What is the most likely cause of inconsistent layer adhesion in FDM printing besides bed leveling?

Besides bed leveling, the most likely cause of inconsistent layer adhesion in FDM (Fused Deposition Modeling) printing is incorrect nozzle temperature. The nozzle temperature determines how well each layer of molten plastic fuses to the layer below. If the nozzle temperature is too low, the extruded filament will not be hot enough to properly melt and bond to the previous layer, resulting in weak layer adhesion. This is because the polymer chains don't have sufficient thermal energy to intermingle and create strong bonds. If the nozzle temperature is too high, it can lead to other problems such as stringing, warping, and degradation of the material, but insufficient temperature is a more common cause of poor layer adhesion. Other factors, such as incorrect printing speed and inadequate cooling, can also contribute to layer adhesion issues, but nozzle temperature is often the primary culprit after bed leveling has been ruled out. A slight increase in nozzle temperature within the material's recommended range can often significantly improve layer adhesion.