screw based extrusion

why screw based extrusion(mini extruder) is not used instead of filament driven method for the industrial purpose 3d printer? Even throughout studying over it I found that screw based extrusion has so many advantages over the filament driven method.
Please clarify my doubt.

What are the advantages?

RichRap just released designs for a pellet extruder...

The pressure is built up by the geometry of the screw to overcome the back pressure from the nozzle, possibility of sticking the molten material inside the nozzle is less as compared to filament driven.

I thought that the main drawback was extrusion speed.

There are some of these extruders to be found if you use the search

You can find more here:
[www.thingiverse.com]

and here:
[forums.reprap.org]

And there was one more topic with lots of info, but I can't find it back that quick.

Edit:
Even more:
[forums.reprap.org]

and more:
[forums.reprap.org]

Edited 1 time(s). Last edit at 01/20/2015 07:42PM by Ohmarinus.

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http://www.marinusdebeer.nl/

It is harder to precisely control the amount of plastic being fed into the nozzle. Given the size of pellets and the space between them, you may not get the exact same number of them into each flight of the screw. If you have a long enough melt zone it might all average out, but keeping a constant, known pressure is more complicated than driving a single strand of plastic of known length and cross-section.

There is also the possibility of introducing air bubbles or other contaminates into the plastic. Most filament making machines have some sort of filter in the system.

Weight might also be a issue. When you pare everything down to fit on a printer you want to keep the printhead as light as possible.

Thanks Mike