For linear low-density polyethylene, the LLDPE Processing Temperature Window: Barrel Profile, Melt and Die Head is not a single fixed value but a function of melt flow index, comonomer type, screw geometry, residence time, and downstream die configuration. Commercially relevant LLDPE grades with densities from 0.915 g/cm³ to 0.940 g/cm³ and melt flow indices from 0.5 g/10 min to 50 g/10 min are processed across blown film, cast film, extrusion coating, and injection molding. Differential scanning calorimetry according to ISO 11357-3:2018 typically places the main melting peak between 119°C and 127°C. The processing window is therefore set far above these values: barrel zones commonly begin at 160°C to 180°C in the feed section, rise to 190°C to 220°C in the metering section, and terminate at a die head from 200°C to 230°C. The upper limit is governed less by crystalline melting than by oxidative degradation and gel formation. For blown film, melt temperatures measured by an immersion probe downstream of the breaker plate are usually maintained between 190°C and 240°C. Specific settings shift with comonomer type: octene-based LLDPE tends to exhibit lower screw torque and higher melt strength than butene-based LLDPE at equivalent melt index, permitting a slightly narrower profile near the die head.

Why Does Narrow Polydispersity Shift the Barrel Temperature Profile?

The barrel profile for LLDPE must account for lower shear rate sensitivity than long-chain branched LDPE. In a 45 mm grooved-feed extruder with a 30:1 L/D ratio, raising the metering zone set point from 200°C to 215°C can reduce screw drive load and melt pressure at the breaker plate without producing equivalent reductions in die-head pressure. For a film resin with a melt flow index of 1.0 g/10 min, breaker-plate pressures between 250 bar and 350 bar are common. The feed zone is typically operated at 160°C to 175°C, the compression zone at 180°C to 195°C, and the metering zone at 195°C to 210°C. If the screw is a high-shear barrier design, the compression-zone temperature may be reduced by 5°C to 8°C because viscous dissipation already supplies a portion of the thermal energy. The temperature window narrows when the melt index drops below 0.8 g/10 min, and the metering-zone set point may need to rise to 220°C to prevent motor overload.

The selection of a rising versus reverse barrel profile also depends on the melting mechanism in grooved-feed extruders. Grooved feed sections generate forced solids conveying and can create pressure peaks in the feed zone. If the feed-zone temperature is too high, pellet melting in the grooves may reduce conveying efficiency; if it is too low, the solid material is compacted and screw torque rises. A typical set point of 165°C to 175°C in zone 1 keeps the feed section below the initial melting point while allowing preheating. The transition from feed to compression zones is therefore treated as a thermal boundary: temperature increases of 15°C to 25°C are provided over one to two screw diameters. In grooved-feed extruders, the barrel temperature may be 5°C to 10°C lower than in smooth-bore machines because the grooves reduce the demand for barrel-supplied heat. This matters when processors transfer a formulation from a smooth-bore cast film line to a grooved-feed blown film line: the same die-head temperature can produce different melt temperatures because screw work and barrel heating are redistributed.

On cast film lines using 75 mm barrier screws at outputs exceeding 300 kg/h, the temperature profile normally becomes flatter. The feed section can be set at 170°C, the middle zones at 200°C to 210°C, the final barrel zone at 210°C to 220°C, and the adaptor at 220°C to 230°C. This flat-to-slightly-rising profile helps maintain stable melt temperature when line speed is above 150 m/min. Lower profile slopes reduce thermal stresses but increase reliance on screw-generated shear heating. When screen pack contamination raises head pressure above 400 bar, a reverse-profile adjustment is often applied: the metering zone is set 5°C to 10°C below the die head to compensate for excessive viscous energy and to suppress overheating at the screw tip.

Thermal Degradation Thresholds Define the Practical Melt Envelope

The upper temperature boundary for LLDPE processing is constrained by oxidative degradation rather than by sudden chain decomposition. In nitrogen, measurable mass loss does not appear until temperatures exceed 300°C, but in air, the formation of hydroperoxides and carbonyl species can occur at extended residence times above 240°C. Blown film extrusion is particularly sensitive because the bubble surface is exposed to air immediately after die exit. A sustained melt temperature above 250°C can raise gel count, reduce machine-direction tear resistance measured by ASTM D1922-21, and cause visible specks. Therefore supplier bulletins for film-grade LLDPE typically specify a maximum melt temperature of 230°C to 240°C. In extruders with high-shear screws, the measured melt temperature can exceed the set-point barrel temperature by 10°C to 20°C due to viscous dissipation alone. This offset must be verified with a hand-held probe or through in-line infrared sensors; set points should then be trimmed downward if the measured melt temperature approaches 240°C. ISO 11357-6:2018 provides a controlled oxidative induction time method for evaluating stabilizer depletion, but it cannot replace real-time melt-temperature measurement on production lines.

