In FDM printing, liquid plastic is laid down layer by layer to build up a 3D model. The print head does this by moving in a pre-defined pattern that is generated from the 3D model, and squirting out a precise amount of plastic which adheres to the layer underneath.
This pre-defined pattern makes assumptions that the plastic squirted out is successfully being laid down in the right place to build up the model. Any time those assumptions don’t hold true, things go wrong, and the model is not going to print well.
Amongst all the possible ways for prints to go wrong, a common one is sagging. This is where an FDM printer attempts to print an unsupported overhang or an arch, but the spanning distance is too wide, allowing the molten plastic to sag. The next layers printed on top of the sagged span is therefore unsupported by the layer underneath and so in turn sags and becomes misaligned. This has a knock-on impact on several if not all of the layers above the span.
Another way prints can go wrong is lifting or warping. This tends to happen at the corners of the print where adhesion to the print bed is weak, allowing thermal stresses caused by the cooling plastic to pull the corners of the print up.
Sagging or lifting involve situations where part of the print is no longer where the printer expects them to be, and causes deformation of the print and some localized spoiling, but by and large the rest of the model prints successfully.
In the extreme case, the entire print separates from the print bed. A detached print means none of the layers underneath the print head are where the printer expects them to be, and instead of laying down neat layers of plastic, the printhead is actually just squirting molten plastic into thin-air, and trailing a tangle of plastic worms behind it in what is known in the industry as “3D printer pubes".
Needless to say, 3D printer pubes mean the entire print is ruined, and the model needs to be printed again. With print times for large prints running several hours, this is a frustrating problem.
However, it can get worse: it’s possible for the detached print to stick to the bottom of the print head, causing it to collect the molten plastic instead of just trailing pubes. In this event, the molten plastic becomes an ever-growing ball that eventually engulfs the entire print head and hardens into an impenetrable amorphous blob I like to call the “3D printer homunculus”. In this extreme case, not only is the print ruined and time wasted, but parts of the 3D printer needs to undergo extreme servicing and/or replacement.
Therefore it’s an extremely big deal for the 3D printed model to be stuck firmly to the print bed; and print-bed adhesion has a lot to do with how well the first layer is being laid down. If the print bed is not levelled either physically or digitally, then the print head is not printing at the correct height over parts of the bed on the first layer, which can cause two types of problems:
If the print head is too high off the bed, then the plastic is not being squished hard enough against the bed for it to stick, and can result in lifting or detached prints as described above.
If the print head is too low off the bed, the print head can crash into the bed, damaging the nozzle and/or the bed.