posted May 19, 2003 05:57 AM               

I've got a strange problem...
I use two identical extruders, in the same conditions.
The problem is that one of the extruder produce bubbles whereas the other doesn't

I don't have a great experience in extrusion process and i'd like to know from where it could come from or/and how to correct this defect.

posted May 19, 2003 06:09 AM               

I'd first check out the hopper area to assure that your not introducing the chance of forming "bubbles". Make sure both are operating at the same conditions - same temperature profile, same cooling loop, and the same RPM's and similiar pressures. The throat may be the most likely culprit. Make sure the cooling loop is the same and that your throat is not intermitantly bridging.

Can you reveal the resin system your using, as this too, may be an issue.

posted May 22, 2003 08:13 AM               

So try resetting the temperatures the same, and the screw spped the same, and the breakerplate screen packs the same.
Throat bridging occurs at the throat or feed port of the process just below the hopper. If the feed throat is to hot, the pre-melted secret resin intermittantly "briges" or for better terms, clogs up the feed throat section depriving a full feed stream. Most throats are cooled with water, and sometimes, someone adjusts the valve wrong depriving the cooling area of coolong water. Heat buils up and slight ot moderate to heavy briding occurs.

The "ideal" temperature is to throttle the valve so just enough water cols that zone. A rule of thunb is to simply feel the warmth of heat with the back of your hand.

Be careful at first, as if it's shut off completely, you can burn your backhand. Move you backhand slowly toward the exterior metal, and if you really feel it being hot as you approach it, then that may be the problem your facing.