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 Extrusion, Single Screw
 Quench tank "Air Gap" breaks - PET extrusion
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| Author | Topic: Quench tank "Air Gap" breaks - PET extrusion |
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StrapGod Senior Member Posts: 23 |
posted March 15, 2003 09:39 PM
  
Here's my question and I will try to give as much detail as I can. Extruding .800-.900 IV PET resin into a quech tank with the distance between the die exit and the water entrance varying in distance. Downstream through the drawing process it has been found that as you decrease the "air gap" or distance bewteen the die exit and the quench entrance breaks can begin to occur - especially when the "air gap" drops below 2.0 inches. When the gap is increased, the breaks stop. I do not want to discuss too much about the process, but feel free to ask questions. Anyone ever experianced this or know why this is happening? One more bit of info - small notches can be seen near the break on the edge of the drawn PET. Hmmmmm..... IP: Logged |
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louis33 Senior Member Posts: 146 |
posted March 25, 2003 08:12 AM
Generally speaking tensile goes up as cooling rate is increased and elongation goes down. Could that be your problem? IP: Logged |
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StrapGod Senior Member Posts: 23 |
posted March 25, 2003 06:27 PM
I don't think that is the problem - it appears that something happens when the PET is quenched quickly, via it entering the water close to the die, that effects the edges of the extrude. Once the PET is drawn/stretched small notches appear on the edges and break at the draw point. When the distance between the die exit and the quench is increased - the notches disappear. Hmmmmmm... any help is appreciated. Thanks - Keith IP: Logged |
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zabielski Senior Member Posts: 386 |
posted March 26, 2003 07:10 AM
Not knowing if your underwater holders are rollers or rods, I would think it may be a matter of entry angle into the primary roller, or the concave rolller not having the correct geometry, or if a rod, the rod itself. The roller may have been mis-sized, or worse yet, the rod may be worn (perhaps to the point where it actually has a sharp edge on it). Yes they wear with time - both rollers and rod's. I guess you may be unkowningly be "etching" the extrudate (almost like a minor embossing effect). IP: Logged |
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louis33 Senior Member Posts: 146 |
posted March 26, 2003 08:15 AM
Sounds like the draw down ratio is too high causing some micro cracking at the edges when you move the tank in. Try examining product that was quenched with the tank in close, but prior to orientation. louis IP: Logged |
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Steve H Moderator Posts: 386 |
posted March 28, 2003 03:03 PM
G'day Keith PET strapping relies on a crystalline form (and orientation) for its performance(330 and 600N/mm2-break strenght between 2 and 10 kN). If you cool your strap too quickly it will be more amorphous in nature, maybe there's something happening between the areas of crystalline and amorphous regions in your strapping. Could the die height above the quench tank be a red herring, and the problem be caused by inadequet drying? Have you talked to your resin supplier and equipment vendor about this? Steve H IP: Logged |
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