posted August 12, 2010 10:40 AM            

PET single screw 140mm extruder with 4 chamber screen changer runnig around 2000 pounds per hour. Sheet die - single pump.

Combination of recycled material along with virgin resin. Recycled material is a combination of washed bottle flake as well as pelletized hi-iv pellet.

Extruder was shutdown for 6 weeks - during shutdown screw was pulled (worn) and sent out for rebuild. Die was removed, split and cleaned - and reinstalled. Screw was return and reinstalled.

Upon start up - "voids" began occurring in extrudate - these are tiny small "bubbles" inside the web - not on the surface.

Attempted to change materials and little effect. Bulk density changes no effect. Runnig 100% virgin has greatest effect - however voids still occur - just much less frequently. Frequentcy runs from every 12 hours to every 20 minutes - not consistant at all. Virgin blend can run 36 hours between voids to 3 hours between voids.

Pretty much at a loss - no issues prior to shutdown - only started happening aftr the shutdown.

Oh, output has no effect - tried slowing machine down to 1600 pph, 1800 pph, etc - no effect.

Any ideas would be appreciated -

Thanks -

SG

posted August 12, 2010 11:27 AM            

Did your specific output (Lbs/screw revolution) change after the rebuild.

Check your output relative to the calculation at http://www.plasticextrusion.info/
and report how you compare.

During a shutdown moisture can get into heaters and short them out on restarting. Check all of your barrel heaters, even if the zones report correct temperatures.

If there is a pressure peak in the screw before all of the resin is melted, then air can be trapped with the unmelted resin. This is a more complex problem to solve.

------------------
Best Regards,

Tom Cunningham
www.ExtrusionTechnicalServices.com

posted August 12, 2010 01:23 PM            

The extruder does not have a vent.

The lbs per rpm did not change from the origianl screw and the rebuild. Hmmm......

All heaters have been checked.

That's what I have right now.

posted August 12, 2010 01:46 PM            

Has the temperature, or preferably moisture content, of the material coming from the dryer been checked?

any places where material can hang up, get wet, then drop into the system later?

------------------
Best Regards,

Tom Cunningham

posted August 12, 2010 02:33 PM            

I changed blenders to eliminate the possibility that something was happening with the normal blender.

We even went to the extreme of changing out extruder hoppers and extruder feed throats - no effect.

We have discussed changing to a different dryer - but as of yet not tried that.

It has been discssed that possibly the screen changer - due to leakage around the bolts - might be contributing. I do not buy into that as again....before shutdown no problem. Nothing was done with the screen changer.

My gut is telling me something happened to the screw during rebuild or installation - just do not know what may have occurred.

posted August 12, 2010 02:45 PM            

Any possibility of air traps inside the screen changer that are slowly working their way out?

If you have a drawing of the screw I could look it over.

------------------
Best Regards,

Tom Cunningham