Moisture Content

property

The amount of water present in plastic resin, typically expressed as a percentage by weight, which must be controlled to prevent processing defects and maintain product quality.

In Simple Terms

Moisture content measures how much water is absorbed in plastic pellets or powder. Most plastics naturally absorb moisture from air, and too much water can cause bubbles, weak spots, or poor surface finish during processing like injection molding or extrusion.

Why It Matters

Excessive moisture causes steam formation during processing, leading to splay marks, bubbles, reduced mechanical properties, and poor surface finish. Different resins have specific moisture limits that must be met before processing to ensure quality parts.

Technical Details

Measured by methods like Karl Fischer titration or loss-on-drying techniques. Hygroscopic resins like polyamides require moisture below 0.1-0.2%, while polyolefins tolerate higher levels. Drying temperature and time vary by resin type - PA6 requires 80-100°C for 4-8 hours, while PC needs 120°C for 4-6 hours.

Real-World Examples

PA66 injection molding preparation

Nylon pellets must be dried to below 0.1% moisture content before molding to prevent hydrolysis and ensure proper mechanical properties in automotive components

PET bottle preform production

PET resin requires drying to less than 0.005% moisture to prevent acetaldehyde formation and maintain clarity in beverage bottles

Resin storage and handling

LDPE pellets can typically process with up to 0.05% moisture, but proper storage in sealed containers prevents contamination and processing issues