Meaning
A dimensionless physical ratio quantifies the resistive force opposing the relative sliding motion of two contacting solid surfaces while they are moving past each other. The dynamic friction coefficient governs how easily a solidified plastic part releases from a steel mold cavity face during the ejection stroke. Standardized tribological tests measure this property to evaluate resin slip additive performance and optimize part release characteristics.
Polymer formulators incorporate internal lubricant additives like erucamide or oleamide to adjust surface slipperiness for high-speed packaging equipment.
Slip Behavior
Internal slip additives migrate from the bulk polymer matrix to the surface layer as the molded part cools down. High additive concentrations reduce frictional resistance significantly, though excessive migration causes surface blooms or printability issues. When evaluating dynamic friction coefficient, film extruders test sheet materials to ensure predictable handling during downstream converting and bag-making operations.
Regrind material alters additive distribution, leading to inconsistent surface friction across production batches.
Mold Ejection
Hydraulic ejector pins exert high mechanical forces against part ribs to overcome vacuum adhesion and frictional drag along cavity walls. Higher surface friction increases required ejection forces, potentially buckling thin pins or stress-whitening clear polymer parts. Monitoring dynamic friction coefficient allows mold designers to select suitable cavity surface polishes or release coatings. draft angles must be increased when processing high-friction resins like thermoplastic elastomers.
Measurement Standard
Laboratory testing according to ISO 8295 uses a weighted sled pulled across a test plaque at a fixed velocity. The resulting tension force divided by normal weight yields the dynamic friction coefficient value.
