posted May 04, 2005 08:34 PM               

Water is not the only source of bubbles. You could have;

Air trapped in the screw

Volitiles from degradation

Shinkage voilds from rapid cooling

Call me if you need additional help.

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Best Regards,

Tom Cunningham

Extrusion Technical Services

posted May 04, 2005 09:59 PM               

Thanks for the tips. Do you suppose that the bubbles could be a result of purging the barrel after each use? In doing so, there would be more of a chance for trapped air than if the material was left in the barrel after each use.

posted May 05, 2005 07:50 AM               

I think the extruder should be fully purged and stable before you collect product, so the condition you suggested does not apply.

Since you have posted under twin screw, please describe the style of twin screw you have. Is it co or counter? Do you have a vent?

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

Tom Cunningham

Extrusion Technical Services

posted May 05, 2005 01:22 PM               

The extruder that I am using is a Leistritz that is currently in a co rotating setting. it has an od of 27mm and the barrell is 1120 mm long, so an L/D of 41.5. There are places on the top of the barrel that can be used for venting; there is nothing that I can see that is allowing the barrel to be vented at these locations.

Another possibility that occured to me is the screw design. Could it be that the material is not mixing enough in the barrel and this is causing air to be trapped? Currently, there is only one mixing section in my screw design with a conveying section before and after the mixing section. The screw design is both length and crosswise open, so the material has opportunities to mix axially and radially.