Standard Melt Flow Test Load Selection Criteria for Polyethylene Sourcing
Polyethylene sourcing requires specifying exact test loads, temperatures, and standards to isolate molecular structure and enforce contractual lot consistency.
Capillary Die
Meaning
A laboratory instrument housing a precision orifice for measuring polymer melt flow rate under controlled thermal and shear conditions. A capillary die establishes the shear rate range for evaluating resin processability prior to large scale extrusion or injection moulding. This geometry defines melt viscosity across variable apparent shear rates by forcing molten polymer through a calibrated channel of known length and diameter.
The operational boundary lies where wall slip or melt fracture distorts the pressure drop reading, rendering apparent viscosity calculations invalid.
Shear Rate
A testing apparatus controls piston descent speed to force molten polymer through the capillary die at defined volumetric rates. Apparent shear rate calculations depend directly on volumetric flow and capillary radius dimensions. Low shear rates simulate barrel conditions during injection moulding whereas high shear rates approximate flow through narrow runner gates or extrusion dies.
Real polymer melts exhibit non Newtonian pseudoplastic behavior where viscosity decreases as the applied shear rate increases.
Pressure Drop
Transducer readings measure force exerted behind the capillary die during material extrusion. Pressure losses accumulate through the entrance region and along the length of the narrow bore. Correcting these entrance pressure losses requires multi length die testing to isolate true wall shear stress from converging flow artifacts.
Excessive pressure spikes during routine testing indicate polymer degradation, gel contamination or incorrect thermal setpoints in the barrel heating zones.
Viscosity Profile
Material specifications rely on capillary die data to predict resin behavior under severe processing conditions. Virgin polymers maintain predictable viscosity curves whereas regrind batches often exhibit shifts driven by molecular weight degradation from prior thermal cycles. Moulders compare incoming resin datasheets against actual laboratory flow curves to verify lot consistency before releasing material to production floors.
Molecular weight distribution broadening alters the slope of the logarithmic viscosity versus shear rate plot derived from these trials.