Material Preparation
A hygroscopic plastic is a material that draws the moisture in the air into the granule; the moisture is held not on the surface but between the polymer chains. PA, PC, PET, PBT, ABS, PMMA and TPU are in this group. At processing temperature this water evaporates and causes silver streaks and bubbles, and in materials such as PET and PC it breaks the chain (hydrolysis) and leaves the part brittle. That is why they must be dried with a dry air dryer.
Non-hygroscopic plastics (PE, PP, PS) carry moisture only on the surface; a hot air dryer is enough to remove surface moisture. With hygroscopic material, however, hot air alone does not work: if the air is already damp, it cannot draw out the water inside the granule. Air with a low dew point — that is, dry air — is needed.
Hydrolysis is the most expensive drying oversight because it is invisible: in a PET preform the intrinsic viscosity falls and the bottle cannot pass the burst test; in a PC part the loss of strength only comes to light in the field.
PA6/PA66, PC, PET, PBT, ABS, PMMA, TPU, PPS, PEEK. The temperature and time values are given with sources in the material processing values table.
Usually not; a hot air dryer is sufficient only if there is surface moisture (condensation, open storage). A dry air dryer is an unnecessary cost for these materials.