Capillary Rheology Qualification Protocols for High Strain Extrusion Regrind
Twin-bore capillary testing at 1,000 to 10,000 s⁻¹ qualifies high-strain regrind by isolating entrance losses, true wall slip, and elastic die swell shifts.
Sharkskin Defect
Meaning
Surface fracturing appears during polymer melt extrusion when high wall shear stresses exceed critical viscoelastic thresholds near the die exit. Sharkskin defect develops during high-speed processing of linear low-density polyethylene and high-density polyethylene resins. Production lines encounter this dimensional failure whenever throughput demands outpace the relaxation rate of stretched polymer chains.
Extrusion tooling design, melt temperature profiles, and molecular weight distribution govern the severity of this periodic roughness.
Shear Boundary
Critical wall shear stress triggers melt fracture inside capillary dies during high output rate operations. Material science isolates this instability at the die lip where constrained polymer chains detach from boundary walls under extreme acceleration. Viscoelastic properties dictate the exact threshold value for linear low-density polyethylene processing before surface irregularities appear.
Production engineers adjust melt temperatures upward to lower viscosity and suppress the onset of this melt instability.
Tooling Geometry
Die land length and exit angle modifications alter the extensional stress history of extruded profiles. Processing equipment manufacturers specify tapered die entries to reduce rapid deformation transitions that exacerbate surface tearing during extrusion runs. Polymer melts experience prolonged relaxation time when tooling engineers optimize die gap dimensions for high-speed output.
Temperature control units maintain uniform thermal profiles across the die periphery to prevent localized variations in melt elasticity.
Economic Impact
Material specifications demand strict visual tolerances that reject parts exhibiting surface roughness due to excessive extrusion speed. Regrind material integration alters molecular weight distribution and lowers the critical shear rate threshold, increasing scrap rates during continuous production runs. Moulders balance productivity targets against surface quality limits because correcting this defect requires lowering line speeds or investing in specialized processing aids.
Final component performance depends entirely on maintaining stable extrusion conditions below the critical wall shear stress limit.