Determining Polyolefin Melt Flow Changes from Repeated Shear Exposure
Polyolefin melt flow shifts reveal chain scission or crosslinking under repeated shear, requiring strict multi-point viscosity and antioxidant monitoring.
Multi-Pass Extrusion
Meaning
Polymer processing cycles repeat material through a heated screw and die assembly to achieve homogeneous melt states or to reduce viscosity through mechanical shearing. Multi-pass extrusion forces a resin through the barrel multiple times, which reduces the molecular weight of long-chain polymers. This repetition facilitates the incorporation of additives that require high shear force or extended thermal history to disperse evenly within a carrier matrix.
Process Variable
Melt temperature and screw speed determine the efficacy of each rotation during the cycle. Excessive passes lead to thermal degradation that weakens the physical properties of the finished product. Virgin resin stability often limits the maximum number of cycles permitted before significant molecular chain scission occurs.
Cost increases as each pass adds energy consumption and labor requirements to the production total.
Material Economics
Regrind quality depends on the history of the polymer during its initial cycles. Recycled pellets frequently arrive with reduced viscosity because the original manufacturer utilized multiple runs to achieve specific flow characteristics. Datasheet values for virgin material represent a single-pass thermal history, whereas moulder reality requires testing the viscosity of the material as it appears in the hopper after subsequent processing.
Flow Control
Die pressure fluctuations indicate inconsistent material density as the polymer enters the final stage. Precise control of the residence time inside the barrel prevents localized overheating that causes color variation or unwanted structural changes. Proper calibration of the cooling system between cycles stops the degradation process and keeps the extrudate within the allowable tolerance for final part geometry.