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| Author | Topic: Cause of extrusion instability |
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greg.barnes Member Posts: 4 |
posted November 23, 2005 08:38 AM
   
Can extrusion die diameter and tip land length cause surges when extruding natural HDPE? An engineer in our office says that our tooling geometry is causing pressure fluctuations and random OD variations. I do not buy in. HDPE extrudate seems to be like a battleship: once it starts moving it does not want to stop. I think it is more likely that there is something under the extrudate (like moisture) causing the lumps. ------------------ IP: Logged |
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Tom C Moderator Posts: 657 |
posted November 23, 2005 09:12 AM
With the use of a pressure gauge and a chart recorder you can see any short and long term pressure variations that come from the screw. From what you are describing it seems as if you are having very short term variations of flow. This can be caused by high shear stress in the die, known as melt fracture and shark skin. As you mentioned moisutre can cause surface defects, as can entrianed air and unmelted pellets. Section the defect and look at it under a microscope. This may give you a clue as to the source. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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greg.barnes Member Posts: 4 |
posted November 23, 2005 03:56 PM
It does not appear to be sharkskin. I saw that before when a die heater was going bad. The surface in this case is smooth to the touch. The OD will suddenly go up and then drop back down to nominal, apparently at random. The cable jacket kind of looks like a python swallowing a rat. We have not yet been able to section a sample because we always make repairs and send the product on. It does cause delays, however. IP: Logged |
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Tom C Moderator Posts: 657 |
posted November 24, 2005 09:06 AM
Barrier screw or conventional screw? Mixer on the screw? ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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greg.barnes Member Posts: 4 |
posted November 28, 2005 08:29 AM
This is a barrier screw and there is no mixer. The OD goes up several millimeters (on a 25 mm tube) within a 2 or 3 cm area. IP: Logged |
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Tom C Moderator Posts: 657 |
posted November 28, 2005 10:12 AM
First you should check some standard stuff before we get into the more exotic. 1) Are your extruder RPMs steady? Sorry for so many requests, but this is what it takes sometimes. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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louis33 Senior Member Posts: 140 |
posted December 05, 2005 12:23 PM
Seems that you are extruding over a cable. That being the case, moisture will cause that problem or volatiles (out gassing) I suspect that you do not have a true "surge" in output, especially if it is in a very short section. That kind of output variation would be very noticeable on the gages of the machine was it to occur. Of course, any interruption in the feed rate of the wire versus the feed rate of the material would show up as a "surge" in the product OD. Check the payout and take-up devices if you have not already done so. Oh, almost forgot, yes, the tooling if it is not sized correctly will cause a similar problem. The Draw Down Balance should be on the positive side. Too high will cause tearing of the melt, too low might put you in a borderline area where slight changes in wire speed cause those short sections of BUMPS. Sounds like tooling or payout/takeup from where I sit.
[This message has been edited by louis33 (edited December 05, 2005).] IP: Logged |
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greg.barnes Member Posts: 4 |
posted December 05, 2005 12:58 PM
Thanks, everyone! We measured the head pressure Friday and it was fluctuating a good bit. Then, we did a trial with different tooling and found a combo (with a positive balance if I am calculating it correctly) that reduced the pressure fluctuations at least 50%. Now we are running with the tooling and will see how it works. Interesting enough, this did not happen on every cable. It was about every 15th cable, which was a nuisance. Fortunately, we were able to make repairs 99% of the time. IP: Logged |
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mademan5284 Senior Member Posts: 23 |
posted December 07, 2005 02:34 PM
Make sure your tip is not protruding past your die. IP: Logged |
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