Meaning
Water content saturation in a polymer occurs when the rate of water absorption matches the rate of evaporation. This state is moisture equilibrium, and it depends on the relative humidity and temperature of the environment. Reaching this point is critical for engineering plastics like nylon, which experience dimensional and mechanical stability only after they are fully conditioned.
Conditioning Process
Molded parts often undergo a water-immersion or high-humidity conditioning stage to accelerate moisture uptake. This step brings the parts to their moisture equilibrium quickly, ensuring they hold their final dimensions before being assembled. Skipping this conditioning means the parts will slowly change shape and size after assembly as they adjust to room air.
Slow post-moulding change can result in tight joints becoming loose or loose joints binding.
Material Performance
Mechanical characteristics like stiffness and impact strength change significantly as moisture is absorbed by the plastic. At moisture equilibrium, polyamide exhibits much higher toughness but lower tensile modulus than in its dry-as-moulded state. Designers must use conditioned material data rather than dry-moulded values when calculating load-bearing limits.
Quality Assurance
Metrology testing should only be conducted on parts that have reached a stable state to ensure repeatable measurements. Variation in moisture content across different batches leads to inconsistent dimensions and false rejects. Documenting that the parts have reached moisture equilibrium before testing eliminates this variable from the quality control process.
