Why is moisture a problem?
Some polymers absorb moisture from the air. During processing this moisture vaporises at high temperature and creates two kinds of problem: visible surface defects and permanent damage to the molecular structure.
In materials such as polyester (PBT), polycarbonate (PC) and polyamide (PA), moisture causes a hydrolysis reaction at high temperature. This reaction shortens the polymer chains — molecular weight drops and the material permanently loses strength.
The critical point is that hydrolysis damage is irreversible. Even if the material is dried and reprocessed, the lost strength does not return.
Symptoms
Typical signs of insufficient drying include silver streaks on the surface, bubbles, matt patches, brittleness and lower than expected impact strength.
Transparent parts can show haze, and glass fibre reinforced parts can show pronounced fibre marks on the surface.
Drying correctly
Drying temperature and time vary by material and grade; these values are stated in the relevant technical data sheet and should not be estimated. Over-drying can also damage the material (colour change, yellowing).
For hygroscopic engineering plastics, hot air dryers are often insufficient; desiccant (dry air) dryers should be used. The dryer's dew point performance should be checked regularly.
Dried material reabsorbs moisture quickly if left open. Material should therefore be conveyed from dryer to machine in a closed system, with waiting times minimised.
Storage
Unopened original packaging provides a moisture barrier. Opened bags must be sealed tightly and stored in a dry, cool environment.
Material taken from cold storage should not be processed directly; condensation can form on the packaging surface. Allow the material to reach room temperature before opening the packaging.
Summary
Drying is not a formality before production but a process step that directly determines material performance. Always use the TDS of the relevant grade for the correct conditions.
We share the official TDS for every product we supply, and our technical team can support you on drying and processing conditions.
This article contains general material information. Numerical technical values vary by grade and appear only in the official technical data sheet (TDS).