Hmm... not to rain on your parade, but I don't think this is a viable option, not with pellets at least. I've built and operated a lyman-esque extruder for the past few months. There are two main reasons:
1) The pellets are 3-5 mm in diameter each. A smaller screw would only feed them a few at a time, and it would be difficult to control the amount of extrusion.
2) Even with a gigantic motor with 12-15 Nm of torque (~10 times what a geared nema17 could put out), a filament extruder can only push out 14" / min of 1.75mm filament. A well calibrated printer doing infill will use more than this rate.
The first problem can be alleviated if you switched to a fine powder. The filastruder folks have used nylon powder from SLS printers with success. With powdered plastic, it's possible to drive a much smaller screw and still have even extrusion. Alternatively, some variety of closed-loop feedback using an optical sensor to measure the amount of plastic pushed out may also work. Also, will powdered plastic be any cheaper than filament?
The second problem is a little less tractable. You can switch to a smaller screw with a higher speed, but the total amount of energy required to form the plastic is fairly large. The resin pellets come full of air pockets and bubbles. I believe the high pressure within the tube is what helps you get nice and even extrusion. In other words, it might be more economical and easier to deal with if you formed the plastic into an even filament with a larger machine, then printed with it. Hence the status quo today with a separate filament extruder :)
BTW, you might want to check this out: [reprapdelft.files.wordpress.com]
They have a drawing on page 23 of a putative direct extruder that you could potentially try building!
1) The pellets are 3-5 mm in diameter each. A smaller screw would only feed them a few at a time, and it would be difficult to control the amount of extrusion.
2) Even with a gigantic motor with 12-15 Nm of torque (~10 times what a geared nema17 could put out), a filament extruder can only push out 14" / min of 1.75mm filament. A well calibrated printer doing infill will use more than this rate.
The first problem can be alleviated if you switched to a fine powder. The filastruder folks have used nylon powder from SLS printers with success. With powdered plastic, it's possible to drive a much smaller screw and still have even extrusion. Alternatively, some variety of closed-loop feedback using an optical sensor to measure the amount of plastic pushed out may also work. Also, will powdered plastic be any cheaper than filament?
The second problem is a little less tractable. You can switch to a smaller screw with a higher speed, but the total amount of energy required to form the plastic is fairly large. The resin pellets come full of air pockets and bubbles. I believe the high pressure within the tube is what helps you get nice and even extrusion. In other words, it might be more economical and easier to deal with if you formed the plastic into an even filament with a larger machine, then printed with it. Hence the status quo today with a separate filament extruder :)
BTW, you might want to check this out: [reprapdelft.files.wordpress.com]
They have a drawing on page 23 of a putative direct extruder that you could potentially try building!