What diagnostic test confirms a nozzle clog beyond visual inspection in FDM?

Beyond visual inspection, a cold pull (also known as an atomic pull) is a diagnostic test that confirms a nozzle clog in FDM (Fused Deposition Modeling). A cold pull involves heating the nozzle to a temperature slightly above the filament's glass transition temperature (the point where it becomes pliable but not fully molten), manually feeding the filament into the nozzle, allowing it to cool significantly, and then quickly and forcefully pulling the filament out. This process ideally pulls any accumulated debris or partially molten material out of the nozzle along with the filament. The shape and appearance of the pulled filament tip can reveal the presence of a clog. A clean, tapered tip indicates a clear nozzle, while a deformed, bulbous tip with embedded debris confirms a clog. This test can be repeated multiple times with the same piece of filament until the pulled tip comes out clean, indicating that the nozzle is clear. The key is to pull the filament out at a temperature where it's solid enough to grab onto the clog but still pliable enough to conform to the nozzle's internal shape.