Why is the first layer critical in 3D printing?

The first layer is the foundation of the entire 3D print. A badly placed first layer = failed print. Key parameters: Z-offset, bed temperature, speed.

First layer parameters

Speed: 20-30 mm/s (slow)
Height: 0.2-0.3 mm (thicker than normal)
Flow: 100-105 % (slightly more)
Cooling: 0 % (to stick)
Z-offset: paper sheet thickness

Common mistakes

Too high - filament won't stick to the plate. Too low - nozzle carves into the plate (elephant foot, clogged nozzle). Cold bed - part lifts off.