Extrusion basics: The relationship of inpush to output

17/02/2016

When pellets fall into the first screw channel in the feed zone, the angled flights of the screw push them downstream, as well as around the barrel circumference. The amount conveyed depends on their size and shape (bulk density), how much they stick to the barrel and how well they slip on the screw root. More sticking to the barrel and less sticking to the root mean more forward-conveying per turn.

However, we mustn't forget the pellets that touch neither the barrel nor the root. These will convey best if they grab onto one another, which is why scrap sometimes feeds better than virgin (for example, heavy-walled high-density polyethylene pipe made from slippery pellets). The deeper the channel, the more important is this "interpelletary grab" and the less important is the barrel temperature.
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