Meaning
Diffusion kinetics define how volatile mass transfers from a solid polymer matrix into a surrounding medium based solely on concentration gradients. Fickian moisture desorption describes a specific rate-controlled loss where the movement of water molecules follows the mathematical laws established for linear diffusion. This phenomenon assumes the polymer structure remains constant while internal molecules migrate toward the surface boundary.
The process relies on the relationship between the square root of time and the quantity of substance lost until equilibrium reaches the level of the ambient environment.
Material Kinetics
Polymers release absorbed water at different speeds depending on the chemical nature of the resin and the internal morphology. Fickian moisture desorption demonstrates how semi-crystalline resins permit different flow paths for water molecules compared to amorphous counterparts. Manufacturers observe this shift during drying cycles when excess humidity threatens the dimensional integrity of moulded parts.
Proper estimation of these rates prevents thermal degradation inside the barrel because the drying time matches the actual exhaustion of volatiles.
Economic Consequence
Excess water trapped inside plastic pellets increases the reject rate during high pressure injection moulding by creating voids or surface defects known as splay. Fickian moisture desorption dictates the duration a dryer must run to pull the material below the critical threshold for defect-free production. Virgin resins typically follow predictable curves whereas regrind batches display accelerated weight loss due to shorter chain lengths or micro-fractures.
Holding a consistent dryness level across a shift avoids the overhead costs associated with secondary inspection and scrapped inventory.
Processing Boundary
Temperature spikes force the rate of mass loss to deviate from the pure mathematical model by causing structural changes in the polymer chain. Fickian moisture desorption remains accurate only while the solid exists below its glass transition range where the matrix does not deform significantly during the loss. Accurate calibration of processing ovens requires the recognition that rapid heating produces non-linear results which falsify standard calculations.
Total volatile removal depends on the distance between the center of the part and the surface contact area.
