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?
TIA