So this is a question that has been bugging me for a while.
When do I need to move to a smaller nozzle?
(I know the extruded filament will not equal the exact diameter of the nozzle. I’m just trying to keep it simple).
Thought 1:
Should I design my parts such that their sizes are a function of the nozzle diameter? Example, wall of the part is 0.5mm thick, nozzle is 0.5mm, one passes equals 0.5mm. Wall thickness of 0.75mm I will have issues?
Thought 2:
Now I could run the 0.5 nozzle and make a single pass or drop down to a 0.25 and make three passes.
Thought 3:
Using Slic3r I could leave the 0.5mm nozzle and adjust the size of the extrusion widths down to 0.25mm and let Slic3r generate the tool path to make a double pass.
I have done this in the past to force Slic3r to make multiple passes across thin areas were I was having problems with porosity.
What is the best process here?
Also, when going to a smaller/larger nozzle should the Retraction Length and Minimum Travel after retraction be adjusted with the nozzle change?
The nozzle diameter parameter should always be the physical opening size. I’ve never thought to vary according to wall thickness…
Extrusion width should always be equal to or larger than the nozzle diameter. There are guidelines on the internet, I think 120 - 150% of the nozzle is typical width.
The physical nozzle size is driven by the type of print and speed for me. A smaller diameter for detail and delicate prints (figurines, toys… etc.). The a larger diameter for functioning parts and general prototyping due to the strength. The larger the nozzle diameter, the faster the print (thicker layer height = fewer layers, wider extrusion = fewer perimeters… etc) and fewer clogs making it almost maintenance free. Most my prints are the latter so the .5mm is my default.
As for designs, I try to consider the nozzle diameter, but often times it boils down the purpose of the object. Again, most my parts are more “functional” than “pretty” and durability is more of a concern. I shoot for 1.5 - 2mm walls and let the slicer figure out how to print it. Slic3r seems to have a variable gap fill function which is nice and saves the printer from wear and tear. S3D not so good, but does the job.
Just to clarify when I was speaking about changing nozzle sizes I was referring to a physical nozzle not a setting the the program. Once the physical change was made then changing the printer profile to match the change.
“Extrusion width should always be equal to or larger than the nozzle diameter. There are guidelines on the internet, I think 120 - 150% of the nozzle is typical width.”
Well if this is what generates the best results then if you have a feature that is 0.375mm wide you should change to a smaller nozzle, correct? If not a single pass of the 0.5mm nozzle will yield a width grater than the 0.375mm you were shooting for.
Where is the Variable Gap Fill function in Slic3r located?
kcchen_00 is absolutely right, you will end up with an instable flow if you go below 1.2x nozzle diameter. If you want to try, 1.05x diameter is the smallest considered to be safe in extreme cases. This is especialy true for perimeter printing, for infill it’s not that bad because than the filament will be constrained by the surounding existing perimeters.
So yes, if you have to print a lot of parts with walls <0.5mm, this would be a reason to change the nozzle size.
Layers and Perimeters -> Quality -> Detect thin walls. Be careful, the gap fill functions is not fully functional in the latest official release. But if you compile the latest code from github, this feature is incredible. It will fill even a variable width small gap with perfection in most cases 
Do you have to heat the hot end when replacing a Nozzle on a new Hexagon that has never been used?
I’ve got a couple of Hotends fom Lulzbot with 0.35 Nozzles and a couple AO Hotends from Reprap Discount with 0.30 Nozzles and I’d like to replace the Nozzles with 0.4 and 0.5 sizes.
Kenny
If it is an extruder on a new TAZ, then yes heat it up because it was test printed at the factory.