posted June 11, 2003 01:49 PM               

It sounds like you're on the right track. Regarding the twin, you may want to try short lead (conveying) elements typically used to create a melt seal prior to devolatization. In this way, you will induce a greater cumulative strain rate at relatively low shear, thereby minimizing any damage to the fiber aspect ratio. It will be important to keep the wood fiber moving as it has a tendency to degrade quickly, and I fear the reverse flight elements may lead to degradation.

Are you currently adding wood fiber to your process or processing a precompounded wood/plastic composite material?

tom black www.ppsincorp.com

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Thomas Black
PPS, Inc.

posted June 12, 2003 09:22 AM               

This sounds interesting. The thing w/ wood fiber compounding is that no technology has emerged as a clear technology leader. In this context, ask a number of different suppliers and you'll get a number of different answers as to what is the correct approach. In my experience, the wood filler is fed first and the screw allowed to run starved for a number of turns in an attempt to help dry out the wood filler (it typically carries a moisture content of as high as 8%). Somewhere downstream, a combi-barrel is incorporated to devolatize moisture from the wood filler and feed the polymer material. Clearly this approach differs from that which you've described which is more in-line w/ traditional fiber processing. If has been found that (in single screw extrusion) fibers are routinely broken down in the compression area at the end of the screw feed section. To avoid this, processors have routinely added their fibers downstream through a vent port, where the fiber can be encapsulated by the polymer melt which reduces its' abrasive character ( ) and its' cumulative shear stress history. However, be forewarned that fiber addition in this location has a tendency to aggregate as the fiber has an affinity for each other, which may lead to the need for shear mixing elements to reduce fiber agglomeration. This is not as bad as it sounds as statistically most fibers orient themselves to the shear plane and go through the zones of high shear with very little change in aspect ratio.

tom

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Thomas Black
PPS, Inc.