First Layer Fundamentals

Why the first layer matters most

Prusa calls the first layer the essential one, because it is the base of the printed object. Every later layer stacks on it, so if it lets go, the whole print fails. The setting that most directly controls it is the height of the nozzle above the bed during that first layer.

On FDM, the nozzle does not just lay plastic down. It presses each line onto the bed. That pressure flattens the round strand into a wider, flatter line that grips the surface and merges with the line beside it. Prusa's description of the target is a line that is flattened but not squished.

What a good first layer looks like

Prusa's first-layer calibration guide describes a correct result on a smooth sheet as an even surface with no gaps between lines and no ridges. In practice:

• The lines touch their neighbors along their whole length. A filled square reads as one continuous sheet, not separate strands.
• The top is flat and even. You can see line direction, but no line stands up above the others.
• The edges are straight and stay down on the bed.
• The same look holds across the whole bed. If one corner differs, the bed is not flat or level there (see Bed Tramming, Probing and Mesh Compensation).

Too high and too low

Nozzle	What you see	What happens
Too high	Round lines with gaps between them; on textured sheets, visibly round strands	The plastic is laid on rather than pressed in. Prusa notes the filament will not stick properly, so the print can lift or come loose.
About right	Flat, even surface, lines merged, no gaps, no ridges	Good grip and a flat base for the next layers
Too low	Ridges between lines, rough or scraped texture, edges curling up; at the extreme, little or no plastic comes out	There is no room for the plastic to flow out. Prusa notes this can leave the first layer or two with no extrusion and can clog the nozzle.

Too low is easy to mistake for success, because a crushed first layer sticks hard. Prusa specifically warns against fixing poor adhesion by lowering the nozzle and squishing the first layer; find the real cause instead.

Setting the height

Every printer has some way to fine-tune the nozzle-to-bed gap for the first layer. Firmware names vary: Prusa calls it Live adjust Z, many others call it Z offset (see Setting the Z Offset). Prusa describes Live adjust Z as setting the vertical distance between the nozzle and the print surface to give better adhesion, and says it should be used only to fine-tune that distance after the printer's calibration has done the main work.

1. Run the first-layer calibration or test print your printer's maker provides. It prints a pattern or a filled square designed to show the problems above.
2. Watch the lines as they go down and compare them to the table. Adjust in small steps while it prints if your printer allows it.
3. When the square looks right, let it finish, then inspect it closely. It should be one even sheet.
4. Note the value. Whether a mid-print change is kept differs by printer: Prusa says it is saved on its older i3 printers but not on its LoadCell printers (such as the CORE One and MK4). Check your printer's guide.

Everything else that affects the first layer

Once the height is right, these are the next things to look at, roughly in order:

• A clean surface. Fingerprints and grease stop plastic gripping. Prusa recommends 90 percent isopropyl alcohol for its smooth, satin and textured steel sheets with PLA, ABS and many other materials Maker spec, and dish soap with warm (not hot) water when adhesion fades over time. Prusa also says not to use acetone on its textured and satin sheets. Follow your own sheet maker's cleaning instructions; coatings differ.
• The right temperatures. Prusa's advice is to use the recommended nozzle and bed temperatures for the material. Use the filament maker's printed range, not a number from someone else's printer.
• A slower first layer. Prusa suggests printing the first layers at about 75 percent of normal speed Starting point.
• A slightly hotter bed for a new material. Prusa suggests trying 5 to 10 °C more on the bed when a new material does not stick Starting point, staying within the filament maker's range.
• The sheet seated properly. A removable sheet that is not sitting flat, or has debris under it, changes the gap in that spot.

If the first layer still lets go after all this, work through First Layer Won't Stick, which orders the causes from most to least common.