Plastic pellets for 3D printing need drying because they contain moisture. But what is the reason they actually contain moisture?
Plastic pellets for large format 3D printing can contain moisture due to the fact that they can be hygroscopic materials. A plastic pellet dryer would be needed to extract any moisture from the plastic pellets. Unless the material is non-hygroscopic, in this case a plastic pellet dryer would be redundant.
Non-hygroscopic materials vs. hygroscopic materials
Non-hygroscopic polymers are those that do not absorb moisture from the environment, while hygroscopic materials are those that readily absorb moisture from the atmosphere. The thermoplastics that are commonly used in large format additive manufacturing are either non-hygroscopic or hygroscopic. Knowing this fundamental aspect about your materials will help you make prints of a higher quality.
Non-hygroscopic polymers
These polymers have low water absorption and are used in applications where moisture resistance is important, such as in subsea or the automotive industry. Examples of non-hygroscopic polymers include polypropylene (PP) and polyethylene (PE). Both these materials can be very useful for large-scale additive manufacturing (LFAM). The PE also happens to be the material that you use to purge your extruder, also known as high-density polyethylene (HPDE).
Hygroscopic materials
These polymers, on the other hand, can absorb moisture from the air, which can affect their physical properties and performance. Examples of hygroscopic materials include polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polyetherimide (PEI) and polyethylene terephthalate (PET). It is very important to dry these materials before using them to print high-quality parts. If you don’t dry the materials beforehand, the moisture will reach its boiling point in the extruder, it will result in air bubbles in the printed parts. Also known as porosity.