Mass Transfer

Thermal Diffusion

Polymers exhibit characteristic rates of additive movement toward the surface of a moulded part based on the local temperature gradient. High processing heat encourages the migration of slip agents or antioxidants which alters the coefficient of friction on the final product. A datasheet specifies the migration limit for a resin, yet the actual performance depends on the thermal history of the individual mould cycle.

Rapid cooling locks these molecules inside the polymer structure whereas slow cycles encourage their depletion from the bulk.

Processing Impact

Inaccurate control of this migration results in surface blooming or plate out on high precision cavities. Surface defects manifest when the concentration of low molecular weight components exceeds the solubility threshold of the host matrix at the wall of the mould. Moulders manage this by adjusting the cycle time and the cooling rate to stabilize the distribution of mobile phases before ejection.

Regrind materials often display higher concentrations of migratory species compared to virgin pellets, which requires adjustments in tool temperatures to maintain consistent part specifications.

Cost Governance

Economic efficiency relies on maintaining the homogeneity of the polymer blend to avoid secondary finishing steps. Material specifications prioritize the retention of active additives to prevent the premature loss of functional properties in the field. When mass transfer occurs too quickly during storage or service, the part fails to meet its intended physical properties.

Stable migration rates prevent the excessive waste of expensive performance additives and ensure the longevity of the finished article.