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Author
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Topic: Flow lines in medical tubing
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donaloriain
Senior Member Posts: 16
From:Ireland
Registered: Apr 2003
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posted August 08, 2005 06:51 AM
  
Can anyone advise on possible causes of flow lines in medical tubing. One specific product with which I am seeing flow lines is based on a Nylon/Pebax blend (80/20). The blend is simply bag-mixed prior to the extrusion stage (single screw).
My own feeling is that there are two possibilities. 1) Inhomogeneous melt due to poor mixing or 2) Die design. Before I look into these further can anyone suggest any other possibilities.IP: Logged |
Tom C
Moderator Posts: 674
From:Brodheadsville, PA USA
Registered: Jun 2001
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posted August 08, 2005 09:25 AM
Dona,Are the lines always in one place? Do they come and go? Do they move around? The behavior would indicate whether the problem is mixing or die related. ------------------
Best Regards, Tom Cunningham Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
donaloriain
Senior Member Posts: 16
From:Ireland
Registered: Apr 2003
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posted August 08, 2005 10:30 AM
As far as I'm aware the lines are continuous and are always there. Generally the problem is not very obvious in the extruded product. It is only when the tubing is post formed into balloons that the the lines are more pronounced.IP: Logged |
louis33
Senior Member Posts: 142
From:Allentown, PA - USA
Registered: Jun 2001
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posted August 08, 2005 10:50 AM
First question - can you see them coming from the die? If so, are there as many lines as you have spider legs, or if a crosshead, flow dividers? If they always come from the die before hitting any sizing apparatus, then it's a tooling/heat profile problem. When one talks about balloon forming, you should always shoot for the most homogenous extrudate that you can get. Therefore, you cannot run it too cold.
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donaloriain
Senior Member Posts: 16
From:Ireland
Registered: Apr 2003
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posted August 08, 2005 10:55 AM
Thanks for the suggestions. I'll have to do a bit of enquiring in the extrusion dept to answer your questions fully. I'll post some more information when I get word back.IP: Logged |
alpertl
Senior Member Posts: 47
From:Dayville, CT USA
Registered: May 2002
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posted September 17, 2005 02:15 AM
quote:
Originally posted by donaloriain:
As far as I'm aware the lines are continuous and are always there. Generally the problem is not very obvious in the extruded product. It is only when the tubing is post formed into balloons that the the lines are more pronounced.
Why on earth would someone hand mix resins for a balloon application? I don't mean to be overly critical here, but what kind of yields do you get with your balloon process? Your 80/20 is probably running from about 70/30 to 90/10 at best for any given section of tubing. Even if using distributive/dispersive mixing screws, if you don't get your RPM's up, it will do you no good (I have done testing on this in the past). Also, if using a crosshead, helicoid design may be a problem with weld lines, especially with a 4 flow diamond pattern. IP: Logged |
donaloriain
Senior Member Posts: 16
From:Ireland
Registered: Apr 2003
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posted September 22, 2005 10:56 AM
I'm new with this project and one of the first things I pointed out was the poor mixing achieved by bag-mixing and single screw. We will ceratinly be changing this aspect of the process. As regards die design, we were using a crosshead but are in the porcess of carrying out development with a straight-through die.
So would I be right in saying that your synopsis would be that the flow lines are probably a combination of poor mixing and possible weld lines?IP: Logged |
alpertl
Senior Member Posts: 47
From:Dayville, CT USA
Registered: May 2002
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posted January 13, 2006 01:00 PM
quote:
Originally posted by donaloriain:
I'm new with this project and one of the first things I pointed out was the poor mixing achieved by bag-mixing and single screw. We will ceratinly be changing this aspect of the process. As regards die design, we were using a crosshead but are in the porcess of carrying out development with a straight-through die.
So would I be right in saying that your synopsis would be that the flow lines are probably a combination of poor mixing and possible weld lines?
I haven't gotten back to this thread in awhile, but in response, the weld lines most likely have little to do with the poor mixing, but mostly from using a diamond pattern helicoid (Most notably a Genca Lovol B series with a 4 flow seems to be prevelent in the industry), though this type of setup does have short land ratio. An inline die may give you problems as well, unless you try a spiderless design (Guill), as Spiders can give you weld lines as well.
Another option is using a spiral helicoid. If you are using a Genca set up, call B&H tool, they have a nice spiral design helicoid that does quite well at eliminating weld lines. Of course, as always, YMMV!!!
Your
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