Hey everyone. So I am in the process of designing a pellet extruder for the Printrbot Big-E we have at River City Labs. @Brook_Drumm I found some design inspiration for the pellet extruder from many people including @Richard_Horne , @Steve_Wygant , along with various filament extruder designs and the E3D V6 from @Sanjay_Mortimer1 and @Joshua_Rowley . I was curious if anyone had any tips for me. I can think of e few problems I might run into. One thing is that it is all made out of 6061 aluminum, and I have heard that molten plastic likes to stick to it which could jam up the auger. I have a feeling that I will have to adjust the auger height so that it isn’t in direct contact with the molten plastic. I know Richard used slightly modified J-head, so that really had a small melt zone. Filament extruders, and the pellet extruder on the PartDaddy, seem to have a much longer melt zone compared to that of a normal hotend usually with multiple heaters and melt zones. The hollow part at the top of the hotend will actually house 2 thrust bearings with a shaft collar in between them. That should allow me to maintain the height of the auger in the barrel, even when disassembling the extruder. I was also planing on using a Volcano nozzle from E3D. One of the things I am worried about is the long 3mm barrel length of the nozzle, and the 3/8" to 3mm cone at the end of the auger. I would love any feedback you could give me based on your years of expertise with hotends. Thanks! Specs: 6061 aluminum with 3mm Volcano nozzle, 3/8" auger (I would have used metric, but it was easier to find imperial augers along with the imperial tools for the lathe here in America), 5:1 geared nema 17, and 2X 25 or 30w heaters in parallel. I should also mention that the actual auger is much longer than the one shown. I hope to eventually put it onto a much larger printer and use locally recycled plastics. Any thoughts? Did the quick thermal analysis with Fusion 360 
Get a rod of Teflon and turn a funnel out of it for near the transition point, then you won’t have sticking issues.
Here is feet of the stuff up to 6" in diameter - http://www.mcmaster.com/#teflon-(made-with-ptfe)/=12ephmf
Yeah, I had considered making it out of peek like a J-head, but maybe I could add a teflon or peek insulator for the auger part. Kind of like the teflon tube in the v6. I will likely have to do that in some fashion, all though I know most filament extruders get along with just using a steel barrel. Granted it doesnt stick to steel like it does aluminum.
You could polish any steel with chromium well enough that it wouldn’t stick at all, it’s just that teflon will handle the temperatures you will be using and it’s way easier to turn and polish than steel… it’s also inherently slippery at a molecular level.
Ok. Back up. You NEED the plastic to stick to the walls. That’s how screw augers advance material. If the outer tube were frictionless, the pellets would just spin in place in the auger grooves and wouldn’t be pushed down.
Are you trying to just hack something together that pushes plastic, or are you wanting to make something high performance for good volume control? Screw extruder design is a complex field. For example, typically the mechanical power input via the screw working semi-molten plastic provides about half the heat to melt the pellets, and the barrel heaters provide the other half. The balance is necessary to avoid air bubbles and unmelted pellets and burnt pockets. To do that properly, you need fairly specific screw geometry. One zone feeds pellets, one zone shears and melts the pellets, one zone generates extrusion pressure. A drill auger is the wrong shape for this. It’ll push plastic, but not very accurately.
is the heat conducted by the screw going to be a problem?
@Ryan_Branch Lots of advanced coatings (TiN, DLC,etc) available for the inside at a lower cost than you might think.
Making the whole hot end of aluminium though, not really recommended, unless you use something really similar to the injection industry - an induction heater (ie: make the auger out of magnetic steel) so that the auger is what heats instead of the aluminium.
I’m pretty worried about using a drill bit as auger though, a custom one would be 100% better (compare the profiles between a real injection auger and this one).
The main issue with an auger/pellet based extruder is retracts/controlling the flow. How are you planning to address it? I could see this used for “vase-mode” (ie:continuous line printing) but for normal printing I’m not sure it will be feasible without lots of stringing.
Instead of step-drilling like that, swap to a 60 degree drill. E3D said it made a difference back in 2015 MRRF
@Ryan_Carlyle I am not trying to make a perfect extruder. It really just needs to function. There are plenty of designs out there that use a drill auger with pretty good success. I should note that this is a budget design aka less than $200. If I had more money to put towards it than I would look into custom machining a custom auger like that used in an injection molding. That is a good point about inside of the barrel. It does need to have some friction, but just enough to keep the pellets moving towards the heater. I am not designing a injection molding extruder with a heated auger that converges near the end which melts and mixes the molten plastic. Mine will simply advance the pellets towards the hotend.
@Ryan_Carlyle is there a resource you could point to about designing these augers?
@Jonathan_Lussier pick up a copy of Plastics Extrusion Technology Handbook by Levy. Good intro to the subject.
@Jonathan_Lussier A custom auger would be better, but a lot more expensive. This is a budget $200 pellet extruder. The retraction will work like a normal hotend for now. The auger will reverse to relieve the pressure. V2 may have a needle valve in the nozzle, but for the first revision, I know that I will have a lot of stringing. I want to print furniture sized pieces. The Part Daddy does use a small injection molding auger, but I don’t think they have a needle valve on their extruder. They have so little stringing it’s amazing! Looking on YouTube and some older reprap posts, I know that using a standard auger bit can make a working pellet extruder.
@ThantiK Yeah, I designed it for the bit I had on hand. I think it is just the standard 118° tip. I will definitely look into getting a bit with a less harsh angle.
For scale, the fan for the heatsink is a 60mm fan.
Phenomenal work. Where can we find out more?
Thanks. @Joseph_Delong I will post updates one here when I get more done. The lathe has been down at our Makerspace, but once it’s back up, I am going to turn the hotend. Still finalising the rest of the extruder design as well. I have most of the parts already though, so hopefully it won’t take me too much longer. I also might post the progress to my website or http://hackaday.io. Not sure yet.
have you considered nickle plating it to handle the sticking possibility. They do that with some hardened brass nozzles. hmm, I wonder what effect anodizing would have on the stickiness. It would make the surface more pitted but possibly more wear resistant.