Mixing Flights

Function

Adequate mixing-zone residence time and correct flight geometry are prerequisites for homogeneous binder distribution — the primary quality indicator of dense-graded asphalt. Under-mixing produces patchy coating, causing ravelling and durability loss in the laid surface course. Over-aggressive flight geometry strips the bitumen film from fine particles and promotes binder drain-down in the hot storage silo. Mixing flights must simultaneously achieve full binder coating and transport the finished mix progressively toward the drum outlet chute at a rate that precisely matches the feeder throughput, maintaining a consistent depth of mix burden in the zone.

Construction & quality

Fabricated from mild steel (IS 2062 / ASTM A36) for the base plate — reducing brittleness at elevated mixing-zone temperatures of 150–200 °C — with a hard-facing weld overlay or HARDOX 450 wear-pad insert at the leading edge where aggregate contact is concentrated. Typical specification: 10–14 mm base plate with a 6–8 mm overlay. Flight profiles are designed as open-blade, angled-scoop, or helical-paddle forms depending on drum diameter and target mix viscosity. Attachment is by bolted flanges or full-length continuous weld. Internal surfaces exposed to bitumen are sandblasted to Sa 2.0 and left uncoated, as bitumen self-passivates the steel surface.

When to replace

• Uneven or patchy bitumen coating visible in discharged mix at the plant output point
• Flights flattened, bent, or missing, reducing the tumbling and forward-transport action
• Mix discharge temperature dropping despite normal aggregate inlet temperature and fuel rate
• Bitumen streaking or pooling inside the drum at the injection point indicating flight loss
• Increased mix segregation or filler clumping observed at the drum outlet or in the hot elevator