Extruder Screw Fatigue

[satchmo]

The company I work for extrudes plastic sheet. We recently had a fatigue failure of one of our extruder screws. The fatigue zone was roughly halfway through the screw. We have other, old machines that may also be wearing in this way. Does anyone know of a way to test the screw continuity while it's still in the extruder? I would like to be able to test the screws for cracks without having to pull and clean the screws. Any ultrasonic methods available?

Any guidance is appreciated.

 

[dwightdixon]

The only way I know is to pull and clean the screw and use the magnetic field equipment or look for a crack using the Zyglow technic of dye penatrant.

 

[CompositeGeek]

If you normally pull, clean and reinstall the screws at your facility then you may want to subject the screw to a dimensional check as well as a non destructvie test as suggested by dwightdixon. I would imagine the screw is changing shape over the course of its life. You may have an early warning of failure that shows up as a change in pitch etc.
I am thinking of a situation that may be similar to a worm gear drive on a mcahine tool where the worm stretches over time.

Of course it is likely I am wrong. Just a thought.



Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[Pud]

Just a thought: if you have access to the end of the screw(s) you may be able to use ultrasound to detect a crack somewhere down the screw length.

Cheers

Harry

 

[Demon3]

As a wild guess it seems to me that one could measure the capacitance. Imagine the screw is one electrode and the extruder barrel the other then the polymer is the dielectric. You would need to regularly run some standard polymer so you know what the capacitance of the system is and it would require that the screw and barrel are electrically insulated from each other. Any crack appearing on the screw should change its ability to act as an electrode so the capacitance should change.

I just made all that up as a guess so no promises.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

A quick rough test on crankshafts in cars is to tap them with a hammer. They ring if not cracked. If they have a small crack, they still ring, but with a large crack they thunk.

The screw must be out and clean. Pulling a screw is not that big a job and should be regular maintenance for this and other reasons.

Regards

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[automatic2]

establish a temperature profile for each machine. Trouble is almost always accompanied by temperature change in that zone.