Is there a better feeder screw ?

Have you dried the PLA? If so, how?

http://www.natureworksllc.com/~/media/T … wn_pdf.pdf

90C for 2-4 hours is appropriate. I would probably aim for 80C and closer to 4 hours to avoid melting the pellets, since Tg is around 70C.

Sealed and silica pack does not mean its dry. Not at all. The pros dry it inline, while its in the hopper - that's how important it is for there to be no moisture.

A hopper with mesh screens on the sides that blow hot, dry air up and through the hopper.

They have to do this for three reasons:

1.) They cannot tolerate any variation in their material properties. A typical industrial hopper would hold 6-12 hours of material, and industrial machines are even more sensitive than DIY machines since they process so much faster.
2.) Factories are typically not HVAC'ed, and often damp/humid.
3.) They do not store the material sealed - it arrives in big cardboard boxes, called gaylords... or sometimes train cars. Thus they must dry it at some point, so best to dry it right before extrusion.

For DIY stuff, it seems drying it well before use, and only adding a few hours at a time of material (so material isn't sitting in the hopper all day) works well. Wouldn't hurt to use a lid. I've only run PLA for 15-20 hours total, others here are probably far more experienced. I prefer ABS because much of what I print has to withstand high temperatures.

Hugh Lyman has an cool filament dryer in one of his videos, but I don't think he ever posted the files.  It wouldn't be hard to recreate.