Correlating Dryer Air Dew Point with Karl Fischer Titration Values
Resin moisture correlates to air dew point only at thermodynamic equilibrium; diffusion kinetics and hopper dwell dynamics govern actual Karl Fischer values.
Polymer Drying Kinetics
Meaning
Mathematical descriptions of the rate at which moisture is removed from a polymer matrix during conditioning define the time-dependent moisture loss of the resin. This rate depends on both the surface evaporation of water and the internal diffusion of moisture through the pellet structure. In industrial injection moulding and extrusion operations, polymer drying kinetics determine the minimum residence time and the temperature required to prepare hygroscopic resins for processing.
Diffusion Phase
The initial phase of drying removes free surface moisture rapidly, but the subsequent phase is governed by the slow migration of water molecules from the core to the surface. This second phase is highly temperature-dependent, as thermal energy increases the diffusion coefficient of the water molecules within the solid polymer matrix. Heating the resin to the correct temperature is therefore necessary to accelerate this migration and achieve deep dryness.
Moulding Consequence
Neglecting the internal drying rate and shortening the cycle can result in pellets that are dry on the outside but still wet on the inside. When these pellets reach the melt zone of the extruder, the trapped moisture causes hydrolysis and lowers the mechanical strength of the moulded parts. This can cause the parts to fail when subjected to load, resulting in recalls or customer complaints.
Dryer Configuration
Sizing the drying hopper correctly depends on understanding the extraction rate under continuous operation. A hopper that is too small reduces the heating time, preventing the pellets from reaching the dryness required for high-quality production.