The skirt lifts and wraps itself around the nozzle. Lines get pushed into ridges instead of staying where they were put. Or the first layer goes down, looks acceptable, and peels off in one sheet when the second layer starts.
Answer first: on a machine that used to work, this is a dirty plate far more often than it is anything else, and washing it takes five minutes.
Make sure it is this and not one of its three neighbours
Four different faults present as "the first layer failed", and they need opposite fixes.
- Lines are round, separated, and sitting on top of the plate rather than pressed into it. That is the nozzle being too far away — see first layer too high, where the fix is downward.
- Lines are wide, translucent and ridged, with plastic squeezed up between them. The opposite error: first layer too squished.
- One region of the plate is perfect and another is not touching. The plate is not flat or not parallel to the toolhead's plane, which is uneven first layer and is not fixed by any adhesion measure.
- It stuck properly, then let go two hours later. That is a completely different competition and belongs on print detaching mid-print.
This page is for the case where the extrusion never bonds anywhere on the plate, from the first line.
What the bond actually is
There is no chemistry here for most materials. A build surface is microscopically rough, and a first layer holds because molten plastic is pressed down into that roughness and freezes there. It is a mechanical key, and it needs three things at once: enough downward squash to force the polymer into the texture, enough heat that it stays soft long enough to flow into it, and a surface with nothing in between.
Remove any one and the bond fails, which is why the cause list below is short but the failures look so varied.
Causes, most common first
- Contamination. Skin oil from handling the plate, mould release on a new sheet, or residue from a previous adhesive. This is the top cause by a wide margin on a printer that worked last week and does not work today.
- Nozzle too far from the plate. Usually after a nozzle change, a probe swap, or a firmware update that reset an offset.
- Bed too cold for the material. PLA is forgiving; PETG, ABS and the polyamides are not.
- First layer printed too fast. Plastic needs a moment in contact to key in, and a fast first layer on a high-speed profile does not give it one.
- A levelling mesh that no longer describes the plate. Covered separately under bed mesh drift, because it has its own slow-onset signature.
Wash it with dish soap, not alcohol
Isopropyl alcohol is the default advice and it is the wrong tool for the commonest contaminant. Skin oil is not a solvent problem; it is an emulsification problem. Alcohol evaporates in seconds and tends to redistribute a thin film of sebum across the sheet rather than lift it off, which is why plates "cleaned" with a wipe often print worse than before.
Hot water and washing-up liquid, worked over the whole surface, then rinsed and dried with a clean towel, removes it properly. Keep alcohol for what it is good at: taking off glue-stick residue and finishing a plate you have already washed.
Two things to avoid: do not use acetone on a textured powder-coated sheet, and do not abrade a smooth PEI plate with anything coarser than a green scouring pad, because the texture is the adhesion.
Setting the offset without guessing
Print a single-layer square patch rather than a real model, and adjust while it runs. You are looking for lines that have merged into a continuous sheet with no gaps between them, and a surface that is matte and even rather than glossy and rippled. Move in 0.02 mm steps and give each change a full patch, because the difference between correct and too low is smaller than most people expect.
Slow the first layer to about 20 mm/s while you are doing this. It removes one variable and it costs seconds.
The material makes this easy or hard
PLA bonds to a clean plate at 60 °C with no help at all. PETG bonds so well to smooth PEI that the opposite problem is the one to worry about. ABS and ASA want the plate at 100 to 110 °C and an enclosure, and will not cooperate without both. Polyamides and polypropylene do not key into PEI usefully at all — they need a surface chosen for them, which for polypropylene means literally printing onto packing tape of the same polymer.
The thing not to do
Reaching for glue stick before washing the plate. Adhesive on a greasy surface adds a second weak layer instead of fixing the first, and it hides the real cause behind an intermittent improvement. Clean, then set the offset, then get the bed temperature right — and only then consider whether the material genuinely needs a release film.
Also resist pushing bed temperature above the material's stated range to force a bond. It softens the base of the part for the rest of the print, which is how a first-layer fix turns into elephant's foot.