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New extruder build questions

Posted by Colin K. 
New extruder build questions
May 24, 2008 03:30PM
Now that I have almost all the materials and drawings gathered and sorted I am ready to get serious about building my extruder head. However reading around here leaves me with a couple of unresolved questions:

1. I was thinking of making the drive screw with a slight taper on the threaded segment so that it engages gradually to reduce drive torque. I'm working with a lathe so I can do this on centers by offsetting the tailstock but not sure whether this is likely to be worth the trouble?

2. I have brass and water hardening tool steel on hand. Is a brass drive screw better in some applications or simply good enough for CAPA and easier to machine? If I use the tool steel is it worth hardening it?

3. The heater -> PTFE barrel -> clamp interfaces seem to be a problem area when running higher temps for HDPE and PLA. As I understand it, the issue here is you want to be able to hold temperature in the heater but keep the section above that nice and cool and the temps for HDPE and PLA are at the upper limit of what you want to do with teflon, which is otherwise a great thermal barrier.

Anyway, this got me to thinking about a multistage design. First, the heater barrel, made out of aluminum. Screw this into a steel barrel, and then screw that into the teflon barrel. optionally add a heatsink at the steel barrel. Putting the heatsink right above the heater seemed to me like a problem since it would draw a lot of heat away from where you want it, while the steel barrel would blunt the temperature gradient before it gets to the more sensitive teflon. Or maybe I'm just reinventing the wheel?

4. Flex drive shaft on the screw: this seems like another weak point and from what I've read in other threads, is not really required for any of the polymers currently being used?

Re: New extruder build questions
May 24, 2008 06:39PM
1. Yes probably a good idea. I found that removing most of the thread reduces the amount of torque required. My drive screw is shown in the second picture on this page: [hydraraptor.blogspot.com]
Hi Colin,

More extruder tweaks here: [hydraraptor.blogspot.com]

2. I am using stainless steel with stainless steel bearings and the bottom bearing land is starting to wear now that I have moved all the force on it. Probably better to use hardened steel and brass bearings.

3. I tried that here :- [hydraraptor.blogspot.com]

But it didn't work very well as described in the article. I haven't completely given up on the idea yet though.

4. No it is not required for HPDE, PCL, PLA or ABS. They will all bend enough to get past the motor.

Regards, Chris

Well, so far I have turned roughly two feet of stock into snapped tapered drive screws. I have them available in aluminum, brass, W-1 drill rod, and 303 stainless :p

What I have been doing^h^h trying to do is turning a taper that goes from full M6 diameter down to 3mm over the central 32mm section of the screw, and then cutting a thread on it. I'm actually working in closely-equivalent inch/metric hybrid since all my tooling is imperial (pls. hold the Mars lander jokes...) but you get the idea.

The main thing with this approach is that you get a fully-formed thread over the whole length, which should be optimal in terms of engaging the thread against the feedstock.

Anyway, somewhere near the end of the thread-cutting, I always lose it as the workpiece climbs up over the bit and snaps itself in the 3mm section. The brass one was the closest I came to success, and probably would have worked but I took a finishing pass and stopped the feed too late and crashed the bit into the shoulder, which of course snapped it.

If I had a 3-point steady I'd have used that, but I don't, and don't really feel like trying to make or buy one right now, so I'm probably going to go back to a piece of threaded rod for the time being.
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