Contributed by: Beth M. Foederer, PE – Optex Process Solutions, LLC
How to get the screw almost completely clean before removal
Several months ago, I submitted an article about proper purging. In this article, I promised to continue with how to go one step further by cleaning and removing a screw. Screw removal can be a tough, time consuming job especially when you consider the screw must be cleaned upon removal. Many years ago, Optex was taught how to clean a screw while it's still in the barrel by the DuPont Lab.
This method uses an Acrylic material that does not melt in the screw but grabs onto the molten polymer and removes it from the screw and barrel. To use this method, the screen changer, valve and downstream must be disconnected from the extruder. The purging compound used is Bamberko Purging Compound.
Once the opening to the barrel is clear of all downstream, clean out the resin from the hopper. The extruder temperatures should be raised to 450F (232C). If you are purging ABS, PA, PC, PET or other high temperature resins, you will run at 450-525F (232-274C). Don't forget to back out the pressure transducer and rupture disc to protect the tip surface.
Once you are at temperature, start the screw at about 80-100 rpm. Let the poly come out the end until it stops coming. Feed the purging compound into the extruder, a small quantity at a time (about a cup or so depending on extruder size). Be sure to keep your hands away from the rotating screw in the barrel. Watch the motor load to insure it can handle the higher load for this material. Slow the screw down if it can't. You should need about 2 pounds (1kg) per inch of screw diameter.
Run the purge little by little through the extruder until the barrel and screw are empty. Stop the screw and immediately push it out of the barrel. Use copper mesh or a wire brush to finish cleaning to prevent any of the purge compound from melting to the screw.
Clean the barrel with a flue brush and follow up with cotton cloth on the brush. Remove the purge dust from the hopper, throat and barrel. Clean pressure transducer and rupture disc holes. Apply anti-seize to the threads and put back into the holes. Reassemble the extruder.


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