Material Drying

Certain materials are hygroscopic; i.e. they absorb moisture from the atmosphere.If appreciably damp materials are injection moulded, the resulting components will exhibit splash, splay or mica marks caused by the bursting of bubbles of steam and other volatiles as the melt flows across the mould. Some materials, for example polycarbonate or the thermoplastic polyesters, suffer severe embrittlement if processed even with very low moisture content that is insufficient to give any trace of splash marks. Hence these materials should always be thoroughly dried before moulding.
Table below gives an indication of the temperatures and drying times needed for a range of thermoplastics.

Drying times

Material    Temperature (C)   Time (h) Method

Nylon 6                     80                    3–4         Vacuum oven
Nylon 66                  80                   3–4           Vacuum oven
Nylon 11                  80                   2–3           Vacuum oven
Acrylic                       70                    3–4          Air circulating oven
Polycarbonate   120                   4–8           Air circulating oven
ABS                              75                   2–3          Air circulating oven
PPO                             90                    2               Air circulating oven
GF PPO                    110                  2               Air circulating oven

Under adverse conditions, (i.e., when the material is exceptionally wet, or when the drying equipment is not particularly effective), drying times may need to be increased.

Ideally Nylon should be dried in a vacuum oven to avoid the risk of degradation causing discoloration and possibly embrittlement due to reaction with oxygen in the atmosphere.

When using air circulating ovens or driers it is strongly recommended that the air being circulated is pre-dried by passing it through a dehumidifier.
It should be borne in mind that some materials that, in theory, may need pre-drying may be used straight from the container. This is only true, however, when they are purchased from a reputable supplier in undamaged sealed containers, and providing
they are used quickly and the humidity in the moulding shop is low enough. If in doubt about drying requirements, consult the material manufacturer for expert advice.

Some filled grades of polymers, which do not normally absorb water, sometimes need drying before moulding.

With materials such as ABS and the acrylics, drying may not be needed if a vented barrel machine is used.