement, should be 90% or more), except you have tried new pumps (or were they old ones)? Also watch amps of pump drive, compare to last year (that's why we should keep records of everything). If temp is fluctuating you'll see it in the amp wiggle (variation). If pump is tight and amps of pump drive don't wiggle too much, it may still relate to the puller. More possibilities: varying internal air pressure or external vacuum, varying tension between puller and sizer. I hope this is useful. A L Griff IP: Logged |
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Technosavy Member Posts: 3 |
posted October 24, 2005 04:16 PM
Thanks for your input Tom, I tried playing with the heats but to no use. I mean the dimensions could not be held good, no matter i go up or down on the heats. We have a long tank (3ft)and I was wondering if the water disturbance is causing it because it is extruded free flow. What do you think about th tank length?
quote: IP: Logged |
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Technosavy Member Posts: 3 |
posted October 24, 2005 04:27 PM
Hello Griffex Thanks for your detailed explaination I really appreciate that. We have been using melt pumps since last year. I am not sure what the life would be considering we run Barium filled materials.So I am going to try a new pump for that. Regarding the heats, the thermocouple does not touch the melt stream. I can certainly look into that. Maybe you are right there saying the way the die heat controller turns on and Off shows the patterns how the OD is jumping. Also, one thing I forgot to mention last time, the screw we are using is a 2.8:1 compression double flighted screw with a mixing section in the metering portion of the screw. Maybe that's causing the heats to vary because of the excessive mixing. What do you think? IP: Logged |
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Tom C Moderator Posts: 631 |
posted October 26, 2005 08:13 AM
Try wrapping the die with some insulation. Sometimes that will help stabilize the die temperature. After insulating look to see if the die holds a stable temperature. If not, perhaps the controller PID parameters need tuning. As Alan mentioned please check the melt with an IR gun. The die temperature should be set close to the melt temperature even if you think it is too high. Otherwise the die may heat or cool the melt unevenly. I have 2 styles of IR guns available if you need assistance. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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alpertl Senior Member Posts: 42 |
posted November 16, 2005 11:20 PM
quote: I know there are issues with pebax grades manufactured in the US around the middle to Q3 of last year. Mw ditribution was much wider then usual and there were high "Visible Impurities" (not sure exactly what they mean by this). I'm not so sure why one would use a melt pump for pebax. I have 3 part numbers of three grades that have ID's from .021 to .027 ± .001 with a wall nominal thickness of .00075 ± .00015 and probably about 50 other UPNs of all grades and fillers and none of these use a melt pump. IP: Logged |
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