posted August 09, 2002 10:29 PM               

We run a polyester strapping line built by SIMA. 180mmm screw with a capable guaranteed output of 1600 pph.
The problem occurred when an operator didn't turn on chillers to cool feed zone section of screw, effectively bridging it. After extended running of this unit without resin flow (other than very little burnt, degraded resin and steam from die) It was determined that the only way to clear this blockage was to extract the screw and clean in.
Upon removal of the screw, it was found to be broken between the metering section and the compression section. Approximatly 2/3 up.
It wasn't a clean break, so it would still rotate, but there is evidence of it breaking and twisting apart by the flight lifted up and gouging the barrel.
The compression section of the barrel ws in good condition. There was no abnormal wear indicating the screw had been broken for a long time before this. The only barrel wear was where the break occurred and the screw flight gouged a 1/16th deep line. This also tells me it probably wasn't broken very long.
Is it possible that the extended running of the screw in the barrel without resin for lubrication overheated it and swelled to the point where it would want to lock up... essentially breaking? With barrel temps increased to 660 deg, and friction from screw and barrel contact, this would seem likely to happen after extended running at max screw rpm. (64.5 is max)
I personally have never seen a broken screw, and certainly one this large would seem to be next to impossible to break. I have heard of this happening but it's incredibly rare.
I would appreciate ony input. Thanks

posted August 11, 2002 06:32 PM            

I would start looking for a fault in the screw in the region of the break.

Regards,

posted August 12, 2002 03:19 PM               

But the event that caused the flaw to fail is the lack of lubrication and support the melt provides a turning screw. Incoming material is the major source of cooling in a plastic extruder, the heat build up as a result of lack of feed would expand the screw causing it to bind.

How long had the screw operated satisfactorily for before it broke? or at least was found to be broken (if the two occurences are unconected)

What is the material the screw is constructed from? Chris Rauwendaal published a paper at ANTEC a few years ago- Screws made from corrosion resistant materials were likely to bind as a result of the differing rates of thermal expansion between the barrel and screw, in this paper he states that Finite Element Analysis indicated that the screw temp in the metering zone could be quite a bit higher than the barrel.

Steve H

posted August 14, 2002 03:02 PM               

Reguarding the possiblity of a structural flaw in this screw that may have led to the breakage, I could probably come up with dozens of contributing reasons.
I guess my main concern is the fact that when the line is shut down for any extended period here, the temps are allowed to cool to room temp, then reheated to the average running temps around 545 to 560 deg.
I was always taught never to let the barrel and screw return to room temp unless there's a specific reason. I was always told (by different people and companys) it's safer to keep your resting temp about 1/2 of your regular running temp. I was told this puts less stress on the equipment and helps reduce the possiblilty of the screw getting "brittle" from repeated heating and cooling in such a wide range.

I may be wrong, this is just a practice I've used in the past, and like I said before, Ive never seen a broken screw. And I've ran lines better than 25 years old with the original screw... with no problems.

posted March 31, 2006 08:31 AM               

I did some research and modeling of screw temperature, http://www.extrudertech.com/antec2000.pdf, with heat conduction in the screw. I found that there is a large temperature difference between the flight tip and the screw root. The gradient is greatest in sections with shallow channels. This leads to thermal stresses in the flight and at the flight root. I believe this is a major contributor to such screw breakage of large screws in areas of shallow channel depth.

Larger radius fillets at the screw root could help. This is also good to elimnate dead spots in the flow, as has been suggested by others.

Correspond, steve@extrudertech.com

posted March 31, 2006 09:28 AM               

Anyone have details on this effect?