Switching a Line from LDPE to LLDPE: Temperature and Screw Changes is not a simple resin substitution in blown or cast film operations. Linear low-density polyethylene is an ethylene copolymer with 1-butene, 1-hexene, or 1-octene, characterized by short-chain branches and the absence of long-chain branching. Under ASTM D792-20, film-grade LLDPE densities typically range from 0.916 g/cm³ to 0.940 g/cm³, whereas LDPE densities occupy 0.910 g/cm³ to 0.930 g/cm³. Density values are cross-checked under ISO 1183-1:2019, and melt flow rate under ISO 1133-1:2022 at 190 °C/2.16 kg. The transition therefore requires recalibration of barrel zone setpoints, screw speed, adapter pressure, die gap, and cooling air flow; otherwise melt-temperature overshoot, head-pressure excursions, or bubble destabilization occur. Feed-throat temperature is also reviewed because LLDPE pellet bulk density is often lower than LDPE and can alter gravimetric hopper calibration at the same extruder throughput.

Thermal Setpoint Adjustments Across Barrel, Adapter, and Die

An LDPE film line typically runs with feed zone at 150 °C to 175 °C, compression zone at 170 °C to 200 °C, metering zone at 185 °C to 210 °C, and die zones at 190 °C to 215 °C. LLDPE requires a higher thermal flux because its differential scanning calorimetry melting peak under ASTM D3418-15 falls at 119 °C to 125 °C, compared with 105 °C to 115 °C for LDPE. Starting profiles for a production-scale 45 mm to 75 mm single-screw extruder running a 0.5 g/10 min to 1.0 g/10 min LLDPE film resin are usually set at 170 °C to 190 °C in the feed section, 190 °C to 220 °C in the compression section, and 210 °C to 230 °C in the metering section. Adapter and die zones are held between 220 °C and 240 °C. The upper melt-temperature boundary is maintained below 245 °C to limit oxidative gel formation.

Shear heating is the primary limitation when raising barrel setpoints. Because LLDPE is more viscous than LDPE at extrusion shear rates, an immersion melt thermocouple in the adapter may read 10 °C to 30 °C above the metering zone setpoint if an LDPE screw is retained. In such cases a reverse temperature profile, with the feed zone set slightly above the metering zone, is sometimes used to reduce viscous dissipation in the solids-conveying region. Setpoint changes alone do not compensate for an incorrect screw geometry; they shift the location of the overheating problem. During start-up after a changeover, the die should be soaked at or below 200 °C before screw rotation begins, and screw speed should be increased in 5 rpm to 10 rpm increments while monitoring adapter pressure and drive current. If the line has only wall-mounted thermocouples, melt temperature can be underestimated by 10 °C to 20 °C under high shear, so an immersion thermocouple protruding 5 mm to 10 mm into the adapter melt stream is preferred.

The apparent viscosity of a 1.0 g/10 min LLDPE at 100 s⁻¹ and 190 °C is frequently higher than that of a 0.8 g/10 min LDPE at the same shear rate. The difference widens above 500 s⁻¹ because long-chain branching in LDPE increases shear thinning, while LLDPE remains more Newtonian. On a grooved-feed extruder, this produces higher head pressure at constant screw speed and often forces a reduction in screw speed to keep die pressure below the screen changer limit. Observed changes on production-scale lines include drive amperage increases of 20% to 40% when changing from LDPE to LLDPE at the same screw speed. Published data for any single line configuration is limited, but the effect is reported across machinery-supplier transition guides. The pressure spike is the main reason screw geometry must be reviewed before the changeover.

What Screw Design Changes Prevent Shear Overheating and Melt Fracture?

Retaining an LDPE-optimized screw with a 3.5:1 to 4.0:1 compression ratio and a high-shear Maddock mixing section is a frequent cause of a failed LLDPE changeover. LLDPE's narrow molecular weight distribution and linear architecture generate less shear thinning, so deep-channel, low-compression screw designs are specified. Machine suppliers commonly recommend compression ratios of 2.2:1 to 2.8:1 for LLDPE film lines, with a gradual transition from feed to metering to reduce melt-temperature non-uniformity. A 24:1 to 30:1 L/D barrier screw with barrier-flight clearance of 0.9 mm to 1.3 mm separates the solid bed from the melt while limiting shear work. By comparison, LDPE screws with compression ratios above 3.5:1 and aggressive Maddock mixers can drive melt temperature beyond 245 °C and generate oxidation-induced gels. If a Maddock section is retained, its clearance should be opened to 0.6 mm to 1.0 mm or replaced with a distributive pin mixer; cavity-transfer mixers have also been used to improve homogeneity without excessive viscous dissipation.

The metering channel depth for a 60 mm LLDPE barrier screw is often 4.5 mm to 6.5 mm, compared with 3.5 mm to 5.0 mm for LDPE. Feed channel depth is kept at 10 mm to 12 mm to maintain solids conveying. If the existing screw has a feed channel depth below 9 mm, the changeover may require a new screw rather than a temperature adjustment. The screw should be chrome-plated and the barrel wall inspected for wear before the resin changeover; a worn barrel with clearance above the barrel-to-screw diameter specification reduces solids conveying and amplifies melt-temperature variation. Screw cooling through the core is reduced or disabled for LLDPE on some lines to avoid temperature stratification, but this depends on screw design and should follow the screw manufacturer's cooling-flow chart.

