Polyethylene Extrusion Temperature Profile: Setting the Zones Right

Polyethylene extrusion temperature control is not a single barrel setting but an axial thermal sequence matched to density, molecular weight distribution, and screw geometry. For high-density grades with an MFR of 0.2–1.2 dg/min under ISO 1133-1:2022 at 190°C and 2.16 kg, a conventional single-screw line operating at 30:1 L/D typically requires a rising profile from the feed section to the die. Low-density and linear low-density grades force different profiles because LLDPE exhibits a narrower molecular weight distribution and higher shear viscosity than LDPE at equivalent melt index. The objective of setting the zones is to complete melting in the compression section without driving the melt into oxidative degradation, while maintaining stable die-head pressure and a melt temperature measured independently of the barrel setpoints.

Industrial polyethylene extrusion lines rarely exceed 230°C for LDPE and 260°C for HDPE because residence-time distribution and temperature overshoot in the metering section can raise local melt temperature by 10–20°C above the setpoint. The feed throat is water-cooled, not heated. Actual melt temperature is therefore governed by three inputs: barrel conduction from the heater bands, screw-transmitted shear heating, and pressure-driven recirculation in the screw channel.

How Does Screw Geometry Shift the Axial Melt Temperature Profile?

The setpoint required in each barrel zone is not an independent variable; it is subordinate to compression ratio and channel depth. A general-purpose single-screw with a compression ratio of 2.5:1 to 3.5:1 and a metering depth of 3–5 mm on a 45 mm extruder generates a different melting onset than a barrier screw with a separate solids channel. Barrier screw designs for LLDPE often lower the melt temperature by 5–8°C at the same output because they isolate solid pellets and reduce viscous dissipation in the existing melt film. Conversely, high-shear mixing sections can add 0.03–0.06 kW/kg of specific mechanical energy, which appears as an increase in melt temperature between the screw tip and the breaker plate.

Viscous dissipation in polyethylene follows a pseudoplastic viscosity decay. LLDPE resins with a melt index of 1.0 dg/min show a shear viscosity near 700–900 Pa·s at 100 s⁻¹ and 190°C. When screw speed is raised from 80 rpm to 120 rpm on a 75 mm extruder, the melt thermocouple may read 12–18°C above the zone 3 setpoint even though the controller holds the barrel within ±1°C. This is the primary reason that zone setpoints are not a direct melt-temperature control; the controller’s thermocouple is buried in the steel, not immersed in the melt.

Feed-throat cooling for HDPE is normally maintained at 30–50°C. If the throat rises above 55°C, pellets soften prematurely and can bridge the feed opening, causing output fluctuation of ±3% to ±7% on a gravimetric line. The first heated barrel zone is therefore often set 10–20°C below the melting onset to allow solids conveying. For HDPE with a DSC peak melting point between 130°C and 137°C under ISO 11357-3, zone 1 does not need to reach the melting point; the pellets are heated mainly by contact with the barrel and by frictional work in the grooved feed section. A groove angle of 45° and a groove depth of 2–3 mm are typical, but published data for every specific configuration remains limited.

Zone Setpoints for HDPE, LDPE, and LLDPE

The following values are aligned with general-purpose extrusion grades and assume a 30:1 L/D single-screw extruder with a compression ratio of 3:1, a die gap of 2.5 mm, and melt pressure of 20–30 MPa. These are baseline profiles; they must be trimmed by melt thermocouple reading, not by barrel setpoint. Values are expressed in °C.

For LLDPE the profile is deliberately shifted 10–15°C higher than LDPE because the linear chains do not strain-soften to the same degree. The same melt index cannot be used as a universal temperature predictor; two resins with an MFR of 1.0 dg/min can differ in melt temperature requirement by 20°C when the molecular weight distribution changes from broad to narrow.

Die-head temperature is not set solely for melt stability. For LLDPE tubular film, a die setpoint of 205–220°C reduces the extensional stress at the lip and delays sharkskin. At a die shear stress above 0.14 MPa, surface roughness can initiate; raising the die temperature by 10°C reduces melt viscosity and local shear stress. However, the adapter temperature should not exceed the die temperature by more than 5°C because stagnation in the spiral mandrel can crosslink HDPE and create gel particles.

When the Die Pressure Exceeds 30 MPa, Melt Temperature Deviates from Setpoint

Head pressure has a direct effect on melt temperature because pressure flow across the screen pack and breaker plate recirculates melt and dissipates energy. At a filtration pack with 20/40/80 mesh screens and a 35 mm screw, a pressure drop of 30 MPa can add 6–10°C to the melt before the die. If the pressure rises above 35 MPa due to a blinded screen pack, the melt temperature can increase by 15°C or more while the barrel zones remain constant. Production operators should replace the screen pack when pressure differential across the screens exceeds 8–12 MPa; otherwise the thermal profile is no longer set by the control system.

