What is considered a 'good' layer height for the first layer? Should it be relatively smaller than the other layers, or should it be considered relative to the nozzle diameter? Can the community suggest an appropriate layer height for the first layer with justification?
As most answers here: "That depends on your preference and needs"
- If you prioritize bed adhesion you might want to compensate for an uneven bed surface/ level and maximize the layer height, which is around 60/75% of your nozzle width. (typical .3mm layer height for .4 nozzles). Couple this with a 200% initial layer width for optimum stickyness.
- If you prioritize for quality you might just use the same layer height as the other layers, although the layer height difference is hardly ever noticeable.
I can't think of a reason to make the first layer height lower than the other layers. It would just make the bed leveling more difficult and add no detail.
It all depends on the bed flatness you can ensure. Do you have a super flat bed? Then you can use the first layer identical to other layers. You don't trust or you can't guarantee flatness? then increase the first layer thickness up to 25%.
Keep in mind that "bed flatness" depends on three elements:
- intrinsic flatness (milled aluminum tooling plate or glass bed, vs rolled and wavy aluminum plate?)
- rigidity of the X and (for coreXY) Y rails/rods plus the weight of the print head (do you have thin rods or a heavy print head, which gets closer to the bed when in the center? or stiff rails and lightweight head which bends only by few microns?)
- availability and accuracy of the bed levelling (are you doing bed levelling? It can compensate for the previous effects if properly done).
Depending on the three factors you may need to print a thicker first layer to ensure that the printed layer is never too thin or too thick, or you may ignore the issue.
Ellis' Print Tuning Guide: First Layer Squish may help as well, after you decide which first layer thickness to use.
If the first layer is so thin the bed clearly shows through, your extruder is skipping steps unable to push enough filament, or the nozzle drags over the bed surface, you're too low.
If the cross-section profile of the extruded thread of filament is round, first layer of a print results in separate threads of filament with gaps in between or parts bulging, detached from the bed, filament doesn't form clear sharp corners where it turns 90 degrees, or becomes detached, you're too high.
The right height is when the filament is smeared somewhat flat, but still with noticeable thickness. And considering inaccuracy of printer rails, bed, drive trains, achieving such profile throughout the entire bed surface, never getting into 'too thin' or 'too thick' is what you strive for. Whether it's 0.07 or 0.13mm, doesn't really matter, what matters is it doesn't go below or above the range at any point of the bed.
A typical 0.4mm nozzle with 0.1mm first layer height is most likely to achieve this, but when in doubt, too thin will only result in dimensional inaccuracy and thinner bottom; too high will result in poor adhesion and may end up with the whole print detaching from the bed.