Posts: 2
From:Wichita Falls, TX. USA
Registered: Oct 2002

posted October 17, 2002 03:31 PM               

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I am experiencing problems with width variation in tubing exiting the die.

Background information:
The process consists of several extruders linked to one die. The die consists of several plates which separate the melt into layers. Depending on die plate configuration and # of extruders, the number of layers of tubing can be set accordingly. Layers consist of seal, abuse, barrier, core, etc....

As the tubing comes out the die and is cooled using chill water, the width variation can be seen by the naked eye. Any suggestions to what is causing this?

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Posts: 38
From:bethesda MD USA
Registered: Sep 2001

posted October 18, 2002 09:45 AM            

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This is sometimes called "breathing."
1. Look at ammeters for puller and extruders. You may have to get a portable one to put on the puller. You are more interested in the variation than the absolute value. "Watch the Wiggle."
If the puller is varying too much, you have a suspect, especially if the puller surfaces are old, glazed, worn, etc. Sometime you can actually see it slipping, and link the slip to the width narrowing.
This is fairly easy to diagnose and the remedy is obvious, but unfortunately it is rare.
2. Air inflation or vacuum is varying, if you are using thoise devices in the takeoff. The gauges should reflect the cycle to match the width change. This, too, is rare.

3. One of the extruders is surging. This will show up on the motor ammeter. If there are gear pumps on the extruders, look at the ammeters of the gear pumps rather than the extruders. If one of them is varying more than usual, the problem lies there.
You can use pressure variation to reflect surging, but that is much harder to do and less reliable.
Surging itself can have several causes such as
a) erratic feed (big particles in small feed channels, fluffy feed, etc.)
b) dielip heater going on and off
c) stick-slip at entry to compression zone (second-zone surge)
d) screw beat.
If you can plot the width vs time, you can distinguish among these:
a) erratic feed will show erratic variation with no regular cycle;
b) dielip heater will show regular variation timed to heater on-off cycle (e.g., 12 sec)
c) stick-slip is erratic but has cycle of 30-90 sec;
d) screw beat is regular and has cycle matching rpm. This happens where die is short, resistance of lips is low, and one or more extruders has a sharp pressure gradient (either way) in the last few flights. This in turn may come from overbite (too much inpush), and may be sensitive to rear barrel temperature.
All of this supports the need to watch all the numbers, or better yet, record them, so you know when things go wrong. "If you know what good is, you'll know what fishy is."
Hope this helps.
Allan Griff
algriff@griffex.com

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Posts: 61
From:Nottingham, Notts., U.K.
Registered: Jun 2001

posted October 18, 2002 07:43 PM            

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This could well be surging. Look to see if the frequency of the variation in diameter is regular. If it is, there is a regular pulse in the output, which results in a variation in the swelling ratio of the extrudate.

Surging is usually associated with bed break-up in the screww. Because the channel depth decreases in the compression zone of the screw the material has to accelerate. The solid bed, having no cohesive forces to hold it together, will break. The melt, on the other hand, being held together by cohesive forces, will flow into the voids caused by the bed break-up resulting in a variation in the output.

See the results of crash cooling and pulling the screw

and on our website at

www.polytechconsultants.com/results.htm

regards,

Peter P

[This message has been edited by Peter P (edited 10-18-2002).]

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