Meaning
Mass transport quantification defines how gas or moisture molecules travel through a polymer matrix under a concentration gradient. Ficks second law calculates this non-steady state diffusion by relating spatial concentration changes over time to the second derivative of concentration with respect to distance. Injection moulding sets the initial concentration profiles during cooling, while subsequent environmental exposure tests measure the resulting barrier degradation.
Parts failing this mass transfer threshold exhibit blistering, dimensional drift, or adhesive delamination.
Boundary Layer
Packaging engineers apply this mathematical relationship to predict shelf-life limitations for food and medical containers. Virgin polymer structures maintain predictable permeability constants, whereas regrind incorporation introduces microvoids that accelerate molecular transfer rates significantly. Material specifications dictate the baseline diffusion coefficient, but actual moulder processing parameters determine local crystallinity and skin layer orientation.
Parts processed with excessive barrel temperatures suffer from localized polymer degradation, which increases internal void density and distorts the predicted diffusion front. Moisture ingress into structural housings reduces glass transition temperatures, leading to premature mechanical failure under load.
Thermal Gradient
Crystallization kinetics alter the amorphous pathways available for diffusing penetrants. Temperature fluctuations during the cooling phase dictate spherulite size and amorphous fraction distribution within the moulded article. Higher mould temperatures promote dense crystalline structures that lengthen the tortuous path for migrating gas molecules.
Process technicians adjust coolant flow rates to balance cycle times against required barrier properties. Deviation from targeted cooling profiles generates internal residual stresses that modify local free volume and invalidate standard datasheet permeability values.
Molecular Transport
Polymer chain orientation induced by high shear rates during cavity filling creates anisotropic diffusion coefficients. Diffusion occurs more rapidly parallel to the melt flow direction than transverse to it, complicating service life predictions for thin-walled packaging. Part specifications must account for this directional bias when predicting barrier performance against oxygen or carbon dioxide transmission.
Regrind blending exacerbates anisotropy by introducing variable molecular weight distributions that disrupt uniform chain packing. Precise control over injection speed and packing pressure minimizes structural heterogeneity, ensuring actual part performance matches predicted mass transfer thresholds.