Drive capacity must be verified before the resin changeover. A 120 mm grooved-feed extruder with 30:1 L/D may require a DC drive rated at 450 kW to 600 kW for high-output LLDPE, while the same line processing LDPE may operate at 60% to 75% of that load. Specific energy consumption for LLDPE film extrusion is typically 0.30 kWh/kg to 0.40 kWh/kg, compared with 0.25 kWh/kg to 0.35 kWh/kg for LDPE, depending on melt index, backpressure, and screw cooling. When the existing extruder cannot maintain melt temperature below 245 °C at acceptable output, the screw must be changed rather than relying on barrel setpoint reductions alone. Torque limitations are more frequently reached with LLDPE because of the higher viscosity at low shear rates; therefore, screw speed is often lower than LDPE for the same die pressure.

When Die Gap and Air Ring Remain Set for LDPE

If the die gap remains at the LDPE setting, LLDPE is likely to exhibit sharkskin melt fracture at lower output rates because the melt experiences higher shear stress in the die land. Blown film dies used for LLDPE are typically set to 1.5 mm to 2.5 mm, rather than the 0.8 mm to 1.2 mm gap common for LDPE. The wider gap reduces die-land shear rate for a given output. Die land length may also be shortened; LDPE dies with 10 mm to 15 mm land length are often modified to shorter lands for LLDPE to limit pressure drop. Air ring selection changes from a single-lip to a dual-lip air ring because LLDPE bubble stability is more sensitive to cooling flow. Lower melt strength is a measurable difference: commercial film-grade LLDPE often shows melt strength of 3 cN to 8 cN, whereas LDPE may show 8 cN to 20 cN at 190 °C capillary break-stretch testing. Lower blow-up ratios reduce transverse orientation and compensate for this difference, but they also reduce transverse-direction tensile properties. Bubble geometry is often run with a blow-up ratio of 2.0:1 to 2.5:1, whereas LDPE can be run at 2.5:1 to 3.0:1.

Internal bubble cooling is commonly used on high-output LLDPE lines. It increases heat transfer and stabilizes the frost line, allowing higher throughput without increasing tower height. External air flow should be adjusted to avoid bubble flutter; excessive air velocity above 20 m/s at the die lip can destabilize the lower melt-strength LLDPE bubble. If the line lacks internal bubble cooling, maximum output may be limited by cooling capacity rather than extruder capacity. On cast film and extrusion coating lines, the melt curtain is quenched against a chilled roll, so bubble stability is absent, but die gap and backpressure effects remain. LLDPE tends to exhibit higher neck-in and draw resonance at low melt strength; die gap is also increased to 0.6 mm to 1.0 mm for cast film, compared with 0.3 mm to 0.5 mm for LDPE, to reduce die-land shear. Air gap is typically minimized to 80 mm to 150 mm, and chill-roll temperature is held at 15 °C to 25 °C. These modifications are independent of barrel temperature and screw compression ratio but are part of a complete line changeover.

Before introducing LLDPE, the extrusion system is purged with a viscous LDPE purge compound or a commercial polyolefin purge with a melt index of 1.0 g/10 min to 2.0 g/10 min at 190 °C. Residual LDPE in the die, adapter, or screen changer masks the changeover because LDPE lowers head pressure and melt temperature. A staged purge from LDPE to a 50/50 blend of LDPE and LLDPE, then to 100% LLDPE, reduces torque spikes and screen-pack plugging. Screen pack configuration should also be reviewed. Typical LDPE filtration may use 40/60/100 mesh, while LLDPE often uses 20/40/60 mesh to avoid excessive backpressure from the higher-viscosity melt. This is consistent with equipment manufacturer guidance for high-viscosity polyolefins. The screen changer should be cycled before and after the transition to remove degraded LDPE and to prevent pressure drop from retained gels.

Film Physicals Measured Under ASTM D882 and ASTM D1709

Film produced from LLDPE differs from LDPE in tensile, impact, and sealing behavior. Under ASTM D882-18, 25 µm to 50 µm blown LLDPE film typically exhibits machine-direction tensile strength of 30 MPa to 50 MPa, compared with 15 MPa to 25 MPa for LDPE film at equivalent thickness. Ultimate elongation under ASTM D882-18 is typically 500% to 700% for LLDPE, compared with 300% to 500% for LDPE. Dart impact measured under ASTM D1709 Method A shows an LLDPE advantage; for a 25 µm film, LDPE dart impact may be 60 g to 120 g, while hexene-based LLDPE can exceed 150 g. Actual values depend on comonomer type, film density, and frost-line height. Heat seal initiation temperature is generally 10 °C to 15 °C lower for LLDPE than for LDPE, which permits faster packaging line speeds but requires a narrower sealing-temperature window in form-fill-seal operations.

Because LLDPE film is often downgauged, the processor must recheck thickness profile and tensile balance after the transition. A 10% to 20% reduction in film thickness may maintain equivalent dart impact performance compared with LDPE, but this must be verified on the specific air ring and die geometry rather than assumed from resin data sheets.

Additive stabilization must be reviewed if melt temperature exceeds 230 °C for extended hold-up times. LLDPE film grades contain slip and antiblock loadings of 500 ppm to 2000 ppm, with processing stabilizer packages often present at 300 ppm to 1000 ppm. Overheating above 245 °C accelerates consumption of hindered phenolic and phosphite stabilizers, leading to die-lip plate-out and gel formation. The line should be purged if die-lip build-up becomes visible within a 2 h run. LLDPE should not be combined with amine-based additives in the presence of residual acidic species because discoloration and reduced long-term thermal stability can result. If a melt-temperature excursion above 250 °C occurs, production should be stopped and a purge compound should be used to remove degraded material before stable film is restored. Food-contact LLDPE film grades must continue to meet FDA 21 CFR 177.1520 and European EU 10/2011 requirements, which are unaffected by the mechanical changeover but can be compromised by degraded polymer or excessive additive consumption.