The first layer decides everything

Bed adhesion and levelling, and why most failed prints fail in the first minute.

The first layer of a print has to be pressed slightly flat against the build plate across the whole of its area before anything printed afterwards can be trusted, because every layer that follows only has whatever foundation the first one left behind, gaps and all.

A filament printer arrived as a kit, took a weekend to build, and then spent most of a year producing failures: brackets that lifted cleanly off the plate at one corner and kept printing into thin air, spools of plastic dragged sideways under the nozzle halfway through, parts that looked finished until the lightest knock sent them skating across the table, never having gripped the plate at all.

The mechanism behind a good first layer

A nozzle laying down the first layer of a fused filament print is not simply placing a string of plastic onto a surface, it is pressing that string into the surface hard enough to squash it slightly flat, and that squashing is what turns a round bead of filament into a wide, gripping ribbon with enough contact area to hold. If the nozzle sits too far from the plate at any point, the filament comes out and lands as a soft round strand resting on top of the surface rather than pressed into it, and a strand only resting on a surface is easy to peel away the moment anything pulls on it. If the nozzle sits too close, the opposite fault appears, since the plastic is scraped and dragged rather than deposited, and it never lays down cleanly enough to bond either. Levelling a bed means finding the height that avoids both faults at every point across the plate, not just at the one point where the check happened to be made, because a plate that is flat in the middle and a fraction higher at one corner will print a perfect first layer everywhere except that corner, and that corner is exactly where a print starts to fail.

The pastry comparison

Rolling out a sheet of pastry with a rolling pin held slightly higher at one end shows the same problem on a kitchen counter. Where the pin travels close to the board, the pastry is pressed thin and even, bonded into one continuous sheet. Where the pin lifts even a little, the dough underneath stays thick and loose, never properly flattened into the rest of the sheet, and it tears away along that line the moment the sheet is picked up. A nozzle sweeping across a build plate has to hold the same height everywhere the rolling pin does, not just where the cook happens to be looking, because plastic laid down under a slightly raised nozzle is exactly as unbonded as dough left thick under a slightly raised rolling pin.

The one number worth remembering

The gap that separates a first layer that grips from one that peels away is often smaller than the thickness of a business card, which is the reason so many printers are levelled by sliding a card between the nozzle and the plate and adjusting until it drags rather than slides freely. A plate tilted by less than that thickness from one corner to the other is enough to turn a print that welds itself down at every point into one that lifts cleanly at the single corner where the gap opened up first.

What follows from this

Because the first layer sets the foundation for everything above it, it is worth treating differently from the rest of the print, slower, and with the nozzle held a touch closer to the plate than later layers need. It is also worth watching rather than leaving unattended, since a failing first layer shows itself within the first minute, in strands that do not stick or corners that visibly lift, and a print that has started on a bad foundation practically never recovers on its own, because nothing printed above a loose first layer has anything solid underneath it to hold onto.