This is especially critical for pipe and profile extrusion using HDPE with an MFR of 0.25 dg/min. The high melt viscosity and low melt index generate high pressure at normal output. A 75 mm grooved-feed extruder producing 450 kg/h of HDPE pipe may operate at a melt pressure of 25–30 MPa before the screen changer. Under these conditions, the melt thermocouple should be placed at the screw tip to record actual temperature; a wall-mounted barrel sensor in zone 3 will under-report by 8–15°C. If the melt thermocouple reaches 240°C with a barrel setpoint of 210°C, screw speed should be reduced or screw cooling engaged rather than lowering only the barrel heaters.

In coextruded barrier film, the polyethylene skins and tie layers must be processed within a narrow interfacial melt-temperature window. If an LDPE skin is run at 190°C and an LLDPE core at 225°C, the interface temperature can exceed the LDPE degradation onset even though each individual melt stream is within range. The feedblock or die is therefore set to 210–220°C for the combined stream, and the LDPE layer is deliberately processed hotter than its single-layer profile to reduce viscosity mismatch. Viscosity ratio at the die lip is maintained between 0.8:1 and 1.2:1; outside this band, flow instability and layer thickness variation increase beyond ±5%.

Thermal Degradation Begins Before Visible Smoke

Polyethylene is thermally stable within a defined processing window, but degradation is a function of time, temperature, and oxygen. LDPE autoclave grades can form carbonyl species and increase melt flow rate when residence time exceeds 5–8 min at 240°C. HDPE more often forms crosslinked gel particles and surface roughness when held above 260°C in dead spots behind the screen pack or in a poorly streamlined adapter. A melt-temperature safety limit of 260°C for HDPE and 240°C for LDPE is therefore common when residence time exceeds 10 min. For short residence times below 3 min, stabilized HDPE can tolerate 280°C, but only if the antioxidant system is sufficient.

Oxidation onset can be monitored by ISO 11357-6 OIT at 200°C. Unstabilized polyethylene may show an OIT of 3–8 min, while a well-stabilized HDPE extrusion grade typically exceeds 30–60 min. When OIT falls below 10 min, the extrusion profile should be reduced by 5–10°C or output increased to shorten residence time. Surface moisture is a separate variable: if resin has been stored at relative humidity above 80%, surface condensation can produce pinholes even in a correctly set thermal profile. Drying at 60–70°C for 1 h is adequate only for surface moisture, because polyethylene does not hydrolyze.

For HDPE sheet fed to a three-roll stack, the die temperature is commonly set 5–10°C below the final metering zone to build a slight pressure gradient and reduce die-lip drool. Melt temperature is controlled to 200–220°C for sheet grades with an MFR of 0.8–1.2 dg/min. If the melt temperature at the die exceeds 230°C, cooling roll chill may not extract heat fast enough and surface haze increases; the contact roll temperature is set at 60–90°C and the second roll at 70–100°C. A melt thermocouple inserted between the die and the roll stack records the effective processing temperature rather than the die steel temperature.

Pipe Wall Sag Depends on Melt Temperature, Not Just Die Temperature

In solid-wall HDPE pipe extrusion, melt temperature is used to manage draw-down and sag. For a 250 mm diameter PE100 pipe with a wall thickness of 22.7 mm based on SDR 11, the melt temperature must remain high enough to fuse the weld line but low enough to prevent the molten tube from sagging before calibration. Typical melt temperature for PE100 is 220–235°C, with the die at 210–225°C. If the melt temperature exceeds 240°C, the extrudate may sag and wall thickness variation can increase above ±3% around the circumference. Vacuum calibration tanks at −0.02 to −0.04 MPa can only correct limited deformation; the primary control is the melt temperature gradient from die to calibrator.

Post-industrial regrind from edge trim can alter the temperature profile because recycled polyethylene has lower bulk density and a mixed particle size distribution. At 20 wt% regrind in HDPE blow molding, the feed zone may require a 5°C increase to compensate for irregular solids compaction, but the metering zone should not be increased because regrind often contains degraded material with a lower OIT. The melt temperature should be checked at the screw tip because regrind fines can melt earlier and increase local shear heating in the compression zone.

Startup and shutdown sequences require deliberate departures from the steady-state zone profile. During startup the die and adapter are typically set 10–15°C above the steady-state target to purge cold material and reduce initial torque; once stable output is reached, the setpoints are lowered to prevent melt-temperature overshoot. During shutdown of an LDPE line, the screw should be run with a low-compression HDPE purge at 180–210°C to displace LDPE from the adapter and die; shutting down with LDPE at 230°C can leave carbonized deposits in the spiral mandrel. Thermocouple response lag also distorts the true temperature profile. Bayonet-style sensors with a response time of 2–4 s are preferred over pressure-well sensors at the die because they detect short-term excursions before the control loop can compensate. A calibration check of the melt thermocouple against a calibrated probe in a temperature bath is usually performed annually, and a drift of ±2°C is considered acceptable for film and pipe lines.