Silvering. Water is essential for human and plant life but can prove inconvenient for thermoplastics. Some materials (polyethylene and polypropylene) are fairly immune. However, others (polyesters, polyamides and polycarbonate) can, in service or storage, absorb water in varying amounts (up to 3 % in the case of PA6) with corresponding changes in dimensions and mechanical properties. I recall a kitchen clock with polyamide gears which worked beautifully anywhere except a high humidity kitchen environment where the gears swelled and jammed.
Water can also do less obvious damage. Polyamides, polyesters and polycarbonate undergo chain breakdown (hydrolysis) at high temperatures in the presence of water, even at ppm levels, with significant deterioration in mechanical properties. Subjecting granules to rigorous drying is always sound advice. Dehumidifying driers can achieve moisture levels of less than 20 ppm. However, this is only if temperature, times and other advice are adhered to. I remember troubleshooting at a company which had the foresight of acquiring the latest dehumidifying drier. However, nobody had thought to change or recharge the desiccant.
Even moisture adsorbed on the surface of granules, which have been transferred from a cold store to a warm, humid process area, can give problems. Likewise condensation on a cold mould cavity is another source of moulding faults. Whether the moisture is absorbed, adsorbed or condensation, it is gasified by the hot melt and ends up as fine bubbles, elongated in the flow direction. The high surface area of the bubbles produces high reflectance visual faults. These are quaintly known as ‘silver streaks’, ‘mica marks’ or ‘splay’.
Ignore drying advice at your peril!