I've noticed this on almost ever print I've ever had. On the initial first line that clears the extruder nozzle tiny little bubbles/craters seem to form on the line. While I don't think these are causing any issue with my prints I'm curious to know the reason why they form at all.
Is this due to water absorption in my filament that turns to steam, which then bursts through the molten plastic? Is it due to air bubbles in the filament that are cause by the manufacturing process of the filament? Or is this more an indication that my nozzle is damaged or clogged in some way?
This image was made using ABS plastic and a heated build plate. I've noticed these same 'bubbles' appearing using PLA, and Nylon.
Edit: Nozzle temperature 240°C, build plate temperature 150°C, Nozzle diameter 0.4 mm, filament diameter (measured 1.75 mm) retraction distance 1.7 mm. Using the Makerbot Desktop Slicer.
The first line that my printer extrudes, where I'm seeing these 'bubbles' is a nearly full line. Makerbot starts from the right side of the image, extruding to the left.