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Curious if anyone else had tested the ideal layer height for different nozzle sizes to produce stronger water tite parts? In house we printed a series of 15" tall rockets using each filament available. Each print was in case mode (single strand of filament) and printed at ~100 MMS. When using .25mm, .4mm, .6mm and .8mm nozzles we found the structures held together the best and were always watertite when printed at 60% nozzle width. So for a .4mm nozzle 240 micron layer height. For the .8mm nozzle 480 micron layer height. These tests were run when the extruder was calibrated and the extrusion width was calibrated to the filament. I run the majority of my prints on a .4mm nozzle at 240 micron and print orders for 250 micron at 240 as a result of this.

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Another fact that I find also affects part strength is (even for a same filament spool) hotend temperature and whether or not it’s adequately cooled.

I haven’t measured the exact percentages in a methodical manner, but I know that if I print in the top 10 degrees of the spool’s range, I usually get a smoother finish and better layer adhesion, all other parameters being equal.

Also, depending on your printer, 240 microns might give better adhesion than 250 because of the stepping precision in relation to the threading on your Z-axis rods. I’ve seen it on a K8200 and a MendelMax at the fablab, changing the layer thickness so the Z-motors’ steps align better with the height gives better results. I assume this is because if the Z motors have to microstep, there’s a little fluctuation which makes the nozzle’s height oscillate a fraction of a millimeter… I’ve never had that problem because I’m using a Kossel Mini, and Z-stepping is done on all three axes.