Saturday, March 08, 2008


Extruder temperatures

I have made some measurements of temperature at various points around my extruder. These are the temperatures I get with my software set to 200°C :-

They should be fairly representative as my latest build is now pretty much faithful to the latest design.

One point of interest is that the heater, and the inside of the heater barrel, are quite a bit hotter than the nozzle, where the thermistor is controlling the temperature. That begs the question of what the actual temperature of the extruded filament is, as that is the important thing to control.

I seem to be getting similar results with temperatures that are 20°C below the values I was using with my single piece nozzle. That would suggest that the filament exits at pretty much the temperature of the heater barrel rather than the nozzle. Mounting the thermistor on the barrel rather than the acorn nut is probably a better option.


Do you wrap insulation round the barrel? We use fibreglass (cut from the roof insulation in my attic...), and I would imagine that would even out the temperatures throughout the entire assembly (and use less heating current).
I haven't use fibreglass yet, I just used a plastic tube to shield the whole nozzle/barrel from my fan.

Without a fan insulation makes little difference I think but it is essential if you extrude with the fan blowing.

Covering the heater with insulation would use less power but the temperature difference between the heater and the nozzle would still be the same unless you insulated the nozzle as well. Losing heat from the heater pushes up the power but doesn't increase its temperature. Losing heat from the nozzle increases the temperature drop for the heater to the nozzle, so raises the heater temperature.

Perhaps a PTFE cover over the nozzle is a good idea. It might also stop the molten plastic from sticking, making it easier to wipe. With the new shape it would be easy to turn such a matching cover.

I am thinking about making a copper nozzle with the heater wound around it and a stainless steel barrel with a heatsink at the top.

SS is a very bad conductor and CU is very good. Both have roughly the same thermal expansion I think so well matched mechanically.

It would reduce the amount of molten plastic as it would melt half way down the SS and so reduce the overrun. It would also reduce the temperature that reaches the PTFE which would just become a spacer and a conduit for the filament and would not need to take any mechanical strain.

I think the result would cover the full temperature range for thermoplastics and also be mechanically rigid.
Thermistor should probably go underneath the insulation.
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