Essentially, this is an issue only because we are melting plastic, and extruding it at temperatures above the boiling point of water.

When plastic is melted with moisture inside, the water molecules turn to steam, and create popping noises, and visible gaps or bubbles during the printing process. Absorbed moisture has been shown to act as a plasticizer, reducing the glass transition temperature and strength of plastic – which is a reversible effect.

This causes a ‘loosening’ of the molecular attraction which causes a reduction in mechanical properties and an increase in fluidity. The equilibrium moisture content (EMC) of a hygroscopic material surrounded at least partially by air is the moisture content at which the material is neither gaining nor losing moisture.

The value of the EMC depends on the material and the relative humidity and temperature of the air with which it is in contact. Because of all this, with almost all the high-performance filaments, like PEEK, PEI, PPSU, and PPS, the filament must be dried before processing (extruding)