Die head set points are normally held at or slightly above the metering zone to prevent viscosity stratification across the die lip. For a spiral mandrel die of 250 mm diameter, typical set points are 210°C to 225°C for the outer die body and 205°C to 215°C for the inner mandrel. The exact offset between die body and mandrel is adjusted to control gauge uniformity. If the die head is too cold, the melt pressure at the die lip increases and the film may exhibit sharkskin or melt fracture at lower output rates. In LLDPE, sharkskin appears at wall shear stresses above 0.14 MPa to 0.20 MPa; raising the die-lip temperature by 5°C to 10°C can delay the onset and permit a throughput increase of 10% to 15% without changing the screw profile. Conversely, an overly hot die head can promote smoke at the die exit and accelerate polymer build-up on the lip. Die-head heaters are therefore zoned into three to six independent control loops on larger dies, with end-seal zones set 2°C to 5°C higher to prevent edge bead solidification in cast film.

When the Die Head Runs 10°C Below the Metering Zone

Operating the die head below the metering zone produces a negative temperature gradient across the adaptor. In LLDPE, this condition can increase melt pressure immediately upstream of the screen pack and die inlet. The higher pressure reduces output if the extruder operates at fixed screw speed, or it raises motor load if line speed is kept constant. On a 65 mm blown film line, a die-head temperature 10°C lower than the metering zone can increase specific energy consumption by 5% to 8% and shift the melt temperature at the die entry to the upper end of the window because of added viscous work. The negative gradient is sometimes used deliberately for extrusion coating grades with melt indices from 4 g/10 min to 10 g/10 min to stabilize the web and reduce neck-in. Published data for this specific configuration is limited, and the practice is usually validated by measuring neck-in with a 0.5 mm die gap and comparing against the supplier reference. Lower melt-strength LLDPE grades are more likely to show draw resonance when the die temperature is too high; a cooler die lip can shift the melt relaxation behavior and stabilize the melt curtain. However, lip temperatures below 200°C may increase surface roughness and reduce adhesion to chill rolls.

Injection molding applications for LLDPE require a different temperature window because the melt must fill thin-wall flow paths under high shear. Barrel temperatures are typically set with the rear zone at 160°C to 180°C, the middle zone at 190°C to 210°C, the front zone at 200°C to 220°C, and the nozzle at 200°C to 220°C. Melt temperatures measured during screw recovery are generally held between 200°C and 240°C. Mold temperatures are kept low, typically 20°C to 40°C, because LLDPE solidifies rapidly and requires efficient cooling. Injection pressures between 600 bar and 1200 bar and holding pressures from 300 bar to 700 bar are common depending on part thickness and gate geometry. The temperature window must be widened for thin-wall containers with flow ratios above 150:1; in such cases the front zone and nozzle can be raised to 230°C to 250°C provided cycle time remains short. Residence time should be kept below 5 min to avoid discoloration and the formation of oxidized layers on the screw root.

ISO 1133-1 and ASTM D1238 as Temperature-Dependent Viscosity References

Melt flow rate alone does not define the full processing window because the standard test applies low shear. ASTM D1238-20 and ISO 1133-1:2022 measure extrudate mass per 10 min at 190°C under 2.16 kg. For LLDPE film grades, acceptance windows of 0.5 g/10 min to 2.0 g/10 min are common, while injection molding grades may range from 20 g/10 min to 50 g/10 min. The same resin can move through a die at shear rates of 100 s⁻¹ to 1000 s⁻¹ in extrusion and above 10,000 s⁻¹ in injection molding; therefore capillary rheometry according to ASTM D3835-16 is used to confirm viscosity at processing-relevant shear rates. Apparent viscosity for a 1.0 g/10 min LLDPE at 190°C and 100 s⁻¹ is typically 500 Pa·s to 1500 Pa·s. When the melt temperature is increased by 10°C, the low-shear viscosity can decrease by 5% to 15%, but high-shear viscosity is less affected. This is why barrel-zone adjustments produce more visible changes in screw torque and melt pressure for low-melt-index grades than for high-flow injection grades.

Feed throat cooling is maintained at 18°C to 30°C to prevent pellet bridging. In high-humidity environments above 60% RH, LLDPE pellets may carry surface moisture, though polyethylene does not hydrolyze; pre-drying is rarely required unless the resin is precompounded or contains hygroscopic additives. The lower processing limit is therefore not determined by moisture but by torque. At barrel-zone temperatures below 160°C, the viscosity increase can exceed the drive capacity on small extruders, and output stability suffers. The processing window closes when either the measured melt temperature exceeds 240°C for film-grade LLDPE, or the die-head pressure exceeds the equipment limit, often 400 bar to 450 bar for older extruders. These boundaries should be recorded with grade-specific lot data because comonomer distribution and stabilizer package have a larger effect on the upper temperature limit than density alone.