In Simplify3D you control print speed as mm/minute and in Cura, it's mm/s, so there's a 1:60 difference there. In Cura, if I set the speed too high, it will warn me that I'm trying to push too much plastic through the nozzle, which seems pretty smart.
I guess the relationship between print quality and speed comes from at least two factors:
- Mechanical accuracy of the printer movements at high speed.
- The ability of the hot end/extruder to melt and output enough filament to fill the required volume.
For this post, I'm just interested in the extrusion volume speed.
If the nozzle width is a constant and we assume that the path is always the width of the nozzle width, then the extrusion volume needed to fill the path depends on the length of the trace and the height of the slice. The length of the trace per time is what you can control in Simplify3D and Cura as the print speed, but it seems this completely ignores the layer height (except maybe in the warnings that you get from Cura).
So if I'm printing 0.2mm layers on my Wanhao i3 at a maximum of 2600 mm/minute with good results, I should be able to print at higher speed when using 0.1mm layers. In fact, the Simplify3D default for my printer is 3600 mm/minute.
Let's make this a feature request:
Instead of controlling extrusion speed with just distance/time, add an extrusion volume limit number as well and cap the extrusion speed to the slower speed of these two. I might then leave the extrusion distance/time setting at 3600 mm/minute and set the extrusion volume setting to 0.4 mm (nozzle width) * 0.2 mm (layer height that works well at 2600 mm/minute) * 2600 mm/minute = 208 mm^3/minute (we could add a pi to the formula to make it more accurate, but the point is to include layer height in the calculation). So using 0.2mm layers, I would get a 2600mm/minute extrusion speed (plus modifiers for skin & infill etc). And at 0.1mm, the 3600 mm/minute limit would have kicked in because the volume extrusion limit would have allowed 5200 mm/s.
I guess the bottom line is that I think that layer height and extrusion speed should be connected somehow because the printer is extruding volume and not flat surfaces. The Wanhao i3 is in my opinion a very good printer that is restricted in speed by the ability of the PTFE-lined hot end to melt plastic.
If the hot end can not melt enough plastic, I suppose you end up with more pressure from the filament feeder, which could interfere with retraction settings as well -> you should end up with stringing when the excess filament has a chance to ooze through the struggling hot end?