Meaning
Solid contact surfaces rely on a microscopic oil film to prevent adhesion while supporting applied loads. Mixed lubrication occurs when that fluid layer thins to a degree where surface asperities intermittently touch during sliding or rolling motions. This regime occupies the transition zone between full hydrodynamic separation and boundary conditions where constant physical contact generates heat.
Contact Physics
High pressure peaks cause local deformation at the summits of roughness profiles on mating parts. These deformations dissipate energy through friction rather than fluid shear. Total load support splits into a proportion carried by the fluid pressure and a remainder borne by the contacting peaks.
Thermal Mechanics
Viscosity drops rapidly as shear rates climb within the narrow gap between moving elements. Increasing heat reduces the load carrying capacity of the oil, forcing more work onto the asperities. Moulders observe this phenomenon when excessive injection pressure or high screw speeds shorten the life of feed throat liners or barrel sleeves.
Surface Wear
Severe mechanical scoring appears if the separation distance falls below the combined average roughness of the two materials. Metallic galling risks mount whenever the additive package in the oil fails to form a stable sacrificial layer under these harsh conditions. Proper surface finish specifications protect components from premature degradation.
