No big reason. The two nozzles I have are both to the standard design (except one is aluminium) so I have made extruders to mate with them rather than change everything at once.
I don't think they cause any problems. In fact I used the brass one (made by Adrian!) for a long time and it worked well, so I concentrate on changing bits that I have had problems with.
There may be some temperature drop but the aluminium should improve that. Also a bit more length to push viscous plastic through, but as it is 3.5mm bore it should be little resistance compared to the exit hole.
The copper welding tips I have to try next are longer but they will shorten the path because they screw into the heater instead of the other way round. If they work I will switch to those as they are much easier than making one from scratch.
The original extruder design worked fine for me but kept breaking down. My motivation for changing the design is to make it last longer, and if possible easier to make.
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