Meaning
Standard procedures for moisture quantification in polymer materials rely on controlled thermal extraction paired with carrier gas transport. ISO 15512 Method B defines the manometric technique specifically designed for measuring water content in thermoplastic granules and powders prior to thermal processing. The procedure applies exclusively to samples where volatile fractions consist solely of water and where moisture release occurs within a defined temperature ceiling without thermal decomposition of the polymer matrix itself.
Operation starts by sealing a weighed resin sample into a heated chamber connected to a calibrated vacuum system.
Thermal Evacuation
Heat applied to the chamber vaporises moisture trapped inside the polymer pellets, driving water molecules out of the material matrix and into the headspace. Pressure increases proportionally inside the closed volume as gas accumulates, allowing transducers to record the total pressure shift after temperature stabilisation. Calculations convert this pressure reading directly into a moisture mass fraction relative to the initial dry sample weight.
Technicians execute this test during material receipt inspections or before feeding hygroscopic resins like polyamides and polyesters into high temperature injection moulding barrels. Maintaining precise barrel temperatures ensures that thermal desorption reaches completion without causing polymer degradation during the measurement cycle.
Moisture Drift
Excess water molecules trapped inside polymer pellets hydrolyse molecular chains during high shear plasticisation inside an injection moulding machine, dropping the intrinsic viscosity of the final part. Processing wet feedstock leads to silvery surface streaks, structural bubble formation, and premature mechanical failure under load in finished mouldings. Virgin pellet shipments arrive with certified moisture limits, whereas regrind materials absorb ambient humidity rapidly during storage and shop floor handling.
Moulded parts require strict feedstock dryness to meet dimensional stability targets, making reliable moisture tracking essential for preventing scrap generation.
Vapour Pressure
Residual humidity values derived from laboratory testing frequently diverge from the actual moisture content present at the moulding machine throat due to ambient humidity exposure during transfer phases. Technicians establish target processing windows by correlating laboratory pressure thresholds with physical part performance under standard stress tests. Extrusion lines processing hygroscopic grades demand continuous verification of drying hopper dew points to counteract seasonal atmospheric changes.
Accurate water quantification prevents costly production interruptions caused by inconsistent melt rheology and structural defects in critical structural components.
