posted August 11, 2003 01:05 AM               

Welcome to the single screw extrusion forum.

You've given us quite a bit of info, well done Have you got the depths of your feed zone, compression and metering sections of your screw.

Are you cooling the feedthroat of your barrel.

With your barrel temps, are you running the feed zone at 280C or 265C. Whats happening with the melt temp, melt pressure and screw motor amps.

posted August 11, 2003 06:56 AM               

The company that owns this forum can supply new screws and barrels. I would recommend a significant screw redesign constructed from wear resisant materials.

posted August 11, 2003 10:26 AM               

Trying to get your post count up?

posted August 12, 2003 12:29 AM               

Could this be a case of the feed bridging in the hopper/ feed throat?

posted August 15, 2003 11:31 AM               

I've had good experiences with CPM-10V on small parts. I think you can only get so big.

On larger screws Stellite and Colmonoy welded overlay grades work good. Be sure to check barrel compatibility charts.

I like Wexco 777 barrel liners. Conforma-clad is also good for smaller barrels like twin screws. Some have had good results with barrels made with the HIP process.

posted August 16, 2003 12:19 PM               

If you are not using very wear resistant materials on the screw and barrel, a screw design like you have could wear out in a single day running glass filled nylon.

If wear is not the problem, then a likely source is screw and barrel temperature. Once you start the extruder it can take 15-20 minutes for temperatures to stabilize.

While not running the screw can heat up in the feed zone. This may provide a temperature where the friction is low on the screw. As the extruder runs, the constant feed of new resin draws heat from the screw. It is possible that a lower screw temperature has higer friction with the resin, and this stops feeding.

Another possibility is related to the above. When you start running the screw is hot. Because of the long feed section of the screw melting begins well before the transition. As the process runs for a while the screws cools as described above, and/or the beginning of melting moves further towards the throat as barrel heaters recover. Some of the melted material then freezes onto the screw causing a buildup. When the extruder is stopped, the buildup could possible be re-melted, or scrubbed off by the stop/start process. Then you can run again until the buildup occurs, again.

Solutions:

1) A correct screw design would be best.
2) Heat the screw.
3) Lower all of the heaters to about 100C up to 6D before the transition. Then set that heater to 300C, then set as usual. This will prevent pre-mature melting.

From this screw you should expect; low rate, excessive wear, and surging as a normal condition.

Tom C

posted August 18, 2003 09:02 PM               

Been busy and sorry to not to have got to this post sooner. Has anyone discussed the idea of nylon slipping in the feed? If so, please ignore the post, however if not, I'ld be happy to offer some comments. I've had lot's of issues with Nylon mis-feeding due to slippage on the barrel and have used small barrel grooves quite successfully to overcome this...

tom black

------------------
Thomas Black
PPS, Inc.