PETG is an abbreviation for Polyethylene Terephthalate (with a glycol modification) which is one of the most common polymers used today. It’s used to make water bottles, food packaging, and countless other common plastic items. As a 3D printing filament, PETG plastic has proven its worth as a durable material that is easy to use. Figuratively speaking, it combines the most useful characteristics of ABS filament (the rigidity and mechanical properties for functional parts) with the ease of printing that PLA filament affords. Kind of a “best of both worlds” scenario.
PETG is hygroscopic, which means it will actively absorb moisture from the air. For this reason, PETG plastic should be stored in a cool, dry place, and dried if exposed to humid air for too long. What constitutes “too long” depends on the relative humidity in the air, but when it comes to 3D printer filament, it’s best to err on the side of making the filament “too dry” rather than allowing it to be a “little wet.”
Printing wet PETG can lead to hydrolysis which will permanently alter the filament on a molecular level, making it significantly weaker than it would be if it were printed dry.
Vacuum-sealed bags and desiccant packs ensure that the filament is exposed to as little moisture as possible. Sometimes a bag can get punctured and lose the vacuum seal, but so long as the whole thing is packaged with a desiccant pack that should be sufficient to absorb enough of the moisture for the filament to print properly– at least until unpacking.