The First Layer Decides Everything Above It

Heat the bed and the nozzle to printing temperature before levelling. Metal expands, and a plate levelled cold moves as it warms — so a cold level is a level for a machine that is not running. The paper test is the traditional method and works: slide a sheet of ordinary paper under the nozzle and adjust until you feel a light drag but can still move it. That is roughly 0.10 mm, which is the batch-124 figure for printer paper arriving in a different workshop. Do all four corners, then the middle, then go round again. The first pass changes the others, which is exactly the compounding problem the marking-out rungs warn about — two passes is the minimum and three is common.

Watch the first layer go down and judge it by its SURFACE. A good first layer is slightly squashed: the lines are flat-topped and just touching their neighbours with no gaps between them. Round lines with visible grooves between them mean the nozzle is too high. A translucent, scraped-looking surface with ridges pushed up at the edges means it is too low. Both are obvious once you have seen them side by side and almost invisible if you only ever see one. Most printers allow live adjustment during the first layer — babystepping the Z offset a few hundredths at a time. Change it in small steps and let a few centimetres print between changes, which is the change-one-thing-at-a-time rung with a very short feedback loop.

Fingerprints are the commonest adhesion failure, and they are invisible. The oil from one touch is enough to stop PLA sticking to a spot the size of a thumb. Wash the plate with warm water and detergent, or wipe it with isopropyl alcohol, and then handle it by the edges. A plate that has been 'cleaned' with a dry cloth has usually just spread the oil around. Different surfaces want different treatment. Smooth PEI likes to be clean and nothing else; textured PEI grips better cold; glass often wants a thin film of adhesive. Adding glue to a surface that did not need it is a common way to make adhesion worse rather than better.

A hotter bed helps adhesion up to a point and then starts causing 'elephant foot' — the bottom few layers bulging outward because the plastic there stays soft under the weight above. A slower first layer helps a great deal and costs almost nothing, because the first layer is a small fraction of the print. 20 mm/s is a common setting and half the speed of the layers above. Print a small test square before a long print, not the long print itself. Twenty minutes of test squares saves a twelve-hour failure, and each square should change only one setting — which is what makes the results mean anything.