Calculating Permissible Moisture Thresholds for High Temperature Extrusion of Polyesters
Extruding polyesters above 280°C demands moisture below 30 ppm to prevent ester hydrolysis, intrinsic viscosity loss, and catastrophic part brittleness.
Die Drool
Meaning
Accumulated polymer degradation residue migrating from the nozzle exit or land edge constitutes die drool during continuous polymer extrusion. Low molecular weight fractions and degraded polymer chains separate under thermal stress inside the flow channel, depositing solid or carbonized material upon the extrudate surface. This thermal buildup causes severe melt fracture and surface blemishes on tubing, profiles, and wire coatings.
Material selection and residence time control govern the severity of this accumulation, which occurs during extended production runs.
Thermal Degradation
Localized overheating inside the adapter and die zones promotes chain scission of the polymer melt before final shaping. Additive packages containing slip agents or plasticizers tend to migrate toward the metal walls under high shear rates, creating a low viscosity boundary layer that separates from the bulk flow. Excessive barrel temperatures accelerate molecular breakdown, leaving carbonized deposits on the land surface that periodically shear off into the extrudate.
Processing polyethylene and polyvinyl chloride requires strict temperature profiling to suppress this chemical breakdown.
Shear Stress
Fluid mechanics at the die exit generate high velocity gradients that strip degraded fractions from the metal boundary. Die geometry and land length dictate the pressure drop across the final shaping zone, influencing how cleanly the melt shears off the tooling surface. High extrusion speeds intensify wall friction, forcing accumulated degradation byproducts into the moving stream.
Polishing the die land reduces surface friction and delays the onset of contaminant buildup during high output periods.
Purging Maintenance
Periodic cleaning protocols remove carbonized residues from the metal surfaces before physical degradation ruins the production run. Implementing high density polyethylene purge compounds with active mechanical scouring agents strips away accumulated deposits without damaging expensive tooling. Production schedules must accommodate these downtime intervals to maintain consistent dimensional tolerances on extruded profiles.
Residual contamination left inside the flow channel accelerates the return of surface defects upon restart.